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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..6833f05 --- /dev/null +++ b/.gitattributes @@ -0,0 +1,3 @@ +* text=auto +*.txt text +*.md text diff --git a/10855-0.txt b/10855-0.txt new file mode 100644 index 0000000..983161e --- /dev/null +++ b/10855-0.txt @@ -0,0 +1,2662 @@ +*** START OF THE PROJECT GUTENBERG EBOOK 10855 *** + +_Is Mars Habitable?_ + +A CRITICAL EXAMINATION OF PROFESSOR PERCIVAL LOWELL'S BOOK +"MARS AND ITS CANALS," WITH AN ALTERNATIVE EXPLANATION + +BY ALFRED RUSSEL WALLACE F.R.S., ETC. + + + +PREFACE. + +This small volume was commenced as a review article on Professor +Percival Lowell's book, _Mars and its Canals_, with the object of +showing that the large amount of new and interesting facts contained in +this work did not invalidate the conclusion I had reached in 1902, and +stated in my book on _Man's Place in the Universe_, that Mars was not +habitable. + +But the more complete presentation of the opposite view in the volume +now under discussion required a more detailed examination of the various +physical problems involved, and as the subject is one of great, popular, +as well as scientific interest, I determined to undertake the task. + +This was rendered the more necessary by the fact that in July last +Professor Lowell published in the _Philosophical Magazine_ an elaborate +mathematical article claiming to demonstrate that, notwithstanding its +much greater distance from the sun and its excessively thin atmosphere, +Mars possessed a climate on the average equal to that of the south of +England, and in its polar and sub-polar regions even less severe than +that of the earth. Such a contention of course required to be dealt +with, and led me to collect information bearing upon temperature in all +its aspects, and so enlarging my criticism that I saw it would be +necessary to issue it in book form. + +Two of my mathematical friends have pointed out the chief omission which +vitiates Professor Lowell's mathematical conclusions--that of a failure +to recognise the very large conservative and _cumulative_ effect of a +dense atmosphere. This very point however I had already myself discussed +in Chapter VI., and by means of some remarkable researches on the heat +of the moon and an investigation of the causes of its very low +temperature, I have, I think, demonstrated the incorrectness of Mr. +Lowell's results. In my last chapter, in which I briefly summarise the +whole argument, I have further strengthened the case for very severe +cold in Mars, by adducing the rapid lowering of temperature universally +caused by diminution of atmospheric pressure, as manifested in the +well-known phenomenon of temperate climates at moderate heights even +close to the equator, cold climates at greater heights even on extensive +plateaux, culminating in arctic climates and perpetual snow at heights +where the air is still far denser than it is on the surface of Mars. +This argument itself is, in my opinion, conclusive; but it is enforced +by two others equally complete, neither of which is adequately met by +Mr. Lowell. + +The careful examination which I have been led to give to the whole of +the phenomena which Mars presents, and especially to the discoveries of +Mr. Lowell, has led me to what I hope will be considered a satisfactory +physical explanation of them. This explanation, which occupies the whole +of my seventh chapter, is founded upon a special mode of origin for +Mars, derived from the Meteoritic Hypothesis, now very widely adopted by +astronomers and physicists. Then, by a comparison with certain +well-known and widely spread geological phenomena, I show how the great +features of Mars--the 'canals' and 'oases'--may have been caused. This +chapter will perhaps be the most interesting to the general reader, as +furnishing a quite natural explanation of features of the planet which +have been termed 'non-natural' by Mr. Lowell. + +Incidentally, also, I have been led to an explanation of the highly +volcanic nature of the moon's surface. This seems to me absolutely to +require some such origin as Sir George Darwin has given it, and thus +furnishes corroborative proof of the accuracy of the hypothesis that our +moon has had an unique origin among the known satellites, in having been +thrown off from the earth itself. + +I am indebted to Professor J. H. Poynting, of the University of +Birmingham, for valuable suggestions on some of the more difficult +points of mathematical physics here discussed, and also for the critical +note (at the end of Chapter V.) on Professor Lowell's estimate of the +temperature of Mars. + +BROADSTONE, DORSET, _October_ 1907. + + + +TABLE OF CONTENTS. + + +CHAPTER I. + +EARLY OBSERVERS OF MARS, +--Mars the only planet the surface of which is + distinctly visible +--Early observation of the snow-caps and seas +--The 'canals' seen by Schiaparelli in 1877 +--Double canals first seen in 1881 +--Round spots at intersection of canals seen + by Pickering in 1892 +--Confirmed by Lowell in 1894 +--Changes of colour seen in 1892 and 1894 +--Existence of seas doubted by Pickering and + Barnard in 1894. + + +CHAPTER II. + +MR. LOWELL'S DISCOVERIES AND THEORIES, +--Observatory at Flagstaff, Arizona +--Illustrated book on his observations of + Mars +--Volume on Mars and its canals, 1906 +--Non-natural features +--The canals as irrigation works of an intelligent + race +--A challenge to the thinking world +--The canals as described and mapped by Mr. Lowell +--The double canals +--Dimensions of the canals +--They cross the supposed seas +--Circular black spots termed oases +--An interesting volume. + + +CHAPTER III. + +THE CLIMATE AND PHYSIOGRAPHY OF MARS, +--No permanent water on Mars +--Rarely any clouds and no rain +--Snow-caps the only source of water +--No mountains, hills, or valleys on Mars +--Two-thirds of the surface a desert +--Water from the snow-caps too scanty to supply + the canals +--Miss Clerke's views as to the water-supply +--Description of some of the chief canals +--Mr. Lowell on the purpose of the canals +--Remarks on the same +--Mr. Lowell on relation of canals to oases and + snow-caps +--Critical remarks on the same. + + +CHAPTER IV. + +IS ANIMAL LIFE POSSIBLE ON MARS? +--Water and air essential for animal life +--Atmosphere of Mars assumed to be like ours +--Blue tint near melting snow the only evidence + of water +--Fallacy of this argument +--Dr. Johnstone Stoney's proof that water-vapour + cannot exist on Mars +--Spectroscope gives no evidence of water. + + +CHAPTER V. + +TEMPERATURE OF MARS--MR. LOWELL'S ESTIMATE, +--Problem of terrestrial temperature +--Ice under recent lava +--Tropical oceans ice-cold at bottom +--Earth's surface-heat all from the sun +--Absolute zero of temperature +--Complex problem of planetary temperatures +--Mr. Lowell's investigation of the problem +--Abstract of Mr. Lowell's paper +--Critical remarks on Mr. Lowell's paper. + + +CHAPTER VI. + +A NEW ESTIMATE OF THE TEMPERATURE OF MARS, +--Langley's determination of lunar heat +--Rapid loss of heat by radiation on the + earth +--Rapid loss of heat on moon during eclipse +--Sir George Darwin's theory of the moon's origin +--Very's researches on the moon's temperature +--Application of these results to the case of Mars +--Cause of great difference of temperatures of earth + and moon +--Special features of Mars influencing its + temperature +--Further criticism of Mr. Lowell's article +--Very low temperature of arctic regions on Mars. + + +CHAPTER VII. + +A SUGGESTION AS TO THE 'CANALS' OF MARS, +--Special features of the canals +--Mr. Pickering's suggested explanation +--The meteoritic hypotheses of origin of planets +--Probable mode of origin of Mars +--Structural straight lines on the earth +--Probable origin of the surface-features of Mars +--Symmetry of basaltic columns +--How this applies to Mars +--Suggested explanation of the oases +--Probable function of the great fissures +--Suggested origin of blue patches adjacent to snow-caps +--The double canals +--Concluding remarks on the canals. + + +CHAPTER VIII. + +PAGE SUMMARY AND CONCLUSION, +--The canals the origin of Mr. Lowell's theory +--Best explained as natural features +--Evaporation difficulty not met by Mr. Lowell +--How did Martians live without the canals +--Radiation due to scanty atmosphere not taken account + of +--Three independent proofs of low temperature and + uninhabitability of Mars +--Conclusion. + + +CHAPTER I. + +EARLY OBSERVERS OF MARS. + +Few persons except astronomers fully realise that of all the planets of +the Solar system the only one whose solid surface has been seen with +certainty is Mars; and, very fortunately, that is also the only one +which is sufficiently near to us for the physical features of the +surface to be determined with any accuracy, even if we could see it in +the other planets. Of Venus we probably see only the upper surface of +its cloudy atmosphere.[1] As regards Jupiter and Saturn this is still +more certain, since their low density will only permit of a +comparatively small proportion of their huge bulk being solid. Their +belts are but the cloud-strata of their upper atmosphere, perhaps +thousands of miles above their solid surfaces, and a somewhat similar +condition seems to prevail in the far more remote planets Uranus and +Neptune. It has thus happened, that, although as telescopic objects of +interest and beauty, the marvellous rings of Saturn, the belts and +ever-changing aspects of the satellites of Jupiter, and the moon-like +phases of Venus, together with its extreme brilliancy, still remain +unsurpassed, yet the greater amount of details of these features when +examined with the powerful instruments of the nineteenth century have +neither added much to our knowledge of the planets themselves or led to +any sensational theories calculated to attract the popular imagination. + +[Footnote 1: Mercury also seems to have a scanty atmosphere, but as its +mass is only one-thirtieth that of the earth it can retain only the +heavier gases, and its atmosphere may be dust-laden, as is that of Mars, +according to Mr. Lowell. Its dusky markings, as seen by Schiaparelli, +seem to be permanent, and they are also for considerable periods +unchangeable in position, indicating that the planet keeps the same face +towards the sun as does Venus. This was confirmed by Mr. Lowell in 1896. +Its distance from us and unfavourable position for observation must +prevent us from obtaining any detailed knowledge of its actual surface, +though its low reflective power indicates that the surface may be really +visible.] + +But in the case of Mars the progress of discovery has had a very +different result. The most obvious peculiarity of this planet--its polar +snow-caps--were seen about 250 years ago, but they were first proved to +increase and decrease alternately, in the summer and winter of each +hemisphere, by Sir William Herschell in the latter part of the +eighteenth century. This fact gave the impulse to that idea of +similarity in the conditions of Mars and the earth, which the +recognition of many large dusky patches and streaks as water, and the +more ruddy and brighter portions as land, further increased. Added to +this, a day only about half an hour longer than our own, and a +succession of seasons of the same character as ours but of nearly double +the length owing to its much longer year, seemed to leave little wanting +to render this planet a true earth on a smaller scale. It was therefore +very natural to suppose that it must be inhabited, and that we should +some day obtain evidence of the fact. + +_The Canals discovered by Schiaparelli._ + +Hence the great interest excited when Schiaparelli, at the Milan +Observatory, during the very favourable opposition of 1877 and 1879, +observed that the whole of the tropical and temperate regions from 60° +N. to 60° S. Lat. were covered with a remarkable network of broader +curved and narrower straight lines of a dark colour. At each successive +favourable opposition, these strange objects called _canali_ (channels) +by their discoverer, but rather misleadingly 'canals' in England and +America, were observed by means of all the great telescopes in the +world, and their reality and general features became well established. +In Schiaparelli's first map they were represented as being much broader +and less sharply defined than he himself and other observers found by +later and equally favourable observations that they really were. + +_Discovery of the Double Canals._ + +In 1881 another strange feature was discovered by Schiaparelli, who +found that about twenty canals which had previously been seen single +were now distinctly double, that is, that they consisted of two parallel +lines, equally distinct and either very close together or a considerable +distance apart. This curious appearance was at first thought to be due +to some instrumental defect or optical illusion; but as it was soon +confirmed by other observers with the best instruments and in widely +different localities it became in time accepted as a real phenomenon of +the planet's surface. + +_Round Spots discovered in_ 1892. + +At the favourable opposition of 1892, Mr. W. H. Pickering noticed that +besides the 'seas' of various sizes there were numerous very small black +spots apparently quite circular and occurring at every intersection or +starting-point of the 'canals.' Many of these had been seen by +Schiaparelli as larger and ill-defined dark patches, and were termed +seas or lakes; but Mr. Pickering's observatory was at Arequipa in Peru, +about 8000 feet above the sea, and with such perfect atmospheric +conditions as were, in his opinion, equal to a doubling of telescopic +aperture. They were soon detected by other observers, especially by Mr. +Lowell in 1894, who thus wrote of them: + +"Scattered over the orange-ochre groundwork of the continental regions +of the planet, are any number of dark round spots. How many there may be +it is not possible to state, as the better the seeing, the more of them +there seem to be. In spite, however, of their great number, there is no +instance of one unconnected with a canal. What is more, there is +apparently none that does not lie at the junction of several canals. +Reversely, all the junctions appear to be provided with spots. Plotted +upon a globe they and their connecting canals make a most curious +network over all the orange-ochre equatorial parts of the planet, a mass +of lines and knots, the one marking being as omnipresent as the other." + +_Changes of Colour recognised._ + +During the oppositions of 1892 and 1894 it was fully recognised that a +regular course of change occurred dependent upon the succession of the +seasons, as had been first suggested by Schiaparelli. As the polar snows +melt the adjacent seas appear to overflow and spread out as far as the +tropics, and are often seen to assume a distinctly green colour. These +remarkable changes and the extraordinary phenomena of perfect straight +lines crossing each other over a large portion of the planet's surface, +with the circular spots at their intersections, had such an appearance +of artificiality that the idea that they were really 'canals' made by +intelligent beings for purposes of irrigation, was first hinted at, and +then adopted as the only intelligible explanation, by Mr. Lowell and a +few other persons. This at once seized upon the public imagination and +was spread by the newspapers and magazines over the whole civilised +world. + +_Existence of Seas doubted._ + +At this time (1894) it began to be doubted whether there were any seas +at all on Mars. Professor Pickering thought they were far more limited +in size than had been supposed, and even might not exist as true seas. +Professor Barnard, with the Lick thirty-six inch telescope, discerned an +astonishing wealth of detail on the surface of Mars, so intricate, +minute, and abundant, that it baffled all attempts to delineate it; and +these peculiarities were seen upon the supposed seas as well as on the +land-surfaces. In fact, under the best conditions these 'seas' lost all +trace of uniformity, their appearance being that of a mountainous +country, broken by ridges, rifts, and canyons, seen from a great +elevation. As we shall see later on these doubts soon became +certainties, and it is now almost universally admitted that Mars +possesses no permanent bodies of water. + + + +CHAPTER II. + + +MR. PERCIVAL LOWELL'S DISCOVERIES AND THEORIES. + +_The Observatory in Arizona._ + +In 1894, after a careful search for the best atmospheric conditions, Mr. +Lowell established his observatory near the town of Flagstaff in +Arizona, in a very dry and uniform climate, and at an elevation of 7300 +feet above the sea. He then possessed a fine equatorial telescope of 18 +inches aperture and 26 feet focal length, besides two smaller ones, all +of the best quality. To these he added in 1896 a telescope with 24 inch +object glass, the last work of the celebrated firm of Alvan Clark & +Sons, with which he has made his later discoveries. He thus became +perhaps more favourably situated than any astronomer in the northern +hemisphere, and during the last twelve years has made a specialty of the +study of Mars, besides doing much valuable astronomical work on other +planets. + +_Mr, Lowell's recent Books upon Mars._ + +In 1905 Mr. Lowell published an illustrated volume giving a full account +of his observations of Mars from 1894 to 1903, chiefly for the use of +astronomers; and he has now given us a popular volume summarising the +whole of his work on the planet, and published both in America and +England by the Macmillan Company. This very interesting volume is fully +illustrated with twenty plates, four of them coloured, and more than +forty figures in the text, showing the great variety of details from +which the larger general maps have been constructed. + +_Non-natural Features of Mars._ + +But what renders this work especially interesting to all intelligent +readers is, that the author has here, for the first time, fully set +forth his views both as to the habitability of Mars and as to its being +actually inhabited by beings comparable with ourselves in intellect. The +larger part of the work is in fact devoted to a detailed description of +what he terms the 'Non-natural Features' of the planet's surface, +including especially a full account of the 'Canals,' single and double; +the 'Oases,' as he terms the dark spots at their intersections; and the +varying visibility of both, depending partly on the Martian seasons; +while the five concluding chapters deal with the possibility of animal +life and the evidence in favour of it. He also upholds the theory of the +canals having been constructed for the purpose of 'husbanding' the +scanty water-supply that exists; and throughout the whole of this +argument he clearly shows that he considers the evidence to be +satisfactory, and that the only intelligible explanation of the whole of +the phenomena he so clearly sets forth is, that the inhabitants of Mars +have carried out on their small and naturally inhospitable planet a vast +system of irrigation-works, far greater both in its extent, in its +utility, and its effect upon their world as a habitation for civilised +beings, than anything we have yet done upon our earth, where our +destructive agencies are perhaps more prominent than those of an +improving and recuperative character. + +_A Challenge to the Thinking World._ + +This volume is therefore in the nature of a challenge, not so much to +astronomers as to the educated world at large, to investigate the +evidence for so portentous a conclusion. To do this requires only a +general acquaintance with modern science, more especially with mechanics +and physics, while the main contention (with which I shall chiefly deal) +that the features termed 'canals' are really works of art and +necessitate the presence of intelligent organic beings, requires only +care and judgment in drawing conclusions from admitted facts. As I have +already paid some attention to this problem and have expressed the +opinion that Mars is not habitable,[2] judging from the evidence then +available, and as few men of science have the leisure required for a +careful examination of so speculative a subject, I propose here to point +out what the facts, as stated by Mr. Lowell himself, do _not_ render +even probable much less prove. Incidentally, I may be able to adduce +evidence of a more or less weighty character, which seems to negative +the possibility of any high form of animal life on Mars, and, _a +fortiori_, the development of such life as might culminate in a being +equal or superior to ourselves. As most popular works on Astronomy for +the last ten years at least, as well as many scientific periodicals and +popular magazines, have reproduced some of the maps of Mars by +Schiaparelli, Lowell, and others, the general appearance of its surface +will be familiar to most readers, who will thus be fully able to +appreciate Mr. Lowell's account of his own further discoveries which I +may have to quote. One of the _best_ of these maps I am able to give as +a frontispiece to this volume, and to this I shall mainly refer. + +[Footnote 2: _Man's Place in the Universe_ p. 267 (1903).] + +_The Canals as described by Mr. Lowell._ + +In the clear atmosphere of Arizona, Mr. Lowell has been able on various +favourable occasions to detect a network of straight lines, meeting or +crossing each other at various angles, and often extending to a thousand +or even over two thousand miles in length. They are seen to cross both +the light and the dark regions of the planet's surface, often extending +up to or starting from the polar snow-caps. Most of these lines are so +fine as only to be visible on special occasions of atmospheric clearness +and steadiness, which hardly ever occur at lowland stations, even with +the best instruments, and almost all are seen to be as perfectly +straight as if drawn with a ruler. + +_The Double Canals._ + +Under exceptionally favourable conditions, many of the lines that have +been already seen single appear double--a pair of equally fine lines +exactly parallel throughout their whole length, and appearing, as Mr. +Lowell says, "clear cut upon the disc, its twin lines like the rails of +a railway track." Both Schiaparelli and Lowell were at first so +surprised at this phenomenon that they thought it must be an optical +illusion, and it was only after many observations in different years, +and by the application of every conceivable test, that they both became +convinced that they witnessed a real feature of the planet's surface. +Mr. Lowell says he has now seen them hundreds of times, and that his +first view of one was 'the most startlingly impressive' sight he has +ever witnessed. + +_Dimensions of the Canals._ + +A few dimensions of these strange objects must be given in order that +readers may appreciate their full strangeness and inexplicability. Out +of more than four hundred canals seen and recorded by Mr. Lowell, +fifty-one, or about one eighth, are either constantly or occasionally +seen to be double, the appearance of duplicity being more or less +periodical. Of 'canals' generally, Mr. Lowell states that they vary in +length from a few hundred to a few thousand miles long, one of the +largest being the Phison, which he terms 'a typical double canal,' and +which is said to be 2250 miles long, while the distance between its two +constituents is about 130 miles.[3] The actual width of each canal is +from a minimum of about a mile up to several miles, in one case over +twenty. A great feature of the doubles is, that they are strictly +parallel throughout their whole course, and that in almost all cases +they are so truly straight as to form parts of a great circle of the +planet's sphere. A few however follow a gradual but very distinct curve, +and such of these as are double present the same strict parallelism as +those which are straight. + +[Footnote 3: This is on the opposite side of Mars from that shown in the +frontispiece.] + +_Canals extend across the Seas._ + +It was only after seventeen years of observation of the canals that it +was found that they extended also into and across the dark spots and +surfaces which by the earlier observers were termed seas, and which then +formed the only clearly distinguishable and permanent marks on the +planet's surface. At the present time, Professor Lowell states that this +"curious triangulation has been traced over almost every portion of the +planet's surface, whether dark or light, whether greenish, ochre, or +brown in colour." In some parts they are much closer together than in +others, "forming a perfect network of lines and spots, so that to +identify them all was a matter of extreme difficulty." Two such portions +are figured at pages 247 and 256 of Mr. Lowell's volume. + +_The Oases._ + +The curious circular black spots which are seen at the intersections of +many of the canals, and which in some parts of the surface are very +numerous, are said to be more difficult of detection than even the +lines, being often blurred or rendered completely invisible by slight +irregularities in our own atmosphere, while the canals themselves +continue visible. About 180 of these have now been found, and the more +prominent of them are estimated to vary from 75 to 100 miles in +diameter. There are however many much smaller, down to minute and barely +visible black points. Yet they all seem a little larger than the canals +which enter them. Where the canals are double, the spots (or 'oases' as +Mr. Lowell terms them) lie between the two parallel canals. + +No one can read this book without admiration for the extreme +perseverance in long continued and successful observation, the results +of which are here recorded; and I myself accept unreservedly the +substantial accuracy of the whole series. It must however always be +remembered that the growth of knowledge of the detailed markings has +been very gradual, and that much of it has only been seen under very +rare and exceptional conditions. It is therefore quite possible that, if +at some future time a further considerable advance in instrumental power +should be made, or a still more favourable locality be found, the new +discoveries might so modify present appearances as to render a +satisfactory explanation of them more easy than it is at present. + +But though I wish to do the fullest justice to Mr. Lowell's technical +skill and long years of persevering work, which have brought to light +the most complex and remarkable appearances that any of the heavenly +bodies present to us, I am obliged absolutely to part company with him +as regards the startling theory of artificial production which he thinks +alone adequate to explain them. So much is this the case, that the very +phenomena, which to him seem to demonstrate the intervention of +intelligent beings working for the improvement of their own environment, +are those which seem to me to bear the unmistakable impress of being due +to natural forces, while they are wholly unintelligible as being useful +works of art. I refer of course to the great system of what are termed +'canals,' whether single or double. Of these I shall give my own +interpretation later on. + + + +CHAPTER III. + + +THE CLIMATE AND PHYSIOGRAPHY OF MARS. + +Mr. Lowell admits, and indeed urges strongly, that there are no +permanent bodies of water on Mars; that the dark spaces and spots, +thought by the early observers to be seas, are certainly not so now, +though they may have been at an earlier period; that true clouds are +rare, even if they exist, the appearances that have been taken for them +being either dust-storms or a surface haze; that there is consequently +no rain, and that large portions (about two-thirds) of the planet's +surface have all the characteristics of desert regions. + +_Snow-caps the only Source of Water._ + +This state of things is supposed to be ameliorated by the fact of the +polar snows, which in the winter cover the arctic and about half the +temperate regions of each hemisphere alternately. The maximum of the +northern snow-caps is reached at a period of the Martian winter +corresponding to the end of February with us. About the end of March the +cap begins to shrink in size (in the Northern Hemisphere), and this goes +on so rapidly that early in the June of Mars it is reduced to its +minimum. About the same time changes of colour take place in the +adjacent darker portions of the surface, which become at first bluish, +and later a decided blue-green; but by far the larger portion, including +almost all the equatorial regions of the planet, remain always of a +reddish-ochre tint.[4] + +[Footnote 4: In 1890 at Mount Wilson, California, Mr. W.H. Pickering's +photographs of Mars on April 9th showed the southern polar cap of +moderate dimensions, but with a large dim adjacent area. Twenty-four +hours later a corresponding plate showed this same area brilliantly +white; the result apparently of a great Martian snowfall. In 1882 the +same observer witnessed the steady disappearance of 1,600,000 square +miles of the southern snow-cap, an area nearly one-third of that +hemisphere of the planet.] + +The rapid and comparatively early disappearance of the white covering +is, very reasonably, supposed to prove that it is of small thickness, +corresponding perhaps to about a foot or two of snow in north-temperate +America and Europe, and that by the increasing amount of sun-heat it is +converted, partly into liquid and partly into vapour. Coincident with +this disappearance and as a presumed result of the water (or other +liquid) producing inundations, the bluish-green tinge which appears on +the previously dark portion of the surface is supposed to be due to a +rapid growth of vegetation. + +But the evidence on this point does not seem to be clear or harmonious, +for in the four coloured plates showing the planet's surface at +successive Martian dates from December 30th to February 21st, not only +is a considerable extent of the south temperate zone shown to change +rapidly from bluish-green to chocolate-brown and then again to +bluish-green, but the portions furthest from the supposed fertilising +overflow are permanently green, as are also considerable portions in the +opposite or northern hemisphere, which one would think would then be +completely dried up. + +_No Hills upon Mars._ + +The special point to which I here wish to call attention is this. Mr. +Lowell's main contention is, that the surface of Mars is wonderfully +smooth and level. Not only are there no mountains, but there are no +hills or valleys or plateaux. This assumption is absolutely essential to +support the other great assumption, that the wonderful network of +perfectly straight lines over nearly the whole surface of the planet are +irrigation canals. It is not alleged that irregularities or undulations +of a few hundreds or even one or two thousands of feet could possibly be +detected, while certainly all we know of planetary formation or +structure point strongly towards _some_ inequalities of surface. Mr. +Lowell admits that the dark portions of the surface, when examined on +the terminator (the margin of the illuminated portion), do _look_ like +hollows and _may be_ the beds of dried-up seas; yet the supposed canals +run across these old sea-beds in perfect straight lines just as they do +across the many thousand miles of what are admitted to be deserts--which +he describes in these forcible terms: "Pitiless as our deserts are, they +are but faint forecasts of the state of things existent on Mars at the +present time." + +It appears, then, that Mr. Lowell has to face this dilemma--_Only if the +whole surface of Mars is an almost perfect level could the enormous +network of straight canals, each from hundreds to thousands of miles +long, have been possibly constructed by intelligent beings for purposes +of irrigation; but, if a complete and universal level surface exists no +such system would be necessary._ For on a level surface--or on a +surface slightly inclined from the poles towards the equator, which +would be advantageous in either case--the melting water would of itself +spread over the ground and naturally irrigate as much of the surface as +it was possible for it to reach. If the surface were not level, but +consisted of slight elevations and expressions to the extent of a few +scores or a few hundreds of feet, then there would be no possible +advantage in cutting straight troughs through these elevations in +various directions with water flowing at the bottom of them. In neither +case, and in hardly any conceivable case, could these perfectly straight +canals, cutting across each other in every direction and at very varying +angles, be of any use, or be the work of an intelligent race, if any +such race could possibly have been developed under the adverse +conditions which exist in Mars. + +_The Scanty Water-supply._ + +But further, if there were any superfluity of water derived from the +melting snow beyond what was sufficient to moisten the hollows indicated +by the darker portions of the surface, which at the time the water +reaches them acquire a green tint (a superfluity under the circumstances +highly improbable), that superfluity could be best utilised by widening, +however little, the borders to which natural overflow had carried it. +Any attempt to make that scanty surplus, by means of overflowing canals, +travel across the equator into the opposite hemisphere, through such a +terrible desert region and exposed to such a cloudless sky as Mr. Lowell +describes, would be the work of a body of madmen rather than of +intelligent beings. It may be safely asserted that not one drop of water +would escape evaporation or insoak at even a hundred miles from its +source. [5] + +[Footnote 5: What the evaporation is likely to be in Mars may be +estimated by the fact, stated by Professor J.W. Gregory in his recent +volume on 'Australia' in _Stanford's Compendium_, that in North-West +Victoria evaporation is at the rate of ten feet per annum, while in +Central Australia it is very much more. The greatly diminished +atmospheric pressure in Mars will probably more than balance the loss of +sun-heat in producing rapid evaporation.] + +_Miss Clerke on the Scanty Water-supply._ + +On this point I am supported by no less an authority than the historian +of modern astronomy, the late Miss Agnes Clerke. In the _Edinburgh +Review_ (of October 1896) there is an article entitled 'New Views about +Mars,' exhibiting the writer's characteristic fulness of knowledge and +charm of style. Speaking of Mr. Lowell's idea of the 'canals' carrying +the surplus water across the equator, far into the opposite hemisphere, +for purposes of irrigation there (which we see he again states in the +present volume), Miss Clerke writes: "We can hardly imagine so shrewd a +people as the irrigators of Thule and Hellas[6] wasting labour, and the +life-giving fluid, after so unprofitable a fashion. There is every +reason to believe that the Martian snow-caps are quite flimsy +structures. Their material might be called snow _soufflé_, since, owing +to the small power of gravity on Mars, snow is almost three times +lighter there than here. Consequently, its own weight can have very +little effect in rendering it compact. Nor, indeed, is there time for +much settling down. The calotte does not form until several months after +the winter solstice, and it begins to melt, as a rule, shortly after the +vernal equinox. (The interval between these two epochs in the southern +hemisphere of Mars is 176 days.) The snow lies on the ground, at the +outside, a couple of months. At times it melts while it is still fresh +fallen. Thus, at the opposition of 1881-82 the spreading of the northern +snows was delayed until seven weeks after the equinox: and they had, +accordingly, no sooner reached their maximum than they began to decline. +And Professor Pickering's photographs of April 9th and 10th, 1890, +proved that the southern calotte may assume its definitive proportions +in a single night. + +[Footnote 6: Areas on Mars so named.] + +"No attempt has yet been made to estimate the quantity of water +derivable from the melting of one of these formations; yet the +experiment is worth trying as a help towards defining ideas. Let us +grant that the average depth of snow in them, of the delicate Martian +kind, is twenty feet, equivalent at the most to one foot of water. The +maximum area covered, of 2,400,000 square miles, is nearly equal to that +of the United States, while the whole globe of Mars measures 55,500,000 +square miles, of which one-third, on the present hypothesis, is under +cultivation, and in need of water. Nearly the whole of the dark areas, +as we know, are situated in the southern hemisphere, of which they +extend over, at the very least, 17,000,000 square miles; that is to say, +they cover an area, in round numbers, seven times that of the snow-cap. +Only one-seventh of a foot of water, accordingly, could possibly be +made available for their fertilisation, supposing them to get the entire +advantage of the spring freshet. Upon a stint of less than two inches of +water these fertile lands are expected to flourish and bear abundant +crops; and since they completely enclose the polar area they are +necessarily served first. The great emissaries for carrying off the +surplus of their aqueous riches, would then appear to be superfluous +constructions, nor is it likely that the share in those riches due to +the canals and oases, intricately dividing up the wide, dry, continental +plains, can ever be realised. + +"We have assumed, in our little calculation, that the entire contents of +a polar hood turn to water; but in actual fact a considerable proportion +of them must pass directly into vapour, omitting the intermediate stage. +Even with us a large quantity of snow is removed aerially; and in the +rare atmosphere of Mars this cause of waste must be especially +effective. Thus the polar reservoirs are despoiled in the act of being +opened. Further objections might be taken to Mr. Lowell's irrigation +scheme, but enough has been said to show that it is hopelessly +unworkable." + +It will be seen that the writer of this article accepted the existence +of water on Mars, on the testimony of Sir W. Huggins, which, in view of +later observations, he has himself acknowledged to be valueless. Dr. +Johnstone Stoney's proof of its absence, derived from the molecular +theory of gases, had not then been made public. + +_Description of some of the Canals._ + +At the end of his volume Mr. Lowell gives a large chart of Mars on +Mercator's projection, showing the canals and other features seen during +the opposition of 1905. This contains many canals not shown on the map +here reproduced (see frontispiece), and some of the differences between +the two are very puzzling. Looking at our map, which shows the +north-polar snow below, so that the south pole is out of the view at the +top of the map, the central feature is the large spot Ascraeeus Lucus, +from which ten canals diverge centrally, and four from the sides, +forming wide double canals, fourteen in all. There is also a canal named +Ulysses, which here passes far to the right of the spot, but in the +large chart enters it centrally. Looking at our map we see, going +downwards a little to the left, the canal Udon, which runs through a +dark area quite to the outer margin. In the dark area, however, there is +shown on the chart a spot Aspledon Lucus, where five canals meet, and if +this is taken as a terminus the Udon canal is almost exactly 2000 miles +long, and another on its right, Lapadon, is the same length, while Ich, +running in a slightly curved line to a large spot (Lucus Castorius on +the chart) is still longer. The Ulysses canal, which (on the chart) runs +straight from the point of the Mare Sirenum to the Astraeeus Lucus is +about 2200 miles long. Others however are even longer, and Mr. Lowell +says: "With them 2000 miles is common; while many exceed 2500; and the +Eumenides-Orcus is 3540 miles from the point where it leaves Lucus +Phoeniceus to where it enters the Trivium Charontis." This last canal is +barely visible on our map, its commencement being indicated by the word +Eumenides. + +The Trivium Charontis is situated just beyond the right-hand margin of +our map. It is a triangular dark area, the sides about 200 miles long, +and it is shown on the chart as being the centre from which radiate +thirteen canals. Another centre is Aquae Calidae situated at the point +of a dark area running obliquely from 55° to 35° N. latitude, and, as +shown on a map of the opposite hemisphere to our map, has nearly twenty +canals radiating from it in almost every direction. Here at all events +there seems to be no special connection with the polar snow-caps, and +the radiating lines seem to have no intelligent purpose whatever, but +are such as might result from fractures in a glass globe produced by +firing at it with very small shots one at a time. Taking the whole +series of them, Mr. Lowell very justly compares them to "a network which +triangulates the surface of the planet like a geodetic survey, into +polygons of all shapes and sizes." + +At the very lowest estimate the total length of the canals observed and +mapped by Mr. Lowell must be over a hundred thousand miles, while he +assures us that numbers of others have been seen over the whole surface, +but so faintly or on such rare occasions as to elude all attempts to fix +their position with certainty. But these, being of the same character +and evidently forming part of the same system, must also be artificial, +and thus we are led to a system of irrigation of almost unimaginable +magnitude on a planet which has no mountains, no rivers, and no rain to +support it; whose whole water-supply is derived from polar snows, the +amount of which is ludicrously inadequate to need any such world-wide +system; while the low atmospheric pressure would lead to rapid +evaporation, thus greatly diminishing the small amount of moisture that +is available. Everyone must, I think, agree with Miss Clerke, that, even +admitting the assumption that the polar snows consist of frozen water, +the excessively scanty amount of water thus obtained would render any +scheme of world-wide distribution of it hopelessly unworkable. + +The very remarkable phenomena of the duplication of many of the lines, +together with the darkspots--the so-called oases--at their +intersections, are doubtless all connected in some unknown way with the +constitution and past history of the planet; but, on the theory of the +whole being works of art, they certainly do _not_ help to remove any of +the difficulties which have been shown to attend the theory that the +single lines represent artificial canals of irrigation with a strip of +verdure on each side of them produced by their overflow. + +_Lowell on the Purpose of the Canals._ + +Before leaving this subject it will be well to quote Mr. Lowell's own +words as to the supposed perfectly level surface of Mars, and his +interpretation of the origin and purpose of the 'canals': + +"A body of planetary size, if unrotating, becomes a sphere, except for +solar tidal deformation; if rotating, it takes on a spheroidal form +exactly expressive, so far as observation goes, of the so-called +centrifugal force at work. Mars presents such a figure, being flattened +out to correspond to its axial rotation. Its surface therefore is in +fluid equilibrium, or, in other words, a particle of liquid at any point +of its surface at the present time would stay where it was devoid of +inclination to move elsewhere. Now the water which quickens the verdure +of the canals moves from the pole down to the equator as the season +advances. This it does then irrespective of gravity. No natural force +propels it, and the inference is forthright and inevitable that it is +artificially helped to its end. There seems to be no escape from this +deduction. Water only flows downhill, and there is no such thing as +downhill on a surface already in fluid equilibrium. A few canals might +presumably be so situated that their flow could, by inequality of +terrane, lie equatorward, but not all....Now it is not in particular but +by general consent that the canal-system of Mars develops from pole to +equator. From the respective times at which the minima take place, it +appears that the canal quickening occupies fifty-two days, as evidenced +by the successive vegetal darkenings, to descend from latitude 72° north +to latitude 0°, a journey of 2650 miles. This gives for the water a +speed of fifty-one miles a day, or 2.1 miles an hour. The rate of +progression is remarkably uniform, and this abets the deduction as to +assisted transference. But the fact is more unnatural yet. The growth +pays no regard to the equator, but proceeds across it as if it did not +exist into the planet's other hemisphere. Here is something still more +telling than travel to this point. For even if we suppose, for the sake +of argument, that natural forces took the water down to the equator, +their action must there be certainly reversed, and the equator prove a +dead-line, to pass which were impossible" (pp. 374-5). + +I think my readers will agree with me that this whole argument is one of +the most curious ever put forth seriously by an eminent man of science. +Because the polar compression of Mars is about what calculation shows it +ought to be in accordance with its rate of rotation, its surface is in a +state of 'fluid equilibrium,' and must therefore be absolutely level +throughout. But the polar compression of the earth equally agrees with +calculation; therefore its surface is also in 'fluid equilibrium'; +therefore it also ought to be as perfectly level on land as it is on the +ocean surface! But as we know this is very far from being the case, why +must it be so in Mars? Are we to suppose Mars to have been formed in +some totally different way from other planets, and that there neither is +nor ever has been any reaction between its interior and exterior forces? +Again, the assumption of perfect flatness is directly opposed to all +observation and all analogy with what we see on the earth and moon. It +gives no account whatever of the numerous and large dark patches, once +termed seas, but now found to be not so, and to be full of detailed +markings and varied depths of shadow. To suppose that these are all the +same dead-level as the light-coloured portions are assumed to be, +implies that the darkness is one of material and colour only, not of +diversified contour, which again is contrary to experience, since +difference of material with us always leads to differences in rate of +degradation, and hence of diversified contour, as these dark spaces +actually show themselves under favourable conditions to independent +observers. + +_Lowell on the System of Canals as a whole._ + +We will now see what Mr. Lowell claims to be the plain teaching of the +'canals' as a whole: + +"But last and all-embracing in its import is the system which the canals +form. Instead of running at hap-hazard, the canals are interconnected in +a most remarkable manner. They seek centres instead of avoiding them. +The centres are linked thus perfectly one with another, an arrangement +which could not result from centres, whether of explosion or otherwise, +which were themselves discrete. Furthermore, the system covers the whole +surface of the planet, dark areas and light ones alike, a world-wide +distribution which exceeds the bounds of natural possibility. Any force +which could act longitudinally on such a scale must be limited +latitudinally in its action, as witness the belts of Jupiter and the +spots upon the sun. Rotational, climatic, or other physical cause could +not fail of zonal expression. Yet these lines are grandly indifferent to +such competing influences. Finally, the system, after meshing the +surface in its entirety, runs straight into the polar caps. + +"It is, then, a system whose end and aim is the tapping of the snow-cap +for the water there semi-annually let loose; then to distribute it over +the planet's face" (p. 373). + +Here, again, we have curiously weak arguments adduced to support the +view that these numerous straight lines imply works of art rather than +of nature, especially in the comparison made with the belts of Jupiter +and the spots on the sun, both purely atmospheric phenomena, whereas the +lines on Mars are on the solid surface of the planet. Why should there +be any resemblance between them? Every fact stated in the above +quotation, always keeping in mind the physical conditions of the +planet--its very tenuous atmosphere and rainless desert-surface--seem +wholly in favour of a purely natural as opposed to an artificial origin; +and at the close of this discussion I shall suggest one which seems to +me to be at least possible, and to explain the whole series of the +phenomena set forth and largely discovered by Mr. Lowell, in a simpler +and more probable manner than does his tremendous assumption of their +being works of art. Readers who may not possess Mr. Lowell's volume will +find three of his most recent maps of the 'canals' reproduced in +_Nature_ of October 11th, 1906. + + + +CHAPTER IV. + + +IS ANIMAL LIFE POSSIBLE ON MARS? + +Having now shown, that, even admitting the accuracy of all Mr. Lowell's +observations, and provisionally accepting all his chief conclusions as +to the climate, the nature of the snow-caps, the vegetation, and the +animal life of Mars, yet his interpretation of the lines on its surface +as being veritably 'canals,' constructed by intelligent beings for the +special purpose of carrying water to the more arid regions, is wholly +erroneous and rationally inconceivable. I now proceed to discuss his +more fundamental position as to the actual habitability of Mars by a +highly organised and intellectual race of material organic beings. + +_Water and Air essential to Life._ + +Here, fortunately, the issue is rendered very simple, because Mr. Lowell +fully recognises the identity of the constitution of matter and of +physical laws throughout the solar-system, and that for any high form of +organic life certain conditions which are absolutely essential on our +earth must also exist in Mars. He admits, for example, that water is +essential, that an atmosphere containing oxygen, nitrogen, aqueous +vapour, and carbonic acid gas is essential, and that an abundant +vegetation is essential; and these of course involve a +surface-temperature through a considerable portion of the year that +renders the existence of these--especially of water--possible and +available for the purposes of a high and abundant animal life. + +_Blue Colour the only Evidence of Water._ + +In attempting to show that these essentials actually exist on Mars he is +not very successful. He adduces evidence of an atmosphere, but of an +exceedingly scanty one, since the greatest amount he can give to it is-- +"not more than about four inches of barometric pressure as we reckon +it";[7] and he assumes, as he has a fair right to do till disproved, +that it consists of oxygen and nitrogen, with carbon-dioxide and +water-vapour, in approximately the same proportions as with us. With +regard to the last item--the water-vapour--there are however many +serious difficulties. The water-vapour of our atmosphere is derived from +the enormous area of our seas, oceans, lakes, and rivers, as well as +from the evaporation from heated lands and tropical forests of much of +the moisture produced by frequent and abundant rains. All these sources +of supply are admittedly absent from Mars, which has no permanent bodies +of water, no rain, and tropical regions which are almost entirely +desert. Many writers have therefore doubted the existence of water in +any form upon this planet, supposing that the snow-caps are not formed +of frozen water but of carbon-dioxide, or some other heavy gas, in a +frozen state; and Mr. Lowell evidently feels this to be a difficulty, +since the only fact he is able to adduce in favour of the melting snows +of the polar caps producing water is, that at the time they are melting +a marginal blue band appears which accompanies them in their retreat, +and this blue colour is said to prove conclusively that the liquid is +not carbonic acid but water. This point he dwells upon repeatedly, +stating, of these blue borders: "This excludes the possibility of their +being formed by carbon-dioxide, and shows that of all the substances we +know the material composing them must be water." + +[Footnote 7: In a paper written since the book appeared the density of +air at the surface of Mars is said to be 1/12 of the earth's.] + +This is the only proof of the existence of _water_ he adduces, and it is +certainly a most extraordinary and futile one. For it is perfectly well +known that although water, in large masses and by transmitted light, is +of a blue colour, yet shallow water by reflected light is not so; and in +the case of the liquid produced by the snow-caps of Mars, which the +whole conditions of the planet show must be shallow, and also be more or +less turbid, it cannot possibly be the cause of the 'deep blue' tint +said to result from the melting of the snow. + +But there is a very weighty argument depending on the molecular theory +of gases against the polar caps of Mars being composed of frozen water +at all. The mass and elastic force of the several gases is due to the +greater or less rapidity of the vibratory motion of their molecules +under identical conditions. The speed of these molecular motions has +been ascertained for all the chief gases, and it is found to be so great +as in certain cases to enable them to overcome the force of gravity and +escape from a planet's surface into space. Dr. G. Johnstone Stoney has +specially investigated this subject, and he finds that the force of +gravity on the earth is sufficient to retain all the gases composing its +atmosphere, but not sufficient to retain hydrogen; and as a consequence, +although this gas is produced in small quantities by volcanoes and by +decomposing vegetation, yet no trace of it is found in our atmosphere. +The moon however, having only one-eightieth the mass of the earth, +cannot retain any gas, hence its airless and waterless condition. + +_Water Vapour cannot exist on Mars._ + +Now, Dr. Stoney finds that in order to retain water vapour permanently a +planet must have a mass at least a quarter that of the earth. But the +mass of Mars is only one-ninth that of the earth; therefore, unless +there are some special conditions that prevent its loss, this gas cannot +be present in the atmosphere. Mr. Lowell does not refer to this argument +against his view, neither does he claim the evidence of spectroscopy in +his favour. This was alleged more than thirty years ago to show the +existence of water-vapour in the atmosphere of Mars, but of late years +it has been doubted, and Mr. Lowell's complete silence on the subject, +while laying stress on such a very weak and inconclusive argument as +that from the tinge of colour that is observed a little distance from +the edge of the diminishing snow-caps, shows that he himself does not +think the fact to be thus proved. If he did he would hardly adduce such +an argument for its presence as the following: "The melting of the caps +on the one hand and their re-forming on the other affirm the presence of +water-vapour in the Martian atmosphere, of whatever else that air +consists" (p. 162). Yet absolutely the only proof he gives that the caps +are frozen water is the almost frivolous colour-argument above referred +to! + +_No Spectroscopic Evidence of Water Vapour._ + +As Sir William Huggins is the chief authority quoted for this fact, and +is referred to as being almost conclusive in the third edition of Miss +Clerke's _History of Astronomy_ in 1893, I have ascertained that his +opinion at the present time is that "there is no conclusive proof of the +presence of aqueous vapour in the atmosphere of Mars, and that +observations at the Lick Observatory (in 1895), where the conditions and +instruments are of the highest order, were negative." He also informs me +that Marchand at the Pic du Midi Observatory was unable to obtain lines +of aqueous vapour in the spectrum of Mars; and that in 1905, Slipher, at +Mr. Lowell's observatory, was unable to detect any indications of +aqueous vapour in the spectrum of Mars. + +It thus appears that spectroscopic observations are quite accordant with +the calculations founded on the molecular theory of gases as to the +absence of aqueous vapour, and therefore presumably of liquid water, +from Mars. It is true that the spectroscopic argument is purely +negative, and this may be due to the extreme delicacy of the +observations required; but that dependent on the inability of the force +of gravity to retain it is positive scientific evidence against its +presence, and, till shown to be erroneous, must be held to be +conclusive. + +This absence of water is of itself conclusive against the existence of +animal life, unless we enter the regions of pure conjecture as to the +possibility of some other liquid being able to take its place, and that +liquid being as omnipresent there as water is here. Mr. Lowell however +never takes this ground, but bases his whole theory on the fundamental +identity of the substance of the bodies of living organisms wherever +they may exist in the solar system. In the next two chapters I shall +discuss an equally essential condition, that of temperature, which +affords a still more conclusive and even crushing argument against the +suitability of Mars for the existence of organic life. + + + +CHAPTER V. + + +THE TEMPERATURE OF MARS--MR. LOWELL'S ESTIMATE. + +We have now to consider a still more important and fundamental question, +and one which Mr. Lowell does not grapple with in this volume, the +actual temperatures on Mars due to its distance from the sun and the +atmospheric conditions on which he himself lays so much stress. If I am +not greatly mistaken we shall arrive at conclusions on this subject +which are absolutely fatal to the conception of any high form of organic +life being possible on this planet. + +_The Problem of Terrestrial Temperatures._ + +In order that the problem may be understood and its importance +appreciated, it is necessary to explain the now generally accepted +principles as to the causes which determine the temperatures on our +earth, and, presumably, on all other planets whose conditions are not +wholly unlike ours. The fact of the internal heat of the earth which +becomes very perceptible even at the moderate depths reached in mines +and deep borings, and in the deepest mines becomes a positive +inconvenience, leads many people to suppose that the surface- +temperatures of the earth are partly due to this cause. But it is now +generally admitted that this is not the case, the reason being that all +rocks and soils, in their natural compacted state, are exceedingly bad +conductors of heat. + +A striking illustration of this is the fact, that a stream of lava often +continues to be red hot at a few feet depth for years after the surface +is consolidated, and is hardly any warmer than that of the surrounding +land. A still more remarkable case is that of a glacier on the +south-east side of the highest cone of Etna underneath a lava stream +with an intervening bed of volcanic sand only ten feet thick. This was +visited by Sir Charles Lyell in 1828, and a second time thirty years +later, when he made a very careful examination of the strata, and was +quite satisfied that the sand and the lava stream together had actually +preserved this mass of ice, which neither the heat of the lava above it +at its first outflow, nor the continued heat rising from the great +volcano below it, had been able to melt or perceptibly to diminish in +thirty years. Another fact that points in the same direction is the +existence over the whole floor of the deepest oceans of ice-cold water, +which, originating in the polar seas, owing to its greater density sinks +and creeps slowly along the ocean bottom to the depths of the Atlantic +and Pacific, and is not perceptibly warmed by the internal heat of the +earth. + +Now the solid crust of the earth is estimated as at least about twenty +miles in average thickness; and, keeping in mind the other facts just +referred to, we can understand that the heat from the interior passes +through it with such extreme slowness as not to be detected at the +surface, the varying temperatures of which are due entirely to solar +heat. A large portion of this heat is stored up in the surface soil, and +especially in the surface layer of the oceans and seas, thus partly +equalising the temperatures of day and night, of winter and summer, so +as greatly to ameliorate the rapid changes of temperature that would +otherwise occur. Our dense atmosphere is also of immense advantage to us +as an equaliser of temperature, charged as it almost always is with a +large quantity of water-vapour. This latter gas, when not condensed into +cloud, allows the solar heat to pass freely to the earth; but it has the +singular and highly beneficial property of absorbing and retaining the +dark or lower-grade heat given off by the earth which would otherwise +radiate into space much more rapidly. We must therefore always remember +that, very nearly if not quite, the _whole_ of _the warmth we experience +on the earth is derived from the sun._[8] + +[Footnote 8: Professor J.H. Poynting, in his lecture to the British +Association at Cambridge in 1904, says: "The surface of the earth +receives, we know, an amount of heat from the inside almost +infinitesimal compared with that which it receives from the sun, and on +the sun, therefore, we depend for our temperature."] + +In order to understand the immense significance of this conclusion we +must know what is meant by the _whole_ heat or warmth; as unless we know +this we cannot define what half or any other proportion of sun-heat +really means. Now I feel pretty sure that nine out of ten of the average +educated public would answer the following question incorrectly: The +mean temperature of the southern half of England is about 48° F. +Supposing the earth received only half the sun-heat it now receives, +what would then be the probable mean temperature of the South of +England? The majority would, I think, answer at once--About 24° F. +Nearly as many would perhaps say--48° F. is 16° above the freezing +point; therefore half the heat received would bring us down to 8° above +the freezing point, or 40° F. Very few, I think, would realise that our +share of half the amount of sun-heat received by the earth would +probably result in reducing our mean temperature to about 100° F. below +the freezing point, and perhaps even lower. This is about the very +lowest temperature yet experienced on the earth's surface. To understand +how such results are obtained a few words must be said about the +absolute zero of temperature. + +_The Zero of Temperature._ + +Heat is now believed to be entirely due to ether-vibration, which +produces a correspondingly rapid vibration of the molecules of matter, +causing it to expand and producing all the phenomena we term 'heat.' We +can conceive this vibration to increase indefinitely, and thus there +would appear to be no necessary limit to the amount of heat possible, +but we cannot conceive it to decrease indefinitely at the same uniform +rate, as it must soon inevitably come to nothing. Now it has been found +by experiment that gases under uniform pressure expand 1/273 of their +volume for each degree Centigrade of increased temperature, so that in +passing from 0° C. to 273° C. they are doubled in volume. They also +decrease in volume at the same rate for each degree below 0° C. (the +freezing point of water). Hence if this goes on to-273° C. a gas will +have no volume, or it will undergo some change of nature. Hence this is +called the zero of temperature, or the temperature to which any matter +falls which receives no heat from any other matter. It is also sometimes +called the temperature of space, or of the ether in a state of rest, if +that is possible. All the gases have now been proved to become, first +liquid and then (most of them) solid, at temperatures considerably above +this zero. + +The only way to compare the proportional temperatures of bodies, whether +on the earth or in space, is therefore by means of a scale beginning at +this natural zero, instead of those scales founded on the artificial +zero of the freezing point of water, or, as in Fahrenheit's, 32° below +it. Only by using the natural zero and measuring continuously from it +can we estimate temperatures in relative proportion to the amount of +heat received. This is termed the absolute zero, and so that we start +reckoning from that point it does not matter whether the scale adopted +is the Centigrade or that of Fahrenheit. + +_The Complex Problem of Planetary Temperatures._ + +Now if, as is the case with Mars, a planet receives only half the amount +of solar heat that we receive, owing to its greater distance from the +sun, and if the mean temperature of our earth is 60° F., this is equal +to 551° F. on the absolute scale. It would therefore appear very simple +to halve this amount and obtain 275.5° F. as the mean temperature of +that planet. But this result is erroneous, because the actual amount of +sun heat intercepted by a planet is only one condition out of many that +determine its resulting temperature. Radiation, that is loss of heat, is +going on concurrently with gain, and the rate of loss varies with the +temperature according to a law recently discovered, the loss being much +greater at high temperatures in proportion to the 4th power of the +absolute temperature. Then, again, the whole heat intercepted by a +planet does not reach its surface unless it has no atmosphere. When it +has one, much is reflected or absorbed according to complex laws +dependent on the density and composition of the atmosphere. Then, again, +the heat that reaches the actual surface is partly reflected and partly +absorbed, according to the nature of that surface--land or water, desert +or forest or snow-clad--that part which is absorbed being the chief +agent in raising the temperature of the surface and of the air in +contact with it. Very important too is the loss of heat by radiation +from these various heated surfaces at different rates; while the +atmosphere itself sends back to the surface an ever varying portion of +both this radiant and reflected heat according to distinct laws. Further +difficulties arise from the fact that much of the sun's heat consists of +dark or invisible rays, and it cannot therefore be measured by the +quantity of light only. + +From this rough statement it will be seen that the problem is an +exceedingly complex one, not to be decided off-hand, or by any simple +method. It has in fact been usually considered as (strictly speaking) +insoluble, and only to be estimated by a more or less rough +approximation, or by the method of general analogy from certain known +facts. It will be seen, from what has been said in previous chapters, +that Mr. Lowell, in his book, has used the latter method, and, by taking +the presence of water and water-vapour in Mars as proved by the +behaviour of the snow-caps and the bluish colour that results from their +melting, has deduced a temperature above the freezing point of water, as +prevalent in the equatorial regions permanently, and in the temperate +and arctic zones during a portion of each year. + +_Mr. Lowell's Mathematical Investigation of the Problem._ + +But as this result has been held to be both improbable in itself and +founded on no valid evidence, he has now, in the _London, Edinburgh, and +Dublin Philosophical Magazine_ of July 1907, published an elaborate +paper of 15 pages, entitled _A General Method for Evaluating the +Surface-Temperatures of the Planets; with special reference to the +Temperature of Mars_, by Professor Percival Lowell; and in this paper, +by what purports to be strict mathematical reasoning based on the most +recent discoveries as to the laws of heat, as well as on measurements or +estimates of the various elements and constants used in the +calculations, he arrives at a conclusion strikingly accordant with that +put forward in the recently published volume. Having myself neither +mathematical nor physical knowledge sufficient to enable me to criticise +this elaborate paper, except on a few points, I will here limit myself +to giving a short account of it, so as to explain its method of +procedure; after which I may add a few notes on what seem to me doubtful +points; while I also hope to be able to give the opinions of some more +competent critics than myself. + +_Mr. Lowell's Mode of Estimating the Surface-temperature of Mars._ + +The author first states, that Professor Young, in his _General +Astronomy_ (1898), makes the mean temperature of Mars 223.6° absolute, +by using Newton's law of heat being radiated in proportion to +temperature, and 363° abs. (=-96° F.) by Dulong and Petit's law; but +adds, that a closer determination has been made by Professor Moulton, +using Stefan's law, that radiation is as the _/4th_ power of the +temperature, whence results a mean temperature of-31° F. These estimates +assume identity of atmospheric conditions of Mars and the Earth. + +But as none of these estimates take account of the many complex factors +which interfere with such direct and simple calculations, Mr. Lowell +then proceeds to enunciate them, and work out mathematically the effects +they produce: + +(1) The whole radiant energy of the sun on striking a planet becomes +divided as follows: Part is reflected back into space, part absorbed by +the atmosphere, part transmitted to the surface of the planet. This +surface again reflects a portion and only the balance left goes to warm +the planet. + +(2) To solve this complex problem we are helped by the _albedoes_ or +intrinsic brilliancy of the planets, which depend on the proportion of +the visible rays which are reflected and which determines the +comparative brightness of their respective surfaces. We also have to +find the ratio of the invisible to the visible rays and the heating +power of each. + +(3) He then refers to the actinometer and pyroheliometer, instruments +for measuring the actual heat derived from the sun, and also to the +Bolometer, an instrument invented by Professor Langley for measuring the +invisible heat rays, which he has proved to extend to more than three +times the length of the whole heat-spectrum as previously known, and +has also shown that the invisible rays contribute 68 per cent, of the +sun's total energy.[9] + +[Footnote 9: For a short account of this remarkable instrument, see my +_Wonderful Century_, new ed., pp. 143-145.] + +(4) Then follows an elaborate estimate of the loss of heat during the +passage of the sun's rays through our atmosphere from experiments made +at different altitudes and from the estimated reflective power of the +various parts of the earth's surface--rocks and soil, ocean, forest and +snow--the final result being that three-fourths of the whole sun-heat +is reflected back into space, forming our _albedo_, while only +one-fourth is absorbed by the soil and goes to warm the air and +determine our mean temperature. + +(5) We now have another elaborate estimate of the comparative amounts of +heat actually received by Mars and the Earth, dependent on their very +different amounts of atmosphere, and this estimate depends almost wholly +on the comparative _albedoes_, that of Mars, as observed by astronomers +being 0.27, while ours has been estimated in a totally different way as +being 0.75, whence he concludes that nearly three-fourths of the +sun-heat that Mars receives reaches the surface and determines its +temperature, while we get only one-fourth of our total amount. Then by +applying Stefan's law, that the radiation is as the 4th power of the +surface temperature, he reaches the final result that the actual heating +power at the surface of Mars is considerably _more_ than on the Earth, +and would produce a mean temperature of 72° F., if it were not for the +greater conservative or blanketing power of our denser and more +water-laden atmosphere. The difference produced by this latter fact he +minimises by dwelling on the probability of a greater proportion of +carbonic-acid gas and water-vapour in the Martian atmosphere, and thus +brings down the mean temperature of Mars to 48° F., which is almost +exactly the same as that of the southern half of England. He has also, +as the result of observations, reduced the probable density of the +atmosphere of Mars to 2-1/2 inches of mercury, or only one-twelfth of +that of the Earth. + +_Critical Remarks on Mr. Lowell's Paper._ + +The last part of this paper, indicated under pars. 4 and 5, is the most +elaborate, occupying eight pages, and it contains much that seems +uncertain, if not erroneous. In particular, it seems very unlikely that +under a clear sky over the whole earth we should only receive at the +sea-level 0.23 of the solar rays which the earth intercepts (p. 167). +These data largely depend on observations made in California and other +parts of the southern United States, where the lower atmosphere is +exceptionally dust-laden. Till we have similar observations made in the +tropical forest-regions, which cover so large an area, and where the +atmosphere is purified by frequent rains, and also on the prairies and +the great oceans, we cannot trust these very local observations for +general conclusions affecting the whole earth. Later, in the same +article (p. 170), Mr. Lowell says: "Clouds transmit approximately 20 per +cent. of the heat reaching them: a clear sky at sea-level 60 per cent. +As the sky is half the time cloudy the mean transmission is 35 per +cent." These statements seem incompatible with that quoted above. + +The figure he uses in his calculations for the actual albedo of the +earth, 0.75, is also not only improbable, but almost self-contradictory, +because the albedo of cloud is 0.72, and that of the great cloud-covered +planet, Jupiter, is given by Lowell as 0.75, while Zollner made it only +0.62. Again, Lowell gives Venus an albedo of 0.92, while Zollner made it +only 0.50 and Mr. Gore 0.65. This shows the extreme uncertainty of these +estimates, while the fact that both Venus and Jupiter are wholly +cloud-covered, while we are only half-covered, renders it almost +certain that our albedo is far less than Mr. Lowell makes it. It is +evident that mathematical calculations founded upon such uncertain data +cannot yield trustworthy results. But this is by no means the only case +in which the data employed in this paper are of uncertain value. +Everywhere we meet with figures of somewhat doubtful accuracy. Here we +have somebody's 'estimate' quoted, there another person's 'observation,' +and these are adopted without further remark and used in the various +calculations leading to the result above quoted. It requires a practised +mathematician, and one fully acquainted with the extensive literature of +this subject, to examine these various data, and track them through the +maze of formulae and figures so as to determine to what extent they +affect the final result. + +There is however one curious oversight which I must refer to, as it is a +point to which I have given much attention. Not only does Mr. Lowell +assume, as in his book, that the 'snows' of Mars consist of frozen +water, and that therefore there _is_ water on its surface and +water-vapour in its atmosphere, not only does he ignore altogether Dr. +Johnstone Stoney's calculations with regard to it, which I have already +referred to, but he uses terms that imply that water-vapour is one of +the heavier components of our atmosphere. The passage is at p. 168 of +the _Philosophical Magazine._ After stating that, owing to the very +small barometric pressure in Mars, water would boil at 110° F., he adds: +"The sublimation at lower temperatures would be correspondingly +increased. Consequently the amount of water-vapour in the Martian air +must on that score be relatively greater than our own." Then follows +this remarkable passage: "Carbon-dioxide, because of its greater +specific gravity, would also be in relatively greater amount so far as +this cause is considered. For the planet would part, _caeteris paribus_, +with its lighter gases the quickest. Whence as regards both water-vapour +and carbon-dioxide we have reason to think them in relatively greater +quantity than in our own air at corresponding barometric pressure." I +cannot understand this passage except as implying that 'water-vapour and +carbon-dioxide' are among the heavier and not among the lighter gases of +the atmosphere--those which the planet 'parts with quickest.' But this +is just what water-vapour _is_, being a little less than two-thirds the +weight of air (0.6225), and one of those which the planet _would_ part +with the quickest, and which, according to Dr. Johnstone Stoney, it +loses altogether. + * * * * * + +Note on Professor Lowell's article in the _Philosophical Magazine_; by +J.H. Poynting, F.R.S., Professor of Physics in the University of +Birmingham. + +"I think Professor Lowell's results are erroneous through his neglect of +the heat stored in the air by its absorption of radiation both from the +sun and from the surface. The air thus heated radiates to the surface +and keeps up the temperature. I have sent to the _Philosophical +Magazine_ a paper in which I think it is shown that when the radiation +by the atmosphere is taken into account the results are entirely +changed. The temperature of Mars, with Professor Lowell's data, still +comes out far below the freezing-point--still further below than the +increased distance alone would make it. Indeed, the lower temperature on +elevated regions of the earth's surface would lead us to expect this. I +think it is impossible to raise the temperature of Mars to anything like +the value obtained by Professor Lowell, unless we assume some quality in +his atmosphere entirely different from any found in our own atmosphere." +J.H. POYNTING. October 19, 1907. + + + +CHAPTER VI. + + +A NEW ESTIMATE OF THE TEMPERATURE OF MARS. + +When we are presented with a complex problem depending on a great number +of imperfectly ascertained data, we may often check the results thus +obtained by the comparison of cases in which some of the more important +of these data are identical, while others are at a maximum or a minimum. +In the present case we can do this by a consideration of the Moon as +compared with the Earth and with Mars. + +_Langley's Determination of the Moon's Temperature._ + +In the moon we see the conditions that prevail in Mars both exaggerated +and simplified. Mars has a very scanty atmosphere, the moon none at all, +or if there is one it is so excessively scanty that the most refined +observations have not detected it. All the complications arising from +the possible nature of the atmosphere, and its complex effects upon +reflection, absorption, and radiation are thus eliminated. The mean +distance of the moon from the sun being identical with that of the +earth, the total amount of heat intercepted must also be identical; only +in this case the whole of it reaches the surface instead of one-fourth +only, according to Mr. Lowell's estimate for the earth. + +Now, by the most refined observations with his Bolometer, Mr. Langley +was able to determine the temperature of the moon's surface exposed to +undimmed sunshine for fourteen days together; and he found that, even in +that portion of it on which the sun was shining almost vertically, the +temperature rarely rose above the freezing point of water. However +extraordinary this result may seem, it is really a striking confirmation +of the accuracy of the general laws determining temperature which I have +endeavoured to explain in the preceding chapter. For the same surface +which has had fourteen days of sunshine has also had a preceding +fourteen days of darkness, during which the heat which it had +accumulated in its surface layers would have been lost by free radiation +into stellar space. It thus acquires during its day a maximum +temperature of only 491° F. absolute, while its minimum, after 14 days' +continuous radiation, must be very low, and is, with much reason, +supposed to approach the absolute zero. + +_Rapid Loss of Heat by Radiation on the Earth._ + +In order better to comprehend what this minimum may be under extreme +conditions, it will be useful to take note of the effects it actually +produces on the earth in places where the conditions are nearest to +those existing on the moon or on Mars, though never quite equalling, or +even approaching very near them. It is in our great desert regions, and +especially on high plateaux, that extreme aridity prevails, and it is in +such districts that the differences between day and night temperatures +reach their maximum. It is stated by geographers that in parts of the +Great Sahara the surface temperature is sometimes 150° F., while during +the night it falls nearly or quite to the freezing point--a difference +of 118 degrees in little more than 12 hours.[10] In the high desert +plains of Central Asia the extremes are said to be even greater.[11] +Again, in his _Universal Geography_, Reclus states that in the Armenian +Highlands the thermometer oscillates between 13° F. and 112°F. We may +therefore, without any fear of exaggeration, take it as proved that a +fall of 100° F. in twelve or fifteen hours not infrequently occurs where +there is a very dry and clear atmosphere permitting continuous +insolation by day and rapid radiation by night. + +[Footnote 10: Keith Johnston's 'Africa' in _Stanford's Compendium._] + +[Footnote 11: _Chambers's Encyclopaedia_, Art. 'Deserts.'] + +Now, as it is admitted that our dense atmosphere, however dry and clear, +absorbs and reflects some considerable portion of the solar heat, we +shall certainly underestimate the radiation from the moon's surface +during its long night if we take as the basis of our calculation a +lowering of temperature amounting to 100° F. during twelve hours, as not +unfrequently occurs with us. Using these data--with Stefan's law of +decrease of radiation as the 4th power of the temperature--a +mathematical friend finds that the temperature of the moon's surface +would be reduced during the lunar night to nearly 200° F. absolute +(equal to-258° F.). + +_More Rapid Loss of Heat by the Moon._ + +Although such a calculation as the above may afford us a good +approximation to the rate of loss of heat by Mars with its very scanty +atmosphere, we have now good evidence that in the case of the moon the +loss is much more rapid. Two independent workers have investigated this +subject with very accordant results--Dr. Boeddicker, with Lord Rosse's +3-foot reflector and a Thermopile to measure the heat, and Mr. Frank +Very, with a glass reflector of 12 inches diameter and the Bolometer +invented by Mr. Langley. The very striking and unexpected fact in which +these observers agree is the sudden disappearance of much of the +stored-up heat during the comparatively short duration of a total +eclipse of the moon--less than two hours of complete darkness, and about +twice that period of partial obscuration. + +Dr. Boeddicker was unable to detect any appreciable heat at the period +of greatest obscuration; but, owing to the extreme sensitiveness of the +Bolometer, Mr. Very ascertained that those parts of the surface which +had been longest in the shadow still emitted heat "to the amount of one +per cent. of the heat to be expected from the full moon." This however +is the amount of radiation measured by the Bolometer, and to get the +temperature of the radiating surface we must apply Stefan's law of the +4th power. Hence the temperature of the moon's dark surface will be the +[fourth root of (1 over 100)] = 1 over 3.2 [A] of the highest temperature + (which we may take at the freezing-point, 491° F. abs.), or 154° F. abs., + just below the liquefaction point of air. This is about 50° lower than the +amount found by calculation from our most rapid radiation; and as this +amount is produced in a few hours, it is not too much to expect that, +when continued for more than two weeks (the lunar night), it might reach +a temperature sufficient to liquefy hydrogen (60° F. abs.), or perhaps +even below it. + +[Note A: LaTex markup $\root 4 \of {1 \over 100} = {1 \over 3.2}$ ] + +_Theory of the Moon's Origin._ + +This extremely rapid loss of heat by radiation, at first sight so +improbable as to be almost incredible, may perhaps be to some extent +explained by the physical constitution of the moon's surface, which, +from a theoretical point of view, does not appear to have received the +attention it deserves. It is clear that our satellite has been long +subjected to volcanic eruptions over its whole visible face, and these +have evidently been of an explosive nature, so as to build up the very +lofty cones and craters, as well as thousands of smaller ones, which, +owing to the absence of any degrading or denuding agencies, have +remained piled up as they were first formed. + +This highly volcanic structure can, I think, be well explained by an +origin such as that attributed to it by Sir George Darwin, and which has +been so well described by Sir Robert Ball in his small volume, _Time and +Tide._ These astronomers adduce strong evidence that the earth once +rotated so rapidly that the equatorial protuberance was almost at the +point of separation from the planet as a ring. Before this occurred, +however, the tension was so great that one large portion of the +protuberance where it was weakest broke away, and began to move around +the earth at some considerable distance from it. As about 1/50 of the +bulk of the earth thus escaped, it must have consisted of a considerable +portion of the solid crust and a much larger quantity of the liquid or +semi-liquid interior, together with a proportionate amount of the gases +which we know formed, and still form, an important part of the earth's +substance. + +As the surface layers of the earth must have been the lightest, they +would necessarily, when broken up by this gigantic convulsion, have come +together to form the exterior of the new satellite, and be soon adjusted +by the forces of gravity and tidal disturbance into a more or less +irregular spheroidal form, all whose interstices and cavities would be +filled up and connected together by the liquid or semi-liquid mass +forced up between them. Thence-forward, as the moon increased its +distance and reduced its time of rotation, in the way explained by Sir +Robert Ball, there would necessarily commence a process of escape of the +imprisoned gases at every fissure and at all points and lines of +weakness, giving rise to numerous volcanic outlets, which, being +subjected only to the small force of lunar gravity (only one-sixth that +of the earth), would, in the course of ages, pile up those gigantic +cones and ridges which form its great characteristic. + +But this small gravitative power of the moon would prevent its retaining +on its surface any of the gases forming our atmosphere, which would all +escape from it and probably be recaptured by the earth. By no process of +external aggregation of solid matter to such a relatively small amount +as that forming the moon, even if the aggregation was so violent as to +produce heat enough to cause liquefaction, could any such +long-continued volcanic action arise by gradual cooling, in the absence +of internal gases. There might be fissures, and even some outflows of +molten rock; but without imprisoned gases, and especially without water +and water-vapour producing explosive outbursts, could any such amount of +scoriae and ashes be produced as were necessary for the building up of +the vast volcanic cones, craters, and craterlets we see upon the moon's +surface. + +I am not aware that either Sir Robert Ball or Sir George Darwin have +adduced this highly volcanic condition of the moon's surface as a +phenomenon which can _only_ be explained by our satellite having been +thrown off a very much larger body, whose gravitative force was +sufficient to acquire and retain the enormous quantity of gases and of +water which we possess, and which are _absolutely essential_ for that +_special form of cone-building volcanic action_ which the moon exhibits +in so pre-eminent a degree. Yet it seems to me clear, that some such +hypothetical origin for our satellite would have had to be assumed if +Sir George Darwin had not deduced it by means of purely mathematical +argument based upon astronomical facts. + +Returning now to the problem of the moon's temperature, I think the +phenomena this presents may be in part due to the mode of formation here +described. For, its entire surface being the result of long-continued +gaseous explosions, all the volcanic products--scoriae, pumice, and +ashes--would necessarily be highly porous throughout; and, never having +been compacted by water-action, as on the earth, and there having been +no winds to carry the finer dust so as to fill up their pores and +fissures, the whole of the surface material to a very considerable depth +must be loose and porous to a high degree. This condition has been +further increased owing to the small power of gravity and the extreme +irregularity of the surface, consisting very largely of lofty cones and +ridges very loosely piled up to enormous heights. + +Now this condition of the substance of the moon's surface is such as +would produce a high specific heat, so that it would absorb a large +amount of heat in proportion to the rise of temperature produced, the +heat being conducted downwards to a considerable depth. Owing, however, +to the total absence of atmosphere radiation would very rapidly cool the +surface, but afterwards more slowly, both on account of the action of +Stefan's law and because the heat stored up in the deeper portions could +be carried to the surface by conduction only, and with extreme slowness. + +_Very's Researches on the Moon's Heat._ + +The results of the eclipse observations are supported by the detailed +examination of the surface-temperature of the moon by Mr. Very in his +_Prize Essay on the Distribution of the Moon's Heat_ (published by the +Utrecht Society of Arts and Sciences in 1891). He shows, by a diagram of +the 'Phase-curve,' that at the commencement of the Lunar day the surface +just within the illuminated limb has acquired about 1/7 of its maximum +temperature, or about 70° F. abs. As the surface exposed to the +Bolometer at each observation is about 1/30 of the moon's surface, and +in order to ensure accuracy the instrument has to be directed to a spot +lying wholly within the edge of the moon, it is evident that the surface +measured has already been for several hours exposed to oblique sunshine. +The curve of temperature then rises gradually and afterwards more +rapidly, till it attains its maximum (of about +30 to 40° F.) a few +hours _before_ noon. This, Mr. Very thinks, is due to the fact that the +half of the moon's face first illuminated for us has, on the average, a +darker surface than that of the afternoon, or second quarter, during +which the curve descends not quite so rapidly, the temperature near +sunset being only a little higher than that near sunrise. This rapid +fall while exposed to oblique sunshine is quite in harmony with the +rapid loss of heat during the few hours of darkness during an eclipse, +both showing the prepotency of radiation over insolation on the moon. + +Two other diagrams show the distribution of heat at the time of +full-moon, one half of the curve showing the temperatures along the +equator from the edge of the disc to the centre, the other along a +meridian from this centre to the pole. This diagram (here reproduced) +exhibits the quick rise of temperature of the oblique rim of the moon +and the nearly uniform heat of the central half of its surface; the +diminution of heat towards the pole, however, is slower for the first +half and more rapid for the latter portion. + +It is an interesting fact that the temperature near the margin of the +full-moon increases towards the centre more rapidly than it does when +the same parts are observed during the early phases of the first +quarter. Mr. Very explains this difference as being due to the fact that +the full-moon to its very edges is fully illuminated, all the shadows of +the ridges and mountains being thrown vertically or obliquely _behind +them._ We thus measure the heat reflected from the _whole_ visible +surface. But at new moon, and somewhat beyond the first quarter, the +deep shadows thrown by the smallest cones and ridges, as well as by the +loftiest mountains, cover a considerable portion of the visible surface, +thus largely reducing the quantity of light and heat reflected or +radiated in our direction. It is only at the full, therefore, that the +maximum temperature of the whole lunar surface can be measured. It must +be considered a proof of the delicacy of the heat-measuring instruments +that this difference in the curves of temperature of the different parts +of the moon's surface and under different conditions is so clearly +shown. + +_The Application of the Preceding Results to the Case of Mars._ + +This somewhat lengthy account of the actual state of the moon's surface +and temperature is of very great importance in our present enquiry, +because it shows us the extraordinary difference in mean and extreme +temperatures of two bodies situated at the same distance from the sun, +and therefore receiving exactly the same amount of solar heat per unit +of surface. We have learned also what are the main causes of this almost +incredible difference, namely: (1) a remarkably rugged surface with +porous and probably cavernous rock-texture, leading to extremely rapid +radiation of heat in the one; as compared with a comparatively even and +well-compacted surface largely clad with vegetation, leading to +comparatively slow and gradual loss by radiation in the other: and (2), +these results being greatly intensified by the total absence of a +protecting atmosphere in the former, while a dense and cloudy atmosphere +with an ever-present supply of water-vapour, accumulates and equalises +the heat received by the latter. + +The only other essential difference in the two bodies which may possibly +aid in the production of this marvellous result, is the fact of our day +and night having a mean length of 12 hours, while those of the moon are +about 14-1/2 of our days. But the altogether unexpected fact, in which +two independent enquirers agree, that during the few hours' duration of +a total eclipse of the moon so large a proportion of the heat is lost by +radiation renders it almost certain that the resulting low temperature +would be not very much less if the moon had a day and night the same +length as our own. + +The great lesson we learn by this extreme contrast of conditions +supplied to us by nature, as if to enable us to solve some of her +problems, is, the overwhelming importance, first, of a dense and +well-compacted surface, due to water-action and strong gravitative +force; secondly, of a more or less general coat of vegetation; and, +thirdly, of a dense vapour-laden atmosphere. These three favourable +conditions result in a mean temperature of about +60° F. with a range +seldom exceeding 40° above or below it, while over more than half the +land-surface of the earth the temperature rarely falls below the +freezing point. On the other hand, we have a globe of the same materials +and at the same distance from the sun, with a maximum temperature of +freezing water, and a minimum not very far from the absolute zero, the +monthly mean being probably much below the freezing point of +carbonic-acid gas--a difference entirely due to the absence of these +three favourable conditions. + +_The Special Features of Mars as influencing Temperature._ + +Coming now to the special feature of Mars and its probable temperature, +we find that most writers have arrived at a very different conclusion +from that of Mr. Lowell, who himself quotes Mr. Moulton as an authority +who 'recently, by the application of Stefan's law,' has found the mean +temperature of this planet to be-35° F. Again, Professor J.H. Poynting, +in his lecture on 'Radiation in the Solar System,' delivered before the +British Association at Cambridge in 1904, gave an estimate of the mean +temperature of the planets, arrived at from measurements of the sun's +emissive power and the application of Stefan's law to the distances of +the several planets, and he thus finds the earth to have a mean +temperature of 17° C. (=62-1/2° F.) and Mars one of-38° C. (=-36-1/2° +F.), a wonderfully close approximation to the mean temperature of the +earth as determined by direct measurement, and therefore, presumably, an +equally near approximation to that of Mars as dependent on distance from +the sun, and '_on the supposition that it is earth-like in all its +conditions._' + +But we know that it is far from being earth-like in the very conditions +which we have found to be those which determine the extremely different +temperatures of the earth, and moon; and, as regards each of these, we +shall find that, so far as it differs from the earth, it approximates to +the less favourable conditions that prevail in the moon. The first of +these conditions which we have found to be essential in regulating the +absorption and radiation of heat, and thus raising the mean temperature +of a planet, is a compact surface well covered with vegetation, two +conditions arising from, and absolutely dependent on, an ample amount of +water. But Mr. Lowell himself assures us, as a fact of which he has no +doubt, that there are no permanent bodies of water, great or small, upon +Mars; that rain, and consequently rivers, are totally wanting; that its +sky is almost constantly clear, and that what appear to be clouds are +not formed of water-vapour but of dust. He dwells, emphatically, on the +terrible desert conditions of the greater part of the surface of the +planet. + +That being the case now, we have no right to assume that it has ever +been otherwise; and, taking full account of the fact, neither denied nor +disputed by Mr. Lowell, that the force of gravity on Mars is not +sufficient to retain water-vapour in its atmosphere, we must conclude +that the surface of that planet, like that of the moon, has been moulded +by some form of volcanic action modified probably by wind, but not by +water. Adding to this, that the force of gravity on Mars is nearer that +of the moon than to that of the earth, and we may r reasonably conclude +that its surface is formed of volcanic matter in a light and porous +condition, and therefore highly favourable for the rapid loss of surface +heat by radiation. The surface-conditions of Mars are therefore, +presumably, much more like those of the moon than like those of the +earth. + +The next condition favourable to the storing up of heat--a covering of +vegetation--is almost certainly absent from Mars except, possibly, over +limited areas and for short periods. In this feature also the surface of +Mars approximates much nearer to lunar than to earth-conditions. The +third condition--a dense, vapour-laden atmosphere--is also wanting in +Mars. For although it possesses an atmosphere it is estimated by Mr. +Lowell (in his latest article) to have a pressure equivalent to only +2-1/2 inches of mercury with us, giving it a density of only one-twelfth +part that of ours; while aqueous vapour, the chief accumulator of heat, +cannot permanently exist in it, and, notwithstanding repeated +spectroscopic observations for the purpose of detecting it, has never +been proved to exist. + +I submit that I have now shown from the statements--and largely as the +result of the long-continued observations--of Mr. Lowell himself, that, +so far as the physical conditions of Mars are known to differ from those +of the earth, the differences are all _unfavourable_ to the conservation +and _favourable_ to the dissipation of the scanty heat it receives from +the sun--that they point unmistakeably towards the temperature +conditions of the moon rather than to those of the earth, and that the +cumulative effect of these adverse conditions, acting upon a +heat-supply, reduced by solar distance to less than one-half of ours, +_must_ result in a mean temperature (as well as in the extremes) nearer +to that of our satellite than to that of our own earth. + +_Further Criticism of Mr. Lowell's Article._ + +We are now in a position to test some further conclusions of Mr. +Lowell's _Phil. Mag._ article by comparison with actual phenomena. We +have seen, in the outline I have given of this article, that he +endeavours to show how the small amount of solar heat received by Mars +is counterbalanced, largely by the greater transparency to light and +heat of its thin and cloudless atmosphere, and partially also by a +greater conservative or 'blanketing' power of its atmosphere due to the +presence in it of a large proportion of carbonic acid gas and aqueous +vapour. The first of these statements may be admitted as a fact which he +is entitled to dwell upon, but the second--the presence of large +quantities of carbon-dioxide and aqueous vapour is a pure hypothesis +unsupported by any item of scientific evidence, while in the case of +aqueous vapour it is directly opposed to admitted results founded upon +the molecular theory of gaseous elasticity. But, although Mr. Lowell +refers to the conservative or 'blanketing' effect of the earth's +atmosphere, he does not consider or allow for its very great cumulative +effect, as is strikingly shown by the comparison with the actual +temperature conditions of the moon. This cumulative effect is due to the +_continuous_ reflection and radiation of heat from the clouds as well as +from the vapour-laden strata of air in our lower atmosphere, which +latter, though very transparent to the luminous and accompanying heat +rays of the sun, are opaque to the dark heat-rays whether radiated or +reflected from the earth's surface. We are therefore in a position +strictly comparable with that of the interior of some huge glass house, +which not only becomes intensely heated by the direct rays of the sun, +but also to a less degree by reflected rays from the sky and those +radiated from the clouds, so that even on a cloudy or misty day its +temperature rises many degrees above that of the outer air. Such a +building, if of large size, of suitable form, and well protected at +night by blinds or other covering, might be so arranged as to accumulate +heat in its soil and walls so as to maintain a tolerably uniform +temperature though exposed to a considerable range of external heat and +cold. It is to such a power of accumulation of heat in our soil and +lower atmosphere that we must impute the overwhelming contrast between +our climate and that of the moon. With us, the solar heat that +penetrates our vapour-laden and cloudy atmosphere is shut in by that +same atmosphere, accumulates there for weeks and months together, and +can only slowly escape. It is this great cumulative power which Mr. +Lowell has not taken account of, while he certainly has not estimated +the enormous loss of heat by free radiation, which entirely neutralises +the effects of increase of sun-heat, however great, when these +cumulative agencies are not present.[12] + +[Footnote 12: The effects of this 'cumulative' power of a dense +atmosphere are further discussed and illustrated in the last chapter of +this book, where I show that the universal fact of steadily diminishing +temperatures at high altitudes is due solely to the diminution of this +cumulative power of our atmosphere, and that from this cause alone the +temperature of Mars must be that which would be found on a lofty plateau +about 18,000 feet higher than the average of the peaks of the Andes!] + +_Temperature on Polar Regions of Mars._ + +There is also a further consideration which I think Mr. Lowell has +altogether omitted to discuss. Whatever may be the _mean_ temperature +of Mars, we must take account of the long nights in its polar and +high-temperate latitudes, lasting nearly twice as long as ours, with the +resulting lowering of temperature by radiation into a constantly clear +sky. Even in Siberia, in Lat. 67-1/2°N. a cold of-88°F. has been +attained; while over a large portion of N. Asia and America above 60° +Lat. the _mean_ January temperature is from-30°F. to-60°F., and the +whole subsoil is permanently frozen from a depth of 6 or 7 feet to +several hundreds. But the winter temperatures, _over the same latitudes_ +in Mars, must be very much lower; and it must require a proportionally +larger amount of its feeble sun-heat to raise the surface even to the +freezing-point, and an additional very large amount to melt any +considerable depth of snow. But this identical area, from a little below +60° to the pole, is that occupied by the snow-caps of Mars, and over the +whole of it the winter temperature must be far lower than the +earth-minimum of-88°F. Then, as the Martian summer comes on, there is +less than half the sun-heat available to raise this low temperature +after a winter nearly double the length of ours. And when the summer +does come with its scanty sun-heat, that heat is not accumulated as it +is by our dense and moisture-laden atmosphere, the marvellous effects of +which we have already shown. Yet with all these adverse conditions, each +assisting the other to produce a climate approximating to that which the +earth would have if it had no atmosphere (but retaining our superiority +over Mars in receiving double the amount of sun-heat), we are asked to +accept a mean temperature for the more distant planet almost exactly the +same as that of mild and equable southern England, and a disappearance +of the vast snowfields of its polar regions as rapid and complete as +what occurs with us! If the moon, even at its equator, has not its +temperature raised above the freezing-point of water, how can the more +_distant_ Mars, with its _oblique_ noon-day sun falling upon the +snow-caps, receive heat enough, first to raise their temperature to 32° +F., and then to melt with marked rapidity the vast frozen plains of its +polar regions? + +Mr. Lowell is however so regardless of the ordinary teachings of +meteorological science that he actually accounts for the supposed mild +climate of the polar regions of Mars by the absence of water on its +surface and in its atmosphere. He concludes his fifth chapter with the +following words: "Could our earth but get rid of its oceans, we too +might have temperate regions stretching to the poles." Here he runs +counter to two of the best-established laws of terrestrial climatology-- +the wonderful equalising effects of warm ocean-currents which are the +chief agents in diminishing polar cold; the equally striking effects of +warm moist winds derived from these oceans, and the great storehouse of +heat we possess in our vapour-laden atmosphere, its vapour being +primarily derived from these same oceans! But, in Mr. Lowell's opinion, +all our meteorologists are quite mistaken. Our oceans are our great +drawbacks. Only get rid of them and we should enjoy the exquisite +climate of Mars--with its absence of clouds and fog, of rain or rivers, +and its delightful expanses of perennial deserts, varied towards the +poles by a scanty snow-fall in winter, the melting of which might, with +great care, supply us with the necessary moisture to grow wheat and +cabbages for about one-tenth, or more likely one-hundredth, of our +present population. I hope I may be excused for not treating such an +argument seriously. The various considerations now advanced, especially +those which show the enormous cumulative and conservative effect of our +dense and water-laden atmosphere, and the disastrous effect--judging by +the actual condition of the moon--which the loss of it would have upon +our temperature, seem to me quite sufficient to demonstrate important +errors in the data or fallacies in the complex mathematical argument by +which Mr. Lowell has attempted to uphold his views as to the temperature +and consequent climatic conditions of Mars. In concluding this portion +of my discussion of the problem of Mars, I wish to call attention to the +fact that my argument, founded upon a comparison of the physical +conditions of the earth and moon with those of Mars, is dependent upon a +small number of generally admitted scientific facts; while the +conclusions drawn from those facts are simple and direct, requiring no +mathematical knowledge to follow them, or to appreciate their weight and +cogency. I claim for them, therefore, that they are in no degree +speculative, but in their data and methods exclusively scientific. In +the next chapter I will put forward a suggestion as to how the very +curious markings upon the surface of Mars may possibly be interpreted, +so as to be in harmony with the planet's actual physical condition and +its not improbable origin and past history. + + + +CHAPTER VII. + + +A SUGGESTION AS TO THE 'CANALS' OF MARS. + +The special characteristics of the numerous lines which intersect the +whole of the equatorial and temperate regions of Mars are, their +straightness combined with their enormous length. It is this which has +led Mr. Lowell to term them 'non-natural features.' Schiaparelli, in his +earlier drawings, showed them curved and of comparatively great width. +Later, he found them to be straight fine lines when seen under the best +conditions, just as Mr. Lowell has always seen them in the pure +atmosphere of his observatory. Both of these observers were at first +doubtful of their reality, but persistent observation continued at many +successive oppositions compelled acceptance of them as actual features +of the planet's disc. So many other observers have now seen them that +the objection of unreality seems no longer valid. + +Mr. Lowell urges, however, that their perfect straightness, their +extreme tenuity, their uniformity throughout their whole length, the +dual character of many of them, their relation to the 'oases' and the +form and position of these round black spots, are all proofs of +artificiality and are suggestive of design. And considering that some of +them are actually as long as from Boston to San Francisco, and +relatively to their globe as long as from London to Bombay, his +objection that "no natural phenomena within our knowledge show such +regularity on such a scale" seems, at first, a mighty one. + +It is certainly true that we can point to nothing exactly like them +either on the earth or on the moon, and these are the only two planetary +bodies we are in a position to compare with Mars. Yet even these do, I +think, afford us some hints towards an interpretation of the mysterious +lines. But as our knowledge of the internal structure and past history +even of our earth is still imperfect, that of the moon only conjectural, +and that of Mars a perfect blank, it is not perhaps surprising that the +surface-features of the latter do not correspond with those of either of +the others. + +_Mr. Pickering's Suggestion._ + +The best clue to a natural interpretation of the strange features of the +surface of Mars is that suggested by the American astronomer Mr. W.H. +Pickering in _Popular Astronomy_ (1904). Briefly it is, that both the +'canals' of Mars and the rifts as well as the luminous streaks on the +moon are cracks in the volcanic crust, caused by internal stresses due +to the action of the heated interior. These cracks he considers to be +symmetrically arranged with regard to small 'craterlets' (Mr. Lowell's +'oases') because they have originated from them, just as the white +streaks on the moon radiate from the larger craters as centres. He +further supposes that water and carbon-dioxide issue from the interior +into these fissures, and, in conjunction with sunlight, promote the +growth of vegetation. Owing to the very rare atmosphere, the vapours, he +thinks, would not ascend but would roll down the outsides of the +craterlets and along the borders of the canals, thus irrigating the +immediate vicinity and serving to promote the growth of some form of +vegetation which renders the canals and oases visible.[13] + +[Footnote 13: _Nature_, vol. 70, p. 536.] + +This opinion is especially important because, next to Mr. Lowell, Mr. +Pickering is perhaps the astronomer who has given most attention to Mars +during the last fifteen years. He was for some time at Flagstaff with +Mr. Lowell, and it was he who discovered the oases or craterlets, and +who originated the idea that we did not see the 'canals' themselves but +only the vegetable growth on their borders. He also observed Mars in the +Southern Hemisphere at Arequipa; and he has since made an elaborate +study of the moon by means of a specially constructed telescope of 135 +feet focal length, which produced a direct image on photographic plates +nearly 16 inches in diameter.[14] + +[Footnote 14: _Nature_, vol. 70, May 5, p.xi, supplement.] + +It is clear therefore that Mr. Lowell's views as to the artificial +nature of the 'canals' of Mars are not accepted by an astronomer of +equal knowledge and still wider experience. Yet Professor Pickering's +alternative view is more a suggestion than an explanation, because there +is no attempt to account for the enormous length and perfect +straightness of the lines on Mars, so different from anything that is +found either on our earth or on the moon. There must evidently be some +great peculiarity of structure or of conditions on Mars to account for +these features, and I shall now attempt to point out what this +peculiarity is and how it may have arisen. + +_The Meteoritic Hypothesis._ + +During the last quarter of a century a considerable change has come over +the opinions of astronomers as regards the probable origin of the Solar +System. The large amount of knowledge of the stellar universe, and +especially of nebulae, of comets and of meteor-streams which we now +possess, together with many other phenomena, such as the constitution of +Saturn's rings, the great number and extent of the minor planets, and +generally of the vast amount of matter in the form of meteor-rings and +meteoric dust in and around our system, have all pointed to a different +origin for the planets and their satellites than that formulated by +Laplace as the Nebular hypothesis. + +It is now seen more clearly than at any earlier period, that most of the +planets possess special characteristics which distinguish them from one +another, and that such an origin as Laplace suggested--the slow cooling +and contraction of one vast sun-mist or nebula, besides presenting +inherent difficulties--many think them impossibilities--in itself does +not afford an adequate explanation of these peculiarities. Hence has +arisen what is termed the Meteoritic theory, which has been ably +advocated for many years by Sir Norman Lockyer, and with some +unimportant modifications is now becoming widely accepted. Briefly, this +theory is, that the planets have been formed by the slow aggregation of +solid particles around centres of greatest condensation; but as many of +my readers may be altogether unacquainted with it, I will here give a +very clear statement of what it is, from Professor J.W. Gregory's +presidential address to the Geological Section of the British +Association of the present year. He began by saying that these modern +views were of far more practical use to men of science than that of +Laplace, and that they give us a history of the world consistent with +the actual records of geology. He then continues: + +"According to Sir Norman Lockyer's Meteoritic Hypothesis, nebulae, +comets, and many so-called stars consist of swarms of meteorites which, +though normally cold and dark, are heated by repeated collisions, and so +become luminous. They may even be volatilised into glowing meteoric +vapour; but in time this heat is dissipated, and the force of gravity +condenses a meteoritic swarm into a single globe. 'Some of the swarms +are,' says Lockyer, 'truly members of the solar system,' and some of +these travel round the sun in nearly circular orbits, like planets. They +may be regarded as infinitesimal planets, and so Chamberlain calls them +'planetismals.' + +"The planetismal theory is a development of the meteoritic theory, and +presents it in an especially attractive guise. It regards meteorites as +very sparsely distributed through space, and gravity as powerless to +collect them into dense groups. So it assigns the parentage of the solar +system to a spiral nebula composed of planetismals, and the planets as +formed from knots in the nebula, where many planetismals had been +concentrated near the intersections of their orbits. These groups of +meteorites, already as dense as a swarm of bees, were then packed closer +by the influence of gravity, and the contracting mass was heated by the +pressure, even above the normal melting-point of the material, which was +kept rigid by the weight of the overlying layers." + +Now, adopting this theory as the last word of science upon the subject +of the origin of planets, we see that it affords immense scope for +diversity in results depending on the total _amount_ of matter available +within the range of attraction of an incipient planetary mass, and the +_rates_ at which this matter becomes available. By a special combination +of these two quantities (which have almost certainly been different for +each planet) I think we may be able to throw some light upon the +structure and physical features of Mars. + +_The Probable Mode of Origin of Mars._ + +This planet, lying between two of much greater mass, has evidently had +less material from which to be formed by aggregation; and if we +assume--as in the absence of evidence to the contrary we have a right to +do--that its beginnings were not much later (or earlier) than those of +the earth, then its smaller size shows that it has in all probability +aggregated very much more slowly. But the internal heat acquired by a +planet while forming in this manner will depend upon the rate at which +it aggregates and the velocity with which the planetismals' fall into +it, and this velocity will increase with its mass and consequent force +of gravity. In the early stages of a planet's growth it will probably +remain cold, the small amount of heat produced by each impact being lost +by radiation before the next one occurs; and with a small and slowly +aggregating planet this condition will prevail till it approaches its +full size. Then only will its gravitative force be sufficient to cause +incoming matter to fall upon it with so powerful an impact as to produce +intense heat. Further, the compressive force of a small planet will be a +less effective heat-producing agency than in the case of a larger one. + +The earth we know has acquired a large amount of internal heat, probably +sufficient to liquefy its whole interior; but Mars has only one-ninth +part the mass of the earth, and it is quite possible, and even probable, +that its comparatively small attractive force would never have liquefied +or even permanently heated the more central portions of its mass. This +being admitted, I suggest the following course of events as quite +possible, and not even improbable, in the case of this planet. During +the whole of its early growth, and till it acquired nearly its present +diameter, its rate of aggregation was so slow that the planetismals +falling upon it, though they might have been heated and even partially +liquefied by the impact, were never in such quantity as to produce any +considerable heating effect on the whole mass, and each local rise of +temperature was soon lost by radiation. The planet thus grew as a solid +and cold mass, compacted together by the impact of the incoming matter +as well as by its slowly increasing gravitative force. But when it had +attained to within perhaps 100, perhaps 50 miles, or less, of its +present diameter, a great change occurred in the opportunity for further +growth. Some large and dense swarm of meteorites, perhaps containing a +number of bodies of the size of the asteroids, came within the range of +the sun's attraction and were drawn by it into an orbit which crossed +that of Mars at such a small angle that the planet was able at each +revolution to capture a considerable number of them. The result might +then be that, as in the case of the earth, the continuous inpour of the +fresh matter first heated, and later on liquefied the greater part of it +as well perhaps as a thin layer of the planet's original surface; so +that when in due course the whole of the meteor-swarm had been captured, +Mars had acquired its present mass, but would consist of an intensely +heated, and either liquid or plastic thin outer shell resting upon a +cold and solid interior. + +The size and position of the two recently discovered satellites of Mars, +which are believed to be not more than ten miles in diameter, the more +remote revolving around its primary very little slower than the planet +rotates, while the nearer one, which is considerably less distant from +the planet's surface than its own antipodes and revolves around it more +than three times during the Martian day, may perhaps be looked upon as +the remnants of the great meteor-swarm which completed the Martian +development, and which are perhaps themselves destined at some distant +period to fall into the planet. Should future astronomers witness the +phenomenon the effect produced upon its surface would be full of +instruction. + +As the result of such an origin as that suggested, Mars would possess a +structure which, in the essential feature of heat-distribution, would be +the very opposite of that which is believed to characterise the earth, +yet it might have been produced by a very slight modification of the +same process. This peculiar heat-distribution, together with a much +smaller mass and gravitative force, would lead to a very different +development of the surface and an altogether diverse geological history +from ours, which has throughout been profoundly influenced by its heated +interior, its vast supply of water, and the continuous physical and +chemical reactions between the interior and the crust. + +These reactions have, in our case, been of substantially the same +nature, and very nearly of the same degree of intensity throughout the +whole vast eons of geological time, and they have resulted in a +wonderfully complex succession of rock-formations--volcanic, plutonic, +and sedimentary--more or less intermingled throughout the whole series, +here remaining horizontal as when first deposited, there upheaved or +depressed, fractured or crushed, inclined or contorted; denuded by rain +and rivers with the assistance of heat and cold, of frost and ice, in an +unceasing series of changes, so that however varied the surface may be, +with hill and dale, plains and uplands, mountain ranges and deep +intervening valleys, these are as nothing to the diversities of interior +structure, as exhibited in the sides of every alpine valley or +precipitous escarpment, and made known to us by the work of the miner +and the well-borer in every part of the world. + +_Structural Straight Lines on the Earth._ + +The great characteristic of the earth, both on its surface and in its +interior, is thus seen to be extreme diversity both of form and +structure, and this is further intensified by the varied texture, +constitution, hardness, and density of the various rocks and debris of +which it is composed. It is therefore not surprising that, with such a +complex outer crust, we should nowhere find examples of those +geometrical forms and almost world-wide straight lines that give such a +remarkable, and as Mr. Lowell maintains, 'non-natural' character to the +surface of Mars, but which, as it seems to me, of themselves afford +_prima facie_ evidence of a corresponding simplicity and uniformity in +its internal structure. + +Yet we are not ourselves by any means devoid of 'straight lines' +structurally produced, in spite of every obstacle of diversity of form +and texture, of softness and hardness, of lamination or crystallisation, +which are adverse to such developments. Examples of these are the +numerous 'faults' which occur in the harder rocks, and which often +extend for great distances in almost perfect straight lines. In our own +country we have the Tyneside and Craven faults in the North of England, +which are 30 miles long and often 20 yards wide; but even more striking +is the great Cleveland Dyke--a wall of volcanic rock dipping slightly +towards the south, but sometimes being almost vertical, and stretching +across the country, over hill and dale, in an almost perfect straight +line from a point on the coast ten miles north of Scarborough, in a +west-by-north direction, passing about two miles south of Stockton and +terminating about six miles north-by-east of Barnard Castle, a distance +of very nearly 60 miles. The great fault between the Highlands and +Lowlands of Scotland extends across the country from Stonehaven to near +Helensburgh, a distance of 120 miles; and there are very many more of +less importance. + +Much more extensive are some of the great continental dislocations, +often forming valleys of considerable width and length. The Upper Rhine +flows in one of these great valleys of subsidence for about 180 miles, +from Mulhausen to Frankfort, in a generally straight line, though +modified by denudation. Vaster still is the valley of the Jordan through +the Sea of Galilee to the Dead Sea, continued by the Wady Arabah to the +Gulf of Akaba, believed to form one vast geological depression or +fracture extending in a straight line for 400 miles. + +Thousands of such faults, dykes, or depressions exist in every part of +the world, all believed to be due to the gradual shrinking of the heated +interior to which the solid crust has to accommodate itself, and they +are especially interesting and instructive for our present purpose as +showing the tendency of such fractures of solid rock-material to extend +to great lengths in straight lines, notwithstanding the extreme +irregularity both in the surface contour as well as in the internal +structures of the varied deposits and formations through which they +pass. + +_Probable Origin of the Surface-features of Mars._ + +Returning now to Mars, let us consider the probable course of events +from the point at which we left it. The heat produced by impact and +condensation would be likely to release gases which had been in +combination with some of the solid matter, or perhaps been itself in a +solid state due to intense cold, and these, escaping outwards to the +surface, would produce on a small scale a certain amount of upheaval and +volcanic disturbance; and as an outer crust rapidly formed, a number of +vents might remain as craters or craterlets in a moderate state of +activity. Owing to the comparatively small force of gravity, the outer +crust would become scoriaceous and more or less permeated by the gases, +which would continue to escape through it, and this would facilitate the +cooling of the whole of the heated outer crust, and allow it to become +rather densely compacted. When the greater portion of the gases had thus +escaped to the outer surface and assisted to form a scanty atmosphere, +such as now exists, there would be no more internal disturbance and the +cooling of the heated outer coating would steadily progress, resulting +at last in a slightly heated, and later in a cold layer of moderate +thickness and great general uniformity. Owing to the absence of rain and +rivers, denudation such as we experience would be unknown, though the +superficial scoriaceous crust might be partially broken up by expansion +and contraction, and suffer a certain amount of atmospheric erosion. + +The final result of this mode of aggregation would be, that the planet +would consist of an outer layer of moderate thickness as compared with +the central mass, which outer layer would have cooled from a highly +heated state to a temperature considerably below the freezing-point, and +this would have been all the time _contracting upon a previously cold, +and therefore non-contracting nucleus._ The result would be that very +early in the process great superficial tensions would be produced, which +could only be relieved by cracks or fissures, which would initiate at +points of weakness--probably at the craterlets already referred to--from +which they would radiate in several directions. Each crack thus formed +near the surface would, as cooling progressed, develop in length and +depth; and owing to the general uniformity of the material, and possibly +some amount of crystalline structure due to slow and continuous cooling +down to a very low temperature, the cracks would tend to run on in +straight lines and to extend vertically downwards, which two +circumstances would necessarily result in their forming portions of +'great circles' on the planet's surface--the two great facts which Mr. +Lowell appeals to as being especially 'non-natural.' + +_Symmetry of Basaltic Columns._ + +We have however one quite natural fact on our earth which serves to +illustrate one of these two features, the direction of the downward +fissure. This is, the comparatively common phenomenon of basaltic +columns and 'Giant's Causeways.' The wonderful regularity of these, and +especially the not unfrequent upright pillars in serried ranks, as in +the palisades of the Hudson river, must have always impressed observers +with their appearance of artificiality. Yet they are undoubtedly the +result of the very slow cooling and contraction of melted rocks under +compression by strata _below and above them_, so that, when once +solidified, the mass was held in position and the tension produced by +contraction could only be relieved by numerous very small cracks at +short distances from each other in every direction, resulting in five, +six, or seven-sided polygons, with sides only a few inches long. This +contraction began of course at the coolest surface, generally the upper +one; and observation of these columns in various positions has +established the rule that their direction lengthways _is always at right +angles to the cooling surface_, and thus, whenever this surface was +horizontal, the columns became almost exactly vertical. + +_How this applies to Mars._ + +One of the features of the surface of Mars that Mr. Lowell describes +with much confidence is, that it is wonderfully uniform and level, which +of course it would be if it had once been in a liquid or plastic state, +and not much disturbed since by volcanic or other internal movements. +The result would be that cracks formed by contraction of the hardened +outer crust would be vertical; and, in a generally uniform material at a +very uniform temperature, these cracks would continue almost +indefinitely in straight lines. The hardened and contracting surface +being free to move laterally on account of there being a more heated and +plastic layer below it, the cracks once initiated above would +continually widen at the surface as they penetrated deeper and deeper +into the slightly heated substratum. Now, as basalt begins to soften at +about 1400° F. and the surface of Mars has cooled to at least the +freezing-point--perhaps very much below it--the contraction would be so +great that if the fissures produced were 500 miles apart they might be +three miles wide at the surface, and, if only 100 miles apart, then +about two-thirds of a mile wide.[15] But as the production of the +fissures might have occupied perhaps millions of years, a considerable +amount of atmospheric denudation would result, however slowly it acted. +Expansion and contraction would wear away the edges and sides of the +fissures, fill up many of them with the debris, and widen them at the +surfaces to perhaps double their original size.[16] + +[Footnote 15: The coefficient of contraction of basalt is 0.000006 for +1° F., which would lead to the results given here.] + +[Footnote 16: Mr. W.H. Pickering observed clouds on Mars 15 miles high; +these are the 'projections' seen on the terminator when the planet is +partially illuminated. They were at first thought to be mountains; but +during the opposition of 1894, more than 400 of them were seen at +Flagstaff during nine months' observation. Usually they are of rare +occurrence. They are seen to change in form and position from day to +day, and Mr. Lowell is strongly of opinion that they are dust-storms, +not what we term clouds. They were mostly about 13 miles high, +indicating considerable aerial disturbance on the planet, and therefore +capable of producing proportional surface denudation.] + +_Suggested Explanation of the 'Oases.'_ + +The numerous round dots seen upon the 'canals,' and especially at points +from which several canals radiate and where they intersect--termed +'oases' by Mr. Lowell and 'craterlets' by Mr. Pickering may be explained +in two ways. Those from which several canals radiate may be true craters +from which the gases imprisoned in the heated surface layers have +gradually escaped. They would be situated at points of weakness in the +crust, and become centres from which cracks would start during +contraction. Those dots which occur at the crossing of two straight +canals or cracks may have originated from the fact that at such +intersections there would be four sharply-projecting angles, which, +being exposed to the influence of alternate heat and cold (during day +and night) on the two opposite surfaces, would inevitably in time become +fractured and crumbled away, resulting in the formation of a roughly +circular chasm which would become partly filled up by the debris. Those +formed by cracks radiating from craterlets would also be subject to the +same process of rounding off to an even greater extent; and thus would +be produced the 'oases' of various sizes up to 50 miles or more in +diameter recorded by Mr. Lowell and other observers. + +_Probable Function of the Great Fissures._ + +Mr. Pickering, as we have seen, supposes that these fissures give out +the gases which, overflowing on each side, favour the growth of the +supposed vegetation which renders the course of the canals visible, and +this no doubt may have been the case during the remote periods when +these cracks gave access to the heated portions of the surface layer. +But it seems more probable that Mars has now cooled down to the almost +uniform mean temperature it derives from solar heat, and that the +fissures--now for the most part broad shallow valleys--serve merely as +channels along which the liquids and heavy gases derived from the +melting of the polar snows naturally flow, and, owing to their nearly +level surfaces, overflow to a certain distance on each side of them. + +_Suggested Origin of the Blue Patches._ + +These heavy gases, mainly perhaps, as has been often suggested, +carbon-dioxide, would, when in large quantity and of considerable depth, +reflect a good deal of light, and, being almost inevitably dust-laden, +might produce that blue tinge adjacent to the melting snow-caps which +Mr. Lowell has erroneously assumed to be itself a proof of the presence +of liquid water. Just as the blue of our sky is undoubtedly due to +reflection from the ultra-minute dust particles in our higher +atmosphere, similar particles brought down by the 'snow' from the higher +Martian atmosphere might produce the blue tinge in the great volumes of +heavy gas produced by its evaporation or liquefaction. + +It may be noted that Mr. Lowell objects to the carbon-dioxide theory of +the formation of the snow-caps, that this gas at low pressures does not +liquefy, but passes at once from the solid to the gaseous state, and +that only water remains liquid sufficiently long to produce the blue +colour' which plays so large a part in his argument for the mild climate +essential for an inhabited planet. But this argument, as I have already +shown, is valueless. For only very deep water can possibly show a blue +colour by reflected light, while a dust-laden atmosphere--especially +with a layer of very dense gas at the bottom of it, as would be the case +with the newly evaporated carbon-dioxide from the diminishing snow-cap +--would provide the very conditions likely to produce this blue tinge of +colour. + +It may be considered a support to this view that carbonic-acid gas +becomes liquid at--140° F. and solid at--162° F., temperatures far +higher than we should expect to prevail in the polar and north temperate +regions of Mars during a considerable part of the year, but such as +might be reached there during the summer solstice when the `snows' so +rapidly disappear, to be re-formed a few months later. + +_The Double Canals._ + +The curious phenomena of the 'double canals' are undoubtedly the most +difficult to explain satisfactorily on any theory that has yet been +suggested. They vary in distance apart from about 100 to 400 miles. In +many cases they appear perfectly parallel, and Mr. Lowell gives us the +impression that they are almost always so. But his maps show, in some +cases, decided differences of width at the two extremities, indicating +considerable want of parallelism. A few of the curved canals are also +double. + +There is one drawing in Mr. Lowell's book (p. 219) of the mouths, or +starting points, of the Euphrates and Phison, two widely separated +double canals diverging at an angle of about 40° from the same two +oases, so that the two inner canals cross each other. Now this suggests +two wide bands of weakness in the planet's crust radiating probably from +within the dark tract called the 'Mare Icarium,' and that some +widespread volcanic outburst initiated diverging cracks on either side +of these bands. Something of this kind may have been the cause of most +of the double canals, or they may have been started from two or more +craterlets not far apart, the direction being at first decided by some +local peculiarity of structure; and where begun continuing in straight +lines owing to homogeneity or uniform density of material. This is very +vague, but the phenomena are so remarkable, and so very imperfectly +known at present, that nothing but suggestion can be attempted. + +_Concluding Remarks on the 'Canals.'_ + +In this somewhat detailed exposition of a possible, and, I hope, a +probable explanation of the surface-features of Mars, I have +endeavoured to be guided by known facts or accepted theories both +astronomical and geological. I think I may claim to have shown that +there are some analogous features of terrestrial rock-structure to +serve as guides towards a natural and intelligible explanation of the +strange geometric markings discovered during the last thirty years, and +which have raised this planet from comparative obscurity into a position +of the very first rank both in astronomical and popular interest. + +This wide-spread interest is very largely due to Mr. Lowell's devotion +to its study, both in seeking out so admirable a position as regards +altitude and climate, and in establishing there a first-class +observatory; and also in bringing his discoveries before the public in +connection with a theory so startling as to compel attention. I venture +to think that his merit as one of our first astronomical observers will +in no way be diminished by the rejection of his theory, and the +substitution of one more in accordance with the actually observed facts. + + + +APPENDIX. + +_A Suggested Experiment to Illustrate the 'Canals' of Mars._ + +If my explanation of the 'canals' should be substantially correct--that +is, if they were produced by the contraction of a heated outward crust +upon a cold, and therefore non-contracting interior, the result of such +a condition might be shown experimentally. + +Several baked clay balls might be formed to serve as cores, say of 8 to +10 inches in diameter. These being fixed within moulds of say half an +inch to an inch greater diameter, the outer layer would be formed by +pouring in some suitable heated liquid material, and releasing it from +the mould as soon as consolidation occurs, so that it may cool rapidly +from the _outside._ Some kinds of impure glass, or the brittle metals +bismuth or antimony or alloys of these might be used, in order to see +what form the resulting fractures would take. It would be well to have +several duplicates of each ball, and, as soon as tension through +contraction manifests itself, to try the effect of firing very small +charges of small shot to ascertain whether such impacts would start +radiating fractures. When taken from the moulds, the balls should be +suspended in a slight current of air, and kept rotating, to reproduce +the planetary condition as nearly as possible. + +The exact size and material of the cores, the thickness of the heated +outer crust, the material best suited to show fracture by contraction, +and the details of their treatment, might be modified in various ways as +suggested by the results first obtained. Such a series of experiments +would probably throw further light on the physical conditions which have +produced the gigantic system of fissures or channels we see upon the +surface of Mars, though it would not, of course, prove that such +conditions actually existed there. In such a speculative matter we can +only be guided by probabilities, based upon whatever evidence is +available. + + + +CHAPTER VIII. + + +SUMMARY AND CONCLUSION. + +This little volume has necessarily touched upon a great variety of +subjects, in order to deal in a tolerably complete manner with the very +extraordinary theories by which Mr. Lowell attempts to explain the +unique features of the surface of the planet, which, by long-continued +study, he has almost made his own. It may therefore be well to sum up +the main points of the arguments against his view, introducing a few +other facts and considerations which greatly strengthen my argument. + +The one great feature of Mars which led Mr. Lowell to adopt the view of +its being inhabited by a race of highly intelligent beings, and, with +ever-increasing discovery to uphold this theory to the present time, is +undoubtedly that of the so-called 'canals'--their straightness, their +enormous length, their great abundance, and their extension over the +planet's whole surface from one polar snow-cap to the other. The very +immensity of this system, and its constant growth and extension during +fifteen years of persistent observation, have so completely taken +possession of his mind, that, after a very hasty glance at analogous +facts and possibilities, he has declared them to be 'non-natural'-- +therefore to be works of art--therefore to necessitate the +presence of highly intelligent beings who have designed and constructed +them. This idea has coloured or governed all his writings on the +subject. The innumerable difficulties which it raises have been either +ignored, or brushed aside on the flimsiest evidence. As examples, he +never even discusses the totally inadequate water-supply for such +worldwide irrigation, or the extreme irrationality of constructing so +vast a canal-system the waste from which, by evaporation, when exposed +to such desert conditions as he himself describes, would use up ten +times the probable supply. + +Again, he urges the 'purpose' displayed in these 'canals.' Their being +_all_ so straight, _all_ describing great circles of the 'sphere,' all +being so evidently arranged (as he thinks) either to carry water to some +'oasis' 2000 miles away, or to reach some arid region far over the +equator in the opposite hemisphere! But he never considers the +difficulties this implies. Everywhere these canals run for thousands of +miles across waterless deserts, forming a system and indicating a +purpose, the wonderful perfection of which he is never tired of dwelling +upon (but which I myself can nowhere perceive). + +Yet he never even attempts to explain how the Martians could have lived +_before_ this great system was planned and executed, or why they did not +_first_ utilise and render fertile the belt of land adjacent to the +limits of the polar snows--why the method of irrigation did not, as with +all human arts, begin gradually, at home, with terraces and channels to +irrigate the land close to the source of the water. How, with such a +desert as he describes three-fourths of Mars to be, did the inhabitants +ever get to _know_ anything of the equatorial regions and its needs, so +as to start right away to supply those needs? All this, to my mind, is +quite opposed to the idea of their being works of art, and altogether in +favour of their being natural features of a globe as peculiar in origin +and internal structure as it is in its surface-features. The explanation +I have given, though of course hypothetical, is founded on known +cosmical and terrestrial facts, and is, I suggest, far more scientific +as well as more satisfactory than Mr. Lowell's wholly unsupported +speculation. This view I have explained in some detail in the preceding +chapter. + +Mr. Lowell never even refers to the important question of loss by +evaporation in these enormous open canals, or considers the undoubted +fact that the only intelligent and practical way to convey a limited +quantity of water such great distances would be by a system of +water-tight and air-tight tubes laid _under the ground._ The mere +attempt to use open canals for such a purpose shows complete ignorance +and stupidity in these alleged very superior beings; while it is certain +that, long before half of them were completed their failure to be of any +use would have led any rational beings to cease constructing them. + +He also fails to consider the difficulty, that, if these canals are +necessary for existence in Mars, how did the inhabitants ever reach a +sufficiently large population with surplus food and leisure enabling +them to rise from the low condition of savages to one of civilisation, +and ultimately to scientific knowledge? Here again is a dilemma which is +hard to overcome. Only a _dense_ population with _ample_ means of +subsistence could possibly have constructed such gigantic works; but, +given these two conditions, no adequate motive existed for the +conception and execution of them--even if they were likely to be of any +use, which I have shown they could not be. + +_Further Considerations on the Climate of Mars._ + +Recurring now to the question of climate, which is all-important, Mr. +Lowell never even discusses the essential point--the temperature that +must _necessarily_ result from an atmospheric envelope one-twelfth (or +at most one-seventh) the density of our own; in either case +corresponding to an altitude far greater than that of our highest +mountains.[17] Surely this phenomenon, everywhere manifested on the +earth even under the equator, of a regular decrease of temperature with +altitude, the only cause of which is a less dense atmosphere, should +have been fairly grappled with, and some attempt made to show why it +should not apply to Mars, except the weak remark that on a level surface +it will not have the same effect as on exposed mountain heights. But it +_does_ have the same effect, or very nearly so, on our lofty plateaux +often hundreds of miles in extent, in proportion to their altitude. +Quito, at 9350 ft. above the sea, has a mean temperature of about 57° +F., giving a lowering of 23° from that of Manaos at the mouth of the Rio +Negro. This is about a degree for each 400 feet, while the general fall +for isolated mountains is about one degree in 340 feet according to +Humboldt, who notes the above difference between the rate of cooling for +altitude of the plains--or more usually sheltered valleys in which the +towns are situated--and the exposed mountain sides. It will be seen that +this lower rate would bring the temperature of Mars at the equator down +to 20° F. below the freezing point of water from this cause alone. + +[Footnote 17: A four inches barometer is equivalent to a height of +40,000 feet above sea-level with us.] + +But all enquirers have admitted, that if conditions as to atmosphere +were the same as on the earth, its greater distance from the sun would +reduce the temperature to-31° F., equal to 63° below the freezing +point. It is therefore certain that the combined effect of both causes +must bring the temperature of Mars down to at least 70° or 80°below the +freezing point. + +The cause of this absolute dependence of terrestrial temperatures upon +density of the air-envelope is seldom discussed in text-books either of +geography or of physics, and there seems to be still some uncertainty +about it. Some impute it wholly to the thinner air being unable to +absorb and retain so much heat as that which is more dense; but if this +were the case the soil at great altitudes not having so much of its heat +taken up by the air should be warmer than below, since it undoubtedly +_receives_ more heat owing to the greater transparency of the air above +it; but it certainly does not become warmer. The more correct view seems +to be that the loss of heat by radiation is increased so much through +the rarity of the air above it as to _more_ than counterbalance the +increased insolation, so that though the surface of the earth at a given +altitude may receive 10 per cent. more direct sun-heat it loses by +direct radiation, combined with diminished air and cloud-radiation, +perhaps 20 or 25 per cent. more, whence there is a resultant cooling +effect of 10 or 15 per cent. This acts by day as well as by night, so +that the greater heat received at high altitudes does not warm the soil +so much as a less amount of heat with a denser atmosphere. + +This effect is further intensified by the fact that a less dense cannot +absorb and transmit so much heat as a more dense atmosphere. Here then +we have an absolute law of nature to be observed operating everywhere on +the earth, and the mode of action of which is fairly well understood. +This law is, that reduced atmospheric pressure increases radiation, or +loss of heat, _more rapidly_ than it increases insolation or gain of +heat, so that the result is _always_ a considerable _lowering_ of +temperature. What this lowering is can be seen in the universal fact, +that even within the tropics perpetual snow covers the higher mountain +summits, while on the high plains of the Andes, at 15,000 or 16,000 feet +altitude, where there is very little or no snow, travellers are often +frozen to death when delayed by storms; yet at this elevation the +atmosphere has much more than double the density of that of Mars! + +The error in Mr. Lowell's argument is, that he claims for the scanty +atmosphere of Mars that it allows more sun-heat to reach the surface; +but he omits to take account of the enormously increased loss of heat by +direct radiation, as well as by the diminution of air-radiation, which +together necessarily produce a great reduction of temperature. + +It is this great principle of the prepotency of radiation over +absorption with a diminishing atmosphere that explains the excessively +low temperature of the moon's surface, a fact which also serves to +indicate a very low temperature for Mars, as I have shown in Chapter VI. +These two independent arguments--from alpine temperatures and from those +of the moon--support and enforce each other, and afford a conclusive +proof (as against anything advanced by Mr. Lowell) that the temperature +of Mars must be far too low to support animal life. + +A third independent argument leading to the same result is Dr. Johnstone +Stoney's proof that aqueous vapour cannot exist on Mars; and this fact +Mr. Lowell does not attempt to controvert. + +To put the whole case in the fewest possible words: + +All physicists are agreed that, owing to the distance of Mars from the +sun, it would have a mean temperature of about-35° F. (= 456° F. abs.) +even if it had an atmosphere as dense as ours. + +(2) But the very low temperatures on the earth under the equator, at a +height where the barometer stands at about three times as high as on +Mars, proves, that from scantiness of atmosphere alone Mars cannot +possibly have a temperature as high as the freezing point of water; and +this proof is supported by Langley's determination of the low _maximum_ +temperature of the full moon. + +The combination of these two results must bring down the temperature of +Mars to a degree wholly incompatible with the existence of animal life. + +(3) The quite independent proof that water-vapour cannot exist on Mars, +and that therefore, the first essential of organic life--water--is +non-existent. + +The conclusion from these three independent proofs, which enforce each +other in the multiple ratio of their respective weights, is therefore +irresistible--that animal life, especially in its higher forms, cannot +exist on the planet. + +Mars, therefore, is not only uninhabited by intelligent beings such as +Mr. Lowell postulates, but is absolutely UNINHABITABLE. + + + + + +End of Project Gutenberg's Is Mars Habitable?, by Alfred Russel Wallace + +*** END OF THE PROJECT GUTENBERG EBOOK 10855 *** diff --git a/LICENSE.txt b/LICENSE.txt new file mode 100644 index 0000000..6312041 --- /dev/null +++ b/LICENSE.txt @@ -0,0 +1,11 @@ +This eBook, including all associated images, markup, improvements, +metadata, and any other content or labor, has been confirmed to be +in the PUBLIC DOMAIN IN THE UNITED STATES. + +Procedures for determining public domain status are described in +the "Copyright How-To" at https://www.gutenberg.org. + +No investigation has been made concerning possible copyrights in +jurisdictions other than the United States. Anyone seeking to utilize +this eBook outside of the United States should confirm copyright +status under the laws that apply to them. diff --git a/README.md b/README.md new file mode 100644 index 0000000..1e49605 --- /dev/null +++ b/README.md @@ -0,0 +1,2 @@ +Project Gutenberg (https://www.gutenberg.org) public repository for +eBook #10855 (https://www.gutenberg.org/ebooks/10855) diff --git a/old/10855-8.txt b/old/10855-8.txt new file mode 100644 index 0000000..927b981 --- /dev/null +++ b/old/10855-8.txt @@ -0,0 +1,3080 @@ +The Project Gutenberg EBook of Is Mars Habitable?, by Alfred Russel Wallace + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: Is Mars Habitable? + +Author: Alfred Russel Wallace + +Release Date: January 28, 2004 [EBook #10855] + +Language: English + +Character set encoding: ISO-8859-1 + +*** START OF THIS PROJECT GUTENBERG EBOOK IS MARS HABITABLE? *** + + + + +Produced by Thaadd and the PG Distributed Proofreading Team + + + + +_Is Mars Habitable?_ + +A CRITICAL EXAMINATION OF PROFESSOR PERCIVAL LOWELL'S BOOK +"MARS AND ITS CANALS," WITH AN ALTERNATIVE EXPLANATION + +BY ALFRED RUSSEL WALLACE F.R.S., ETC. + + + +PREFACE. + +This small volume was commenced as a review article on Professor +Percival Lowell's book, _Mars and its Canals_, with the object of +showing that the large amount of new and interesting facts contained in +this work did not invalidate the conclusion I had reached in 1902, and +stated in my book on _Man's Place in the Universe_, that Mars was not +habitable. + +But the more complete presentation of the opposite view in the volume +now under discussion required a more detailed examination of the various +physical problems involved, and as the subject is one of great, popular, +as well as scientific interest, I determined to undertake the task. + +This was rendered the more necessary by the fact that in July last +Professor Lowell published in the _Philosophical Magazine_ an elaborate +mathematical article claiming to demonstrate that, notwithstanding its +much greater distance from the sun and its excessively thin atmosphere, +Mars possessed a climate on the average equal to that of the south of +England, and in its polar and sub-polar regions even less severe than +that of the earth. Such a contention of course required to be dealt +with, and led me to collect information bearing upon temperature in all +its aspects, and so enlarging my criticism that I saw it would be +necessary to issue it in book form. + +Two of my mathematical friends have pointed out the chief omission which +vitiates Professor Lowell's mathematical conclusions--that of a failure +to recognise the very large conservative and _cumulative_ effect of a +dense atmosphere. This very point however I had already myself discussed +in Chapter VI., and by means of some remarkable researches on the heat +of the moon and an investigation of the causes of its very low +temperature, I have, I think, demonstrated the incorrectness of Mr. +Lowell's results. In my last chapter, in which I briefly summarise the +whole argument, I have further strengthened the case for very severe +cold in Mars, by adducing the rapid lowering of temperature universally +caused by diminution of atmospheric pressure, as manifested in the +well-known phenomenon of temperate climates at moderate heights even +close to the equator, cold climates at greater heights even on extensive +plateaux, culminating in arctic climates and perpetual snow at heights +where the air is still far denser than it is on the surface of Mars. +This argument itself is, in my opinion, conclusive; but it is enforced +by two others equally complete, neither of which is adequately met by +Mr. Lowell. + +The careful examination which I have been led to give to the whole of +the phenomena which Mars presents, and especially to the discoveries of +Mr. Lowell, has led me to what I hope will be considered a satisfactory +physical explanation of them. This explanation, which occupies the whole +of my seventh chapter, is founded upon a special mode of origin for +Mars, derived from the Meteoritic Hypothesis, now very widely adopted by +astronomers and physicists. Then, by a comparison with certain +well-known and widely spread geological phenomena, I show how the great +features of Mars--the 'canals' and 'oases'--may have been caused. This +chapter will perhaps be the most interesting to the general reader, as +furnishing a quite natural explanation of features of the planet which +have been termed 'non-natural' by Mr. Lowell. + +Incidentally, also, I have been led to an explanation of the highly +volcanic nature of the moon's surface. This seems to me absolutely to +require some such origin as Sir George Darwin has given it, and thus +furnishes corroborative proof of the accuracy of the hypothesis that our +moon has had an unique origin among the known satellites, in having been +thrown off from the earth itself. + +I am indebted to Professor J. H. Poynting, of the University of +Birmingham, for valuable suggestions on some of the more difficult +points of mathematical physics here discussed, and also for the critical +note (at the end of Chapter V.) on Professor Lowell's estimate of the +temperature of Mars. + +BROADSTONE, DORSET, _October_ 1907. + + + +TABLE OF CONTENTS. + + +CHAPTER I. + +EARLY OBSERVERS OF MARS, +--Mars the only planet the surface of which is + distinctly visible +--Early observation of the snow-caps and seas +--The 'canals' seen by Schiaparelli in 1877 +--Double canals first seen in 1881 +--Round spots at intersection of canals seen + by Pickering in 1892 +--Confirmed by Lowell in 1894 +--Changes of colour seen in 1892 and 1894 +--Existence of seas doubted by Pickering and + Barnard in 1894. + + +CHAPTER II. + +MR. LOWELL'S DISCOVERIES AND THEORIES, +--Observatory at Flagstaff, Arizona +--Illustrated book on his observations of + Mars +--Volume on Mars and its canals, 1906 +--Non-natural features +--The canals as irrigation works of an intelligent + race +--A challenge to the thinking world +--The canals as described and mapped by Mr. Lowell +--The double canals +--Dimensions of the canals +--They cross the supposed seas +--Circular black spots termed oases +--An interesting volume. + + +CHAPTER III. + +THE CLIMATE AND PHYSIOGRAPHY OF MARS, +--No permanent water on Mars +--Rarely any clouds and no rain +--Snow-caps the only source of water +--No mountains, hills, or valleys on Mars +--Two-thirds of the surface a desert +--Water from the snow-caps too scanty to supply + the canals +--Miss Clerke's views as to the water-supply +--Description of some of the chief canals +--Mr. Lowell on the purpose of the canals +--Remarks on the same +--Mr. Lowell on relation of canals to oases and + snow-caps +--Critical remarks on the same. + + +CHAPTER IV. + +IS ANIMAL LIFE POSSIBLE ON MARS? +--Water and air essential for animal life +--Atmosphere of Mars assumed to be like ours +--Blue tint near melting snow the only evidence + of water +--Fallacy of this argument +--Dr. Johnstone Stoney's proof that water-vapour + cannot exist on Mars +--Spectroscope gives no evidence of water. + + +CHAPTER V. + +TEMPERATURE OF MARS--MR. LOWELL'S ESTIMATE, +--Problem of terrestrial temperature +--Ice under recent lava +--Tropical oceans ice-cold at bottom +--Earth's surface-heat all from the sun +--Absolute zero of temperature +--Complex problem of planetary temperatures +--Mr. Lowell's investigation of the problem +--Abstract of Mr. Lowell's paper +--Critical remarks on Mr. Lowell's paper. + + +CHAPTER VI. + +A NEW ESTIMATE OF THE TEMPERATURE OF MARS, +--Langley's determination of lunar heat +--Rapid loss of heat by radiation on the + earth +--Rapid loss of heat on moon during eclipse +--Sir George Darwin's theory of the moon's origin +--Very's researches on the moon's temperature +--Application of these results to the case of Mars +--Cause of great difference of temperatures of earth + and moon +--Special features of Mars influencing its + temperature +--Further criticism of Mr. Lowell's article +--Very low temperature of arctic regions on Mars. + + +CHAPTER VII. + +A SUGGESTION AS TO THE 'CANALS' OF MARS, +--Special features of the canals +--Mr. Pickering's suggested explanation +--The meteoritic hypotheses of origin of planets +--Probable mode of origin of Mars +--Structural straight lines on the earth +--Probable origin of the surface-features of Mars +--Symmetry of basaltic columns +--How this applies to Mars +--Suggested explanation of the oases +--Probable function of the great fissures +--Suggested origin of blue patches adjacent to snow-caps +--The double canals +--Concluding remarks on the canals. + + +CHAPTER VIII. + +PAGE SUMMARY AND CONCLUSION, +--The canals the origin of Mr. Lowell's theory +--Best explained as natural features +--Evaporation difficulty not met by Mr. Lowell +--How did Martians live without the canals +--Radiation due to scanty atmosphere not taken account + of +--Three independent proofs of low temperature and + uninhabitability of Mars +--Conclusion. + + +CHAPTER I. + +EARLY OBSERVERS OF MARS. + +Few persons except astronomers fully realise that of all the planets of +the Solar system the only one whose solid surface has been seen with +certainty is Mars; and, very fortunately, that is also the only one +which is sufficiently near to us for the physical features of the +surface to be determined with any accuracy, even if we could see it in +the other planets. Of Venus we probably see only the upper surface of +its cloudy atmosphere.[1] As regards Jupiter and Saturn this is still +more certain, since their low density will only permit of a +comparatively small proportion of their huge bulk being solid. Their +belts are but the cloud-strata of their upper atmosphere, perhaps +thousands of miles above their solid surfaces, and a somewhat similar +condition seems to prevail in the far more remote planets Uranus and +Neptune. It has thus happened, that, although as telescopic objects of +interest and beauty, the marvellous rings of Saturn, the belts and +ever-changing aspects of the satellites of Jupiter, and the moon-like +phases of Venus, together with its extreme brilliancy, still remain +unsurpassed, yet the greater amount of details of these features when +examined with the powerful instruments of the nineteenth century have +neither added much to our knowledge of the planets themselves or led to +any sensational theories calculated to attract the popular imagination. + +[Footnote 1: Mercury also seems to have a scanty atmosphere, but as its +mass is only one-thirtieth that of the earth it can retain only the +heavier gases, and its atmosphere may be dust-laden, as is that of Mars, +according to Mr. Lowell. Its dusky markings, as seen by Schiaparelli, +seem to be permanent, and they are also for considerable periods +unchangeable in position, indicating that the planet keeps the same face +towards the sun as does Venus. This was confirmed by Mr. Lowell in 1896. +Its distance from us and unfavourable position for observation must +prevent us from obtaining any detailed knowledge of its actual surface, +though its low reflective power indicates that the surface may be really +visible.] + +But in the case of Mars the progress of discovery has had a very +different result. The most obvious peculiarity of this planet--its polar +snow-caps--were seen about 250 years ago, but they were first proved to +increase and decrease alternately, in the summer and winter of each +hemisphere, by Sir William Herschell in the latter part of the +eighteenth century. This fact gave the impulse to that idea of +similarity in the conditions of Mars and the earth, which the +recognition of many large dusky patches and streaks as water, and the +more ruddy and brighter portions as land, further increased. Added to +this, a day only about half an hour longer than our own, and a +succession of seasons of the same character as ours but of nearly double +the length owing to its much longer year, seemed to leave little wanting +to render this planet a true earth on a smaller scale. It was therefore +very natural to suppose that it must be inhabited, and that we should +some day obtain evidence of the fact. + +_The Canals discovered by Schiaparelli._ + +Hence the great interest excited when Schiaparelli, at the Milan +Observatory, during the very favourable opposition of 1877 and 1879, +observed that the whole of the tropical and temperate regions from 60° +N. to 60° S. Lat. were covered with a remarkable network of broader +curved and narrower straight lines of a dark colour. At each successive +favourable opposition, these strange objects called _canali_ (channels) +by their discoverer, but rather misleadingly 'canals' in England and +America, were observed by means of all the great telescopes in the +world, and their reality and general features became well established. +In Schiaparelli's first map they were represented as being much broader +and less sharply defined than he himself and other observers found by +later and equally favourable observations that they really were. + +_Discovery of the Double Canals._ + +In 1881 another strange feature was discovered by Schiaparelli, who +found that about twenty canals which had previously been seen single +were now distinctly double, that is, that they consisted of two parallel +lines, equally distinct and either very close together or a considerable +distance apart. This curious appearance was at first thought to be due +to some instrumental defect or optical illusion; but as it was soon +confirmed by other observers with the best instruments and in widely +different localities it became in time accepted as a real phenomenon of +the planet's surface. + +_Round Spots discovered in_ 1892. + +At the favourable opposition of 1892, Mr. W. H. Pickering noticed that +besides the 'seas' of various sizes there were numerous very small black +spots apparently quite circular and occurring at every intersection or +starting-point of the 'canals.' Many of these had been seen by +Schiaparelli as larger and ill-defined dark patches, and were termed +seas or lakes; but Mr. Pickering's observatory was at Arequipa in Peru, +about 8000 feet above the sea, and with such perfect atmospheric +conditions as were, in his opinion, equal to a doubling of telescopic +aperture. They were soon detected by other observers, especially by Mr. +Lowell in 1894, who thus wrote of them: + +"Scattered over the orange-ochre groundwork of the continental regions +of the planet, are any number of dark round spots. How many there may be +it is not possible to state, as the better the seeing, the more of them +there seem to be. In spite, however, of their great number, there is no +instance of one unconnected with a canal. What is more, there is +apparently none that does not lie at the junction of several canals. +Reversely, all the junctions appear to be provided with spots. Plotted +upon a globe they and their connecting canals make a most curious +network over all the orange-ochre equatorial parts of the planet, a mass +of lines and knots, the one marking being as omnipresent as the other." + +_Changes of Colour recognised._ + +During the oppositions of 1892 and 1894 it was fully recognised that a +regular course of change occurred dependent upon the succession of the +seasons, as had been first suggested by Schiaparelli. As the polar snows +melt the adjacent seas appear to overflow and spread out as far as the +tropics, and are often seen to assume a distinctly green colour. These +remarkable changes and the extraordinary phenomena of perfect straight +lines crossing each other over a large portion of the planet's surface, +with the circular spots at their intersections, had such an appearance +of artificiality that the idea that they were really 'canals' made by +intelligent beings for purposes of irrigation, was first hinted at, and +then adopted as the only intelligible explanation, by Mr. Lowell and a +few other persons. This at once seized upon the public imagination and +was spread by the newspapers and magazines over the whole civilised +world. + +_Existence of Seas doubted._ + +At this time (1894) it began to be doubted whether there were any seas +at all on Mars. Professor Pickering thought they were far more limited +in size than had been supposed, and even might not exist as true seas. +Professor Barnard, with the Lick thirty-six inch telescope, discerned an +astonishing wealth of detail on the surface of Mars, so intricate, +minute, and abundant, that it baffled all attempts to delineate it; and +these peculiarities were seen upon the supposed seas as well as on the +land-surfaces. In fact, under the best conditions these 'seas' lost all +trace of uniformity, their appearance being that of a mountainous +country, broken by ridges, rifts, and canyons, seen from a great +elevation. As we shall see later on these doubts soon became +certainties, and it is now almost universally admitted that Mars +possesses no permanent bodies of water. + + + +CHAPTER II. + + +MR. PERCIVAL LOWELL'S DISCOVERIES AND THEORIES. + +_The Observatory in Arizona._ + +In 1894, after a careful search for the best atmospheric conditions, Mr. +Lowell established his observatory near the town of Flagstaff in +Arizona, in a very dry and uniform climate, and at an elevation of 7300 +feet above the sea. He then possessed a fine equatorial telescope of 18 +inches aperture and 26 feet focal length, besides two smaller ones, all +of the best quality. To these he added in 1896 a telescope with 24 inch +object glass, the last work of the celebrated firm of Alvan Clark & +Sons, with which he has made his later discoveries. He thus became +perhaps more favourably situated than any astronomer in the northern +hemisphere, and during the last twelve years has made a specialty of the +study of Mars, besides doing much valuable astronomical work on other +planets. + +_Mr, Lowell's recent Books upon Mars._ + +In 1905 Mr. Lowell published an illustrated volume giving a full account +of his observations of Mars from 1894 to 1903, chiefly for the use of +astronomers; and he has now given us a popular volume summarising the +whole of his work on the planet, and published both in America and +England by the Macmillan Company. This very interesting volume is fully +illustrated with twenty plates, four of them coloured, and more than +forty figures in the text, showing the great variety of details from +which the larger general maps have been constructed. + +_Non-natural Features of Mars._ + +But what renders this work especially interesting to all intelligent +readers is, that the author has here, for the first time, fully set +forth his views both as to the habitability of Mars and as to its being +actually inhabited by beings comparable with ourselves in intellect. The +larger part of the work is in fact devoted to a detailed description of +what he terms the 'Non-natural Features' of the planet's surface, +including especially a full account of the 'Canals,' single and double; +the 'Oases,' as he terms the dark spots at their intersections; and the +varying visibility of both, depending partly on the Martian seasons; +while the five concluding chapters deal with the possibility of animal +life and the evidence in favour of it. He also upholds the theory of the +canals having been constructed for the purpose of 'husbanding' the +scanty water-supply that exists; and throughout the whole of this +argument he clearly shows that he considers the evidence to be +satisfactory, and that the only intelligible explanation of the whole of +the phenomena he so clearly sets forth is, that the inhabitants of Mars +have carried out on their small and naturally inhospitable planet a vast +system of irrigation-works, far greater both in its extent, in its +utility, and its effect upon their world as a habitation for civilised +beings, than anything we have yet done upon our earth, where our +destructive agencies are perhaps more prominent than those of an +improving and recuperative character. + +_A Challenge to the Thinking World._ + +This volume is therefore in the nature of a challenge, not so much to +astronomers as to the educated world at large, to investigate the +evidence for so portentous a conclusion. To do this requires only a +general acquaintance with modern science, more especially with mechanics +and physics, while the main contention (with which I shall chiefly deal) +that the features termed 'canals' are really works of art and +necessitate the presence of intelligent organic beings, requires only +care and judgment in drawing conclusions from admitted facts. As I have +already paid some attention to this problem and have expressed the +opinion that Mars is not habitable,[2] judging from the evidence then +available, and as few men of science have the leisure required for a +careful examination of so speculative a subject, I propose here to point +out what the facts, as stated by Mr. Lowell himself, do _not_ render +even probable much less prove. Incidentally, I may be able to adduce +evidence of a more or less weighty character, which seems to negative +the possibility of any high form of animal life on Mars, and, _a +fortiori_, the development of such life as might culminate in a being +equal or superior to ourselves. As most popular works on Astronomy for +the last ten years at least, as well as many scientific periodicals and +popular magazines, have reproduced some of the maps of Mars by +Schiaparelli, Lowell, and others, the general appearance of its surface +will be familiar to most readers, who will thus be fully able to +appreciate Mr. Lowell's account of his own further discoveries which I +may have to quote. One of the _best_ of these maps I am able to give as +a frontispiece to this volume, and to this I shall mainly refer. + +[Footnote 2: _Man's Place in the Universe_ p. 267 (1903).] + +_The Canals as described by Mr. Lowell._ + +In the clear atmosphere of Arizona, Mr. Lowell has been able on various +favourable occasions to detect a network of straight lines, meeting or +crossing each other at various angles, and often extending to a thousand +or even over two thousand miles in length. They are seen to cross both +the light and the dark regions of the planet's surface, often extending +up to or starting from the polar snow-caps. Most of these lines are so +fine as only to be visible on special occasions of atmospheric clearness +and steadiness, which hardly ever occur at lowland stations, even with +the best instruments, and almost all are seen to be as perfectly +straight as if drawn with a ruler. + +_The Double Canals._ + +Under exceptionally favourable conditions, many of the lines that have +been already seen single appear double--a pair of equally fine lines +exactly parallel throughout their whole length, and appearing, as Mr. +Lowell says, "clear cut upon the disc, its twin lines like the rails of +a railway track." Both Schiaparelli and Lowell were at first so +surprised at this phenomenon that they thought it must be an optical +illusion, and it was only after many observations in different years, +and by the application of every conceivable test, that they both became +convinced that they witnessed a real feature of the planet's surface. +Mr. Lowell says he has now seen them hundreds of times, and that his +first view of one was 'the most startlingly impressive' sight he has +ever witnessed. + +_Dimensions of the Canals._ + +A few dimensions of these strange objects must be given in order that +readers may appreciate their full strangeness and inexplicability. Out +of more than four hundred canals seen and recorded by Mr. Lowell, +fifty-one, or about one eighth, are either constantly or occasionally +seen to be double, the appearance of duplicity being more or less +periodical. Of 'canals' generally, Mr. Lowell states that they vary in +length from a few hundred to a few thousand miles long, one of the +largest being the Phison, which he terms 'a typical double canal,' and +which is said to be 2250 miles long, while the distance between its two +constituents is about 130 miles.[3] The actual width of each canal is +from a minimum of about a mile up to several miles, in one case over +twenty. A great feature of the doubles is, that they are strictly +parallel throughout their whole course, and that in almost all cases +they are so truly straight as to form parts of a great circle of the +planet's sphere. A few however follow a gradual but very distinct curve, +and such of these as are double present the same strict parallelism as +those which are straight. + +[Footnote 3: This is on the opposite side of Mars from that shown in the +frontispiece.] + +_Canals extend across the Seas._ + +It was only after seventeen years of observation of the canals that it +was found that they extended also into and across the dark spots and +surfaces which by the earlier observers were termed seas, and which then +formed the only clearly distinguishable and permanent marks on the +planet's surface. At the present time, Professor Lowell states that this +"curious triangulation has been traced over almost every portion of the +planet's surface, whether dark or light, whether greenish, ochre, or +brown in colour." In some parts they are much closer together than in +others, "forming a perfect network of lines and spots, so that to +identify them all was a matter of extreme difficulty." Two such portions +are figured at pages 247 and 256 of Mr. Lowell's volume. + +_The Oases._ + +The curious circular black spots which are seen at the intersections of +many of the canals, and which in some parts of the surface are very +numerous, are said to be more difficult of detection than even the +lines, being often blurred or rendered completely invisible by slight +irregularities in our own atmosphere, while the canals themselves +continue visible. About 180 of these have now been found, and the more +prominent of them are estimated to vary from 75 to 100 miles in +diameter. There are however many much smaller, down to minute and barely +visible black points. Yet they all seem a little larger than the canals +which enter them. Where the canals are double, the spots (or 'oases' as +Mr. Lowell terms them) lie between the two parallel canals. + +No one can read this book without admiration for the extreme +perseverance in long continued and successful observation, the results +of which are here recorded; and I myself accept unreservedly the +substantial accuracy of the whole series. It must however always be +remembered that the growth of knowledge of the detailed markings has +been very gradual, and that much of it has only been seen under very +rare and exceptional conditions. It is therefore quite possible that, if +at some future time a further considerable advance in instrumental power +should be made, or a still more favourable locality be found, the new +discoveries might so modify present appearances as to render a +satisfactory explanation of them more easy than it is at present. + +But though I wish to do the fullest justice to Mr. Lowell's technical +skill and long years of persevering work, which have brought to light +the most complex and remarkable appearances that any of the heavenly +bodies present to us, I am obliged absolutely to part company with him +as regards the startling theory of artificial production which he thinks +alone adequate to explain them. So much is this the case, that the very +phenomena, which to him seem to demonstrate the intervention of +intelligent beings working for the improvement of their own environment, +are those which seem to me to bear the unmistakable impress of being due +to natural forces, while they are wholly unintelligible as being useful +works of art. I refer of course to the great system of what are termed +'canals,' whether single or double. Of these I shall give my own +interpretation later on. + + + +CHAPTER III. + + +THE CLIMATE AND PHYSIOGRAPHY OF MARS. + +Mr. Lowell admits, and indeed urges strongly, that there are no +permanent bodies of water on Mars; that the dark spaces and spots, +thought by the early observers to be seas, are certainly not so now, +though they may have been at an earlier period; that true clouds are +rare, even if they exist, the appearances that have been taken for them +being either dust-storms or a surface haze; that there is consequently +no rain, and that large portions (about two-thirds) of the planet's +surface have all the characteristics of desert regions. + +_Snow-caps the only Source of Water._ + +This state of things is supposed to be ameliorated by the fact of the +polar snows, which in the winter cover the arctic and about half the +temperate regions of each hemisphere alternately. The maximum of the +northern snow-caps is reached at a period of the Martian winter +corresponding to the end of February with us. About the end of March the +cap begins to shrink in size (in the Northern Hemisphere), and this goes +on so rapidly that early in the June of Mars it is reduced to its +minimum. About the same time changes of colour take place in the +adjacent darker portions of the surface, which become at first bluish, +and later a decided blue-green; but by far the larger portion, including +almost all the equatorial regions of the planet, remain always of a +reddish-ochre tint.[4] + +[Footnote 4: In 1890 at Mount Wilson, California, Mr. W.H. Pickering's +photographs of Mars on April 9th showed the southern polar cap of +moderate dimensions, but with a large dim adjacent area. Twenty-four +hours later a corresponding plate showed this same area brilliantly +white; the result apparently of a great Martian snowfall. In 1882 the +same observer witnessed the steady disappearance of 1,600,000 square +miles of the southern snow-cap, an area nearly one-third of that +hemisphere of the planet.] + +The rapid and comparatively early disappearance of the white covering +is, very reasonably, supposed to prove that it is of small thickness, +corresponding perhaps to about a foot or two of snow in north-temperate +America and Europe, and that by the increasing amount of sun-heat it is +converted, partly into liquid and partly into vapour. Coincident with +this disappearance and as a presumed result of the water (or other +liquid) producing inundations, the bluish-green tinge which appears on +the previously dark portion of the surface is supposed to be due to a +rapid growth of vegetation. + +But the evidence on this point does not seem to be clear or harmonious, +for in the four coloured plates showing the planet's surface at +successive Martian dates from December 30th to February 21st, not only +is a considerable extent of the south temperate zone shown to change +rapidly from bluish-green to chocolate-brown and then again to +bluish-green, but the portions furthest from the supposed fertilising +overflow are permanently green, as are also considerable portions in the +opposite or northern hemisphere, which one would think would then be +completely dried up. + +_No Hills upon Mars._ + +The special point to which I here wish to call attention is this. Mr. +Lowell's main contention is, that the surface of Mars is wonderfully +smooth and level. Not only are there no mountains, but there are no +hills or valleys or plateaux. This assumption is absolutely essential to +support the other great assumption, that the wonderful network of +perfectly straight lines over nearly the whole surface of the planet are +irrigation canals. It is not alleged that irregularities or undulations +of a few hundreds or even one or two thousands of feet could possibly be +detected, while certainly all we know of planetary formation or +structure point strongly towards _some_ inequalities of surface. Mr. +Lowell admits that the dark portions of the surface, when examined on +the terminator (the margin of the illuminated portion), do _look_ like +hollows and _may be_ the beds of dried-up seas; yet the supposed canals +run across these old sea-beds in perfect straight lines just as they do +across the many thousand miles of what are admitted to be deserts--which +he describes in these forcible terms: "Pitiless as our deserts are, they +are but faint forecasts of the state of things existent on Mars at the +present time." + +It appears, then, that Mr. Lowell has to face this dilemma--_Only if the +whole surface of Mars is an almost perfect level could the enormous +network of straight canals, each from hundreds to thousands of miles +long, have been possibly constructed by intelligent beings for purposes +of irrigation; but, if a complete and universal level surface exists no +such system would be necessary._ For on a level surface--or on a +surface slightly inclined from the poles towards the equator, which +would be advantageous in either case--the melting water would of itself +spread over the ground and naturally irrigate as much of the surface as +it was possible for it to reach. If the surface were not level, but +consisted of slight elevations and expressions to the extent of a few +scores or a few hundreds of feet, then there would be no possible +advantage in cutting straight troughs through these elevations in +various directions with water flowing at the bottom of them. In neither +case, and in hardly any conceivable case, could these perfectly straight +canals, cutting across each other in every direction and at very varying +angles, be of any use, or be the work of an intelligent race, if any +such race could possibly have been developed under the adverse +conditions which exist in Mars. + +_The Scanty Water-supply._ + +But further, if there were any superfluity of water derived from the +melting snow beyond what was sufficient to moisten the hollows indicated +by the darker portions of the surface, which at the time the water +reaches them acquire a green tint (a superfluity under the circumstances +highly improbable), that superfluity could be best utilised by widening, +however little, the borders to which natural overflow had carried it. +Any attempt to make that scanty surplus, by means of overflowing canals, +travel across the equator into the opposite hemisphere, through such a +terrible desert region and exposed to such a cloudless sky as Mr. Lowell +describes, would be the work of a body of madmen rather than of +intelligent beings. It may be safely asserted that not one drop of water +would escape evaporation or insoak at even a hundred miles from its +source. [5] + +[Footnote 5: What the evaporation is likely to be in Mars may be +estimated by the fact, stated by Professor J.W. Gregory in his recent +volume on 'Australia' in _Stanford's Compendium_, that in North-West +Victoria evaporation is at the rate of ten feet per annum, while in +Central Australia it is very much more. The greatly diminished +atmospheric pressure in Mars will probably more than balance the loss of +sun-heat in producing rapid evaporation.] + +_Miss Clerke on the Scanty Water-supply._ + +On this point I am supported by no less an authority than the historian +of modern astronomy, the late Miss Agnes Clerke. In the _Edinburgh +Review_ (of October 1896) there is an article entitled 'New Views about +Mars,' exhibiting the writer's characteristic fulness of knowledge and +charm of style. Speaking of Mr. Lowell's idea of the 'canals' carrying +the surplus water across the equator, far into the opposite hemisphere, +for purposes of irrigation there (which we see he again states in the +present volume), Miss Clerke writes: "We can hardly imagine so shrewd a +people as the irrigators of Thule and Hellas[6] wasting labour, and the +life-giving fluid, after so unprofitable a fashion. There is every +reason to believe that the Martian snow-caps are quite flimsy +structures. Their material might be called snow _soufflé_, since, owing +to the small power of gravity on Mars, snow is almost three times +lighter there than here. Consequently, its own weight can have very +little effect in rendering it compact. Nor, indeed, is there time for +much settling down. The calotte does not form until several months after +the winter solstice, and it begins to melt, as a rule, shortly after the +vernal equinox. (The interval between these two epochs in the southern +hemisphere of Mars is 176 days.) The snow lies on the ground, at the +outside, a couple of months. At times it melts while it is still fresh +fallen. Thus, at the opposition of 1881-82 the spreading of the northern +snows was delayed until seven weeks after the equinox: and they had, +accordingly, no sooner reached their maximum than they began to decline. +And Professor Pickering's photographs of April 9th and 10th, 1890, +proved that the southern calotte may assume its definitive proportions +in a single night. + +[Footnote 6: Areas on Mars so named.] + +"No attempt has yet been made to estimate the quantity of water +derivable from the melting of one of these formations; yet the +experiment is worth trying as a help towards defining ideas. Let us +grant that the average depth of snow in them, of the delicate Martian +kind, is twenty feet, equivalent at the most to one foot of water. The +maximum area covered, of 2,400,000 square miles, is nearly equal to that +of the United States, while the whole globe of Mars measures 55,500,000 +square miles, of which one-third, on the present hypothesis, is under +cultivation, and in need of water. Nearly the whole of the dark areas, +as we know, are situated in the southern hemisphere, of which they +extend over, at the very least, 17,000,000 square miles; that is to say, +they cover an area, in round numbers, seven times that of the snow-cap. +Only one-seventh of a foot of water, accordingly, could possibly be +made available for their fertilisation, supposing them to get the entire +advantage of the spring freshet. Upon a stint of less than two inches of +water these fertile lands are expected to flourish and bear abundant +crops; and since they completely enclose the polar area they are +necessarily served first. The great emissaries for carrying off the +surplus of their aqueous riches, would then appear to be superfluous +constructions, nor is it likely that the share in those riches due to +the canals and oases, intricately dividing up the wide, dry, continental +plains, can ever be realised. + +"We have assumed, in our little calculation, that the entire contents of +a polar hood turn to water; but in actual fact a considerable proportion +of them must pass directly into vapour, omitting the intermediate stage. +Even with us a large quantity of snow is removed aerially; and in the +rare atmosphere of Mars this cause of waste must be especially +effective. Thus the polar reservoirs are despoiled in the act of being +opened. Further objections might be taken to Mr. Lowell's irrigation +scheme, but enough has been said to show that it is hopelessly +unworkable." + +It will be seen that the writer of this article accepted the existence +of water on Mars, on the testimony of Sir W. Huggins, which, in view of +later observations, he has himself acknowledged to be valueless. Dr. +Johnstone Stoney's proof of its absence, derived from the molecular +theory of gases, had not then been made public. + +_Description of some of the Canals._ + +At the end of his volume Mr. Lowell gives a large chart of Mars on +Mercator's projection, showing the canals and other features seen during +the opposition of 1905. This contains many canals not shown on the map +here reproduced (see frontispiece), and some of the differences between +the two are very puzzling. Looking at our map, which shows the +north-polar snow below, so that the south pole is out of the view at the +top of the map, the central feature is the large spot Ascraeeus Lucus, +from which ten canals diverge centrally, and four from the sides, +forming wide double canals, fourteen in all. There is also a canal named +Ulysses, which here passes far to the right of the spot, but in the +large chart enters it centrally. Looking at our map we see, going +downwards a little to the left, the canal Udon, which runs through a +dark area quite to the outer margin. In the dark area, however, there is +shown on the chart a spot Aspledon Lucus, where five canals meet, and if +this is taken as a terminus the Udon canal is almost exactly 2000 miles +long, and another on its right, Lapadon, is the same length, while Ich, +running in a slightly curved line to a large spot (Lucus Castorius on +the chart) is still longer. The Ulysses canal, which (on the chart) runs +straight from the point of the Mare Sirenum to the Astraeeus Lucus is +about 2200 miles long. Others however are even longer, and Mr. Lowell +says: "With them 2000 miles is common; while many exceed 2500; and the +Eumenides-Orcus is 3540 miles from the point where it leaves Lucus +Phoeniceus to where it enters the Trivium Charontis." This last canal is +barely visible on our map, its commencement being indicated by the word +Eumenides. + +The Trivium Charontis is situated just beyond the right-hand margin of +our map. It is a triangular dark area, the sides about 200 miles long, +and it is shown on the chart as being the centre from which radiate +thirteen canals. Another centre is Aquae Calidae situated at the point +of a dark area running obliquely from 55° to 35° N. latitude, and, as +shown on a map of the opposite hemisphere to our map, has nearly twenty +canals radiating from it in almost every direction. Here at all events +there seems to be no special connection with the polar snow-caps, and +the radiating lines seem to have no intelligent purpose whatever, but +are such as might result from fractures in a glass globe produced by +firing at it with very small shots one at a time. Taking the whole +series of them, Mr. Lowell very justly compares them to "a network which +triangulates the surface of the planet like a geodetic survey, into +polygons of all shapes and sizes." + +At the very lowest estimate the total length of the canals observed and +mapped by Mr. Lowell must be over a hundred thousand miles, while he +assures us that numbers of others have been seen over the whole surface, +but so faintly or on such rare occasions as to elude all attempts to fix +their position with certainty. But these, being of the same character +and evidently forming part of the same system, must also be artificial, +and thus we are led to a system of irrigation of almost unimaginable +magnitude on a planet which has no mountains, no rivers, and no rain to +support it; whose whole water-supply is derived from polar snows, the +amount of which is ludicrously inadequate to need any such world-wide +system; while the low atmospheric pressure would lead to rapid +evaporation, thus greatly diminishing the small amount of moisture that +is available. Everyone must, I think, agree with Miss Clerke, that, even +admitting the assumption that the polar snows consist of frozen water, +the excessively scanty amount of water thus obtained would render any +scheme of world-wide distribution of it hopelessly unworkable. + +The very remarkable phenomena of the duplication of many of the lines, +together with the darkspots--the so-called oases--at their +intersections, are doubtless all connected in some unknown way with the +constitution and past history of the planet; but, on the theory of the +whole being works of art, they certainly do _not_ help to remove any of +the difficulties which have been shown to attend the theory that the +single lines represent artificial canals of irrigation with a strip of +verdure on each side of them produced by their overflow. + +_Lowell on the Purpose of the Canals._ + +Before leaving this subject it will be well to quote Mr. Lowell's own +words as to the supposed perfectly level surface of Mars, and his +interpretation of the origin and purpose of the 'canals': + +"A body of planetary size, if unrotating, becomes a sphere, except for +solar tidal deformation; if rotating, it takes on a spheroidal form +exactly expressive, so far as observation goes, of the so-called +centrifugal force at work. Mars presents such a figure, being flattened +out to correspond to its axial rotation. Its surface therefore is in +fluid equilibrium, or, in other words, a particle of liquid at any point +of its surface at the present time would stay where it was devoid of +inclination to move elsewhere. Now the water which quickens the verdure +of the canals moves from the pole down to the equator as the season +advances. This it does then irrespective of gravity. No natural force +propels it, and the inference is forthright and inevitable that it is +artificially helped to its end. There seems to be no escape from this +deduction. Water only flows downhill, and there is no such thing as +downhill on a surface already in fluid equilibrium. A few canals might +presumably be so situated that their flow could, by inequality of +terrane, lie equatorward, but not all....Now it is not in particular but +by general consent that the canal-system of Mars develops from pole to +equator. From the respective times at which the minima take place, it +appears that the canal quickening occupies fifty-two days, as evidenced +by the successive vegetal darkenings, to descend from latitude 72° north +to latitude 0°, a journey of 2650 miles. This gives for the water a +speed of fifty-one miles a day, or 2.1 miles an hour. The rate of +progression is remarkably uniform, and this abets the deduction as to +assisted transference. But the fact is more unnatural yet. The growth +pays no regard to the equator, but proceeds across it as if it did not +exist into the planet's other hemisphere. Here is something still more +telling than travel to this point. For even if we suppose, for the sake +of argument, that natural forces took the water down to the equator, +their action must there be certainly reversed, and the equator prove a +dead-line, to pass which were impossible" (pp. 374-5). + +I think my readers will agree with me that this whole argument is one of +the most curious ever put forth seriously by an eminent man of science. +Because the polar compression of Mars is about what calculation shows it +ought to be in accordance with its rate of rotation, its surface is in a +state of 'fluid equilibrium,' and must therefore be absolutely level +throughout. But the polar compression of the earth equally agrees with +calculation; therefore its surface is also in 'fluid equilibrium'; +therefore it also ought to be as perfectly level on land as it is on the +ocean surface! But as we know this is very far from being the case, why +must it be so in Mars? Are we to suppose Mars to have been formed in +some totally different way from other planets, and that there neither is +nor ever has been any reaction between its interior and exterior forces? +Again, the assumption of perfect flatness is directly opposed to all +observation and all analogy with what we see on the earth and moon. It +gives no account whatever of the numerous and large dark patches, once +termed seas, but now found to be not so, and to be full of detailed +markings and varied depths of shadow. To suppose that these are all the +same dead-level as the light-coloured portions are assumed to be, +implies that the darkness is one of material and colour only, not of +diversified contour, which again is contrary to experience, since +difference of material with us always leads to differences in rate of +degradation, and hence of diversified contour, as these dark spaces +actually show themselves under favourable conditions to independent +observers. + +_Lowell on the System of Canals as a whole._ + +We will now see what Mr. Lowell claims to be the plain teaching of the +'canals' as a whole: + +"But last and all-embracing in its import is the system which the canals +form. Instead of running at hap-hazard, the canals are interconnected in +a most remarkable manner. They seek centres instead of avoiding them. +The centres are linked thus perfectly one with another, an arrangement +which could not result from centres, whether of explosion or otherwise, +which were themselves discrete. Furthermore, the system covers the whole +surface of the planet, dark areas and light ones alike, a world-wide +distribution which exceeds the bounds of natural possibility. Any force +which could act longitudinally on such a scale must be limited +latitudinally in its action, as witness the belts of Jupiter and the +spots upon the sun. Rotational, climatic, or other physical cause could +not fail of zonal expression. Yet these lines are grandly indifferent to +such competing influences. Finally, the system, after meshing the +surface in its entirety, runs straight into the polar caps. + +"It is, then, a system whose end and aim is the tapping of the snow-cap +for the water there semi-annually let loose; then to distribute it over +the planet's face" (p. 373). + +Here, again, we have curiously weak arguments adduced to support the +view that these numerous straight lines imply works of art rather than +of nature, especially in the comparison made with the belts of Jupiter +and the spots on the sun, both purely atmospheric phenomena, whereas the +lines on Mars are on the solid surface of the planet. Why should there +be any resemblance between them? Every fact stated in the above +quotation, always keeping in mind the physical conditions of the +planet--its very tenuous atmosphere and rainless desert-surface--seem +wholly in favour of a purely natural as opposed to an artificial origin; +and at the close of this discussion I shall suggest one which seems to +me to be at least possible, and to explain the whole series of the +phenomena set forth and largely discovered by Mr. Lowell, in a simpler +and more probable manner than does his tremendous assumption of their +being works of art. Readers who may not possess Mr. Lowell's volume will +find three of his most recent maps of the 'canals' reproduced in +_Nature_ of October 11th, 1906. + + + +CHAPTER IV. + + +IS ANIMAL LIFE POSSIBLE ON MARS? + +Having now shown, that, even admitting the accuracy of all Mr. Lowell's +observations, and provisionally accepting all his chief conclusions as +to the climate, the nature of the snow-caps, the vegetation, and the +animal life of Mars, yet his interpretation of the lines on its surface +as being veritably 'canals,' constructed by intelligent beings for the +special purpose of carrying water to the more arid regions, is wholly +erroneous and rationally inconceivable. I now proceed to discuss his +more fundamental position as to the actual habitability of Mars by a +highly organised and intellectual race of material organic beings. + +_Water and Air essential to Life._ + +Here, fortunately, the issue is rendered very simple, because Mr. Lowell +fully recognises the identity of the constitution of matter and of +physical laws throughout the solar-system, and that for any high form of +organic life certain conditions which are absolutely essential on our +earth must also exist in Mars. He admits, for example, that water is +essential, that an atmosphere containing oxygen, nitrogen, aqueous +vapour, and carbonic acid gas is essential, and that an abundant +vegetation is essential; and these of course involve a +surface-temperature through a considerable portion of the year that +renders the existence of these--especially of water--possible and +available for the purposes of a high and abundant animal life. + +_Blue Colour the only Evidence of Water._ + +In attempting to show that these essentials actually exist on Mars he is +not very successful. He adduces evidence of an atmosphere, but of an +exceedingly scanty one, since the greatest amount he can give to it is-- +"not more than about four inches of barometric pressure as we reckon +it";[7] and he assumes, as he has a fair right to do till disproved, +that it consists of oxygen and nitrogen, with carbon-dioxide and +water-vapour, in approximately the same proportions as with us. With +regard to the last item--the water-vapour--there are however many +serious difficulties. The water-vapour of our atmosphere is derived from +the enormous area of our seas, oceans, lakes, and rivers, as well as +from the evaporation from heated lands and tropical forests of much of +the moisture produced by frequent and abundant rains. All these sources +of supply are admittedly absent from Mars, which has no permanent bodies +of water, no rain, and tropical regions which are almost entirely +desert. Many writers have therefore doubted the existence of water in +any form upon this planet, supposing that the snow-caps are not formed +of frozen water but of carbon-dioxide, or some other heavy gas, in a +frozen state; and Mr. Lowell evidently feels this to be a difficulty, +since the only fact he is able to adduce in favour of the melting snows +of the polar caps producing water is, that at the time they are melting +a marginal blue band appears which accompanies them in their retreat, +and this blue colour is said to prove conclusively that the liquid is +not carbonic acid but water. This point he dwells upon repeatedly, +stating, of these blue borders: "This excludes the possibility of their +being formed by carbon-dioxide, and shows that of all the substances we +know the material composing them must be water." + +[Footnote 7: In a paper written since the book appeared the density of +air at the surface of Mars is said to be 1/12 of the earth's.] + +This is the only proof of the existence of _water_ he adduces, and it is +certainly a most extraordinary and futile one. For it is perfectly well +known that although water, in large masses and by transmitted light, is +of a blue colour, yet shallow water by reflected light is not so; and in +the case of the liquid produced by the snow-caps of Mars, which the +whole conditions of the planet show must be shallow, and also be more or +less turbid, it cannot possibly be the cause of the 'deep blue' tint +said to result from the melting of the snow. + +But there is a very weighty argument depending on the molecular theory +of gases against the polar caps of Mars being composed of frozen water +at all. The mass and elastic force of the several gases is due to the +greater or less rapidity of the vibratory motion of their molecules +under identical conditions. The speed of these molecular motions has +been ascertained for all the chief gases, and it is found to be so great +as in certain cases to enable them to overcome the force of gravity and +escape from a planet's surface into space. Dr. G. Johnstone Stoney has +specially investigated this subject, and he finds that the force of +gravity on the earth is sufficient to retain all the gases composing its +atmosphere, but not sufficient to retain hydrogen; and as a consequence, +although this gas is produced in small quantities by volcanoes and by +decomposing vegetation, yet no trace of it is found in our atmosphere. +The moon however, having only one-eightieth the mass of the earth, +cannot retain any gas, hence its airless and waterless condition. + +_Water Vapour cannot exist on Mars._ + +Now, Dr. Stoney finds that in order to retain water vapour permanently a +planet must have a mass at least a quarter that of the earth. But the +mass of Mars is only one-ninth that of the earth; therefore, unless +there are some special conditions that prevent its loss, this gas cannot +be present in the atmosphere. Mr. Lowell does not refer to this argument +against his view, neither does he claim the evidence of spectroscopy in +his favour. This was alleged more than thirty years ago to show the +existence of water-vapour in the atmosphere of Mars, but of late years +it has been doubted, and Mr. Lowell's complete silence on the subject, +while laying stress on such a very weak and inconclusive argument as +that from the tinge of colour that is observed a little distance from +the edge of the diminishing snow-caps, shows that he himself does not +think the fact to be thus proved. If he did he would hardly adduce such +an argument for its presence as the following: "The melting of the caps +on the one hand and their re-forming on the other affirm the presence of +water-vapour in the Martian atmosphere, of whatever else that air +consists" (p. 162). Yet absolutely the only proof he gives that the caps +are frozen water is the almost frivolous colour-argument above referred +to! + +_No Spectroscopic Evidence of Water Vapour._ + +As Sir William Huggins is the chief authority quoted for this fact, and +is referred to as being almost conclusive in the third edition of Miss +Clerke's _History of Astronomy_ in 1893, I have ascertained that his +opinion at the present time is that "there is no conclusive proof of the +presence of aqueous vapour in the atmosphere of Mars, and that +observations at the Lick Observatory (in 1895), where the conditions and +instruments are of the highest order, were negative." He also informs me +that Marchand at the Pic du Midi Observatory was unable to obtain lines +of aqueous vapour in the spectrum of Mars; and that in 1905, Slipher, at +Mr. Lowell's observatory, was unable to detect any indications of +aqueous vapour in the spectrum of Mars. + +It thus appears that spectroscopic observations are quite accordant with +the calculations founded on the molecular theory of gases as to the +absence of aqueous vapour, and therefore presumably of liquid water, +from Mars. It is true that the spectroscopic argument is purely +negative, and this may be due to the extreme delicacy of the +observations required; but that dependent on the inability of the force +of gravity to retain it is positive scientific evidence against its +presence, and, till shown to be erroneous, must be held to be +conclusive. + +This absence of water is of itself conclusive against the existence of +animal life, unless we enter the regions of pure conjecture as to the +possibility of some other liquid being able to take its place, and that +liquid being as omnipresent there as water is here. Mr. Lowell however +never takes this ground, but bases his whole theory on the fundamental +identity of the substance of the bodies of living organisms wherever +they may exist in the solar system. In the next two chapters I shall +discuss an equally essential condition, that of temperature, which +affords a still more conclusive and even crushing argument against the +suitability of Mars for the existence of organic life. + + + +CHAPTER V. + + +THE TEMPERATURE OF MARS--MR. LOWELL'S ESTIMATE. + +We have now to consider a still more important and fundamental question, +and one which Mr. Lowell does not grapple with in this volume, the +actual temperatures on Mars due to its distance from the sun and the +atmospheric conditions on which he himself lays so much stress. If I am +not greatly mistaken we shall arrive at conclusions on this subject +which are absolutely fatal to the conception of any high form of organic +life being possible on this planet. + +_The Problem of Terrestrial Temperatures._ + +In order that the problem may be understood and its importance +appreciated, it is necessary to explain the now generally accepted +principles as to the causes which determine the temperatures on our +earth, and, presumably, on all other planets whose conditions are not +wholly unlike ours. The fact of the internal heat of the earth which +becomes very perceptible even at the moderate depths reached in mines +and deep borings, and in the deepest mines becomes a positive +inconvenience, leads many people to suppose that the surface- +temperatures of the earth are partly due to this cause. But it is now +generally admitted that this is not the case, the reason being that all +rocks and soils, in their natural compacted state, are exceedingly bad +conductors of heat. + +A striking illustration of this is the fact, that a stream of lava often +continues to be red hot at a few feet depth for years after the surface +is consolidated, and is hardly any warmer than that of the surrounding +land. A still more remarkable case is that of a glacier on the +south-east side of the highest cone of Etna underneath a lava stream +with an intervening bed of volcanic sand only ten feet thick. This was +visited by Sir Charles Lyell in 1828, and a second time thirty years +later, when he made a very careful examination of the strata, and was +quite satisfied that the sand and the lava stream together had actually +preserved this mass of ice, which neither the heat of the lava above it +at its first outflow, nor the continued heat rising from the great +volcano below it, had been able to melt or perceptibly to diminish in +thirty years. Another fact that points in the same direction is the +existence over the whole floor of the deepest oceans of ice-cold water, +which, originating in the polar seas, owing to its greater density sinks +and creeps slowly along the ocean bottom to the depths of the Atlantic +and Pacific, and is not perceptibly warmed by the internal heat of the +earth. + +Now the solid crust of the earth is estimated as at least about twenty +miles in average thickness; and, keeping in mind the other facts just +referred to, we can understand that the heat from the interior passes +through it with such extreme slowness as not to be detected at the +surface, the varying temperatures of which are due entirely to solar +heat. A large portion of this heat is stored up in the surface soil, and +especially in the surface layer of the oceans and seas, thus partly +equalising the temperatures of day and night, of winter and summer, so +as greatly to ameliorate the rapid changes of temperature that would +otherwise occur. Our dense atmosphere is also of immense advantage to us +as an equaliser of temperature, charged as it almost always is with a +large quantity of water-vapour. This latter gas, when not condensed into +cloud, allows the solar heat to pass freely to the earth; but it has the +singular and highly beneficial property of absorbing and retaining the +dark or lower-grade heat given off by the earth which would otherwise +radiate into space much more rapidly. We must therefore always remember +that, very nearly if not quite, the _whole_ of _the warmth we experience +on the earth is derived from the sun._[8] + +[Footnote 8: Professor J.H. Poynting, in his lecture to the British +Association at Cambridge in 1904, says: "The surface of the earth +receives, we know, an amount of heat from the inside almost +infinitesimal compared with that which it receives from the sun, and on +the sun, therefore, we depend for our temperature."] + +In order to understand the immense significance of this conclusion we +must know what is meant by the _whole_ heat or warmth; as unless we know +this we cannot define what half or any other proportion of sun-heat +really means. Now I feel pretty sure that nine out of ten of the average +educated public would answer the following question incorrectly: The +mean temperature of the southern half of England is about 48° F. +Supposing the earth received only half the sun-heat it now receives, +what would then be the probable mean temperature of the South of +England? The majority would, I think, answer at once--About 24° F. +Nearly as many would perhaps say--48° F. is 16° above the freezing +point; therefore half the heat received would bring us down to 8° above +the freezing point, or 40° F. Very few, I think, would realise that our +share of half the amount of sun-heat received by the earth would +probably result in reducing our mean temperature to about 100° F. below +the freezing point, and perhaps even lower. This is about the very +lowest temperature yet experienced on the earth's surface. To understand +how such results are obtained a few words must be said about the +absolute zero of temperature. + +_The Zero of Temperature._ + +Heat is now believed to be entirely due to ether-vibration, which +produces a correspondingly rapid vibration of the molecules of matter, +causing it to expand and producing all the phenomena we term 'heat.' We +can conceive this vibration to increase indefinitely, and thus there +would appear to be no necessary limit to the amount of heat possible, +but we cannot conceive it to decrease indefinitely at the same uniform +rate, as it must soon inevitably come to nothing. Now it has been found +by experiment that gases under uniform pressure expand 1/273 of their +volume for each degree Centigrade of increased temperature, so that in +passing from 0° C. to 273° C. they are doubled in volume. They also +decrease in volume at the same rate for each degree below 0° C. (the +freezing point of water). Hence if this goes on to-273° C. a gas will +have no volume, or it will undergo some change of nature. Hence this is +called the zero of temperature, or the temperature to which any matter +falls which receives no heat from any other matter. It is also sometimes +called the temperature of space, or of the ether in a state of rest, if +that is possible. All the gases have now been proved to become, first +liquid and then (most of them) solid, at temperatures considerably above +this zero. + +The only way to compare the proportional temperatures of bodies, whether +on the earth or in space, is therefore by means of a scale beginning at +this natural zero, instead of those scales founded on the artificial +zero of the freezing point of water, or, as in Fahrenheit's, 32° below +it. Only by using the natural zero and measuring continuously from it +can we estimate temperatures in relative proportion to the amount of +heat received. This is termed the absolute zero, and so that we start +reckoning from that point it does not matter whether the scale adopted +is the Centigrade or that of Fahrenheit. + +_The Complex Problem of Planetary Temperatures._ + +Now if, as is the case with Mars, a planet receives only half the amount +of solar heat that we receive, owing to its greater distance from the +sun, and if the mean temperature of our earth is 60° F., this is equal +to 551° F. on the absolute scale. It would therefore appear very simple +to halve this amount and obtain 275.5° F. as the mean temperature of +that planet. But this result is erroneous, because the actual amount of +sun heat intercepted by a planet is only one condition out of many that +determine its resulting temperature. Radiation, that is loss of heat, is +going on concurrently with gain, and the rate of loss varies with the +temperature according to a law recently discovered, the loss being much +greater at high temperatures in proportion to the 4th power of the +absolute temperature. Then, again, the whole heat intercepted by a +planet does not reach its surface unless it has no atmosphere. When it +has one, much is reflected or absorbed according to complex laws +dependent on the density and composition of the atmosphere. Then, again, +the heat that reaches the actual surface is partly reflected and partly +absorbed, according to the nature of that surface--land or water, desert +or forest or snow-clad--that part which is absorbed being the chief +agent in raising the temperature of the surface and of the air in +contact with it. Very important too is the loss of heat by radiation +from these various heated surfaces at different rates; while the +atmosphere itself sends back to the surface an ever varying portion of +both this radiant and reflected heat according to distinct laws. Further +difficulties arise from the fact that much of the sun's heat consists of +dark or invisible rays, and it cannot therefore be measured by the +quantity of light only. + +From this rough statement it will be seen that the problem is an +exceedingly complex one, not to be decided off-hand, or by any simple +method. It has in fact been usually considered as (strictly speaking) +insoluble, and only to be estimated by a more or less rough +approximation, or by the method of general analogy from certain known +facts. It will be seen, from what has been said in previous chapters, +that Mr. Lowell, in his book, has used the latter method, and, by taking +the presence of water and water-vapour in Mars as proved by the +behaviour of the snow-caps and the bluish colour that results from their +melting, has deduced a temperature above the freezing point of water, as +prevalent in the equatorial regions permanently, and in the temperate +and arctic zones during a portion of each year. + +_Mr. Lowell's Mathematical Investigation of the Problem._ + +But as this result has been held to be both improbable in itself and +founded on no valid evidence, he has now, in the _London, Edinburgh, and +Dublin Philosophical Magazine_ of July 1907, published an elaborate +paper of 15 pages, entitled _A General Method for Evaluating the +Surface-Temperatures of the Planets; with special reference to the +Temperature of Mars_, by Professor Percival Lowell; and in this paper, +by what purports to be strict mathematical reasoning based on the most +recent discoveries as to the laws of heat, as well as on measurements or +estimates of the various elements and constants used in the +calculations, he arrives at a conclusion strikingly accordant with that +put forward in the recently published volume. Having myself neither +mathematical nor physical knowledge sufficient to enable me to criticise +this elaborate paper, except on a few points, I will here limit myself +to giving a short account of it, so as to explain its method of +procedure; after which I may add a few notes on what seem to me doubtful +points; while I also hope to be able to give the opinions of some more +competent critics than myself. + +_Mr. Lowell's Mode of Estimating the Surface-temperature of Mars._ + +The author first states, that Professor Young, in his _General +Astronomy_ (1898), makes the mean temperature of Mars 223.6° absolute, +by using Newton's law of heat being radiated in proportion to +temperature, and 363° abs. (=-96° F.) by Dulong and Petit's law; but +adds, that a closer determination has been made by Professor Moulton, +using Stefan's law, that radiation is as the _/4th_ power of the +temperature, whence results a mean temperature of-31° F. These estimates +assume identity of atmospheric conditions of Mars and the Earth. + +But as none of these estimates take account of the many complex factors +which interfere with such direct and simple calculations, Mr. Lowell +then proceeds to enunciate them, and work out mathematically the effects +they produce: + +(1) The whole radiant energy of the sun on striking a planet becomes +divided as follows: Part is reflected back into space, part absorbed by +the atmosphere, part transmitted to the surface of the planet. This +surface again reflects a portion and only the balance left goes to warm +the planet. + +(2) To solve this complex problem we are helped by the _albedoes_ or +intrinsic brilliancy of the planets, which depend on the proportion of +the visible rays which are reflected and which determines the +comparative brightness of their respective surfaces. We also have to +find the ratio of the invisible to the visible rays and the heating +power of each. + +(3) He then refers to the actinometer and pyroheliometer, instruments +for measuring the actual heat derived from the sun, and also to the +Bolometer, an instrument invented by Professor Langley for measuring the +invisible heat rays, which he has proved to extend to more than three +times the length of the whole heat-spectrum as previously known, and +has also shown that the invisible rays contribute 68 per cent, of the +sun's total energy.[9] + +[Footnote 9: For a short account of this remarkable instrument, see my +_Wonderful Century_, new ed., pp. 143-145.] + +(4) Then follows an elaborate estimate of the loss of heat during the +passage of the sun's rays through our atmosphere from experiments made +at different altitudes and from the estimated reflective power of the +various parts of the earth's surface--rocks and soil, ocean, forest and +snow--the final result being that three-fourths of the whole sun-heat +is reflected back into space, forming our _albedo_, while only +one-fourth is absorbed by the soil and goes to warm the air and +determine our mean temperature. + +(5) We now have another elaborate estimate of the comparative amounts of +heat actually received by Mars and the Earth, dependent on their very +different amounts of atmosphere, and this estimate depends almost wholly +on the comparative _albedoes_, that of Mars, as observed by astronomers +being 0.27, while ours has been estimated in a totally different way as +being 0.75, whence he concludes that nearly three-fourths of the +sun-heat that Mars receives reaches the surface and determines its +temperature, while we get only one-fourth of our total amount. Then by +applying Stefan's law, that the radiation is as the 4th power of the +surface temperature, he reaches the final result that the actual heating +power at the surface of Mars is considerably _more_ than on the Earth, +and would produce a mean temperature of 72° F., if it were not for the +greater conservative or blanketing power of our denser and more +water-laden atmosphere. The difference produced by this latter fact he +minimises by dwelling on the probability of a greater proportion of +carbonic-acid gas and water-vapour in the Martian atmosphere, and thus +brings down the mean temperature of Mars to 48° F., which is almost +exactly the same as that of the southern half of England. He has also, +as the result of observations, reduced the probable density of the +atmosphere of Mars to 2-1/2 inches of mercury, or only one-twelfth of +that of the Earth. + +_Critical Remarks on Mr. Lowell's Paper._ + +The last part of this paper, indicated under pars. 4 and 5, is the most +elaborate, occupying eight pages, and it contains much that seems +uncertain, if not erroneous. In particular, it seems very unlikely that +under a clear sky over the whole earth we should only receive at the +sea-level 0.23 of the solar rays which the earth intercepts (p. 167). +These data largely depend on observations made in California and other +parts of the southern United States, where the lower atmosphere is +exceptionally dust-laden. Till we have similar observations made in the +tropical forest-regions, which cover so large an area, and where the +atmosphere is purified by frequent rains, and also on the prairies and +the great oceans, we cannot trust these very local observations for +general conclusions affecting the whole earth. Later, in the same +article (p. 170), Mr. Lowell says: "Clouds transmit approximately 20 per +cent. of the heat reaching them: a clear sky at sea-level 60 per cent. +As the sky is half the time cloudy the mean transmission is 35 per +cent." These statements seem incompatible with that quoted above. + +The figure he uses in his calculations for the actual albedo of the +earth, 0.75, is also not only improbable, but almost self-contradictory, +because the albedo of cloud is 0.72, and that of the great cloud-covered +planet, Jupiter, is given by Lowell as 0.75, while Zollner made it only +0.62. Again, Lowell gives Venus an albedo of 0.92, while Zollner made it +only 0.50 and Mr. Gore 0.65. This shows the extreme uncertainty of these +estimates, while the fact that both Venus and Jupiter are wholly +cloud-covered, while we are only half-covered, renders it almost +certain that our albedo is far less than Mr. Lowell makes it. It is +evident that mathematical calculations founded upon such uncertain data +cannot yield trustworthy results. But this is by no means the only case +in which the data employed in this paper are of uncertain value. +Everywhere we meet with figures of somewhat doubtful accuracy. Here we +have somebody's 'estimate' quoted, there another person's 'observation,' +and these are adopted without further remark and used in the various +calculations leading to the result above quoted. It requires a practised +mathematician, and one fully acquainted with the extensive literature of +this subject, to examine these various data, and track them through the +maze of formulae and figures so as to determine to what extent they +affect the final result. + +There is however one curious oversight which I must refer to, as it is a +point to which I have given much attention. Not only does Mr. Lowell +assume, as in his book, that the 'snows' of Mars consist of frozen +water, and that therefore there _is_ water on its surface and +water-vapour in its atmosphere, not only does he ignore altogether Dr. +Johnstone Stoney's calculations with regard to it, which I have already +referred to, but he uses terms that imply that water-vapour is one of +the heavier components of our atmosphere. The passage is at p. 168 of +the _Philosophical Magazine._ After stating that, owing to the very +small barometric pressure in Mars, water would boil at 110° F., he adds: +"The sublimation at lower temperatures would be correspondingly +increased. Consequently the amount of water-vapour in the Martian air +must on that score be relatively greater than our own." Then follows +this remarkable passage: "Carbon-dioxide, because of its greater +specific gravity, would also be in relatively greater amount so far as +this cause is considered. For the planet would part, _caeteris paribus_, +with its lighter gases the quickest. Whence as regards both water-vapour +and carbon-dioxide we have reason to think them in relatively greater +quantity than in our own air at corresponding barometric pressure." I +cannot understand this passage except as implying that 'water-vapour and +carbon-dioxide' are among the heavier and not among the lighter gases of +the atmosphere--those which the planet 'parts with quickest.' But this +is just what water-vapour _is_, being a little less than two-thirds the +weight of air (0.6225), and one of those which the planet _would_ part +with the quickest, and which, according to Dr. Johnstone Stoney, it +loses altogether. + * * * * * + +Note on Professor Lowell's article in the _Philosophical Magazine_; by +J.H. Poynting, F.R.S., Professor of Physics in the University of +Birmingham. + +"I think Professor Lowell's results are erroneous through his neglect of +the heat stored in the air by its absorption of radiation both from the +sun and from the surface. The air thus heated radiates to the surface +and keeps up the temperature. I have sent to the _Philosophical +Magazine_ a paper in which I think it is shown that when the radiation +by the atmosphere is taken into account the results are entirely +changed. The temperature of Mars, with Professor Lowell's data, still +comes out far below the freezing-point--still further below than the +increased distance alone would make it. Indeed, the lower temperature on +elevated regions of the earth's surface would lead us to expect this. I +think it is impossible to raise the temperature of Mars to anything like +the value obtained by Professor Lowell, unless we assume some quality in +his atmosphere entirely different from any found in our own atmosphere." +J.H. POYNTING. October 19, 1907. + + + +CHAPTER VI. + + +A NEW ESTIMATE OF THE TEMPERATURE OF MARS. + +When we are presented with a complex problem depending on a great number +of imperfectly ascertained data, we may often check the results thus +obtained by the comparison of cases in which some of the more important +of these data are identical, while others are at a maximum or a minimum. +In the present case we can do this by a consideration of the Moon as +compared with the Earth and with Mars. + +_Langley's Determination of the Moon's Temperature._ + +In the moon we see the conditions that prevail in Mars both exaggerated +and simplified. Mars has a very scanty atmosphere, the moon none at all, +or if there is one it is so excessively scanty that the most refined +observations have not detected it. All the complications arising from +the possible nature of the atmosphere, and its complex effects upon +reflection, absorption, and radiation are thus eliminated. The mean +distance of the moon from the sun being identical with that of the +earth, the total amount of heat intercepted must also be identical; only +in this case the whole of it reaches the surface instead of one-fourth +only, according to Mr. Lowell's estimate for the earth. + +Now, by the most refined observations with his Bolometer, Mr. Langley +was able to determine the temperature of the moon's surface exposed to +undimmed sunshine for fourteen days together; and he found that, even in +that portion of it on which the sun was shining almost vertically, the +temperature rarely rose above the freezing point of water. However +extraordinary this result may seem, it is really a striking confirmation +of the accuracy of the general laws determining temperature which I have +endeavoured to explain in the preceding chapter. For the same surface +which has had fourteen days of sunshine has also had a preceding +fourteen days of darkness, during which the heat which it had +accumulated in its surface layers would have been lost by free radiation +into stellar space. It thus acquires during its day a maximum +temperature of only 491° F. absolute, while its minimum, after 14 days' +continuous radiation, must be very low, and is, with much reason, +supposed to approach the absolute zero. + +_Rapid Loss of Heat by Radiation on the Earth._ + +In order better to comprehend what this minimum may be under extreme +conditions, it will be useful to take note of the effects it actually +produces on the earth in places where the conditions are nearest to +those existing on the moon or on Mars, though never quite equalling, or +even approaching very near them. It is in our great desert regions, and +especially on high plateaux, that extreme aridity prevails, and it is in +such districts that the differences between day and night temperatures +reach their maximum. It is stated by geographers that in parts of the +Great Sahara the surface temperature is sometimes 150° F., while during +the night it falls nearly or quite to the freezing point--a difference +of 118 degrees in little more than 12 hours.[10] In the high desert +plains of Central Asia the extremes are said to be even greater.[11] +Again, in his _Universal Geography_, Reclus states that in the Armenian +Highlands the thermometer oscillates between 13° F. and 112°F. We may +therefore, without any fear of exaggeration, take it as proved that a +fall of 100° F. in twelve or fifteen hours not infrequently occurs where +there is a very dry and clear atmosphere permitting continuous +insolation by day and rapid radiation by night. + +[Footnote 10: Keith Johnston's 'Africa' in _Stanford's Compendium._] + +[Footnote 11: _Chambers's Encyclopaedia_, Art. 'Deserts.'] + +Now, as it is admitted that our dense atmosphere, however dry and clear, +absorbs and reflects some considerable portion of the solar heat, we +shall certainly underestimate the radiation from the moon's surface +during its long night if we take as the basis of our calculation a +lowering of temperature amounting to 100° F. during twelve hours, as not +unfrequently occurs with us. Using these data--with Stefan's law of +decrease of radiation as the 4th power of the temperature--a +mathematical friend finds that the temperature of the moon's surface +would be reduced during the lunar night to nearly 200° F. absolute +(equal to-258° F.). + +_More Rapid Loss of Heat by the Moon._ + +Although such a calculation as the above may afford us a good +approximation to the rate of loss of heat by Mars with its very scanty +atmosphere, we have now good evidence that in the case of the moon the +loss is much more rapid. Two independent workers have investigated this +subject with very accordant results--Dr. Boeddicker, with Lord Rosse's +3-foot reflector and a Thermopile to measure the heat, and Mr. Frank +Very, with a glass reflector of 12 inches diameter and the Bolometer +invented by Mr. Langley. The very striking and unexpected fact in which +these observers agree is the sudden disappearance of much of the +stored-up heat during the comparatively short duration of a total +eclipse of the moon--less than two hours of complete darkness, and about +twice that period of partial obscuration. + +Dr. Boeddicker was unable to detect any appreciable heat at the period +of greatest obscuration; but, owing to the extreme sensitiveness of the +Bolometer, Mr. Very ascertained that those parts of the surface which +had been longest in the shadow still emitted heat "to the amount of one +per cent. of the heat to be expected from the full moon." This however +is the amount of radiation measured by the Bolometer, and to get the +temperature of the radiating surface we must apply Stefan's law of the +4th power. Hence the temperature of the moon's dark surface will be the +[fourth root of (1 over 100)] = 1 over 3.2 [A] of the highest temperature + (which we may take at the freezing-point, 491° F. abs.), or 154° F. abs., + just below the liquefaction point of air. This is about 50° lower than the +amount found by calculation from our most rapid radiation; and as this +amount is produced in a few hours, it is not too much to expect that, +when continued for more than two weeks (the lunar night), it might reach +a temperature sufficient to liquefy hydrogen (60° F. abs.), or perhaps +even below it. + +[Note A: LaTex markup $\root 4 \of {1 \over 100} = {1 \over 3.2}$ ] + +_Theory of the Moon's Origin._ + +This extremely rapid loss of heat by radiation, at first sight so +improbable as to be almost incredible, may perhaps be to some extent +explained by the physical constitution of the moon's surface, which, +from a theoretical point of view, does not appear to have received the +attention it deserves. It is clear that our satellite has been long +subjected to volcanic eruptions over its whole visible face, and these +have evidently been of an explosive nature, so as to build up the very +lofty cones and craters, as well as thousands of smaller ones, which, +owing to the absence of any degrading or denuding agencies, have +remained piled up as they were first formed. + +This highly volcanic structure can, I think, be well explained by an +origin such as that attributed to it by Sir George Darwin, and which has +been so well described by Sir Robert Ball in his small volume, _Time and +Tide._ These astronomers adduce strong evidence that the earth once +rotated so rapidly that the equatorial protuberance was almost at the +point of separation from the planet as a ring. Before this occurred, +however, the tension was so great that one large portion of the +protuberance where it was weakest broke away, and began to move around +the earth at some considerable distance from it. As about 1/50 of the +bulk of the earth thus escaped, it must have consisted of a considerable +portion of the solid crust and a much larger quantity of the liquid or +semi-liquid interior, together with a proportionate amount of the gases +which we know formed, and still form, an important part of the earth's +substance. + +As the surface layers of the earth must have been the lightest, they +would necessarily, when broken up by this gigantic convulsion, have come +together to form the exterior of the new satellite, and be soon adjusted +by the forces of gravity and tidal disturbance into a more or less +irregular spheroidal form, all whose interstices and cavities would be +filled up and connected together by the liquid or semi-liquid mass +forced up between them. Thence-forward, as the moon increased its +distance and reduced its time of rotation, in the way explained by Sir +Robert Ball, there would necessarily commence a process of escape of the +imprisoned gases at every fissure and at all points and lines of +weakness, giving rise to numerous volcanic outlets, which, being +subjected only to the small force of lunar gravity (only one-sixth that +of the earth), would, in the course of ages, pile up those gigantic +cones and ridges which form its great characteristic. + +But this small gravitative power of the moon would prevent its retaining +on its surface any of the gases forming our atmosphere, which would all +escape from it and probably be recaptured by the earth. By no process of +external aggregation of solid matter to such a relatively small amount +as that forming the moon, even if the aggregation was so violent as to +produce heat enough to cause liquefaction, could any such +long-continued volcanic action arise by gradual cooling, in the absence +of internal gases. There might be fissures, and even some outflows of +molten rock; but without imprisoned gases, and especially without water +and water-vapour producing explosive outbursts, could any such amount of +scoriae and ashes be produced as were necessary for the building up of +the vast volcanic cones, craters, and craterlets we see upon the moon's +surface. + +I am not aware that either Sir Robert Ball or Sir George Darwin have +adduced this highly volcanic condition of the moon's surface as a +phenomenon which can _only_ be explained by our satellite having been +thrown off a very much larger body, whose gravitative force was +sufficient to acquire and retain the enormous quantity of gases and of +water which we possess, and which are _absolutely essential_ for that +_special form of cone-building volcanic action_ which the moon exhibits +in so pre-eminent a degree. Yet it seems to me clear, that some such +hypothetical origin for our satellite would have had to be assumed if +Sir George Darwin had not deduced it by means of purely mathematical +argument based upon astronomical facts. + +Returning now to the problem of the moon's temperature, I think the +phenomena this presents may be in part due to the mode of formation here +described. For, its entire surface being the result of long-continued +gaseous explosions, all the volcanic products--scoriae, pumice, and +ashes--would necessarily be highly porous throughout; and, never having +been compacted by water-action, as on the earth, and there having been +no winds to carry the finer dust so as to fill up their pores and +fissures, the whole of the surface material to a very considerable depth +must be loose and porous to a high degree. This condition has been +further increased owing to the small power of gravity and the extreme +irregularity of the surface, consisting very largely of lofty cones and +ridges very loosely piled up to enormous heights. + +Now this condition of the substance of the moon's surface is such as +would produce a high specific heat, so that it would absorb a large +amount of heat in proportion to the rise of temperature produced, the +heat being conducted downwards to a considerable depth. Owing, however, +to the total absence of atmosphere radiation would very rapidly cool the +surface, but afterwards more slowly, both on account of the action of +Stefan's law and because the heat stored up in the deeper portions could +be carried to the surface by conduction only, and with extreme slowness. + +_Very's Researches on the Moon's Heat._ + +The results of the eclipse observations are supported by the detailed +examination of the surface-temperature of the moon by Mr. Very in his +_Prize Essay on the Distribution of the Moon's Heat_ (published by the +Utrecht Society of Arts and Sciences in 1891). He shows, by a diagram of +the 'Phase-curve,' that at the commencement of the Lunar day the surface +just within the illuminated limb has acquired about 1/7 of its maximum +temperature, or about 70° F. abs. As the surface exposed to the +Bolometer at each observation is about 1/30 of the moon's surface, and +in order to ensure accuracy the instrument has to be directed to a spot +lying wholly within the edge of the moon, it is evident that the surface +measured has already been for several hours exposed to oblique sunshine. +The curve of temperature then rises gradually and afterwards more +rapidly, till it attains its maximum (of about +30 to 40° F.) a few +hours _before_ noon. This, Mr. Very thinks, is due to the fact that the +half of the moon's face first illuminated for us has, on the average, a +darker surface than that of the afternoon, or second quarter, during +which the curve descends not quite so rapidly, the temperature near +sunset being only a little higher than that near sunrise. This rapid +fall while exposed to oblique sunshine is quite in harmony with the +rapid loss of heat during the few hours of darkness during an eclipse, +both showing the prepotency of radiation over insolation on the moon. + +Two other diagrams show the distribution of heat at the time of +full-moon, one half of the curve showing the temperatures along the +equator from the edge of the disc to the centre, the other along a +meridian from this centre to the pole. This diagram (here reproduced) +exhibits the quick rise of temperature of the oblique rim of the moon +and the nearly uniform heat of the central half of its surface; the +diminution of heat towards the pole, however, is slower for the first +half and more rapid for the latter portion. + +It is an interesting fact that the temperature near the margin of the +full-moon increases towards the centre more rapidly than it does when +the same parts are observed during the early phases of the first +quarter. Mr. Very explains this difference as being due to the fact that +the full-moon to its very edges is fully illuminated, all the shadows of +the ridges and mountains being thrown vertically or obliquely _behind +them._ We thus measure the heat reflected from the _whole_ visible +surface. But at new moon, and somewhat beyond the first quarter, the +deep shadows thrown by the smallest cones and ridges, as well as by the +loftiest mountains, cover a considerable portion of the visible surface, +thus largely reducing the quantity of light and heat reflected or +radiated in our direction. It is only at the full, therefore, that the +maximum temperature of the whole lunar surface can be measured. It must +be considered a proof of the delicacy of the heat-measuring instruments +that this difference in the curves of temperature of the different parts +of the moon's surface and under different conditions is so clearly +shown. + +_The Application of the Preceding Results to the Case of Mars._ + +This somewhat lengthy account of the actual state of the moon's surface +and temperature is of very great importance in our present enquiry, +because it shows us the extraordinary difference in mean and extreme +temperatures of two bodies situated at the same distance from the sun, +and therefore receiving exactly the same amount of solar heat per unit +of surface. We have learned also what are the main causes of this almost +incredible difference, namely: (1) a remarkably rugged surface with +porous and probably cavernous rock-texture, leading to extremely rapid +radiation of heat in the one; as compared with a comparatively even and +well-compacted surface largely clad with vegetation, leading to +comparatively slow and gradual loss by radiation in the other: and (2), +these results being greatly intensified by the total absence of a +protecting atmosphere in the former, while a dense and cloudy atmosphere +with an ever-present supply of water-vapour, accumulates and equalises +the heat received by the latter. + +The only other essential difference in the two bodies which may possibly +aid in the production of this marvellous result, is the fact of our day +and night having a mean length of 12 hours, while those of the moon are +about 14-1/2 of our days. But the altogether unexpected fact, in which +two independent enquirers agree, that during the few hours' duration of +a total eclipse of the moon so large a proportion of the heat is lost by +radiation renders it almost certain that the resulting low temperature +would be not very much less if the moon had a day and night the same +length as our own. + +The great lesson we learn by this extreme contrast of conditions +supplied to us by nature, as if to enable us to solve some of her +problems, is, the overwhelming importance, first, of a dense and +well-compacted surface, due to water-action and strong gravitative +force; secondly, of a more or less general coat of vegetation; and, +thirdly, of a dense vapour-laden atmosphere. These three favourable +conditions result in a mean temperature of about +60° F. with a range +seldom exceeding 40° above or below it, while over more than half the +land-surface of the earth the temperature rarely falls below the +freezing point. On the other hand, we have a globe of the same materials +and at the same distance from the sun, with a maximum temperature of +freezing water, and a minimum not very far from the absolute zero, the +monthly mean being probably much below the freezing point of +carbonic-acid gas--a difference entirely due to the absence of these +three favourable conditions. + +_The Special Features of Mars as influencing Temperature._ + +Coming now to the special feature of Mars and its probable temperature, +we find that most writers have arrived at a very different conclusion +from that of Mr. Lowell, who himself quotes Mr. Moulton as an authority +who 'recently, by the application of Stefan's law,' has found the mean +temperature of this planet to be-35° F. Again, Professor J.H. Poynting, +in his lecture on 'Radiation in the Solar System,' delivered before the +British Association at Cambridge in 1904, gave an estimate of the mean +temperature of the planets, arrived at from measurements of the sun's +emissive power and the application of Stefan's law to the distances of +the several planets, and he thus finds the earth to have a mean +temperature of 17° C. (=62-1/2° F.) and Mars one of-38° C. (=-36-1/2° +F.), a wonderfully close approximation to the mean temperature of the +earth as determined by direct measurement, and therefore, presumably, an +equally near approximation to that of Mars as dependent on distance from +the sun, and '_on the supposition that it is earth-like in all its +conditions._' + +But we know that it is far from being earth-like in the very conditions +which we have found to be those which determine the extremely different +temperatures of the earth, and moon; and, as regards each of these, we +shall find that, so far as it differs from the earth, it approximates to +the less favourable conditions that prevail in the moon. The first of +these conditions which we have found to be essential in regulating the +absorption and radiation of heat, and thus raising the mean temperature +of a planet, is a compact surface well covered with vegetation, two +conditions arising from, and absolutely dependent on, an ample amount of +water. But Mr. Lowell himself assures us, as a fact of which he has no +doubt, that there are no permanent bodies of water, great or small, upon +Mars; that rain, and consequently rivers, are totally wanting; that its +sky is almost constantly clear, and that what appear to be clouds are +not formed of water-vapour but of dust. He dwells, emphatically, on the +terrible desert conditions of the greater part of the surface of the +planet. + +That being the case now, we have no right to assume that it has ever +been otherwise; and, taking full account of the fact, neither denied nor +disputed by Mr. Lowell, that the force of gravity on Mars is not +sufficient to retain water-vapour in its atmosphere, we must conclude +that the surface of that planet, like that of the moon, has been moulded +by some form of volcanic action modified probably by wind, but not by +water. Adding to this, that the force of gravity on Mars is nearer that +of the moon than to that of the earth, and we may r reasonably conclude +that its surface is formed of volcanic matter in a light and porous +condition, and therefore highly favourable for the rapid loss of surface +heat by radiation. The surface-conditions of Mars are therefore, +presumably, much more like those of the moon than like those of the +earth. + +The next condition favourable to the storing up of heat--a covering of +vegetation--is almost certainly absent from Mars except, possibly, over +limited areas and for short periods. In this feature also the surface of +Mars approximates much nearer to lunar than to earth-conditions. The +third condition--a dense, vapour-laden atmosphere--is also wanting in +Mars. For although it possesses an atmosphere it is estimated by Mr. +Lowell (in his latest article) to have a pressure equivalent to only +2-1/2 inches of mercury with us, giving it a density of only one-twelfth +part that of ours; while aqueous vapour, the chief accumulator of heat, +cannot permanently exist in it, and, notwithstanding repeated +spectroscopic observations for the purpose of detecting it, has never +been proved to exist. + +I submit that I have now shown from the statements--and largely as the +result of the long-continued observations--of Mr. Lowell himself, that, +so far as the physical conditions of Mars are known to differ from those +of the earth, the differences are all _unfavourable_ to the conservation +and _favourable_ to the dissipation of the scanty heat it receives from +the sun--that they point unmistakeably towards the temperature +conditions of the moon rather than to those of the earth, and that the +cumulative effect of these adverse conditions, acting upon a +heat-supply, reduced by solar distance to less than one-half of ours, +_must_ result in a mean temperature (as well as in the extremes) nearer +to that of our satellite than to that of our own earth. + +_Further Criticism of Mr. Lowell's Article._ + +We are now in a position to test some further conclusions of Mr. +Lowell's _Phil. Mag._ article by comparison with actual phenomena. We +have seen, in the outline I have given of this article, that he +endeavours to show how the small amount of solar heat received by Mars +is counterbalanced, largely by the greater transparency to light and +heat of its thin and cloudless atmosphere, and partially also by a +greater conservative or 'blanketing' power of its atmosphere due to the +presence in it of a large proportion of carbonic acid gas and aqueous +vapour. The first of these statements may be admitted as a fact which he +is entitled to dwell upon, but the second--the presence of large +quantities of carbon-dioxide and aqueous vapour is a pure hypothesis +unsupported by any item of scientific evidence, while in the case of +aqueous vapour it is directly opposed to admitted results founded upon +the molecular theory of gaseous elasticity. But, although Mr. Lowell +refers to the conservative or 'blanketing' effect of the earth's +atmosphere, he does not consider or allow for its very great cumulative +effect, as is strikingly shown by the comparison with the actual +temperature conditions of the moon. This cumulative effect is due to the +_continuous_ reflection and radiation of heat from the clouds as well as +from the vapour-laden strata of air in our lower atmosphere, which +latter, though very transparent to the luminous and accompanying heat +rays of the sun, are opaque to the dark heat-rays whether radiated or +reflected from the earth's surface. We are therefore in a position +strictly comparable with that of the interior of some huge glass house, +which not only becomes intensely heated by the direct rays of the sun, +but also to a less degree by reflected rays from the sky and those +radiated from the clouds, so that even on a cloudy or misty day its +temperature rises many degrees above that of the outer air. Such a +building, if of large size, of suitable form, and well protected at +night by blinds or other covering, might be so arranged as to accumulate +heat in its soil and walls so as to maintain a tolerably uniform +temperature though exposed to a considerable range of external heat and +cold. It is to such a power of accumulation of heat in our soil and +lower atmosphere that we must impute the overwhelming contrast between +our climate and that of the moon. With us, the solar heat that +penetrates our vapour-laden and cloudy atmosphere is shut in by that +same atmosphere, accumulates there for weeks and months together, and +can only slowly escape. It is this great cumulative power which Mr. +Lowell has not taken account of, while he certainly has not estimated +the enormous loss of heat by free radiation, which entirely neutralises +the effects of increase of sun-heat, however great, when these +cumulative agencies are not present.[12] + +[Footnote 12: The effects of this 'cumulative' power of a dense +atmosphere are further discussed and illustrated in the last chapter of +this book, where I show that the universal fact of steadily diminishing +temperatures at high altitudes is due solely to the diminution of this +cumulative power of our atmosphere, and that from this cause alone the +temperature of Mars must be that which would be found on a lofty plateau +about 18,000 feet higher than the average of the peaks of the Andes!] + +_Temperature on Polar Regions of Mars._ + +There is also a further consideration which I think Mr. Lowell has +altogether omitted to discuss. Whatever may be the _mean_ temperature +of Mars, we must take account of the long nights in its polar and +high-temperate latitudes, lasting nearly twice as long as ours, with the +resulting lowering of temperature by radiation into a constantly clear +sky. Even in Siberia, in Lat. 67-1/2°N. a cold of-88°F. has been +attained; while over a large portion of N. Asia and America above 60° +Lat. the _mean_ January temperature is from-30°F. to-60°F., and the +whole subsoil is permanently frozen from a depth of 6 or 7 feet to +several hundreds. But the winter temperatures, _over the same latitudes_ +in Mars, must be very much lower; and it must require a proportionally +larger amount of its feeble sun-heat to raise the surface even to the +freezing-point, and an additional very large amount to melt any +considerable depth of snow. But this identical area, from a little below +60° to the pole, is that occupied by the snow-caps of Mars, and over the +whole of it the winter temperature must be far lower than the +earth-minimum of-88°F. Then, as the Martian summer comes on, there is +less than half the sun-heat available to raise this low temperature +after a winter nearly double the length of ours. And when the summer +does come with its scanty sun-heat, that heat is not accumulated as it +is by our dense and moisture-laden atmosphere, the marvellous effects of +which we have already shown. Yet with all these adverse conditions, each +assisting the other to produce a climate approximating to that which the +earth would have if it had no atmosphere (but retaining our superiority +over Mars in receiving double the amount of sun-heat), we are asked to +accept a mean temperature for the more distant planet almost exactly the +same as that of mild and equable southern England, and a disappearance +of the vast snowfields of its polar regions as rapid and complete as +what occurs with us! If the moon, even at its equator, has not its +temperature raised above the freezing-point of water, how can the more +_distant_ Mars, with its _oblique_ noon-day sun falling upon the +snow-caps, receive heat enough, first to raise their temperature to 32° +F., and then to melt with marked rapidity the vast frozen plains of its +polar regions? + +Mr. Lowell is however so regardless of the ordinary teachings of +meteorological science that he actually accounts for the supposed mild +climate of the polar regions of Mars by the absence of water on its +surface and in its atmosphere. He concludes his fifth chapter with the +following words: "Could our earth but get rid of its oceans, we too +might have temperate regions stretching to the poles." Here he runs +counter to two of the best-established laws of terrestrial climatology-- +the wonderful equalising effects of warm ocean-currents which are the +chief agents in diminishing polar cold; the equally striking effects of +warm moist winds derived from these oceans, and the great storehouse of +heat we possess in our vapour-laden atmosphere, its vapour being +primarily derived from these same oceans! But, in Mr. Lowell's opinion, +all our meteorologists are quite mistaken. Our oceans are our great +drawbacks. Only get rid of them and we should enjoy the exquisite +climate of Mars--with its absence of clouds and fog, of rain or rivers, +and its delightful expanses of perennial deserts, varied towards the +poles by a scanty snow-fall in winter, the melting of which might, with +great care, supply us with the necessary moisture to grow wheat and +cabbages for about one-tenth, or more likely one-hundredth, of our +present population. I hope I may be excused for not treating such an +argument seriously. The various considerations now advanced, especially +those which show the enormous cumulative and conservative effect of our +dense and water-laden atmosphere, and the disastrous effect--judging by +the actual condition of the moon--which the loss of it would have upon +our temperature, seem to me quite sufficient to demonstrate important +errors in the data or fallacies in the complex mathematical argument by +which Mr. Lowell has attempted to uphold his views as to the temperature +and consequent climatic conditions of Mars. In concluding this portion +of my discussion of the problem of Mars, I wish to call attention to the +fact that my argument, founded upon a comparison of the physical +conditions of the earth and moon with those of Mars, is dependent upon a +small number of generally admitted scientific facts; while the +conclusions drawn from those facts are simple and direct, requiring no +mathematical knowledge to follow them, or to appreciate their weight and +cogency. I claim for them, therefore, that they are in no degree +speculative, but in their data and methods exclusively scientific. In +the next chapter I will put forward a suggestion as to how the very +curious markings upon the surface of Mars may possibly be interpreted, +so as to be in harmony with the planet's actual physical condition and +its not improbable origin and past history. + + + +CHAPTER VII. + + +A SUGGESTION AS TO THE 'CANALS' OF MARS. + +The special characteristics of the numerous lines which intersect the +whole of the equatorial and temperate regions of Mars are, their +straightness combined with their enormous length. It is this which has +led Mr. Lowell to term them 'non-natural features.' Schiaparelli, in his +earlier drawings, showed them curved and of comparatively great width. +Later, he found them to be straight fine lines when seen under the best +conditions, just as Mr. Lowell has always seen them in the pure +atmosphere of his observatory. Both of these observers were at first +doubtful of their reality, but persistent observation continued at many +successive oppositions compelled acceptance of them as actual features +of the planet's disc. So many other observers have now seen them that +the objection of unreality seems no longer valid. + +Mr. Lowell urges, however, that their perfect straightness, their +extreme tenuity, their uniformity throughout their whole length, the +dual character of many of them, their relation to the 'oases' and the +form and position of these round black spots, are all proofs of +artificiality and are suggestive of design. And considering that some of +them are actually as long as from Boston to San Francisco, and +relatively to their globe as long as from London to Bombay, his +objection that "no natural phenomena within our knowledge show such +regularity on such a scale" seems, at first, a mighty one. + +It is certainly true that we can point to nothing exactly like them +either on the earth or on the moon, and these are the only two planetary +bodies we are in a position to compare with Mars. Yet even these do, I +think, afford us some hints towards an interpretation of the mysterious +lines. But as our knowledge of the internal structure and past history +even of our earth is still imperfect, that of the moon only conjectural, +and that of Mars a perfect blank, it is not perhaps surprising that the +surface-features of the latter do not correspond with those of either of +the others. + +_Mr. Pickering's Suggestion._ + +The best clue to a natural interpretation of the strange features of the +surface of Mars is that suggested by the American astronomer Mr. W.H. +Pickering in _Popular Astronomy_ (1904). Briefly it is, that both the +'canals' of Mars and the rifts as well as the luminous streaks on the +moon are cracks in the volcanic crust, caused by internal stresses due +to the action of the heated interior. These cracks he considers to be +symmetrically arranged with regard to small 'craterlets' (Mr. Lowell's +'oases') because they have originated from them, just as the white +streaks on the moon radiate from the larger craters as centres. He +further supposes that water and carbon-dioxide issue from the interior +into these fissures, and, in conjunction with sunlight, promote the +growth of vegetation. Owing to the very rare atmosphere, the vapours, he +thinks, would not ascend but would roll down the outsides of the +craterlets and along the borders of the canals, thus irrigating the +immediate vicinity and serving to promote the growth of some form of +vegetation which renders the canals and oases visible.[13] + +[Footnote 13: _Nature_, vol. 70, p. 536.] + +This opinion is especially important because, next to Mr. Lowell, Mr. +Pickering is perhaps the astronomer who has given most attention to Mars +during the last fifteen years. He was for some time at Flagstaff with +Mr. Lowell, and it was he who discovered the oases or craterlets, and +who originated the idea that we did not see the 'canals' themselves but +only the vegetable growth on their borders. He also observed Mars in the +Southern Hemisphere at Arequipa; and he has since made an elaborate +study of the moon by means of a specially constructed telescope of 135 +feet focal length, which produced a direct image on photographic plates +nearly 16 inches in diameter.[14] + +[Footnote 14: _Nature_, vol. 70, May 5, p.xi, supplement.] + +It is clear therefore that Mr. Lowell's views as to the artificial +nature of the 'canals' of Mars are not accepted by an astronomer of +equal knowledge and still wider experience. Yet Professor Pickering's +alternative view is more a suggestion than an explanation, because there +is no attempt to account for the enormous length and perfect +straightness of the lines on Mars, so different from anything that is +found either on our earth or on the moon. There must evidently be some +great peculiarity of structure or of conditions on Mars to account for +these features, and I shall now attempt to point out what this +peculiarity is and how it may have arisen. + +_The Meteoritic Hypothesis._ + +During the last quarter of a century a considerable change has come over +the opinions of astronomers as regards the probable origin of the Solar +System. The large amount of knowledge of the stellar universe, and +especially of nebulae, of comets and of meteor-streams which we now +possess, together with many other phenomena, such as the constitution of +Saturn's rings, the great number and extent of the minor planets, and +generally of the vast amount of matter in the form of meteor-rings and +meteoric dust in and around our system, have all pointed to a different +origin for the planets and their satellites than that formulated by +Laplace as the Nebular hypothesis. + +It is now seen more clearly than at any earlier period, that most of the +planets possess special characteristics which distinguish them from one +another, and that such an origin as Laplace suggested--the slow cooling +and contraction of one vast sun-mist or nebula, besides presenting +inherent difficulties--many think them impossibilities--in itself does +not afford an adequate explanation of these peculiarities. Hence has +arisen what is termed the Meteoritic theory, which has been ably +advocated for many years by Sir Norman Lockyer, and with some +unimportant modifications is now becoming widely accepted. Briefly, this +theory is, that the planets have been formed by the slow aggregation of +solid particles around centres of greatest condensation; but as many of +my readers may be altogether unacquainted with it, I will here give a +very clear statement of what it is, from Professor J.W. Gregory's +presidential address to the Geological Section of the British +Association of the present year. He began by saying that these modern +views were of far more practical use to men of science than that of +Laplace, and that they give us a history of the world consistent with +the actual records of geology. He then continues: + +"According to Sir Norman Lockyer's Meteoritic Hypothesis, nebulae, +comets, and many so-called stars consist of swarms of meteorites which, +though normally cold and dark, are heated by repeated collisions, and so +become luminous. They may even be volatilised into glowing meteoric +vapour; but in time this heat is dissipated, and the force of gravity +condenses a meteoritic swarm into a single globe. 'Some of the swarms +are,' says Lockyer, 'truly members of the solar system,' and some of +these travel round the sun in nearly circular orbits, like planets. They +may be regarded as infinitesimal planets, and so Chamberlain calls them +'planetismals.' + +"The planetismal theory is a development of the meteoritic theory, and +presents it in an especially attractive guise. It regards meteorites as +very sparsely distributed through space, and gravity as powerless to +collect them into dense groups. So it assigns the parentage of the solar +system to a spiral nebula composed of planetismals, and the planets as +formed from knots in the nebula, where many planetismals had been +concentrated near the intersections of their orbits. These groups of +meteorites, already as dense as a swarm of bees, were then packed closer +by the influence of gravity, and the contracting mass was heated by the +pressure, even above the normal melting-point of the material, which was +kept rigid by the weight of the overlying layers." + +Now, adopting this theory as the last word of science upon the subject +of the origin of planets, we see that it affords immense scope for +diversity in results depending on the total _amount_ of matter available +within the range of attraction of an incipient planetary mass, and the +_rates_ at which this matter becomes available. By a special combination +of these two quantities (which have almost certainly been different for +each planet) I think we may be able to throw some light upon the +structure and physical features of Mars. + +_The Probable Mode of Origin of Mars._ + +This planet, lying between two of much greater mass, has evidently had +less material from which to be formed by aggregation; and if we +assume--as in the absence of evidence to the contrary we have a right to +do--that its beginnings were not much later (or earlier) than those of +the earth, then its smaller size shows that it has in all probability +aggregated very much more slowly. But the internal heat acquired by a +planet while forming in this manner will depend upon the rate at which +it aggregates and the velocity with which the planetismals' fall into +it, and this velocity will increase with its mass and consequent force +of gravity. In the early stages of a planet's growth it will probably +remain cold, the small amount of heat produced by each impact being lost +by radiation before the next one occurs; and with a small and slowly +aggregating planet this condition will prevail till it approaches its +full size. Then only will its gravitative force be sufficient to cause +incoming matter to fall upon it with so powerful an impact as to produce +intense heat. Further, the compressive force of a small planet will be a +less effective heat-producing agency than in the case of a larger one. + +The earth we know has acquired a large amount of internal heat, probably +sufficient to liquefy its whole interior; but Mars has only one-ninth +part the mass of the earth, and it is quite possible, and even probable, +that its comparatively small attractive force would never have liquefied +or even permanently heated the more central portions of its mass. This +being admitted, I suggest the following course of events as quite +possible, and not even improbable, in the case of this planet. During +the whole of its early growth, and till it acquired nearly its present +diameter, its rate of aggregation was so slow that the planetismals +falling upon it, though they might have been heated and even partially +liquefied by the impact, were never in such quantity as to produce any +considerable heating effect on the whole mass, and each local rise of +temperature was soon lost by radiation. The planet thus grew as a solid +and cold mass, compacted together by the impact of the incoming matter +as well as by its slowly increasing gravitative force. But when it had +attained to within perhaps 100, perhaps 50 miles, or less, of its +present diameter, a great change occurred in the opportunity for further +growth. Some large and dense swarm of meteorites, perhaps containing a +number of bodies of the size of the asteroids, came within the range of +the sun's attraction and were drawn by it into an orbit which crossed +that of Mars at such a small angle that the planet was able at each +revolution to capture a considerable number of them. The result might +then be that, as in the case of the earth, the continuous inpour of the +fresh matter first heated, and later on liquefied the greater part of it +as well perhaps as a thin layer of the planet's original surface; so +that when in due course the whole of the meteor-swarm had been captured, +Mars had acquired its present mass, but would consist of an intensely +heated, and either liquid or plastic thin outer shell resting upon a +cold and solid interior. + +The size and position of the two recently discovered satellites of Mars, +which are believed to be not more than ten miles in diameter, the more +remote revolving around its primary very little slower than the planet +rotates, while the nearer one, which is considerably less distant from +the planet's surface than its own antipodes and revolves around it more +than three times during the Martian day, may perhaps be looked upon as +the remnants of the great meteor-swarm which completed the Martian +development, and which are perhaps themselves destined at some distant +period to fall into the planet. Should future astronomers witness the +phenomenon the effect produced upon its surface would be full of +instruction. + +As the result of such an origin as that suggested, Mars would possess a +structure which, in the essential feature of heat-distribution, would be +the very opposite of that which is believed to characterise the earth, +yet it might have been produced by a very slight modification of the +same process. This peculiar heat-distribution, together with a much +smaller mass and gravitative force, would lead to a very different +development of the surface and an altogether diverse geological history +from ours, which has throughout been profoundly influenced by its heated +interior, its vast supply of water, and the continuous physical and +chemical reactions between the interior and the crust. + +These reactions have, in our case, been of substantially the same +nature, and very nearly of the same degree of intensity throughout the +whole vast eons of geological time, and they have resulted in a +wonderfully complex succession of rock-formations--volcanic, plutonic, +and sedimentary--more or less intermingled throughout the whole series, +here remaining horizontal as when first deposited, there upheaved or +depressed, fractured or crushed, inclined or contorted; denuded by rain +and rivers with the assistance of heat and cold, of frost and ice, in an +unceasing series of changes, so that however varied the surface may be, +with hill and dale, plains and uplands, mountain ranges and deep +intervening valleys, these are as nothing to the diversities of interior +structure, as exhibited in the sides of every alpine valley or +precipitous escarpment, and made known to us by the work of the miner +and the well-borer in every part of the world. + +_Structural Straight Lines on the Earth._ + +The great characteristic of the earth, both on its surface and in its +interior, is thus seen to be extreme diversity both of form and +structure, and this is further intensified by the varied texture, +constitution, hardness, and density of the various rocks and debris of +which it is composed. It is therefore not surprising that, with such a +complex outer crust, we should nowhere find examples of those +geometrical forms and almost world-wide straight lines that give such a +remarkable, and as Mr. Lowell maintains, 'non-natural' character to the +surface of Mars, but which, as it seems to me, of themselves afford +_prima facie_ evidence of a corresponding simplicity and uniformity in +its internal structure. + +Yet we are not ourselves by any means devoid of 'straight lines' +structurally produced, in spite of every obstacle of diversity of form +and texture, of softness and hardness, of lamination or crystallisation, +which are adverse to such developments. Examples of these are the +numerous 'faults' which occur in the harder rocks, and which often +extend for great distances in almost perfect straight lines. In our own +country we have the Tyneside and Craven faults in the North of England, +which are 30 miles long and often 20 yards wide; but even more striking +is the great Cleveland Dyke--a wall of volcanic rock dipping slightly +towards the south, but sometimes being almost vertical, and stretching +across the country, over hill and dale, in an almost perfect straight +line from a point on the coast ten miles north of Scarborough, in a +west-by-north direction, passing about two miles south of Stockton and +terminating about six miles north-by-east of Barnard Castle, a distance +of very nearly 60 miles. The great fault between the Highlands and +Lowlands of Scotland extends across the country from Stonehaven to near +Helensburgh, a distance of 120 miles; and there are very many more of +less importance. + +Much more extensive are some of the great continental dislocations, +often forming valleys of considerable width and length. The Upper Rhine +flows in one of these great valleys of subsidence for about 180 miles, +from Mulhausen to Frankfort, in a generally straight line, though +modified by denudation. Vaster still is the valley of the Jordan through +the Sea of Galilee to the Dead Sea, continued by the Wady Arabah to the +Gulf of Akaba, believed to form one vast geological depression or +fracture extending in a straight line for 400 miles. + +Thousands of such faults, dykes, or depressions exist in every part of +the world, all believed to be due to the gradual shrinking of the heated +interior to which the solid crust has to accommodate itself, and they +are especially interesting and instructive for our present purpose as +showing the tendency of such fractures of solid rock-material to extend +to great lengths in straight lines, notwithstanding the extreme +irregularity both in the surface contour as well as in the internal +structures of the varied deposits and formations through which they +pass. + +_Probable Origin of the Surface-features of Mars._ + +Returning now to Mars, let us consider the probable course of events +from the point at which we left it. The heat produced by impact and +condensation would be likely to release gases which had been in +combination with some of the solid matter, or perhaps been itself in a +solid state due to intense cold, and these, escaping outwards to the +surface, would produce on a small scale a certain amount of upheaval and +volcanic disturbance; and as an outer crust rapidly formed, a number of +vents might remain as craters or craterlets in a moderate state of +activity. Owing to the comparatively small force of gravity, the outer +crust would become scoriaceous and more or less permeated by the gases, +which would continue to escape through it, and this would facilitate the +cooling of the whole of the heated outer crust, and allow it to become +rather densely compacted. When the greater portion of the gases had thus +escaped to the outer surface and assisted to form a scanty atmosphere, +such as now exists, there would be no more internal disturbance and the +cooling of the heated outer coating would steadily progress, resulting +at last in a slightly heated, and later in a cold layer of moderate +thickness and great general uniformity. Owing to the absence of rain and +rivers, denudation such as we experience would be unknown, though the +superficial scoriaceous crust might be partially broken up by expansion +and contraction, and suffer a certain amount of atmospheric erosion. + +The final result of this mode of aggregation would be, that the planet +would consist of an outer layer of moderate thickness as compared with +the central mass, which outer layer would have cooled from a highly +heated state to a temperature considerably below the freezing-point, and +this would have been all the time _contracting upon a previously cold, +and therefore non-contracting nucleus._ The result would be that very +early in the process great superficial tensions would be produced, which +could only be relieved by cracks or fissures, which would initiate at +points of weakness--probably at the craterlets already referred to--from +which they would radiate in several directions. Each crack thus formed +near the surface would, as cooling progressed, develop in length and +depth; and owing to the general uniformity of the material, and possibly +some amount of crystalline structure due to slow and continuous cooling +down to a very low temperature, the cracks would tend to run on in +straight lines and to extend vertically downwards, which two +circumstances would necessarily result in their forming portions of +'great circles' on the planet's surface--the two great facts which Mr. +Lowell appeals to as being especially 'non-natural.' + +_Symmetry of Basaltic Columns._ + +We have however one quite natural fact on our earth which serves to +illustrate one of these two features, the direction of the downward +fissure. This is, the comparatively common phenomenon of basaltic +columns and 'Giant's Causeways.' The wonderful regularity of these, and +especially the not unfrequent upright pillars in serried ranks, as in +the palisades of the Hudson river, must have always impressed observers +with their appearance of artificiality. Yet they are undoubtedly the +result of the very slow cooling and contraction of melted rocks under +compression by strata _below and above them_, so that, when once +solidified, the mass was held in position and the tension produced by +contraction could only be relieved by numerous very small cracks at +short distances from each other in every direction, resulting in five, +six, or seven-sided polygons, with sides only a few inches long. This +contraction began of course at the coolest surface, generally the upper +one; and observation of these columns in various positions has +established the rule that their direction lengthways _is always at right +angles to the cooling surface_, and thus, whenever this surface was +horizontal, the columns became almost exactly vertical. + +_How this applies to Mars._ + +One of the features of the surface of Mars that Mr. Lowell describes +with much confidence is, that it is wonderfully uniform and level, which +of course it would be if it had once been in a liquid or plastic state, +and not much disturbed since by volcanic or other internal movements. +The result would be that cracks formed by contraction of the hardened +outer crust would be vertical; and, in a generally uniform material at a +very uniform temperature, these cracks would continue almost +indefinitely in straight lines. The hardened and contracting surface +being free to move laterally on account of there being a more heated and +plastic layer below it, the cracks once initiated above would +continually widen at the surface as they penetrated deeper and deeper +into the slightly heated substratum. Now, as basalt begins to soften at +about 1400° F. and the surface of Mars has cooled to at least the +freezing-point--perhaps very much below it--the contraction would be so +great that if the fissures produced were 500 miles apart they might be +three miles wide at the surface, and, if only 100 miles apart, then +about two-thirds of a mile wide.[15] But as the production of the +fissures might have occupied perhaps millions of years, a considerable +amount of atmospheric denudation would result, however slowly it acted. +Expansion and contraction would wear away the edges and sides of the +fissures, fill up many of them with the debris, and widen them at the +surfaces to perhaps double their original size.[16] + +[Footnote 15: The coefficient of contraction of basalt is 0.000006 for +1° F., which would lead to the results given here.] + +[Footnote 16: Mr. W.H. Pickering observed clouds on Mars 15 miles high; +these are the 'projections' seen on the terminator when the planet is +partially illuminated. They were at first thought to be mountains; but +during the opposition of 1894, more than 400 of them were seen at +Flagstaff during nine months' observation. Usually they are of rare +occurrence. They are seen to change in form and position from day to +day, and Mr. Lowell is strongly of opinion that they are dust-storms, +not what we term clouds. They were mostly about 13 miles high, +indicating considerable aerial disturbance on the planet, and therefore +capable of producing proportional surface denudation.] + +_Suggested Explanation of the 'Oases.'_ + +The numerous round dots seen upon the 'canals,' and especially at points +from which several canals radiate and where they intersect--termed +'oases' by Mr. Lowell and 'craterlets' by Mr. Pickering may be explained +in two ways. Those from which several canals radiate may be true craters +from which the gases imprisoned in the heated surface layers have +gradually escaped. They would be situated at points of weakness in the +crust, and become centres from which cracks would start during +contraction. Those dots which occur at the crossing of two straight +canals or cracks may have originated from the fact that at such +intersections there would be four sharply-projecting angles, which, +being exposed to the influence of alternate heat and cold (during day +and night) on the two opposite surfaces, would inevitably in time become +fractured and crumbled away, resulting in the formation of a roughly +circular chasm which would become partly filled up by the debris. Those +formed by cracks radiating from craterlets would also be subject to the +same process of rounding off to an even greater extent; and thus would +be produced the 'oases' of various sizes up to 50 miles or more in +diameter recorded by Mr. Lowell and other observers. + +_Probable Function of the Great Fissures._ + +Mr. Pickering, as we have seen, supposes that these fissures give out +the gases which, overflowing on each side, favour the growth of the +supposed vegetation which renders the course of the canals visible, and +this no doubt may have been the case during the remote periods when +these cracks gave access to the heated portions of the surface layer. +But it seems more probable that Mars has now cooled down to the almost +uniform mean temperature it derives from solar heat, and that the +fissures--now for the most part broad shallow valleys--serve merely as +channels along which the liquids and heavy gases derived from the +melting of the polar snows naturally flow, and, owing to their nearly +level surfaces, overflow to a certain distance on each side of them. + +_Suggested Origin of the Blue Patches._ + +These heavy gases, mainly perhaps, as has been often suggested, +carbon-dioxide, would, when in large quantity and of considerable depth, +reflect a good deal of light, and, being almost inevitably dust-laden, +might produce that blue tinge adjacent to the melting snow-caps which +Mr. Lowell has erroneously assumed to be itself a proof of the presence +of liquid water. Just as the blue of our sky is undoubtedly due to +reflection from the ultra-minute dust particles in our higher +atmosphere, similar particles brought down by the 'snow' from the higher +Martian atmosphere might produce the blue tinge in the great volumes of +heavy gas produced by its evaporation or liquefaction. + +It may be noted that Mr. Lowell objects to the carbon-dioxide theory of +the formation of the snow-caps, that this gas at low pressures does not +liquefy, but passes at once from the solid to the gaseous state, and +that only water remains liquid sufficiently long to produce the blue +colour' which plays so large a part in his argument for the mild climate +essential for an inhabited planet. But this argument, as I have already +shown, is valueless. For only very deep water can possibly show a blue +colour by reflected light, while a dust-laden atmosphere--especially +with a layer of very dense gas at the bottom of it, as would be the case +with the newly evaporated carbon-dioxide from the diminishing snow-cap +--would provide the very conditions likely to produce this blue tinge of +colour. + +It may be considered a support to this view that carbonic-acid gas +becomes liquid at--140° F. and solid at--162° F., temperatures far +higher than we should expect to prevail in the polar and north temperate +regions of Mars during a considerable part of the year, but such as +might be reached there during the summer solstice when the `snows' so +rapidly disappear, to be re-formed a few months later. + +_The Double Canals._ + +The curious phenomena of the 'double canals' are undoubtedly the most +difficult to explain satisfactorily on any theory that has yet been +suggested. They vary in distance apart from about 100 to 400 miles. In +many cases they appear perfectly parallel, and Mr. Lowell gives us the +impression that they are almost always so. But his maps show, in some +cases, decided differences of width at the two extremities, indicating +considerable want of parallelism. A few of the curved canals are also +double. + +There is one drawing in Mr. Lowell's book (p. 219) of the mouths, or +starting points, of the Euphrates and Phison, two widely separated +double canals diverging at an angle of about 40° from the same two +oases, so that the two inner canals cross each other. Now this suggests +two wide bands of weakness in the planet's crust radiating probably from +within the dark tract called the 'Mare Icarium,' and that some +widespread volcanic outburst initiated diverging cracks on either side +of these bands. Something of this kind may have been the cause of most +of the double canals, or they may have been started from two or more +craterlets not far apart, the direction being at first decided by some +local peculiarity of structure; and where begun continuing in straight +lines owing to homogeneity or uniform density of material. This is very +vague, but the phenomena are so remarkable, and so very imperfectly +known at present, that nothing but suggestion can be attempted. + +_Concluding Remarks on the 'Canals.'_ + +In this somewhat detailed exposition of a possible, and, I hope, a +probable explanation of the surface-features of Mars, I have +endeavoured to be guided by known facts or accepted theories both +astronomical and geological. I think I may claim to have shown that +there are some analogous features of terrestrial rock-structure to +serve as guides towards a natural and intelligible explanation of the +strange geometric markings discovered during the last thirty years, and +which have raised this planet from comparative obscurity into a position +of the very first rank both in astronomical and popular interest. + +This wide-spread interest is very largely due to Mr. Lowell's devotion +to its study, both in seeking out so admirable a position as regards +altitude and climate, and in establishing there a first-class +observatory; and also in bringing his discoveries before the public in +connection with a theory so startling as to compel attention. I venture +to think that his merit as one of our first astronomical observers will +in no way be diminished by the rejection of his theory, and the +substitution of one more in accordance with the actually observed facts. + + + +APPENDIX. + +_A Suggested Experiment to Illustrate the 'Canals' of Mars._ + +If my explanation of the 'canals' should be substantially correct--that +is, if they were produced by the contraction of a heated outward crust +upon a cold, and therefore non-contracting interior, the result of such +a condition might be shown experimentally. + +Several baked clay balls might be formed to serve as cores, say of 8 to +10 inches in diameter. These being fixed within moulds of say half an +inch to an inch greater diameter, the outer layer would be formed by +pouring in some suitable heated liquid material, and releasing it from +the mould as soon as consolidation occurs, so that it may cool rapidly +from the _outside._ Some kinds of impure glass, or the brittle metals +bismuth or antimony or alloys of these might be used, in order to see +what form the resulting fractures would take. It would be well to have +several duplicates of each ball, and, as soon as tension through +contraction manifests itself, to try the effect of firing very small +charges of small shot to ascertain whether such impacts would start +radiating fractures. When taken from the moulds, the balls should be +suspended in a slight current of air, and kept rotating, to reproduce +the planetary condition as nearly as possible. + +The exact size and material of the cores, the thickness of the heated +outer crust, the material best suited to show fracture by contraction, +and the details of their treatment, might be modified in various ways as +suggested by the results first obtained. Such a series of experiments +would probably throw further light on the physical conditions which have +produced the gigantic system of fissures or channels we see upon the +surface of Mars, though it would not, of course, prove that such +conditions actually existed there. In such a speculative matter we can +only be guided by probabilities, based upon whatever evidence is +available. + + + +CHAPTER VIII. + + +SUMMARY AND CONCLUSION. + +This little volume has necessarily touched upon a great variety of +subjects, in order to deal in a tolerably complete manner with the very +extraordinary theories by which Mr. Lowell attempts to explain the +unique features of the surface of the planet, which, by long-continued +study, he has almost made his own. It may therefore be well to sum up +the main points of the arguments against his view, introducing a few +other facts and considerations which greatly strengthen my argument. + +The one great feature of Mars which led Mr. Lowell to adopt the view of +its being inhabited by a race of highly intelligent beings, and, with +ever-increasing discovery to uphold this theory to the present time, is +undoubtedly that of the so-called 'canals'--their straightness, their +enormous length, their great abundance, and their extension over the +planet's whole surface from one polar snow-cap to the other. The very +immensity of this system, and its constant growth and extension during +fifteen years of persistent observation, have so completely taken +possession of his mind, that, after a very hasty glance at analogous +facts and possibilities, he has declared them to be 'non-natural'-- +therefore to be works of art--therefore to necessitate the +presence of highly intelligent beings who have designed and constructed +them. This idea has coloured or governed all his writings on the +subject. The innumerable difficulties which it raises have been either +ignored, or brushed aside on the flimsiest evidence. As examples, he +never even discusses the totally inadequate water-supply for such +worldwide irrigation, or the extreme irrationality of constructing so +vast a canal-system the waste from which, by evaporation, when exposed +to such desert conditions as he himself describes, would use up ten +times the probable supply. + +Again, he urges the 'purpose' displayed in these 'canals.' Their being +_all_ so straight, _all_ describing great circles of the 'sphere,' all +being so evidently arranged (as he thinks) either to carry water to some +'oasis' 2000 miles away, or to reach some arid region far over the +equator in the opposite hemisphere! But he never considers the +difficulties this implies. Everywhere these canals run for thousands of +miles across waterless deserts, forming a system and indicating a +purpose, the wonderful perfection of which he is never tired of dwelling +upon (but which I myself can nowhere perceive). + +Yet he never even attempts to explain how the Martians could have lived +_before_ this great system was planned and executed, or why they did not +_first_ utilise and render fertile the belt of land adjacent to the +limits of the polar snows--why the method of irrigation did not, as with +all human arts, begin gradually, at home, with terraces and channels to +irrigate the land close to the source of the water. How, with such a +desert as he describes three-fourths of Mars to be, did the inhabitants +ever get to _know_ anything of the equatorial regions and its needs, so +as to start right away to supply those needs? All this, to my mind, is +quite opposed to the idea of their being works of art, and altogether in +favour of their being natural features of a globe as peculiar in origin +and internal structure as it is in its surface-features. The explanation +I have given, though of course hypothetical, is founded on known +cosmical and terrestrial facts, and is, I suggest, far more scientific +as well as more satisfactory than Mr. Lowell's wholly unsupported +speculation. This view I have explained in some detail in the preceding +chapter. + +Mr. Lowell never even refers to the important question of loss by +evaporation in these enormous open canals, or considers the undoubted +fact that the only intelligent and practical way to convey a limited +quantity of water such great distances would be by a system of +water-tight and air-tight tubes laid _under the ground._ The mere +attempt to use open canals for such a purpose shows complete ignorance +and stupidity in these alleged very superior beings; while it is certain +that, long before half of them were completed their failure to be of any +use would have led any rational beings to cease constructing them. + +He also fails to consider the difficulty, that, if these canals are +necessary for existence in Mars, how did the inhabitants ever reach a +sufficiently large population with surplus food and leisure enabling +them to rise from the low condition of savages to one of civilisation, +and ultimately to scientific knowledge? Here again is a dilemma which is +hard to overcome. Only a _dense_ population with _ample_ means of +subsistence could possibly have constructed such gigantic works; but, +given these two conditions, no adequate motive existed for the +conception and execution of them--even if they were likely to be of any +use, which I have shown they could not be. + +_Further Considerations on the Climate of Mars._ + +Recurring now to the question of climate, which is all-important, Mr. +Lowell never even discusses the essential point--the temperature that +must _necessarily_ result from an atmospheric envelope one-twelfth (or +at most one-seventh) the density of our own; in either case +corresponding to an altitude far greater than that of our highest +mountains.[17] Surely this phenomenon, everywhere manifested on the +earth even under the equator, of a regular decrease of temperature with +altitude, the only cause of which is a less dense atmosphere, should +have been fairly grappled with, and some attempt made to show why it +should not apply to Mars, except the weak remark that on a level surface +it will not have the same effect as on exposed mountain heights. But it +_does_ have the same effect, or very nearly so, on our lofty plateaux +often hundreds of miles in extent, in proportion to their altitude. +Quito, at 9350 ft. above the sea, has a mean temperature of about 57° +F., giving a lowering of 23° from that of Manaos at the mouth of the Rio +Negro. This is about a degree for each 400 feet, while the general fall +for isolated mountains is about one degree in 340 feet according to +Humboldt, who notes the above difference between the rate of cooling for +altitude of the plains--or more usually sheltered valleys in which the +towns are situated--and the exposed mountain sides. It will be seen that +this lower rate would bring the temperature of Mars at the equator down +to 20° F. below the freezing point of water from this cause alone. + +[Footnote 17: A four inches barometer is equivalent to a height of +40,000 feet above sea-level with us.] + +But all enquirers have admitted, that if conditions as to atmosphere +were the same as on the earth, its greater distance from the sun would +reduce the temperature to-31° F., equal to 63° below the freezing +point. It is therefore certain that the combined effect of both causes +must bring the temperature of Mars down to at least 70° or 80°below the +freezing point. + +The cause of this absolute dependence of terrestrial temperatures upon +density of the air-envelope is seldom discussed in text-books either of +geography or of physics, and there seems to be still some uncertainty +about it. Some impute it wholly to the thinner air being unable to +absorb and retain so much heat as that which is more dense; but if this +were the case the soil at great altitudes not having so much of its heat +taken up by the air should be warmer than below, since it undoubtedly +_receives_ more heat owing to the greater transparency of the air above +it; but it certainly does not become warmer. The more correct view seems +to be that the loss of heat by radiation is increased so much through +the rarity of the air above it as to _more_ than counterbalance the +increased insolation, so that though the surface of the earth at a given +altitude may receive 10 per cent. more direct sun-heat it loses by +direct radiation, combined with diminished air and cloud-radiation, +perhaps 20 or 25 per cent. more, whence there is a resultant cooling +effect of 10 or 15 per cent. This acts by day as well as by night, so +that the greater heat received at high altitudes does not warm the soil +so much as a less amount of heat with a denser atmosphere. + +This effect is further intensified by the fact that a less dense cannot +absorb and transmit so much heat as a more dense atmosphere. Here then +we have an absolute law of nature to be observed operating everywhere on +the earth, and the mode of action of which is fairly well understood. +This law is, that reduced atmospheric pressure increases radiation, or +loss of heat, _more rapidly_ than it increases insolation or gain of +heat, so that the result is _always_ a considerable _lowering_ of +temperature. What this lowering is can be seen in the universal fact, +that even within the tropics perpetual snow covers the higher mountain +summits, while on the high plains of the Andes, at 15,000 or 16,000 feet +altitude, where there is very little or no snow, travellers are often +frozen to death when delayed by storms; yet at this elevation the +atmosphere has much more than double the density of that of Mars! + +The error in Mr. Lowell's argument is, that he claims for the scanty +atmosphere of Mars that it allows more sun-heat to reach the surface; +but he omits to take account of the enormously increased loss of heat by +direct radiation, as well as by the diminution of air-radiation, which +together necessarily produce a great reduction of temperature. + +It is this great principle of the prepotency of radiation over +absorption with a diminishing atmosphere that explains the excessively +low temperature of the moon's surface, a fact which also serves to +indicate a very low temperature for Mars, as I have shown in Chapter VI. +These two independent arguments--from alpine temperatures and from those +of the moon--support and enforce each other, and afford a conclusive +proof (as against anything advanced by Mr. Lowell) that the temperature +of Mars must be far too low to support animal life. + +A third independent argument leading to the same result is Dr. Johnstone +Stoney's proof that aqueous vapour cannot exist on Mars; and this fact +Mr. Lowell does not attempt to controvert. + +To put the whole case in the fewest possible words: + +All physicists are agreed that, owing to the distance of Mars from the +sun, it would have a mean temperature of about-35° F. (= 456° F. abs.) +even if it had an atmosphere as dense as ours. + +(2) But the very low temperatures on the earth under the equator, at a +height where the barometer stands at about three times as high as on +Mars, proves, that from scantiness of atmosphere alone Mars cannot +possibly have a temperature as high as the freezing point of water; and +this proof is supported by Langley's determination of the low _maximum_ +temperature of the full moon. + +The combination of these two results must bring down the temperature of +Mars to a degree wholly incompatible with the existence of animal life. + +(3) The quite independent proof that water-vapour cannot exist on Mars, +and that therefore, the first essential of organic life--water--is +non-existent. + +The conclusion from these three independent proofs, which enforce each +other in the multiple ratio of their respective weights, is therefore +irresistible--that animal life, especially in its higher forms, cannot +exist on the planet. + +Mars, therefore, is not only uninhabited by intelligent beings such as +Mr. Lowell postulates, but is absolutely UNINHABITABLE. + + + + + +End of Project Gutenberg's Is Mars Habitable?, by Alfred Russel Wallace + +*** END OF THIS PROJECT GUTENBERG EBOOK IS MARS HABITABLE? *** + +***** This file should be named 10855-8.txt or 10855-8.zip ***** +This and all associated files of various formats will be found in: + https://www.gutenberg.org/1/0/8/5/10855/ + +Produced by Thaadd and the PG Distributed Proofreading Team + +Updated editions will replace the previous one--the old editions +will be renamed. + +Creating the works from public domain print editions means that no +one owns a United States copyright in these works, so the Foundation +(and you!) can copy and distribute it in the United States without +permission and without paying copyright royalties. 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You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: Is Mars Habitable? + +Author: Alfred Russel Wallace + +Release Date: January 28, 2004 [EBook #10855] + +Language: English + +Character set encoding: ASCII + +*** START OF THIS PROJECT GUTENBERG EBOOK IS MARS HABITABLE? *** + + + + +Produced by Thaadd and the PG Distributed Proofreading Team + + + + +_Is Mars Habitable?_ + +A CRITICAL EXAMINATION OF PROFESSOR PERCIVAL LOWELL'S BOOK +"MARS AND ITS CANALS," WITH AN ALTERNATIVE EXPLANATION + +BY ALFRED RUSSEL WALLACE F.R.S., ETC. + + + +PREFACE. + +This small volume was commenced as a review article on Professor +Percival Lowell's book, _Mars and its Canals_, with the object of +showing that the large amount of new and interesting facts contained in +this work did not invalidate the conclusion I had reached in 1902, and +stated in my book on _Man's Place in the Universe_, that Mars was not +habitable. + +But the more complete presentation of the opposite view in the volume +now under discussion required a more detailed examination of the various +physical problems involved, and as the subject is one of great, popular, +as well as scientific interest, I determined to undertake the task. + +This was rendered the more necessary by the fact that in July last +Professor Lowell published in the _Philosophical Magazine_ an elaborate +mathematical article claiming to demonstrate that, notwithstanding its +much greater distance from the sun and its excessively thin atmosphere, +Mars possessed a climate on the average equal to that of the south of +England, and in its polar and sub-polar regions even less severe than +that of the earth. Such a contention of course required to be dealt +with, and led me to collect information bearing upon temperature in all +its aspects, and so enlarging my criticism that I saw it would be +necessary to issue it in book form. + +Two of my mathematical friends have pointed out the chief omission which +vitiates Professor Lowell's mathematical conclusions--that of a failure +to recognise the very large conservative and _cumulative_ effect of a +dense atmosphere. This very point however I had already myself discussed +in Chapter VI., and by means of some remarkable researches on the heat +of the moon and an investigation of the causes of its very low +temperature, I have, I think, demonstrated the incorrectness of Mr. +Lowell's results. In my last chapter, in which I briefly summarise the +whole argument, I have further strengthened the case for very severe +cold in Mars, by adducing the rapid lowering of temperature universally +caused by diminution of atmospheric pressure, as manifested in the +well-known phenomenon of temperate climates at moderate heights even +close to the equator, cold climates at greater heights even on extensive +plateaux, culminating in arctic climates and perpetual snow at heights +where the air is still far denser than it is on the surface of Mars. +This argument itself is, in my opinion, conclusive; but it is enforced +by two others equally complete, neither of which is adequately met by +Mr. Lowell. + +The careful examination which I have been led to give to the whole of +the phenomena which Mars presents, and especially to the discoveries of +Mr. Lowell, has led me to what I hope will be considered a satisfactory +physical explanation of them. This explanation, which occupies the whole +of my seventh chapter, is founded upon a special mode of origin for +Mars, derived from the Meteoritic Hypothesis, now very widely adopted by +astronomers and physicists. Then, by a comparison with certain +well-known and widely spread geological phenomena, I show how the great +features of Mars--the 'canals' and 'oases'--may have been caused. This +chapter will perhaps be the most interesting to the general reader, as +furnishing a quite natural explanation of features of the planet which +have been termed 'non-natural' by Mr. Lowell. + +Incidentally, also, I have been led to an explanation of the highly +volcanic nature of the moon's surface. This seems to me absolutely to +require some such origin as Sir George Darwin has given it, and thus +furnishes corroborative proof of the accuracy of the hypothesis that our +moon has had an unique origin among the known satellites, in having been +thrown off from the earth itself. + +I am indebted to Professor J. H. Poynting, of the University of +Birmingham, for valuable suggestions on some of the more difficult +points of mathematical physics here discussed, and also for the critical +note (at the end of Chapter V.) on Professor Lowell's estimate of the +temperature of Mars. + +BROADSTONE, DORSET, _October_ 1907. + + + +TABLE OF CONTENTS. + + +CHAPTER I. + +EARLY OBSERVERS OF MARS, +--Mars the only planet the surface of which is + distinctly visible +--Early observation of the snow-caps and seas +--The 'canals' seen by Schiaparelli in 1877 +--Double canals first seen in 1881 +--Round spots at intersection of canals seen + by Pickering in 1892 +--Confirmed by Lowell in 1894 +--Changes of colour seen in 1892 and 1894 +--Existence of seas doubted by Pickering and + Barnard in 1894. + + +CHAPTER II. + +MR. LOWELL'S DISCOVERIES AND THEORIES, +--Observatory at Flagstaff, Arizona +--Illustrated book on his observations of + Mars +--Volume on Mars and its canals, 1906 +--Non-natural features +--The canals as irrigation works of an intelligent + race +--A challenge to the thinking world +--The canals as described and mapped by Mr. Lowell +--The double canals +--Dimensions of the canals +--They cross the supposed seas +--Circular black spots termed oases +--An interesting volume. + + +CHAPTER III. + +THE CLIMATE AND PHYSIOGRAPHY OF MARS, +--No permanent water on Mars +--Rarely any clouds and no rain +--Snow-caps the only source of water +--No mountains, hills, or valleys on Mars +--Two-thirds of the surface a desert +--Water from the snow-caps too scanty to supply + the canals +--Miss Clerke's views as to the water-supply +--Description of some of the chief canals +--Mr. Lowell on the purpose of the canals +--Remarks on the same +--Mr. Lowell on relation of canals to oases and + snow-caps +--Critical remarks on the same. + + +CHAPTER IV. + +IS ANIMAL LIFE POSSIBLE ON MARS? +--Water and air essential for animal life +--Atmosphere of Mars assumed to be like ours +--Blue tint near melting snow the only evidence + of water +--Fallacy of this argument +--Dr. Johnstone Stoney's proof that water-vapour + cannot exist on Mars +--Spectroscope gives no evidence of water. + + +CHAPTER V. + +TEMPERATURE OF MARS--MR. LOWELL'S ESTIMATE, +--Problem of terrestrial temperature +--Ice under recent lava +--Tropical oceans ice-cold at bottom +--Earth's surface-heat all from the sun +--Absolute zero of temperature +--Complex problem of planetary temperatures +--Mr. Lowell's investigation of the problem +--Abstract of Mr. Lowell's paper +--Critical remarks on Mr. Lowell's paper. + + +CHAPTER VI. + +A NEW ESTIMATE OF THE TEMPERATURE OF MARS, +--Langley's determination of lunar heat +--Rapid loss of heat by radiation on the + earth +--Rapid loss of heat on moon during eclipse +--Sir George Darwin's theory of the moon's origin +--Very's researches on the moon's temperature +--Application of these results to the case of Mars +--Cause of great difference of temperatures of earth + and moon +--Special features of Mars influencing its + temperature +--Further criticism of Mr. Lowell's article +--Very low temperature of arctic regions on Mars. + + +CHAPTER VII. + +A SUGGESTION AS TO THE 'CANALS' OF MARS, +--Special features of the canals +--Mr. Pickering's suggested explanation +--The meteoritic hypotheses of origin of planets +--Probable mode of origin of Mars +--Structural straight lines on the earth +--Probable origin of the surface-features of Mars +--Symmetry of basaltic columns +--How this applies to Mars +--Suggested explanation of the oases +--Probable function of the great fissures +--Suggested origin of blue patches adjacent to snow-caps +--The double canals +--Concluding remarks on the canals. + + +CHAPTER VIII. + +PAGE SUMMARY AND CONCLUSION, +--The canals the origin of Mr. Lowell's theory +--Best explained as natural features +--Evaporation difficulty not met by Mr. Lowell +--How did Martians live without the canals +--Radiation due to scanty atmosphere not taken account + of +--Three independent proofs of low temperature and + uninhabitability of Mars +--Conclusion. + + +CHAPTER I. + +EARLY OBSERVERS OF MARS. + +Few persons except astronomers fully realise that of all the planets of +the Solar system the only one whose solid surface has been seen with +certainty is Mars; and, very fortunately, that is also the only one +which is sufficiently near to us for the physical features of the +surface to be determined with any accuracy, even if we could see it in +the other planets. Of Venus we probably see only the upper surface of +its cloudy atmosphere.[1] As regards Jupiter and Saturn this is still +more certain, since their low density will only permit of a +comparatively small proportion of their huge bulk being solid. Their +belts are but the cloud-strata of their upper atmosphere, perhaps +thousands of miles above their solid surfaces, and a somewhat similar +condition seems to prevail in the far more remote planets Uranus and +Neptune. It has thus happened, that, although as telescopic objects of +interest and beauty, the marvellous rings of Saturn, the belts and +ever-changing aspects of the satellites of Jupiter, and the moon-like +phases of Venus, together with its extreme brilliancy, still remain +unsurpassed, yet the greater amount of details of these features when +examined with the powerful instruments of the nineteenth century have +neither added much to our knowledge of the planets themselves or led to +any sensational theories calculated to attract the popular imagination. + +[Footnote 1: Mercury also seems to have a scanty atmosphere, but as its +mass is only one-thirtieth that of the earth it can retain only the +heavier gases, and its atmosphere may be dust-laden, as is that of Mars, +according to Mr. Lowell. Its dusky markings, as seen by Schiaparelli, +seem to be permanent, and they are also for considerable periods +unchangeable in position, indicating that the planet keeps the same face +towards the sun as does Venus. This was confirmed by Mr. Lowell in 1896. +Its distance from us and unfavourable position for observation must +prevent us from obtaining any detailed knowledge of its actual surface, +though its low reflective power indicates that the surface may be really +visible.] + +But in the case of Mars the progress of discovery has had a very +different result. The most obvious peculiarity of this planet--its polar +snow-caps--were seen about 250 years ago, but they were first proved to +increase and decrease alternately, in the summer and winter of each +hemisphere, by Sir William Herschell in the latter part of the +eighteenth century. This fact gave the impulse to that idea of +similarity in the conditions of Mars and the earth, which the +recognition of many large dusky patches and streaks as water, and the +more ruddy and brighter portions as land, further increased. Added to +this, a day only about half an hour longer than our own, and a +succession of seasons of the same character as ours but of nearly double +the length owing to its much longer year, seemed to leave little wanting +to render this planet a true earth on a smaller scale. It was therefore +very natural to suppose that it must be inhabited, and that we should +some day obtain evidence of the fact. + +_The Canals discovered by Schiaparelli._ + +Hence the great interest excited when Schiaparelli, at the Milan +Observatory, during the very favourable opposition of 1877 and 1879, +observed that the whole of the tropical and temperate regions from 60 deg. +N. to 60 deg. S. Lat. were covered with a remarkable network of broader +curved and narrower straight lines of a dark colour. At each successive +favourable opposition, these strange objects called _canali_ (channels) +by their discoverer, but rather misleadingly 'canals' in England and +America, were observed by means of all the great telescopes in the +world, and their reality and general features became well established. +In Schiaparelli's first map they were represented as being much broader +and less sharply defined than he himself and other observers found by +later and equally favourable observations that they really were. + +_Discovery of the Double Canals._ + +In 1881 another strange feature was discovered by Schiaparelli, who +found that about twenty canals which had previously been seen single +were now distinctly double, that is, that they consisted of two parallel +lines, equally distinct and either very close together or a considerable +distance apart. This curious appearance was at first thought to be due +to some instrumental defect or optical illusion; but as it was soon +confirmed by other observers with the best instruments and in widely +different localities it became in time accepted as a real phenomenon of +the planet's surface. + +_Round Spots discovered in_ 1892. + +At the favourable opposition of 1892, Mr. W. H. Pickering noticed that +besides the 'seas' of various sizes there were numerous very small black +spots apparently quite circular and occurring at every intersection or +starting-point of the 'canals.' Many of these had been seen by +Schiaparelli as larger and ill-defined dark patches, and were termed +seas or lakes; but Mr. Pickering's observatory was at Arequipa in Peru, +about 8000 feet above the sea, and with such perfect atmospheric +conditions as were, in his opinion, equal to a doubling of telescopic +aperture. They were soon detected by other observers, especially by Mr. +Lowell in 1894, who thus wrote of them: + +"Scattered over the orange-ochre groundwork of the continental regions +of the planet, are any number of dark round spots. How many there may be +it is not possible to state, as the better the seeing, the more of them +there seem to be. In spite, however, of their great number, there is no +instance of one unconnected with a canal. What is more, there is +apparently none that does not lie at the junction of several canals. +Reversely, all the junctions appear to be provided with spots. Plotted +upon a globe they and their connecting canals make a most curious +network over all the orange-ochre equatorial parts of the planet, a mass +of lines and knots, the one marking being as omnipresent as the other." + +_Changes of Colour recognised._ + +During the oppositions of 1892 and 1894 it was fully recognised that a +regular course of change occurred dependent upon the succession of the +seasons, as had been first suggested by Schiaparelli. As the polar snows +melt the adjacent seas appear to overflow and spread out as far as the +tropics, and are often seen to assume a distinctly green colour. These +remarkable changes and the extraordinary phenomena of perfect straight +lines crossing each other over a large portion of the planet's surface, +with the circular spots at their intersections, had such an appearance +of artificiality that the idea that they were really 'canals' made by +intelligent beings for purposes of irrigation, was first hinted at, and +then adopted as the only intelligible explanation, by Mr. Lowell and a +few other persons. This at once seized upon the public imagination and +was spread by the newspapers and magazines over the whole civilised +world. + +_Existence of Seas doubted._ + +At this time (1894) it began to be doubted whether there were any seas +at all on Mars. Professor Pickering thought they were far more limited +in size than had been supposed, and even might not exist as true seas. +Professor Barnard, with the Lick thirty-six inch telescope, discerned an +astonishing wealth of detail on the surface of Mars, so intricate, +minute, and abundant, that it baffled all attempts to delineate it; and +these peculiarities were seen upon the supposed seas as well as on the +land-surfaces. In fact, under the best conditions these 'seas' lost all +trace of uniformity, their appearance being that of a mountainous +country, broken by ridges, rifts, and canyons, seen from a great +elevation. As we shall see later on these doubts soon became +certainties, and it is now almost universally admitted that Mars +possesses no permanent bodies of water. + + + +CHAPTER II. + + +MR. PERCIVAL LOWELL'S DISCOVERIES AND THEORIES. + +_The Observatory in Arizona._ + +In 1894, after a careful search for the best atmospheric conditions, Mr. +Lowell established his observatory near the town of Flagstaff in +Arizona, in a very dry and uniform climate, and at an elevation of 7300 +feet above the sea. He then possessed a fine equatorial telescope of 18 +inches aperture and 26 feet focal length, besides two smaller ones, all +of the best quality. To these he added in 1896 a telescope with 24 inch +object glass, the last work of the celebrated firm of Alvan Clark & +Sons, with which he has made his later discoveries. He thus became +perhaps more favourably situated than any astronomer in the northern +hemisphere, and during the last twelve years has made a specialty of the +study of Mars, besides doing much valuable astronomical work on other +planets. + +_Mr, Lowell's recent Books upon Mars._ + +In 1905 Mr. Lowell published an illustrated volume giving a full account +of his observations of Mars from 1894 to 1903, chiefly for the use of +astronomers; and he has now given us a popular volume summarising the +whole of his work on the planet, and published both in America and +England by the Macmillan Company. This very interesting volume is fully +illustrated with twenty plates, four of them coloured, and more than +forty figures in the text, showing the great variety of details from +which the larger general maps have been constructed. + +_Non-natural Features of Mars._ + +But what renders this work especially interesting to all intelligent +readers is, that the author has here, for the first time, fully set +forth his views both as to the habitability of Mars and as to its being +actually inhabited by beings comparable with ourselves in intellect. The +larger part of the work is in fact devoted to a detailed description of +what he terms the 'Non-natural Features' of the planet's surface, +including especially a full account of the 'Canals,' single and double; +the 'Oases,' as he terms the dark spots at their intersections; and the +varying visibility of both, depending partly on the Martian seasons; +while the five concluding chapters deal with the possibility of animal +life and the evidence in favour of it. He also upholds the theory of the +canals having been constructed for the purpose of 'husbanding' the +scanty water-supply that exists; and throughout the whole of this +argument he clearly shows that he considers the evidence to be +satisfactory, and that the only intelligible explanation of the whole of +the phenomena he so clearly sets forth is, that the inhabitants of Mars +have carried out on their small and naturally inhospitable planet a vast +system of irrigation-works, far greater both in its extent, in its +utility, and its effect upon their world as a habitation for civilised +beings, than anything we have yet done upon our earth, where our +destructive agencies are perhaps more prominent than those of an +improving and recuperative character. + +_A Challenge to the Thinking World._ + +This volume is therefore in the nature of a challenge, not so much to +astronomers as to the educated world at large, to investigate the +evidence for so portentous a conclusion. To do this requires only a +general acquaintance with modern science, more especially with mechanics +and physics, while the main contention (with which I shall chiefly deal) +that the features termed 'canals' are really works of art and +necessitate the presence of intelligent organic beings, requires only +care and judgment in drawing conclusions from admitted facts. As I have +already paid some attention to this problem and have expressed the +opinion that Mars is not habitable,[2] judging from the evidence then +available, and as few men of science have the leisure required for a +careful examination of so speculative a subject, I propose here to point +out what the facts, as stated by Mr. Lowell himself, do _not_ render +even probable much less prove. Incidentally, I may be able to adduce +evidence of a more or less weighty character, which seems to negative +the possibility of any high form of animal life on Mars, and, _a +fortiori_, the development of such life as might culminate in a being +equal or superior to ourselves. As most popular works on Astronomy for +the last ten years at least, as well as many scientific periodicals and +popular magazines, have reproduced some of the maps of Mars by +Schiaparelli, Lowell, and others, the general appearance of its surface +will be familiar to most readers, who will thus be fully able to +appreciate Mr. Lowell's account of his own further discoveries which I +may have to quote. One of the _best_ of these maps I am able to give as +a frontispiece to this volume, and to this I shall mainly refer. + +[Footnote 2: _Man's Place in the Universe_ p. 267 (1903).] + +_The Canals as described by Mr. Lowell._ + +In the clear atmosphere of Arizona, Mr. Lowell has been able on various +favourable occasions to detect a network of straight lines, meeting or +crossing each other at various angles, and often extending to a thousand +or even over two thousand miles in length. They are seen to cross both +the light and the dark regions of the planet's surface, often extending +up to or starting from the polar snow-caps. Most of these lines are so +fine as only to be visible on special occasions of atmospheric clearness +and steadiness, which hardly ever occur at lowland stations, even with +the best instruments, and almost all are seen to be as perfectly +straight as if drawn with a ruler. + +_The Double Canals._ + +Under exceptionally favourable conditions, many of the lines that have +been already seen single appear double--a pair of equally fine lines +exactly parallel throughout their whole length, and appearing, as Mr. +Lowell says, "clear cut upon the disc, its twin lines like the rails of +a railway track." Both Schiaparelli and Lowell were at first so +surprised at this phenomenon that they thought it must be an optical +illusion, and it was only after many observations in different years, +and by the application of every conceivable test, that they both became +convinced that they witnessed a real feature of the planet's surface. +Mr. Lowell says he has now seen them hundreds of times, and that his +first view of one was 'the most startlingly impressive' sight he has +ever witnessed. + +_Dimensions of the Canals._ + +A few dimensions of these strange objects must be given in order that +readers may appreciate their full strangeness and inexplicability. Out +of more than four hundred canals seen and recorded by Mr. Lowell, +fifty-one, or about one eighth, are either constantly or occasionally +seen to be double, the appearance of duplicity being more or less +periodical. Of 'canals' generally, Mr. Lowell states that they vary in +length from a few hundred to a few thousand miles long, one of the +largest being the Phison, which he terms 'a typical double canal,' and +which is said to be 2250 miles long, while the distance between its two +constituents is about 130 miles.[3] The actual width of each canal is +from a minimum of about a mile up to several miles, in one case over +twenty. A great feature of the doubles is, that they are strictly +parallel throughout their whole course, and that in almost all cases +they are so truly straight as to form parts of a great circle of the +planet's sphere. A few however follow a gradual but very distinct curve, +and such of these as are double present the same strict parallelism as +those which are straight. + +[Footnote 3: This is on the opposite side of Mars from that shown in the +frontispiece.] + +_Canals extend across the Seas._ + +It was only after seventeen years of observation of the canals that it +was found that they extended also into and across the dark spots and +surfaces which by the earlier observers were termed seas, and which then +formed the only clearly distinguishable and permanent marks on the +planet's surface. At the present time, Professor Lowell states that this +"curious triangulation has been traced over almost every portion of the +planet's surface, whether dark or light, whether greenish, ochre, or +brown in colour." In some parts they are much closer together than in +others, "forming a perfect network of lines and spots, so that to +identify them all was a matter of extreme difficulty." Two such portions +are figured at pages 247 and 256 of Mr. Lowell's volume. + +_The Oases._ + +The curious circular black spots which are seen at the intersections of +many of the canals, and which in some parts of the surface are very +numerous, are said to be more difficult of detection than even the +lines, being often blurred or rendered completely invisible by slight +irregularities in our own atmosphere, while the canals themselves +continue visible. About 180 of these have now been found, and the more +prominent of them are estimated to vary from 75 to 100 miles in +diameter. There are however many much smaller, down to minute and barely +visible black points. Yet they all seem a little larger than the canals +which enter them. Where the canals are double, the spots (or 'oases' as +Mr. Lowell terms them) lie between the two parallel canals. + +No one can read this book without admiration for the extreme +perseverance in long continued and successful observation, the results +of which are here recorded; and I myself accept unreservedly the +substantial accuracy of the whole series. It must however always be +remembered that the growth of knowledge of the detailed markings has +been very gradual, and that much of it has only been seen under very +rare and exceptional conditions. It is therefore quite possible that, if +at some future time a further considerable advance in instrumental power +should be made, or a still more favourable locality be found, the new +discoveries might so modify present appearances as to render a +satisfactory explanation of them more easy than it is at present. + +But though I wish to do the fullest justice to Mr. Lowell's technical +skill and long years of persevering work, which have brought to light +the most complex and remarkable appearances that any of the heavenly +bodies present to us, I am obliged absolutely to part company with him +as regards the startling theory of artificial production which he thinks +alone adequate to explain them. So much is this the case, that the very +phenomena, which to him seem to demonstrate the intervention of +intelligent beings working for the improvement of their own environment, +are those which seem to me to bear the unmistakable impress of being due +to natural forces, while they are wholly unintelligible as being useful +works of art. I refer of course to the great system of what are termed +'canals,' whether single or double. Of these I shall give my own +interpretation later on. + + + +CHAPTER III. + + +THE CLIMATE AND PHYSIOGRAPHY OF MARS. + +Mr. Lowell admits, and indeed urges strongly, that there are no +permanent bodies of water on Mars; that the dark spaces and spots, +thought by the early observers to be seas, are certainly not so now, +though they may have been at an earlier period; that true clouds are +rare, even if they exist, the appearances that have been taken for them +being either dust-storms or a surface haze; that there is consequently +no rain, and that large portions (about two-thirds) of the planet's +surface have all the characteristics of desert regions. + +_Snow-caps the only Source of Water._ + +This state of things is supposed to be ameliorated by the fact of the +polar snows, which in the winter cover the arctic and about half the +temperate regions of each hemisphere alternately. The maximum of the +northern snow-caps is reached at a period of the Martian winter +corresponding to the end of February with us. About the end of March the +cap begins to shrink in size (in the Northern Hemisphere), and this goes +on so rapidly that early in the June of Mars it is reduced to its +minimum. About the same time changes of colour take place in the +adjacent darker portions of the surface, which become at first bluish, +and later a decided blue-green; but by far the larger portion, including +almost all the equatorial regions of the planet, remain always of a +reddish-ochre tint.[4] + +[Footnote 4: In 1890 at Mount Wilson, California, Mr. W.H. Pickering's +photographs of Mars on April 9th showed the southern polar cap of +moderate dimensions, but with a large dim adjacent area. Twenty-four +hours later a corresponding plate showed this same area brilliantly +white; the result apparently of a great Martian snowfall. In 1882 the +same observer witnessed the steady disappearance of 1,600,000 square +miles of the southern snow-cap, an area nearly one-third of that +hemisphere of the planet.] + +The rapid and comparatively early disappearance of the white covering +is, very reasonably, supposed to prove that it is of small thickness, +corresponding perhaps to about a foot or two of snow in north-temperate +America and Europe, and that by the increasing amount of sun-heat it is +converted, partly into liquid and partly into vapour. Coincident with +this disappearance and as a presumed result of the water (or other +liquid) producing inundations, the bluish-green tinge which appears on +the previously dark portion of the surface is supposed to be due to a +rapid growth of vegetation. + +But the evidence on this point does not seem to be clear or harmonious, +for in the four coloured plates showing the planet's surface at +successive Martian dates from December 30th to February 21st, not only +is a considerable extent of the south temperate zone shown to change +rapidly from bluish-green to chocolate-brown and then again to +bluish-green, but the portions furthest from the supposed fertilising +overflow are permanently green, as are also considerable portions in the +opposite or northern hemisphere, which one would think would then be +completely dried up. + +_No Hills upon Mars._ + +The special point to which I here wish to call attention is this. Mr. +Lowell's main contention is, that the surface of Mars is wonderfully +smooth and level. Not only are there no mountains, but there are no +hills or valleys or plateaux. This assumption is absolutely essential to +support the other great assumption, that the wonderful network of +perfectly straight lines over nearly the whole surface of the planet are +irrigation canals. It is not alleged that irregularities or undulations +of a few hundreds or even one or two thousands of feet could possibly be +detected, while certainly all we know of planetary formation or +structure point strongly towards _some_ inequalities of surface. Mr. +Lowell admits that the dark portions of the surface, when examined on +the terminator (the margin of the illuminated portion), do _look_ like +hollows and _may be_ the beds of dried-up seas; yet the supposed canals +run across these old sea-beds in perfect straight lines just as they do +across the many thousand miles of what are admitted to be deserts--which +he describes in these forcible terms: "Pitiless as our deserts are, they +are but faint forecasts of the state of things existent on Mars at the +present time." + +It appears, then, that Mr. Lowell has to face this dilemma--_Only if the +whole surface of Mars is an almost perfect level could the enormous +network of straight canals, each from hundreds to thousands of miles +long, have been possibly constructed by intelligent beings for purposes +of irrigation; but, if a complete and universal level surface exists no +such system would be necessary._ For on a level surface--or on a +surface slightly inclined from the poles towards the equator, which +would be advantageous in either case--the melting water would of itself +spread over the ground and naturally irrigate as much of the surface as +it was possible for it to reach. If the surface were not level, but +consisted of slight elevations and expressions to the extent of a few +scores or a few hundreds of feet, then there would be no possible +advantage in cutting straight troughs through these elevations in +various directions with water flowing at the bottom of them. In neither +case, and in hardly any conceivable case, could these perfectly straight +canals, cutting across each other in every direction and at very varying +angles, be of any use, or be the work of an intelligent race, if any +such race could possibly have been developed under the adverse +conditions which exist in Mars. + +_The Scanty Water-supply._ + +But further, if there were any superfluity of water derived from the +melting snow beyond what was sufficient to moisten the hollows indicated +by the darker portions of the surface, which at the time the water +reaches them acquire a green tint (a superfluity under the circumstances +highly improbable), that superfluity could be best utilised by widening, +however little, the borders to which natural overflow had carried it. +Any attempt to make that scanty surplus, by means of overflowing canals, +travel across the equator into the opposite hemisphere, through such a +terrible desert region and exposed to such a cloudless sky as Mr. Lowell +describes, would be the work of a body of madmen rather than of +intelligent beings. It may be safely asserted that not one drop of water +would escape evaporation or insoak at even a hundred miles from its +source. [5] + +[Footnote 5: What the evaporation is likely to be in Mars may be +estimated by the fact, stated by Professor J.W. Gregory in his recent +volume on 'Australia' in _Stanford's Compendium_, that in North-West +Victoria evaporation is at the rate of ten feet per annum, while in +Central Australia it is very much more. The greatly diminished +atmospheric pressure in Mars will probably more than balance the loss of +sun-heat in producing rapid evaporation.] + +_Miss Clerke on the Scanty Water-supply._ + +On this point I am supported by no less an authority than the historian +of modern astronomy, the late Miss Agnes Clerke. In the _Edinburgh +Review_ (of October 1896) there is an article entitled 'New Views about +Mars,' exhibiting the writer's characteristic fulness of knowledge and +charm of style. Speaking of Mr. Lowell's idea of the 'canals' carrying +the surplus water across the equator, far into the opposite hemisphere, +for purposes of irrigation there (which we see he again states in the +present volume), Miss Clerke writes: "We can hardly imagine so shrewd a +people as the irrigators of Thule and Hellas[6] wasting labour, and the +life-giving fluid, after so unprofitable a fashion. There is every +reason to believe that the Martian snow-caps are quite flimsy +structures. Their material might be called snow _souffle_, since, owing +to the small power of gravity on Mars, snow is almost three times +lighter there than here. Consequently, its own weight can have very +little effect in rendering it compact. Nor, indeed, is there time for +much settling down. The calotte does not form until several months after +the winter solstice, and it begins to melt, as a rule, shortly after the +vernal equinox. (The interval between these two epochs in the southern +hemisphere of Mars is 176 days.) The snow lies on the ground, at the +outside, a couple of months. At times it melts while it is still fresh +fallen. Thus, at the opposition of 1881-82 the spreading of the northern +snows was delayed until seven weeks after the equinox: and they had, +accordingly, no sooner reached their maximum than they began to decline. +And Professor Pickering's photographs of April 9th and 10th, 1890, +proved that the southern calotte may assume its definitive proportions +in a single night. + +[Footnote 6: Areas on Mars so named.] + +"No attempt has yet been made to estimate the quantity of water +derivable from the melting of one of these formations; yet the +experiment is worth trying as a help towards defining ideas. Let us +grant that the average depth of snow in them, of the delicate Martian +kind, is twenty feet, equivalent at the most to one foot of water. The +maximum area covered, of 2,400,000 square miles, is nearly equal to that +of the United States, while the whole globe of Mars measures 55,500,000 +square miles, of which one-third, on the present hypothesis, is under +cultivation, and in need of water. Nearly the whole of the dark areas, +as we know, are situated in the southern hemisphere, of which they +extend over, at the very least, 17,000,000 square miles; that is to say, +they cover an area, in round numbers, seven times that of the snow-cap. +Only one-seventh of a foot of water, accordingly, could possibly be +made available for their fertilisation, supposing them to get the entire +advantage of the spring freshet. Upon a stint of less than two inches of +water these fertile lands are expected to flourish and bear abundant +crops; and since they completely enclose the polar area they are +necessarily served first. The great emissaries for carrying off the +surplus of their aqueous riches, would then appear to be superfluous +constructions, nor is it likely that the share in those riches due to +the canals and oases, intricately dividing up the wide, dry, continental +plains, can ever be realised. + +"We have assumed, in our little calculation, that the entire contents of +a polar hood turn to water; but in actual fact a considerable proportion +of them must pass directly into vapour, omitting the intermediate stage. +Even with us a large quantity of snow is removed aerially; and in the +rare atmosphere of Mars this cause of waste must be especially +effective. Thus the polar reservoirs are despoiled in the act of being +opened. Further objections might be taken to Mr. Lowell's irrigation +scheme, but enough has been said to show that it is hopelessly +unworkable." + +It will be seen that the writer of this article accepted the existence +of water on Mars, on the testimony of Sir W. Huggins, which, in view of +later observations, he has himself acknowledged to be valueless. Dr. +Johnstone Stoney's proof of its absence, derived from the molecular +theory of gases, had not then been made public. + +_Description of some of the Canals._ + +At the end of his volume Mr. Lowell gives a large chart of Mars on +Mercator's projection, showing the canals and other features seen during +the opposition of 1905. This contains many canals not shown on the map +here reproduced (see frontispiece), and some of the differences between +the two are very puzzling. Looking at our map, which shows the +north-polar snow below, so that the south pole is out of the view at the +top of the map, the central feature is the large spot Ascraeeus Lucus, +from which ten canals diverge centrally, and four from the sides, +forming wide double canals, fourteen in all. There is also a canal named +Ulysses, which here passes far to the right of the spot, but in the +large chart enters it centrally. Looking at our map we see, going +downwards a little to the left, the canal Udon, which runs through a +dark area quite to the outer margin. In the dark area, however, there is +shown on the chart a spot Aspledon Lucus, where five canals meet, and if +this is taken as a terminus the Udon canal is almost exactly 2000 miles +long, and another on its right, Lapadon, is the same length, while Ich, +running in a slightly curved line to a large spot (Lucus Castorius on +the chart) is still longer. The Ulysses canal, which (on the chart) runs +straight from the point of the Mare Sirenum to the Astraeeus Lucus is +about 2200 miles long. Others however are even longer, and Mr. Lowell +says: "With them 2000 miles is common; while many exceed 2500; and the +Eumenides-Orcus is 3540 miles from the point where it leaves Lucus +Phoeniceus to where it enters the Trivium Charontis." This last canal is +barely visible on our map, its commencement being indicated by the word +Eumenides. + +The Trivium Charontis is situated just beyond the right-hand margin of +our map. It is a triangular dark area, the sides about 200 miles long, +and it is shown on the chart as being the centre from which radiate +thirteen canals. Another centre is Aquae Calidae situated at the point +of a dark area running obliquely from 55 deg. to 35 deg. N. latitude, and, as +shown on a map of the opposite hemisphere to our map, has nearly twenty +canals radiating from it in almost every direction. Here at all events +there seems to be no special connection with the polar snow-caps, and +the radiating lines seem to have no intelligent purpose whatever, but +are such as might result from fractures in a glass globe produced by +firing at it with very small shots one at a time. Taking the whole +series of them, Mr. Lowell very justly compares them to "a network which +triangulates the surface of the planet like a geodetic survey, into +polygons of all shapes and sizes." + +At the very lowest estimate the total length of the canals observed and +mapped by Mr. Lowell must be over a hundred thousand miles, while he +assures us that numbers of others have been seen over the whole surface, +but so faintly or on such rare occasions as to elude all attempts to fix +their position with certainty. But these, being of the same character +and evidently forming part of the same system, must also be artificial, +and thus we are led to a system of irrigation of almost unimaginable +magnitude on a planet which has no mountains, no rivers, and no rain to +support it; whose whole water-supply is derived from polar snows, the +amount of which is ludicrously inadequate to need any such world-wide +system; while the low atmospheric pressure would lead to rapid +evaporation, thus greatly diminishing the small amount of moisture that +is available. Everyone must, I think, agree with Miss Clerke, that, even +admitting the assumption that the polar snows consist of frozen water, +the excessively scanty amount of water thus obtained would render any +scheme of world-wide distribution of it hopelessly unworkable. + +The very remarkable phenomena of the duplication of many of the lines, +together with the darkspots--the so-called oases--at their +intersections, are doubtless all connected in some unknown way with the +constitution and past history of the planet; but, on the theory of the +whole being works of art, they certainly do _not_ help to remove any of +the difficulties which have been shown to attend the theory that the +single lines represent artificial canals of irrigation with a strip of +verdure on each side of them produced by their overflow. + +_Lowell on the Purpose of the Canals._ + +Before leaving this subject it will be well to quote Mr. Lowell's own +words as to the supposed perfectly level surface of Mars, and his +interpretation of the origin and purpose of the 'canals': + +"A body of planetary size, if unrotating, becomes a sphere, except for +solar tidal deformation; if rotating, it takes on a spheroidal form +exactly expressive, so far as observation goes, of the so-called +centrifugal force at work. Mars presents such a figure, being flattened +out to correspond to its axial rotation. Its surface therefore is in +fluid equilibrium, or, in other words, a particle of liquid at any point +of its surface at the present time would stay where it was devoid of +inclination to move elsewhere. Now the water which quickens the verdure +of the canals moves from the pole down to the equator as the season +advances. This it does then irrespective of gravity. No natural force +propels it, and the inference is forthright and inevitable that it is +artificially helped to its end. There seems to be no escape from this +deduction. Water only flows downhill, and there is no such thing as +downhill on a surface already in fluid equilibrium. A few canals might +presumably be so situated that their flow could, by inequality of +terrane, lie equatorward, but not all....Now it is not in particular but +by general consent that the canal-system of Mars develops from pole to +equator. From the respective times at which the minima take place, it +appears that the canal quickening occupies fifty-two days, as evidenced +by the successive vegetal darkenings, to descend from latitude 72 deg. north +to latitude 0 deg., a journey of 2650 miles. This gives for the water a +speed of fifty-one miles a day, or 2.1 miles an hour. The rate of +progression is remarkably uniform, and this abets the deduction as to +assisted transference. But the fact is more unnatural yet. The growth +pays no regard to the equator, but proceeds across it as if it did not +exist into the planet's other hemisphere. Here is something still more +telling than travel to this point. For even if we suppose, for the sake +of argument, that natural forces took the water down to the equator, +their action must there be certainly reversed, and the equator prove a +dead-line, to pass which were impossible" (pp. 374-5). + +I think my readers will agree with me that this whole argument is one of +the most curious ever put forth seriously by an eminent man of science. +Because the polar compression of Mars is about what calculation shows it +ought to be in accordance with its rate of rotation, its surface is in a +state of 'fluid equilibrium,' and must therefore be absolutely level +throughout. But the polar compression of the earth equally agrees with +calculation; therefore its surface is also in 'fluid equilibrium'; +therefore it also ought to be as perfectly level on land as it is on the +ocean surface! But as we know this is very far from being the case, why +must it be so in Mars? Are we to suppose Mars to have been formed in +some totally different way from other planets, and that there neither is +nor ever has been any reaction between its interior and exterior forces? +Again, the assumption of perfect flatness is directly opposed to all +observation and all analogy with what we see on the earth and moon. It +gives no account whatever of the numerous and large dark patches, once +termed seas, but now found to be not so, and to be full of detailed +markings and varied depths of shadow. To suppose that these are all the +same dead-level as the light-coloured portions are assumed to be, +implies that the darkness is one of material and colour only, not of +diversified contour, which again is contrary to experience, since +difference of material with us always leads to differences in rate of +degradation, and hence of diversified contour, as these dark spaces +actually show themselves under favourable conditions to independent +observers. + +_Lowell on the System of Canals as a whole._ + +We will now see what Mr. Lowell claims to be the plain teaching of the +'canals' as a whole: + +"But last and all-embracing in its import is the system which the canals +form. Instead of running at hap-hazard, the canals are interconnected in +a most remarkable manner. They seek centres instead of avoiding them. +The centres are linked thus perfectly one with another, an arrangement +which could not result from centres, whether of explosion or otherwise, +which were themselves discrete. Furthermore, the system covers the whole +surface of the planet, dark areas and light ones alike, a world-wide +distribution which exceeds the bounds of natural possibility. Any force +which could act longitudinally on such a scale must be limited +latitudinally in its action, as witness the belts of Jupiter and the +spots upon the sun. Rotational, climatic, or other physical cause could +not fail of zonal expression. Yet these lines are grandly indifferent to +such competing influences. Finally, the system, after meshing the +surface in its entirety, runs straight into the polar caps. + +"It is, then, a system whose end and aim is the tapping of the snow-cap +for the water there semi-annually let loose; then to distribute it over +the planet's face" (p. 373). + +Here, again, we have curiously weak arguments adduced to support the +view that these numerous straight lines imply works of art rather than +of nature, especially in the comparison made with the belts of Jupiter +and the spots on the sun, both purely atmospheric phenomena, whereas the +lines on Mars are on the solid surface of the planet. Why should there +be any resemblance between them? Every fact stated in the above +quotation, always keeping in mind the physical conditions of the +planet--its very tenuous atmosphere and rainless desert-surface--seem +wholly in favour of a purely natural as opposed to an artificial origin; +and at the close of this discussion I shall suggest one which seems to +me to be at least possible, and to explain the whole series of the +phenomena set forth and largely discovered by Mr. Lowell, in a simpler +and more probable manner than does his tremendous assumption of their +being works of art. Readers who may not possess Mr. Lowell's volume will +find three of his most recent maps of the 'canals' reproduced in +_Nature_ of October 11th, 1906. + + + +CHAPTER IV. + + +IS ANIMAL LIFE POSSIBLE ON MARS? + +Having now shown, that, even admitting the accuracy of all Mr. Lowell's +observations, and provisionally accepting all his chief conclusions as +to the climate, the nature of the snow-caps, the vegetation, and the +animal life of Mars, yet his interpretation of the lines on its surface +as being veritably 'canals,' constructed by intelligent beings for the +special purpose of carrying water to the more arid regions, is wholly +erroneous and rationally inconceivable. I now proceed to discuss his +more fundamental position as to the actual habitability of Mars by a +highly organised and intellectual race of material organic beings. + +_Water and Air essential to Life._ + +Here, fortunately, the issue is rendered very simple, because Mr. Lowell +fully recognises the identity of the constitution of matter and of +physical laws throughout the solar-system, and that for any high form of +organic life certain conditions which are absolutely essential on our +earth must also exist in Mars. He admits, for example, that water is +essential, that an atmosphere containing oxygen, nitrogen, aqueous +vapour, and carbonic acid gas is essential, and that an abundant +vegetation is essential; and these of course involve a +surface-temperature through a considerable portion of the year that +renders the existence of these--especially of water--possible and +available for the purposes of a high and abundant animal life. + +_Blue Colour the only Evidence of Water._ + +In attempting to show that these essentials actually exist on Mars he is +not very successful. He adduces evidence of an atmosphere, but of an +exceedingly scanty one, since the greatest amount he can give to it is-- +"not more than about four inches of barometric pressure as we reckon +it";[7] and he assumes, as he has a fair right to do till disproved, +that it consists of oxygen and nitrogen, with carbon-dioxide and +water-vapour, in approximately the same proportions as with us. With +regard to the last item--the water-vapour--there are however many +serious difficulties. The water-vapour of our atmosphere is derived from +the enormous area of our seas, oceans, lakes, and rivers, as well as +from the evaporation from heated lands and tropical forests of much of +the moisture produced by frequent and abundant rains. All these sources +of supply are admittedly absent from Mars, which has no permanent bodies +of water, no rain, and tropical regions which are almost entirely +desert. Many writers have therefore doubted the existence of water in +any form upon this planet, supposing that the snow-caps are not formed +of frozen water but of carbon-dioxide, or some other heavy gas, in a +frozen state; and Mr. Lowell evidently feels this to be a difficulty, +since the only fact he is able to adduce in favour of the melting snows +of the polar caps producing water is, that at the time they are melting +a marginal blue band appears which accompanies them in their retreat, +and this blue colour is said to prove conclusively that the liquid is +not carbonic acid but water. This point he dwells upon repeatedly, +stating, of these blue borders: "This excludes the possibility of their +being formed by carbon-dioxide, and shows that of all the substances we +know the material composing them must be water." + +[Footnote 7: In a paper written since the book appeared the density of +air at the surface of Mars is said to be 1/12 of the earth's.] + +This is the only proof of the existence of _water_ he adduces, and it is +certainly a most extraordinary and futile one. For it is perfectly well +known that although water, in large masses and by transmitted light, is +of a blue colour, yet shallow water by reflected light is not so; and in +the case of the liquid produced by the snow-caps of Mars, which the +whole conditions of the planet show must be shallow, and also be more or +less turbid, it cannot possibly be the cause of the 'deep blue' tint +said to result from the melting of the snow. + +But there is a very weighty argument depending on the molecular theory +of gases against the polar caps of Mars being composed of frozen water +at all. The mass and elastic force of the several gases is due to the +greater or less rapidity of the vibratory motion of their molecules +under identical conditions. The speed of these molecular motions has +been ascertained for all the chief gases, and it is found to be so great +as in certain cases to enable them to overcome the force of gravity and +escape from a planet's surface into space. Dr. G. Johnstone Stoney has +specially investigated this subject, and he finds that the force of +gravity on the earth is sufficient to retain all the gases composing its +atmosphere, but not sufficient to retain hydrogen; and as a consequence, +although this gas is produced in small quantities by volcanoes and by +decomposing vegetation, yet no trace of it is found in our atmosphere. +The moon however, having only one-eightieth the mass of the earth, +cannot retain any gas, hence its airless and waterless condition. + +_Water Vapour cannot exist on Mars._ + +Now, Dr. Stoney finds that in order to retain water vapour permanently a +planet must have a mass at least a quarter that of the earth. But the +mass of Mars is only one-ninth that of the earth; therefore, unless +there are some special conditions that prevent its loss, this gas cannot +be present in the atmosphere. Mr. Lowell does not refer to this argument +against his view, neither does he claim the evidence of spectroscopy in +his favour. This was alleged more than thirty years ago to show the +existence of water-vapour in the atmosphere of Mars, but of late years +it has been doubted, and Mr. Lowell's complete silence on the subject, +while laying stress on such a very weak and inconclusive argument as +that from the tinge of colour that is observed a little distance from +the edge of the diminishing snow-caps, shows that he himself does not +think the fact to be thus proved. If he did he would hardly adduce such +an argument for its presence as the following: "The melting of the caps +on the one hand and their re-forming on the other affirm the presence of +water-vapour in the Martian atmosphere, of whatever else that air +consists" (p. 162). Yet absolutely the only proof he gives that the caps +are frozen water is the almost frivolous colour-argument above referred +to! + +_No Spectroscopic Evidence of Water Vapour._ + +As Sir William Huggins is the chief authority quoted for this fact, and +is referred to as being almost conclusive in the third edition of Miss +Clerke's _History of Astronomy_ in 1893, I have ascertained that his +opinion at the present time is that "there is no conclusive proof of the +presence of aqueous vapour in the atmosphere of Mars, and that +observations at the Lick Observatory (in 1895), where the conditions and +instruments are of the highest order, were negative." He also informs me +that Marchand at the Pic du Midi Observatory was unable to obtain lines +of aqueous vapour in the spectrum of Mars; and that in 1905, Slipher, at +Mr. Lowell's observatory, was unable to detect any indications of +aqueous vapour in the spectrum of Mars. + +It thus appears that spectroscopic observations are quite accordant with +the calculations founded on the molecular theory of gases as to the +absence of aqueous vapour, and therefore presumably of liquid water, +from Mars. It is true that the spectroscopic argument is purely +negative, and this may be due to the extreme delicacy of the +observations required; but that dependent on the inability of the force +of gravity to retain it is positive scientific evidence against its +presence, and, till shown to be erroneous, must be held to be +conclusive. + +This absence of water is of itself conclusive against the existence of +animal life, unless we enter the regions of pure conjecture as to the +possibility of some other liquid being able to take its place, and that +liquid being as omnipresent there as water is here. Mr. Lowell however +never takes this ground, but bases his whole theory on the fundamental +identity of the substance of the bodies of living organisms wherever +they may exist in the solar system. In the next two chapters I shall +discuss an equally essential condition, that of temperature, which +affords a still more conclusive and even crushing argument against the +suitability of Mars for the existence of organic life. + + + +CHAPTER V. + + +THE TEMPERATURE OF MARS--MR. LOWELL'S ESTIMATE. + +We have now to consider a still more important and fundamental question, +and one which Mr. Lowell does not grapple with in this volume, the +actual temperatures on Mars due to its distance from the sun and the +atmospheric conditions on which he himself lays so much stress. If I am +not greatly mistaken we shall arrive at conclusions on this subject +which are absolutely fatal to the conception of any high form of organic +life being possible on this planet. + +_The Problem of Terrestrial Temperatures._ + +In order that the problem may be understood and its importance +appreciated, it is necessary to explain the now generally accepted +principles as to the causes which determine the temperatures on our +earth, and, presumably, on all other planets whose conditions are not +wholly unlike ours. The fact of the internal heat of the earth which +becomes very perceptible even at the moderate depths reached in mines +and deep borings, and in the deepest mines becomes a positive +inconvenience, leads many people to suppose that the surface- +temperatures of the earth are partly due to this cause. But it is now +generally admitted that this is not the case, the reason being that all +rocks and soils, in their natural compacted state, are exceedingly bad +conductors of heat. + +A striking illustration of this is the fact, that a stream of lava often +continues to be red hot at a few feet depth for years after the surface +is consolidated, and is hardly any warmer than that of the surrounding +land. A still more remarkable case is that of a glacier on the +south-east side of the highest cone of Etna underneath a lava stream +with an intervening bed of volcanic sand only ten feet thick. This was +visited by Sir Charles Lyell in 1828, and a second time thirty years +later, when he made a very careful examination of the strata, and was +quite satisfied that the sand and the lava stream together had actually +preserved this mass of ice, which neither the heat of the lava above it +at its first outflow, nor the continued heat rising from the great +volcano below it, had been able to melt or perceptibly to diminish in +thirty years. Another fact that points in the same direction is the +existence over the whole floor of the deepest oceans of ice-cold water, +which, originating in the polar seas, owing to its greater density sinks +and creeps slowly along the ocean bottom to the depths of the Atlantic +and Pacific, and is not perceptibly warmed by the internal heat of the +earth. + +Now the solid crust of the earth is estimated as at least about twenty +miles in average thickness; and, keeping in mind the other facts just +referred to, we can understand that the heat from the interior passes +through it with such extreme slowness as not to be detected at the +surface, the varying temperatures of which are due entirely to solar +heat. A large portion of this heat is stored up in the surface soil, and +especially in the surface layer of the oceans and seas, thus partly +equalising the temperatures of day and night, of winter and summer, so +as greatly to ameliorate the rapid changes of temperature that would +otherwise occur. Our dense atmosphere is also of immense advantage to us +as an equaliser of temperature, charged as it almost always is with a +large quantity of water-vapour. This latter gas, when not condensed into +cloud, allows the solar heat to pass freely to the earth; but it has the +singular and highly beneficial property of absorbing and retaining the +dark or lower-grade heat given off by the earth which would otherwise +radiate into space much more rapidly. We must therefore always remember +that, very nearly if not quite, the _whole_ of _the warmth we experience +on the earth is derived from the sun._[8] + +[Footnote 8: Professor J.H. Poynting, in his lecture to the British +Association at Cambridge in 1904, says: "The surface of the earth +receives, we know, an amount of heat from the inside almost +infinitesimal compared with that which it receives from the sun, and on +the sun, therefore, we depend for our temperature."] + +In order to understand the immense significance of this conclusion we +must know what is meant by the _whole_ heat or warmth; as unless we know +this we cannot define what half or any other proportion of sun-heat +really means. Now I feel pretty sure that nine out of ten of the average +educated public would answer the following question incorrectly: The +mean temperature of the southern half of England is about 48 deg. F. +Supposing the earth received only half the sun-heat it now receives, +what would then be the probable mean temperature of the South of +England? The majority would, I think, answer at once--About 24 deg. F. +Nearly as many would perhaps say--48 deg. F. is 16 deg. above the freezing +point; therefore half the heat received would bring us down to 8 deg. above +the freezing point, or 40 deg. F. Very few, I think, would realise that our +share of half the amount of sun-heat received by the earth would +probably result in reducing our mean temperature to about 100 deg. F. below +the freezing point, and perhaps even lower. This is about the very +lowest temperature yet experienced on the earth's surface. To understand +how such results are obtained a few words must be said about the +absolute zero of temperature. + +_The Zero of Temperature._ + +Heat is now believed to be entirely due to ether-vibration, which +produces a correspondingly rapid vibration of the molecules of matter, +causing it to expand and producing all the phenomena we term 'heat.' We +can conceive this vibration to increase indefinitely, and thus there +would appear to be no necessary limit to the amount of heat possible, +but we cannot conceive it to decrease indefinitely at the same uniform +rate, as it must soon inevitably come to nothing. Now it has been found +by experiment that gases under uniform pressure expand 1/273 of their +volume for each degree Centigrade of increased temperature, so that in +passing from 0 deg. C. to 273 deg. C. they are doubled in volume. They also +decrease in volume at the same rate for each degree below 0 deg. C. (the +freezing point of water). Hence if this goes on to-273 deg. C. a gas will +have no volume, or it will undergo some change of nature. Hence this is +called the zero of temperature, or the temperature to which any matter +falls which receives no heat from any other matter. It is also sometimes +called the temperature of space, or of the ether in a state of rest, if +that is possible. All the gases have now been proved to become, first +liquid and then (most of them) solid, at temperatures considerably above +this zero. + +The only way to compare the proportional temperatures of bodies, whether +on the earth or in space, is therefore by means of a scale beginning at +this natural zero, instead of those scales founded on the artificial +zero of the freezing point of water, or, as in Fahrenheit's, 32 deg. below +it. Only by using the natural zero and measuring continuously from it +can we estimate temperatures in relative proportion to the amount of +heat received. This is termed the absolute zero, and so that we start +reckoning from that point it does not matter whether the scale adopted +is the Centigrade or that of Fahrenheit. + +_The Complex Problem of Planetary Temperatures._ + +Now if, as is the case with Mars, a planet receives only half the amount +of solar heat that we receive, owing to its greater distance from the +sun, and if the mean temperature of our earth is 60 deg. F., this is equal +to 551 deg. F. on the absolute scale. It would therefore appear very simple +to halve this amount and obtain 275.5 deg. F. as the mean temperature of +that planet. But this result is erroneous, because the actual amount of +sun heat intercepted by a planet is only one condition out of many that +determine its resulting temperature. Radiation, that is loss of heat, is +going on concurrently with gain, and the rate of loss varies with the +temperature according to a law recently discovered, the loss being much +greater at high temperatures in proportion to the 4th power of the +absolute temperature. Then, again, the whole heat intercepted by a +planet does not reach its surface unless it has no atmosphere. When it +has one, much is reflected or absorbed according to complex laws +dependent on the density and composition of the atmosphere. Then, again, +the heat that reaches the actual surface is partly reflected and partly +absorbed, according to the nature of that surface--land or water, desert +or forest or snow-clad--that part which is absorbed being the chief +agent in raising the temperature of the surface and of the air in +contact with it. Very important too is the loss of heat by radiation +from these various heated surfaces at different rates; while the +atmosphere itself sends back to the surface an ever varying portion of +both this radiant and reflected heat according to distinct laws. Further +difficulties arise from the fact that much of the sun's heat consists of +dark or invisible rays, and it cannot therefore be measured by the +quantity of light only. + +From this rough statement it will be seen that the problem is an +exceedingly complex one, not to be decided off-hand, or by any simple +method. It has in fact been usually considered as (strictly speaking) +insoluble, and only to be estimated by a more or less rough +approximation, or by the method of general analogy from certain known +facts. It will be seen, from what has been said in previous chapters, +that Mr. Lowell, in his book, has used the latter method, and, by taking +the presence of water and water-vapour in Mars as proved by the +behaviour of the snow-caps and the bluish colour that results from their +melting, has deduced a temperature above the freezing point of water, as +prevalent in the equatorial regions permanently, and in the temperate +and arctic zones during a portion of each year. + +_Mr. Lowell's Mathematical Investigation of the Problem._ + +But as this result has been held to be both improbable in itself and +founded on no valid evidence, he has now, in the _London, Edinburgh, and +Dublin Philosophical Magazine_ of July 1907, published an elaborate +paper of 15 pages, entitled _A General Method for Evaluating the +Surface-Temperatures of the Planets; with special reference to the +Temperature of Mars_, by Professor Percival Lowell; and in this paper, +by what purports to be strict mathematical reasoning based on the most +recent discoveries as to the laws of heat, as well as on measurements or +estimates of the various elements and constants used in the +calculations, he arrives at a conclusion strikingly accordant with that +put forward in the recently published volume. Having myself neither +mathematical nor physical knowledge sufficient to enable me to criticise +this elaborate paper, except on a few points, I will here limit myself +to giving a short account of it, so as to explain its method of +procedure; after which I may add a few notes on what seem to me doubtful +points; while I also hope to be able to give the opinions of some more +competent critics than myself. + +_Mr. Lowell's Mode of Estimating the Surface-temperature of Mars._ + +The author first states, that Professor Young, in his _General +Astronomy_ (1898), makes the mean temperature of Mars 223.6 deg. absolute, +by using Newton's law of heat being radiated in proportion to +temperature, and 363 deg. abs. (=-96 deg. F.) by Dulong and Petit's law; but +adds, that a closer determination has been made by Professor Moulton, +using Stefan's law, that radiation is as the _/4th_ power of the +temperature, whence results a mean temperature of-31 deg. F. These estimates +assume identity of atmospheric conditions of Mars and the Earth. + +But as none of these estimates take account of the many complex factors +which interfere with such direct and simple calculations, Mr. Lowell +then proceeds to enunciate them, and work out mathematically the effects +they produce: + +(1) The whole radiant energy of the sun on striking a planet becomes +divided as follows: Part is reflected back into space, part absorbed by +the atmosphere, part transmitted to the surface of the planet. This +surface again reflects a portion and only the balance left goes to warm +the planet. + +(2) To solve this complex problem we are helped by the _albedoes_ or +intrinsic brilliancy of the planets, which depend on the proportion of +the visible rays which are reflected and which determines the +comparative brightness of their respective surfaces. We also have to +find the ratio of the invisible to the visible rays and the heating +power of each. + +(3) He then refers to the actinometer and pyroheliometer, instruments +for measuring the actual heat derived from the sun, and also to the +Bolometer, an instrument invented by Professor Langley for measuring the +invisible heat rays, which he has proved to extend to more than three +times the length of the whole heat-spectrum as previously known, and +has also shown that the invisible rays contribute 68 per cent, of the +sun's total energy.[9] + +[Footnote 9: For a short account of this remarkable instrument, see my +_Wonderful Century_, new ed., pp. 143-145.] + +(4) Then follows an elaborate estimate of the loss of heat during the +passage of the sun's rays through our atmosphere from experiments made +at different altitudes and from the estimated reflective power of the +various parts of the earth's surface--rocks and soil, ocean, forest and +snow--the final result being that three-fourths of the whole sun-heat +is reflected back into space, forming our _albedo_, while only +one-fourth is absorbed by the soil and goes to warm the air and +determine our mean temperature. + +(5) We now have another elaborate estimate of the comparative amounts of +heat actually received by Mars and the Earth, dependent on their very +different amounts of atmosphere, and this estimate depends almost wholly +on the comparative _albedoes_, that of Mars, as observed by astronomers +being 0.27, while ours has been estimated in a totally different way as +being 0.75, whence he concludes that nearly three-fourths of the +sun-heat that Mars receives reaches the surface and determines its +temperature, while we get only one-fourth of our total amount. Then by +applying Stefan's law, that the radiation is as the 4th power of the +surface temperature, he reaches the final result that the actual heating +power at the surface of Mars is considerably _more_ than on the Earth, +and would produce a mean temperature of 72 deg. F., if it were not for the +greater conservative or blanketing power of our denser and more +water-laden atmosphere. The difference produced by this latter fact he +minimises by dwelling on the probability of a greater proportion of +carbonic-acid gas and water-vapour in the Martian atmosphere, and thus +brings down the mean temperature of Mars to 48 deg. F., which is almost +exactly the same as that of the southern half of England. He has also, +as the result of observations, reduced the probable density of the +atmosphere of Mars to 2-1/2 inches of mercury, or only one-twelfth of +that of the Earth. + +_Critical Remarks on Mr. Lowell's Paper._ + +The last part of this paper, indicated under pars. 4 and 5, is the most +elaborate, occupying eight pages, and it contains much that seems +uncertain, if not erroneous. In particular, it seems very unlikely that +under a clear sky over the whole earth we should only receive at the +sea-level 0.23 of the solar rays which the earth intercepts (p. 167). +These data largely depend on observations made in California and other +parts of the southern United States, where the lower atmosphere is +exceptionally dust-laden. Till we have similar observations made in the +tropical forest-regions, which cover so large an area, and where the +atmosphere is purified by frequent rains, and also on the prairies and +the great oceans, we cannot trust these very local observations for +general conclusions affecting the whole earth. Later, in the same +article (p. 170), Mr. Lowell says: "Clouds transmit approximately 20 per +cent. of the heat reaching them: a clear sky at sea-level 60 per cent. +As the sky is half the time cloudy the mean transmission is 35 per +cent." These statements seem incompatible with that quoted above. + +The figure he uses in his calculations for the actual albedo of the +earth, 0.75, is also not only improbable, but almost self-contradictory, +because the albedo of cloud is 0.72, and that of the great cloud-covered +planet, Jupiter, is given by Lowell as 0.75, while Zollner made it only +0.62. Again, Lowell gives Venus an albedo of 0.92, while Zollner made it +only 0.50 and Mr. Gore 0.65. This shows the extreme uncertainty of these +estimates, while the fact that both Venus and Jupiter are wholly +cloud-covered, while we are only half-covered, renders it almost +certain that our albedo is far less than Mr. Lowell makes it. It is +evident that mathematical calculations founded upon such uncertain data +cannot yield trustworthy results. But this is by no means the only case +in which the data employed in this paper are of uncertain value. +Everywhere we meet with figures of somewhat doubtful accuracy. Here we +have somebody's 'estimate' quoted, there another person's 'observation,' +and these are adopted without further remark and used in the various +calculations leading to the result above quoted. It requires a practised +mathematician, and one fully acquainted with the extensive literature of +this subject, to examine these various data, and track them through the +maze of formulae and figures so as to determine to what extent they +affect the final result. + +There is however one curious oversight which I must refer to, as it is a +point to which I have given much attention. Not only does Mr. Lowell +assume, as in his book, that the 'snows' of Mars consist of frozen +water, and that therefore there _is_ water on its surface and +water-vapour in its atmosphere, not only does he ignore altogether Dr. +Johnstone Stoney's calculations with regard to it, which I have already +referred to, but he uses terms that imply that water-vapour is one of +the heavier components of our atmosphere. The passage is at p. 168 of +the _Philosophical Magazine._ After stating that, owing to the very +small barometric pressure in Mars, water would boil at 110 deg. F., he adds: +"The sublimation at lower temperatures would be correspondingly +increased. Consequently the amount of water-vapour in the Martian air +must on that score be relatively greater than our own." Then follows +this remarkable passage: "Carbon-dioxide, because of its greater +specific gravity, would also be in relatively greater amount so far as +this cause is considered. For the planet would part, _caeteris paribus_, +with its lighter gases the quickest. Whence as regards both water-vapour +and carbon-dioxide we have reason to think them in relatively greater +quantity than in our own air at corresponding barometric pressure." I +cannot understand this passage except as implying that 'water-vapour and +carbon-dioxide' are among the heavier and not among the lighter gases of +the atmosphere--those which the planet 'parts with quickest.' But this +is just what water-vapour _is_, being a little less than two-thirds the +weight of air (0.6225), and one of those which the planet _would_ part +with the quickest, and which, according to Dr. Johnstone Stoney, it +loses altogether. + * * * * * + +Note on Professor Lowell's article in the _Philosophical Magazine_; by +J.H. Poynting, F.R.S., Professor of Physics in the University of +Birmingham. + +"I think Professor Lowell's results are erroneous through his neglect of +the heat stored in the air by its absorption of radiation both from the +sun and from the surface. The air thus heated radiates to the surface +and keeps up the temperature. I have sent to the _Philosophical +Magazine_ a paper in which I think it is shown that when the radiation +by the atmosphere is taken into account the results are entirely +changed. The temperature of Mars, with Professor Lowell's data, still +comes out far below the freezing-point--still further below than the +increased distance alone would make it. Indeed, the lower temperature on +elevated regions of the earth's surface would lead us to expect this. I +think it is impossible to raise the temperature of Mars to anything like +the value obtained by Professor Lowell, unless we assume some quality in +his atmosphere entirely different from any found in our own atmosphere." +J.H. POYNTING. October 19, 1907. + + + +CHAPTER VI. + + +A NEW ESTIMATE OF THE TEMPERATURE OF MARS. + +When we are presented with a complex problem depending on a great number +of imperfectly ascertained data, we may often check the results thus +obtained by the comparison of cases in which some of the more important +of these data are identical, while others are at a maximum or a minimum. +In the present case we can do this by a consideration of the Moon as +compared with the Earth and with Mars. + +_Langley's Determination of the Moon's Temperature._ + +In the moon we see the conditions that prevail in Mars both exaggerated +and simplified. Mars has a very scanty atmosphere, the moon none at all, +or if there is one it is so excessively scanty that the most refined +observations have not detected it. All the complications arising from +the possible nature of the atmosphere, and its complex effects upon +reflection, absorption, and radiation are thus eliminated. The mean +distance of the moon from the sun being identical with that of the +earth, the total amount of heat intercepted must also be identical; only +in this case the whole of it reaches the surface instead of one-fourth +only, according to Mr. Lowell's estimate for the earth. + +Now, by the most refined observations with his Bolometer, Mr. Langley +was able to determine the temperature of the moon's surface exposed to +undimmed sunshine for fourteen days together; and he found that, even in +that portion of it on which the sun was shining almost vertically, the +temperature rarely rose above the freezing point of water. However +extraordinary this result may seem, it is really a striking confirmation +of the accuracy of the general laws determining temperature which I have +endeavoured to explain in the preceding chapter. For the same surface +which has had fourteen days of sunshine has also had a preceding +fourteen days of darkness, during which the heat which it had +accumulated in its surface layers would have been lost by free radiation +into stellar space. It thus acquires during its day a maximum +temperature of only 491 deg. F. absolute, while its minimum, after 14 days' +continuous radiation, must be very low, and is, with much reason, +supposed to approach the absolute zero. + +_Rapid Loss of Heat by Radiation on the Earth._ + +In order better to comprehend what this minimum may be under extreme +conditions, it will be useful to take note of the effects it actually +produces on the earth in places where the conditions are nearest to +those existing on the moon or on Mars, though never quite equalling, or +even approaching very near them. It is in our great desert regions, and +especially on high plateaux, that extreme aridity prevails, and it is in +such districts that the differences between day and night temperatures +reach their maximum. It is stated by geographers that in parts of the +Great Sahara the surface temperature is sometimes 150 deg. F., while during +the night it falls nearly or quite to the freezing point--a difference +of 118 degrees in little more than 12 hours.[10] In the high desert +plains of Central Asia the extremes are said to be even greater.[11] +Again, in his _Universal Geography_, Reclus states that in the Armenian +Highlands the thermometer oscillates between 13 deg. F. and 112 deg.F. We may +therefore, without any fear of exaggeration, take it as proved that a +fall of 100 deg. F. in twelve or fifteen hours not infrequently occurs where +there is a very dry and clear atmosphere permitting continuous +insolation by day and rapid radiation by night. + +[Footnote 10: Keith Johnston's 'Africa' in _Stanford's Compendium._] + +[Footnote 11: _Chambers's Encyclopaedia_, Art. 'Deserts.'] + +Now, as it is admitted that our dense atmosphere, however dry and clear, +absorbs and reflects some considerable portion of the solar heat, we +shall certainly underestimate the radiation from the moon's surface +during its long night if we take as the basis of our calculation a +lowering of temperature amounting to 100 deg. F. during twelve hours, as not +unfrequently occurs with us. Using these data--with Stefan's law of +decrease of radiation as the 4th power of the temperature--a +mathematical friend finds that the temperature of the moon's surface +would be reduced during the lunar night to nearly 200 deg. F. absolute +(equal to-258 deg. F.). + +_More Rapid Loss of Heat by the Moon._ + +Although such a calculation as the above may afford us a good +approximation to the rate of loss of heat by Mars with its very scanty +atmosphere, we have now good evidence that in the case of the moon the +loss is much more rapid. Two independent workers have investigated this +subject with very accordant results--Dr. Boeddicker, with Lord Rosse's +3-foot reflector and a Thermopile to measure the heat, and Mr. Frank +Very, with a glass reflector of 12 inches diameter and the Bolometer +invented by Mr. Langley. The very striking and unexpected fact in which +these observers agree is the sudden disappearance of much of the +stored-up heat during the comparatively short duration of a total +eclipse of the moon--less than two hours of complete darkness, and about +twice that period of partial obscuration. + +Dr. Boeddicker was unable to detect any appreciable heat at the period +of greatest obscuration; but, owing to the extreme sensitiveness of the +Bolometer, Mr. Very ascertained that those parts of the surface which +had been longest in the shadow still emitted heat "to the amount of one +per cent. of the heat to be expected from the full moon." This however +is the amount of radiation measured by the Bolometer, and to get the +temperature of the radiating surface we must apply Stefan's law of the +4th power. Hence the temperature of the moon's dark surface will be the +[fourth root of (1 over 100)] = 1 over 3.2 [A] of the highest temperature + (which we may take at the freezing-point, 491 deg. F. abs.), or 154 deg. F. abs., + just below the liquefaction point of air. This is about 50 deg. lower than the +amount found by calculation from our most rapid radiation; and as this +amount is produced in a few hours, it is not too much to expect that, +when continued for more than two weeks (the lunar night), it might reach +a temperature sufficient to liquefy hydrogen (60 deg. F. abs.), or perhaps +even below it. + +[Note A: LaTex markup $\root 4 \of {1 \over 100} = {1 \over 3.2}$ ] + +_Theory of the Moon's Origin._ + +This extremely rapid loss of heat by radiation, at first sight so +improbable as to be almost incredible, may perhaps be to some extent +explained by the physical constitution of the moon's surface, which, +from a theoretical point of view, does not appear to have received the +attention it deserves. It is clear that our satellite has been long +subjected to volcanic eruptions over its whole visible face, and these +have evidently been of an explosive nature, so as to build up the very +lofty cones and craters, as well as thousands of smaller ones, which, +owing to the absence of any degrading or denuding agencies, have +remained piled up as they were first formed. + +This highly volcanic structure can, I think, be well explained by an +origin such as that attributed to it by Sir George Darwin, and which has +been so well described by Sir Robert Ball in his small volume, _Time and +Tide._ These astronomers adduce strong evidence that the earth once +rotated so rapidly that the equatorial protuberance was almost at the +point of separation from the planet as a ring. Before this occurred, +however, the tension was so great that one large portion of the +protuberance where it was weakest broke away, and began to move around +the earth at some considerable distance from it. As about 1/50 of the +bulk of the earth thus escaped, it must have consisted of a considerable +portion of the solid crust and a much larger quantity of the liquid or +semi-liquid interior, together with a proportionate amount of the gases +which we know formed, and still form, an important part of the earth's +substance. + +As the surface layers of the earth must have been the lightest, they +would necessarily, when broken up by this gigantic convulsion, have come +together to form the exterior of the new satellite, and be soon adjusted +by the forces of gravity and tidal disturbance into a more or less +irregular spheroidal form, all whose interstices and cavities would be +filled up and connected together by the liquid or semi-liquid mass +forced up between them. Thence-forward, as the moon increased its +distance and reduced its time of rotation, in the way explained by Sir +Robert Ball, there would necessarily commence a process of escape of the +imprisoned gases at every fissure and at all points and lines of +weakness, giving rise to numerous volcanic outlets, which, being +subjected only to the small force of lunar gravity (only one-sixth that +of the earth), would, in the course of ages, pile up those gigantic +cones and ridges which form its great characteristic. + +But this small gravitative power of the moon would prevent its retaining +on its surface any of the gases forming our atmosphere, which would all +escape from it and probably be recaptured by the earth. By no process of +external aggregation of solid matter to such a relatively small amount +as that forming the moon, even if the aggregation was so violent as to +produce heat enough to cause liquefaction, could any such +long-continued volcanic action arise by gradual cooling, in the absence +of internal gases. There might be fissures, and even some outflows of +molten rock; but without imprisoned gases, and especially without water +and water-vapour producing explosive outbursts, could any such amount of +scoriae and ashes be produced as were necessary for the building up of +the vast volcanic cones, craters, and craterlets we see upon the moon's +surface. + +I am not aware that either Sir Robert Ball or Sir George Darwin have +adduced this highly volcanic condition of the moon's surface as a +phenomenon which can _only_ be explained by our satellite having been +thrown off a very much larger body, whose gravitative force was +sufficient to acquire and retain the enormous quantity of gases and of +water which we possess, and which are _absolutely essential_ for that +_special form of cone-building volcanic action_ which the moon exhibits +in so pre-eminent a degree. Yet it seems to me clear, that some such +hypothetical origin for our satellite would have had to be assumed if +Sir George Darwin had not deduced it by means of purely mathematical +argument based upon astronomical facts. + +Returning now to the problem of the moon's temperature, I think the +phenomena this presents may be in part due to the mode of formation here +described. For, its entire surface being the result of long-continued +gaseous explosions, all the volcanic products--scoriae, pumice, and +ashes--would necessarily be highly porous throughout; and, never having +been compacted by water-action, as on the earth, and there having been +no winds to carry the finer dust so as to fill up their pores and +fissures, the whole of the surface material to a very considerable depth +must be loose and porous to a high degree. This condition has been +further increased owing to the small power of gravity and the extreme +irregularity of the surface, consisting very largely of lofty cones and +ridges very loosely piled up to enormous heights. + +Now this condition of the substance of the moon's surface is such as +would produce a high specific heat, so that it would absorb a large +amount of heat in proportion to the rise of temperature produced, the +heat being conducted downwards to a considerable depth. Owing, however, +to the total absence of atmosphere radiation would very rapidly cool the +surface, but afterwards more slowly, both on account of the action of +Stefan's law and because the heat stored up in the deeper portions could +be carried to the surface by conduction only, and with extreme slowness. + +_Very's Researches on the Moon's Heat._ + +The results of the eclipse observations are supported by the detailed +examination of the surface-temperature of the moon by Mr. Very in his +_Prize Essay on the Distribution of the Moon's Heat_ (published by the +Utrecht Society of Arts and Sciences in 1891). He shows, by a diagram of +the 'Phase-curve,' that at the commencement of the Lunar day the surface +just within the illuminated limb has acquired about 1/7 of its maximum +temperature, or about 70 deg. F. abs. As the surface exposed to the +Bolometer at each observation is about 1/30 of the moon's surface, and +in order to ensure accuracy the instrument has to be directed to a spot +lying wholly within the edge of the moon, it is evident that the surface +measured has already been for several hours exposed to oblique sunshine. +The curve of temperature then rises gradually and afterwards more +rapidly, till it attains its maximum (of about +30 to 40 deg. F.) a few +hours _before_ noon. This, Mr. Very thinks, is due to the fact that the +half of the moon's face first illuminated for us has, on the average, a +darker surface than that of the afternoon, or second quarter, during +which the curve descends not quite so rapidly, the temperature near +sunset being only a little higher than that near sunrise. This rapid +fall while exposed to oblique sunshine is quite in harmony with the +rapid loss of heat during the few hours of darkness during an eclipse, +both showing the prepotency of radiation over insolation on the moon. + +Two other diagrams show the distribution of heat at the time of +full-moon, one half of the curve showing the temperatures along the +equator from the edge of the disc to the centre, the other along a +meridian from this centre to the pole. This diagram (here reproduced) +exhibits the quick rise of temperature of the oblique rim of the moon +and the nearly uniform heat of the central half of its surface; the +diminution of heat towards the pole, however, is slower for the first +half and more rapid for the latter portion. + +It is an interesting fact that the temperature near the margin of the +full-moon increases towards the centre more rapidly than it does when +the same parts are observed during the early phases of the first +quarter. Mr. Very explains this difference as being due to the fact that +the full-moon to its very edges is fully illuminated, all the shadows of +the ridges and mountains being thrown vertically or obliquely _behind +them._ We thus measure the heat reflected from the _whole_ visible +surface. But at new moon, and somewhat beyond the first quarter, the +deep shadows thrown by the smallest cones and ridges, as well as by the +loftiest mountains, cover a considerable portion of the visible surface, +thus largely reducing the quantity of light and heat reflected or +radiated in our direction. It is only at the full, therefore, that the +maximum temperature of the whole lunar surface can be measured. It must +be considered a proof of the delicacy of the heat-measuring instruments +that this difference in the curves of temperature of the different parts +of the moon's surface and under different conditions is so clearly +shown. + +_The Application of the Preceding Results to the Case of Mars._ + +This somewhat lengthy account of the actual state of the moon's surface +and temperature is of very great importance in our present enquiry, +because it shows us the extraordinary difference in mean and extreme +temperatures of two bodies situated at the same distance from the sun, +and therefore receiving exactly the same amount of solar heat per unit +of surface. We have learned also what are the main causes of this almost +incredible difference, namely: (1) a remarkably rugged surface with +porous and probably cavernous rock-texture, leading to extremely rapid +radiation of heat in the one; as compared with a comparatively even and +well-compacted surface largely clad with vegetation, leading to +comparatively slow and gradual loss by radiation in the other: and (2), +these results being greatly intensified by the total absence of a +protecting atmosphere in the former, while a dense and cloudy atmosphere +with an ever-present supply of water-vapour, accumulates and equalises +the heat received by the latter. + +The only other essential difference in the two bodies which may possibly +aid in the production of this marvellous result, is the fact of our day +and night having a mean length of 12 hours, while those of the moon are +about 14-1/2 of our days. But the altogether unexpected fact, in which +two independent enquirers agree, that during the few hours' duration of +a total eclipse of the moon so large a proportion of the heat is lost by +radiation renders it almost certain that the resulting low temperature +would be not very much less if the moon had a day and night the same +length as our own. + +The great lesson we learn by this extreme contrast of conditions +supplied to us by nature, as if to enable us to solve some of her +problems, is, the overwhelming importance, first, of a dense and +well-compacted surface, due to water-action and strong gravitative +force; secondly, of a more or less general coat of vegetation; and, +thirdly, of a dense vapour-laden atmosphere. These three favourable +conditions result in a mean temperature of about +60 deg. F. with a range +seldom exceeding 40 deg. above or below it, while over more than half the +land-surface of the earth the temperature rarely falls below the +freezing point. On the other hand, we have a globe of the same materials +and at the same distance from the sun, with a maximum temperature of +freezing water, and a minimum not very far from the absolute zero, the +monthly mean being probably much below the freezing point of +carbonic-acid gas--a difference entirely due to the absence of these +three favourable conditions. + +_The Special Features of Mars as influencing Temperature._ + +Coming now to the special feature of Mars and its probable temperature, +we find that most writers have arrived at a very different conclusion +from that of Mr. Lowell, who himself quotes Mr. Moulton as an authority +who 'recently, by the application of Stefan's law,' has found the mean +temperature of this planet to be-35 deg. F. Again, Professor J.H. Poynting, +in his lecture on 'Radiation in the Solar System,' delivered before the +British Association at Cambridge in 1904, gave an estimate of the mean +temperature of the planets, arrived at from measurements of the sun's +emissive power and the application of Stefan's law to the distances of +the several planets, and he thus finds the earth to have a mean +temperature of 17 deg. C. (=62-1/2 deg. F.) and Mars one of-38 deg. C. (=-36-1/2 deg. +F.), a wonderfully close approximation to the mean temperature of the +earth as determined by direct measurement, and therefore, presumably, an +equally near approximation to that of Mars as dependent on distance from +the sun, and '_on the supposition that it is earth-like in all its +conditions._' + +But we know that it is far from being earth-like in the very conditions +which we have found to be those which determine the extremely different +temperatures of the earth, and moon; and, as regards each of these, we +shall find that, so far as it differs from the earth, it approximates to +the less favourable conditions that prevail in the moon. The first of +these conditions which we have found to be essential in regulating the +absorption and radiation of heat, and thus raising the mean temperature +of a planet, is a compact surface well covered with vegetation, two +conditions arising from, and absolutely dependent on, an ample amount of +water. But Mr. Lowell himself assures us, as a fact of which he has no +doubt, that there are no permanent bodies of water, great or small, upon +Mars; that rain, and consequently rivers, are totally wanting; that its +sky is almost constantly clear, and that what appear to be clouds are +not formed of water-vapour but of dust. He dwells, emphatically, on the +terrible desert conditions of the greater part of the surface of the +planet. + +That being the case now, we have no right to assume that it has ever +been otherwise; and, taking full account of the fact, neither denied nor +disputed by Mr. Lowell, that the force of gravity on Mars is not +sufficient to retain water-vapour in its atmosphere, we must conclude +that the surface of that planet, like that of the moon, has been moulded +by some form of volcanic action modified probably by wind, but not by +water. Adding to this, that the force of gravity on Mars is nearer that +of the moon than to that of the earth, and we may r reasonably conclude +that its surface is formed of volcanic matter in a light and porous +condition, and therefore highly favourable for the rapid loss of surface +heat by radiation. The surface-conditions of Mars are therefore, +presumably, much more like those of the moon than like those of the +earth. + +The next condition favourable to the storing up of heat--a covering of +vegetation--is almost certainly absent from Mars except, possibly, over +limited areas and for short periods. In this feature also the surface of +Mars approximates much nearer to lunar than to earth-conditions. The +third condition--a dense, vapour-laden atmosphere--is also wanting in +Mars. For although it possesses an atmosphere it is estimated by Mr. +Lowell (in his latest article) to have a pressure equivalent to only +2-1/2 inches of mercury with us, giving it a density of only one-twelfth +part that of ours; while aqueous vapour, the chief accumulator of heat, +cannot permanently exist in it, and, notwithstanding repeated +spectroscopic observations for the purpose of detecting it, has never +been proved to exist. + +I submit that I have now shown from the statements--and largely as the +result of the long-continued observations--of Mr. Lowell himself, that, +so far as the physical conditions of Mars are known to differ from those +of the earth, the differences are all _unfavourable_ to the conservation +and _favourable_ to the dissipation of the scanty heat it receives from +the sun--that they point unmistakeably towards the temperature +conditions of the moon rather than to those of the earth, and that the +cumulative effect of these adverse conditions, acting upon a +heat-supply, reduced by solar distance to less than one-half of ours, +_must_ result in a mean temperature (as well as in the extremes) nearer +to that of our satellite than to that of our own earth. + +_Further Criticism of Mr. Lowell's Article._ + +We are now in a position to test some further conclusions of Mr. +Lowell's _Phil. Mag._ article by comparison with actual phenomena. We +have seen, in the outline I have given of this article, that he +endeavours to show how the small amount of solar heat received by Mars +is counterbalanced, largely by the greater transparency to light and +heat of its thin and cloudless atmosphere, and partially also by a +greater conservative or 'blanketing' power of its atmosphere due to the +presence in it of a large proportion of carbonic acid gas and aqueous +vapour. The first of these statements may be admitted as a fact which he +is entitled to dwell upon, but the second--the presence of large +quantities of carbon-dioxide and aqueous vapour is a pure hypothesis +unsupported by any item of scientific evidence, while in the case of +aqueous vapour it is directly opposed to admitted results founded upon +the molecular theory of gaseous elasticity. But, although Mr. Lowell +refers to the conservative or 'blanketing' effect of the earth's +atmosphere, he does not consider or allow for its very great cumulative +effect, as is strikingly shown by the comparison with the actual +temperature conditions of the moon. This cumulative effect is due to the +_continuous_ reflection and radiation of heat from the clouds as well as +from the vapour-laden strata of air in our lower atmosphere, which +latter, though very transparent to the luminous and accompanying heat +rays of the sun, are opaque to the dark heat-rays whether radiated or +reflected from the earth's surface. We are therefore in a position +strictly comparable with that of the interior of some huge glass house, +which not only becomes intensely heated by the direct rays of the sun, +but also to a less degree by reflected rays from the sky and those +radiated from the clouds, so that even on a cloudy or misty day its +temperature rises many degrees above that of the outer air. Such a +building, if of large size, of suitable form, and well protected at +night by blinds or other covering, might be so arranged as to accumulate +heat in its soil and walls so as to maintain a tolerably uniform +temperature though exposed to a considerable range of external heat and +cold. It is to such a power of accumulation of heat in our soil and +lower atmosphere that we must impute the overwhelming contrast between +our climate and that of the moon. With us, the solar heat that +penetrates our vapour-laden and cloudy atmosphere is shut in by that +same atmosphere, accumulates there for weeks and months together, and +can only slowly escape. It is this great cumulative power which Mr. +Lowell has not taken account of, while he certainly has not estimated +the enormous loss of heat by free radiation, which entirely neutralises +the effects of increase of sun-heat, however great, when these +cumulative agencies are not present.[12] + +[Footnote 12: The effects of this 'cumulative' power of a dense +atmosphere are further discussed and illustrated in the last chapter of +this book, where I show that the universal fact of steadily diminishing +temperatures at high altitudes is due solely to the diminution of this +cumulative power of our atmosphere, and that from this cause alone the +temperature of Mars must be that which would be found on a lofty plateau +about 18,000 feet higher than the average of the peaks of the Andes!] + +_Temperature on Polar Regions of Mars._ + +There is also a further consideration which I think Mr. Lowell has +altogether omitted to discuss. Whatever may be the _mean_ temperature +of Mars, we must take account of the long nights in its polar and +high-temperate latitudes, lasting nearly twice as long as ours, with the +resulting lowering of temperature by radiation into a constantly clear +sky. Even in Siberia, in Lat. 67-1/2 deg.N. a cold of-88 deg.F. has been +attained; while over a large portion of N. Asia and America above 60 deg. +Lat. the _mean_ January temperature is from-30 deg.F. to-60 deg.F., and the +whole subsoil is permanently frozen from a depth of 6 or 7 feet to +several hundreds. But the winter temperatures, _over the same latitudes_ +in Mars, must be very much lower; and it must require a proportionally +larger amount of its feeble sun-heat to raise the surface even to the +freezing-point, and an additional very large amount to melt any +considerable depth of snow. But this identical area, from a little below +60 deg. to the pole, is that occupied by the snow-caps of Mars, and over the +whole of it the winter temperature must be far lower than the +earth-minimum of-88 deg.F. Then, as the Martian summer comes on, there is +less than half the sun-heat available to raise this low temperature +after a winter nearly double the length of ours. And when the summer +does come with its scanty sun-heat, that heat is not accumulated as it +is by our dense and moisture-laden atmosphere, the marvellous effects of +which we have already shown. Yet with all these adverse conditions, each +assisting the other to produce a climate approximating to that which the +earth would have if it had no atmosphere (but retaining our superiority +over Mars in receiving double the amount of sun-heat), we are asked to +accept a mean temperature for the more distant planet almost exactly the +same as that of mild and equable southern England, and a disappearance +of the vast snowfields of its polar regions as rapid and complete as +what occurs with us! If the moon, even at its equator, has not its +temperature raised above the freezing-point of water, how can the more +_distant_ Mars, with its _oblique_ noon-day sun falling upon the +snow-caps, receive heat enough, first to raise their temperature to 32 deg. +F., and then to melt with marked rapidity the vast frozen plains of its +polar regions? + +Mr. Lowell is however so regardless of the ordinary teachings of +meteorological science that he actually accounts for the supposed mild +climate of the polar regions of Mars by the absence of water on its +surface and in its atmosphere. He concludes his fifth chapter with the +following words: "Could our earth but get rid of its oceans, we too +might have temperate regions stretching to the poles." Here he runs +counter to two of the best-established laws of terrestrial climatology-- +the wonderful equalising effects of warm ocean-currents which are the +chief agents in diminishing polar cold; the equally striking effects of +warm moist winds derived from these oceans, and the great storehouse of +heat we possess in our vapour-laden atmosphere, its vapour being +primarily derived from these same oceans! But, in Mr. Lowell's opinion, +all our meteorologists are quite mistaken. Our oceans are our great +drawbacks. Only get rid of them and we should enjoy the exquisite +climate of Mars--with its absence of clouds and fog, of rain or rivers, +and its delightful expanses of perennial deserts, varied towards the +poles by a scanty snow-fall in winter, the melting of which might, with +great care, supply us with the necessary moisture to grow wheat and +cabbages for about one-tenth, or more likely one-hundredth, of our +present population. I hope I may be excused for not treating such an +argument seriously. The various considerations now advanced, especially +those which show the enormous cumulative and conservative effect of our +dense and water-laden atmosphere, and the disastrous effect--judging by +the actual condition of the moon--which the loss of it would have upon +our temperature, seem to me quite sufficient to demonstrate important +errors in the data or fallacies in the complex mathematical argument by +which Mr. Lowell has attempted to uphold his views as to the temperature +and consequent climatic conditions of Mars. In concluding this portion +of my discussion of the problem of Mars, I wish to call attention to the +fact that my argument, founded upon a comparison of the physical +conditions of the earth and moon with those of Mars, is dependent upon a +small number of generally admitted scientific facts; while the +conclusions drawn from those facts are simple and direct, requiring no +mathematical knowledge to follow them, or to appreciate their weight and +cogency. I claim for them, therefore, that they are in no degree +speculative, but in their data and methods exclusively scientific. In +the next chapter I will put forward a suggestion as to how the very +curious markings upon the surface of Mars may possibly be interpreted, +so as to be in harmony with the planet's actual physical condition and +its not improbable origin and past history. + + + +CHAPTER VII. + + +A SUGGESTION AS TO THE 'CANALS' OF MARS. + +The special characteristics of the numerous lines which intersect the +whole of the equatorial and temperate regions of Mars are, their +straightness combined with their enormous length. It is this which has +led Mr. Lowell to term them 'non-natural features.' Schiaparelli, in his +earlier drawings, showed them curved and of comparatively great width. +Later, he found them to be straight fine lines when seen under the best +conditions, just as Mr. Lowell has always seen them in the pure +atmosphere of his observatory. Both of these observers were at first +doubtful of their reality, but persistent observation continued at many +successive oppositions compelled acceptance of them as actual features +of the planet's disc. So many other observers have now seen them that +the objection of unreality seems no longer valid. + +Mr. Lowell urges, however, that their perfect straightness, their +extreme tenuity, their uniformity throughout their whole length, the +dual character of many of them, their relation to the 'oases' and the +form and position of these round black spots, are all proofs of +artificiality and are suggestive of design. And considering that some of +them are actually as long as from Boston to San Francisco, and +relatively to their globe as long as from London to Bombay, his +objection that "no natural phenomena within our knowledge show such +regularity on such a scale" seems, at first, a mighty one. + +It is certainly true that we can point to nothing exactly like them +either on the earth or on the moon, and these are the only two planetary +bodies we are in a position to compare with Mars. Yet even these do, I +think, afford us some hints towards an interpretation of the mysterious +lines. But as our knowledge of the internal structure and past history +even of our earth is still imperfect, that of the moon only conjectural, +and that of Mars a perfect blank, it is not perhaps surprising that the +surface-features of the latter do not correspond with those of either of +the others. + +_Mr. Pickering's Suggestion._ + +The best clue to a natural interpretation of the strange features of the +surface of Mars is that suggested by the American astronomer Mr. W.H. +Pickering in _Popular Astronomy_ (1904). Briefly it is, that both the +'canals' of Mars and the rifts as well as the luminous streaks on the +moon are cracks in the volcanic crust, caused by internal stresses due +to the action of the heated interior. These cracks he considers to be +symmetrically arranged with regard to small 'craterlets' (Mr. Lowell's +'oases') because they have originated from them, just as the white +streaks on the moon radiate from the larger craters as centres. He +further supposes that water and carbon-dioxide issue from the interior +into these fissures, and, in conjunction with sunlight, promote the +growth of vegetation. Owing to the very rare atmosphere, the vapours, he +thinks, would not ascend but would roll down the outsides of the +craterlets and along the borders of the canals, thus irrigating the +immediate vicinity and serving to promote the growth of some form of +vegetation which renders the canals and oases visible.[13] + +[Footnote 13: _Nature_, vol. 70, p. 536.] + +This opinion is especially important because, next to Mr. Lowell, Mr. +Pickering is perhaps the astronomer who has given most attention to Mars +during the last fifteen years. He was for some time at Flagstaff with +Mr. Lowell, and it was he who discovered the oases or craterlets, and +who originated the idea that we did not see the 'canals' themselves but +only the vegetable growth on their borders. He also observed Mars in the +Southern Hemisphere at Arequipa; and he has since made an elaborate +study of the moon by means of a specially constructed telescope of 135 +feet focal length, which produced a direct image on photographic plates +nearly 16 inches in diameter.[14] + +[Footnote 14: _Nature_, vol. 70, May 5, p.xi, supplement.] + +It is clear therefore that Mr. Lowell's views as to the artificial +nature of the 'canals' of Mars are not accepted by an astronomer of +equal knowledge and still wider experience. Yet Professor Pickering's +alternative view is more a suggestion than an explanation, because there +is no attempt to account for the enormous length and perfect +straightness of the lines on Mars, so different from anything that is +found either on our earth or on the moon. There must evidently be some +great peculiarity of structure or of conditions on Mars to account for +these features, and I shall now attempt to point out what this +peculiarity is and how it may have arisen. + +_The Meteoritic Hypothesis._ + +During the last quarter of a century a considerable change has come over +the opinions of astronomers as regards the probable origin of the Solar +System. The large amount of knowledge of the stellar universe, and +especially of nebulae, of comets and of meteor-streams which we now +possess, together with many other phenomena, such as the constitution of +Saturn's rings, the great number and extent of the minor planets, and +generally of the vast amount of matter in the form of meteor-rings and +meteoric dust in and around our system, have all pointed to a different +origin for the planets and their satellites than that formulated by +Laplace as the Nebular hypothesis. + +It is now seen more clearly than at any earlier period, that most of the +planets possess special characteristics which distinguish them from one +another, and that such an origin as Laplace suggested--the slow cooling +and contraction of one vast sun-mist or nebula, besides presenting +inherent difficulties--many think them impossibilities--in itself does +not afford an adequate explanation of these peculiarities. Hence has +arisen what is termed the Meteoritic theory, which has been ably +advocated for many years by Sir Norman Lockyer, and with some +unimportant modifications is now becoming widely accepted. Briefly, this +theory is, that the planets have been formed by the slow aggregation of +solid particles around centres of greatest condensation; but as many of +my readers may be altogether unacquainted with it, I will here give a +very clear statement of what it is, from Professor J.W. Gregory's +presidential address to the Geological Section of the British +Association of the present year. He began by saying that these modern +views were of far more practical use to men of science than that of +Laplace, and that they give us a history of the world consistent with +the actual records of geology. He then continues: + +"According to Sir Norman Lockyer's Meteoritic Hypothesis, nebulae, +comets, and many so-called stars consist of swarms of meteorites which, +though normally cold and dark, are heated by repeated collisions, and so +become luminous. They may even be volatilised into glowing meteoric +vapour; but in time this heat is dissipated, and the force of gravity +condenses a meteoritic swarm into a single globe. 'Some of the swarms +are,' says Lockyer, 'truly members of the solar system,' and some of +these travel round the sun in nearly circular orbits, like planets. They +may be regarded as infinitesimal planets, and so Chamberlain calls them +'planetismals.' + +"The planetismal theory is a development of the meteoritic theory, and +presents it in an especially attractive guise. It regards meteorites as +very sparsely distributed through space, and gravity as powerless to +collect them into dense groups. So it assigns the parentage of the solar +system to a spiral nebula composed of planetismals, and the planets as +formed from knots in the nebula, where many planetismals had been +concentrated near the intersections of their orbits. These groups of +meteorites, already as dense as a swarm of bees, were then packed closer +by the influence of gravity, and the contracting mass was heated by the +pressure, even above the normal melting-point of the material, which was +kept rigid by the weight of the overlying layers." + +Now, adopting this theory as the last word of science upon the subject +of the origin of planets, we see that it affords immense scope for +diversity in results depending on the total _amount_ of matter available +within the range of attraction of an incipient planetary mass, and the +_rates_ at which this matter becomes available. By a special combination +of these two quantities (which have almost certainly been different for +each planet) I think we may be able to throw some light upon the +structure and physical features of Mars. + +_The Probable Mode of Origin of Mars._ + +This planet, lying between two of much greater mass, has evidently had +less material from which to be formed by aggregation; and if we +assume--as in the absence of evidence to the contrary we have a right to +do--that its beginnings were not much later (or earlier) than those of +the earth, then its smaller size shows that it has in all probability +aggregated very much more slowly. But the internal heat acquired by a +planet while forming in this manner will depend upon the rate at which +it aggregates and the velocity with which the planetismals' fall into +it, and this velocity will increase with its mass and consequent force +of gravity. In the early stages of a planet's growth it will probably +remain cold, the small amount of heat produced by each impact being lost +by radiation before the next one occurs; and with a small and slowly +aggregating planet this condition will prevail till it approaches its +full size. Then only will its gravitative force be sufficient to cause +incoming matter to fall upon it with so powerful an impact as to produce +intense heat. Further, the compressive force of a small planet will be a +less effective heat-producing agency than in the case of a larger one. + +The earth we know has acquired a large amount of internal heat, probably +sufficient to liquefy its whole interior; but Mars has only one-ninth +part the mass of the earth, and it is quite possible, and even probable, +that its comparatively small attractive force would never have liquefied +or even permanently heated the more central portions of its mass. This +being admitted, I suggest the following course of events as quite +possible, and not even improbable, in the case of this planet. During +the whole of its early growth, and till it acquired nearly its present +diameter, its rate of aggregation was so slow that the planetismals +falling upon it, though they might have been heated and even partially +liquefied by the impact, were never in such quantity as to produce any +considerable heating effect on the whole mass, and each local rise of +temperature was soon lost by radiation. The planet thus grew as a solid +and cold mass, compacted together by the impact of the incoming matter +as well as by its slowly increasing gravitative force. But when it had +attained to within perhaps 100, perhaps 50 miles, or less, of its +present diameter, a great change occurred in the opportunity for further +growth. Some large and dense swarm of meteorites, perhaps containing a +number of bodies of the size of the asteroids, came within the range of +the sun's attraction and were drawn by it into an orbit which crossed +that of Mars at such a small angle that the planet was able at each +revolution to capture a considerable number of them. The result might +then be that, as in the case of the earth, the continuous inpour of the +fresh matter first heated, and later on liquefied the greater part of it +as well perhaps as a thin layer of the planet's original surface; so +that when in due course the whole of the meteor-swarm had been captured, +Mars had acquired its present mass, but would consist of an intensely +heated, and either liquid or plastic thin outer shell resting upon a +cold and solid interior. + +The size and position of the two recently discovered satellites of Mars, +which are believed to be not more than ten miles in diameter, the more +remote revolving around its primary very little slower than the planet +rotates, while the nearer one, which is considerably less distant from +the planet's surface than its own antipodes and revolves around it more +than three times during the Martian day, may perhaps be looked upon as +the remnants of the great meteor-swarm which completed the Martian +development, and which are perhaps themselves destined at some distant +period to fall into the planet. Should future astronomers witness the +phenomenon the effect produced upon its surface would be full of +instruction. + +As the result of such an origin as that suggested, Mars would possess a +structure which, in the essential feature of heat-distribution, would be +the very opposite of that which is believed to characterise the earth, +yet it might have been produced by a very slight modification of the +same process. This peculiar heat-distribution, together with a much +smaller mass and gravitative force, would lead to a very different +development of the surface and an altogether diverse geological history +from ours, which has throughout been profoundly influenced by its heated +interior, its vast supply of water, and the continuous physical and +chemical reactions between the interior and the crust. + +These reactions have, in our case, been of substantially the same +nature, and very nearly of the same degree of intensity throughout the +whole vast eons of geological time, and they have resulted in a +wonderfully complex succession of rock-formations--volcanic, plutonic, +and sedimentary--more or less intermingled throughout the whole series, +here remaining horizontal as when first deposited, there upheaved or +depressed, fractured or crushed, inclined or contorted; denuded by rain +and rivers with the assistance of heat and cold, of frost and ice, in an +unceasing series of changes, so that however varied the surface may be, +with hill and dale, plains and uplands, mountain ranges and deep +intervening valleys, these are as nothing to the diversities of interior +structure, as exhibited in the sides of every alpine valley or +precipitous escarpment, and made known to us by the work of the miner +and the well-borer in every part of the world. + +_Structural Straight Lines on the Earth._ + +The great characteristic of the earth, both on its surface and in its +interior, is thus seen to be extreme diversity both of form and +structure, and this is further intensified by the varied texture, +constitution, hardness, and density of the various rocks and debris of +which it is composed. It is therefore not surprising that, with such a +complex outer crust, we should nowhere find examples of those +geometrical forms and almost world-wide straight lines that give such a +remarkable, and as Mr. Lowell maintains, 'non-natural' character to the +surface of Mars, but which, as it seems to me, of themselves afford +_prima facie_ evidence of a corresponding simplicity and uniformity in +its internal structure. + +Yet we are not ourselves by any means devoid of 'straight lines' +structurally produced, in spite of every obstacle of diversity of form +and texture, of softness and hardness, of lamination or crystallisation, +which are adverse to such developments. Examples of these are the +numerous 'faults' which occur in the harder rocks, and which often +extend for great distances in almost perfect straight lines. In our own +country we have the Tyneside and Craven faults in the North of England, +which are 30 miles long and often 20 yards wide; but even more striking +is the great Cleveland Dyke--a wall of volcanic rock dipping slightly +towards the south, but sometimes being almost vertical, and stretching +across the country, over hill and dale, in an almost perfect straight +line from a point on the coast ten miles north of Scarborough, in a +west-by-north direction, passing about two miles south of Stockton and +terminating about six miles north-by-east of Barnard Castle, a distance +of very nearly 60 miles. The great fault between the Highlands and +Lowlands of Scotland extends across the country from Stonehaven to near +Helensburgh, a distance of 120 miles; and there are very many more of +less importance. + +Much more extensive are some of the great continental dislocations, +often forming valleys of considerable width and length. The Upper Rhine +flows in one of these great valleys of subsidence for about 180 miles, +from Mulhausen to Frankfort, in a generally straight line, though +modified by denudation. Vaster still is the valley of the Jordan through +the Sea of Galilee to the Dead Sea, continued by the Wady Arabah to the +Gulf of Akaba, believed to form one vast geological depression or +fracture extending in a straight line for 400 miles. + +Thousands of such faults, dykes, or depressions exist in every part of +the world, all believed to be due to the gradual shrinking of the heated +interior to which the solid crust has to accommodate itself, and they +are especially interesting and instructive for our present purpose as +showing the tendency of such fractures of solid rock-material to extend +to great lengths in straight lines, notwithstanding the extreme +irregularity both in the surface contour as well as in the internal +structures of the varied deposits and formations through which they +pass. + +_Probable Origin of the Surface-features of Mars._ + +Returning now to Mars, let us consider the probable course of events +from the point at which we left it. The heat produced by impact and +condensation would be likely to release gases which had been in +combination with some of the solid matter, or perhaps been itself in a +solid state due to intense cold, and these, escaping outwards to the +surface, would produce on a small scale a certain amount of upheaval and +volcanic disturbance; and as an outer crust rapidly formed, a number of +vents might remain as craters or craterlets in a moderate state of +activity. Owing to the comparatively small force of gravity, the outer +crust would become scoriaceous and more or less permeated by the gases, +which would continue to escape through it, and this would facilitate the +cooling of the whole of the heated outer crust, and allow it to become +rather densely compacted. When the greater portion of the gases had thus +escaped to the outer surface and assisted to form a scanty atmosphere, +such as now exists, there would be no more internal disturbance and the +cooling of the heated outer coating would steadily progress, resulting +at last in a slightly heated, and later in a cold layer of moderate +thickness and great general uniformity. Owing to the absence of rain and +rivers, denudation such as we experience would be unknown, though the +superficial scoriaceous crust might be partially broken up by expansion +and contraction, and suffer a certain amount of atmospheric erosion. + +The final result of this mode of aggregation would be, that the planet +would consist of an outer layer of moderate thickness as compared with +the central mass, which outer layer would have cooled from a highly +heated state to a temperature considerably below the freezing-point, and +this would have been all the time _contracting upon a previously cold, +and therefore non-contracting nucleus._ The result would be that very +early in the process great superficial tensions would be produced, which +could only be relieved by cracks or fissures, which would initiate at +points of weakness--probably at the craterlets already referred to--from +which they would radiate in several directions. Each crack thus formed +near the surface would, as cooling progressed, develop in length and +depth; and owing to the general uniformity of the material, and possibly +some amount of crystalline structure due to slow and continuous cooling +down to a very low temperature, the cracks would tend to run on in +straight lines and to extend vertically downwards, which two +circumstances would necessarily result in their forming portions of +'great circles' on the planet's surface--the two great facts which Mr. +Lowell appeals to as being especially 'non-natural.' + +_Symmetry of Basaltic Columns._ + +We have however one quite natural fact on our earth which serves to +illustrate one of these two features, the direction of the downward +fissure. This is, the comparatively common phenomenon of basaltic +columns and 'Giant's Causeways.' The wonderful regularity of these, and +especially the not unfrequent upright pillars in serried ranks, as in +the palisades of the Hudson river, must have always impressed observers +with their appearance of artificiality. Yet they are undoubtedly the +result of the very slow cooling and contraction of melted rocks under +compression by strata _below and above them_, so that, when once +solidified, the mass was held in position and the tension produced by +contraction could only be relieved by numerous very small cracks at +short distances from each other in every direction, resulting in five, +six, or seven-sided polygons, with sides only a few inches long. This +contraction began of course at the coolest surface, generally the upper +one; and observation of these columns in various positions has +established the rule that their direction lengthways _is always at right +angles to the cooling surface_, and thus, whenever this surface was +horizontal, the columns became almost exactly vertical. + +_How this applies to Mars._ + +One of the features of the surface of Mars that Mr. Lowell describes +with much confidence is, that it is wonderfully uniform and level, which +of course it would be if it had once been in a liquid or plastic state, +and not much disturbed since by volcanic or other internal movements. +The result would be that cracks formed by contraction of the hardened +outer crust would be vertical; and, in a generally uniform material at a +very uniform temperature, these cracks would continue almost +indefinitely in straight lines. The hardened and contracting surface +being free to move laterally on account of there being a more heated and +plastic layer below it, the cracks once initiated above would +continually widen at the surface as they penetrated deeper and deeper +into the slightly heated substratum. Now, as basalt begins to soften at +about 1400 deg. F. and the surface of Mars has cooled to at least the +freezing-point--perhaps very much below it--the contraction would be so +great that if the fissures produced were 500 miles apart they might be +three miles wide at the surface, and, if only 100 miles apart, then +about two-thirds of a mile wide.[15] But as the production of the +fissures might have occupied perhaps millions of years, a considerable +amount of atmospheric denudation would result, however slowly it acted. +Expansion and contraction would wear away the edges and sides of the +fissures, fill up many of them with the debris, and widen them at the +surfaces to perhaps double their original size.[16] + +[Footnote 15: The coefficient of contraction of basalt is 0.000006 for +1 deg. F., which would lead to the results given here.] + +[Footnote 16: Mr. W.H. Pickering observed clouds on Mars 15 miles high; +these are the 'projections' seen on the terminator when the planet is +partially illuminated. They were at first thought to be mountains; but +during the opposition of 1894, more than 400 of them were seen at +Flagstaff during nine months' observation. Usually they are of rare +occurrence. They are seen to change in form and position from day to +day, and Mr. Lowell is strongly of opinion that they are dust-storms, +not what we term clouds. They were mostly about 13 miles high, +indicating considerable aerial disturbance on the planet, and therefore +capable of producing proportional surface denudation.] + +_Suggested Explanation of the 'Oases.'_ + +The numerous round dots seen upon the 'canals,' and especially at points +from which several canals radiate and where they intersect--termed +'oases' by Mr. Lowell and 'craterlets' by Mr. Pickering may be explained +in two ways. Those from which several canals radiate may be true craters +from which the gases imprisoned in the heated surface layers have +gradually escaped. They would be situated at points of weakness in the +crust, and become centres from which cracks would start during +contraction. Those dots which occur at the crossing of two straight +canals or cracks may have originated from the fact that at such +intersections there would be four sharply-projecting angles, which, +being exposed to the influence of alternate heat and cold (during day +and night) on the two opposite surfaces, would inevitably in time become +fractured and crumbled away, resulting in the formation of a roughly +circular chasm which would become partly filled up by the debris. Those +formed by cracks radiating from craterlets would also be subject to the +same process of rounding off to an even greater extent; and thus would +be produced the 'oases' of various sizes up to 50 miles or more in +diameter recorded by Mr. Lowell and other observers. + +_Probable Function of the Great Fissures._ + +Mr. Pickering, as we have seen, supposes that these fissures give out +the gases which, overflowing on each side, favour the growth of the +supposed vegetation which renders the course of the canals visible, and +this no doubt may have been the case during the remote periods when +these cracks gave access to the heated portions of the surface layer. +But it seems more probable that Mars has now cooled down to the almost +uniform mean temperature it derives from solar heat, and that the +fissures--now for the most part broad shallow valleys--serve merely as +channels along which the liquids and heavy gases derived from the +melting of the polar snows naturally flow, and, owing to their nearly +level surfaces, overflow to a certain distance on each side of them. + +_Suggested Origin of the Blue Patches._ + +These heavy gases, mainly perhaps, as has been often suggested, +carbon-dioxide, would, when in large quantity and of considerable depth, +reflect a good deal of light, and, being almost inevitably dust-laden, +might produce that blue tinge adjacent to the melting snow-caps which +Mr. Lowell has erroneously assumed to be itself a proof of the presence +of liquid water. Just as the blue of our sky is undoubtedly due to +reflection from the ultra-minute dust particles in our higher +atmosphere, similar particles brought down by the 'snow' from the higher +Martian atmosphere might produce the blue tinge in the great volumes of +heavy gas produced by its evaporation or liquefaction. + +It may be noted that Mr. Lowell objects to the carbon-dioxide theory of +the formation of the snow-caps, that this gas at low pressures does not +liquefy, but passes at once from the solid to the gaseous state, and +that only water remains liquid sufficiently long to produce the blue +colour' which plays so large a part in his argument for the mild climate +essential for an inhabited planet. But this argument, as I have already +shown, is valueless. For only very deep water can possibly show a blue +colour by reflected light, while a dust-laden atmosphere--especially +with a layer of very dense gas at the bottom of it, as would be the case +with the newly evaporated carbon-dioxide from the diminishing snow-cap +--would provide the very conditions likely to produce this blue tinge of +colour. + +It may be considered a support to this view that carbonic-acid gas +becomes liquid at--140 deg. F. and solid at--162 deg. F., temperatures far +higher than we should expect to prevail in the polar and north temperate +regions of Mars during a considerable part of the year, but such as +might be reached there during the summer solstice when the `snows' so +rapidly disappear, to be re-formed a few months later. + +_The Double Canals._ + +The curious phenomena of the 'double canals' are undoubtedly the most +difficult to explain satisfactorily on any theory that has yet been +suggested. They vary in distance apart from about 100 to 400 miles. In +many cases they appear perfectly parallel, and Mr. Lowell gives us the +impression that they are almost always so. But his maps show, in some +cases, decided differences of width at the two extremities, indicating +considerable want of parallelism. A few of the curved canals are also +double. + +There is one drawing in Mr. Lowell's book (p. 219) of the mouths, or +starting points, of the Euphrates and Phison, two widely separated +double canals diverging at an angle of about 40 deg. from the same two +oases, so that the two inner canals cross each other. Now this suggests +two wide bands of weakness in the planet's crust radiating probably from +within the dark tract called the 'Mare Icarium,' and that some +widespread volcanic outburst initiated diverging cracks on either side +of these bands. Something of this kind may have been the cause of most +of the double canals, or they may have been started from two or more +craterlets not far apart, the direction being at first decided by some +local peculiarity of structure; and where begun continuing in straight +lines owing to homogeneity or uniform density of material. This is very +vague, but the phenomena are so remarkable, and so very imperfectly +known at present, that nothing but suggestion can be attempted. + +_Concluding Remarks on the 'Canals.'_ + +In this somewhat detailed exposition of a possible, and, I hope, a +probable explanation of the surface-features of Mars, I have +endeavoured to be guided by known facts or accepted theories both +astronomical and geological. I think I may claim to have shown that +there are some analogous features of terrestrial rock-structure to +serve as guides towards a natural and intelligible explanation of the +strange geometric markings discovered during the last thirty years, and +which have raised this planet from comparative obscurity into a position +of the very first rank both in astronomical and popular interest. + +This wide-spread interest is very largely due to Mr. Lowell's devotion +to its study, both in seeking out so admirable a position as regards +altitude and climate, and in establishing there a first-class +observatory; and also in bringing his discoveries before the public in +connection with a theory so startling as to compel attention. I venture +to think that his merit as one of our first astronomical observers will +in no way be diminished by the rejection of his theory, and the +substitution of one more in accordance with the actually observed facts. + + + +APPENDIX. + +_A Suggested Experiment to Illustrate the 'Canals' of Mars._ + +If my explanation of the 'canals' should be substantially correct--that +is, if they were produced by the contraction of a heated outward crust +upon a cold, and therefore non-contracting interior, the result of such +a condition might be shown experimentally. + +Several baked clay balls might be formed to serve as cores, say of 8 to +10 inches in diameter. These being fixed within moulds of say half an +inch to an inch greater diameter, the outer layer would be formed by +pouring in some suitable heated liquid material, and releasing it from +the mould as soon as consolidation occurs, so that it may cool rapidly +from the _outside._ Some kinds of impure glass, or the brittle metals +bismuth or antimony or alloys of these might be used, in order to see +what form the resulting fractures would take. It would be well to have +several duplicates of each ball, and, as soon as tension through +contraction manifests itself, to try the effect of firing very small +charges of small shot to ascertain whether such impacts would start +radiating fractures. When taken from the moulds, the balls should be +suspended in a slight current of air, and kept rotating, to reproduce +the planetary condition as nearly as possible. + +The exact size and material of the cores, the thickness of the heated +outer crust, the material best suited to show fracture by contraction, +and the details of their treatment, might be modified in various ways as +suggested by the results first obtained. Such a series of experiments +would probably throw further light on the physical conditions which have +produced the gigantic system of fissures or channels we see upon the +surface of Mars, though it would not, of course, prove that such +conditions actually existed there. In such a speculative matter we can +only be guided by probabilities, based upon whatever evidence is +available. + + + +CHAPTER VIII. + + +SUMMARY AND CONCLUSION. + +This little volume has necessarily touched upon a great variety of +subjects, in order to deal in a tolerably complete manner with the very +extraordinary theories by which Mr. Lowell attempts to explain the +unique features of the surface of the planet, which, by long-continued +study, he has almost made his own. It may therefore be well to sum up +the main points of the arguments against his view, introducing a few +other facts and considerations which greatly strengthen my argument. + +The one great feature of Mars which led Mr. Lowell to adopt the view of +its being inhabited by a race of highly intelligent beings, and, with +ever-increasing discovery to uphold this theory to the present time, is +undoubtedly that of the so-called 'canals'--their straightness, their +enormous length, their great abundance, and their extension over the +planet's whole surface from one polar snow-cap to the other. The very +immensity of this system, and its constant growth and extension during +fifteen years of persistent observation, have so completely taken +possession of his mind, that, after a very hasty glance at analogous +facts and possibilities, he has declared them to be 'non-natural'-- +therefore to be works of art--therefore to necessitate the +presence of highly intelligent beings who have designed and constructed +them. This idea has coloured or governed all his writings on the +subject. The innumerable difficulties which it raises have been either +ignored, or brushed aside on the flimsiest evidence. As examples, he +never even discusses the totally inadequate water-supply for such +worldwide irrigation, or the extreme irrationality of constructing so +vast a canal-system the waste from which, by evaporation, when exposed +to such desert conditions as he himself describes, would use up ten +times the probable supply. + +Again, he urges the 'purpose' displayed in these 'canals.' Their being +_all_ so straight, _all_ describing great circles of the 'sphere,' all +being so evidently arranged (as he thinks) either to carry water to some +'oasis' 2000 miles away, or to reach some arid region far over the +equator in the opposite hemisphere! But he never considers the +difficulties this implies. Everywhere these canals run for thousands of +miles across waterless deserts, forming a system and indicating a +purpose, the wonderful perfection of which he is never tired of dwelling +upon (but which I myself can nowhere perceive). + +Yet he never even attempts to explain how the Martians could have lived +_before_ this great system was planned and executed, or why they did not +_first_ utilise and render fertile the belt of land adjacent to the +limits of the polar snows--why the method of irrigation did not, as with +all human arts, begin gradually, at home, with terraces and channels to +irrigate the land close to the source of the water. How, with such a +desert as he describes three-fourths of Mars to be, did the inhabitants +ever get to _know_ anything of the equatorial regions and its needs, so +as to start right away to supply those needs? All this, to my mind, is +quite opposed to the idea of their being works of art, and altogether in +favour of their being natural features of a globe as peculiar in origin +and internal structure as it is in its surface-features. The explanation +I have given, though of course hypothetical, is founded on known +cosmical and terrestrial facts, and is, I suggest, far more scientific +as well as more satisfactory than Mr. Lowell's wholly unsupported +speculation. This view I have explained in some detail in the preceding +chapter. + +Mr. Lowell never even refers to the important question of loss by +evaporation in these enormous open canals, or considers the undoubted +fact that the only intelligent and practical way to convey a limited +quantity of water such great distances would be by a system of +water-tight and air-tight tubes laid _under the ground._ The mere +attempt to use open canals for such a purpose shows complete ignorance +and stupidity in these alleged very superior beings; while it is certain +that, long before half of them were completed their failure to be of any +use would have led any rational beings to cease constructing them. + +He also fails to consider the difficulty, that, if these canals are +necessary for existence in Mars, how did the inhabitants ever reach a +sufficiently large population with surplus food and leisure enabling +them to rise from the low condition of savages to one of civilisation, +and ultimately to scientific knowledge? Here again is a dilemma which is +hard to overcome. Only a _dense_ population with _ample_ means of +subsistence could possibly have constructed such gigantic works; but, +given these two conditions, no adequate motive existed for the +conception and execution of them--even if they were likely to be of any +use, which I have shown they could not be. + +_Further Considerations on the Climate of Mars._ + +Recurring now to the question of climate, which is all-important, Mr. +Lowell never even discusses the essential point--the temperature that +must _necessarily_ result from an atmospheric envelope one-twelfth (or +at most one-seventh) the density of our own; in either case +corresponding to an altitude far greater than that of our highest +mountains.[17] Surely this phenomenon, everywhere manifested on the +earth even under the equator, of a regular decrease of temperature with +altitude, the only cause of which is a less dense atmosphere, should +have been fairly grappled with, and some attempt made to show why it +should not apply to Mars, except the weak remark that on a level surface +it will not have the same effect as on exposed mountain heights. But it +_does_ have the same effect, or very nearly so, on our lofty plateaux +often hundreds of miles in extent, in proportion to their altitude. +Quito, at 9350 ft. above the sea, has a mean temperature of about 57 deg. +F., giving a lowering of 23 deg. from that of Manaos at the mouth of the Rio +Negro. This is about a degree for each 400 feet, while the general fall +for isolated mountains is about one degree in 340 feet according to +Humboldt, who notes the above difference between the rate of cooling for +altitude of the plains--or more usually sheltered valleys in which the +towns are situated--and the exposed mountain sides. It will be seen that +this lower rate would bring the temperature of Mars at the equator down +to 20 deg. F. below the freezing point of water from this cause alone. + +[Footnote 17: A four inches barometer is equivalent to a height of +40,000 feet above sea-level with us.] + +But all enquirers have admitted, that if conditions as to atmosphere +were the same as on the earth, its greater distance from the sun would +reduce the temperature to-31 deg. F., equal to 63 deg. below the freezing +point. It is therefore certain that the combined effect of both causes +must bring the temperature of Mars down to at least 70 deg. or 80 deg.below the +freezing point. + +The cause of this absolute dependence of terrestrial temperatures upon +density of the air-envelope is seldom discussed in text-books either of +geography or of physics, and there seems to be still some uncertainty +about it. Some impute it wholly to the thinner air being unable to +absorb and retain so much heat as that which is more dense; but if this +were the case the soil at great altitudes not having so much of its heat +taken up by the air should be warmer than below, since it undoubtedly +_receives_ more heat owing to the greater transparency of the air above +it; but it certainly does not become warmer. The more correct view seems +to be that the loss of heat by radiation is increased so much through +the rarity of the air above it as to _more_ than counterbalance the +increased insolation, so that though the surface of the earth at a given +altitude may receive 10 per cent. more direct sun-heat it loses by +direct radiation, combined with diminished air and cloud-radiation, +perhaps 20 or 25 per cent. more, whence there is a resultant cooling +effect of 10 or 15 per cent. This acts by day as well as by night, so +that the greater heat received at high altitudes does not warm the soil +so much as a less amount of heat with a denser atmosphere. + +This effect is further intensified by the fact that a less dense cannot +absorb and transmit so much heat as a more dense atmosphere. Here then +we have an absolute law of nature to be observed operating everywhere on +the earth, and the mode of action of which is fairly well understood. +This law is, that reduced atmospheric pressure increases radiation, or +loss of heat, _more rapidly_ than it increases insolation or gain of +heat, so that the result is _always_ a considerable _lowering_ of +temperature. What this lowering is can be seen in the universal fact, +that even within the tropics perpetual snow covers the higher mountain +summits, while on the high plains of the Andes, at 15,000 or 16,000 feet +altitude, where there is very little or no snow, travellers are often +frozen to death when delayed by storms; yet at this elevation the +atmosphere has much more than double the density of that of Mars! + +The error in Mr. Lowell's argument is, that he claims for the scanty +atmosphere of Mars that it allows more sun-heat to reach the surface; +but he omits to take account of the enormously increased loss of heat by +direct radiation, as well as by the diminution of air-radiation, which +together necessarily produce a great reduction of temperature. + +It is this great principle of the prepotency of radiation over +absorption with a diminishing atmosphere that explains the excessively +low temperature of the moon's surface, a fact which also serves to +indicate a very low temperature for Mars, as I have shown in Chapter VI. +These two independent arguments--from alpine temperatures and from those +of the moon--support and enforce each other, and afford a conclusive +proof (as against anything advanced by Mr. Lowell) that the temperature +of Mars must be far too low to support animal life. + +A third independent argument leading to the same result is Dr. Johnstone +Stoney's proof that aqueous vapour cannot exist on Mars; and this fact +Mr. Lowell does not attempt to controvert. + +To put the whole case in the fewest possible words: + +All physicists are agreed that, owing to the distance of Mars from the +sun, it would have a mean temperature of about-35 deg. F. (= 456 deg. F. abs.) +even if it had an atmosphere as dense as ours. + +(2) But the very low temperatures on the earth under the equator, at a +height where the barometer stands at about three times as high as on +Mars, proves, that from scantiness of atmosphere alone Mars cannot +possibly have a temperature as high as the freezing point of water; and +this proof is supported by Langley's determination of the low _maximum_ +temperature of the full moon. + +The combination of these two results must bring down the temperature of +Mars to a degree wholly incompatible with the existence of animal life. + +(3) The quite independent proof that water-vapour cannot exist on Mars, +and that therefore, the first essential of organic life--water--is +non-existent. + +The conclusion from these three independent proofs, which enforce each +other in the multiple ratio of their respective weights, is therefore +irresistible--that animal life, especially in its higher forms, cannot +exist on the planet. + +Mars, therefore, is not only uninhabited by intelligent beings such as +Mr. Lowell postulates, but is absolutely UNINHABITABLE. + + + + + +End of Project Gutenberg's Is Mars Habitable?, by Alfred Russel Wallace + +*** END OF THIS PROJECT GUTENBERG EBOOK IS MARS HABITABLE? *** + +***** This file should be named 10855.txt or 10855.zip ***** +This and all associated files of various formats will be found in: + https://www.gutenberg.org/1/0/8/5/10855/ + +Produced by Thaadd and the PG Distributed Proofreading Team + +Updated editions will replace the previous one--the old editions +will be renamed. + +Creating the works from public domain print editions means that no +one owns a United States copyright in these works, so the Foundation +(and you!) can copy and distribute it in the United States without +permission and without paying copyright royalties. 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