Is Mars habitable? A critical examination of Professor Percival Lowell's book "Mars and its canals," with an alternative explanation
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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.18 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.
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.
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.
2
Man's Place in the Universe p. 267 (1903).
3
This is on the opposite side of Mars from that shown in the frontispiece.
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.
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.
6
Areas on Mars so named.
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.
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."
9
For a short account of this remarkable instrument, see my Wonderful Century, new ed., pp. 143-145.
10
Keith Johnston's 'Africa' in Stanford's Compendium.
11
Chambers's Encyclopaedia, Art. 'Deserts.'
12
LaTex markup $\root 4 \of {1 \over 100} = {1 \over 3.2}$
13
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!
14
Nature, vol. 70, p. 536.
15
Nature, vol. 70, May 5, p.xi, supplement.
16
The coefficient of contraction of basalt is 0.000006 for 1° F., which would lead to the results given here.
17
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.
18
A four inches barometer is equivalent to a height of 40,000 feet above sea-level with us.