CHAPTER XI
LIFE IN OTHER WORLDS
If life was not brought to the earth from another planet, then life was created, or originated, on the earth. Some of the conditions which attended its birth have been considered, and they amount to this: that the temperature of the earth, the elements of the earth, the conditions of the earth's surface, oceans, and atmosphere were exactly those which favoured the origination, the continuance, and the development of living things. The earth, among all the heavenly bodies which we can examine at all closely, is probably the only one on which life, as we know it, would have much chance of survival.
The Sun as an abode of life we may at once put out of the question. Taking the planets in order of distance from the Sun: of Mercury we know very little, but astronomers like Schiaparelli and Lowell have pronounced it to be an airless, dead planet with a surface cracked in cooling untold ages ago. Venus is believed to be much like the earth, not differing greatly in size, and probably having an atmosphere of considerable extent;[10] but its appearance is so bright, viewed from the earth, that it has been surmised that what we see is not the planet itself but its atmosphere always charged with clouds or possibly snow. Of Mars as an abode of life, and of the Moon, which is the body nearest to us, we shall speak more fully in the present chapter. Coming to the outer major planets, the giant Jupiter—with a bulk more than a thousand times as great as the earth—has a constitution by no means so solid. For many reasons the belief seems justified that Jupiter must be a still hot, and almost gaseous body, without a solid crust. If Jupiter's comparatively small weight for its size and its wonderful and varying system of cloud coverings are evidence of an early stage of development and a high internal temperature, still more is this the case with Saturn. In bulk it is not far inferior to Jupiter; but it is so much lighter than water that if some of its fragments fell into one of the earth's oceans they would float there. Its outer coverings or envelopes must consist of heated gases in active circulation. Of Uranus and Neptune, still farther off, we know very little, and the progress of knowledge concerning them is very slow; but it is more probable that they are in the early stage of development attributed to Jupiter and Saturn than in the solid stage of planets like the earth. So that we may fairly dismiss the probability of the existence of life as we know it on any of them—and neglect incidentally, therefore, any possibility that life could have been carried in a meteorite from them to us. Whether there are other forms of "life" than any we know is a question hardly needing discussion.
[10] W. W. Bryant, A History of Astronomy (Methuen), 1907.
There remains the question of the probability of life on the Moon or on Mars; and the question of possible life on the Moon is specially worth consideration, because the earth's satellite was once part of the earth's mass. We may first repeat briefly the explanation which Sir G. H. Darwin has given of the Moon's separation from the earth. If a flexible hoop or ring is spun very rapidly it will be seen to flatten itself at top and bottom (or, as we might say, at its poles) and broaden itself out at its middle or equator. The semi-liquid earth once rotated so rapidly that its swelling equatorial belt was almost at the point of separation from the parent body. 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 a considerable distance from it. There are several estimates of the bulk of the earth thus shot off; but we may assume that about one-fiftieth of the earth escaped thus. It must have consisted of a considerable portion of the earth's solid crust, and a much larger quantity of the molten rocks of the earth's interior.
The Moon is much lighter than the earth. The earth taken as a whole weighs about five and a half times as much as water. If we consider its surface alone, this weighs rather more than two and a half times the weight of water—from which it can be seen that the interior of the earth is very much denser than the earth's surface crust. [11] The Moon weighs rather less than three and a half times its bulk in water. This shows clearly that the Moon is composed of material scraped off from the outer surface of the earth, rather than of matter obtained from a considerable depth. At the same time the layer of material removed had an appreciable thickness. The volume of the Moon is equivalent to a solid body whose surface is equal to the area of all the earth's oceans, and whose depth would be thirty-six miles. It seems probable, therefore, that at the time when the Moon was torn off, or shot off, from the earth, the parent body had a solid crust averaging at least thirty-six miles in thickness, while beneath this crust the temperature was so high that the materials underneath were molten or liquid, and in other places were only kept solid by the enormous pressure of the material above them. When the Moon separated from the earth three-quarters of this crust was carried away. It has sometimes been supposed that the remainder was torn into two parts, one of which formed the great land area of the Eastern Hemisphere and the other the great land area of which North and South America are the relics in the Western Hemisphere. These two great areas, at that time, floated on the semi-liquid surface like two large ice-floes. But they were, of course, a good deal heavier than ice, and the molten stuff on which they floated was a good deal heavier than water. Later on this liquid stuff cooled and hardened. But its bottom was still a good deal lower than the surface of the great areas of land which had "floated" on it; and therefore it formed great depressions all about and surrounding them. Thus the depressions were there ready formed, into which the waters of the earth, beginning as rain and ending as rivers or lakes, could flow as into reservoirs. On the whole scientific men incline to believe that, according to a popular tradition, the Moon may have been torn from the earth where the Pacific Ocean now lies, and may have left that hollow behind it when it went.
[11] The figures are: earth's specific gravity = 5·6. The specific gravity of the surface material ranges from, in general, between 2·2 and 3·2, with an average of 2·7. The specific gravity of the Moon is 3·4.
Many people, scientific men and astronomers among them, have imagined the possibilities of life on the Moon. In his clever romance, The First Man on the Moon, Mr. H. G. Wells has gathered together all the more reasoned speculations on the subject. They all turn on one point—Is there an atmosphere on the Moon which would support life? There are gases of some kind on the Moon. There must be gases, for example, shut up in the moon's rocks; there may be gases in the Moon's interior. Mr. Wells imagined that there was a good deal of gas inside the Moon; indeed, he went so far as to suppose that the Moon was partly hollow. If it were we should perhaps be able to accept that as an explanation of the fact that the Moon is, bulk for bulk, considerably lighter than the earth, and is, in short, rather lighter than we should expect it to be. If the Moon were hollow there might be an atmosphere and water inside it, and a race of beings might live there—in the underworld of the Moon. The "Selenites," as Mr. Wells called them, would probably be not in the least like human beings, though they might be immeasurably more intelligent, because, seeing that the earth cooled at a later period than the moon, life might have begun earlier on the Moon, and would have had, perhaps, many hundreds of thousands of years in which to develop. Mr. Wells therefore imagined the "Selenites" to resemble in some respects a race of very large insects with enormous brains. However, we need pursue these romantic speculations no further, but must turn to inquire not whether life might exist in the interior of the Moon (which we can never see), but what would be the kind of life that could exist on the part of the Moon that we can see.
In the first place, we believe that the atmosphere there would be very, very thin; as thin as the atmosphere which is left in the bell-receiver of an ordinary air-pump when the experimenter has done the best he can to exhaust it of air altogether.[12] In the second place, the atmosphere would not be one of oxygen and nitrogen as that of the earth is, but of some heavy gas like carbon-dioxide (which will not support animal life). The question is whether it would support vegetable life. Several astronomers (no less eminent a one than W. H. Pickering, of Harvard University, among them) have supposed that it might, and they have imagined great jungles of vegetation springing up on the surface of the Moon under the influence of the Sun's rays—jungles which are stricken down again when they are four days old under the oncoming of night. For the Moon's day is equal to several of ours, and when night comes there the temperature sinks to a level colder far than that of the earth's Arctic regions; so cold, in fact, that even gases would be turned to liquid and then frozen solid. It is by no means certain that the gases we have mentioned would support vegetable life, but assuming that they would, we should then expect the vegetation to spring up with extraordinary rapidity—because it would be so little hampered by its own weight—when the vertical rays of the Sun were beating down on the Moon. When that was the case the temperature there would be from 500° (F.) to 600° (F.) higher than during the night.
[12] The exhaustion produced by an ordinary air-pump is never a complete vacuum. Exhaustion which leaves only 1/2000th of the original air is unusually efficient.
Perhaps we may here explain some of the reasons why vegetation would be little hampered by its own weight on the Moon. It is similar to the reasons why light gases escape from the Moon. The mass of the Moon—that is to say, the amount of matter it contains—is 1/80th that of the earth. Therefore since the weight of a body means the measure of the force by which gravity attracts it (to the earth or to the Moon), bodies on the Moon's surface are much lighter than they are here. The ratio is almost exactly one-sixth; consequently a man weighing 180 lbs. on the earth if transplanted to the Moon would find that he only weighed 30 lbs. there, and could carry two men at once on his back for twenty miles much more easily than he could walk that distance without a load here. He could throw a stone six times as far as on the earth, and jump six times as high. Indeed, jumping over a moderate-sized house would be a gymnastic feat scarcely worth mentioning on the Moon.
After consideration of all these facts; and despite the belief of some distinguished astronomers that changes are sometimes perceptible on the Moon's surface; and that hoar frost can be perceived there; and perhaps volcanic eruptions—the general conclusion arrived at by astronomical authority is that organic life does not exist either on the Moon or in it; and we may conclude this outline of the speculations concerning it by quoting the American astronomer, Professor N. S. Shaler: "It is naturally painful to conclude that the Moon is and always has been deprived of those features of existence which we deem the nobler; that it has never known the stir of air or water or the higher life of beings who inherit the profit of experience, and thereby climb the way that has led upward to man. That these large gifts have been denied to the nearest companion of the earth has its lesson for the naturalist. How vast are the effects arising from the interrelation of actions.... If the gases could have been retained in the Moon (by its attractive force) there is no reason why it should not have had the history of a miniature earth. As it is, from the beginning it appears to have been determined that the Sun should not warm it in the same way as the earth; that rain should not fall on it, nor the stirrings of life and energy be visible on it. There is no imaginable accident that can alter its state. Just as it is, our Moon is likely to see the Sun go out."
This chapter may be ended by a brief application of some of these considerations to the case of the planet Mars. Next to the Moon Mars is our nearest neighbour, and the erection of great telescopes in America, one of them at Flagstaff Observatory, Arizona, where the air is extraordinarily clear and telescopic vision unusually penetrating, has stimulated the observation of the planet to a very great degree during recent years. Mars has an atmosphere not nearly so dense as that of the earth, but still dense enough in all probability to support some form of organic life. It may, for example, support vegetation. In some other respects Mars resembles the earth. It has arctic circles; it has clouds, though whether these are of vapour or of dust is not quite certain; and it has a less variable temperature by far than that of the Moon. There are, at any rate, some of the conditions to support and perhaps to encourage life; and if we could be certain that the atmosphere in Mars more nearly resembles that of the earth, and that its temperature was such as to be sometimes above that of our Arctic regions, then it would be difficult to deny that life, and probably intelligent beings, existed there. One very able and intelligent astronomer is convinced that life and intelligent beings do exist there. This is Professor Lowell, of Flagstaff Observatory, who has devoted a number of years and a great deal of money to the careful observation of the planet. He has brought forward many cogent arguments to show that Mars might be inhabited, and that great telescopes can discover signs on it, and may discover further signs, which are a reason for supposing it to be so. It is not, however, within the range of this book to examine these reasons in detail; and we need only say that in the first decade of the twentieth century most astronomers, despite the close examination of Mars and its markings, which had been conducted for more than a generation, were still not convinced that life as we know it could exist there.