CHAPTER XIV
THE EARTH IS THE ONLY HABITABLE PLANET IN THE SOLAR SYSTEM
Having shown in the last three chapters how numerous and how complex are the conditions which alone render life possible on our earth, how nicely balanced are opposing forces, and how curious and delicate are the means by which the essential combinations of the elements are brought about, it will be a comparatively easy task to show how totally unfitted are all the other planets either to develop or to preserve the higher forms of life, and, in most cases, any forms above the lowest and most rudimentary. In order to make this clear we will take the most important of the conditions in order, and see how the various planets fulfil them.
Mass of a Planet and its Atmosphere
The height and density of the atmosphere of a planet is important as regards life in several ways. On its density depends its power of carrying moisture; of holding a sufficient supply of dust-particles for the formation of clouds; of carrying ultra-microscopic particles to such a height and in such quantity as to diffuse the light of the sun by reflection from the whole sky; of raising waves in the ocean and thus aerating its waters, and of producing the ocean currents which so greatly equalise temperature. Now this density depends on two factors: the mass of the planet and the quantity of the atmospheric gases. But there is good reason to think that the latter depends directly upon the former, because it is only when a certain mass is attained that any of the lighter permanent gases can be held on the surface of a planet. Thus, according to Dr. G. Johnstone Stoney, who has specially studied this subject, the moon cannot retain even such a heavy gas as carbonic acid, or the still heavier carbon disulphide; while no particle of oxygen, nitrogen, or water-vapour can possibly remain on it, owing to the fact of its mass being only about one-eightieth that of the earth. It is believed that there are considerable quantities of gases in the stellar spaces, and probably also within the solar system, but perhaps in the liquid or solid form. In that state they might be attracted by any small mass such as the moon, but the heat of its surface when exposed to the solar rays would quickly restore them to the gaseous condition, when they would at once escape.
It is only when a planet attains a mass at least a quarter that of the earth that it is capable of retaining water-vapour, one of the most essential of the gases; but with so small a mass as this, its whole atmosphere would probably be so limited in amount and so rare at the planet's surface that it would be quite unable to fulfil the various purposes for which an atmosphere is required in order to support life. For their adequate fulfilment the mass of a planet cannot be much less than that of the earth. Here we come to one of those nice adjustments of which so many have been already pointed out. Dr. Johnstone Stoney arrives at the conclusion that hydrogen escapes from the earth. It is continually produced in small quantities by submarine volcanoes, by fissures in volcanic regions, from decaying vegetation, and from some other sources; yet, though sometimes found in minute quantities, it forms no regular constituent of our atmosphere.18
The quantity of hydrogen combined with oxygen to form the mass of water in our vast and deep oceans is enormous. Yet if it had been only one-tenth more than it actually is, the present land-surface would have been almost all submerged. How the adjustments occurred so that there was exactly enough hydrogen to fill the vast ocean basins with water to such a depth as to leave enough land-surface for the ample development of vegetable and animal life, and yet not so much as to be injurious to climate, it is difficult to imagine. Yet the adjustment stares us in the face. First, we have a satellite unique in size as compared with its primary, and apparently in lateness of origin; then we have a mode of origin for that satellite said to be certainly unique in the solar system; as a consequence of this origin, it is believed, we have enormously deep ocean basins symmetrically placed with regard to the equator—an arrangement which is very important for ocean circulation; then we must have had the right quantity of hydrogen, obtained in some unknown way, which formed water enough to fill these chasms, so as to leave an ample area of dry land, but which one-tenth more water would have ingulfed; and, lastly, we have oxygen enough left to form an atmosphere of sufficient density for all the requirements of life. It could not be that the surplus hydrogen escaped when the water had been produced, because it escapes very slowly, and it combines so easily with free oxygen by means of even a spark, as to make it certain that all the available hydrogen was used up in the oceanic waters, and that the supply from the earth's interior has been since comparatively small in amount.
There is yet one more adjustment to be noticed. All the facts now referred to show that the earth's mass is sufficient to bring about the conditions favourable for life. But if our globe had been a little larger, and proportionately denser, in all probability no life would have been possible. Between a planet of 8000 and one of 9500 miles diameter is not a large difference, when compared with the enormous range of size of the other planets. Yet this slight increase in diameter would give two-thirds increase in bulk, and, with a corresponding increase of density due to the greater gravitative force, the mass would be about double what it is. But with double the mass the quantity of gases of all sorts attracted and retained by gravity would probably have been double; and in that case there would have been double the quantity of water produced, as no hydrogen could then escape. But the surface of the globe would only be one half greater than at present, in which case the water would have sufficed to cover the whole surface several miles deep.
Habitability of Other Planets
When we look to the other planets of our system we see everywhere illustrations of the relation of size and mass to habitability. The smaller planets, Mercury and Mars, have not sufficient mass to retain water-vapour, and, without it, they cannot be habitable. All the larger planets can have very little solid matter, as indicated by their very low density notwithstanding their enormous mass. There is, therefore, very good reason for the belief that the adaptability of a planet for a full development of life is primarily dependent, within very narrow limits, on its size and, more directly, on its mass. But if the earth owes its specially constituted atmosphere and its nicely adjusted quantity of water to such general causes as here indicated, and the same causes apply to the other planets of the solar system, then the only planet on which life can be possible is Venus. As, however, it may be urged that exceptional causes may have given other planets an equal advantage in the matter of air and water, we will briefly consider some of the other conditions which we have found to be essential in the case of the earth, but which it is almost impossible to conceive as existing, to the required extent, on any of the other planets of the solar system.
A Small and Definite Range of Temperature
We have already seen within what narrow limits the temperature on a planet's surface must be maintained in order to develop and support life. We have also seen how numerous and how delicate are the conditions, such as density of atmosphere, extent and permanence of oceans, and distribution of sea and land, which are requisite, even with us, in order to render possible the continuous preservation of a sufficiently uniform temperature. Slight alterations one way or another might render the earth almost uninhabitable, through its being liable to alternations of too great heat or excessive cold. How then can we suppose that any other of the planets, which have either very much more or very much less sun-heat than we receive, could, by any possible modification of conditions, be rendered capable of producing and supporting a full and varied life-development?
Mars receives less than half the amount of sun-heat per unit of surface that we do. And as it is almost certain that it contains no water (its polar snows being caused by carbonic acid or some other heavy gas) it follows that, although it may produce vegetable life of some low kinds, it must be quite unsuited for that of the higher animals. Its small size and mass, the latter only one-ninth that of the earth, may probably allow it to possess a very rare atmosphere of oxygen and nitrogen, if those gases exist there, and this lack of density would render it unable to retain during the night the very moderate amount of heat it might absorb during the day. This conclusion is supported by its low reflecting power, showing that it has hardly any clouds in its scanty atmosphere. During the greater part of the twenty-four hours, therefore, its surface-temperature would probably be much below the freezing point of water; and this, taken in conjunction with the total absence of aqueous vapour or liquid water, would add still further to its unsuitability for animal life.
In Venus the conditions are equally adverse in the other direction. It receives from the sun almost double the amount of heat that we receive, and this alone would render necessary some extraordinary combination of modifying agencies in order to reduce and render uniform the excessively high temperature. But it is now known that Venus has one peculiarity which is in itself almost prohibitive of animal life, and probably of even the lowest forms of vegetable life. This peculiarity is, that through tidal action caused by the sun, its day has been made to coincide with its year, or, more properly, that it rotates on its axis in the same time that it revolves round the sun. Hence it always presents the same face to the sun; and while one half has a perpetual day, the other half has perpetual night, with perpetual twilight through refraction in a narrow belt adjoining the illuminated half. But the side that never receives the direct rays of the sun must be intensely cold, approximating, in the central portions, to the zero of temperature, while the half exposed to perpetual sunshine of double intensity to ours must almost certainly rise to a temperature far too great for the existence of protoplasm, and probably, therefore, of any form of animal life.
Venus appears to have a dense atmosphere, and its brilliancy suggests that we see the upper surface of a cloud-canopy, and this would no doubt greatly reduce the excessive solar heat. Its mass, being a little more than three-fourths that of the earth, would enable it to retain the same gases as we possess. But under the extraordinary conditions that prevail on the surface of this planet, it is hardly possible that the temperature of the illuminated side can be preserved in a sufficient state of uniformity for the development of life in any of its higher forms.
Mercury possesses the same peculiarity of keeping one face always towards the sun, and as it is so much smaller and so much nearer the sun its contrasts of heat and cold must be still more excessive, and we need hardly discuss the possibility of this planet being habitable. Its mass being only one-thirtieth that of the earth, water-vapour will certainly escape from it, and, most probably, nitrogen and oxygen also, so that it can possess very little atmosphere; and this is indicated by its low reflecting power, no less than 83 per cent. of the sun's light being absorbed, and only 17 per cent. reflected, whereas clouds reflect 72 per cent. This planet is therefore intensely heated on one side and frozen on the other; it has no water and hardly any atmosphere, and is therefore, from every point of view, totally unfitted for supporting living organisms.
Even if it is supposed that, in the case of Venus, its perpetual cloud-canopy may keep down the surface temperature within the limits necessary for animal life, the extraordinary turmoil in its atmosphere caused by the excessively contrasted temperatures of its dark and light hemispheres must be extremely inimical to life, if not absolutely prohibitive of it. For on the greater part of the hemisphere that never receives a ray of light or heat from the sun all the water and aqueous vapour must be turned into ice or snow, and it seems almost impossible that the air itself can escape congelation. It could only do so by a very rapid circulation of the whole atmosphere, and this would certainly be produced by the enormous and permanent difference of temperature between the two hemispheres. Indications of refraction by a dense atmosphere are visible during the planet's transit over the sun's disc, and also when it is in conjunction with the sun, and the refraction is so great that Venus is believed to have an atmosphere much higher than ours. But during the rapid circulation of such an atmosphere, heated on one half the planet and cooled on the other, most of the aqueous vapour must be taken out of it on the dark side as fast as it is produced on the heated side, though sufficient may remain to produce a canopy of very lofty clouds analogous to our cirri. The occasional visibility of the dark side of Venus may be caused by an electrical glow due to the friction of the perpetually overflowing and inflowing atmosphere, this being increased by reflection from a vast surface of perpetual snow. If we consider all the exceptional features of this planet, it appears certain that the conditions as regards climate cannot now be such as to maintain a temperature within the narrow limits essential for life, while there is little probability that at any earlier period it can have possessed and maintained the necessary stability during the long epochs which are requisite for its development.
Before considering the condition of the larger planets, it will be well to refer to an argument which has been supposed to minimise the difficulties already stated as to those planets which approach nearest to the earth in size and distance from the sun.
The Argument from Extreme Conditions, on the Earth
In reply to the evidence showing how nice are the adaptations required for life-development, it is often objected that life does now exist under very extreme conditions—under tropic heat and arctic snows; in the burnt-up desert as well as in the moist tropical forest; in the air as well as in the water; on lofty mountains as well as on the level lowlands. This is no doubt true, but it does not prove that life could have been developed in a world where any of these extremes of climate characterised the whole surface. The deserts are inhabited because there are oases where water is attainable, as well as in the surrounding fertile areas. The arctic regions are inhabited because there is a summer, and during that summer there is vegetation. If the surface of the ground were always frozen, there would be no vegetation and no animal life.
The late Mr. R.A. Proctor put this argument of the diversity of conditions under which life actually does exist on the earth as well probably as it can be put. He says: 'When we consider the various conditions under which life is found to prevail, that no difference of climatic relations, or of elevation, of land, or of air, or of water, of soil in land, of freshness or saltness in water, of density in air, appears (so far as our researches have extended) to render life impossible, we are compelled to infer that the power of supporting life is a quality which has an exceedingly wide range in nature.'
This is true, but with certain reservations. The only species of animal which does really exist under the most varied conditions of climate is man, and he does so because his intellect renders him to some extent the ruler of nature. None of the lower animals have such a wide range, and the diversity of conditions is not really so great as it appears to be. The strict limits are nowhere permanently overpassed, and there is always the change from winter to summer, and the possibility of migration to less inhospitable areas.
The Great Planets all Uninhabitable
Having already shown that the condition of Mars, both as regards water, atmosphere, and temperature, is quite unfitted to maintain life, a view in which both general principles and telescopic examination perfectly agree, we may pass on to the outer planets, which, however, have long been given up as adapted for life even by the most ardent advocates for 'life in other worlds.' Their remoteness from the sun—even Jupiter being five times as far as the earth, and therefore receiving only one twenty-fifth of the light and heat that we receive per unit of surface—renders it almost impossible, even if other conditions were favourable, that they should possess surface-temperatures adequate to the necessities of organic life. But their very low densities, combined with very large size, renders it certain that they none of them have a solidified surface, or even the elements from which such a surface could be formed.
It is supposed that Jupiter and Saturn, as well as Uranus and Neptune, retain a considerable amount of internal heat, but certainly not sufficient to keep the metallic and other elements of which the sun and earth consist in a state of vapour, for if so they would be planetary stars and would shine by their own light. And if any considerable portion of their bulk consisted of these elements, whether in a solid or a liquid state, their densities would necessarily be much greater than that of the earth instead of very much less—Jupiter is under one-fourth the density of the earth, Saturn under an eighth, while Uranus and Neptune are of intermediate densities, though much less in bulk even than Saturn.
It thus appears that the solar system consists of two groups of planets which differ widely from each other. The outer group of four very large planets are almost wholly gaseous, and probably consist of the permanent gases—those which can only be liquefied or solidified at a very low temperature. In no other way can their small density combined with enormous bulk be accounted for.
The inner group also of four planets are totally unlike the preceding. They are all of small size, the earth being the largest. They are all of a density roughly proportionate to their bulk. The earth is both the largest and the densest of the group; not only is it situated at that distance from the sun which, through solar heat alone, allows water to remain in the liquid state over almost the whole of its surface, but it possesses numerous characteristics which secure a very equable temperature, and which have secured to it very nearly the same temperature during those enormous geological periods in which terrestrial life has existed. We have already shown that no other planet possesses these characteristics now, and it is almost equally certain that they never have possessed them in the past, and never will possess them in the future.
A Last Argument for Habitability of the Planets
Although it has been admitted by the late Mr. Proctor and some other astronomers that most of the planets are not now habitable, yet, it is often urged, they may have been so in the past or may become so in the future. Some are now too hot, others are now too cold; some have now no water, others have too much; but all go through their appointed series of stages, and during some of these stages life may be or may have been possible. This argument, although vague, will appeal to some readers, and it may, therefore, be necessary to reply to it. This is the more necessary as it is still made use of by astronomers. In a criticism of my article in The Fortnightly Review, M. Camille Flammarion, of the Paris Observatory, dramatically remarks: 'Yes, life is universal, and eternal, for time is one of its factors. Yesterday the moon, to-day the earth, to-morrow Jupiter. In space there are both cradles and tombs.'19
It is thus suggested that the moon was once inhabited and that Jupiter will be inhabited in some remote future; but no attempt is made to deal with the essential physical conditions of these very diverse objects, rendering them not only now, but always, unfitted to develop and to maintain terrestrial or aerial life. This vague supposition—it can hardly be termed an argument—as regards past or future adaptability for life, of all the planets and some of the satellites in the solar system, is, however, rendered invalid by an equally general objection to which its upholders appear never to have given a moment's consideration; and as it is an objection which still further enforces the view as to the unique position of the earth in the solar system, it will be well to submit it to the judgment of our readers.
Limitation of the Sun's Heat
It is well known that there is, and has been for nearly half a century, a profound difference of opinion between geologists and physicists as to the actual or possible duration in years of life upon the earth. The geologists, being greatly impressed with the vast results produced by the slow processes of the wearing away of the rocks and the deposit of the material in seas or lakes, to be again upheaved to form dry land, and to be again carved out by rain and wind, by heat and cold, by snow and ice, into hills and valleys and grand mountain ranges; and further, by the fact that the highest mountains in every part of the globe very often exhibit on their loftiest summits stratified rocks which contain marine organisms, and were therefore originally laid down beneath the sea; and, yet again, by the fact that the loftiest mountains are often the most recent, and that these grand features of the earth's surface are but the latest examples of the action of forces that have been at work throughout all geological time—studying all their lives the detailed evidences of all these changes, have come to the conclusion that they imply enormous periods only to be measured by scores or hundreds of millions of years.
And the collateral study of fossil remains in the long series of rock-formations enforces this view. In the whole epoch of human history, and far back into prehistoric times during which man existed on the earth, although several animals have become extinct, yet there is no proof that any new one has been developed. But this human era, so far as yet known, going back certainly to the glacial epoch and almost certainly to pre-glacial times, cannot be estimated at less than a million, some think even several million years; and as there have certainly been some considerable alterations of level, excavation of valleys, deposits of great beds of gravel, and other superficial changes during this period, some kind of a scale of measurement of geological time has been obtained, by comparison with the very minute changes that have occurred during the historical period. This scale is admittedly a very imperfect one, but it is better than none at all; and it is by comparing these small changes with the far greater ones which have occurred during every successive step backward in geological history that these estimates of geological time have been arrived at. They are also supported by the palæontologists, to whom the vast panorama of successive forms of life is an ever-present reality. Directly they pass into the latest stage of the Tertiary period—the Pliocene of Sir Charles Lyell—all over the world new forms of life appear which are evidently the forerunners of many of our still existing species; and as they go a little further back, into the Miocene, there are indications of a warmer climate in Europe, and large numbers of mammals resembling those which now inhabit the tropics, but of quite distinct species and often of distinct genera and families. And here, though we have only reached to about the middle of the Tertiary period, the changes in the forms of life, in the climate, and in the land-surfaces are so great when compared with the very minute changes during the human epoch, as to require us to multiply the time elapsed many times over. Yet the whole of the Tertiary period, during which all the great groups of the higher animals were developed from a comparatively few generalised ancestral forms, is yet the shortest by far of the three great geological periods—the Mesozoic or Secondary, having been much longer, with still vaster changes both in the earth's crust and in the forms of life; while the Palæozoic or Primary, which carries us back to the earliest forms of life as represented by fossilised remains, is always estimated by geologists to be at least as long as the other two combined, and probably very much longer.
From these various considerations most geologists who have made any estimates of geological time from the period of the earliest fossiliferous rocks, have arrived at the conclusion that about 200 millions of years are required. But from the variety of the forms of life at this early period it is concluded that a very much greater duration is needed for the whole epoch of life. Speaking of the varied marine fauna of the Cambrian period, the late Professor Ramsay says:—'In this earliest known varied life we find no evidence of its having lived near the beginning of the zoological series. In a broad sense, compared with what must have gone before, both biologically and physically, all the phenomena connected with this old period seem, to my mind, to be of quite a recent description; and the climates of seas and lands were of the very same kind as those the world enjoys at the present day.' And Professor Huxley held very similar views when he declared: 'If the very small differences which are observable between the crocodiles of the older Secondary formations and those of the present day furnish any sort of an approximation towards an estimate of the average rate of change among reptiles, it is almost appalling to reflect how far back in Palæozoic times we must go before we can hope to arrive at that common stock from which the crocodiles, lizards, Ornithoscelida, and Plesiosauria, which had attained so great a development in the Triassic epoch, must have been derived.'
Now, in opposition to these demands of the geologists, in which they are almost unanimous, the most celebrated physicists, after full consideration of all possible sources of the heat of the sun, and knowing the rate at which it is now expending heat, declare, with complete conviction, that our sun cannot have existed as a heat-giving body for so long a period, and they would therefore reduce the time during which life can possibly have existed on the earth to about one-fourth of that demanded by geologists. In one of his latest articles, Lord Kelvin says:—'Now we have irrefragable dynamics proving that the whole life of our sun as a luminary is a very moderate number of million years, probably less than 50 million, possibly between 50 and 100' (Phil. Mag., vol. ii., Sixth Ser., p. 175, Aug. 1901). In my Island Life (chap. X.) I have myself given reasons for thinking that both the stratigraphical and biological changes may have gone on more quickly than has been supposed, and that geological time (meaning thereby the time during which the development of life upon the earth has been going on) may be reduced so as possibly to be brought within the maximum period allowed by physicists; but there will certainly be no time to spare, and any planets dependent on our sun whose period of habitability is either past or to come, cannot possibly have, or have had, sufficient time for the necessarily slow evolution of the higher life-forms. Again, all physicists hold that the sun is now cooling, and that its future life will be much less than its past. In a lecture at the Royal Institution (published in Nature Series, in 1889), Lord Kelvin says:—'It would, I think, be exceedingly rash to assume as probable anything more than twenty million years of the sun's light in the past history of the earth, or to reckon more than five or six million years of sunlight for time to come.'
These extracts serve to show that, unless either geologists or physicists are very far from any approach to accuracy in their estimates of past or future age of the sun, there is very great difficulty in bringing them into harmony or in accounting for the actual facts of the geological history of the earth and of the whole course of life-development upon it. We are, therefore, again brought to the conclusion that there has been, and is, no time to spare; that the whole of the available past life-period of the sun has been utilised for life-development on the earth, and that the future will be not much more than may be needed for the completion of the grand drama of human history, and the development of the full possibilities of the mental and moral nature of man.
We have here, then, a very powerful argument, from a different point of view than any previously considered, for the conclusion that man's place in the solar system is altogether unique, and that no other planet either has developed or can develop such a full and complete life-series as that which the earth has actually developed. Even if the conditions had been more favourable than they are seen to be on other planets, Mercury, Venus, and Mars could not possibly have preserved equability of conditions long enough for life-development, since for unknown ages they must have been passing slowly towards their present wholly unsuitable conditions; while Jupiter and the planets beyond him, whose epoch of life-development is supposed to be in the remote future when they shall have slowly cooled down to habitability, will then be still more faintly illuminated and scantily warmed by a rapidly cooling sun, and may thus become, at the best, globes of solid ice. This is the teaching of science—of the best science of the twentieth century. Yet we find even astronomers who, more than any other exponents of science, should give heed to the teachings of the sister sciences to which they owe so much, indulging in such rhapsodies as the following:—'In our solar system, this little earth has not obtained any special privileges from Nature, and it is strange to wish to confine life within the circle of terrestrial chemistry.' And again: 'Infinity encompasses us on all sides, life asserts itself, universal and eternal, our existence is but a fleeting moment, the vibration of an atom in a ray of the sun, and our planet is but an island floating in the celestial archipelago, to which no thought will ever place any bounds.'20