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.