How Water Protects Earth by Night
In the first place the atmosphere must be of a sufficient density, this being needed in order that it may be an adequate storer up of solar heat, and also in order that it may be able to supply sufficient oxygen, water-vapour, and carbonic-acid gas for the requirements of both vegetable and animal life. We have a striking example of the use of air as a storer-up and distributor of heat and moisture in the very different character of our south-west and north-east winds. The effect of the density of the air is equally well shown when we ascend lofty mountains where we find perpetual snow and ice, due simply to the fact that the air is not dense enough to retain the heat of the sun—which is actually greater than at low levels—so that at night the temperature regularly falls below the freezing point. On the other hand a very much denser atmosphere would absorb so much water vapour as probably to shut out the light of the sun, and thus have a prejudicial effect on vegetable life.
Again, there is good reason to believe that the proportions of the various gases in the atmosphere are, within certain narrow limits, such as are most favourable not only for the life that actually exists, but for any life that could be developed from the elements that constitute the universe. Oxygen has properties which seem absolutely essential to organic life; but nitrogen, though only serving to dilute the oxygen so far as the higher animals are directly concerned, is yet indirectly essential for them, since it is in vegetables a constituent of that protoplasm which is the very substance of their bodies.
Use of Thunderstorms
Now, plants obtain their nitrogen mainly from the minute proportion of ammonia that exists in the atmosphere, and this ammonia is formed by the union of the nitrogen of the air with the hydrogen of the water-vapour under the influence of electric discharges—that is, of thunderstorms. It is evident, then, that the required amount of this essential compound will depend upon a due adjustment of the quantities of nitrogen and aqueous vapour always present; while the electric discharges seem to be due to the friction of various strata of air with each other and with the earth’s surface, due to the winds and storms; and winds are due to highly complex causes, involving the rate of the earth’s rotation, the rise and fall of the tide, the density of the atmosphere, the quantity of its aqueous vapour, and the amount of solar heat which it receives. Unless all these very diverse factors existed in their due proportion, some of the results might be highly prejudicial if not quite inimical to the development of life. To these various adaptations of our gaseous envelope we must add one other. Carbonic acid gas in the atmosphere is absolutely essential to vegetable life, while it is directly antagonistic to that of the higher animals. Its quantity must, therefore, be strictly proportionate to the needs of both; and that beneficial proportion must have been preserved throughout the whole period of the existence of the higher air-breathing animals.
These various considerations show us that our atmosphere, consisting as it does mainly of two common gases mixed together, and therefore seeming to most people one of the simplest things possible, is really a wonderfully complex arrangement which is adapted to serve the purposes of living organisms in a great variety of ways. But this by no means exhausts the subject of its adaptation to support and develop organic life, because its very existence on the earth in a suitable quantity and composed of the essential elements can be shown to depend on other and deeper relations which will now be pointed out.
The older writers on the subject of the habitability of the planets took no account whatever of the importance of size, distance from the sun, period of rotation, and obliquity of the ecliptic as determining the possibility of organic life, but simply assumed that, because the earth possessed an abundant life-development, all the other planets must also possess it. But we know that the above-mentioned factors are of very high importance, as we will proceed briefly to point out.
Earth’s Envelope of Gas
It is now believed that the amount of atmosphere possessed by a planet is due mainly, perhaps entirely, to the planet’s mass, and its consequent gravitative power. Spectrum-analysis has shown that vast masses of gaseous matter exist in the universe, and it is probable that, in a state of extreme tenuity, these are very widely diffused. Just as meteoric dust is constantly attracted to the earth, and periodically in larger quantities, so are gases, and supposing the aggregations of free gaseous matter to have been distributed with some approach to uniformity, then, as planets grew in size, they would also tend to secure a larger amount of the diffused gases, thus forming deeper atmospheres. The observed facts agree with this view. The largest planets, Jupiter and Saturn, have such a depth of atmosphere as permanently to obscure any solid interior they may possess. The only planet closely approaching the earth in size and density—Venus—has an atmosphere which appears to be loftier than ours, but it may be composed of different gases. Mars, which has only one-ninth the mass of the earth, has a lofty but very tenuous atmosphere, and probably no water, the Polar snows being due probably to the freezing of some dense gas. The climate and physical condition of Mars is, however, still a subject of much controversy, which I hope to discuss in a separate work dealing with the arguments of Professor Lowell [see [page 105]]. In that volume the reader will find, fully set forth my reasons, on scientific grounds, against the supposed habitability of Mars.