The Earth Selects and Uses Gas
But, besides attracting cosmic masses of gaseous matter to form its atmosphere, there is another equally important function of the mass of a planet—its selective power on the kind of gases it can permanently retain in a free state. The molecules of gases are in a condition of rapid motion in all directions, which explains the elastic force they exhibit. The speed of this motion has been determined for all the chief gases, and also the gravitative force necessary to prevent them from continually escaping into space from the upper limit of the atmosphere. Thus the moon, which has a mass only one-eightieth that of the earth, can retain no free gas whatever on its surface. Mars can retain only the very heavy gases, but neither hydrogen nor water-vapour. The earth, however, has force enough to retain all the gases except hydrogen, which is just beyond its limit; and this may explain why it is that there is no free hydrogen in the atmosphere, although this gas is continually produced in small quantities by submarine volcanoes, is emitted sometimes from fissures in volcanic regions, and is a product of decaying vegetation. Once united with oxygen to form water, it becomes amenable to gravity in the form of invisible aqueous vapour, and is thenceforth a permanent possession for us in its most valuable form.
EARLY ICE AGE, WHEN MAMMOTHS ROAMED THE EARTH AND MAN WAS ARISING
LARGER IMAGE
The very accurate adjustments that render our earth suitable for the production and long-continued development of organic life, culminating in man, may be well shown by another consideration. If our earth had been 9,600 miles instead of 8,000 miles in diameter—a very small increase in view of the immense range of planetary magnitudes from Mercury to Jupiter—with a slight proportionate increase in density, due to its greater force of gravitative compression, its mass would have been about double what it is now. This would probably have led to its having attracted and retained double the amount of gases, in which case the water produced would have been double what it is—perhaps even more, because hydrogen gas would not then escape into space as it does now. But the surface of the globe would have been only one-half greater than at present; so that, unless the ocean cavities were twice as deep as they actually are, the whole surface of the earth—except, perhaps, a few tops of submarine volcanoes—would have been covered several miles deep in water, and all terrestrial life would have been impossible.
The Deep Atmosphere of Venus
From the various considerations here set forth it appears clear to me that no other planet of the solar system makes any approach to the conditions essential for the development of a rich and varied organic life such as adorns our earth. One only—Venus—has a sufficient bulk and density to give it the needful atmosphere; but as it receives about twice as much solar heat as does the earth, it is probable that its very deep atmosphere may be mainly due to the fact that a large proportion of its water is held in a state of vapour, its seas and oceans being proportionately reduced in extent. Judging from what happens on the earth, this would probably lead to an excessive area of deserts, and thus be inimical to life. But this planet appears to possess one feature which renders it fundamentally unsuitable for organic life.
Why there is no Life on Venus
Several modern observers have found that the older astronomers were all in error in giving Venus a rotation-period almost exactly the same as ours, an error due to the indefinite and variable markings of its surface. They have now deduced a period about equal to that of its revolution round the sun—a rate which has been confirmed by spectrum-analysis, and further confirmed by the fact that this planet has no measurable polar compression. As during transits of Venus over the sun’s disc the conditions for the accurate measurement of the compression, if any exist, are the best possible, and as none has been found, this alone affords a demonstration that the rate of rotation must be very slow, because the laws of motion necessitate a definite amount of equatorial protuberance corresponding to that rate. Half the surface has, therefore, perpetual day and the other half perpetual night, leading to violent contrasts of heat and cold for the two hemispheres with, in all probability, correspondingly violent winds, rains, and electrical disturbances—conditions so entirely opposed to the uniformity of temperatures and stability of meteorological phenomena during long geological epochs which are essential for the full development of organic life, that such development is perhaps less probable on this planet than on any other.