[XXI]
IS THE MOON A DEAD WORLD?
It has been a generally accepted belief among astronomers for years that the moon is a dead world devoid of air and water and so, necessarily, lifeless. It is certain that the moon has no extensive atmosphere such as envelops our own planet. There is abundant proof of this fact. The edge of the lunar disk is clear-cut. Whenever, as happens frequently, the moon passes between us and a star the disappearance of the star is instantaneous. There is no gradual dimming or refraction of the star's light by atmospheric vapors. Moreover, lunar shadows are harsh and black. There is no evidence of diffusion of light on the moon by atmospheric gases.
The absence of water or water vapor on the visible surface of the moon, at least in any appreciable quantity, is plainly evident to anyone who observes the moon through the telescope. Even with small telescopes, objects five miles or so in diameter can be readily detected and clouds drifting over the surface could not possibly escape our observation if they existed.
Bodies of water, great or small, would be plainly visible and would besides give rise to water vapor and clouds, which we would not fail to detect.
Since the surface of the moon is unscreened by air and water vapor to absorb the incoming rays from the sun, and the outgoing radiations from the surface, the extremes of temperature between day and night are very great, and are augmented by the fact that the lunar day equals the lunar month in length, so that fourteen days of untempered heat are followed by fourteen days of frigid darkness. Observations of the rate of radiation from the moon's surface during total eclipses of the moon indicate that the moon's radiation is very rapid, and that its temperature during the height of the lunar day probably approaches 200° F., while at the lunar midnight it may have fallen to 100° below zero F., or even lower.
With air and water both lacking and such extremes of temperature existing why should we seriously consider the question of life on the moon?
This is the point of view of the majority of astronomers and it seems well taken. Yet many astronomers who have made a special study of the lunar surface for years under all conditions of illumination and phase, and have most carefully observed and mapped and photographed its characteristic markings, are agreed that there are evidences that changes are taking place on the moon, and recently Prof. W. H. Pickering has expressed the belief, substantiated by drawings, that there is a progressive change of color or darkening within certain lunar craters with the advance of the lunar day, indicating, in his opinion, a rapid vegetational growth that springs up in the height of the lunar day and dies out as the lunar night approaches.
Some years ago certain selenographers suggested that there might exist in the numberless crater-pits and craters, in the deep-lying maria or "seas," and in the clefts and rills and cracks that form intricate systems all over the lunar surface, certain exhalations from the surface and heavy vapors including possibly carbon dioxide and water vapor to temper the extremes of the long lunar days and night and furnish the necessary medium for the support of certain forms of animal and vegetable life.