THE PLANETS OF OUR SYSTEM
(We continue the translation of the essay of the Potsdam astronomer.)
In the light of contemporary astronomical knowledge let us make a rapid survey of the other worlds.
The Moon
Our lady readers will not raise any objection if we occupy ourselves at first with the Moon, this confidante of every heart, either happy or unhappy. The most important thing about her for the moment can be put into one sentence: she has neither air nor water, and her temperature oscillates between extremes separated by more than 200 degrees centigrade. None of the conditions stated above is therefore fulfilled, and accordingly no organic life could exist on her. It is also interesting to see how the cessation on the Moon of one of these vital conditions has entailed the cessation of the two others. There is no occasion to doubt that the Moon formerly possessed an atmosphere; by analogy with the planets this must indeed be assumed as altogether certain. The feeble mass of our satellite which, on the one hand, has been the cause of its rapid cooling, has, on the other hand, brought about the dissipation into space of its atmosphere, which was probably always of small density.
But the smaller the pressure of the air the more rapid is the evaporation of water, and this is why its disappearance coincides with that of the atmosphere. Besides, the complete absence of air allows the rays of the Sun to penetrate without hindrance to the ground and to heat it to a high temperature during the 14 times 24 hours of duration of the lunar day. During the night, which is of equal duration, there is radiation of heat into celestial space, and the soil cools down to a temperature which cannot be very different from the absolute zero of the temperature of empty space.
(I cannot entirely accept these allegations with regard to the habitability of our satellite. The absence of air and even of water is pot proved. The variations actually observed even prove that the moon is not altogether a dead world. But let the author continue.)
Such a fate is also in store for the Earth, and nothing can save it. Our Earth will also one day become a barren body, incapable of supporting organic life, a deserted grave of the civilisation created by the human spirit. Just as the isolated individual disappears, so will humanity one day disappear entirely. In a limited domain death is always finally victorious; but, on the other hand, a new life flourishes elsewhere, and when that new life is developed, some day perhaps on another planet of our system a scientific article will be written on the question whether the Earth is still habitable.
Mercury
Our knowledge of the physical constitution of Mercury is very slight. It seems to be surrounded by a light atmosphere which contains water-vapour. Since the solar heat on Mercury is about seven times stronger than on us, the extreme limits of temperature above indicated must here be very considerably surpassed, and water can hardly exist except in the form of vapour. But the argument is entirely changed if we accept the recent discovery of Schiaparelli, according to which the duration of rotation of Mercury would be identical with its period of revolution, so that, like the Moon to the Earth, Mercury would always turn the same face to the Sun. On that side the temperature would naturally be very high, while the most intense frost would reign on the other side. But between these two extremes there ought to be a mixed zone in which our three conditions might possibly be realised, so that we may assume for Mercury a limited habitability.