Are the Planets Inhabited?

by E. Walter Maunder

The difference in appearance of a given area on the Moon when viewed under a low Sun and when the Sun is on the meridian is very striking. In the first case everything is in the boldest relief; the shadows are long and intensely black; the whole area under examination in the telescope seems as if it might be handled. Under the high Sun, the contrasts are gone; the scenery appears flat, many of the large conspicuous markings are only recognized with difficulty. Thus the terse remark of Mädler, “The full Moon knows no Maginus,” has become a proverb amongst selenographers; yet Maginus is a fine walled plain some eighty miles in diameter, and its rampart attains a height in parts of 14,000 feet. Maginus lies near Tycho, which has been well named “the lunar metropolis,” for from it radiates the principal system of bright streaks conspicuous on the full Moon. These white streaks appear when the shadows have vanished or are growing short; they are not seen under a low Sun.

The changes which appear to take place in the lunar formations owing to the change in their illumination are much more striking and varied than would be anticipated. But the question arises whether all the changes that are associated with the progress of the lunar day can be ascribed to this effect. Thus, Prof. W. H. Pickering writes concerning a well-known pair of little craters of about nine miles in diameter, “known as Messier and Messier A, situated side by side not far from the centre of the Mare Fecunditatis. When the Sun rises first on them, the eastern one, A, is triangular and larger than Messier, which latter is somewhat pear-shaped. About three days after sunrise they both suddenly turn white, Messier rapidly grows in size, soon surpasses A, and also becomes triangular in shape. Six days after sunrise the craters are again nearly of the same size, owing to the diminution of Messier. The shape of A has become irregular, and differs in different lunations. At nine days after sunrise the craters are exactly alike in size and shape, both now being elliptical, with their major axes lying in a nearly N. and S. direction. Just before sunset A is again the larger, being almost twice the size of Messier.”[12]

Some observers explain this cycle of changes as due merely to the peculiar contour of the two objects, the change in the lighting during the lunar day altering their apparent figures. Prof. W. H. Pickering, on the other hand, while recognizing that some portion of the change of shape is probably due to the contour of the ground, conceives that, in order to explain the whole phenomenon, it is necessary to suppose that a white layer of hoar frost is formed periodically round the two craters. It is also alleged that whereas Mädler described the two craters as being exactly alike eighty years ago, Messier A is now distinctly the larger; but it is very doubtful whether Mädler’s description can be trusted to this degree of nicety. If it could, this would establish a permanent change in the actual structure of the lunar surface at this point.

There are several other cases of the same order of ambiguity. The most celebrated is Linné, a white spot about six miles in diameter on the Mare Serentatis. This object appears to change in size during the progress of the lunar day, and, as with Messier, some selenographers consider that it has also suffered an actual permanent change in shape within the last sixty or seventy years. Here again the evidence is not decisive; Neison is by no means convinced that a change has taken place, yet does not think it impossible that Linné may once have been a crater with steep walls which have collapsed into its interior through the force of gravity.

Another type of suspected change is associated with the neighbourhood of Aristarchus, the brightest formation on the Moon, so bright indeed that Sir William Herschel, observing it when illuminated by earthshine in the dark portion of the Moon, thought that he was watching a lunar volcano in eruption. In 1897, on September 21, the late Major Molesworth noticed that the crater was at that time under the rays of the setting Sun, and filled with shadow, and the inner terraces, which should have been invisible, were seen as faint, knotted, glimmering streaks under both the eastern and western walls, and the central peak was also dimly discernible. He thought this unusual lighting up of rocks on which the Sun had already set might be due either to phosphorescence produced by long exposure to the Sun’s rays, or to inherent heat, or to reflected glare from the western rampart. Still more important, both Major Molesworth and Mr. Walter Goodacre, each on more than one occasion, observed what seemed to be a faint bluish mist on the inner slope of the east wall, soon after sunrise, but this was visible only for a short time. Other selenographers too, on rare occasions, have made observations accordant with these, relating to various regions on the Moon.

These, and a few other similar instances, are all that selenography has to offer by way of evidence of actual lunar change. Of seeming change there is abundance, but beyond that we have only cases for controversy, and one of the most industrious of the present-day observers of the Moon, M. Philip Fauth, declares that “as a student of the Moon for the last twenty years, and as probably one of the few living investigators who have kept in practical touch with the results of selenography, he is bound to express his conviction that no eye has ever seen a physical change in the plastic features of the Moon’s surface.”[13]

In this matter of change, then, the Earth and Moon stand in the greatest contrast to each other. As we have seen, from the view-point of the Moon, the appearance of the Earth would change so manifestly with the progress of the seasons that no one could fail to remark the difference, even though observing with the naked eye. But from the view-point of the Earth, the Moon when examined by our most experienced observers, armed with our most powerful telescopes, offers us only a few doubtful enigmatical instances of possible change confined to small isolated localities; we see no evidence that the “gloom of iron substance” below is ever concealed by a veil of changing vegetation, or that “between the burning light and deep vacuity” of the heavens above, the veil of the flying vapour has ever been spread out. We see the Moon so clearly that we are assured it holds no water to nourish plant life; we see it so clearly because there is no air to carry the vapour that might dim our view.

Life is change, and a planet where there is no change, or where that change is very small, can be no home for life. The “stability and insensibility” are indeed required in the platform upon which life is to appear, but there must be the presence of “the passion and the perishing,” or life will be unable to find a home.

We infer the absence of water and air from the Moon not only from the unchanging character of its features and the distinctness with which we see them; we are able to make direct observations. Galileo, the first man to observe the Moon to better advantage than with the naked eye, was not long before he decided that the Moon contained no water, for though Milton, in a well-known passage, makes Galileo discover

“Rivers or mountains on her spotty globe,”