Other Worlds<br />Their Nature, Possibilities and Habitability in the Light of the Latest Discoveries, Garrett Putman Serviss [reading diary .txt] 📗

carried with it both air and water, or the[Pg 237] gases from which they were to be formed? If the moon ever had enough of these prime requisites to enable it to support forms of life comparable with those of the earth, the disappearance of that life must have been a direct consequence of the gradual vanishing of the lunar air and water. The secular drying up of the oceans and wasting away of the atmosphere on our little neighbor world involved a vast, all-embracing tragedy, some of the earlier scenes of which, if theories be correct, are now reenacted on the half-desiccated planet Mars—a planet, by the way, which in size, mass, and ability to retain vital gases stands about half-way between the earth and the moon.

One of the most interesting facts about the moon is that its surface affords evidence of a cataclysm which has wiped out many, and perhaps nearly all, of the records of its earlier history, that were once written upon its face. Even on the earth there have been geological catastrophes destroying or burying the accumulated results of ages of undisturbed progress, but on the moon these[Pg 238] effects have been transcendent. The story of the tremendous disaster that overtook the moon is partly written in its giant volcanoes. Although it may be true, as some maintain, that there is yet volcanic action going on upon the lunar surface, it is evident that such action must be insignificant in comparison with that which took place ages ago.

There is a spot in the western hemisphere of the moon, on the border of a placid bay or "sea," that I can never look at without a feeling of awe and almost of shrinking. There, within a space about 250 miles in length by 100 in width, is an exhibition of the most terrifying effects of volcanic energy that the eye of man can anywhere behold. Three immense craters—Theophilus, 64 miles across and 3-1/2 miles deep; Cyrillus, 60 miles across and 15,000 feet deep; and Catharina, 70 miles across and from 8,000 to 16,000 feet deep—form an interlinked chain of mountain rings, ridges, precipices, chasms, and bottomless pits that take away one's breath.[Pg 239]

But when the first impression of astonishment and dismay produced by this overwhelming spectacle has somewhat abated, the thoughtful observer will note that here the moon is telling him a part of her wonderful story, depicted in characters so plain that he needs no instruction in order to decipher their meaning. He will observe that this ruin was not all wrought at once or simultaneously. Theophilus, the crater-mountain at the northwestern end of the chain, whose bottom lies deepest of all, is the youngest of these giants, though the most imposing. For a distance of forty miles the lofty wall of Theophilus has piled itself upon the ruins of the wall of Cyrillus, and the circumference of the circle of its tremendous crater has been forcibly thrust within the original rim of the more ancient crater, which was thus rudely compelled to make room for its more vigorous rival and successor.

The observer will also notice that Catharina, the huge pit at the southeastern end of the chain, bears evidence of yet greater[Pg 240] age. Its original walls, fragments of which still stand in broken grandeur, towering to a height of 16,000 feet, have, throughout the greater part of their circuit, been riddled by the outbreak of smaller craters, and torn asunder and thrown down on all sides.

In the vast enclosure that was originally the floor of the crater-mountain Catharina, several crater rings, only a third, a quarter, or a fifth as great in diameter, have broken forth, and these in turn have been partially destroyed, while in the interior of the oldest of them yet smaller craters, a nest of them, mere Etnas, Cotopaxis, and Kilaueas in magnitude, simple pinheads on the moon, have opened their tiny jaws in weak and ineffective expression of the waning energies of a still later epoch, which followed the truly heroic age of lunar vulcanicity.

This is only one example among hundreds, scattered all over the moon, which show how the surface of our satellite has suffered upheaval after upheaval. It is possible that some of the small craters, not included within the walls of the greater ones,[Pg 241] may represent an early stage in the era of volcanic activity that wrecked the moon, but where larger and smaller are grouped together a certain progression can be seen, tending finally to extinction. The internal energies reached a maximum and then fell off in strength until they died out completely.

It can hardly be supposed that the life-bearing phase of lunar history—if there ever was one—could survive the outbreak of the volcanic cataclysm. North America, or Europe, if subjected to such an experience as the continental areas of the moon have passed through, would be, in proportion, worse wrecked than the most fearfully battered steel victim of a modern sea fight, and one can readily understand that, in such circumstances, those now beautiful and populous continents would exhibit, from a distance, scarcely any token of their present topographical features, to say nothing of any relics of their occupation by living creatures.

There are other interesting glimpses to[Pg 242] be had of an older world in the moon than that whose scarred face is now beautified for us by distance. Not far from Theophilus and the other great crater-mountains just described, at the upper, or southern, end of the level expanse called the "Sea of Nectar," is a broad, semicircular bay whose shores are formed by the walls of a partially destroyed crater named Fracastorius. It is evident that this bay, and the larger part of the "Sea of Nectar," have been created by an outwelling of liquid lavas, which formed a smooth floor over a portion of the pre-existing surface of the moon, and broke down and submerged a large part of the mountain ring of Fracastorius, leaving the more ancient walls standing at the southern end, while, outlined by depressions and corrugations in the rocky blanket, are certain half-defined forms belonging to the buried world beneath.

Near Copernicus, some years ago, as Dr. Edward S. Holden pointed out, photographs made with the great Lick telescope, then under his direction, showed, in skeleton out[Pg 243]line, a huge ring buried beneath some vast outflow of molten matter and undiscerned by telescopic observers. And Mr. Elger, who was a most industrious observer and careful interpreter of lunar scenery, speaks of "the undoubted existence of the relics of an earlier lunar world beneath the smooth superficies of the maria."

Although, as already remarked, it seems necessary to assume that any life existing in the moon prior to its great volcanic outburst must have ceased at that time, yet the possibility may be admitted that life could reappear upon the moon after its surface had again become quiet and comparatively undisturbed. Germs of the earlier life might have survived, despite the terrible nature of the catastrophe. But the conditions on the moon at present are such that even the most confident advocates of the view that the lunar world is not entirely dead do not venture to assume that anything beyond the lowest and simplest organic forms—mainly, if not wholly, in the shape of vegetation—can exist there. The[Pg 244] impression that even such life is possible rests upon the accumulating evidence of the existence of a lunar atmosphere, and of visible changes, some apparently of a volcanic character and some not, on the moon's surface.

Prof. William H. Pickering, who is, perhaps, more familiar with the telescopic and photographic aspects of the moon than any other American astronomer, has recorded numberless instances of change in minute details of the lunar landscapes. He regards some of his observations made at Arequipa as "pointing very strongly to the existence of vegetation upon the surface of the moon in large quantities at the present time." The mountain-ringed valley of Plato is one of the places in the lunar world where the visible changes have been most frequently observed, and more than one student of the moon has reached the conclusion that something very like the appearances that vegetation would produce is to be seen in that valley.

Professor Pickering has thoroughly dis[Pg 245]cussed the observations relating to a celebrated crater named Linné in the Mare Serenitatis, and after reading his description of its changes of appearance one can hardly reject his conclusion that Linné is an active volcanic vent, but variable in its manifestations. This is only one of a number of similar instances among the smaller craters of the moon. The giant ones are evidently entirely extinct, but some of the minor vents give occasional signs of activity. Nor should it be assumed that these relatively slight manifestations of volcanic action are really insignificant. As Professor Pickering shows, they may be regarded as comparable with the greatest volcanic phenomena now witnessed on the earth, and, speaking again of Plato, he says of its evidences of volcanic action:

"It is, I believe, more active than any area of similar size upon the earth. There seems to be no evidences of lava, but the white streaks indicate apparently something analogous to snow or clouds. There must be a certain escape of gases, presum[Pg 246]ably steam and carbonic acid, the former of which, probably, aids in the production of the white markings."[19]

To Professor Pickering we owe the suggestion that the wonderful rays emanating from Tycho consist of some whitish substance blown by the wind, not from Tycho itself, but from lines of little volcanic vents or craters lying along the course of the rays. This substance may be volcanic powder or snow, in the form of minute ice crystals. Mr. Elger remarks of this theory that the "confused network of streaks" around Copernicus seems to respond to it more happily than the rays of Tycho do, because of the lack of definiteness of direction so manifest in the case of the rays.

As an encouragement to amateur observers who may be disposed to find out for themselves whether or not changes now take place in the moon, the following sentence from the introduction to Professor Pickering's chapter on Plato in the Harvard[Pg 247] Observatory Annals, volume xxxii, will prove useful and interesting:

"In reviewing the history of selenography, one must be impressed by the singular fact that, while most of the astronomers who have made a special study of the moon, such as Schroeter, Maedler, Schmidt, Webb, Neison, and Elger, have all believed that its surface was still subject to changes readily visible from the earth, the great majority of astronomers who have paid little attention to the subject have quite as strenuously denied the existence of such changes."

In regard to the lunar atmosphere, it may be said, in a word, that even those who advocate the existence of vegetation and of clouds of dust or ice crystals on the moon do not predicate any greater amount, or greater density, of atmosphere than do those who consider the moon to be wholly dead and inert. Professor Pickering himself showed, from his observations, that the horizontal refraction of the lunar atmosphere, instead of being less than 2″, as formerly stated, was less than 0.4″. Yet he[Pg 248] found visual evidence that on the sunlit side of the moon this rare atmosphere was filled to a height of four miles with some absorbing medium which was absent on the dark side, and which was apparently an emanation from the lunar crust, occurring after sunrise. And Messrs. Loewy and Puiseux, of the Paris Observatory, say, after showing reasons for thinking that the great volcanic eruptions belong to a recent period in the history of the moon, that "the diffusion of cinders to great distances infers a gaseous envelope of a certain density.... The resistance of the atmosphere must have been sufficient to retard the fall of this dust [the reference is to the white trails, like those from Tycho], during its transport over a distance of more than 1,000 kilometers [620 miles]."[20]

We come now to a brief consideration of certain peculiarities in the motions of the moon, and in the phenomena of day and night on its surface. The moon keeps the[Pg 249] same side forever turned toward the earth, behaving, in this respect, as Mercury does with regard to the sun. The consequence is that the lunar globe makes but one rotation on its axis in the course of a month, or in the course of one revolution about the earth. Some of the results of this practical identity of the periods of rotation and revolution are illustrated in the diagram on page 250. The moon really undergoes considerable libration, recalling the libration of Mercury, which was explained