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

It has been suggested that these gigantic rings are only "basal wrecks" of volcanic mountains, whose conical summits have been blown away, leaving vast crateriform hollows where the mighty peaks once stood; but the better opinion seems to be that which assumes that the rings were formed by volcanic action very much as we now see them. If such a crater as Copernicus[Pg 225] or the still larger one named Theophilus, which is situated in the western hemisphere of the moon, on the shore of the "Sea of Nectar," ever had a conical mountain rising from its rim, the height attained by the peak, if the average slope were about 30°, would have been truly stupendous—fifteen or eighteen miles!

There is a kind of ring mountains, found in many places on the moon, whose forms and surroundings do not, as the craters heretofore described do, suggest at first sight a volcanic origin. These are rather level plains of an oval or circular outline, enclosed by a wall of mountains. The finest example is, perhaps, the dark-gray Plato, situated in 50° of north latitude, near an immense mountain uplift named the Lunar Alps, and on the northern shore of the Mare Imbrium, or "Sea of Showers." Plato appears as an oval plain, very smooth and level, about 60 miles in length, and completely surrounded by mountains, quite precipitous on the inner side, and rising in their highest peaks to an elevation of 6,000 to[Pg 226] 7,000 feet. Enclosed plains, bearing more or less resemblance to Plato—sometimes smooth within, and sometimes broken with small peaks and craters or hilly ridges—are to be found scattered over almost all parts of the moon. If our satellite was ever an inhabited world like the earth, while its surface was in its present condition, these valleys must have presented an extraordinary spectacle. It has been thought that they may once have been filled with water, forming lakes that recall the curious Crater Lake of Oregon.

THE MOON AT FIRST AND LAST QUARTER (WESTERN AND EASTERN HEMISPHERES).
Photographed with the Lick Telescope.

It is not my intention to give a complete description of the various lunar features, and I mention but one other—the "clefts" or "rills," which are to be seen running across the surface like cracks. One of the most remarkable of these is found in the Oceanus Procellarum, near the crater-mountain Aristarchus, which is famed for the intense brilliance of its central peak, whose reflective power is so great that it was once supposed to be aflame with volcanic fire. The cleft, or crack, in question[Pg 227] is very erratic in its course, and many miles in length, and it terminates in a ringed plain named Herodotus not far east of Aristarchus, breaking through the wall of the plain and entering the interior. Many other similar chasms or cañons exist on the moon, some crossing plains, some cleaving mountain walls, and some forming a network of intersecting clefts. Mr. Thomas Gwyn Elger has this to say on the subject of the lunar clefts:

"If, as seems most probable, these gigantic cracks are due to contractions of the moon's surface, it is not impossible, in spite of the assertions of the text-books to the effect that our satellite is now a 'changeless world,' that emanations may proceed from these fissures, even if, under the monthly alternations of extreme temperatures, surface changes do not now occasionally take place from this cause also. Should this be so, the appearance of new rills and the extension and modification of those already existing may reasonably be looked for."[Pg 228]

Mr. Elger then proceeds to describe his discovery in 1883, in the ring-plain Mersenius, of a cleft never noticed before, and which seems to have been of recent formation.[15]

We now return to the question of the force of lunar gravity. This we find to be only one sixth as great as gravity on the surface of the earth. It is by far the smallest force of gravity that we have found anywhere except on the asteroids. Employing the same method of comparison that was made in the case of Mars, we compute that a man on the moon could attain a height of thirty-six feet without being relatively more unwieldy than a six-foot descendant of Adam is on the earth.

Whether this furnishes a sound reason[Pg 229] for assuming that the lunar inhabitants, if any exist or have ever existed, should be preposterous giants is questionable; yet such an assumption receives a certain degree of support from the observed fact that the natural features of the moon are framed on an exaggerated scale as compared with the earth's. We have just observed that the moon is characterized by vast mountain rings, attaining in many cases a diameter exceeding fifty miles. If these are volcanic craters, it is evident, at a glance, that the mightiest volcanoes of the earth fall into insignificance beside them. Now, the slight force of gravity on the moon has been appealed to as a reason why volcanic explosions on the lunar globe should produce incomparably greater effects than upon the earth, where the ejected materials are so much heavier. The same force that would throw a volcanic bomb a mile high on the earth could throw it six miles high on the moon. The giant cannon that we have placed in one of our coast forts, which is said to be able to hurl a projectile to a dis[Pg 230]tance of fifteen miles, could send the same projectile ninety miles on the moon. An athlete who can clear a horizontal bar at a height of six feet on the earth could clear the same bar at a height of thirty-six feet on the moon. In other words, he could jump over a house, unless, indeed, the lunarians really are giants, and live in houses proportioned to their own dimensions and to the size of their mountains. In that case, our athlete would have to content himself with jumping over a lunarian, whose head he could just clear—with the hat off.

These things are not only amusing, but important. There can be no question that the force of gravity on the moon actually is as slight as it has just been described. So, even without calling in imaginary inhabitants to lend it interest, the comparative inability of the moon to arrest bodies in motion becomes a fact of much significance. It has led to the theory that meteorites may have originally been shot out of the moon's great volcanoes, when those volcanoes were active, and may have circulated about the[Pg 231] sun until various perturbations have brought them down upon the earth. A body shot radially from the surface of the moon would need to have a velocity of only about a mile and a half in a second in order to escape from the moon's control, and we can believe that a lunar volcano when in action could have imparted such a velocity, all the more readily because with modern gunpowders we have been able to give to projectiles a speed one half as great as that needed for liberation from lunar gravity.

Another consequence of the small gravitative power of the moon bears upon the all-important question of atmosphere. According to the theory of Dr. Johnstone Stoney, heretofore referred to, oxygen, nitrogen, and water vapor would all gradually escape from the moon, if originally placed upon it, because, by the kinetic theory, the maximum velocities of their molecules are greater than a mile and a half per second. The escape would not occur instantly, nor all at once, for it would be only the molecules at the upper surface of the atmos[Pg 232]phere which were moving with their greatest velocity, and in a direction radial to the center of the moon, that would get away; but in the course of time this gradual leakage would result in the escape of all of those gases.[16]

After it had been found that, to ordinary tests, the moon offered no evidence of the possession of an atmosphere, and before Dr. Stoney's theory was broached, it was supposed by many that the moon had lost its original supply of air by absorption into its interior. The oxygen was supposed to have entered into combination with the cooling rocks and minerals, thus being withdrawn from the atmosphere, and the nitrogen was imagined to have disappeared also[Pg 233] within the lunar crust. For it seems to have always been tacitly assumed that the phenomenon to be accounted for was not so much the absence of a lunar atmosphere as its disappearance. But disappearance, of course, implies previous existence. In like manner it has always been a commonly accepted view that the moon probably once had enough water to form lakes and seas.

These, it has been calculated, could have been absorbed into the lunar globe as it cooled off. But Johnstone Stoney's theory offers another method by which they could have escaped, through evaporation and the gradual flight of the molecules into open space. Possibly both methods have been in operation, a portion of the constituents of the former atmosphere and oceans having entered into chemical combinations in the lunar crust, and the remainder having vanished in consequence of the lack of sufficient gravitative force to retain them.

But why, it may be asked, should it be assumed that the moon ever had things which it does not now possess? Perhaps no[Pg 234] entirely satisfactory reply can be made. Some observers have believed that they detected unmistakable indications of alluvial deposits on lunar plains, and of the existence of beaches on the shores of the "seas." Messrs. Loewy and Puiseux, of the Paris Observatory, whose photographs of the moon are perhaps the finest yet made, say on this subject:

"There exists, from the point of view of relief, a general similarity between the 'seas' of the moon and the plateaux which are covered to-day by terrestrial oceans. In these convex surfaces are more frequent than concave basins, thrown back usually toward the verge of the depressed space. In the same way the 'seas' of the moon present, generally at the edges, rather pronounced depressions. In one case, as in the other, we observe normal deformations of a shrinking globe shielded from the erosive action of rain, which tends, on the contrary, in all the abundantly watered parts of the earth to make the concave surfaces predominate. The explanation of this structure,[Pg 235] such as is admitted at present by geologists, seems to us equally valid for the moon."[17]

It might be urged that there is evidence of former volcanic activity on the moon of such a nature that explosions of steam must have played a part in the phenomena, and if there was steam, of course there was water.

But perhaps the most convincing argument tending to show that the moon once had a supply of water, of which some remnant may yet remain below the surface of the lunar globe, is based upon the probable similarity in composition of the earth and the moon. This similarity results almost equally whether we regard the moon as having originated in a ring of matter left off from the contracting mass that became the earth, or whether we accept the suggestion of Prof. G.H. Darwin, that the moon is the veritable offspring of the earth, brought into being by the assistance of the tidal influence of the sun. The latter hypothesis[Pg 236] is the more picturesque of the two, and, at present, is probably the more generally favored. It depends upon the theory of tidal friction, which was referred to in Chapter III, as offering an explanation of the manner in which the rotation of the planet Mercury has been slowed down until its rotary period coincides with that of its revolution.

The gist of the hypothesis in question is that at a very early period in its history, when the earth was probably yet in a fluid condition, it rotated with extreme rapidity on its axis, and was, at the same time, greatly agitated by the tidal attraction of the sun, and finally huge masses were detached from the earth which, ultimately uniting, became the moon.[18]

Born in this manner from the very substance of the earth, the moon would necessarily be composed, in the main, of the same elements as the globe on which we dwell, and is it conceivable that it should not have