The more massive and extended mountain ranges of the moon are found in the Northern Hemisphere, and (what is significant) in that portion of it which exhibits few indications of other superficial disturbances. The most prominently developed systems, the Alps, the Caucasus, and the Apennines, forming a mighty western rampart to the Mare Imbrium and giving it all the appearance of a vast walled-plain, present few points of resemblance to any terrestrial chain. The former include many hundred peaks, among which Mont Blanc rises to a height of 12,000 feet, and a second, some distance west of Plato, to nearly as great an altitude; while others ranging from 5,000 to 8,000 feet are common. They extend in a southwest direction from Plato to the Caucasus, terminating somewhat abruptly, a little west of the central meridian in about N. lat. 42°. One of the most interesting features associated with this range is the so-called great Alpine valley, which cuts through it west of Plato.
The Caucasus consist of a massive wedge-shaped mountain land, projecting southward, and partially dividing the Mare Imbrium from the Mare Serenitatis, both of which they flank. Though without peaks so lofty as those pertaining to the Alps, there is one, immediately east of the ring-plain Calippus, which, towering to 19,000 feet, surpasses any of which the latter system can boast. The Apennines, however, are by far the most magnificent range on the visible surface, including as they do some 3,000 peaks, and extending in an almost continuous curve of more than 400 miles in length from Mount Hadley, on the north, to the fine ring-plain Eratosthenes, which forms a fitting termination, on the south. The great headland Mount Hadley rises more than 15,000 feet, while a neighboring promontory on the southeast of it is fully 14,000 feet, and another, close by, is still higher above the Mare. Mount Huyghens, again in N. lat. 20°, and the square-shaped mass Mount Wolf, near the southern end of the chain, include peaks standing 18,000 and 12,000 feet respectively above the plain to which their flanks descend with a steep declivity. The counterscarp of the Apennines, in places 160 miles in width from east to west, runs down to the Mare Vaporum, with a comparatively gentle inclination. It is everywhere traversed by winding valleys of a very intricate type, all trending toward the southwest, and includes some very bright craters and mountain-rings.
Whether variations in the visibility of lunar details, when observed under apparently similar conditions, actually occur from time to time from some unknown cause, is one of those vexed questions which will only be determined when the moon is systematically studied by experienced observers using the finest instruments at exceptionally good stations; but no one who examines existing records of rills by Gruithuisen, Lohrmann, Mädler, Schmidt, and other observers, can well avoid the conclusion that the anomalies brought to light therein point strongly to the probability of the existence of some agency which occasionally modifies their appearance or entirely conceals them from view. In short, the more direct telescopic observations accumulate, and the more the study of minute detail is extended, the stronger becomes the conviction that, in spite of the absence of an appreciable atmosphere, there may be something resembling low-lying exhalations from some parts of the surface which from time to time are sufficiently dense to obscure, or even obliterate, the region beneath them.
Sir John Herschel maintained that “the actual illumination of the lunar surface is not much superior to that of weathered sandstone rock in full sunshine. I have,” he says, “frequently compared the moon setting behind the gray perpendicular façade of the Table Mountain, illumined by the sun just risen in the opposite quarter of the horizon when it has been scarcely distinguishable in brightness from the rock in contact with it. The sun and moon being at nearly equal altitudes, and the atmosphere perfectly free from cloud or vapor, its effect is alike on both luminaries.” Zöllner’s elaborate researches on this question are closely in accord with the above observational result. Though he considers that the brightest parts of the surface are as white as the whitest objects with which we are acquainted, yet, taking the reflected light as a whole, he finds that the moon is more nearly black than white. The most brilliant object on the surface is the central peak of the ring-plain Aristarchus, the darkest the floor of Grimaldi, or perhaps a portion of that of the neighboring Riccioli. Between these extremes there is every gradation of tone. Proctor, discussing this question on the basis of Zöllner’s experiments respecting the light reflected by various substances, concludes that the dark area just mentioned must be notably darker than the dark gray syenite which figures in his tables, while the floor of Aristarchus is as white as newly fallen snow.
MARS.—Agnes M. Clerke
The furthest terrestrial planet from the sun is Mars, the “star of strength.” No other heavenly body, except the moon, is so well placed for observation from our position in space.
The diameter of Mars is 4,200 miles; its surface is equal to two-sevenths, its volume to one-seventh those of the earth. But, in consequence of its inferior mean density, nine such spheres would go to make up the mass of our world. The superficial force of gravity on Mars, compared with its terrestrial value, is as thirty-eight to a hundred. A man could leap there a wall eight feet four inches in height with no more effort than it would cost him here to spring over a two-foot fence.
The planet’s rotation is performed in 24 hours, 37 minutes, on an axis deviating from the vertical by 24° 50′. Hence its seasons resemble our own, except in being nearly twice as long, for the Martian year is of 687 days.
The disk of Mars is diversified with three shades of color—reddish, or dull orange, dark grayish-green, and pure white. The last shows mainly in two diametrically opposite patches. Each pole is surrounded by a brilliant cap, suggesting the deposition of ice or snow over the chilly spaces corresponding to our arctic and antarctic regions. Nor is this all. Each of the polar hoods shrinks to a mere remnant as the local summer advances, but regains its original size when wintry influences are again in the ascendant. Here, and nowhere else in the planetary system, we meet evidence of seasonal change; and seasonal change is associated with vital possibilities. Again, a globe upon which snow visibly melts must contain water; hence the green markings can not but image to our minds seas and inlets subdividing continents, the blond complexion of which may be caused by some native peculiarity of the soil. It is in no way connected with vegetation, since it neither fades nor flushes with the advent of spring; and an atmospheric origin is excluded by the circumstance that it becomes effaced by a whitish haze near the limb, just where the densest atmospheric strata are traversed by the line of sight.
The spots on Mars are by no means so sharply defined as lunar craters and maria; yet they are fundamentally permanent. Some can be recognized from drawings made over two hundred years ago; and these antique records have served modern astronomers to determine with minute accuracy the rotation-period of the planet. Continents are somewhat vaguely outlined. Great tracts of them are of an uncertain and variable hue, as if subject to inundations. This peculiarity, thoroughly certified during the favorable opposition of 1892, makes a strong distinction between Mars and the Earth. Terrestrial oceans keep within the limits assigned to them. On the neighboring planet—as M. Faye observed in 1892—“water seems to march about at its ease,” flooding from time to time regions as wide as France. The imperfect separation of the two elements recalls the conditions prevailing during the terrestrial carboniferous era.