The part of Mercury which is turned from the Sun must be characterized by a tremendous cold due to radiation into space. The temperature stays probably about 200° C. (360° F.) below the freezing point of water (328° below zero F.). Even the most concentrated solutions we know of freeze to ice precipitating the salt considerably above this temperature. Moisture in fluid state can, therefore, not very well exist on this side. On the sunny hemisphere it must be lacking as well, due to evaporation over to the cold side. As a result, the desolation on Mercury must be far greater than that on Mars and surface changes caused by variations in temperature are almost precluded. On account of the so-called libration, certain dark portions near the boundary of the illuminated hemisphere occasionally enter the sunlight. But, during this interval, all traces of moisture are undoubtedly driven away from these parts, also never to return. The Earth’s moon is not entirely as stagnant as Mercury, although on the whole it closely resembles this planet. The Moon always turns the same side to the Earth--a small libration exists here also--so that each part of its surface is illumined by the Sun during one half of the synodical month (29.53 days). This time, however, is so long that the moon’s surface in the meantime almost assumes the temperatures due to continuous sunlight and continuous night. Some investigators, as W. H. Pickering, are persuaded that portions of the moon just emerged from the shadow show a lighter colour than after a short time of illumination. These observations, however, have not been accepted as correct. According to Pickering, the light colour should result from a slight formation of snow or hoar-frost during the long night of 355 hours. If an appreciable trace of vapour existed on the Moon, it ought to evaporate and form white caps over the poles where the Sun’s heat is not sufficiently strong to melt them. As no such signs have ever been observed, the faith in snow on the Moon is not likely to find many defenders. The lunar mountains are not attacked by water or sandstorms, nor do they peel off due to rapid heating by the Sun. They rise, therefore, to full stature over their surroundings. Their height can be measured by the length of their shadows. Mädler, in this manner, computed one of the peaks of the Mountain Newton to rise 7300 m. (24,000 ft.) above the territory on which its shadow falls. Six peaks reach between 6000 and 7000 m. (19,500 and 24,000 ft.), 21 between 5000 and 6000 m. (16,500 and 19,500 ft.), 82 between 4000 and 5000 m. (13,000 and 16,500 ft.), and 582 reach 2000 m. (6500 ft.) and more. These figures show the extraordinary mountainous character of the Moon’s surface compared to that of the Earth which is thirteen times larger. In Fig. 26, we see a picture of the portion of the Moon most rich in volcanoes, with the crater Tycho in the centre and Clavius above.