Fig. 25. Drawing by Lowell, representing the planet Mercury with “canals.” ]
Fig. 26. A part of the moon near its south pole. The big crater above, in the interior and on the walls of which a large number of smaller craters appear, is Clavius. A little below and to the right is Longomontanus just at the edge of the shadow; almost in the middle of the picture appears Tycho with its central cone. The moon diameter corresponds to 43.4 cm. Photo by Yerkes Observatory. ]
Fig. 27. Mare Serenitatis (below), Mare Tranquillitatis (upper left) and vicinity. To the left of Mare Serenitatis the great crater Posidonius; 2.8 cm. from the right edge a small white spot may be seen. This is the remarkable crater Linné, said to have undergone changes. The moon diameter corresponds to 35.7 cm. Photo by Yerkes Observatory. ]
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.