The lack of air is considered by many astronomers to furnish the
explanation of the enormous number of "craters" which pit the moon's
surface. There are about a hundred thousand of these strange rings, and
it is now believed by many that they are spots where very large
meteorites, or even planetoids, splashed into the moon when its surface
was still soft. Other astronomers think that they are the remains of
gigantic bubbles which were raised in the moon's "skin," when the globe
was still molten, by volcanic gases from below. A few astronomers think
that they are, as is popularly supposed, the craters of extinct
volcanoes. Our craters, on the earth, are generally deep cups, whereas
these ring-formations on the moon are more like very shallow and broad
saucers. Clavius, the largest of them, is 123 miles across the interior,
yet its encircling rampart is not a mile high.
The mountains on the moon (Fig. 16) rise to a great height, and are
extraordinarily gaunt and rugged. They are like fountains of lava,
rising in places to 26,000 and 27,000 feet. The lunar Apennines have
three thousand steep and weird peaks. Our terrestrial mountains are
continually worn down by frost acting on moisture and by ice and water,
but there are none of these agencies operating on the moon. Its
mountains are comparatively "everlasting hills."
The moon is interesting to us precisely because it is a dead world. It
seems to show how the earth, or any cooling metal globe, will evolve in
the remote future. We do not know if there was ever life on the moon,
but in any case it cannot have proceeded far in development. At the most
we can imagine some strange lowly forms of vegetation lingering here and
there in pools of heavy gas, expanding during the blaze of the sun's
long day, and frozen rigid during the long night.