The Meteoritic Impact Origin of the Moon's Surface Features

The craters on the moon appear to have been formed by meteoritic impact rather than by volcanism. This mode of origin is suggested by their size, shape, distribution, associated features, and other considerations. Many of the aspects of lunar craters, such as their circular shape and the presence of central peaks, which have been considered as disproof of their impact origin, are the result of the crater's being formed by explosion rather than by percussion. Variation of the physiographic form of these craters, with increase in their size from small, cup-shaped pits to large, walled plains, is mainly due to modification and melting by superheated molten lava, generated by the impact in the case of the larger craters. Maria are probably extensive lava plains generated by the impact of bodies of asteroidal dimensions and were formed relatively late in lunar history, as is indicated by the physiographic youth of the superimposed post-maria craters. Aside from the shape of the craters and the maria, the small mass of the moon and its low internal pressure make volcanism unlikely in the lunar environment.