CRATERS ON THE LUNAR SURFACE
A tremendous number of impact craters pit the moon, evidence of cataclysms that altered the crust during its past. More than 30,000 are visible by telescope. The total, down to bushel-basket size, may well exceed a million. The great walled plains, or supercraters with low profiles, like Clavius or Grimaldi have structures similar to those of the maria but on a smaller scale. Thei r diameters are between 200 and 300 kilometers.
Next in size on the moon's front side are some three dozen impact craters from 80 to 200 kilometers in diameter. A third of them have conspicuous Iightcolored streaks, called rays, radiating outward in all directions up to several hundred kilometers long, such as the well-known ray craters- Tycho, Copernicus, Kepler, and Aristarchus. Many of the small secondary craters, as well as the ray systems, were apparently formed by a rain of debris ejected from the primary crater after a large body struck the surface.
Impact craters are reasonable circular, with the interior rim steeper than the outer rim. The larger craters have terraces on their inner walls and frequently have a fairly smooth floor from which a few low peaks rise. Beyond the craters the terrain is hummocky and overlain with the ejected material from the cratering activity. Even moderate-sized craters, like the larger ones, have high walls. In these craters having a central peak, the peak is believed to have been created by the elastic rebound of rock from below the surface after the initial impact. Others have bare floors, presumably because they were flooded with lava; the crater Plato is a good example.
The impact craters are not all of the same age with the crater Clavius. The rim of the major crater is eroded and worn, whereas the half dozen or so small ones in the center have sharper and higher rims. Clearly the impacting bodies that produced the small craters superimposed on the rim of the large crater must have fallen more recently than that one which formed the large crater. An inspection of crater photographs shows that they have not only a spectrum of size but also a range in erosion and wear; that is, they are not all the same age. As an example, the craters Copernicus and Tycho are about 600 and 200 million years old, respectively.
Volcanically produced craters, formed mostly during the moon's early history, are present but in smaller numbers than those of impact origin. Volcanic craters are not always circular., Their rims slope at about the same angle, both inside and outside, and their rims and floors are shallower than those of impact craters. Thus thermal activity as a terrain-forming mechanism has not been absent from lunar history, although it is markedly less important than on the earth.
