1. Various Explanations for Shallow Craters
But even the vast
majority of craters on the Moon which obviously are the result of
the impact on its surface of space debris have a problem. Depending
on the size and velocity of the meteor, the impact crater it leaves
behind will vary in the depth of the crater. Thus, one would expect
that the largest craters would have, by comparison, a relatively
greater depth. Thus, the most familiar feature of the Moon, its
many craters, some of them quite large, are ironically the most
anomalous features of them all, octagons, pyramids, domes, shards
and pie slices notwithstanding.
Despite their fantastic size, even the largest gaping holes are surprisingly shallow. Craters fifty, even one hundred miles in diameter are no more than a mere two to three miles deep... Even conservative estimates by scientists indicate that meteors 10 miles or more in diameter should have penetrated the Moon surface to a depth of 4 or 5 times that diameter; yet the deepest Moon crater we know about (the Gagarin Crater) is 186 miles across but less than 4 miles deep. The conclusion that some scientists drew is that there is something extremely tough and strong under the shallow lunar surface, beneath the thin layers of rock and dirt covering this mysterious sphere, that prevented deep holes.613
In one case of a 148
mile wide crater, the depth is a mere 3 miles, about half of which
is accounted for merely by the surrounding wall of rock thrown up
around it. Weirder still is the fact that the floor of the crater
is convex, following the curvature of
the Moon’s surface, rather than
concave.614
The mechanism most
familiar to current science that can cause such shallow craters are
...thermonuclear explosions.