Crustal thickness and the forms of impact craters

Abstract

Most impact craters with flat lavalike floors are attributed to (1) hypervelocity impact piercing a floating crust and admitting liquid magma to the crater floor or (2) impacts that pierce a solid crust with sufficient energy to relieve the load on high-temperature material held in a solid state, so that it melts and partially fills the crater. It is suggested that these two processes may represent successive stages in the history of a once-molten cooling planetary surface. A circular mare is considered to be a very massive event of type (2), producing a large surface of molten material that, on cooling, follows an accelerated cycle of the same general type as the original planetary surface. Observations of impact craters on both the earth and the moon suggest that cooling eventually results in a crust too thick to be pierced, even by massive impacts, so that a majority of craters are of a type associated with solidstate phenomena and exhibit raised rims and central uplifts similar to large experimental explosion craters.