Impact crater collapse

The detailed morphology of impact craters is now believed to be mainly caus ed by the collapse of a geometrically simple, bowl-shaped "transient crater ." The transient crater forms immediately after the impact. In small crater s, those less than approximately 15 km diameter on the Moon, the steepest p art of the rim collapses into the crater bowl to produce a lens of broken r ock in an otherwise unmodified transient crater. Such craters are called "s imple" and have a depth-to-diameter ratio near 1:5. Large craters collapse more spectacularly, giving rise to central peaks, wall terraces, and intern al rings in still larger craters. These are called "complex" craters. The t ransition between simple and complex craters depends on 1/g, suggesting tha t the collapse occurs when a strength threshold is exceeded. The apparent s trength, however, is very low: only a few bars, and with little or no inter nal friction. This behavior requires a mechanism for temporary strength deg radation in the rocks surrounding the impact site. Several models for this process, including acoustic fluidization and shock weakening, have been con sidered by recent investigations. Acoustic fluidization, in particular, app ears to produce results in good agreement with observations, although bette r understanding is still needed.