Abstract
The detailed morphology of impact craters is now believed to be mainly
caused by the collapse of a geometrically simple, bowl-shaped "transient
crater." The transient crater forms immediately after the impact.
In small craters, those less than approximately 15 km diameter on
the Moon, the steepest part of the rim collapses into the crater
bowl to produce a lens of broken rock in an otherwise unmodified
transient crater. Such craters are called "simple" and have a depth-to-diameter
ratio near 1:5. Large craters collapse more spectacularly, giving
rise to central peaks, wall terraces, and internal rings in still
larger craters. These are called "complex" craters. The transition
between simple and complex craters depends on 1/g, suggesting that
the collapse occurs when a strength threshold is exceeded. The apparent
strength, however, is very low: only a few bars, and with little
or no internal friction. This behavior requires a mechanism for temporary
strength degradation in the rocks surrounding the impact site. Several
models for this process, including acoustic fluidization and shock
weakening, have been considered by recent investigations. Acoustic
fluidization, in particular, appears to produce results in good agreement
with observations, although better understanding is still needed.
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