IMPACT CRATERING: CONSTRAINTS FROM LUNAR CRATERS

Terrestrial craters are typically eroded or buried making analysis of morphometric, morphologic and geologic properties difficult or impossible. Extraterrestrial impacts on silicate bodies (Moon, Mars, Mercury, Venus) provide detailed morphology and morphometry to study impact processes, although at resolutions less than for terrestrial structures. The Lunar Reconnaissance Orbiter (LRO) and other international missions have provided a wealth of data for lunar impacts. The extremely high resolution (0.5 m/pixel) of the LRO imaging (LROC) plus thermal (Diviner) and altimetry (LOLA) data provide a database allowing detailed examination of impact processes. Lunar impact crater diameters range from microns to thousands of kilometers and occur on the mare (target: regolith - basalt - impact basin derived megaregolith) and in the highlands (target: regolith - megaregolith). Within the mare, for very small craters (m to 10’s m) the regolith and basalt form a two-layer (weak-over-strong) target producing unique morphologies. At intermediate diameters, the impact process is unaffected by the thin regolith and represents a single strong layer (basalt). At the largest diameters (kms), the target can represent a strong over an intermediate strength target (basalt over megaregolith) or relative intermediate strength target (megaregolith) when the mare basalts are thin relative to the impact diameter. Highlands have both a relative thin, weak regolith and an underlying intermediate strength megaregolith producing an effective size-dependent strength regime; at the small diameters - weak over intermediate; at larger diameters - intermediate. Impact melts are observed in many fresh mare craters at diameters <1 km (presumably it occurs in many more craters but is hidden by the development of regolith). Melt occurs as pools on the crater floor, as isolated pools on the ejecta and interior terraces, and most dramatically as rootless flows around the margins. Impact melt is the last ejecta component to be deposited.