WHAT HAVE WE LEARNED ABOUT IMPACT CRATERING FROM THE MOON?

The recent suite of lunar orbital and surface missions have provided a wealth of high spatial resolution data allowing new insight into the formation and evolution of impact craters. Impact craters are one of two dominant geologic processes on the Moon and understanding the dynamics of their formation and deposits is key to understanding the geologic history. We review here some of the more important advances. More precise measurements of crater and ejecta volume allow a better estimate of the ejected volume and mass as a function of size (energy). Data have shown that recognizable deposits of impact melt can be identified in craters having diameters of only a couple of hundred meters. Digital elevation models indicate the volume of melt produced as a function of crater size and target materials. Stringers of dark material extending across the rim at smaller and larger diameters can be attributed to rays of melt material ejected from the crater. Self-secondary craters occur on the continuous ejecta blanket indicating that significant material is launched into vertical trajectories during excavation such that sufficient time elapses for the rim-encircling continuous ejecta to be emplaced. Presumably, self-secondary cratering is occurring during deposition of the continuous ejecta and that such craters are buried by the ejecta. Thermal data has shown some craters exhibit a cold thermal anomaly at night indicating that the surface regolith cools very rapidly and that it has been altered with respect to the surrounding regolith. Such thermal anomalies extend for many km away from the rim, well beyond the observed continuous and discontinuous ejecta. Material on the continuous ejecta can be remobilized after deposition forming lobes that are redeposited downslope. It is unclear if this material is impact melt that has leaked from the interior of the ejecta or simply remobilized clastic ejecta. High-resolution images (both orbital and surface) of boulders around craters provide information about the erosion and breakup of surficial rocks by micrometeors and thermal fatigue. The distribution of rocks around craters provides insight into the regolith thickness, the subsurface distribution of rock and patterns of ejecta distribution.