LUNAR CRATERS: INSIGHTS INTO THE CRATERING PROCESS

The wealth of new data for the Moon reveals a range of impact crater morphologies. suggesting the cratering process is perhaps more complicated than typically realized. Several different types illustrate the complexity. Craters (<100 m diameter) on melt surfaces have an unusual morphology (shallow floor, central peaks, no ejecta). Such craters are absent on adjacent ejecta. Geologic relations suggest these are impacts into still molten impact melt. Such craters must be "self-secondary" craters caused by ejecta from the main impact as the timing is instantaneous on a geologic time scale (sec. to min.) and must form before the melt solidifies. Craters on slopes have an asymmetric morphology. A crater floor is offset downslope from the geometric crater center, for larger craters impact melt occurs on the floor and typically overtops a low (or missing) downslope rim. Such craters are also observed on small bodies (e.g., Vesta). These craters are formed on topographic slopes; presumably as the crater cavity expands, it grows beyond the surface on the downslope side creating an asymmetric crater. Subsequent collapse is also asymmetric with most debris deposited on the downslope side. Craters formed by spacecraft impacts have unique morphology. Craters formed by the upper stage of the Saturn V launch vehicle have diameters of ~33 m but are asymmetric. A characteristic feature is a large mound of material in the crater center that covers much of the floor. A unique aspect of such impacts is that the vehicles have very low bulk densities (28 kg m^-3) and low impact velocities (1.2 km s^-1). The unusual morphology is interpreted to result from the low projectile density and interactions of impact excavation with the lunar regolith. Details of ejecta emplacement are also clearly revealed. The observed sequence includes the continuous clastic ejecta blanket, bouldery debris around the rim exterior and scattered boulders on the ejecta blanket, and flow of impact melt on the margins. The presence of isolated boulders on the ejecta, unusual craters on melt deposits, and variations in the size-frequency distribution of small craters on the ejecta clearly indicate the debris is lofted during the impact event and only re-impacts the surface late in the ejecta emplacement process. Such late-stage ejecta was also observed at the Sedan nuclear crater.