INSIGHTS INTO THE CRATERING PROCESS FROM LUNAR IMPACT CRATERS

Recent LRO/LROC images of lunar impact craters have provided important insights into the cratering mechanics. Lunar impact craters, because of the range of sizes and, in some cases, pristine morphology allow insight into the details of the impact cratering process that are not possible with terrestrial craters, due to erosion and burial. Impact craters on the Moon range in diameter from microns (zap pits) to thousands of km (SPA basin). Target properties vary on the Moon as a function of the geology. The highlands consist of a regolith some meters thick overlying an impact-basin generated megaregolith which is possibly many km thick. On the mare, a meters-thick regolith overlies a section of basalt that is hundreds of meters to a few km thick overlying highlands megaregolith. Thus, depending upon the size of the crater and the target, the strength contrast between different layers can influence crater development. Ejecta deposits are observed to consist of inter-layered lobes of material deposited in a complex process. Numerous small impact craters form on the continuous ejecta blanket due to self-secondary cratering in which ejected clasts impact the continuous ejecta blanket during and after it is deposited. Impact melt forming ponds on crater floors, which is typically observed in large complex craters, is observed in some fresh craters as small as 200 m diameter; these craters are interpreted to result from infrequent vertical impacts in which shock is maximized and most of the melt produced is retained in the crater. Impact melt is observed on the flanks of many craters forming discrete sheets and flows; relations indicate that the melt is deposited at the end of the ejecta deposition. Artificial impacts from a variety of spacecraft (Ranger, Saturn SIVB booster, Lunar Module Ascent Stage) provide constraints on cratering mechanics as they serve as large-scale experiments. These events are unique in that the projectiles (the spacecraft) have very low bulk densities (e.g., 0.02-0.07 g cm-3). The SIVB craters (30-40 m diameter), for example, show a unique elongate ridge in the crater interior that covers most of the crater floor. Interaction between the shock wave and the thin regolith overlying stronger basalt presumably has resulted in this interior morphology.