Lunar Surface Geology From Analysis of Impact Craters and Their Ejecta

Analysis of impact craters and their ejecta addresses someunanswered questions about the lunar surface. First I estimatethe regolith depth on the south farside of the Moon to be about40 m, which is significantly deeper than the nearside regolith,estimated to be 3-16 m. This result is obtained by studyinghundred meter diameter flat floored craters, using the method ofQuaide and Oberbeck (J. Geophys. Res., 1968, 73, 5247-5270). This measurement has implications for the formation of the lunarregolith, and for interpretation of samples returned in thefuture by astronauts or automated sample return missions.Next, I report the discovery of a method that distinguishesbetween primary and distant secondary craters in high resolutionplanetary images. For a given crater size, the largest bouldersof secondary craters are significantly larger than those ofprimary craters. The ability to identify distant secondarycraters will help constrain primary production rates of smallcraters and improve surface age determination of small areasbased on small crater counts.Third, I characterize the distributions of boulders ejected from18 lunar impact craters. I find that in large craters, thelargest boulders are preferentially ejected at low velocities(closer to the crater), whereas the largest boulders from smallcraters are ejected over a wider range of ejection velocities. Also, for a given crater size, deeper regolith reduces themaximum ejection velocity attained by a boulder ejected from acrater. I show that this is a logical result of the streamlinesof excavation in an impact when there are no coherent boulders inthe regolith. Cumulative plots of the boulders have slopessteeper than -2, as do secondary craters. This result isexpected because ejecta fragments produce secondary craters.Finally, I describe the morphology of some lunar crater walllandslides that strongly resemble martian gullies, despite thelack of geologically active water on the Moon today or in thepast. The lunar features indicate that alcove-channel-apronmorphology, attributed on Mars to seepage of liquid water, canalso form via a dry landslide mechanism. Therefore alcove-channel-apron morphology is not diagnostic of water carvedgullies