Lunar surface geology from analysis of impact craters and their ejecta.

摘要

Analysis of impact craters and their ejecta addresses some unanswered questions about the lunar surface. First I estimate the regolith depth on the south farside of the Moon to be about 40 m, which is significantly deeper than the nearside regolith, estimated to be 3-16 m. This result is obtained by studying hundred meter diameter flat floored craters, using the method of Quaide and Oberbeck (J. Geophys. Res., 1968, 73, 5247-5270). This measurement has implications for the formation of the lunar regolith, and for interpretation of samples returned in the future by astronauts or automated sample return missions.; Next, I report the discovery of a method that distinguishes between primary and distant secondary craters in high resolution planetary images. For a given crater size, the largest boulders of secondary craters are significantly larger than those of primary craters. The ability to identify distant secondary craters will help constrain primary production rates of small craters and improve surface age determination of small areas based on small crater counts.; Third, I characterize the distributions of boulders ejected from 18 lunar impact craters. I find that in large craters, the largest boulders are preferentially ejected at low velocities (closer to the crater), whereas the largest boulders from small craters are ejected over a wider range of ejection velocities. Also, for a given crater size, deeper regolith reduces the maximum ejection velocity attained by a boulder ejected from a crater. I show that this is a logical result of the streamlines of excavation in an impact when there are no coherent boulders in the regolith. Cumulative plots of the boulders have slopes steeper than -2, as do secondary craters. This result is expected because ejecta fragments produce secondary craters.; Finally, I describe the morphology of some lunar crater wall landslides that strongly resemble martian gullies, despite the lack of geologically active water on the Moon today or in the past. The lunar features indicate that alcove-channel-apron morphology, attributed on Mars to seepage of liquid water, can also form via a dry landslide mechanism. Therefore alcove-channel-apron morphology is not diagnostic of water carved gullies.