Effects of Space Weathering and Porosity on the Far‐UV Reflectance of Amundsen Crater

Abstract

AbstractThe lunar South Pole crater Amundsen is a prime location to study the effects of space weathering in the far ultraviolet. Amundsen's equator‐facing terrace walls are highly illuminated while the northern side of the crater has permanently shaded regions (PSRs). Using data from the Lunar Reconnaissance Orbiter Lyman Alpha Mapping Project, we investigate signatures of space weathering in different regions of Amundsen. We find that regions of the surface that receive large amounts of solar illumination and solar wind flux (e.g., the southern terrace walls) display high Lyman‐α albedos and blue spectral slopes in the 175–190‐nm region, indicative of increased regolith maturity due to solar wind weathering and thermal cycling. Amundsen's PSRs, however, receive no direct solar illumination and very little solar wind flux and have a lower albedo across Lyman Alpha Mapping Project's entire band pass (57–197 nm) than illuminated regions of the crater. We conclude that the low PSR albedos correspond to high regolith porosity in the PSRs. These PSRs are extremely cold regions with very minor thermal cycling. Thermal cycling might be a process that reduces porosity in regions of the crater exposed to a wide range of temperatures, thus increasing their albedos across the entire wavelength range. However, our analysis of the present data set was unable to uniquely identify its role. The low albedos in the PSRs may also result from extreme charging effects inside the PSRs, causing lofting and redeposition of dust, as well as dielectric breakdown, which would act to increase regolith porosity.