Physical and compositional properties of impact melts for Jackson and Tycho craters: Implications for space weathering and degradation of lunar impact melts

Abstract

Impact melts are ubiquitous across the Moon, occurring in settings ranging from massive basin deposits to flows and ponds in and around small craters. Recent high spatial resolution imaging and spectroscopy datasets for the Moon have enhabled the identification and study of impact melt units at increasingly small spatial scales, including on the central peaks of complex craters. Lunar impact melts have unique physical properties at many scales (e.g. smooth appearances in visual imagery, low rock concentration implying smoothness and regolith cover at meter scale, high S-band radar returns implying roughness at decimeter scale, and possible anomalously low OMAT values), which may be due to their unique mode of emplacement or post-emplacement modification processes. We isolate impact melt regions in the crater interior of Jackson and Tycho as well as in the continuous ejecta blanket of Tycho using the geologic maps of Dhingra et al. (2017) and analyze their spectral, compositional, and physical properties utilizing datasets from the SELENE Multiband Imager and Terrain Camera, the LROC Narrow Angle Camera, and Diviner Lunar Radiometer. We leverage the unique high-slope setting of impact melts on the central peaks of these two craters to assess the influence of slope on post-emplacement modification processes, compared with unique physical properties of impact melts, on rock concentration and optical maturation. We find that slope is the primary control on optical maturity, while rock concentration plays a secondary role. We also find that melt units are generally more optically mature than their non-melt counterparts, and that this difference attenuates with distance from the crater center until no difference is noted in the continuous ejecta blanket. This suggests that melt units have different surface properties than the non-melt units in similar settings potentially due to the increased degree of shock they experienced during crater formation.