PHYSICAL AND COMPOSITIONAL PROPERTIES OF IMPACT MELTS FOR JACKSON AND TYCHO CRATERS: IMPLICATIONS FOR SPACE WEATHERING AND DEGRADATION OF LUNAR IMPACT MELTS

Impact melts form when decompression from high shock pressure and temperatures associated with hypervelocity impacts leads to the melting (or vaporization) of target rock material. The morphology and physical properties of these rocks can be linked to their initial emplacement conditions such as temperature and viscosity through modeling. Their composition is in turn derived from the target lithology. As a result, impact melts can provide information about both the dynamics of the impact process (e.g. melt mobility and mixing), and the composition and structure of the lunar crust. 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 permitted the identification and study of impact melt units at increasingly small spatial scales, including on the central peaks of complex craters. For example, impact melt units have been mapped in details for Jackson and Tycho craters, on their floor, central peak and continuous ejecta. Here we study the composition, optical maturity and rock concentrations of these units compared to nearby non-melt units. We also leverage the unique high slope setting of impact melts on the central peak of these two craters to assess the influence of slope on post-emplacement modification processes. The datasets we use comprise the SELENE Multiband Imager and Terrain Camera data, the LROC Narrow Angle Camera data, and the Diviner Lunar Radiometer data.

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, from the highest shock on the central peak melt units, to the least amount of shock in the ejecta.