Petro, Noah - Sampling Impact Melt From the South Pole-Aitken Basin and SPA Pre-Nectarian Basins

Abstract: 
The South Pole-Aitken Basin (SPA), possibly the oldest lunar basin, contains an array of basin, crater, and volcanic deposits. For many years, the nature of regolith components has been modeled in order to predict their origin, both vertically and horizontally, on the crust (local vs. foreign, lower crust/mantle vs. upper crust). These modeling efforts have concluded that the non-volcanic components of the regolith within SPA is dominantly locally derived, containing SPA derived impact melt with a small component of possible mantle derived materials (derived from two, large basins within SPA over very thin crust). Apollo and Luna regolith samples inform us that the regolith at any location of the Moon contain a diversity of materials, both in time and composition. As Bottke et al. present at this meeting PreNectarian (pN) basins may have formed in a narrow window of time following the formation of the Moon and SPA. Here we assess the likely contribution of pN basin ejecta to SPA regolith. We consider the contributions to the regolith of five pN basins identified by Fassett et al (2012; JGR), four interior to SPA (Amundsen-Ganswindt, Poincaré, Ingenii, Apollo) and the Australe basin located exterior to SPA.

Following the approach described by Cohen and Cooker (2010; LPSC) we evaluate the fraction of melt from pN basins as part of ejecta deposits that are incorporated into the SPA regolith. First we consider the ejecta from Australe, a heavily degraded 880 km diameter basin. Impact melt from the formation of this basin may extend as deep as ~190 km (Cintala and Grieve, 1998) and may comprise ~8% of its ejecta. Given that Australe may be extremely old (see Bottke et al., this meeting), its ejecta may be extremely diluted within the SPA regolith, or absent entirely if the crustal properties at the time of formation are as unique as Bottke et al. describe. The other four pN basins are located inside or on the rim of SPA (listed above). These basins range in diameter from 315-480km in diameter with depths of melting likely from 60-100 km depth (assuming impact velocities of 15-20 km/s; Cintala and Grieve, 1998). As Petro and Jolliff described in 2012, both Apollo and Poincare have extremely thin crust in their interiors, implying that their melt could be derived from within the mantle. The fraction of melt in their ejecta deposits from each these basins is ~5%, not a large volume, but when the total contribution of basin ejecta to SPA is small (Petro and Pieters, 2008), their contribution to the regolith is not-insignificant.

   A well selected sample-return site within SPA would allow access to both SPA impact melt as well as material that can constrain the “SPA impact chronology” including SPA, and post-SPA basins. These in turn are ideal locales to test hypotheses of early impact fluxes.