The Cratering History of the Interior of South Pole-Aitken Basin

The South Pole-Aitken Basin (SPA) is the largest and oldest identified basin on the Moon. The formation of SPA is thought to have excavated deep into the lunar crust, exposing unique lithologies and compositional units. Unlike many of the large nearside basins, extensive mare flooding did not cover SPA's interior, allowing the ancient, compositionally distinct materials to remain exposed at the lunar surface. However, a number of subsequent large impacts, as well as secondary craters from nearby basins, have significantly modified portions of the basin interior. Existing geologic maps of SPA identify ancient regions that contain materials possibly related to SPA's formation. However, these ancient units are not, in general, associated with the regional geochemical anomalies that may represent either SPA-derived melt-breccias or subsequent processes.

We have previously shown that, on average, the interior of SPA is older than the ancient heavily cratered feldspathic highlands terrane (FHT) north of the basin. This suggests that the formation of subsequent basins in and around the FHT effectively resurfaced much of the terrane, while such a process did not as significantly modify the interior of SPA. Therefore, identifying the regions within SPA that are the oldest is important in identifying regions that contain the highest proportion of SPA-derived materials. In order to define compositional units within SPA, we begin by identifying areas that correspond to the strongest geochemical anomalies, specifically the high Thorium and high Iron regions. We will perform crater counting in these regions in order to determine relative ages, which may indicate if these geochemical anomalies are related to SPA formation. If these units are found to be younger than pre-Nectarian in age, and therefore not related to SPA formation, then they will likely contain little or no SPA-derived material and therefore should not be considered for sample return sites.