Gravity Mapping of Lunar Mantle Material in South Pole‐Aitken Basin Ejecta

Abstract The South Pole‐Aitken basin (SPA) is the largest and oldest impact basin on the Moon and was excavated through the crust and into the mantle early in lunar history. Here, we use gravity data to identify heterogeneities in crustal density that could indicate deeply sourced material in the crust despite a lack of surface spectral signatures. We show the presence of a ∼400‐km‐wide annulus of large‐amplitude, small‐scale gravity anomalies associated with the basin's rim region, which we interpret as the SPA ejecta blanket. We perform a pair of gravity inversions, varying either the thickness of a dense layer or the concentration of dense material in a mixed layer to match the gravity observations, and determine that the SPA ejecta blanket likely contains ≳5 million km 3 of mantle‐derived material. Three craters excavate the sources of these anomalies, revealing varied compositions that imply that the lunar mantle may have had orthopyroxene‐ and clinopyroxene‐rich components at similar depths at the time of the SPA impact, with important implications for the timing of mantle overturn. Gravity anomalies also reveal a previously unnoticed 1,440‐km‐diameter peak‐ring within SPA, delineating the transition between the collapsed ejecta and the central melt sheet. The annulus of strong anomalies associated with the ejecta encompasses the south polar region, suggesting a strongly heterogeneous subsurface near several future missions' landing targets, including the upcoming Artemis missions. These results place an important constraint on basin formation, reveal the composition of the mantle, and will guide future studies in targeting this material.