A southward differentiated impactor forms the tapered shape of the South Pole–Aitken impact basin on the Moon

The South Pole–Aitken (SPA) basin-forming impact was a critical event in the Moon’s history. Despite being the oldest and largest acknowledged basin, critical details including the impactor’s size, nature, direction, and fate of the ejecta remain uncertain. Here, we simulate SPA’s formation and resultant crustal structure. We find that SPA’s observed shape of an ellipse tapered toward the south is best reproduced by a 260-km-diameter differentiated impactor striking with a north-to-south trajectory. In this scenario, the impact disperses lunar mantle materials in the cross-range and the downrange directions away from the lunar farside. Much of the mantle ejecta collapses into the basin interior, consistent with the distribution inferred from gravity. These results suggest that the Artemis landing sites near the lunar south pole should contain abundant SPA ejecta including mantle materials. Samples returned from these regions should therefore reveal the age of SPA and the composition of the lunar mantle.