Southward impact excavated magma ocean at the lunar South Pole-Aitken basin

The ancient South Pole-Aitken (SPA) impact basin provides a key datapoint for our understanding of the evolution of the Moon, since it formed during the earliest pre-Nectarian epoch of lunar history1, excavated more deeply than any other known impact basin2,3, and is found on the lunar farside about which less is known than the well-explored nearside. Here we show that the tapering of the basin outline and the more gradual topographic and crustal thickness transition toward the south support a southward impact trajectory, opposite of that commonly assumed. A broad thorium- and iron-rich ejecta deposit southwest of the basin is consistent with partial excavation of late-stage magma ocean liquids. These observations indicate that Th-rich magma ocean liquids persisted only beneath the southwestern half of the basin at the time of impact, matching predictions for the transition from a global magma ocean to a localized nearside Procellarum KREEP terrane. These results have important implications for the upcoming human exploration of the lunar south pole by Artemis, as proposed landing sites are now recognized to sit on the downrange rim and Th-rich impact ejecta of the basin.