The South Pole-Aitken basin constrains the early evolution and differentiation of the Moon

The South Pole-Aitken basin (SPA) is the oldest and largest known impact basin on the Moon.  We use gravity, topography, and surface remote sensing data together with impact simulations to reveal new details of the structure and formation of the basin and to place new constraints on the structure, differentiation, and early evolution of the Moon. The geophysical expression of SPA reveals an elongated, tapered basin formed in a southward-directed oblique impact. Impact simulations show that the downrange excavation from the core of a differentiated impactor can explain the tapered shape of the basin. Remote sensing reveals an asymmetric ejecta blanket rich in thorium, consistent with asymmetric excavation of late-stage lunar magma ocean liquids enriched in incompatible elements such as potassium, rare earth elements, and phosphorus (KREEP). The distribution of Th-rich ejecta can be explained in the context of models of magma ocean crystallization, in which progressive solidification of the magma ocean caused it to become concentrated beneath regions of thinner crust, eventually pinching out to zero thickness beneath the farside highlands and finally concentrating within the nearside Procellarum KREEP terrane.  At an intermediate stage, a thin and discontinuous layer of late-stage magma ocean liquids would have been present beneath the southwestern half of the basin extending onto the nearside, which explains the observed distribution of Th-rich SPA ejecta. Material excavated by SPA on the farside and the younger Imbrium basin on the nearside reveal the evolution of the late-stage magma ocean products in space and time. The ages of these basins and Th concentrations of their ejecta match the modeled compositional evolution of the magma ocean.  Thus, the ejecta of SPA provides a means to sample the late-stage magma ocean as well as the lunar mantle.  High-resolution gravity data reveals an annulus of large-amplitude, short-wavelength gravity anomalies surrounding the basin, consistent with the predicted distribution of material excavated from the lunar mantle. Remote sensing observations of craters excavating into this material indicate a heterogeneous mantle at the time of impact, containing both orthopyroxene-rich and clinopyroxene-rich material. Experimental work predicts that these distinct compositions should form early and late in the magma ocean crystallization sequence, respectively. Thus, the observed compositions are consistent with partial or ongoing overturn of the lunar mantle at the time of the SPA impact. Together, these analyses show how the Moon’s oldest known impact basin provides a key constraint on the interior structure, differentiation, and early evolution of the Moon.  This work provides context for recent, ongoing, and future missions exploring the lunar farside that offer the opportunity for in situ exploration of materials derived from the SPA impact.