Jones, and 5 co-authors, reported in late 2020 that the lunar South Pole–Aitken (SPA) basin is the largest and oldest recognized impact basin on the Moon . Located on the farside hemisphere. SPA is nearly antipodal to the nearside geochemical province referred to as the Procellarum KREEP Terrane (PKT). (KREEP is a geochemical component named for its enrichment in incompatible elements including Potassium (K), Rare Earth Elements (REE), and Phosphorous (P).
The PKT represents an as-of-yet unexplained compositional asymmetry on the lunar surface, but the relative position of the two regions may be indicative of the SPA impact playing a role in forming the PKT. Large basin-forming impacts create substantial thermal anomalies in planetary interiors; the authors demonstrate here that the SPA impact-induced thermal anomaly would have greatly influenced lunar thermochemical evolution, leading to an interior asymmetry and offering an explanation for the existence of the PKT.
The KREEP component detected in the PKT is proposed to be derived from partial melting of incompatible-rich late-stage LMO cumulates, so
the surface distribution of KREEP could be representative of its subsurface distribution. Pre-SPA interior heterogeneities may have shifted the center of KREEP concentration, or the obliquity of the SPA impact may have shifted the center of the associated thermal anomaly, but the authors' models indicate that concentration of late-stage LMO cumulates beneath the nearside lunar crust is highly likely.
The authors' results robustly show a connection between a known event in lunar history (i.e., the SPA basin-forming impact) and a process which transports and concentrates KREEP in the lunar nearside. The post-overturn scenario requires further examination, but it shows the same degree-1 convection patterns as the pre-overturn scenario. Furthermore, all models show a mechanism by which a deeper lunar interior asymmetry could form.
Such a mantle asymmetry could affect characterizations of the interior of the Moon, e.g. a mantle density asymmetry would change values of crustal thickness or could lead to true polar wander. A mantle geochemical asymmetry created by degree-1 convection could also lead to observable surface features, such as azimuthally varying Mg# due to the vertical gradient in mantle Mg# from fractional LMO crystallization. The major reorganization of lunar interior dynamics caused by the SPA impact has not yet been fully factored into studies of lunar thermochemical evolution. However, the authors have demonstrated here that this process has broad implications for the geologic history of the Moon.
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