Basins created by giant impacts can excavate material from deep within a planetary body and expose its underlying crustal and mantle stratigraphy. The South Pole-Aitken basin (SPA) is the largest impact-basin structure on the Moon at some 2,494.5 kilometers (1,550 miles) in diameter, meaning the entire state of Texas could fit comfortably inside it. In general, the larger the diameter of the crater or basin, the deeper the impact has likely excavated, so scientists are confident that SPA has created the deepest window into the Moon that we have available, and provides a unique observational opportunity to learn more about the structure of the lunar interior.
A study led by Dan Moriarty at NASA Goddard Space Flight Center used orbital data acquired by the Moon Mineralogy Mapper (M3) instrument aboard Chandrayaan-1 to investigate mantle-derived lithologies exposed at SPA. Moriarty and colleagues found that the thorium and potassium enrichments in SPA ejecta are consistent with material (known as KREEP) thought to have been produced during the final stages of lunar magma ocean crystallization as the Moon cooled and solidified after formation. Impact melts at SPA are consistent with low-calcium, pyroxene-dominated materials thought to have formed in the upper lunar mantle during lunar magma ocean crystallization. Previous studies have suggested that the Moon’s upper mantle and late-stage materials were sequestered into the lower lunar mantle during a period of gravitational overturn in which deep, buoyant material rose upward and shallow, dense material sank downward. However, these new findings indicate that this process was either inefficient or had not occurred at the time of the SPA-forming impact.
Additionally, thorium enrichments have long been associated with the Procellarum KREEP Terrane, a region on the Earth-facing side of the Moon, suggesting that these late magma ocean products were restricted to the lunar nearside. The discovery of thorium enrichments at SPA suggests instead that thorium and potassium enrichments (i.e., KREEP) may have been distributed more uniformly around the Moon than previously believed. These results demonstrate the importance of ongoing efforts by NASA’s Artemis program to return to and sample the Moon, specifically by visiting the SPA basin. Samples directly recovered from the SPA basin and returned to Earth could help elucidate many unanswered questions about the lunar interior and improve our understanding of planetary differentiation. READ MORE