Lunar Farside South Pole‐Aitken Basin Interior: Evidence for More Extensive Central Cryptomaria in the South Pole‐Aitken Compositional Anomaly (SPACA).

In the central area of the South Pole‐Aitken (SPA) basin, an intermediate albedo, mafic compositional anomaly (SPA Compositional Anomaly, SPACA) has been documented by previous studies, but its origin remains uncertain. We conducted an investigation of stratigraphic units defined based on morphology and composition and their relative ages, and placed these in the context of basin topography and the observed sequence of geological events, all helping to distinguish between SPACA origins from: (a) SPA impact melt, (b) volcanism induced by the SPA event and (c) lunar cryptomaria. We conclude that SPACA represents extensive traditional cryptomare deposits overlying the SPA impact melt. We interpret the basin center to be filled with cryptomare deposits at least one km thick (>1 × 105 km3 in volume) with ages not younger than Early Imbrian. We attribute the relatively high albedo of SPACA to lateral mixing of ejecta from nearby highlands craters and basins, and conclude that the cryptomaria basalts are likely to be very similar to basalts on the nearside. Our findings imply a 0.5%–1.8% increase in the total volume of global lunar mare and cryptomare deposits. These results show that mare volcanism was common only in areas of thinnest crust on the lunar farside, a factor important in understanding lunar nearside‐farside asymmetries. Despite this significant increase in total cryptomare volume in the SPA basin center, SPA remains underfilled relative to nearside mascon basins. Return of mare basalts from the SPA region by Chang'E‐6 will help determine potential mantle source region differences and petrogenetic pathways. Plain Language Summary: On the enigmatic farside of the Moon, a huge impact basin called SPA basin occupies an immense area (∼2,500 km diameter). Previous spectroscopic investigations identified four compositional zones across SPA, and the central one shows a distinctive high‐Ca pyroxene anomaly, designated the "South Pole‐Aitken compositional anomaly" (SPACA). SPACA has a higher albedo compared to typical maria but lower than lunar highlands and has a paucity of impact craters relative to other SPA units. Previous studies concluded that SPACA is either associated with (1) the SPA impact melt sheet or (2) subsequent volcanic resurfacing events of a special composition induced by the SPA impact. Our reassessment favors the hypothesis that SPACA represents cryptomaria erupted into the basin center during a period of higher impact flux, resulting in lateral dusting of the dark mare surfaces by highland material, thereby raising their albedos. Consequently, the SPACA is similar to other lunar basins in that cryptomaria were deposited in the basin center, followed by maria when the impact flux declined and lateral dusting was minimal. Our SPA cryptomare volume estimates represent a 0.5%–1.8% increase in the total volume of global lunar mare and cryptomare deposits, but SPA remains underfilled relative to nearside giant basins. Key Points: The central South‐Pole Aitken basin compositional anomaly is formed of cryptomare deposits rather than a differentiated impact melt sheetSouth‐Pole Aitken basin resembles other lunar basins having mare/cryptomare infilling the center, adding 0.5%–1.8% to the global basalt inventoryCrustal thickness variations may account for the concentration of farside maria in the South‐Pole Aitken basin, yet it remains underfilled [ABSTRACT FROM AUTHOR]