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

Abstract

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.