South Pole - Aitken Sample Return: Insights From Moon Mineralogy Mapper Data
As noted in the previous two Decadal Surveys, sample return from the South Pole - Aitken Basin (SPA) is a high-priority goal with implications for several critical lunar science questions relevant to (1)basin formation chronology, (2)lower crust/upper mantle composition, (3)large impact processes, (4)lunar formation/thermal evolution, and (5)lunar volcanism. In-situ experiments are insufficient, as they do not permit the detailed analyses required to address these questions. Therefore, sample return is required. It is paramount to carefully evaluate potential landing sites with remote sensing data to maximize science return and minimize risk. Here, we describe a recent analysis of the compositional structure of SPA using Moon Mineralogy Mapper (M3) data, including implications for landing sites. Spectral variations across SPA are dominated by differences in pyroxene composition and abundance (Fig. 1). From these data, four distinct compositional zones are identified: 1. The SPA Compositional Anomaly (SPACA) in central SPA exhibits pyroxene compositions unlike typical mare basalts or norites. Based on crater observations, the source of this material is ~5 km thick and is underlain by Mg-rich pyroxenes. Geo-logically, SPACA correlates with a region of fairly smooth topography and an apparent paucity of im-pact craters, suggesting resurfacing by a potentially unsampled magmatic composition. Underlying materials probably represent part of the SPA im-pact melt complex. 2. The Mg-Pyroxene Annulus surrounds SPACA and is fairly uniform in pyroxene composition. Based on the ubiquitous presence of Mg-rich py-roxenes in impact structures across the region, Mg-rich material persists to depths of several km. Ow-ing to the pervasive nature of this material, it most likely represents impact melt/breccia from the SPA impact. 3. The Heterogeneous Annulus exhibits localized exposures of Mg-rich pyroxenes interspersed in a more feldspathic matrix. Mg-rich pyroxene expo-sures probably represent SPA proximal ejecta. 4. The SPA Exterior is primarily feldspathic high-lands materials and contains little SPA impact melt or breccia. To fully address the science questions raised in the Decadal Survey, it is important to sample both SPA impact melt and post-SPA volcanic products, including the enigmatic SPACA material. Given this constraint, ideal sample sites are confined to impact craters within SPACA that have excavated SPA melt/breccia (Mg-pyroxenes) from beneath the resurfaced zone. Using M3 data, several candidate locations are identified, including Bhabha, Bose, and Finsen. Integrated characterization (LROC, LOLA, M3, etc.) is currently under-way. Fig. 1: A schematic overview of the compositional structure of SPA. Basemap: LOLA topography. Colored pixels represent the Estimated Band Center of the 2 µm absorption band derived from M3 images. Shorter wavelengths (purple/blue) indicate Mg-rich pyroxenes; longer wavelengths (yellow/red), Fe,Ca-rich pyrox-enes. Mare basalt regions have been removed to emphasize variations in nonmare areas. A survey of large impact structures was performed to assess materials exposed from depth. The overlaid colored ellipses and triangles correspond to the dominant pyroxene compositions observed in these crater walls/rims and central peaks (white symbols represent feldspathic structures). Integrating these analyses, four compositional zones were identified: (1) the central SPA Compositional Anomaly (SPACA), (2) the Mg-Pyroxene Annulus, (3) the Heterogeneous Annu-lus, and (4) the SPA Exterior.