Regolith in the South Pole-aitken Basin Is Mainly

Introduction: This abstract is concerned with the probability that a mission to a site within the South Pole-Aitken basin (SPA) would yield a meaningful sample of typical SPA floor material. The probability seems favorable, barring a highly atypical landing site, because the chemical composition of the SPA interior, as determined remotely from orbit, is different from that of the surrounding lunar surface. How representative would the sample be? To what extent have lateral transport or later events compromised the original chemical and mineralogical composition of the floor material? Where or in what kind of deposit should the mission land to provide the best example? We address these questions from the point of view of modeling of impact ejecta deposits. SPA is the largest lunar impact basin [e.g., 1,2]. Shallow for its diameter, it has a subdued gravity signature [3], a lower albedo [4], and a more Th-and Fe-rich interior [5,6] than the surrounding highlands (the Feldspathic Highlands Terrane, FHT) [7]. Its floor may represent noritic or perhaps (but less abundant) gab-broic lower crust of the FHT [8,9], the upper crust stripped away by the basin-forming impact, possibly an oblique one [10]. The minor Th enrichment exposed on the SPA floor could reflect a KREEP component, part of the hypothesized global layer of trace-element-rich magma-ocean residue [11]. If KREEP, however, which is somewhat magnesian, is concentrated primarily in the Procellarum KREEP Terrane (PKT), the SPA Th enrichment could arise from two sources: 1) KREEP impact ejecta from the PKT in the region of grooved ter-rane antipodal to the Imbrium basin [12; but see 13] and 2) residual melt with KREEP-like inter-element ratios, which geochemical modeling shows would be a result of crystallization of magmas in the lunar crust. Such melts would be poorer in Th and richer in Fe than the KREEP of Apollos 12 and 14 [14,15], unless their origin, too, included mixing with a magnesian mantle component as suggested for KREEP associated with the PKT. The original materials may have been incorporated into a melt sheet, perhaps a differentiated one [16]. A sample from the floor of SPA could confirm or refute these hypotheses and will surely yield surprises. Impact ejecta deposit modeling: The impact mod-eling shown here was done using the method described by [17]. It depends on ejection of fragmented material from a primary crater [18] and ballistic transport of the ejecta and excavation by the ejecta …