Landing Sites For South Pole-Aitken Basin Sample Return
Science Priority: Sample return from South Pole-Aitken (SPA) basin has priority rooted in the Planetary Science Decadal Survey . A mission concept (MoonRise) was proposed to NASA’s New Frontiers program to accomplish such a sample return with science objectives that include (1) determining the age of SPA basin formation to constrain models for the heavy impact bombardment of the Moon and inner Solar System, and test models for early Solar System planetary dynamics; (2) gaining a new understanding of the giant impact basin-forming process; and (3) testing hypotheses for internal differentiation and thermal history of the Moon and effects of SPA basin formation on the Moon’s internal evolution, magnetism, and volcanism . Key to these objectives is collecting impact-derived melt rocks from the basin-forming event  for age determination and to unravel the lithologic materials associated with the impact, including deep-seated target rocks of lower crustal and upper mantle origins. Collected materials may also include volcanics that formed after the SPA event as well as some proportion of ejecta from post-SPA impact basins. Site Selection: Orbital remote sensing reveals a strong geochemical signature associated with SPA, which is interpreted as the SPA impact-melt sheet [3-5]. Regolith production on this substrate means that landing sites within the basin will yield a proportion of materials whose geochemistry can be directly tied to orbital signatures and whose radiometric ages were thoroughly reset by melting during the SPA event . A suitable landing site for a robotic sample return mission must be (1) certifiably safe for automated landing and lift-off operations, (2) located within the region indicated by geochemical and mineralogical data to yield abundant original SPA impact-melt substrate, and (3) near, but not on, volcanic materials (mare, cryptomare), suitable for evaluating the volcanic contribution to basin materials . These criteria rule out areas in or very close to large impact craters and areas of post-SPA volcanic resurfacing. Although many large areas exist that meet these criteria, areas of relatively smooth plains in the central part of SPA basin (55-60º S) are well suited. In this region, as a result of impact mixing, rock components of SPA regolith will include a mixture of (1) impact-melt rocks and breccias from the SPA impact; (2) impact-melt rocks and breccia from large post-SPA craters and basins that represent the post-SPA late-heavy bombardment interval; (3) ancient volcanics or cryptomare, and (4) more recent basalts derived from the post-SPA mantle. All of these rock types are expected to be present in a well-mixed regolith and are desired in the returned samples. As an example, inter-crater plains E-SE of Bhabha crater are expected to contain ejecta from large craters such as Bhabha, Bose, and Stoney; mare and cryptomare deposits delivered from the local vicinity and north of the site, and mixed into the regolith by small impacts, as well as material from the “Mafic Mound” (Fig. 1). Landing Site Safety Assessment: High-spatial-resolution imaging (LROC NAC) and LRO topographic data sufficient to fully characterize potential hazards and to select and certify safe landing sites are in hand. At the lander scale, DTMs derived from NAC images provide slope and surface roughness data at a 2 m baseline. LROC NAC and LRO Diviner data are sufficient to assess the presence of boulders. Examples of these data will be shown at the workshop. Acknowledgements: NASA for support of LRO, and LRO science & operations teams for collection and production of data used to support landing site analysis. References:  NRC (2011) Vision and Voyages for Planetary Science in the Decade 2013-2022;  Jolliff et al. (2017) LPSC 48, #1236.  Moriarty et al., this Workshop;  Petro et al. (2011) GSA Spec. Paper 477, 129-140.  Petro & Pieters (2004) JGR-P 109, E06004.