Sampling the Spa Basin: Some Considerations Based on the Apollo Experience

The evolution of the crust and the timing of early impact events is important in the scientific exploration of the Moon [1]. The recent National Research Council decadal study [2] made sampling the deposits of the South Pole-Aitken basin a major priority for planetary exploration. This basin, located on the southern far side, is the largest and oldest multi-ring basin on the Moon and may have excavated most, if not all, of the lunar crust in its target region [3-5]. Study of this feature can illuminate the early cratering history of the Earth-Moon system, lunar crustal composition and structure, and the nature of the large impact process [2,6]. The NRC study recommended a robotic mission to sample the SPA deposits [2]. The issue of robot samplers v. human geological field study in lunar exploration was considered by Ryder et al. [7]. They suggested that to collect samples relevant to the issues under study, complex, geological targets requires the guiding presence of human intelligence. Given the scientific questions about SPA and its evolution , where should the basin be sampled and how? I here review the Apollo experience with basin sampling and draw some parallels the South Pole-Aitken basin. Apollo Sites and Sampling Results The Apollo landing sites were selected to advance our understanding of the composition and history of the lunar highlands, including the role of large multi-ring basins, particularly Imbrium, a dividing event in lunar history [8, 9]. Key objectives were to document the absolute age of the Imbrium impact and to characterize the deep lunar crust through the examination of basin ejecta [8]. The Apollo missions to the lunar highlands (14-17) returned carefully selected and documented samples [10], probably representing regional units, although dissenting opinions exist [11]. Apollo 14. The Apollo 14 mission was sent to Fra Mauro because: a) the context of the regional deposit was thought clear (i.e., Imbrium basin ejecta); and 2) a young, fresh crater (Cone) was available to provide a " drill hole " into the thick basin deposits recognized there, ensuring proper sampling. It was considered critical to sample the deposits of Cone crater to be certain that genuine Fra Mauro Fm. (Imbrium-related) was collected, not later surficial contamination [8]. The Apollo 14 samples are impact breccias of broadly basaltic composition; all are enriched in KREEP. True anorthosite (thought to be the principal component of the highlands) is uncommon. Plutonic rocks are …