South Pole-aitken Basin: Geology, Basin Floor, and Unit Compositions

At 2600 km diameter, the South Pole-Aitken (SPA) basin is the largest on the Moon and has profoundly influenced lunar crustal configuration and subsequent evolution [1-4]. SPA is significant for two principal reasons: 1) its immense size suggests that it has excavated most or all of the lunar crust at its target site, thus giving us a window into the very deep lunar interior; 2) because it is the oldest basin in relative terms (i.e., it has the highest density of superposed impact craters [2]), determination of its absolute age through the radiometric dating of melt samples could establish the existence and characteristics of the terminal lunar “cataclysm”, a proposed sharp increase in the bombardment rate of the Moon early in its history [5]. For these reasons, the SPA basin has been identified as a high-priority target for future exploration, including a possible robotic sample return mission [6]. We have compiled a variety of remote sensing data in order to understand as completely as possible, the geological setting of basin units, with the aim of identifying targets where specific exploration goals can be addressed. We have used Apollo, Galileo, Clementine, and Lunar Prospector data to make compositional and geological maps of the basin and its environs. Our objectives are to address the following questions: 1) Does a basin “impact melt sheet” exist in such form that samples of it could be identified and returned to Earth for analysis?; 2) Do units of mantle provenance exist within the floor of SPA basin?; 3) How deep into the Moon did the SPA impact excavate and what is the crustal provenance of various basin units?; 4) Do post-SPA basin geologic units obscure the original basin configuration and if so, to what degree and where? This paper is a preliminary report on our attempts to address these questions.