Anorthosite distribution and its implication in the Lunar South Pole-Aitken basin based on data derived from SELENE Multiband Imager

The South Pole-Aitken (SPA) basin is one of the biggest basins (2500 km in diameter [1]) on the lunar far side. Previous studies have suggested that most of the upper crustal material in this basin has been excavated and that the lower crust or mantle materials have been exposed [1], [2]. However, the basin formation process and consequent mineralogy of this basin are still unclear because of the degradation after the supposedly ancient SPA basin-generated impact. For example, Pieters et al. [2] and Ohtake et al. [3] reported that anorthosite is exposed within the estimated transient cavity of the SPA basin [1], suggesting that possible upper crustal material remained in the SPA basin. In this study, we analyzed more than 50 craters within the SPA basin and investigated the distribution of anorthosite to understand the extent of excavation of the upper crustal material. We chose the craters to analyze by selecting higher reflectance locations based on the Clementine 750 nm-base map because the reflectance of anorthosite is higher than that of rocks with mafic-rich compositions. We used images acquired by the SELENE Multiband Imager (MI) to investigate the mineralogy at a high spatial resolution (20 m x 20 m per pixel) within the craters and their surrounding area in the SPA basin. The MI data correction and calibration procedures we used are similar to those described in [3]. We estimated the mineralogy by using the absorption band generated by plagioclase (around 1250 nm), olivine (around 1050 nm) and pyroxene (around 1000 nm). The presence of anorthosite is determined by the absorption depth around 1250 nm. Additionally, we checked and confirmed that the purest anorthosite is exposed at fresh surfaces by comparing the maturity map generated using an expression of OMAT by Lucey et al. [4]. We found anorthosite both near and outside of the estimated transient cavity (and within the SPA basin) and in the northwest area within the estimated transient cavity. However, our current analyses provide no evidence of anorthosite in the other part of the estimated transient cavity. In these non-anorthosite craters, the spectrally most prominent mineral species is pyroxene with spectral absorption deepest around 950 nm. The presence of anorthosite only in the northwest part within the estimated transient cavity possibly suggests heterogeneous excavation depth within the SPA basin and the upper crustal material remained in the SPA basin at least in some extent. References: [1] Head J.W. (1993) JGR, 98, 17149-17181. [2] Pieters C.M. et al. (2001) JGR, 106, 28001-28022. [3] Ohtake M. et al. (2009) Nature, 461, 236-401. [4] Lucey P. et al. (2000) JGR, 105, 20377-20386.