Regional Elemental Abundances within South Pole-Aitken Basin as Measured with Lunar Prospector

Introduction: South Pole-Aitken (SPA) basin is one of the largest impact basins in the solar system [1] and is a target of intense study since it is thought that SPA basin excavated deep into the lunar crust and possibly even the mantle [2,3,4]. These conclusions are supported by the observed mafic and thorium composition anomalies seen across the entire basin [5,6,7]. One of the major goals of lunar and planetary science [8] has been to measure and understand the composition of the non-mare materials within SPA basin. It is expected that this information will help to increase our understanding of the formation and differentiation processes that occurred early on the Moon. We now have information from a fully reduced Lunar Prospector Gamma-ray Spectrometer (LP-GRS) dataset [9] so we can evaluate the broad compositional differences within SPA basin. Here we present the results of a study where we have examined the abundances for both mare and non-mare materials within SPA basin using the LP-GRS data. In addition, we compare these results to the abundances that are seen in the anorthositic highlands to the north of SPA basin. Data Selection and analysis: For this study, we are using the following data: FeO [10] and Th [11] abundances are taken from the low-altitude (30km) portion of the LP mission and mapped onto 0.5°x0.5° pixels (15km x 15km at the equator); Al 2 O 3 , FeO and MgO abundances are taken from the high-altitude (100km) portion of the LP mission and mapped onto 150km x 150km pixels [9]. Our study area (80°S–20°S, 140°E–240°E) is shown in Figs. 1 and 2. Our initial selection is to define SPA basin as being all locations in the study area having an elevation of less than –2.5 km using Clementine topography data [12]. The contours in Figs. 1 and 2 show the SPA boundary as selected. In order to select mare regions, we used geologic maps [13,14] that have been digitized onto 0.5°x0.5° pixels, as shown by the red regions in Fig. 1. Finally, to make sure the measurements of non-mare materials are not contaminated by mare materials because of footprint spreading [11,15], we have masked out all locations that are closer than 90km to a mare unit but still within the SPA contour (blue regions in Fig. 1). Results: Figs. 3 – 5 show example results from the low-altitude FeO and Th data as well as …