Thorium abundances on the lunar surface

Measurements of absolute thorium abundances on the lunar surface are presented using both the high- and low-altitude data taken with the Lunar Prospector Gamma-Ray Spectrometer. An analysis of the uncertainties shows that the measured uncertainties are <0.5 μg/g and are close to the theoretical limit of Poisson statistics. Currently, the overall systematic uncertainties are likely dominated by variations in background counts. The relative systematic uncertainties are thought to be no larger than 30% for the high-altitude data and 15% for the low-altitude data. A comparison of high- and low-altitude data show that most regions having thorium abundances >7 μg/g are likely small area regions ≤(150 km)². Using lunar topographic data, we have shown that the thorium abundances in the lunar high-lands and portions of South Pole-Aitken (SPA) Basin are larger for lower elevations. We have also studied a number of regions with anomalously high thorium abundances such as the northwestern region of SPA Basin, the crater Arago in western Tranquillitatis, and the Compton/Belkovich region in the northeastern highlands. The Compton/Belkovich region appears to be enriched with evolved rocks such as alkali anorthosite and currently represents the only such extended region on the Moon that has been identified. In contrast, Tycho crater has very low thorium abundances which suggests that KREEP was not assimilated at depth in this portion of the Moon.