Small-area thorium features on the lunar surface

Abstract

Using an improved understanding of the Lunar Prospector Gamma-Ray Spectrometer (LP-GRS) spatial footprint, we have derived a new map of global thorium abundances on the lunar surface. This map has a full-width, half-maximum spatial resolution of ∼(80 km)2 and is mapped on the lunar surface using 0.5° × 0.5° pixels. This map has allowed the identification and classification of 42 small-area ( 3 μg/g) in southwestern Mare Tranquillitatis, near the Apollo 11 landing site. With our better understanding of the LP-GRS spatial footprint, we have been able to constrain the surface thorium abundance at the Compton/Belkovich thorium anomaly to 40-55 μg/g, which is higher than any other measured location on the lunar surface and higher than most samples. Finally, using 1 km/pixel FeO abundances from Clementine and LP-GRS spatial footprint information, we have been able to obtain plausible thorium distributions around Kepler crater at a resolution of 1 km/pixel. The materials around Kepler crater appear to be a relatively simple mixing of thorium-rich MIB compositions and high-thorium mare basalts.