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 (<[80 km]2) thorium features across the lunar surface. Twenty of these features, all of which are located in the nearside Procellarum KREEP terrane, show a thorium‐iron anticorrelation that is indicative of mixing between mare basalts and thorium‐rich mafic impact‐melt breccias (MIB). However, there exists at least one example of a farside location (Dewar crater) that appears to have abundances similar to the thorium‐rich MIBs. This new map has also allowed the identification of mare basalts having high thorium abundances (>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.