Lunar rare earth element distribution and ramifications for FeO and TiO2: Lunar Prospector neutron spectrometer observations

Abstract

Lunar Prospector neutron spectrometer data have been used to map the global surface distribution of the incompatible rare earth elements gadolinium and samarium from the low‐altitude (30±15 km) mapping orbit. These results afford improved surface resolution and detailed views of the potassium, rare earth elements, and phosphorus (KREEP) distribution within and around Mare Imbrium and elsewhere. The Gd and Sm results serve as a complementary and independent check of the distribution of KREEP on the Moon, in contrast with the Lunar Prospector gamma ray spectrometer results for thorium. The neutron spectrometer observations reflect the presence of Fe and Ti as well as Gd and Sm. The contributions of Fe and Ti are removed using high spatial resolution Clementine spectral reflectance determinations of FeO and TiO2 abundances. Overall, the resulting Gd and Sm abundance map agrees with the Th abundance map determined using the Lunar Prospector gamma ray spectrometer. In general, the detailed features of the Procellarum/Imbrium KREEP terrane are found in both. For example, distinct highs in Gd, Sm, and Th abundances are resolved over the craters Mairan, Aristarchus, Kepler, Reinhold, Lalande, and Aristillus, over the Apennine Bench and Fra Mauro regions, and over the Montes Jura and Montes Carpatus, indicating an enhanced abundance of KREEP in these locations. The neutron observations also provide constraints on FeO and TiO2 abundances; for some high‐Ti locales, there is a significant disagreement with TiO2 abundances inferred from Clementine spectral reflectance.