A New View of the Moon in Light of Data from Clementine and Prospector Missions

Abstract

Results from the 1994 Clementine and 1998–99 Lunar Prospector orbital missions are forcing a reevaluation of existing models of the origin and evolution of the Moon. Data on global topography and gravity from Clementine gravity and LIDAR experiments indicate a 16 km elevation range on the farside of the Moon and a wide range in computed crustal thickness. The data confirm the presence of mascons under mare-filled basins and validate earlier models of lunar hemispherical asymmetry. High resolution global maps of FeO and TiO2 derived from the Clementine UV-VIS data and Th maps from the Prospector gamma-ray data indicate that the lunar crust and the uppermost lunar mantle are laterally and vertically inhomogeneous on a global scale. An area of enhanced Th (and other incompatible element) abundances, known as the Procellarum KREEP Terrane (PKT), makes up approximately 16% of the nearside lunar surface. Although the highest Th abundances appear to be in upper crustal impact deposits, the close association of mare basalts with the PKT indicates that anomalously high Th, U, and K concentrations extend to mantle depths. Anorthosites are very rare within the PKT and the pre-mare crust in this region is composed of breccias and pristine rocks of the magnesian-suite and alkali-suite. The upper crust outside the PKT is anorthositic (∼4% FeO and <1 ppm Th) and appears to be only slightly modified from the crust produced in the early magmasphere differentiation. A glimpse into the lower crust is provided in the South Pole-Aitken (SP-A) basin where the upper crust has been removed by a giant impact. The lower crust in SP-A is noritic in composition (FeO from 8–12%, Th from 2–4 ppm) and this material may represent impact-melted crustal cumulates that originally crystallized from the magma ocean. It is suggested that the enrichment in Th, and all other incompatible elements, occurred early in lunar history as a consequence of the migration of late-stage residual melts along a pressure gradient induced by impact removal or thinning of the anorthositic crust.