A new view of the moon in light of data from Clementine and Prospector missions

Results from the 1994 Clementine and 1998-99 Lunar Prospector orbital missi ons are forcing a reevaluation of existing models of the origin and evoluti on of the Moon. Data on global topography and gravity from Clementine gravi ty 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 model s of lunar hemispherical asymmetry. High resolution global maps of FeO and TiO2 derived from the Clementine UV-VIS data and Th maps from the Prospecto r gamma-ray data indicate that the lunar crust and the uppermost lunar mant le are laterally and vertically inhomogeneous on a global scale. An area of enhanced Th (and other incompatible element) abundances, known as the Proc ellarum KREEP Terrane (PKT), makes up approximately 16% of the nearside lun ar surface. Although the highest Th abundances appear to be in upper crusta l impact deposits, the close association of mare basalts with the PKT indic ates that anomalously high Th, U, and K concentrations extend to mantle dep ths. Anorthosites are very rare within the PKT and the pre-mare crust in th is region is composed of breccias and pristine rocks of the magnesian-suite and alkali-suite. The upper crust outside the PKT is anorthositic (similar to4% 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 up per crust has been removed by a giant impact. The lower crust in SP-A is no ritic in composition (FeO from 8-12%, Th from 2-4 ppm) and this material ma y represent impact-melted crustal cumulates that originally crystallized fr om the magma ocean. It is suggested that the enrichment in Th, and all othe r incompatible elements, occurred early in lunar history as a consequence o f the migration of late-stage residual melts along a pressure gradient indu ced by impact removal or thinning of the anorthositic crust.