Crustal attributes of lunar basins from terrain-correlated free-air gravity anomalies

For the major lunar basins between ±64° latitude, uncompensated mass components as well as crustal ring and transient cavity attributes were evaluated from the free-air anomaly components of the Lunar Prospector gravity model (LP75G) that are spectrally correlated at 100-km altitude with the gravity effects of the Clementine terrain model (GLTM-2). Inversion of the terrain-correlated anomalies inferred incipient radial adjustments of the Moho that equilibrate the basin topography, assuming the lunar crust is mainly compensated by thickness variations. Nearly all nearside multiring basins are marked by mascon gravity anomalies that reflect mass concentrations from superisostatic mantle plugs plus mare fill. Farside basins, by contrast, are mostly characterized by maslite gravity anomalies from mass deficiencies due to subisostatic mantle plugs with marginal or no mare fill. The preponderance of maslites on the farside is consistent with crust that responded more rigidly to impacts than the nearside crust, which had a higher thermal gradient due to enhanced abundances of radioactive elements in the nearside mantle and crust. However, the extended thermal evolution of the lunar lithosphere also may have promoted the development of interfering mascon and maslite effects for tectonically complicated regions like the farside Freundlich-Sharonov basin. The Moho adjustments also revealed well-defined concentric zones of maxima and minima about the central basins that are strongly correlated with photogeologically inferred basin rings. In addition, the innermost zero-contour of the radial adjustments provides an effective estimate for the diameter of the transient cavity. The inferred transient cavity diameters correlated negatively with crustal thickness for basins with superisostatic mantle plugs and positively with the relative ages of the nearside basins. On the farside, superisostatic mantle plugs apparently developed in thinner crust up to thicknesses of about 30 km, whereas in thicker crust mantle plugs developed only to subisostatic levels. For the nearside, mass balance calculations between the equilibrium mantle plug and excavated basin materials support the proportional scaling relation of roughly 0.1 for the excavation depth-to-diameter ratio of the transient cavity that photogeologic studies have obtained. However, for the Serenitatis and Imbrium basins with possibly atypical nearside crustal properties, this proportionality may overestimate excavation depths by 28% and 37%, respectively. For nearside and farside basins apparently lacking mare flooding, shallow excavation depths were also inferred.