The compensation state of intermediate size lunar craters

The compensation state of 49 intermediate size (120 to 600 km diameter) lun ar craters are investigated using the most recent spherical harmonic models of the lunar topography and gravity, truncated at degree n = 110. The tota l mass anomalies per unit area (i.e., the lateral variations of the vertica lly integrated density perturbations per unit area) within an otherwise uni form crust of 60 km thickness are determined such that, together with the s urface topography, give rise to the model gravity anomalies. Crustal thickn esses of 40 and 80 km are also considered, but the general results of this study are not significantly affected. Excess mass anomalies are obtained by subtracting from the total mass anomalies the mass anomalies that are requ ired for the isostatic compensation of the surface topography. The excess m ass anomaly of a crater denotes its particular state of compensation. Depen dencies of the excess mass anomalies on crater location, size, and age are investigated, but in general few discernable trends are evident. Although t he vast majority of craters indicate some compensation, no correlation exis ts between age or size and the state of compensation. Roughly 16% of the cr aters show no compensation, and in some cases have mass deficiencies most l ikely due to the shock fractured bedrock: the breccia lens of lower density . The crust in these regions was likely cold and rigid enough at the time o f impact to rigidly support the stress caused by crater excavation. These f eatures are seen throughout different geological periods, demonstrating tha t the lunar crust cooled quickly and strengthened soon after formation. A c omparison of the compensation state of craters Apollo, Korolev, and Hertzsp rung suggests that the thermal and mechanical properties of the crust prior to impact had an appreciable effect on the compensation, and that crustal thickness may be the single most important factor controlling the compensat ion of intermediate size craters. The characteristics of the excess mass an omaly profiles of the eight well-known near side mascon basins are used to identify new mascon-like craters. Ten newly, found mascons are confirmed: H umboldtianum, Moscoviense, Mendel-Rydberg, Lorentz, Hertzsprung, Korolev, S chrodinger, Freundlich-Sharonov, Coulomb-Sarton, and Schiller-Zucchius, whi le two more, Deslandres and Dirichlet-Jackson, are very plausible. These re sults show that mare flow is not necessarily required to produce mascon-lik e characteristics. (C) 2001 Academic Press.