The Compensation State of Intermediate Size Lunar Craters

The compensation state of 49 intermediate size (120 to 600 km diameter) lunar craters are investigated using the most recent spherical harmonic models of the lunar topography and gravity, truncated at degree n =110. The total mass anomalies per unit area (i.e., the lateral variations of the vertically integrated density perturbations per unit area) within an otherwise uniform crust of 60 km thickness are determined such that, together with the surface topography, give rise to the model gravity anomalies. Crustal thicknesses 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 required for the isostatic compensation of the surface topography. The excess mass anomaly of a crater denotes its particular state of compensation. Dependencies of the excess mass anomalies on crater location, size, and age are investigated, but in general few discernable trends are evident. Although the vast majority of craters indicate some compensation, no correlation exists between age or size and the state of compensation. Roughly 16% of the craters show no compensation, and in some cases have mass deficiencies most likely 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 of impact to rigidly support the stress caused by crater excavation. These features are seen throughout different geological periods, demonstrating that the lunar crust cooled quickly and strengthened soon after formation. A comparison of the compensation state of craters Apollo, Korolev, and Hertzsprung 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 compensation of intermediate size craters. The characteristics of the excess mass a nomaly profiles of the eight well-known near side mascon basins are used to identify new mascon-like craters. Ten newly found mascons are confirmed: Humboldtianum, Moscoviense, Mendel–Rydberg, Lorentz, Hertzsprung, Korolev, Schrodinger, Freundlich–Sharonov, Coulomb-Sarton, and Schiller–Zucchius, while two more, Deslandres and Dirichlet–Jackson, are very plausible. These results show that mare flow is not necessarily required to produce mascon-like characteristics.