The lunar mascons revisited

The entire spectra of the surface topography and gravity field of lunar mascons are well presented by the high-resolution Clementine data. The negative correlation between the topography and gravitational potential of the mascons suggests that they are dynamically supported. Both elastic support and viscous decay models are examined. For the elastic support models, the spectral characteristics and the lateral variations of the thickness of the elastic layer are determined on the basis of the thin spherical shell flexure formulation and using only the antivarying harmonics of the topography and potential. An elastic layer thickness of about 50 km is required to support Imbrium, Serenitatis, and Nectaris mascons, about 35 km for Crisium mascon, and about 30 km for Smythii and Humorum mascons. A layer of about 20 km thickness can support Orientale mascon. The strength envelopes of the upper 100 km of the Moon within 4-3 Gyr ago show that the elastic layer was not thick enough to support the mascons in the early history. They decayed through viscous deformationof the lunar interior. A lower limit of 6 x 10(24) Pa s is estimated for the lunar viscosity within 3.6-3 Gyr ago. We also determine the thicknesses of the crust and mare flows of the mascon basins on the basis of the assumption that the basins were isostatically compensated prior to the mascon formation. The crust beneath the mascon basins is about 30-40 km except for Crisium and Orientale, where the crust is about 20 km. The mare flows of about 3-6 km are obtained for almost all of the mascon basins. The topography and gravitational potential of Aitken basin shows that the basin is compensated at 52-54 km depth. The mantle beneath the basin has rebounded upward by about 20-30 km and is overlain by about 20-25 km of a mixture of the excavated crustal and mantle material. The lack of a mascon and pervasive mare flows in the basin brings into question the current models of the mascon formation.