THE LUNAR MASCONS REVISITED

The entire spectra of the surface topography and gravity field of luna r mascons are well presented by the high-resolution Clementine data. T he negative correlation between the topography and gravitational poten tial of the mascons suggests that they are dynamically supported. Both elastic support and viscous decay models are examined. For the elasti c support models, the spectral characteristics and the lateral variati ons of the thickness of the elastic layer are determined on the basis of the thin spherical shell flexure formulation and using only the ant ivarying harmonics of the topography and potential. An elastic layer t hickness of about 50 km is required to support Imbrium, Serenitatis, a nd Nectaris mascons, about 35 km for Crisium mascon, and about 30 km f or Smythii and Humorum mascons. A layer of about 20 km thickness can s upport 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 thro ugh 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 mas con basins on the basis of the assumption that the basins were isostat ically compensated prior to the mascon formation. The crust beneath th e mascon basins is about 30-40 km except for Crisium and Orientale, wh ere the crust is about 20 km. The mare flows of about 3-6 km are obtai ned for almost all of the mascon basins. The topography and gravitatio nal potential of Aitken basin shows that the basin is compensated at 5 2-54 km depth. The mantle beneath the basin has rebounded upward by ab out 20-30 km and is overlain by about 20-25 km of a mixture of the exc avated crustal and mantle material. The lack of a mascon and pervasive mare flows in the basin brings into question the current models of th e mascon formation.