Measurements of the elastic thickness under ancient lunar terrain

The part of a planet's gravity that is coherent with its topography provides information about the deflection of its crust after loading, and hence the resistance of its lithosphere to bending at the time the load was emplaced. We used observed line of sight accelerations from Lunar Prospector, together with the accelerations we would have expected if anomalies in the gravity field were only caused by topography, to estimate the admittance and coherence between topography and gravity over several regions of the lunar nearside. We then compared our estimates to what we would expect if the lithosphere behaved as a floating elastic plate or shell, assuming a linear relationship between topography and gravity. We found in the region surrounding the southern highland crater Clavius that the data can be modeled using a thin plate with Te=12±5 km and uncorrelated loads at the surface and base of the upper crust. A spherical shell model with surface loading is less satisfactory: to fit the admittance adequately requires topography with wavelengths over 400 km to be formed when Te≈ 1 km and the remainder when Te≈7.5 km. By contrast, the apparent lack of compensation around the youngest giant impact basins requires a plate with Te>80 km or a shell with Te>25 km. Our results indicate the thickness of the lunar lithosphere increased from ≈12 km in the pre-Nectarian to >25 km in the Nectarian.