Lunar gravity determinations and their implications

Abstract

The variations in speed of the orbiting Apollo spacecraft as observed from Earth-based radiometric data have provided a direct measure of the local gravitational field. The gravity data were used to infer mass distributions that relate to topography in varying degrees. The mascons exist as mass excesses in topographic lows in all the near-side ringed basins and are best represented as near surface disks with excess loads of 800 kg/cm2. Large 100 km size craters like Langrenus, Theophilus, and Copernicus have mass deficits that are consistent with the craters’ volumes. Both of these results imply a relatively rigid surface layer that allowed little isostatic adjustment over lunar time. However, the Apennine mountains, presumably formed at the time of the Imbrium impact event, reveal only a small gravitational anomaly compared to their topographic size. This suggests that at this era the Moon was more plastic and isostatically compensated. By using the orbital element history of the subsatellites, the first realistic far-side field has been determined. The far-side ringed basins are mass deficits consistent with the lack of maria filling. The 2 km centre-ofgravity offset from the geometric centre implies a thicker far-side crust that possibly prevented far-side maria flooding. The homogeneity parameter (C/MR2) is near that of a homogeneous sphere having possibly a small core with a slight density increase towards its centre.