High‐Resolution Gravity Field Models from GRAIL Data and Implications for Models of the Density Structure of the Moon's Crust

Abstract We present our latest high‐resolution lunar gravity field model of degree and order 1200 in spherical harmonics using Gravity Recovery and Interior Laboratory (GRAIL) data. In addition to a model with the standard spectral Kaula regularization constraint, we determine models by applying a constraint based on topography called rank‐minus‐one (RM1). The new models using this RM1 constraint have high correlations with topography over the entire degree range by design. The RM1 models allow the determination of apparent crustal densities at all spatial scales (called effective density) covered by the model, whereas the Kaula‐constrained model can only be used globally up to spherical harmonic degree 700. We find that the effective density spectrum has a smaller slope for the high degrees when compared to the medium degrees. We interpret this as indicative of a global average surface density, as opposed to an ever‐decreasing effective density as one approaches the surface. We use the RM1 models to derive maps of lateral and vertical density variations in the lunar crust. These models allow us to increase the resolution of this analysis compared to previous studies, by increasing the degree range over which to fit theoretical models of vertical density variations, and by decreasing the size of the spherical caps used in a localized analysis. Several regions on the Moon, such as South Pole‐Aitken and Mare Orientale, are distinct from their surroundings in terms of surface densities. The RM1 models are especially valuable in (localized) spectral studies of the structure of the lunar crust.