The moon before mare

The crust of the Moon experienced a unique geodynamic evolution, beginning with its crystallization from a magma ocean, continuing through a period of heavy impact bombardment, and followed by extensive basaltic mare volcanism. All these events have left crucial records imprinted in the form of topographic features and gravity anomalies. Here, we invert gravity and topography data using a two-layer thin-shell loading model under the premise of pre-mare isostasy to investigate the global structure of the crust and solve for feldspathic crust and mare thickness, together with mare-induced flexure. The tectonic record and partially buried crater population are used to constrain the bulk of mare volcanism to have been emplaced on a 40 km elastic lithosphere, although mare within large impact basins may have formed on a thinner elastic lithosphere. The mare thickness and associated flexure are removed to calculate a map of the surface and crust of the Moon before mare volcanism. The pre-mare surface in the Oceanus Procellarum region is found to be -2 km lower than the surrounding nearside, and several possible explanations, including a giant impact, pore space annealing, isostatic adjustment, and crustal erosion induced by a mantle plume or thermal anomaly, are discussed. The pre-mare elevation map further sheds light on the ring structure of Imbrium, which is seen to resemble that of Orientale. Imbrium's outermost ring is observed to be at a larger radial distance to the northeast relative to the south, indicating that some level of lithospheric variability affected ring formation at the time of impact. The western part of Imbrium's ring within Oceanus Procellarum is not found in the pre-mare topography, implying that it either never formed or that some processes erased its signature from gravity and topography. The feldspathic, pre-mare, crust is found to be -7 km thinner within large nearside basins than in models not accounting for the high-density mare. The pre-fill floor of these basins was -6 km deeper than currently observed, and together with their updated crustal structure, these new insights have implications for impact simulations that try to reproduce the crustal structure of nearside mare basins.