The origin of mascons on Ceres as constrained by crater morphology

Abstract

Positive, superisostatic gravity anomalies (also known as mass concentrations or “mascons”) are found beneath some large impact basins on Ceres, including Kerwan and Yalode. Such anomalies may be due to high-density mantle material that has been uplifted towards the surface, a process comparable to that on rocky bodies. Alternatively, the anomalies may be caused by impact energy devolatilizing the icy crust beneath the impact site, leaving a denser, ice-poor portion of material below the surface. The two hypotheses would result in different simple-to-complex transition diameters of craters superposing the larger basins. We analyzed crater morphology in order to deduce transition diameters of superposed craters in the Kerwan and Yalode basins. The surveyed locations produced transition diameters between 7 and 14 km. This transition diameter is consistent with that of average cerean crust composed of a mixture of both rock and ice, and not of a purely rocky crust which would produce a transition diameter of about 30 km. Our results have two possible interpretations. First, positive gravity anomalies beneath the basins are due to mantle uplift, rather than devolatilization of local crust. Second, impact-induced devolatilization is an incomplete process, removing enough ice to substantially increase crustal density but leaving enough ice to cause later impact processes to be ice-controlled. We favor the former explanation as the simpler interpretation, which would imply that mascons formed by impact-induced mantle uplift is a common process in the Solar System, and not just confined to rocky planets.