Large Impact Basins on Ceres: Probing Beneath the Icy Mantle

Abstract:

Interior evolution models of Ceres suggest that the dwarf planet likely differentiated due radiogenic heating in its early history. These models suggest that Ceres now consists of a rocky silicate core overlain by a H₂O ice mantle between 50 and 125 kilometers in thickness (McCord and Sotin, 2005, JGR, 100, EO5009). At depth, this water layer may exist in the form of a subsurface liquid ocean. While upcoming gravity measurements made by NASA’s Dawn spacecraft will be extremely useful in determining the depth and state of Ceres’ ice mantle, large impact basins on Ceres provide an independent means by which to estimate the thickness of Ceres’ icy shell. The crater size frequency distribution observed on Vesta implies that approximately 8 bodies larger than 40 km diameter have struck Ceres over the past 4.5 billion years, with the largest expected impactor being ~70 km in diameter (Rivkin and Asphaug, 2013, Proc. LPSC XLV, 1649). Numerical simulations suggest that the transient craters formed by such large collisions are deep enough to probe below Ceres’ icy mantle and deform the silicate core. Because of material differences between H₂O and silicate, the presence of the core suppresses the downward flow of material during the crater excavation process. This results in shallower, wider transient cavities, and should cause an increase in the crater size frequency distribution at large sizes. This widening effect may be enhanced by the presence of a strength-less liquid H₂O ocean above Ceres’ core mantle boundary. While large basins in H₂O ice will likely relax viscously over relatively short timescales, Ceres’ largest impactors produce more permanent depressions at the crust mantle boundary, which may be detected as negative Bouguer anomalies in the Dawn recovered gravity field.