Investigating Ceres' Interior Structure by Simulating Relaxation of Large Basin

Ceres’ interior structure has always been a puzzle. The gravity and shape data returned by the Dawn spacecraft show that Ceres is in hydrostatic equilibrium with a mean normalized moment of inertia of 0.37, which suggests that Ceres is at least partially differentiated [Park et al., 2016]. This gravity data was interpreted as a two-layer interior structure, with a rocky core overlaid by a volatile-rich shell. Kerwan is the largest recognized basin on Ceres, and it is also the most likely crater to have experienced topographic relaxation given its size and location (close to equator). I simulated the viscoelastic relaxation of Ceres' large basins by using finite element method, the peculiar morphology of Kerwan (and by extension Yalode) can be achieved by relaxation in a thin, dense, and high-viscosity crust layer ~25 km thick, over a lower density, softer mantle ~75 km thick. The predominance of deep craters on Ceres tells us its near-surface is not composed of ice-dominated material. Therefore, I propose a three-layer structure with a dense core, surrounded by a high viscosity icy-dirt crust on top of a relatively less dense dirty-ice mantle. It can preserve not only topography of smaller impact craters but also produce the peculiar morphology of the largest craters. Furthermore, this structure sets conditions for mantle overturn, which may drive late stage cryovolcanic activity on Ceres.