The water regime of asteroid (1) Ceres

Abstract

An analytical model is presented which describes the water regime of asteroid Ceres. The model predicts surface and subsurface temperatures, water fluxes, and ice depths as well as integrated water supply rates and residual atmospheric/surface water as a function of time, lattitude, and assumed regolith properties. We find that ice could have survived for 4.5 byr at depths of only 10–100 m near the equator and less than 1.0 to 10 m at latitudes greater than 40°. The current global water supply rate is expected to be between 30 and 300 g sec −1, which corresponds to a near surface number density of ∼1 × 10 4molecules cm −3. At least one current interpretation of the near-infrared reflectance of Ceres requires coverage by a very thin layer of ice down to 48° lat. Our model suggests that ice with the overall albedo of Ceres (0.09) could exist in transient steady state with the above supply rate only at latitudes greater than 80°. Otherwise the water loss rate from Ceres would exceed the supply rate by orders of magnitude. Stirring of the regolith substantially alters the distribution of ice only at latitudes greater than 70°. At lower latitudes ice depletion is faster than impact rehomogenization at all relevant depths and time scales. Possible contributors to the spectral feature at 3.1 μm other than free surface ice, such as interlayer ice within the optical surface, are considered.