Role of natron in delaying retreat of buried ice tables on Ceres

Ceres is the nearest ocean world to the Sun after Earth, and evidence of its extensive hydration is reflected in the wealth of data from the Dawn mission. The role that hydrated salts have played in the evolution of Ceres’ near-surface hydrogen budget has not been well-understood. Examples of hydrated and dehydrated salts occur in many locations across Ceres, notably at Occator crater (∼90 km, ∼20° N) where additional hydrogen enhancement was detected during the final phase of the Dawn mission. The presence of hydrated salts in the regolith is expected to slow the retreat of the ice table. To model the effect of hydrated salts entrained in a retreating ice table, as may be the case for Ceres, we modify well-established models in which saturation vapor pressure and Knudsen diffusion play major governing roles in surface vapor mass flux. We use this modified model, with a variety of temperatures and salt contents, to constrain the delay in ice retreat time to decimeter depths, as would be relevant for nuclear spectroscopy instruments. We find that Ceres’ equatorial annual average temperatures show delays in ice table retreat by a few to tens of million years, with variations due to salt content. Based on Occator’s inferred age, hydrated salts are not likely to be primarily responsible for hydrogen detected at that crater location compared to buried water ice. Ceres’ current equatorial thermal regime is where the effect of salt dehydration matters the most in ice table retreat.