Thermal Stability of Ice on Ceres with Rough Topography

The dwarf planet Ceres may have an ice-rich crust, and subsurface ice exposed by impacts or endogenic activity would be subject to sublimation. We model surface and subsurface temperatures on Ceres to assess lifetimes of water ice and other volatiles. Topographic shadowing allows a small, but non-negligible fraction (∼0.4%) of Ceres’ surface to be perennially below the ∼110 K criterion for 1 Gyr of stability. These areas are found above 60∘ latitude. Other molecules (CH3OH, NH3, SO2, CO2) may be cold-trapped in smaller abundances. A model for the transport, gravitational escape and photoionization of H2O molecules suggests net accumulation in the cold traps. Buried ice is stable within a meter for > 1 Gyr at latitudes higher than ∼50∘. An illuminated polar cap of water ice would be stable within a few degrees of the poles only if it maintained a high albedo (>0.5) at present obliquity. If the obliquity exceeded 5∘ in the geologically recent past, then a putative polar cap would have been erased. At latitudes 0∘–30∘, ice is stable under solar illumination only briefly (∼10–100 yr), unless it has high albedo and thermal inertia, in which case lifetimes of > 104 yr are possible. Finally, a small hemispheric asymmetry exists due to the timing of Ceres’ perihelion passage, which would lead to a detectable enhancement of ice in the northern hemisphere if the orbital elements vary slowly relative to the ice accumulation rate. Our model results are potentially testable during the Dawn science mission.