Polar Ice Caps on Ceres: Predictions for Dawn

Observations of Mercury and Earth's Moon support the presence of substantial deposits of water ice and other volatile species in polar regions persistently shadowed by topography. This motivates investigation into whether similar thermal regimes exist on the dwarf planet Ceres. We investigated the effects of topographic shadowing using models for the temperatures inside craters at different latitudes, placing constraints on the extent and total surface areas that may be occupied by water ice and other volatile species. The model calculates heat diffusion subject to the effects of Ceres’ obliquity and orbital eccentricity, as well as multiple scattering of solar and infrared radiation between surface elements. Our results indicate that peak surface temperatures in perennially shadowed parts of craters are always below the 110 K cutoff for water ice to be stable on billion-year time scales. For an obliquity of 3° and depth-to-diameter ratio of 1/6 typical of simple craters, perennial shadow occurs at latitudes > 53°. We therefore expect that Ceres could harbor substantial ice deposits on shadowed crater slopes down to this critical latitude, provided non-thermal losses are slow compared to the influx of molecules to the cold traps. Furthermore, a hemispherical asymmetry exists due to the timing of Ceres’ perihelion passage, which would lead to ice buildup in the southern hemisphere if the orbital elements vary slowly relative to the accumulation rate. Our model results are potentially testable by the Dawn mission during its expected 2015 encounter with Ceres.