HIDDEN ICE ON DWARF PLANET CERES: RESULTS FROM THE DAWN MISSION

Ceres, the only dwarf planet in the asteroid belt, is orbited by the Dawn spacecraft since March 2015. The cerean surface is almost entirely devoid of macroscopic ice deposits, but measurements by the Gamma Ray and Neutron Detector (GRaND) on Dawn reveal extensive latitude-dependent H-enrichment in the shallow subsurface. These measurements are consistent with a gradual and temperature-driven loss of water ice over solar system history, with ice remaining extremely close to the surface in the cold polar regions. The measurements point to the following: a) Desiccation proceeded exceptionally slow, suggesting a dust mantle of remarkably small grain size, and b) Impact de-volatization was insignificant over the age of the solar system. Ceres also has perennially shadowed craters near its rotational poles. Dawn has mapped hundreds of these persistently shadowed regions (PSRs) in the northern hemisphere and their cumulative area is about 2,000 km². Bright deposits are visible in a small fraction of them due to scattered sunlight, whereas most PSRs are void of them. The location of bright deposits may coincide with the minimum extent of PSRs over axial tilt history, consistent with the notion that the bright deposits consist of a volatile. One sunlit bright deposit in the polar region is spectroscopically identified as H2O ice, which suggests a recent origin. Overall, Dawn has found evidence for water ice at all places where ice could have survived over geological long time periods (and a few more).