Impacts could have served to keep the crust of Ceres replenished with water and other volatiles, helping to counteract their sublimation by the heat of the Sun. Credit:  NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/Justin Cowart.

The crust of dwarf planet Ceres is primarily composed of water ice mixed with hydrated silicate minerals. The water ice in the outermost layer is susceptible to sublimation due to heating by the Sun, which should deplete the ice on the surface of Ceres, especially near the equator where the sunlight is most direct. However, surface ice has been detected in and around mid-latitude craters. Geological evidence such as flow features and pitted terrain suggests that water is abundant in the near-surface, requiring a mechanism to recharge water from Ceres’ interior to its outer crust.

An international team led by Thomas Prettyman of the Planetary Science Institute used the Dawn spacecraft’s Gamma Ray and Neutron Detector (GRaND) instrument to investigate this issue during the mission’s final phase in 2017. The spacecraft was put into a low-altitude, eccentric orbit that allowed data collection with ten times the spatial resolution of previous mission phases. The team mapped the distribution of hydrogen in the near-surface regolith around Occator Crater, a young (less than 20 million years old) complex crater found in Ceres’ mid-latitudes that shows geologic features consistent with near-surface water ice.

The team found that Occator’s interior and ejecta blanket were enriched in hydrogen relative to other regions at the same latitude. This is consistent with subsurface ice having been excavated and distributed by the impact that formed Occator. This process would preferentially enrich and replenish surface ice and volatiles in the regolith of Ceres near impact craters. This work serves as the first direct evidence that Ceres’ interior is water-rich and that such water ice can survive and be distributed by impact processes. It will allow researchers to re-interpret previous hypotheses of Ceres’ geology and surface age that were made under the assumption of a dryer equatorial regolith. READ MORE