Water Vapor Contribution to Ceres' Exosphere From Observed Surface Ice and Postulated Ice-Exposing Impacts

Telescopic observations have detected an exosphere around Ceres, composed of either water vapor or its photolytic products. Proposed mechanisms for its formation include sublimation or sputtering from solar energetic particles of buried ice, surface ice, or an optically thin seasonal polar cap. We estimate the amount of water vapor produced by known exposures of water ice, detected in Dawn spacecraft image and spectral data and by ice exposures from subresolution impact craters. We use thermal and sublimation modeling to take into account slope, orientation, and, in the case of water ice within craters, shadowing due to crater walls. We use a Monte Carlo approach to calculate the number of ice-exposing impacts, where they occur on Ceres' surface, and how long the ice within the impact crater remains bright (e.g., less than one monolayer of sublimation lag). We find that the observed water ice patches on Ceres could account for 0.06kg/s of water vapor to (with Oxo crater as the main contributor) and that ice-exposing impacts that remain bright in appearance after one Ceres year supply 0.08-0.56kg/s of vapor, depending on the regolith volume fraction of the ice. While water ice has not been detected to date at Occator crater, if it were present we find that Occator is unlikely to be a major contributor of vapor. We find a typical background water vapor production rate from all of Ceres, combining surface and buried ice, of about a few tenths of a kilogram per second. Plain Language Summary Ceres, the closest dwarf planet to the Sun, may be periodically surrounded with water vapor. This water vapor could be coming from water ice, either exposed at the surface or buried, undergoing temperature changes during Ceres' day and year. Vapor could also be released from ice that is bombarded by solar energetic particles. In this paper, we calculate how much water vapor could be produced from ice currently exposed on the surface of Ceres due to changes in temperature. We calculate the water vapor produced from observed surface water ice patches identified in Dawn spacecraft mission data. We also calculate water vapor produced from likely water ice patches that are exposed to space after a new impact crater forms but are too small to be seen by the Dawn spacecraft's instruments. We find that from these sources plus water vapor produced from buried water ice tables (calculated in other papers), about a few tenths of a kilogram per second of water vapor is likely to be escaping Ceres at the present day.