Conditions for Sublimating Water Ice to Supply Ceres' Exosphere

Observations of a water vapor exosphere around Ceres suggest that the dwarf planet may be episodically outgassing at a rate of similar to 6 kg s(-1) from unknown sources. With data from the Dawn mission as constraints, we use a coupled thermal and vapor diffusion model to explore three different configurations of water ice (global buried pore-filling ice, global buried excess ice, and local exposed surface ice) that could be present on Ceres. We conclude that a buried ice table cannot alone explain the vapor production rates previously measured, but newly exposed surface ice, given the right conditions, can exceed that vapor production rate. Sublimation lag deposits form that bury and darken this surface ice over a large range of timescales (from < 1 year to approximately hundreds of kyr) that depend on latitude and ice regolith content. Sublimating water vapor can loft regolith particles from the surface of exposed ice, possibly prolonging the visible lifespan of those areas. We find that this process is only effective for regolith grains smaller than approximately ones of microns. Plain Language Summary A thin water vapor atmosphere has been previously detected around Ceres, a dwarf planet in the asteroid belt, from space telescope observations. Based on data collected before the Dawn spacecraft's arrival, Ceres potentially contained a large mass of water ice. However, how that water, frozen within Ceres, could be fueling a tenuous water vapor atmosphere has been previously unknown. We explore the possibility of water vapor coming from either a buried ice table or buried surface ice at the same rate detected by previous observations. We use published data from the Dawn spacecraft mission to compare to our thermal and sublimation models. We conclude that given the right place and time, exposed surface ice can generate enough water vapor to replicate Ceres' tenuous atmosphere.