Conditions for Sublimating Water Ice to Supply Ceres' Exosphere

Abstract

Observations of a water vapor exosphere around Ceres suggest that the dwarf planet may be episodically outgassing at a rate of ~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.

The Putative Cerean Exosphere

“…It could be caused by mass wasting events or impacts, but such events are rare. Ice patches exposed on the sunlit surface (Combe et al 2016) are few and small, and produce little vapor (Landis et al 2017). Ice deposits in perennially shadowed craters are also small and few in number (Ermakov et al 2017;Platz et al 2017), and moreover the Herschel observations suggest that the vapor did not originate from the polar regions (Küppers 2017).…”

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

“…The lack of a sublimation lag (and diffusive barrier) results in more sublimation than for buried ice (Schörghofer, ). Previous work shows that, given enough surface area and the right time of year, exposed surface ice can result in vapor production rates approaching those reported by Küppers et al () of ~6 kg/s (Landis et al, ). However, the current exposures appear on the poleward facing slopes within craters.…”

“…Several explanations have been put forward to explain the water vapor production inferred: sublimation from subsurface ice tables (Fanale & Salvail, ; Hayne & Aharonson, ; Landis et al, ; Prettyman et al, ; Schörghofer, ; Titus, ), sublimation from transient surface exposures of water ice (Landis et al, ), sputtering by solar energetic particle events (SEPs) from surface ice (Villarreal et al, ), or a seasonal, optically thin water ice polar deposit (Schörghofer et al, ). Sublimation from ground ice is estimated to produce between 0.003–0.37 kg/s of water vapor globally, depending on the parameters used for Ceres regolith (Fanale & Salvail, ; Landis et al, ; Prettyman et al, ; Schörghofer, ). Sublimation from transient exposures of water ice have to be either particularly large (several km 2 ) or near the equator to match the ~6 kg/s level (Landis et al, ), but the likelihood of geologic processes resulting in large exposed water ice patches in the recent past has not been examined.…”

“…Sublimation from ground ice is estimated to produce between 0.003–0.37 kg/s of water vapor globally, depending on the parameters used for Ceres regolith (Fanale & Salvail, ; Landis et al, ; Prettyman et al, ; Schörghofer, ). Sublimation from transient exposures of water ice have to be either particularly large (several km 2 ) or near the equator to match the ~6 kg/s level (Landis et al, ), but the likelihood of geologic processes resulting in large exposed water ice patches in the recent past has not been examined. Sputtering from SEPs matches the timing of many of the telescopic observations (Villarreal et al, ), but Ceres' surface is not dominated by water ice, unlike icy satellites where radiation sputtering of H 2 O can liberate significant water vapor (e.g., for Ganymede; Bahr et al, ).…”

“…With six of the nine reported water ice exposures on Ceres related to impacts (Combe et al, ), understanding how impacts generate additional areas of exposed surface ice is key to evaluating further how sublimation (Landis et al, ) and sputtering (Villarreal et al, ) contribute to the exosphere. Both processes require a certain amount of exposed surface ice to produce vapor.…”