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

Abstract

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 ~0.06 kg/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 0.08–0.56 kg/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.

The varied sources of faculae-forming brines in Ceres’ Occator crater emplaced via hydrothermal brine effusion

“…While the composition of the melted material is different (water ice versus silicate rock), Occator’s lobate material is the Cerean equivalent of crater-fill impact melt rocks and melt-bearing breccias found in the floors of impact craters throughout the inner solar system 22 . The now-solidified lobate material is comparatively rich in water ice when compared with the surrounding terrain 23 , and is covered by a desiccated sublimation lag most likely 1-m thick 24 . Hydrated salts are more stable than water ice at the same depth, and would stay hydrated in the presence of water ice 25 , 26 .…”

Volatiles and Refractories in Surface-Bounded Exospheres in the Inner Solar System

“…However, the cases of non-detection of water-group species are just as numerous (Rousselot et al 2011(Rousselot et al , 2019Roth et al 2016;Roth 2018). Right now there is no clear explanation for the origin of Ceres' transient exosphere: sublimation rates from the known distribution of surface ice patches is two orders of magnitude lower than the water production rate derived from the observations (Landis et al 2019). Villarreal et al (2017) showed a correlation with solar energetic particle events, but ion sputtering is not effective enough (Küppers 2019).…”

Water Group Exospheres and Surface Interactions on the Moon, Mercury, and Ceres

“…Small grain sizes can act as a strong barrier to vapor diffusion, and quantitative estimates place the grain size at one micron to be consistent with the low rate of vapor diffusion (Prettyman et al 2017;). The GRaND measurements, combined with models, suggest that the equatorial regions are ice-free to at least 1 m depth whereas the polar regions may have ice even at cm depth (Schorghofer 2016;Prettyman et al 2017;Landis et al 2017Landis et al , 2019.…”