The Putative Cerean Exosphere

Abstract

The ice-rich crust of dwarf planet 1 Ceres is the source of a tenuous water exosphere, and the behavior of this putative exosphere is investigated with model calculations. Outgassing water molecules seasonally condense around the winter pole in an optically thin layer. This seasonal cap reaches an estimated mass of at least 2 10 kg 3 , and the aphelion summer pole may even retain water throughout summer. If this reservoir is suddenly released by a solar energetic particle event, it would form a denser transient water exosphere. Our model calculations also explore species other than H 2 O. Light exospheric species escape rapidly from Ceres due to its low gravity, and hence their exospheres dissipate soon after their respective source has faded. For example, the theoretical turn-over time in a water exosphere is only 7 hr. A significant fraction of CO 2 and SO 2 molecules can get trapped and stored in perennially shadowed regions at the current spin axis orientation, but not at the higher spin axis tilt, leaving H 2 O as the only common volatile expected to accumulate in polar cold traps over long timescales. The D/H fractionation during migration to the cold traps is only about 10%.

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

“…Total vapor production from the nine exposed water ice spots on Ceres identified by Combe et al (). The residence time of water vapor around Ceres is ~7 hr (Schörghofer et al, ), close to the rotation period of Ceres. This will result in the diurnal variations (shown by the width of the curve above) being negligible when considering potential observations.…”

“…Water ice patches that have undergone sublimation lag buildup could be obscured from remote sensing instruments but still could be producing large amounts of vapor compared to their surroundings (e.g., Landis et al, ; Prettyman et al, ; Schörghofer et al, ). This would add additional vapor to the observed ice exposures and ice‐exposing impacts previously discussed.…”

“…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, ).…”

“…An optically thin water ice deposit could accumulate near the winter pole and its release via sputtering from an SEP event could match the exospheric observations. However, the number of times this could generate an exosphere in one Ceres year is limited by the recharge time of the seasonal deposit (Schörghofer et al, ). Determining what contribution each mechanism could make toward the current exosphere production rate is a key step in determining whether the ~6 kg/s production inferred by Küppers et al () is typical for Ceres or requires special circumstances.…”

“…In addition to calculating timing and water vapor mass produced, additional work has characterized how long the exosphere would be predicted to last around Ceres (Schörghofer et al, ), and found that the residence time can be as short as ~7 hr (less than one Ceres day). This implies that if successive observations of Ceres' exosphere occur separated by a day or more, like the observations in October 2013 reported by Küppers et al (), then there is an active source that has been continuously feeding the exosphere.…”