The Putative Cerean Exosphere

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 × 10³ kg, 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 H₂O. 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 CO₂ and SO₂ 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 H₂O 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%.