CONTRIBUTION OF SUBLIMATION FROM BURIED AND SURFACE WATER ICE TO CERES’ EXOSPHERE (Invited Presentation)

Dwarf planet Ceres, the largest object in the main asteroid belt, was thought to contain a significant amount of water ice and an exosphere [e.g., 1,2] even before the arrival of the Dawn mission in 2015. Telescopic observations suggested a H₂O vapor exosphere was being produced episodically at a rate of ~10²⁶ molecule/s (or ~ 6 kg /s) [2]. We used thermal and vapor diffusion modeling alongside observations from Dawn Visible and Infrared mapping spectrometer (VIR), Framing Camera (FC), and Gamma Ray and Neutron Detector (GRaND) to determine the contribution sublimating water ice has to Ceres’ exosphere.

We first determined whether sublimation from a global buried H₂O ice table could explain the observed vapor flux. We found that a buried H₂O ice table could not explain the full ~6 kg/s vapor production and simultaneously satisfy the latitudinal hydrogen variation observed by GRaND [3,4].

Other sources were considered for Ceres’ exospheric H₂O. We modeled the loss of vapor from nine known exposed H₂O ice patches [5,6] and found that their total maximum vapor production was ~0.06 kg/s. We considered local, shallow ice at the ~92 km diameter Occator impact crater [e.g., 7]. Expected H₂O ice concentrations [6] resulted in a maximum of ~0.01 kg/s of vapor. New, <100 m diameter, impact craters could also expose H₂O ice patches, with vapor production widely varying up to ~0.5 kg/s [6].

The contributions from each current source of surface and near-surface H₂O ice on Ceres are individually small, but together they could deliver several tenths of a kg/s of H₂O vapor to Ceres’ exosphere. While other processes like solar energetic particle sputtering events [e.g., 8] could also contribute to the exosphere, sublimation from buried and surface water ice plays an observationally-constrained and consistent role in Ceres’ exosphere. Previous inferred vapor production rates [e.g., 2] should be revisited in light of Dawn-based results.

[1] Castillo-Rogez & McCord, 2010, Icarus 205

[2] Kuppers et al., 2014, Nature 505

[3] Prettyman et al., 2017, Science 355

[4] Landis et al., 2017, JGR: Planets, 122

[5] Combe et al., 2019, Icarus, 319

[6] Landis et al., 2019, JGR: Planets,124

[7] Scully et al., 2019, Icarus, 320

[8] Villarreal et al. 2017, AJ, 838