H2O-rich materials are exposed at the surface of Ceres as discovered from VIR spectra  of the Dawn mission . Oxo crater exhibits the most diagnostic absorption bands of the H2O molecule at 1.65 and 1.28 µm . These features exist in at least four other locations, and they are consistent with H2O ice mixed with low-albedo components [3,4]. Spectra of mineral hydrates such as salts are also characterized by H2O absorption overtones, however they do not fit VIR observations as well as H2O ice spectra. In order to further constrain the composition, the thermophysical and chemical stability of exposed H2O-rich compounds on Ceres and results from chemical models of Ceres interior are being investigated. One meter of pure H2O ice exposed to direct sunlight would sublimate within a few tens of years [5-7].The sublimation of a H2O ice-cemented regolith would leave a low-albedo lag deposit that would also decrease detectability over time . All the reported H2O exposures occur at latitudes higher than 30°N in fresh craters near rim shadows, have surface area < 3 km2, and relatively high albedo. The exposed H2O ice that is observed by VIR is likely due to a recent impact or a landslide. In some occurrences, high-albedo materials observed within these shadows by the Framing Camera (FC) are contiguous to the observed H2O; several of them could be in permanently shadowed regions. The surface shape model and history of illumination shall indicate whether these areas could be cold traps where H2O ice could be preserved from sublimation .
The funding for this research was provided under the NASA Dawn mission through a subcontract 2090-S-MB516 from the University of California, Los Angeles and the Dawn Guest Investigator Program. The VIR instrument and VIR team are funded by ASI (Italian Space Agency) and INAF (Istituto Nazionale di Astrofisica).
1. De Sanctis M.C. et al.,2011, SSR 163
2. Russell C. T. et al., 2011, SSR 163
3. Combe J-Ph. et al., 2016, LPSC #1820
4. Raponi A. et al., 2016, DPS-EPSC
5. Hayne P. O., Aharonson O., 2015, JGR 120
6. Formisano M. et al., 2016, MNRAS 455
7. Titus T., 2015, GRL 42
8. Hayne P. O., Aharonson O., 2016, LPSC #2736
9. Schorghofer et al., 2016, GRL accepted