Constraints on an exosphere at Ceres from Hubble Space Telescope observations

We report far ultraviolet observations of Ceres obtained with the Cosmic Origin Spectrograph (COS) of the Hubble Space Telescope in the search for atomic emissions from an exosphere. The derived brightnesses at the oxygen lines at 1304 angstrom and 1356 angstrom are consistent with zero signals within the 1 sigma propagated statistical uncertainties. The OI 1304 angstrom brightness of 0.12 0.20 Rayleighs can be explained by solar resonant scattering from an atomic oxygen column density of (8.2 13.4) x 10(10)cm(-2). Assuming that O is produced by photodissociation of H2O, we derive an upper limit for H2O abundance and compare it to previous observations. Our upper limit is well above the expected O brightness for a tenuous sublimated H2O exosphere, but it suggests that H2O production with a rate higher than 4 x 10(26)moleculess(-1) was not present at the time of the COS observation. Additionally, we derive an extremely low geometric albedo of approximate to 1% in the 1300 angstrom to 1400 angstrom range.