The possible source mechanisms of Ceres' exosphere

Ceres is the largest object (with a diameter of ~ 950 km) among the main-belt asteroids. The OH cloud, the photodissociated products of H2O, was suggested by the IUE (International Ultraviolet Explorer) observations to be present around Ceres (A'Hearn and Feldman, 1992). Recently, Kuppers et al. (2014) reported a direct detection of water vapor by Herschel that > 1026 molecules s-1 was produced from localized sources on Ceres' surface. Most excitingly, the Dawn images showed that a haze layer consisting of water-ice particles and dust was found above the Occator crater (Nathues et al., 2015). The haze was also shown to have a diurnal change of brightness, indicating a comet-like sublimation activity. In addition, subsurface outgassing (e.g., cryovolcanism and/or the plume activity similar to that found in Enceladus) is another possible source mechanism, which is not fully understood yet. The formation of Ceres' exosphere by sputtering on the surface ice associated with violent solar activities has also been suggested (Villarreal et al., 2017). Following the Ceres' exospheric model of Tu et al. (2014), here we focus on O2, O3 and H2O2 molecules, which are the primary products of radiolytic and photolytic decomposition of water ice, and possibly the CO2 and SO2 molecules produced from the impurities. We will compute the source rates from various production mechanisms and simulate the cloud morphologies. Then we will compare with the available data in the ground-based millimeter/submillimeter observations. Understanding the chemical composition of Ceres and its evolved exosphere, in analogy to comets and the icy satellites, would reveal the secret of the origin and evolution of the solar system.