Surface-bounded oxygen atmosphere of Europa

The very tenuous O_2 atmosphere of Europa is a surface-bounded atmosphere (Johnson, 2002). It is produced by the radiolysis of Europa's surface due to exposure to solar ultraviolet radiation and energetic magnetospheric plasma. Earlier we developed a collisional Monte Carlo model of Europa's atmosphere (Shematovich and Johnson, 2001) accounting for adsorption, thermalization and re-emission of condensed O_2, a stable decomposition product of H_2O radiolysis. In this report we discuss a modified collisional Monte Carlo model of Europa's atmosphere in which the sublimation and sputtering sources of H_2O molecules and their molecular fragments are accounted for. Dissociation and ionization of H_2O and O_2 by magnetospheric electron and solar UV-photon impact, and collisional ejection from the atmosphere by the low energy plasma were taken into account. The surface-bounded oxygen atmosphere of Europa origins from a balance between radiolysis of the satellite icy surface by the solar UV radiation and high-energy magnetospheric plasma and the collisional ejection from the near-surface atmosphere by the low-energy plasma. Calculations showed that atmospheric chemical composition is determined by both the water and oxygen photochemistry in the near-surface atmospheric region, escape of suprathermal oxygen and hydroxyl into the Jovian system, and the adsorption- desorption exchange by radiolytic water products with the satellite surface. It was found that to account for the production of oxygen emission observed by HST (Hall et al., 1998) larger surface fluxes of O_2 are required than those assumed in earlier work from measured fluxes of magnetospheric particles (Cooper et al., 2001). The effective yield of O_2 is enhanced because the radiolysis is occurring in a regolith and not on a laboratory surface (Johnson et al., 2003). The model will eventually be expanded to include the effect of the surface catalytic chemistry and the release of trace amounts of SO_2 and CO_2 that are trapped in the surface ice (Johnson et al., 2003). This work is supported by NASA Planetary Atmospheres Program. Cooper et al., 2001, Icarus, v.149, 133. Hall et al., 1998, Astrophys. J., v.499, 475. Johnson, R.E., 2002, in Atmospheres in the Solar System: Comparative Aeronomy, Geophys. Monograph AGU, 203. Johnson et al., 2003, In Jupiter: Satellites, Atmosphere, Magnetosphere. Ed. by F. Bagenal, Univ. of Arizona Press, 2003. Shematovich and Johnson, 2001, Adv. Space Res., v.27, 1881.