Near-surface oxygen atmosphere at Europa

The bombardment of the surface of Europa by charged particles trapped in the jovian magnetosphere causes sputtering and decomposition of surface materials leading to the production of a tenuous O-2 atmosphere. In this paper we use a direct Monte Carlo simulation of the atmosphere and we account for adsorption, thermalization and re-emission of condensed O-2, dissociation and ionization by magnetospheric electron and photon impact, and collisional ejection by the low energy plasma in the magnetosphere. Since the ion flux to the surface and the O-2 production are not well constrained, we treat the surface source as a variable parameter. To account for the production of atomic O seen by HST (Hall et al., 1995, 1998), we find that larger surface fluxes are required than those predicted (Cooper et al., 2001). This is due primarily to the inclusion of photo-dissociation which leads to loss. The variation of atmospheric O-2 and O from near equilibrium near the surface to highly non-equilibrium is shown and is poorly approximated by the escape atmosphere assumed by Saur et al. (1998). (C) 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved.