Europa `s Surface Sputtering by the Thermal Plasma of the Torus Considering the Electro-magnetic Interaction Between the Plasma and the Atmosphere

Europa's atmosphere is very tenuous and is mainly composed of O2. It is thought to be produced by ion bombardment of its icy surface. Several ion populations may contribute to this sputtering: 1) the thermal plasma of the torus ( 1keV including ram velocity), which may be partially diverted around the moon by the ionospheric currents. 2) the energetic sulfur and hydrogen ions ( 10 keV-MeV), which diffuse inward toward Europa's orbit. 3) and possibly the newly ionized O2 molecules that are picked up by the torus flow and hit the surface. The relative contribution of each sputtering ion population has been debated for more than three decades with estimated O2 sputtering rates varying by 2 order of magnitude. Modelers have historically focused on a single piece of the puzzle: plasma modelers assume a static atmosphere and tend not to check that their sources and losses are consistent with their prescribed atmosphere; while atmospheric modelers neglect the electro-dynamic interaction that diverts torus plasma around the moon, and limits the ion flux to the surface. In this work, we compute self-consistently the atmospheric production by the bombardment of the thermal plasma and pickup O2+ ions. We 1) Calculate the plasma flow around Europa with a MHD model 2) Use this flow in a multi-species physical chemistry model of the plasma-atmosphere interaction to compute the ion fluxes into Europa's surface. 3) Compute the production rate of O2 resulting from the ice sputtering by thermal and pickup ions. Finally, we compare the resulting atmospheric source rate to previously published results.