Comparing the radiolytic oxidation of sulfur and chloride within ice on Europa and Mars

The surface of Europa is heavily irradiated by high-energy particles with the energy ranges of 1-100 MeV for primary electrons and -10 keV for secondary electrons. These irradiations may cause oxidation of surface materials, such as chlorides and sulfur -bearing species, through reactions with oxidative radicals formed by dissociation of H2O ice. Similar irradiation -induced oxidation could have occurred in near -surface ice on Mars. However, most of previous experiments simulated -10 keV electron irradiation onto ice materials on Europa and Mars, and few studies have investigated the oxidation of chlorides and sulfur by MeV electron irradiation. Here we performed laboratory experiments on MeV electron irradiation, as well as 10 keV electron and UV irradiations, onto mixtures of H2O ice and chlorides, or elemental sulfur, at low temperatures. Our experimental results indicate selective oxidation of sulfur against chlorides, with negligible formation of oxychlorides on irradiation of H2O ice and chloride mixtures, regardless of experimental conditions (temperature, electron energy, or presence of oxidants in ice). Sulfate is effectively formed by the electron irradiation. This selective oxidation may be caused by the different stabilities of atoms within irradiated H2O ice. If sulfate on Europa's surface is transported into the subsurface ocean, this could provide energy sources for possible chemoautotrophic life in the ocean. On Europa, sulfur would be one of key elements in redox chemistry on the surface and possibly in the subsurface ocean.