The role of electron-stimulated production of O₂ from water ice in the radiation processing of outer solar system surfaces

We present measurements of the electron energy threshold, fluence, and temperature dependence for O₂ production during low-energy (5-100 eV) electron bombardment of thin (∼ 40 bilayer) amorphous and crystalline D₂O ice films deposited on a Pt(1 1 1) substrate in vacuum. The electron energy threshold (referenced to the vacuum level) for O₂ formation is approximately 10 ± 2 eV. This threshold is due to valence level electronic excitations or ionization of condensed water molecules. Comparison of the fluence dependence with kinetic models shows that the formation of O₂ is dominated by direct excitation and dissociation of a stable precursor molecule, and not by diffusion and recombination of oxygen atoms. The O₂ yield is also strongly dependent upon the temperature of ice, is different for crystalline and amorphous ice films, and is indicative of surface and bulk structural transitions. We demonstrate that electron-stimulated production of O₂ is important in radiation processing of low-temperature icy surfaces present on outer solar system bodies. This is particularly true in regions with strong magnetospheres such as the Saturnine and Jovian systems.