The signature of condensed molecular oxygen has been reported in recent optical-reflectance measurements of the jovian moon Ganymede, and a tenuous oxygen atmosphere has been observed on Europa. The surfaces of these moons contain large amounts of water ice, and it is thought that O2 is formed by the sputtering of ice by energetic particles from the jovian magnetosphere. Understanding how O2 might be formed from low-temperature ice is crucial for theoretical and experimental simulations of the surfaces and atmospheres of icy bodies in the Solar System. Here we report laboratory measurements of the threshold energy, cross-section and temperature dependence of O2 production by electronic excitation of ice in vacuum, following electron-beam irradiation. Molecular oxygen is formed by direct excitation and dissociation of a stable precursor molecule, rather than (as has been previously thought) by diffusion and chemical recombination of precursor fragments. The large cross-section for O2 production suggests that electronic excitation plays an important part in the formation of O2 on Ganymede and Europa.

The yield and kinetic energy ͑KE͒ distributions of D Ϫ ions produced via dissociative electron attachment ͑DEA͒ resonances in nanoscale D 2 O ice films are collected as a function of film thickness. The 2 B 1 , 2 A 1 , and 2 B 2 DEA resonances shift to higher energies and their D Ϫ ion yields first increase and then decrease as the D 2 O films thicken. The D Ϫ KE distributions also shift to higher energy with increasing film thickness. We interpret the changes in the DEA yield and the D Ϫ KE distributions in terms of modifications in the electronic and geometric structure of the surface of the film as it thickens. A small amount of charge build-up occurs following prolonged electron beam exposure at certain energies, which primarily affects the D Ϫ KE distributions. Charge trapping measurements indicate that an enhancement in the trapping cross section occurs at energies near zero and between 6 and 10 eV.