Production of O2 on icy satellites by electronic excitation of low-temperature water ice

Abstract

The signature of condensed molecular oxygen has been reported in recent optical-reflectance measurements of the jovian moon Ganymede¹, and a tenuous oxygen atmosphere has been observed on Europa². The surfaces of these moons contain large amounts of water ice, and it is thought that O₂ is formed by the sputtering ofice by energetic particles from the jovian magnetosphere^3,4,5,6,7,8. 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.