PRODUCTION OF O-2 ON ICY SATELLITES BY ELECTRONIC EXCITATION OF LOW-TEMPERATURE WATER ICE

The signature of condensed molecular oxygen has been reported in recen t optical-reflectance measurements of the jovian moon Ganymede(1), and a tenuous oxygen atmosphere has been observed on Europa(2). The surfa ces of these moons contain large amounts of water ice, and it is thoug ht that O-2 is formed by the sputtering of ice by energetic particles from the jovian magnetosphere(3-8). Understanding how O-2 might be for med from low-temperature ice is crucial for theoretical and experiment al simulations of the surfaces and atmospheres of icy bodies in the So lar System. Here we report laboratory measurements of the threshold en ergy, cross-section and temperature dependence of O-2 production by el ectronic excitation of ice in vacuum, following electron-beam irradiat ion. Molecular oxygen is formed by direct excitation and dissociation of a stable precursor molecule, rather than (as has been previously th ought) by diffusion and chemical recombination of precursor fragments. The large cross-section for O-2 production suggests that electronic e xcitation plays an important part in the formation of O-2 on Ganymede and Europa.