O-2/O-3 microatmospheres in the surface of Ganymede

Radiation-induced bubble formation, a process known to cause the deterioration of reactor materials, produces O-2/O-3 microatmospheres in the icy surface of Ganymede. Energetic ions in Jupiter's magnetosphere bombard this surface, producing vacancies and radicals in the ice. At the equatorial temperatures on Ganymede, the radicals migrate and react efficiently, forming new volatile species H-2 and O-2, and the vacancies aggregate to form voids. Whereas the H-2 is lost readily, the O-2 accumulates, permeating the regolith, producing a weak atmosphere, and becoming trapped in the voids, forming gas-filled bubbles. Such bubbles (microatmospheres) form efficiently above similar to 0.3-0.5 times the melting temperature of the material and can attain a high density of O-2, consistent with the observation of ''condensed'' O-2 on Ganymede. Dissociation of O-2 in a microatmosphere by UV photons or the incident ions leads to formation of O-3. Including the O(D-1) interactions in the Chapman equations, a ratio O-3/O-2 similar to 2 x 10(-4) is obtained, close to the estimate based on observations of Ganymede.