O[TINF]2[/TINF]/O[TINF]3[/TINF] Microatmospheres in the Surface of Ganymede

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Abstract
Radiation-induced bubble formation, a process known to cause the deterioration of reactor materials, produces O2/O3 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 H2 and O2, and the vacancies aggregate to form voids. Whereas the H2 is lost readily, the O2 accumulates, permeating the regolith, producing a weak atmosphere, and becoming trapped in the voids, forming gas-filled bubbles. Such bubbles (microatmospheres) form efficiently above ~0.3-0.5 times the melting temperature of the material and can attain a high density of O2, consistent with the observation of "condensed" O2 on Ganymede. Dissociation of O2 in a microatmosphere by UV photons or the incident ions leads to formation of O3. Including the O(1D) interactions in the Chapman equations, a ratio O3/O2 ~ 2 × 10-4 is obtained, close to the estimate based on observations of Ganymede.