O2/O3 Microatmospheres in the Surface of Ganymede

Radiation-inducedbubbleformation,aprocessknowntocausethedeteriorationofreactormaterials,produces O2/O3microatmospheres 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 radicalsmigrateandreactefficiently,formingnewvolatilespeciesH2andO2,andthevacanciesaggregatetoform voids.WhereastheH2islostreadily,theO2accumulates,permeatingtheregolith,producingaweakatmosphere, and becoming trapped in the voids, forming gas-filled bubbles. Such bubbles (microatmospheres) form efficiently above10.3‐0.5timesthemeltingtemperatureofthematerialandcanattainahighdensityofO2,consistentwith the observation of “condensed” O2on Ganymede. Dissociation of O2in a microatmosphere by UV photons or the incident ions leads to formation of O3. Including the O( 1 D) interactions in the Chapman equations, a ratio O3/O212 310 24 is obtained, close to the estimate based on observations of Ganymede. Subject headings: planets and satellites: individual (Ganymede, Europa)—molecular processes— radiation mechanisms: nonthermal

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