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C. Alexander (Caltech / JPL), A. Lee, Y. Yung (Caltech), B. Butler (NRAO), K. Hibbits (Univeristy of Hawaii), C. Paranicas (Johns Hopkins University / APL)
The Ganymede exosphere may be one populated by hot hydrogen liberated via ion sputtering of the surface, cold H2O liberated via sublimation, and other cold molecules formed within the surface matrix itself, liberated by a slow diffusion process. Any O2 produced would not be recondensed on the surface but remain suspended, forming a cold (and heretofore undetected) background gas. A model has been developed to examine the sources and sinks of the exosphere, and the interaction of the hot and cold components. The model accounts for sublimation, sputtering, and molecule migration. The magnitude of both sublimation and sputtering, as described in earlier work, is calculated as a function of latitude, longitude, and local time for H2O, O2, H, and H2. A statistical study of the trajectories and energies of molecules released from the surface has been conducted with the migration model, in which molecules migrate across the surface until they encounter a cold trap or are removed in a simple photodissocation process. In the presence of a background gas, the suspension of molecules in the air via collisions is modeled. Most released volatiles do not appear to be sufficiently energetic for large lateral excursions in the atmosphere, however a small population of molecules does stay suspended for an entire Ganymede day, and travel large distances laterally. In the absence of other considerations, a positive feedback mechanism may exist between increases in atmospheric density due to photodissociation and increasingly long suspension times in the atmosphere, resulting in increasing background concentrations of O2. The presence of such a background gas has implications for a chemical cycle between the surface and atmosphere of Ganymede involving oxygen.