Ganymede’s tenuous atmosphere

Ganymede possesses a tenuous water-based atmosphere, thought to be produced by the solar and Jovian plasma irradiation of its icy surface. Observational evidence for Ganymede's atmosphere is provided by far-ultraviolet (FUV) emission from the atomic oxygen (O) and hydrogen (H) constituents. The relative strengths of the two observed FUV atomic oxygen emission multiplets, which are produced by electron-impact on the atmosphere, allow inference of molecular O2 and H2O components and their relative abundances. Minor species besides O and H have not been detected and the observational constraints can be considered loose overall, partly due to the inherent ambiguity and difficulty in relating the electron-excited emissions to atmospheric properties. Much of our understanding of the configuration of Ganymede’s atmosphere is based on modelling efforts. The modelling studies suggest that O2 is produced primarily through radiolysis and sputtering by the plasma precipitation in the icy polar regions. Since it does not condense, O2 is yet abundant everywhere and likely the dominant species over most of the surface. Ice sublimation increases the abundance of H2O near the sub-solar point, locally exceeding the O2 abundance. The high altitudes, in contrast, are expected to be primarily populated by the lighter H2 molecules everywhere. Inferred column densities for the primary molecules, O2 and H2O, are on the order of 1014 cm-2 to 1015 cm-2. In addition, models suggest that the atmosphere is collisional only up to a few tens of kilometers above the surface, particular in the dense sub-solar region.QC 20220125