A kinetic model of Ganymedeʹs atmosphere

A multispecies, 2-D axisymmetric, kinetic model that accounts for all kinetic regimes is applied to the neutral atmosphere of Ganymede. Using reasonable interpretations of the limited observations, it is found that Ganymede has a two-part atmosphere, with H2O being dominant between the subsolar point and a subsolar latitude of about 45 degrees, and O2 dominating elsewhere at the lower altitudes. H2 is dominant everywhere above a few hundred kilometers. Except for a small region near the subsolar point, the atmosphere is quasicollisional or collisionless. The resulting nonequilibrium is manifest in the atmospheric constituents having generally different temperatures and bulk velocities. Escape rates are similar to those of Europa, but a less visible torus is expected. The escape rates are also highly latitude and species dependent. The effect on the atmosphere by a fast ion plasma is investigated and found to primarily affect the O2 scale height above a few hundred kilometers. It is also found that Lyman α emission from collision of electrons with H2 may be significant near the surface.