A Model for the Ganymede Atmosphere

C. Alexander (JPL), J. Spencer (Lowell Observatory), R. Carlson, W. Smythe (JPL), L. Frank, B. Paterson (University of Iowa), S.J. Bolton (JPL)

The presence of an atmosphere around the icy Jovian satellite Ganymede suggests the efficacy of mechanisms to provide a neutral source from sputtering and from sublimation. The model presented here is a preliminary look at processes within the atmosphere near the surface, including feedback with the surface. The magnitude of the neutral atmosphere source from both sublimation and sputtering is calculated as a function of latitude and local hour angle, and these neutral sources are used as input to an atmospheric photochemical model to provide diurnal variations in water group ion/neutral mixing ratios. The surface-thermal model is an integration of a 1-D heat conduction equation over a diurnal cycle, subject to a surface boundary condition. The temperature distribution of the satellite surface, as well as the depth of a frosty, or silicate/icy overlayer, and the magnitude of volatile sublimation, is calculated. Surface brightness temperatures from the Galileo PPR experiment, as well as albedo variations and morphology conditions measured by the Galileo NIMS experiment are used to constrain the thermal model. A functional dependence of volatile production from sputtering is assumed, with latitude and longitude dependencies incorporated into the model. The photochemical model employs a diffusion factor to simulate the effects of a corotation wind upon the photochemical ion production, and accounts for adsorption of atmospheric molecules by the surface. Temperature measurements from the Galileo PLS experiment are used with some temperature dependent reactions of the photochemical model.

Preliminary results from this model include a maximum (noon) subsolar surface temperature of 153 K, total global water production from (upward) sublimation of 1.1 x1027 molecules/s, and a ratio of atmospheric [H]/[H2O] =  500.