Theoretical studies of Io's atmosphere

A range of theoretical models of the compositional structure of Io's dayside atmosphere and ionosphere are developed. The dominant neutral gas, SO2, is provided by sublimation of surface frost. Photochemical processes lead to the build up of O, S, SO, and O2 as minor gasses near Io's surface while O becomes the dominant gas near the exobase. The vertical column density of O2 in all models considered is less than 1014 cm-2. The dayside ionosphere is formed as a result of ionization of neutral species by solar UV radiation. Charge exchange and rearrangement reactions are important for determining the ionic composition of the ionosphere. The dominant ion in the models considered is SO+. The number of charge exchange reactions are identified whose rates need to be better determined in order to refine the present model of the ionosphere. The best matches of the model ionospheres to that observed by the Pioneer 10 radio occulation experiment require atmospheric surface concentrations of SO2 in the range of 2.5 x 109 to 1 x 1011 cm-3, and an exospheric temperature in the range of 960 K to 1230 K. The ratio of the escape fluxes of O to S from the exobase is greater than or equal to2 in the models considered, while the models which allow surface deposition of minor constituents always have a total sulfur depositional rate greater than 1/2 of the total oxygen depositional rate, thus a surface enrichment of S relative to that predicted by pure SO2 surface. The depositional rate of this excess sulfur is in the range 100 m to 1 km thickness per billion years