| A range of theoretical models of the compositional structure of Io's dayside atmosphere and ionosphere are developed. The dominant neutral gas, SO(,2), is provided by sublimation of surface frost. Photochemical processes lead to the build up of O, S, SO, and O(,2) as minor gasses near Io's surface while O becomes the dominant gas near the exobase. The vertical column density of O(,2) in all models considered is less than 10('14) 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('+). A 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 occultation experiment require atmospheric surface concentrations of SO(,2) in the range of 2.5 x 10('9) to 1 x 10('11) 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 (GREATERTHEQ)2 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 a pure SO(,2) surface. The depositional rate of this "excess" sulfur is in the range 100 m to 1 km thickness per billion years.; Atmospheric Na is provided by surface sputtering of SO(,2) surface frost with Na impurities by MeV type magnetospheric ions. An upward flux of Na(,2)O of 5 x 10('7) cm('-2) s('-1) leads to an escape flux of Na from the exobase of 1 x 10('7) cm('-2) s('-1). The chemistry (ion and neutral) of Na species in the atmosphere has only minor effects on the major characteristics of the atmosphere and ionosphere.; It is generally accepted that Io is the source of S, O, Na, and K which, subsequent to ionization, form the constituents of the Io plasma torus. It is shown that the escape of S and O from Io can be understood in terms of the photochemistry of a predominantly SO(,2) atmosphere created by the high vapor pressure of SO(,2). However, the vapor pressures of Na(,2)S, K(,2)S and other common compounds containing Na and K are negligible at the surface temperature of Io. (Abstract shortened with permission of author.)... |
