Predicted Abundances of Carbon Compounds in Volcanic Gases on IO

We use chemical equilibrium calculations to model the speciation of carbon involcanic gases on Io. The calculations cover wide temperature (500-2000 K),pressure (10^-8 to 10^+2 bars), and composition ranges (bulk O/S atomic ratios\~0 to 3), which overlap the nominal conditions at Pele (1760 K, 0.01 bar, O/S~ 1.5). Bulk C/S atomic ratios ranging from 10^-6 to 10^-1 in volcanic gasesare used with a nominal value of 10^-3 based upon upper limits from Voyager forcarbon in the Loki plume on Io. Carbon monoxide and CO2 are the two majorcarbon gases under all conditions studied. Carbonyl sulfide and CS2 are ordersof magnitude less abundant. Consideration of different loss processes(photolysis, condensation, kinetic reactions in the plume) indicates thatphotolysis is probably the major loss process for all gases. Both CO and CO2should be observable in volcanic plumes and in Io's atmosphere at abundances ofseveral hundred parts per million by volume for a bulk C/S ratio of 10^-3.Comment: 21 pages, 4 figures, 4 tables; accepted by Astrophysical Journa