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J. I. Moses (LPI), M. Yu. Zolotov, B. Fegley, Jr. (Washington Univ.)
To determine how active volcanism might affect the standard picture of sulfur dioxide photochemistry on Io, we have developed a one-dimensional model in which a variety of sulfur-, oxygen-, sodium-, potassium-, and chlorine-bearing volatiles are volcanically outgassed from Io's surface and then evolve due to photolysis and chemical kinetics. Thermochemical equilibrium calculations in combination with recent observations of gases in the Pele plume are used to help constrain the composition and physical properties of the exsolved volcanic vapors. We find that although SO2 continues to remain the dominant atmospheric gas, other species such as S2, SO, and S are more abundant than in a pure SO2 frost sublimation-driven atmosphere. Some important species (e.g., S2, S3, S4, S2O, NaCl, and KCl) will be rapidly destroyed by photochemical processes once the volcanic plumes shut off; however, these species might be observable while the plumes are active. Many of the volcanic gases will condense near their source, thereby altering the surface composition near active vents. We will identify observable quantities (e.g., S2/SO2, S/SO2 and SO/SO2 ratios) that might help distinguish between a sublimation-driven atmosphere and a volcanic atmosphere, and we will discuss the implications of our modeling for the observed composition of Io's atmosphere, surface, torus, and neutral clouds.