Photochemistry of a volcanically driven atmosphere on Io: Sulfur and oxygen species from a pele-type eruption

To determine how active volcanism might affect the standard picture of sulfur dioxide photochemistry on Io, we have developed a one-dimensional atmospheric model in which a variety of sulfur-, oxygen-, sodium-, potassium-, and chlorine-bearing volatiles are volcanically outgassed at Io's surface and then evolve due to photolysis, chemical kinetics, and diffusion. 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. Both thermochemical equilibrium calculations (Zolotov and Fegley 1999, Icarus 141, 40-52) and the Pele plume observations of Spencer et al. (2000; Science 288, 1208-1210) suggest that S₂may be a common gas emitted in volcanic eruptions on Io. If so, our photochemical models indicate that the composition of Io's atmosphere could differ significantly from the case of an atmosphere in equilibrium with SO₂frost. The major differences as they relate to oxygen and sulfur species are an increased abundance of S, S₂, S₃, S₄, SO, and S₂O and a decreased abundance of O and O₂in the Pele-type volcanic models as compared with frost sublimation models. The high observed SO/SO₂ratio on Io might reflect the importance of a contribution from volcanic SO rather than indicate low eddy diffusion coefficients in Io's atmosphere or low SO "sticking" probabilities at Io's surface; in that case, the SO/SO₂ratio could be temporally and/or spatially variable as volcanic activity fluctuates. Many of the interesting volcanic species (e.g., S₂, S₃, S₄, and S₂O) are short lived and will be rapidly destroyed once the volcanic plumes shut off; condensation of these species near the source vent is also likely. The diffuse red deposits associated with active volcanic centers on Io may be caused by S₄radicals that are created and temporarily preserved when sulfur vapor (predominantly S₂) condenses around the volcanic vent. Condensation of SO across the surface and, in particular, in the polar regions might also affect the surface spectral properties. We predict that the S/O ratio in the torus and neutral clouds might be correlated with volcanic activity-during periods when volcanic outgassing of S₂(or other molecular sulfur vapors) is prevalent, we would expect the escape of sulfur to be enhanced relative to that of oxygen, and the S/O ratio in the torus and neutral clouds could be correspondingly increased.