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W. H. Smyth (Atmospheric and Environmental Research, Inc.), M. L. Marconi (Fresh Pond Research Institute), M. C. Wong (Jet Propulsion Laboratory)
The nature of Io's atmosphere with its strong interaction with the Io plasma torus is a complex three-dimensional problem of significant scientific interest. To gain more physical insight into this problem, new numerical calculations for Io's atmosphere have been undertaken based upon an improved version of the 2-D (altitude and solar zenith angle) multi-species diffusive hydrodynamic model of Wong and Smyth (Icarus 146, 60-74, 2000) for an SO2 sublimation atmosphere with photo- and gas-phase chemistry for the species SO2, SO, O2, O, S, Na, NaO, and NaS. First, plasma-neutral chemistry (in particular, electron impact molecular dissociation) has been incorporated in the 2-D model in a simplified manner. Model calculations show that electron impact molecular dissociation is important at higher altitudes in the dayside atmosphere, over a larger altitude range in the thinner nightside atmosphere, and in Io's entire atmosphere when eclipsed by Jupiter's shadow. In particular, O, S and SO are enhanced dramatically relative to the SO2 density at high altitudes in the dayside atmosphere and are the dominant species in the nightside atmosphere. Plasma-neutral chemistry also significantly stabilizes the abundance of O and S in Io's eclipsed atmosphere, so as to be more compatible with eclipse and post-eclipse changes observed in O and S ultraviolet intensities obtained with HST (Ballester et al., BAAS 29, 980, 1997). Second, relevant photo- and gas-phase chemistry for chlorine species (which can be strongly coupled through Cl2 to sodium species) has been incorporated in the 2-D hydrodynamic model, and exploratory calculations have being undertaken for different source scenarios. Atomic chlorine was recently detected in UV emission lines acquired by HST/STIS measurements (Retherford et al., BAAS 32, 1055, 2000). Model calculations will be presented to illustrate the impact of plasma-neutral chemistry and the distribution of chlorine and sodium species in Io's atmosphere.