nic gases exsolved from magmas (Symonds et al. 1994). Thus, Ionian volcanic gases exsolved from magmas during eruption may also chemically
We use thermochemical equilibrium calculations to show equilibrate (see below). that SO is expected in volcanic gases erupted on Io, and derive
We modeled volcanic gas chemistry on Io in the 1000-2000 K range the range of temperatures, pressures, and elemental composithat covers the eruption temperatures of many terrestrial and lunar silicate tions that provide the observed SO/SO 2 ratio of 3-10% in Io's magmas. We used 1400 K as a nominal temperature because it is a typical atmosphere. Our predictions could be tested during the Galileo eruption temperature for basaltic magmas on Earth, such as at Kilauea (Symonds et al. 1994). The total pressure and bulk elemental composition
Europa Mission (GEM), by Earth-based and Hubble Space
of the volcanic gases are also inputs to our modeling. Although the Telescope observations in the millimeter and UV regions, or pressures at which Ionian volcanic gases exsolve from magmas are unby a mass spectrometer and/or infrared spectrometer on an Io known, pressures up to ศ100 bars in volcanic conduits on Io appear