Oxidation State of Volcanic Gases and the Interior of Io

We used thermochemical equilibrium calculations to constrain the oxygen fugacity (f_O2) of volcanic gases on Io. Three types of calculations were done: (1) Upper limits for f_O2 from Voyager IRIS upper limits for the SO₃/SO₂ ratio and the O₃ abundance in the Loki volcanic plume; (2) lower limits for f_O2 from the observed SO/SO₂ ratio in Io's atmosphere; (3) oxygen fugacities as a function of temperature, total pressure, and O/S ratio for volcanic gases. We find that hot SO₂ (e.g., Loki volcanic gases) has oxygen fugacities between the Ni-NiO and hematite-magnetite oxygen fugacity buffers. Pele-type volcanic gases (i.e., SO₂-S₂ mixtures) have f_O2 values ranging from Ni-NiO to a few log f_O2 units lower. These f_O2 values are similar to those for most terrestrial volcanic gases and magmas. This coincidence indirectly indicates the predominantly silicate character of volcanism on Io. The oxidized nature of volcanic gases and their probable source magmas indicates that Io is differentiated and that metallic iron and free carbon are not present (or at least not abundant) in bulk silicate Io. This deduction agrees with the earlier inference from Galileo data of an iron core in Io. The inferred oxidation state of bulk silicate Io is plausibly due to loss of most of its initial water inventory via hydrogen escape and consequent oxidation of Fe and Fe²⁺-bearing minerals to magnetite and other Fe³⁺-bearing phases. Geochemical analyses of Io's surface and volcanic plumes, while difficult, are possible and can test our predictions.