Oxidation State of Volcanic Gases and the Interior of Io

Mikhail Yu Zolotov, Bruce Fegley

Research output: Contribution to journalArticlepeer-review

43 Scopus citations


We used thermochemical equilibrium calculations to constrain the oxygen fugacity (fO2) of volcanic gases on Io. Three types of calculations were done: (1) Upper limits for fO2 from Voyager IRIS upper limits for the SO3/SO2 ratio and the O3 abundance in the Loki volcanic plume; (2) lower limits for fO2 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 fO2 values ranging from Ni-NiO to a few log fO2 units lower. These fO2 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.

Original languageEnglish (US)
Pages (from-to)40-52
Number of pages13
Issue number1
StatePublished - Sep 1999
Externally publishedYes


  • Io
  • Lithosphere
  • Mantle
  • Oxidation state
  • Oxygen fugacity
  • Sulfur
  • Volcanic gases
  • Volcanism

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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