Volcanic Origin of Disulfur Monoxide (S₂O) on Io

We use chemical equilibrium calculations to model S₂O formation by high temperature thermochemical reactions in volcanic gases on Io. The calculations predict formation of 1-6% S₂O gas at observed hot spot temperatures in SO₂-S₂gas mixtures at pressures of 1-100 bars in volcanoes on Io. The S₂O abundance increases with increasing pressure (up to ~6% at 100 bars) and is relatively insensitive to temperature and the bulk O/S ratio over wide ranges. Condensation of volcanic S₂O provides a plausible explanation for the solid S₂O that has been suggested to be present on Io's surface (e.g., around the Pele volcano).