What is eating ozone? thermal reactions between SO₂ and O₃: IMPLICATIONS for ICY ENVIRONMENTS

Laboratory studies are presented, showing for the first time that thermally driven reactions in solid H₂O+SO₂+O₃ mixtures can occur below 150 K, with the main sulfur-containing product being bisulfate (). Using a technique not previously applied to the low-temperature kinetics of either interstellar or solar-system ice analogs, we estimate an activation energy of 32 kJ mol^-1 for formation. These results show that at the temperatures of the Jovian satellites, SO₂ and O₃ will efficiently react making detection of these molecules in the same vicinity unlikely. Our results also explain why O₃ has not been detected on Callisto and why the SO₂ concentration on Callisto appears to be highest on that world's leading hemisphere. Furthermore, our results predict that the SO₂ concentration on Ganymede will be lowest in the trailing hemisphere, where the concentration of O₃ is the highest. Our work suggests that thermal reactions in ices play a much more important role in surface and sub-surface chemistry than generally appreciated, possibly explaining the low abundance of sulfur-containing molecules and the lack of ozone observed in comets and interstellar ices.