WHAT IS EATING OZONE? THERMAL REACTIONS BETWEEN SO 2 AND O 3 : IMPLICATIONS FOR ICY ENVIRONMENTS

Abstract

Laboratory studies are presented, showing for the first time that thermally driven reactions in solid H 2 O + SO 2  + O 3 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, SO 2 and O 3 will efficiently react making detection of these molecules in the same vicinity unlikely. Our results also explain why O 3 has not been detected on Callisto and why the SO 2 concentration on Callisto appears to be highest on that world’s leading hemisphere. Furthermore, our results predict that the SO 2 concentration on Ganymede will be lowest in the trailing hemisphere, where the concentration of O 3 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.