Efficient Thermal Reactions of Sulfur Dioxide on Ice Surfaces at Low Temperature: A Combined Experimental and Theoretical Study

The interaction of sulfur dioxide (SO2) gas with a crystalline ice surface at low temperature was studied by analyzing the surface species with low energy sputtering (LES) and reactive ion scattering methods and the desorbing gases with temperature-programmed desorption mass spectrometry. The study gives direct evidence for the occurrence of efficient hydrolysis of SO2 with low energy barriers on the ice surface. Adsorbed SO2 molecules react with the ice surface at temperatures above ∼90 K to form anionic molecular species, which can be detected by OH–, SO2–, and HSO3– emission signals in the LES experiments. Heating the sample above ∼120 K causes the desorption of SO2 gas from the surface-bound hydrolysis products. As a result, the hydrolysis of SO2 on an ice surface is most efficient at 100–120 K. The surface products formed at these temperatures correspond to metastable states, which are kinetically isolated on the cold surface. Quantum mechanical calculations of a model ice system suggest plausible mechanistic pathways for how physisorbed SO2 is transformed into chemisorbed HSO3– species. HSO3– is formed either by direct conversion of physisorbed SO2 or through the formation of a stable H2SO3 surface complex, both involving proton transfer on the ice surface with low energy barriers. These findings suggest the possibility that thermal reactions of SO2 occur efficiently on the ice surface of Jovian satellites even without bombardment by high-energy radiation.