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 â90 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 â120 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.