Thermal Reactions between H₂S and O₃: Implications for Europa Surface Chemistry

Here we present a laboratory study demonstrating a low-temperature thermal oxidation reaction within H₂O + H₂S + O₃ solid ice mixtures that produces observable sulfur anion products at temperatures as low as 90 K. This reaction primarily produces SO₂, sulfur anions (including HSO^-₃ , HSO^-₄ , and SO²₄^-), and O₂ at lower temperatures (90–140 K) and hydrated states of sulfuric acid (H₂SO₄: nH₂O, where n = 0, 1, 4) at higher temperatures (150–250 K). We estimate that the overall activation energy to initiate these reactions is 20 ± 3 kJ mol⁻¹, which is significantly lower than the activation energy required to oxidize SO₂ to the sulfate ion. Given the detection of sulfur species on the surfaces of the Galilean satellites and the prevalence of radiolytically produced oxidants, we expect that these thermal reactions will play an important role in explaining the results obtained from future observations and missions that can measure the spatial distribution of these species.