Electrical breakdown products of SO2 in CO2 and N2 mixtures and the implication to the mysterious UV absorber in Venus's atmosphere

Sulfur-containing species are suggested as the UV-absorbers in Venus’s atmosphere, which can be generated by photochemistry or electrochemistry. Here we report an electrical discharge experiment in gas mixtures of SO₂ with CO₂ and N₂, under the pressure and temperature conditions relevant to the Venus cloud layer. We directly observed the primary breakdown products of SO₂ as free radicals SO*, S_I*, S_II*, S₂*, O_I*, O_II* using plasma spectroscopy; and the stable ending products as S₈ particles and H₂SO₄ droplets using Raman spectroscopy. Their co-exhibitions after a few minutes of electrical discharge imply the formation of short-lived intermediate phases (from the radicals as precursors), including polysulfur and sulfur-oxides, both are recognized candidates for the mysterious UV-absorber. The simultaneous observations of plasma lines of SO*$$and S₂* under all experimental conditions suggest that the two major breakdown paths of SO₂, which require similar electron energy, are likely to occur simultaneously. In Venus’s cloud layer where electric activity may occur, the high breakdown rate of SO₂ by glow-to-arc electric discharge would generate various S-bearing radicals with very high transient density regionally, ∼3 orders of magnitude higher than the global mixing ratio of similar species from photochemistry. The high density of reactive S-species from regional electrochemistry could be responsible for the inhomogeneous distribution and temporal changes of dark features in Venus UV images.