2024ApJ...973L..38R

Among the atmospheric gases that have been proposed as possible biosignatures in exoplanetary atmospheres, organosulfur gases are currently considered one of the more robust indicators of extant life. These gases include dimethyl sulfide (DMS; CH₃SCH₃), carbonyl sulfide (OCS), and carbon disulfide (CS₂), which are predominantly secondary metabolic products of living organisms on Earth. Here we present results that challenge this interpretation and provide constraints on the robustness of organosulfur gases as biosignatures. Through laboratory photochemical experiments, we show the abiotic production of organosulfur gases, including DMS, OCS, methane thiol (CH₃SH), ethane thiol (C₂H₅SH), CS_2, and ethyl methyl sulfide (CH₃CH₂SCH₃) via photochemistry in analog atmospheres. Gas-phase products of H₂S/CH₄/N₂ haze photochemistry, with or without CO₂, were collected and analyzed using gas chromatography equipped with sulfur chemiluminescence detection. Depending on the starting conditions, we estimate that DMS, OCS, CH₃SH, CH₃CH₂SH, CS₂, and CH₃CH₂SCH₃ are produced in mixing ratios >10^–1 ppm_v. We further demonstrate that as the mixing ratio of CO₂ increases, so does the relative importance of OCS compared to DMS. Although our results constrain the robustness of common organosulfur gases as biosignatures, the presence of these compounds may serve as an indicator of metabolic potential on exoplanets.