Abiotic Production of Dimethyl Sulfide, Carbonyl Sulfide, and Other Organosulfur Gases via Photochemistry: Implications for Biosignatures and Metabolic Potential

Abstract

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; CH _3 SCH _3 ), carbonyl sulfide (OCS), and carbon disulfide (CS _2 ), 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 (CH _3 SH), ethane thiol (C _2 H _5 SH), CS _2, and ethyl methyl sulfide (CH _3 CH _2 SCH _3 ) via photochemistry in analog atmospheres. Gas-phase products of H _2 S/CH _4 /N _2 haze photochemistry, with or without CO _2 , were collected and analyzed using gas chromatography equipped with sulfur chemiluminescence detection. Depending on the starting conditions, we estimate that DMS, OCS, CH _3 SH, CH _3 CH _2 SH, CS _2 , and CH _3 CH _2 SCH _3 are produced in mixing ratios >10 ^−1 ppm _v . We further demonstrate that as the mixing ratio of CO _2 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.

Keywords

• Pre-biotic astrochemistry
• Astrobiology
• Exoplanet atmospheric composition
• Atmospheric composition
• Complex organic molecules
• Biosignatures