Sulfur-driven haze formation in warm CO 2 -rich exoplanet atmospheres

Sulfur gases substantially affect the photochemistry of planetary atmospheres in our Solar System, and are expected to be important components in exoplanet atmospheres. However, sulfur photochemistry in the context of exoplanets is poorly understood due to a lack of chemical kinetics information for sulfur species under relevant conditions. Here, we study the photochemical role of hydrogen sulfide (H2S) in warm CO2-rich exoplanet atmospheres (800 K) by carrying out laboratory simulations. We find that H2S plays a prominent role in photochemistry, even when present in the atmosphere at relatively low concentrations (1.6%). It participates in both gas and solid phase chemistry, leading to the formation of other sulfur gas products (CH3SH/SO, C2H4S/OCS, SO2/S2 and CS2) and to an increase in solid haze particle production and compositional complexity. Our study shows that we may expect thicker haze with small particle sizes (20–140 nm) for warm CO2-rich exoplanet atmospheres that possess H2S. Laboratory experiments show that the inclusion of even small quantities of sulfur in the atmospheres of exoplanets at 800 K significantly increases photochemical complexity, both in the vapour and in the solid phase: many sulfur gas products are created (including potential biosignature gases) and the production of organic haze particles increases threefold.