Sulfur Promotes Haze Formation in Warm CO₂-Rich Exoplanet Atmospheres

Sulfur gases significantly affect the photochemistry in the atmospheres of Solar System bodies, such as Earth, Venus, Jupiter, and its moon, Io. Sulfur gases are expected to be present and play an important role in exoplanet atmospheres. However, sulfur photochemistry in planetary atmospheres remains unclear in general because the chemical-kinetics information of sulfur species is largely unknown under relevant conditions. Sulfur laboratory experiments are much needed to understand current and future observations of exoplanet atmospheres, but performing such experiments is very challenging due to the complexity of sulfur chemistry and the high reactivity of sulfur species.

We carried out laboratory simulations to study the photochemical role of hydrogen sulfide (H₂S) in warm CO₂-rich exoplanet atmospheres (800 K). We found that even a small amount of H₂S (1.6%) plays a significant role in photochemistry. Specifically, H₂S participates in both gas and solid phase chemistry. In the gas phase, we identified new sulfur gas products (CH₃SH/SO, C₂H₄S/OCS, SO₂/S₂, and CS₂); while in the solid phase, the inclusion of H₂S increases the haze production rate by three times. Our result shows that sulfur photochemistry could produce some potential biosignatures abiotically, and may provide a source of materials for prebiotic evolution because the solid particles are mainly organic compounds, either non-sulfur-containing or organosulfur compounds.

1. He, C. et al. 2020, Nat. Astron., DOI:10.1038/s41550-020-1072-9