Organosulfur Chemistry on sub-Neptunes: Implications for hazes and biosignatures

TL;DR

This study investigates the formation pathways of organosulfur gases on sub-Neptune exoplanets, assessing their potential as biosignatures and their role in atmospheric hazes, with implications for interpreting JWST observations.

Contribution

The paper critically evaluates abiotic formation pathways of DMS and DMDS, explores alternative hydrocarbon explanations, and links sulfur haze formation to atmospheric composition variations.

Findings

Abiotic DMS and DMDS formation depends on unmeasured energy barriers.

Hydrocarbon formation can mimic organosulfur spectral features.

Sulfur hazes can form from trace H2S, affecting atmospheric observations.

Abstract

The organosulfur biosignature gases dimethylsulfide (DMS) and dimethlydisulfide (DMDS) have recently been claimed to be present in the atmosphere of sub-Neptune exoplanet K2-18b, leading to the suggestion of possible extraterrestrial life. Abiotic formation pathways for DMS and DMDS in reducing atmospheres have also been proposed, raising concern over the use of DMS and DMDS as biosignature gases more generally. In this paper we independently test and contrast the proposed abiotic formation pathways for DMS and DMDS using K2-18b as a case study, and explore the wider implications for the atmospheric carbon and sulfur chemistry of hydrogen-rich sub-Neptunes. We demonstrate that one proposed formation pathway is capable of producing observable abundances of abiotic DMS and DMDS, however it depends sensitively on the energy barrier of the limiting step, which remains unmeasured experimentally. The formation of hydrocarbons including C2H6, however, occurs abundantly and offers a plausible alternative explanation to the reported suggestions of organosulfur compounds on K2-18b, having previously been shown to share similar spectral features with DMS and DMDS at near-IR wavelengths. Finally, we demonstrate that sulfur hazes form via the photochemistry of H2S and condense in the atmosphere of K2-18b even at trace abundances. We propose that variation in atmospheric sulfur abundance can explain the diversity of haziness observed across the sub-Neptune population so far with JWST.