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

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. In contrast, hydrocarbons including C2H6 are formed abundantly in such atmospheres and offer a plausible alternative explanation to the reported suggestions of organosulfur compounds on K2-18b, having previously been shown to be degenerate observationally. 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.

Figures (4)

• Figure 1: The network of important reactions for the carbon, sulfur, and coupled organosulfur chemistry in the atmosphere of K2-18b. Pathway B (following Hu2025) dominates the DMS and DMDS production and is highlighted in green. Reaction steps in pathway A (following Reed2024) that are not in pathway B are highlighted in grey. Destruction pathways of DMS and DMDS are highlighted in red. Unimolecular reactions are denoted with dashed lines, bimolecular reactions with solid single lines, and termolecular reactions with solid double lines.
• Figure 2: Maximum atmospheric mixing ratios (top) and integrated column number densities (bottom) of DMS (left) and DMDS (right) in a 1 bar H2 atmosphere containing 10% mole fraction of CH4 and varying H2S abundances. The results of pathway B are highlighted in green and those of pathway A in grey. A range of reference DMS mixing ratios observed over algea blooms on Earth are highlighted in red Park2017 along with their equivalent integrated column density assuming uniform mixing ratio.
• Figure 3: Mixing ratio profiles of DMS, DMDS, hydrocarbons C2H2+C2H4+C2H6, and H2S, in the atmosphere of K2-18b. Observational data points from Madhu2025 and Hu2025 are shown alongside, initially attributed to DMS and/or DMDS but later shown to be consistent with alternative hydrocarbon species Luque2025. DMS and DMDS production is sensitive to the input H2S abundance (left) and the energy barrier to reaction \ref{['eq9']} in pathway B (right), while the hydrocarbon production is not.
• Figure 4: (Left) The temperature structure of K2-18b across the range of input H2S abundances. The Antoine equations for the saturation vapour pressure of sulfur allotropes, H2O, and C2H6 are shown in bold dashed lines, and in lighter coloured lines the vapour pressure curves are progressively scaled down from 100% to 100 ppm to illustrate how sulfur allotropes still condense out at trace mixing ratios. (Right) Profiles of gaseous H2O and condensed S2 and S8 in the atmosphere of K2-18b.