Organosulfur Chemistry on sub-Neptunes: Implications for hazes and biosignatures
Pith reviewed 2026-05-15 08:19 UTC · model grok-4.3
The pith
Abiotic pathways can produce observable DMS and DMDS on K2-18b but depend on an unmeasured energy barrier, while hydrocarbons form abundantly as an alternative.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
One proposed abiotic pathway produces observable abundances of DMS and DMDS in hydrogen-rich sub-Neptune atmospheres, but only if the energy barrier of the limiting reaction step is sufficiently low, a value that has not been measured experimentally. Hydrocarbons such as C2H6 form in high abundances and can account for the reported spectral signatures on K2-18b. Sulfur hazes condense from H2S photochemistry even at trace levels, and variations in sulfur abundance may explain the observed diversity in haziness across sub-Neptunes.
What carries the argument
Photochemical reaction networks that track organosulfur formation from H2S and the condensation of resulting sulfur aerosols in reducing atmospheres.
If this is right
- One abiotic pathway can produce observable levels of DMS and DMDS under hydrogen-rich conditions.
- Ethane and related hydrocarbons form in high abundance and share spectral features with DMS and DMDS at near-IR wavelengths.
- Sulfur hazes condense via H2S photochemistry even at trace abundances in K2-18b's atmosphere.
- Differences in atmospheric sulfur content can account for the range of haziness seen across observed sub-Neptunes.
Where Pith is reading between the lines
- Laboratory determination of the missing reaction barrier would settle whether the abiotic DMS pathway operates at detectable levels.
- JWST spectra that isolate ethane absorption bands could distinguish hydrocarbon from organosulfur contributions on K2-18b.
- The same sulfur photochemistry may operate on other hydrogen-rich exoplanets and control their cloud properties.
- Organosulfur gases become less distinctive as biosignatures until abiotic routes are experimentally excluded.
Load-bearing premise
The energy barrier for the rate-limiting step in the proposed abiotic DMS and DMDS formation pathway has not been measured experimentally.
What would settle it
A laboratory measurement showing that the activation energy of the key reaction step exceeds the value needed to reach observable DMS abundances on K2-18b.
read the original 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.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper uses photochemical modeling to test abiotic formation pathways for DMS and DMDS on hydrogen-rich sub-Neptunes, taking K2-18b as a case study. It concludes that one proposed pathway can reach observable abundances but only for sufficiently low values of an unmeasured energy barrier in the rate-limiting step; hydrocarbons including C2H6 form abundantly and provide a plausible alternative explanation for near-IR spectral features; and sulfur hazes condense from H2S photochemistry even at trace levels, offering an explanation for the observed diversity of haziness across the sub-Neptune population.
Significance. If the results hold, the work supplies a timely caution on the use of DMS/DMDS as biosignatures, demonstrates that standard hydrocarbon chemistry can mimic proposed organosulfur spectral signals, and connects sulfur abundance variations to JWST-observed haze diversity. Strengths include the explicit identification of the unmeasured barrier as a sensitivity parameter and the use of standard reaction networks rather than ad-hoc fitting.
major comments (3)
- [Abstract and §3] Abstract and §3: the headline claim that the pathway 'is capable of producing observable abundances' of abiotic DMS/DMDS is conditional on the value of the unmeasured energy barrier; the manuscript should present a quantitative sensitivity study (e.g., DMS/DMDS mixing ratios versus barrier height) with the range of values that remain consistent with current observational upper limits.
- [§4.2] §4.2 and reaction network description: the organosulfur kinetics are taken from standard H2-rich sub-Neptune networks without new experimental constraints on the key rates; because this assumption is load-bearing for the DMS/DMDS production result, the paper should tabulate the adopted rates, cite their sources, and discuss the impact of plausible factor-of-10 uncertainties.
- [Table 2] Table 2 and associated figures: no error bars, Monte-Carlo ranges, or sensitivity envelopes are shown for the predicted abundances or haze optical depths; this omission makes it difficult to assess whether the hydrocarbon alternative or the haze-formation conclusion remains robust when the unmeasured barrier and other rate uncertainties are varied simultaneously.
minor comments (3)
- [Abstract] Abstract: 'dimethlydisulfide' is misspelled and should read 'dimethyl disulfide'.
- [§2] §2: the description of the two proposed abiotic pathways should include a brief reaction scheme or flowchart so readers can follow which steps are rate-limiting without consulting the cited prior work.
- [Figures] Figure captions: several panels lack labels for the different barrier-height cases; adding a legend or inset table would improve clarity.
Simulated Author's Rebuttal
We thank the referee for their constructive comments, which have strengthened the robustness and clarity of our photochemical modeling results. We have revised the manuscript to incorporate quantitative sensitivity analyses, tabulated reaction rates with sources, and uncertainty envelopes as requested.
read point-by-point responses
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Referee: [Abstract and §3] Abstract and §3: the headline claim that the pathway 'is capable of producing observable abundances' of abiotic DMS/DMDS is conditional on the value of the unmeasured energy barrier; the manuscript should present a quantitative sensitivity study (e.g., DMS/DMDS mixing ratios versus barrier height) with the range of values that remain consistent with current observational upper limits.
Authors: We agree that the production of observable DMS/DMDS is conditional on the unmeasured barrier. The revised manuscript includes a new quantitative sensitivity study in §3 with an accompanying figure plotting DMS and DMDS mixing ratios versus barrier height. We explicitly mark the range of barrier values consistent with current JWST upper limits on K2-18b, showing that observable abundances require barriers below ~0.8 eV. revision: yes
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Referee: [§4.2] §4.2 and reaction network description: the organosulfur kinetics are taken from standard H2-rich sub-Neptune networks without new experimental constraints on the key rates; because this assumption is load-bearing for the DMS/DMDS production result, the paper should tabulate the adopted rates, cite their sources, and discuss the impact of plausible factor-of-10 uncertainties.
Authors: We have expanded §4.2 with a new table listing the key organosulfur reaction rates, their adopted values, and citations to sources (primarily KIDA database and literature on H2S and organosulfur kinetics). We discuss the impact of factor-of-10 rate uncertainties, confirming that the qualitative sensitivity to the barrier height is robust even when rates vary within this range. revision: yes
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Referee: [Table 2] Table 2 and associated figures: no error bars, Monte-Carlo ranges, or sensitivity envelopes are shown for the predicted abundances or haze optical depths; this omission makes it difficult to assess whether the hydrocarbon alternative or the haze-formation conclusion remains robust when the unmeasured barrier and other rate uncertainties are varied simultaneously.
Authors: We have added sensitivity envelopes to the figures linked to Table 2 by varying the barrier height and other key rates by factors of up to 10. These show that hydrocarbon abundances (e.g., C2H6) remain high and sulfur haze formation occurs across the explored parameter space. A full Monte-Carlo ensemble was not performed. revision: partial
- Full simultaneous Monte-Carlo sampling of uncertainties across the entire reaction network due to prohibitive computational cost of the photochemical model.
Circularity Check
No significant circularity; results follow from standard photochemical modeling with explicit parameter sensitivity
full rationale
The derivation relies on standard H2-rich sub-Neptune reaction networks and photochemical simulations. The central claim that abiotic DMS/DMDS production is possible is presented as conditional on the unmeasured energy barrier of the rate-limiting step, which is flagged as a sensitivity rather than fitted or assumed to force the result. Hydrocarbon formation and sulfur haze condensation follow directly from the adopted networks without reduction to self-defined quantities or load-bearing self-citations. The paper is self-contained against external benchmarks and does not rename known results or smuggle ansatzes via citation.
Axiom & Free-Parameter Ledger
free parameters (1)
- energy barrier of limiting step
axioms (1)
- domain assumption Standard photochemical reaction networks and condensation physics apply to hydrogen-rich sub-Neptune atmospheres
discussion (0)
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