Examining the Potential for Methyl Halide Accumulation and Detectability in Possible Hycean-Type Atmospheres
Some sub-Neptune planets may host habitable conditions; for example “Hycean” worlds with H2 envelopes over liquid water oceans can maintain potentially hospitable pressures and temperatures at their surface.
Recent JWST observations of K2-18b and TOI-270d have shown that such worlds could be compelling targets for biosignature searches, given their extended scale heights and therefore large atmospheric signatures.
Methylated biosignatures, a broad group of gases that can be generated by biological attachment of a CH3 group to an environmental substrate, have been proposed as candidate signs of life for Earth-like exoplanets. However, methyl halides (CH3 + halogen) have not yet been robustly examined with self-consistent photochemical and spectral models for planets with H2-dominated atmospheres.
Here we demonstrate that methyl chloride (CH3Cl), predominantly produced by marine microbes, could be detected using JWST in tens of transits or fewer for Hycean planets, comparable to detection requirements for other potential atmospheric biosignatures.
The threshold atmospheric mixing ratio for detectability is ∼10 ppm, which can accumulate with global fluxes comparable to moderately productive local environments on Earth.
(a) Comparison of stellar surface spectra for both the Sun and GJ 436, used here as a stand in for K2-18, an M2.8V star (top) and photochemical cross sections for CH3X gases and DMS (bottom). (b) Comparison of photolysis for CH3X gases for Earth-like biological production flux levels simulated using K2-18b parameters. (c) Comparison of mixing ratio profiles for 1D (dashed) and 2D (solid, shaded by longitude) simulations using Earth-like flux levels. Modest enhancements of CH3Cl are shown, with similar ratios of the other gases. Largest mixing ratio range is seen for CH3I which shows some longitudinal dependency. (d) Mixing ratios similar to (c) for 50x Earth-like flux, around where fluxes becomes potentially spectrally detectable. Enhancements of 5-10x are seen for all gases compared to 1D results. — astro-ph.EP
Top: Methyl halide opacities used in spectral simulations. Data sourced from the HITRAN database Gordon et al. (2022). Middle: Comparison of haze precursor molecules in Hycean atmospheres for a variety of methyl halide flux levels. These gases are investigated here because Tsai et al. (2024) showed that that CO and C2H6 are highly responsive to DMS levels, with CO showing a large drop off and C2H6 a large increase as the organosulfur gas level increases. However, this trend does not hold here due to the rates of methyl halide reformation consuming the methyl radical and preventing further downstream chemical impacts. This is especially apparent in the large drop in C2H6 levels between the “no Methyl Halide flux” case and the “Earthlike Flux” . Bottom: Comparison of average volume mixing ratios of methyl halide gases in Hycean-type atmospheres for the Sun and GJ 436, used here as a well characterized test case for a relatively inactive M2 star like K2-18. The triangles show the GJ 436 stellar environment and circle markers the solar environment. The difference in accumulation level for CH3Cl is smaller in comparison to the other methyl halides, because CH3Cl has less opacity in the NUV and is less affected by the increased brightness of the Sun at these shorter wavelengths (see Figure 1). One part-per-million is shown with the black horizontal line as an approximate threshold for spectral relevance. Dashed yellow horizontal lines indicate mixing ratios reported for the most productive biological production flux scenario (1000x globally averaged) under modern Earth-like (O2-rich) bulk conditions in Leung et al. (2022). — astro-ph.EP
Michaela Leung, Shang-Min Tsai, Edward W Schwieterman, Daniel Angerhausen, Janina Hansen
Comments: 13 pages, 4 figures, 1 appendix; accepted at ApJL updated version includes clarified color scheme and axis in Figure 4
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2502.13856 [astro-ph.EP] (or arXiv:2502.13856v2 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2502.13856
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Submission history
From: Michaela Leung
[v1] Wed, 19 Feb 2025 16:20:24 UTC (7,691 KB)
[v2] Thu, 20 Feb 2025 19:56:15 UTC (7,687 KB)
https://arxiv.org/abs/2502.13856
Astrobiology
