Summary
Fujisawa et al. test whether self-consistent Hycean atmospheric models for k2-18b are compatible with current jwst NIRISS and NIRSpec transmission spectra (0.8–5.2 µm). Using 1D photochemical modelling coupled with radiative-convective equilibrium calculations, the study finds that H₂-CH₄-H₂O atmospheres over a liquid ocean — with ~1 bar H₂, percent-level CH₄ and CO, and CO₂ at ~10⁻³–10⁻² — reproduce the observed spectra without invoking DMS or other potential biosignatures. The photochemical network naturally drives CO to ~1–2% mixing ratios, and a 1 bar H₂ envelope with percent-level CH₄ requires interior replenishment on gigayear timescales. The paper directly rebuts the argument that oxidized carbon abundances rule out the hycean scenario, while not excluding the mini-Neptune interpretation either. This is a significant methodological contribution to the K2-18b “parallel interpretations” debate, extending the work of src-k2-18b-dms-insufficient-evidence-2025 and src-jwst-biosignature-prospects-2025.
Key Claims
- Hycean models with 1 bar H₂, percent-level CH₄ and CO, and CO₂ ~10⁻³–10⁻² reproduce NIRISS + NIRSpec spectra from 0.8 to 5.2 µm without DMS or additional species.
- H₂-CH₄-H₂O photochemical networks generically produce CO at ~1–2% mixing ratios — no exotic chemistry required.
- ~99% of methane-derived carbon remains within the C–H loop; only ~1% is oxidized.
- Liquid oceans can exist across a wide range of temperatures and pressures within the modeled hycean configurations.
- A ~1 bar H₂ envelope with percent-level CH₄ requires interior replenishment on gigayear timescales.
- Current CO and CO₂ constraints alone are insufficient to rule out hycean interpretations.
- Mini-Neptune scenarios remain viable; the data are not yet discriminating.
Notable Quotes
“Hycean configurations are likewise consistent with the data, and current CO and CO₂ constraints alone are not yet sufficient to rule out Hycean interpretations of K2-18b.”
“Approximately 99% of the carbon from CH₄ remains within the C–H circulation loop, while about 1% is directed towards oxidation.”