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Considerations for Photochemical Modeling of Possible Hycean Worlds

By Keith Cowing

Status Report

astro-ph.EP

October 11, 2024

Filed under astro-ph.EP, astrochemistry, atmosphere, exoplanet, Hycean worlds, JWST, K2-18 b, metallicity, organic chemistry, stellar spectrum, sub-Neptune, surface pressure, UV albedo

Considerations for Photochemical Modeling of Possible Hycean Worlds — Hycean and mini-Neptune scenarios with two photochemical models. Top row: Original simulations from Wogan et al. (2024), showing the mixing ratio of molecules against atmospheric pressure in bar. Bottom row: VULCAN simulations of the same scenarios with equivalent assumptions (see main text). The scenarios are: an uninhabited Hycean (left column), an inhabited Hycean (middle column), and a mini-Neptune case (right column). The given molecular constraints are from the “One offset” case (Madhusudhan et al. 2023b) as this case gives the best fit to the observed data. 1-σ error bars are given for detected molecules (CH4 – orange; CO2 – green), and 2-σ upper limits are given for CO (purple), H2O (blue), and NH3 (grey), which were not detected by Madhusudhan et al. (2023b). The photosphere, the region in the atmosphere which impacts the transmission spectrum observed, is given in a light grey shade between 10−2 – 10−4 bar. A variety of simulations were run with both the W24 setup (see Table A1) and VULCAN (see Table A2). All of the simulations shown in this figure use the GJ 176 spectrum. Only the inhabited Hycean scenarios in both models are consistent with the retrieved abundances. — astro-ph.EP

JWST is revolutionising the study of temperate sub-Neptunes, starting with the first detection of carbon-bearing molecules in the habitable-zone sub-Neptune K2-18 b. The retrieved abundances of CH₄ and CO₂ and non-detection of NH₃ and CO in K2-18 b are consistent with prior predictions of photochemical models for a Hycean world with a habitable ocean.

However, recent photochemical modeling raised the prospect that the observed abundances may be explained by a mini-Neptune scenario instead. In this study, we explore these scenarios using independent photochemical modeling with K2-18 b as a case study.

We find the previous results to be sensitive to a range of model assumptions, such as the photochemical cross sections, incident stellar spectrum, surface pressure, UV albedo, and metallicity, significantly affecting the resulting abundances.

We explore a wide model space to investigate scenarios that are compatible with the retrieved molecular abundances for K2-18 b. Our analysis shows that the previously favoured mini-Neptune scenario is not compatible with most of the retrieved abundances, while the Hycean scenarios, both inhabited and uninhabited, provide better agreement.

An uninhabited Hycean scenario explains most of the abundance constraints, except CH₄ which is generally underabundant but dependent on the model assumptions. The inhabited Hycean scenario is compatible with all the abundances if the observed CH₄ is assumed to be predominantly biogenic.

Our results underscore the importance of systematic photochemical modeling and accurate interpretation of chemical abundance constraints for candidate Hycean worlds.

Photodissociation cross sections for twelve molecules are compared across three models. Cross sections (σλ) in terms of cm² molecule^-1 are shown for H₂O, CH₄, H₂, CO, CO₂, NH₃ in the left column on a log scale and the right column shows the cross section for HO₂, HCN, CS₂, H₂S, C₂H₆, and OCS and linear (right) scale versus wavelength in nm. Available cross sections in Atmos and Photochem are shown in red and light blue, respectively. VULCAN has cross sections given in terms of absorption (grey) and dissociation (black). The vertical magenta dotted line shows where the Photochem photolysis grid starts at 92.5 nm. — astro-ph.EP

Gregory J. Cooke, Nikku Madhusudhan

Comments: 29 pages, 12 figures, in review in ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2410.07313 [astro-ph.EP] (or arXiv:2410.07313v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2410.07313
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Submission history
From: Gregory Cooke
[v1] Wed, 9 Oct 2024 18:00:01 UTC (6,326 KB)
https://arxiv.org/abs/2410.07313

Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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