Exoplanets, -moons, -comets

Three-dimensional Transport-induced Chemistry On Temperate Sub-Neptune K2-18b, Part II: The Combined Effects Of Atmospheric Dynamics And Chemical Reactions

By Keith Cowing

Status Report

astro-ph.EP

April 12, 2026

Filed under disequilibrium chemistry, exoplanet, JWST, K2-18b, mini-Neptune, Spectroscopy, sub-Neptune

Three-dimensional Transport-induced Chemistry On Temperate Sub-Neptune K2-18b, Part II: The Combined Effects Of Atmospheric Dynamics And Chemical Reactions — Wind structures for the synchronous and 10:1 SOR simulations, reproduced from fig. 5 of Part I and included here for reference. Columns from left to right show the horizontal distribution of zonal wind, meridional wind, and vertical wind at 0.001 bar. Streamlines indicate the direction of the horizontal flow. Results are shown in the heliocentric frame to more clearly illustrate the wind patterns. Black star-shaped markers indicate the location of the substellar point. The first and second rows show results from the synchronous and 10:1 SOR simulations as examples, with the wind patterns in other simulations being similar. — astro-ph.EP

The upper atmospheres of temperate sub-Neptunes are strongly influenced by atmospheric dynamics due to their cool equilibrium temperature and thereby longer chemical timescales than the atmospheric dynamical timescales.

In this study, we used a three-dimensional (3D) general circulation model to investigate the transport-induced disequilibrium chemistry and vertical mixing on temperate gas-rich mini-Neptunes, using K2-18b as an example. We model K2-18b assuming 180 times solar metallicity and consider it as either a synchronous or an asynchronous rotator, exploring spin-orbit resonances of 2:1, 6:1, and 10:1.

We find that the vertical transport affects the chemical structure significantly, making CO₂ and CO more abundant (∼10−3) in the upper atmosphere compared to the chemical equilibrium abundance (<10−15), and horizontal winds further homogenize the chemical composition zonally in this region. Molecular abundances in the photosphere generally agree across different rotation periods.

We employ a passive tracer in the model to estimate the one-dimensional (1D) equivalent eddy-diffusion coefficient (K_zz) of K2-18b, providing a parameter useful for future 1D atmospheric models.

Additionally, synthetic transmission spectra generated from our model are compared with the JWST observations, and we find that our model can provide a comparable fit to the observations. This work offers a 3D perspective on transport-induced chemistry on a temperate sub-Neptune and derives vertical mixing parameters to support 1D modelling.

Jiachen Liu, Duncan Christie, Jun Yang, Krisztian Kohary

Comments: Resubmitted to MNRAS after addressing referee’s comments
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2604.07987 [astro-ph.EP] (or arXiv:2604.07987v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2604.07987
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Submission history
From: Jiachen Liu
[v1] Thu, 9 Apr 2026 08:55:46 UTC (3,159 KB)
https://arxiv.org/abs/2604.07987
Astrobiology, exoplanet,

Keith Cowing

Biologist, Explorers Club Fellow, ex-NASA Space Biologist and Payload integrator, Editor of NASAWatch.com and Astrobiology.com, Lapsed climber, Explorer, Synaesthete, Former Challenger Center board member 🖖🏻

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