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

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
Focus to learn more
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,