Volatile-rich Sub-Neptunes as Hydrothermal Worlds: The Case of K2-18 b

TL;DR

This study explores whether a global supercritical water ocean on the sub-Neptune K2-18 b can explain its atmospheric composition, providing insights into its interior structure and potential habitability implications.

Contribution

It introduces the first equilibrium geochemical model linking supercritical water oceans with observable atmospheric signatures on sub-Neptunes.

Findings

A supercritical water ocean on K2-18 b is plausible based on atmospheric ratios.

Ocean temperature estimates range from ~710 K to ~1070 K.

Future observations could help distinguish interior compositions.

Abstract

Temperate exoplanets between the sizes of Earth and Neptune, known as "sub-Neptunes", have emerged as intriguing targets for astrobiology. It is unknown whether these planets resemble Earth-like terrestrial worlds with a habitable surface, Neptune-like giant planets with deep atmospheres and no habitable surface, or something exotic in between. Recent JWST transmission spectroscopy observations of the canonical sub-Neptune, K2-18 b, revealed ~1% CH4, ~1% CO2, and a non-detection of CO in the atmosphere. While previous studies proposed that the observed atmospheric composition could help constrain the lower atmosphere's conditions and determine the interior structure of sub-Neptunes like K2-18 b, the possible interactions between the atmosphere and a hot, supercritical water ocean at its base remain unexplored. In this work, we investigate whether a global supercritical water ocean, resembling a planetary-scale hydrothermal system, can explain these observations on K2-18 b-like sub-Neptunes through equilibrium aqueous geochemical calculations. We find that the observed atmospheric CH4/CO2 ratio implies a minimum ocean temperature of ~710 K, whereas the corresponding CO/CO2 ratio allows ocean temperatures up to ~1070 K. These results indicate that a global supercritical water ocean on K2-18 b is plausible. While life cannot survive in such an ocean, this work represents the first step towards understanding how a global supercritical water ocean may influence observable atmospheric characteristics on volatile-rich sub-Neptunes. Future observations with better constrained CO and NH$_3$ mixing ratios could further help distinguish between possible interior compositions of K2-18 b.