Chemical Mapping of Temperate Sub-Neptune Atmospheres: Constraining the Deep Interior H₂O/H₂ Ratio from the Atmospheric CO₂/CH₄ Ratio

Abstract

Understanding the planetary envelope composition of sub-Neptune-type exoplanets is challenging due to the inherent degeneracy in their interior composition scenarios. Particularly, the planetary envelope’s H2O/H2 ratio, which can also be expressed as the O/H ratio, provides crucial insights into its original location relative to the ice line during planetary formation. Using self-consistent radiative transfer modeling and a rate-based automatic chemical network generator combined with 1D photochemical kinetic-transport atmospheric modeling, we investigate various atmospheric scenarios of temperate sub-Neptunes, ranging from H2-dominated to H2O-dominated atmospheres with equilibrium temperatures (T eq) of 250—400 K. This study includes examples such as K2-18 b (T eq = 255 K), LP 791-18 c (T eq = 324 K), and TOI-270 d (T eq = 354 K). Our models indicate that the atmospheric CO2/CH4 ratio can be used to infer the deep interior H2O/H2 ratio. Applying this method to recent JWST observations, our findings suggest that K2-18 b likely has an interior that is 50% highly enriched in water, exceeding the water content in a 100 × Z ⊙ scenario and suggesting a planetary formation mechanism involving substantial accretion of ices. In contrast, our model suggests that approximately 25% of TOI-270 d’s interior is composed of H2O, which aligns with the conventional metallicity framework with a metallicity higher than 100 × Z ⊙. Furthermore, our models identify carbonyl sulfide (OCS) and sulfur dioxide (SO2) as strong indicators for temperate sub-Neptunes with at least 10% of their interior composed of water. These results provide a method to delineate the internal composition and formation mechanisms of temperate sub-Neptunes (T eq < ∼ 500 K) via atmospheric characterization through transmission spectroscopy.