Consistently Simulating a Wide Range of Atmospheric Scenarios for K2-18b with a Flexible Radiative Transfer Module

The atmospheres of small, potentially rocky exoplanets are expected to cover a diverse range in composition and mass. Studying such objects therefore requires flexible and wide-ranging modeling capabilities. We present here our newly developed, flexible radiative transfer module, REDFOX, validated for the Solar system planets Earth, Venus and Mars, as well as steam atmospheres. REDFOX is a k-distribution model using the correlated-k approach with random overlap method for the calculation of opacities used in the δ-two-stream approximation for radiative transfer. Opacity contributions from Rayleigh scattering, UV / visible cross sections and continua can be added selectively.

With the improved capabilities of our new model, we calculate various atmospheric scenarios for K2-18b, a super-Earth / sub-Neptune with ∼8 M⊕ orbiting in the temperate zone around an M-star, with recently observed H₂O spectral features in the infrared. We model Earth-like, Venus-like, as well as H₂-He primary atmospheres of different Solar metallicity and show resulting climates and spectral characteristics, compared to observed data. Our results suggest that K2-18b has an H₂-He atmosphere with limited amounts of H₂O and CH₄. Results do not support the possibility of K2-18b having a water reservoir directly exposed to the atmosphere, which would reduce atmospheric scale heights, hence too the amplitudes of spectral features inconsistent with the observations. We also performed tests for H₂-He atmospheres up to 50 times Solar metallicity, all compatible with the observations.