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

Abstract

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 in this work the essential development steps that lead to our flexible radiative transfer module, REDFOX, and validate REDFOX for the solar system planets Earth, Venus, and Mars, as well as for steam atmospheres. REDFOX is a k-distribution model using the correlated-k approach with the 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, and 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, and with it the amplitudes of spectral features, making the latter inconsistent with the observations. We also performed tests for H2–He atmospheres up to 50 times solar metallicity, all compatible with the observations.

1D atmospheric study of the temperate sub-Neptune K2-18b

“…While our standard models, presented in Section 4, are statistically able to reproduce the observed spectrum of K2-18b, here we test some scenarios allowing for CH 4 VMR to be as small as retrieved by all the other teams who analysed the data so far. Indeed, our nominal model gives a CH 4 VMR of ≈ 5 +1 −2 % (for metallicities between 65 and 500 (Z/H) ), while Benneke et al (2019a), Madhusudhan et al (2020) and Scheucher et al (2020) give, respectively, a 2σ upper limit of 0.248%, a 99% upper limit of 3.47%, and an upper limit of 460 ppm. Benneke et al (2019b) propose three explanations to this depletion: (i) a high internal temperature, either from residual heat of formation or tidal heat-ing, (ii) a low C/O ratio resulting from planetary formation process, and (iii) a catalytic destruction of CH 4 by photolysis.…”

“…Using an interior model, Madhusudhan et al (2020) indicated that if the planet had a small rocky core and a thin H 2 /He atmosphere, an ocean of liquid H 2 O could exist. However, Scheucher et al (2020) ruled out this possibility, arguing that an H 2 O ocean would partially evaporate in the atmosphere, giving a spectrum that would be incompatible with the data. In the present work, we use Exo-REM, our self-consistent one-dimensional (1D) atmospheric model adapted for transiting exoplanets to study K2-18b atmospheric composition assuming a thick H 2 -He atmosphere.…”

“…These data have already been analysed by several teams (Benneke et al 2019a;Tsiaras et al 2019;Madhusudhan et al 2020;Scheucher et al 2020;Bézard et al 2020). Also, Scheucher et al (2020) and Bézard et al (2020) used self-consistent models (respectively, '1D-TERRA' and 'Exo-REM') to analyse the data , while the other used free retrieval algorithms. The first five investigations concluded that there is a presence of H 2 O as well as a significant amount of H 2 -He.…”

“…Nine transits were acquired us-ing the Wide Field Camera 3 on the Hubble Space Telescope (HST/WFC3), as well as data from Kepler K2 and Spitzer IRAC channels 1 and 2 (Benneke et al 2019a). These data have already been analysed by several teams (Benneke et al 2019a;Tsiaras et al 2019;Madhusudhan et al 2020;Scheucher et al 2020;Bézard et al 2020). Also, Scheucher et al (2020) and Bézard et al (2020) used self-consistent models (respectively, '1D-TERRA' and 'Exo-REM') to analyse the data , while the other used free retrieval algorithms.…”

A Heavy Molecular Weight Atmosphere for the Super-Earth π Men c

“…For the IR opacities, we took the Rosseland mean κ R for solar composition (Freedman et al 2008) as our baseline. In our opacity 1 setting, we adopted γ = 0.123 and κ IR = 1 × 〈κ R 〉 (bracket 〈〉 denotes average over wavelength) which was confirmed to reproduce well the p-T profiles published for the H 2 /He/H 2 O atmosphere of K2-18 (Scheucher et al 2020). As π Men cʼs atmosphere may also contain large abundances of CO 2 , our opacity 2 setting adopted γ = 0.500 and κ IR = 2 × 〈κ R 〉, which is appropriate for a CO 2 -dominated atmosphere with some admixture of H 2 /He.…”