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 random overlap method for the calculation of opacities used in the $\delta$-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 $\sim$8 M${}_{\oplus }$ orbiting in the temperate zone around an M-star, with recently observed H${}_2$O spectral features in the infrared. We model Earth-like, Venus-like, as well as H${}_2$-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 H${}_2$-He atmosphere with limited amounts of H${}_2$O and CH${}_4$. 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 H${}_2$-He atmospheres up to 50 times Solar metallicity, all compatible with the observations.

28 pages, 13 figures, accepted for publication in ApJ

References in corpus (20)

• Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1
• A terrestrial planet candidate in a temperate orbit around Proxima Centauri
• Atomic and Molecular Opacities for Brown Dwarf and Giant Planet Atmospheres
• Warming early Mars with CO2 and H2
• A Sub-Neptune Exoplanet with a Low-Metallicity Methane-Depleted Atmosphere and Mie-Scattering Clouds
• Trends in Atmospheric Properties of Neptune-Size Exoplanets
• Treatment of overlapping gaseous absorption with the correlated-k method in hot Jupiter and brown dwarf atmosphere models
• Is Gliese 581d habitable? Some constraints from radiative-convective climate modeling
• The interior and atmosphere of the habitable-zone exoplanet K2-18b
• Detectability of atmospheric features of Earth-like planets in the habitable zone around M dwarfs
• Revised mass-radius relationships for water-rich rocky planets more irradiated than the runaway greenhouse limit
• Warming the early Earth - CO2 reconsidered
• Biomarker Response to Galactic Cosmic Ray-Induced NOx and the Methane Greenhouse Effect in the Atmosphere of an Earthlike Planet Orbiting an M-Dwarf Star
• Clouds in the atmospheres of extrasolar planets. I. Climatic effects of multi-layered clouds for Earth-like planets and implications for habitable zones
• The habitability of stagnant-lid Earths around dwarf stars
• A New Model Suite to Determine the Influence of Cosmic Rays on (Exo)planetary Atmospheric Biosignatures -- Validation based on Modern Earth
• Proxima Centauri b: A Strong Case for including Cosmic-Ray-induced Chemistry in Atmospheric Biosignature Studies
• Evolution and Spectral Response of a Steam Atmosphere for Early Earth with a coupled climate-interior model
• Effect of O3 on the atmospheric temperature structure of early Mars
• The effect of varying atmospheric pressure upon habitability and biosignatures of Earth-like planets

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• 1D atmospheric study of the temperate sub-Neptune K2-18b
• Distinguishing oceans of water from magma on mini-Neptune K2-18b
• Unveiling shrouded oceans on temperate sub-Neptunes via transit signatures of solubility equilibria vs. gas thermochemistry
• How to identify exoplanet surfaces using atmospheric trace species in hydrogen-dominated atmospheres
• Formation and dynamics of water clouds on temperate sub-Neptunes: The example of K2-18b
• A Heavy Molecular Weight Atmosphere for the Super-Earth π Men c
• On the Temperature Profiles and Emission Spectra of Mini-Neptune Atmospheres
• The runaway greenhouse on subNeptune waterworlds
• Distinguishing between wet and dry atmospheres of TRAPPIST-1 e and f
• Effect of mantle oxidation state and escape upon the evolution of Earth's magma ocean atmosphere
• Chemical mapping of temperate sub-Neptune atmospheres: Constraining the deep-interior H2O/H2 using the atmospheric CO2/CH4
• Detectability of biosignatures on LHS 1140 b
• Detecting Biosignatures in the Atmospheres of Gas Dwarf Planets with the James Webb Space Telescope
• Prebiosignature Molecules Can Be Detected in Temperate Exoplanet Atmospheres with JWST
• Redox state and interior structure control on the long-term habitability of stagnant-lid planets
• Atmospheric dynamics of temperate sub-Neptunes. Part I: dry dynamics
• Radiative Transfer and Inversion codes for characterizing planetary atmospheres: an overview
• Modeling the astrosphere of LHS~1140
• Influence of Biomass Emissions upon Habitability, Biosignatures and Detectability in Earth-like Atmospheres
• Inferring Shallow Surfaces on sub-Neptune Exoplanets with JWST
• A joint effort to discover and characterize two resonant mini Neptunes around TOI-1803 with TESS, HARPS-N and CHEOPS
• HyDRo: Atmospheric Retrieval of Rocky Exoplanets in Thermal Emission
• The effect of a biosphere on the habitable timespan of stagnant-lid planets and implications for the atmospheric spectrum