SpeCT: A state-of-the-art tool to calculate correlated-k tables and continua of CO₂-H₂O-N₂ gas mixtures

¹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
² Life in the Universe Center, Geneva, Switzerland
³ Laboratoire de Météorologie Dynamique/IPSL, CNRS, Sorbonne Université, École Normale Supérieure, PSL Research University, École Polytechnique, Institut Poytechnique de Paris, 75005 Paris, France
⁴ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁵ Univ. Grenoble Alpes, CNRS, LIPhy, 38000 Grenoble, France
⁶ Observatoire astronomique de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland

^★ Corresponding author: guillaume.chaverot@univ-grenoble-alpes.fr

Received: 31 May 2025
Accepted: 14 August 2025

Abstract

A key challenge in modeling (exo)planetary atmospheres lies in generating extensive opacity datasets that cover the wide variety of possible atmospheric composition, pressure, and temperature conditions. This critical step requires specific knowledge and can be considerably time-consuming. To circumvent this issue, most available codes approximate the total opacity by summing the contributions of individual molecular species during the radiative transfer calculation. This approach neglects inter-species interactions, which can be an issue for precisely estimating the climate of planets. To produce accurate opacity data, such as correlated-k tables, χ factor corrections of the far wings of the line profile are required. We propose an update of the χ factors of CO₂ absorption lines that are relevant for terrestrial planets (pure CO₂, CO₂-N₂, and CO₂-H₂O). These new factors are already implemented in an original user-friendly open-source tool, named SpeCT, designed to calculate high-resolution spectra. This tool produces correlated-k tables for mixtures made of H₂O, CO₂, and N₂, and accounts for inter-species broadening. In order to facilitate future updates of these χ factors, we also provide a review of all the relevant laboratory measurements available in the literature for the considered mixtures. Finally, we provide in this work eight different correlated-k tables and continua for pure CO₂, CO₂-N₂, CO₂-H₂O, and CO₂-H₂O-N₂ mixtures based on the MT_CKD formalism (for H₂O), and calculated using SpeCT. These opacity data can be used to study various planets and atmospheric conditions, such as Earth’s paleo-climates, Mars, Venus, Magma ocean exoplanets, and telluric exoplanets.