Measurements and semi-empirical calculations of CO₂ + CH₄ and CO₂ + H₂ collision-induced absorption across a wide range of wavelengths and temperatures. Application for the prediction of early Mars surface temperature

Reducing atmospheres have recently emerged as a promising scenario to warm the surface of early Mars enough to drive the formation of valley networks and other ancient aqueous features that have been detected so far on the surface of Mars. Here we present a series of experiments and calculations to better constrain CO₂ + CH₄ and CO₂ + H₂ collision-induced absorptions (CIAs) as well as their effect on the prediction of early Mars surface temperature. First, we carried out a new set of experimental measurements (using the AILES line of the SOLEIL synchrotron) of both CO₂ + CH₄ and CO₂ + H₂ CIAs. These measurements confirm the previous results of Turbet et al. (2019), Icarus vol. 321, while significantly reducing the experimental uncertainties. Secondly, we fitted a semi-empirical model to these CIAs measurements, allowing us to compute the CO₂ + CH₄ and CO₂ + H₂ CIAs across a broad spectral domain (0–1500 cm⁻¹) and for a wide range of temperatures (100–600 K). Last, we performed 1-D numerical radiative-convective climate calculations (using the LMD Generic Model) to compute the surface temperature expected on the surface of early Mars for several CO₂, CH₄ and H₂ atmospheric contents, taking into account the radiative effect of these revised CIAs. These calculations demonstrate that thick CO₂ + H₂-dominated atmospheres remain a viable solution for warming the surface of Mars above the melting point of water, but not CO₂ + CH₄-dominated atmospheres. Our calculated CO₂ + CH₄ and CO₂ + H₂ CIA spectra and predicted early Mars surface temperatures are provided to the community for future uses.