Far infrared measurements of absorptions by CH$_4 +$ CO$_2$ and H$_2 +$ CO$_2$ mixtures and implications for greenhouse warming on early Mars

Abstract

We present an experimental study of the absorption, between 40 and 640 cm^-1, by CO₂, CH₄ and H₂ gases as well as by H₂ + CO₂ and CH₄ + CO₂ mixtures at room temperature. A Fourier transform spectrometer associated to a multi-pass cell, whose optics were adjusted to obtain a 152 m path length, were used to record transmission spectra at total pressures up to about 0.98 bar. These measurements provide information concerning the collision-induced absorption (CIA) bands as well as about the wing of the CO₂ 15 μm band. Our results for the CIAs of pure gases are, within uncertainties, in agreement with previous determinations, validating our experimental and data analysis procedures. We then consider the CIAs by H₂ + CO₂ and CH₄ + CO₂ and the low frequency wing of the pure CO₂ 15 μm band, for which there are, to our knowledge, no previous measurements. We confirm experimentally the theoretical prediction of Wordsworth et al. (2017) that the H₂ + CO₂ and CH₄ + CO₂ CIAs are significantly stronger in the 50-550 cm^-1 region than those of H₂ + N₂ and CH₄ + N₂, respectively. However, we find that the shape and the strength of these recorded CIAs differ from the aforementioned predictions. For the pure CO₂ line-wings, we show that both the $\chi$-factor deduced from measurements near 4 μm and a line-mixing model very well describe the observed strongly sub-Lorentzian behavior in the 500-600 cm^-1 region. These experimental results open renewed perspectives for studies of the past climate of Mars and extrasolar analogues.

Olivier Pirali

Director of Research

My research interests includes high resolution molecular spectroscopy, laboratory astrophysics, and THz generation.