Far infrared measurements of absorptions by CH4 + CO2 and H2 + CO2 mixtures and implications for greenhouse warming on early Mars

International audienceWe present an experimental study of the absorption, between 40 and 640 cm-1 , by CO 2 , CH 4 and H 2 gases as well as by H 2 +CO 2 and CH 4 +CO 2 mixtures at room temperature. A Fourier transform spectrometer associated to a multi-pass cell, whose optics were adjusted to obtain a 152m 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 CO 2 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 H 2 +CO 2 and CH 4 +CO 2 and the low frequency wing of the pure CO 2 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 H 2 +CO 2 and CH 4 +CO 2 CIAs are significantly stronger in the 50-550 cm-1 region than those of H 2 +N 2 and CH 4 +N 2 , respectively. However, we find that the shape and the strength of these recorded CIAs differ from the aforementioned predictions. For the pure CO 2 line-wings, we show that both the χ-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