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

We present an experimental study of the absorption, between 40 and 640 cm(-1), by CO2, CH4 and H-2 gases as well as by H-2 + 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 mu m 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 + CO2 and CH4 + CO2 and the low frequency wing of the pure CO2 15 mu m 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 + CO2 and CH4 + CO2 CIAs are significantly stronger in the 50-550 cm(-1) region than those of H-2 + N-2 and CH4 + N-2, 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 mu m 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.