Far infrared measurements of absorptions by CH4+CO2 and H2+CO2 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 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 152 m pathlength, 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 \(\mu\)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\(_2\)+CO\(_2\) and CH\(_4\)+CO\(_2\) and the low frequency wing of the pure CO\(_2\) 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\(_2\)+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 \(\chi\)-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.