paper

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

arXiv:1805.02595 · doi:10.1016/j.icarus.2018.11.021

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 $I^¼$ 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 $I^¼$ 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 $I ¨ $ -factor deduced from measurements near 4 $I^¼$ 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.

Accepted for publication in Icarus. 22 pages, 7 figures, 2 tables