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

Martin Turbet (1) , Ha Tran (1) , Olivier Pirali (2, 3) , François Forget (1) , Christian Boulet (2) , Jean-Michel Hartmann (1)

1 LMD - Laboratoire de Météorologie Dynamique (UMR 8539)
2 ISMO - Institut des Sciences Moléculaires d'Orsay
3 SSOLEIL - Synchrotron SOLEIL

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 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.

Mots clés: methane; climate; collision induced absorptions; hydrogen; Mars; spectroscopy; measurement

Domaines: Physique Atmosphérique et Océanique [physics.ao-ph]; Climatologie; Chimie; Planète et Univers [physics]

Martin Turbet, Ha Tran, Olivier Pirali, François Forget, Christian Boulet, et al.. Far infrared measurements of absorptions by CH4 + CO2 and H2 + CO2 mixtures and implications for greenhouse warming on early Mars. Icarus, 2019, 321, pp.189-199. ⟨10.1016/j.icarus.2018.11.021⟩. ⟨hal-02181726⟩