Experimental measurements of CO2+CH4 and CO2+H2 far-infrared absorptions

Understanding how the early Martian climate could have been warm enough for liquid water to flow on the surface remains one of the major enigmas of planetary and no consensus scenario has yet been reached. Thick, CO2-dominated atmospheres do not provide the necessary greenhouse effect to warm the surface of early Mars above the melting point of water.

In addition to CO2, reducing gases (CH4, H2) could have accumulated in the atmosphere of early Mars through (i) volcanic outgassing by a reducing mantle, (ii) delivery by a large comet impacting Mars, (iii) release of clathrate hydrates or (iv) serpentinization.

In reducing atmosphere conditions, absorptions can arise from the Collision-Induced Absorptions (CIAs; CIAs are produced by multipolar absorptions produced during a collision between two molecules) of H2-CO2 and CH4-CO2 pairs. Such absorptions can potentially absorb in the 15-50 microns region corresponding to a carbon dioxide spectral window and thus produce a strong greenhouse warming in a CO2-dominated atmosphere. Up to now, the modelings of these two contributions were based, due to lack of relevant data, on the CIAs of H2-N2 and CH4-N2 pairs, respectively. Wordsworth et al. 2017 recently calculated that the CIAs by H2+CO2 and CH4+CO2 mixtures should be significantly stronger than those for H2+N2 and CH4+N2, respectively.

(A) Experimental setup of the AILES experiment located at the SOLEIL synchrotron facility, used to measure the CO2+H2 and CO2+CH4 far-infrared collision-induced absorptions. (B) Comparison between the CO2+H2 experimental CIA (Turbet et al. 2019, Icarus), CO2+H2 theoretical CIA (Wordsworth et al. 2017) and the N2+H2 CIA (from HITRAN) used in Ramirez et al. 2014. Credit: M. Turbet et al.

We performed a series of experiments (at the SOLEIL synchrotron, using the AILES experimental setup; see Figure A) to measure far-infrared CIAs of CO2+H2 and CO2+CH4 mixtures (Turbet et al. 2019a). We confirmed experimentally the theoretical prediction of Wordsworth et al. 2017 that CO2+H2 (and CO2+CH4) CIAs are significantly stronger than those of N2+H2 (and N2+CH4), as illustrated in Figure B. This demonstrates that these CIAs could have produced a strong greenhouse effect in early Mars atmosphere.

CO2 line mixing effects on early Mars and exoplanets.

Modeling the entire CO2 spectrum, including both the regions near the lines centers and the far wings, is an extremely difficult task for which no rigorous model is available so far.

A measured CO2 spectrum can be significantly different from that calculated with the usual Lorentz (or Voigt) profile, due to the line mixing effect. This effect is associated with the collisional transfers of rotational populations between absorption lines. It modifies the shape of clusters of closely spaced lines and results in transfers of absorption from the band wing region to the band center, leading to the strongly sub-Lorentzian behavior observed in CO2 band wings.

Here we modeled line mixing effects in the far wings of CO2 bands using two different approaches: (1) using an empirical correction of the Lorentzian shape adjusted to laboratory measurements of the absorption in some band wings (e.g. Perrin and Hartmann 1989). (2) using a line mixing model (e.g. Tran et al. 2011).

We show that early Mars surface temperatures calculated when using a proper line mixing model (i.e. accounting for CO2 foreign broadening, instead of air-broadening, as done in Ozak et al. 2016) are very close to those obtained from spectra calculations based on an empirical correction of the Lorentzian shape. This good match comes from the good agreement between the two calculations above 20 microns, in the atmospheric window of a CO2-rich early Mars.

Pure CO2 spectrum at 10bar and 250~K calculated using the two different aforementionned approaches. Credit: M. Turbet.

However, we also found that the two models can produce significant differences in absorption in other spectral regions (e.g. around 2.5, 3.5 and 6 microns). This result has potentially huge implications for the detectability of thermal emission of warm CO2-rich exoplanets, using secondary eclipses or thermal phase curves techniques. We submitted the 'COMPLEAT' ANR proposal to obtain funding to measure experimentally absorptions in the far wings of CO2 lines.

By Martin Turbet | Design by Andreas Viklund | Inspired by Aymeric Spiga