Infrared absorption continua: Laboratory studies, calculations and applications to the modeling of the atmospheres of telluric planets – COMPLEAT

The measurement of weak absorption continua is challenging as it shows up as a small variation of the baseline spectra when the studied gas is injected in the absorption cell. Although limited in spectral coverage, the cavity enhanced absorption spectroscopy (CEAS) techniques are particularly well suited. Cavity ring down spectroscopy (CRDS) and optical-feedback (OF-) CEAS techniques will be used to accurately measure the H2O-CO2 and H2-CO2 absorption continua in the mid- and near-IR. In the far infrared, where absorption continua are stronger, the FTS technique associated with long path absorption cells will be employed.
Concerning modelling, direct predictions will be first carried for each absorption of interest based on literature values for the needed input parameters. Their results will be compared to the available measurements. We will then tune (within reasonable ranges) some of the input parameters for the best agreement with the measurements. Calculations will then be made over the temperature and wavelength regions relevant for planetary studies and the results of the various models will be combined, keeping the results of a given model in the regions where it does “the best job”. Look-up tables of absorption coefficients and/or simple analytical empirical representations will then be created and implemented in 3-dimensional numerical climate simulations performed with the LMD Generic Model, to conduct three main investigations:
- exploring how reducing atmospheres may have affected the environment of early Mars,
- improving our understanding of the so-called “magma ocean” stage of the telluric planets,
- exploring how continua will affect the climate state and observability of the atmosphere of nearby terrestrial exoplanets with a focus on the TRAPPIST-1 system

A temperature-regulated high-finesse cell has been developed. This cell can record CRDS spectra at temperatures between 240K and 350K with a noise level of ~ 10-10 cm-1 and a baseline stability of 5 × 10-10 cm-1.
Several campaigns to measure the absorption induced by CH4 + CO2 mixtures in the far infrared (50-650 cm-1) were carried out by Fourier transform spectrometry (FTS). The results obtained provided data available in [1,2]. Very recently, new measurements were taken between 700 and 3500 cm-1, the results of which are being analyzed.
Spectra of CO2 + H2 mixtures were recorded at room temperature by CRDS and OF-CEAS, between 2.12-2.35 µm. The binary coefficients BCO2-H2 + BH2-CO2 were extracted and compared to the values ??provided by a semi-empirical model. This comparison shows the limits of the model, which is the only one available [4].
The absorption induced by H2 + CO2 mixtures was also studied by FTS. The results obtained have been published in [1,2]. Very recently, new measurements were made between 700 and 1400 cm-1, the results of which are being analyzed.
Humidified CO2 spectra were recorded at room temperature by CRDS and OF-CEAS between 2.20-2.35 µm, around 3.50 µm and between 1.68-1.75 µm. The binary coefficients BCO2-H2O + BH2O-CO2 were extracted and compared to the only models available based on sub-Lorentz line profiles. The calculated binary coefficients are underestimated by factors between 2 and 5 compared to our experimental data.
The absorption by H2O + CO2 mixtures has also been studied by FTS between 50 and 650 cm-1, from which we were able to extract the very first values ??of the H2O continuum widened by CO2 in the far infrared [3]. Very recently, new measurements were taken between 700 and 3500 cm-1, the results of which are being analyzed.

Concerning the theoretical approaches and models for calculations of the continua, several benefits are expected from this project. For those which have been developed in the past, testing them further for the (yet unstudied) molecular systems that we will here plan to investigate is obviously of interest in order to further assess their strengths and weaknesses. For the very recently developed computations of the far wings using molecular dynamics simulations, applying them to complex systems such as H2O and CH4 in CO2 is a challenge. It is likely that we will learn a lot in this exercise with probable (but yet unpredictable) advances in this field.
Results related to the investigation of the effect of continua on planetary atmospheres are likely to bring significant contributions to our understanding of planetary atmospheres and habitability in the Solar System and beyond: first, we will learn whether or not early Mars may have been warmed by reducing gases, which will have important implication for the understanding of Mars history and Mars past and present habitability. Second, we will improve our knowledge on the duration, evolution and observational fingerprints of magma ocean planets. This will provide very important information on the very early evolution of all Solar System telluric planets, and help us to assess the possibility to detect such magma ocean planets around other stars. Last, we will better constrain the possibility and even strategies to characterize the atmosphere of nearby, temperate, terrestrial-size planets with forthcoming telescopes such as JWST. This characterization is of prime importance because it has the potential to revolution all what we know about the evolution, the atmosphere and habitability of terrestrial worlds.

[1] M. Turbet, H. Tran, O. Pirali, F. Forget, C. Boulet, J.-M. Hartmann, Far infrared measurements of absorptions by CH4?+?CO2 and H2?+?CO2 mixtures and implications for greenhouse warming on early Mars, Icarus 321, 189-199 (2019).
[2] M. Turbet, C. Boulet, T. Karman, Measurements and semi-empirical calculations of CO2?+?CH4 and CO2?+?H2 collision-induced absorption across a wide range of wavelengths and temperatures. Application for the prediction of early Mars surface temperature, Icarus 346, 113762 (2020).
[3] H. Tran, M. Turbet, S. Hanoufa, X. Landsheere, P. Chelin, Q. Ma, J.-M. Hartmann, The CO2–broadened H2O continuum in the 100–1500?cm-1 region: Measurements, predictions and empirical model, Journal of Quantitative Spectroscopy and Radiative Transfer 230, 75-80 (2019).
[4] H. Tran, P.-M. Flaud, T. Gabard, F. Hase, T. von Clarmann, C. Camy-Peyret, S. Payan, J.-M. Hartmann, Model, software and database for line-mixing effects in the ?3 and ?4 bands of CH4 and tests using laboratory and planetary measurements—I: N2 (and air) broadenings and the earth atmosphere, Journal of Quantitative Spectroscopy and Radiative Transfer 101, 284-305 (2006).
[5] D. Mondelain, C. Boulet, J.-M. Hartmann, The binary absorption coefficients for H2 + CO2 mixtures in the 2.12–2.35 µm spectral region determined by CRDS and by semi-empirical calculations, Journal of Quantitative Spectroscopy and Radiative Transfer 260, 107454 (2021).

The aim of this project is to characterize by experiment and theory the absorption continua of H2O-CO2, CH4-CO2 and H2-CO2 gas mixture in support of numerical climate simulations of CO2-enriched planetary atmospheres. At present, these continua are mostly unknown. State-of the-art measurements will be performed at different temperatures by recording spectra with FTS, CRDS and OF-CEAS techniques in the wings of the CO2 bands from the far to the short wave-infrared. CRDS and OF-CEAS techniques have proven their ability to measure weak continua of water vapour in transparency windows with much reduced uncertainties thanks to their high sensitivity and very stable spectra base line. In the frame of this project we will develop a temperature regulated high finesse cell that will be installed in the available CRDS spectrometers allowing studying H2-CO2 and H2O-CO2 continua in the short-wave infrared (SWIR) between 250 K and 350 K. A new laser source, emitting near 2853 cm-1, will be installed in the OF-CEAS instrument allowing studies of H2O-CO2 around this wavenumber between room temperature and 325 K. In parallel, mid-IR as well as far-IR spectra will be recorded using the 150 m multi-pass absorption cell associated to the Fourier Transform spectrometer located on the AILES beam line of synchrotron SOLEIL. This cell offers the possibility to be heated up to 330 K. In the project, we will replace the current polypropyle films windows by wedged diamond windows to improve significantly the baseline stability of the set-up when dealing with relatively high pressures (in the order of 1 atm). Using this set-up, we will perform cutting-edge recordings of H2-CO2 , CH4-CO2 and H2O-CO2 in the far-IR and mid-IR ranges at room temperature and 330 K. At the end of the experimental research action, accurate data sets will be available for validation tests of the theoretical models developed at LMD, the calculated continua having the advantage to cover a wide variety of spectral and temperature conditions. For this, several theoretical approaches, previously developed and tested for molecular systems (e.g., CH4-N2) close to those of interest here, will be used. The input parameters of these models will be adjusted for the best agreement with the available measurements. Then, accurate predictions tools and absorption spectra (absorption coefficient look-up tables) will be built-up for the wavelength and temperatures ranges relevant for the targeted planetary applications. The obtained H2O-CO2, CH4-CO2 and H2-CO2 continua will be implemented in a 3-D Planetary Global Climate Model, for three distinct applications: first, to model the climate of early Mars to understand the conditions in which the enigmatic valley networks and lakes were carved; then, to simulate the evolution of magma ocean planets, which is a crucial step to understand why Earth and Venus evolved so differently; last, to produce synthetic observables of nearby Earth-size exoplanets - at all stages of their evolution - to investigate the possibility to characterize the atmosphere of these worlds with forthcoming astronomical observatories.