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Home > Teams > LAsers, Molecules and Environment > Themes > Ultra-sensitive absorption spectroscopy

Spectroscopy by CRDS of the methane in the near infrared region

published on , updated on

 

 

Alain CAMPARGUE
Samir KASSI
Didier MONDELAIN

 

 

The insufficient knowledge of the methane spectrum at low temperature was, during a long time, a major limitation to the spectral data analysis for some planets. On the basis of laboratory spectra recorded using high sensitivity laser techniques; the LAME team has recently managed to build an absorption line list for methane (CH4) in the 1.26-1.71 µm range. The WKMC (Wang Kassi, Mondelain & Campargue) allows simulating the methane spectrum between 80 and 300 K. It is about to become the reference list for planetology applications and it has already been applied with success to Titan, Pluto, Uranus, Saturn and Jupiter.

The context
In the framework of a project funded by the Agence Nationale pour la Recherche (ANR CH4@Titan), we collaborate with the planetologues from the LESIA involved in the Cassini-Huygens mission to study Saturn and Titan. Methane is one of the main infrared absorbers in the spectra of the atmospheres of Titan and of the giant planets (Saturn, Jupiter,…). For example, Titan’s atmosphere is composed of 2-5% of methane in nitrogen at a pressure of 1.5 bar and temperatures between 70 and 115 K in the troposphere and up to 180-200 K in the stratosphere.


In the absence of satisfying theoretical models for the methane spectrum, accurate and exhaustive experimental data at low temperature are essential to study these atmospheres. Transparency windows corresponding to the weakest absorption regions between the strong absorption bands of methane (see Fig. below), are of particular interest to determine the low atmosphere composition and the surface albedo of these planets.

 Titan's spectra recorded with the ULIS instrument (looking upward) of DISR at different altitudes. The solar light is more and more absorbed in the methane absorption bands as the instrument goes down whereas the windows around 930, 1070, 1290 and 1580 nm remain relatively transparent (Tomasko et al. 2008).
Titan’s spectra recorded with the ULIS instrument (looking upward) of DISR at different altitudes. The solar light is more and more absorbed in the methane absorption bands as the instrument goes down whereas the windows around 930, 1070, 1290 and 1580 nm remain relatively transparent (Tomasko et al. 2008).

Our goal in the frame of the ANR CH4@Titan project is to provide astronomers with a complete list for methane applicable in a large range of temperature conditions from 80 to 300 K. We have recently completed the WKMC (Wang, Kassi, Mondelain, Campargue) lists at 80\pm3 K and 296\pm3 K for methane in “natural” isotopic abundance in the 5852-7919 cm-1 region. The positions and intensities included in the WKMC lists were obtained from spectra recorded at 80 K and room temperature by:
(i) differential absorption spectroscopy (DAS) in the high energy part of the tetradecad (5852-6195 cm-1) and in the icosad (6717-7589 cm-1);
(ii) high sensitivity CW-Cavity Ring Down Spectroscopy (CRDS) in the 1.58 µm and 1.28 µm transparency windows (6165-6750 cm-1 and 7541-7919 cm-1, respectively).

WKLMC line lists
Due to the systematic line overlapping, the construction of the WKMC list was an extremely laborious and difficult task. The line profile of more than 90000 lines was adjusted line by line in order to reproduce the measured spectra and derive the position and intensities at 296 K and 80 K. A Voigt profile was adjusted for each line. Two lists including about 45000 lines each were constructed for the entire 5852-7920 cm-1 region : the WKMC@80K et WKMC@296K lists. They are provided as Supplementary Material attached to the publication:
- Refinements of the WKMC empirical line lists (5852-7919 cm-1) for methane between 80 K and 296 K

  • Author(s): Campargue, A.; Leshchishina, O.; Wang, L.; Mondelain, D.; Kassi, S.; Nikitin, A.V.
  • Source: JOURNAL OF QUANTITATIVE SPECTROSCOPY & RADIATIVE TRANSFER
  • Volume: 113 Issue: 15
  • Pages: 1855-1873
  • DOI: 10.1016/j.jqsrt.2012.05.011
  • Published: OCT 2012
 Fig. 2. WKLMC line lists
Fig. 2. WKLMC line lists

CH4 and CH3D were recorded by differential absorption spectroscopy, in order to identify the lines of these two isotopologues present in natural abundance in the methane sample. A major result of this work is that, in spite of its low relative abundance (5x10-4), CH3D is the major absorber in natural methane in the 1.58 µm transparency window. This is very important because CH3D relative abundance varies importantly according to the studied atmosphere (a factor of 6 for Jupiter) and the absorption lines must be scaled according to this abundance.

 Overview of the WKMC line lists for methane at 80 K and 296 K where transitions the 13CH4 (blue) and CH3D (orange) isotopologues have been underlined.
Overview of the WKMC line lists for methane at 80 K and 296 K where transitions the 13CH4 (blue) and CH3D (orange) isotopologues have been underlined.

Applications of the WKMC lists: the case of Titan (collaboration with LESIA-Meudon)
The obtained WKMC list at 80 K has already been successfully applied to a large range of temperature conditions existing on Titan, Pluto, Uranus, Saturn and Jupiter. As an example , we present a comparison performed by our partners at LESIA-Meudon, of the high resolution spectrum recorded from the Earth with the CRIRES instrument to a simulation of the atmosphere of Titan based on our line list at 80 K. The figure below shows the very good agreement obtained between the two spectra. All the observed absorption features are well reproduced, indicating that methane is the only important absorber in the considered region.

Titan’s spectrum observed by the CRIRES instrument through the Earth atmosphere and its modelization including the methane line (from WKMC list). A zoom of one part of the spectrum is presented below.


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