An empirical line list for methane in the 1.26-1.71 μm region for planetary investigations (T = 80-300 K). Application to Titan
Résumé
Insufficient knowledge of the near infrared spectrum of methane is an important limitation for the analysis of the spectra of Titan and of the outer planetary atmospheres in general. The work reported here is the result of a long-term project aiming to provide astronomers with a line by line list for precise calculations of the methane absorption in the near infrared region. We thus present here our best to date empirical line list between 5854 and 7919 cm-1 (1.71-1.26 μm) and apply it to Titan, demonstrating its capability to significantly improve planetary spectral analysis. In recent contributions, we have obtained empirical line lists at room temperature and at 80 K (suitable for Titan conditions) from spectra recorded by (i) Differential Absorption Spectroscopy (DAS) in the high energy part of the tetradecad (5854-6195 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). In this work, we construct the global line lists for methane in "natural" isotopic abundance, covering the entire spectral region from 5854 to 7919 cm-1. These WKMC (for Wang, Kassi, Mondelain, Campargue) empirical lists include 42,988 and 46,320 lines at 80 ± 3 K and 296 ± 3 K, respectively and are assembled here with some important improvements: New spectroscopic parameters for the 5854-6148 cm-1 region at 80 K which increase significantly the number of observations and lower state energy determinations. Transitions of 13CH4 and CH3D were systematically identified by comparison with DAS spectra of highly enriched 13CH4 and CH3D, recorded at the same temperatures. In the 1.58 μm transparency window where CH3D lines contribute importantly to the methane spectrum at 80 K, the set of CH3D lower state energy values was completed by using recent DAS results for pure CH3D. The "two temperature method" provided lower state energy values for about 24,000 transitions from the ratios of their line intensities at 80 K and 296 K. The clear propensity of the derived low J values of 12CH4 and 13CH4 to be integer illustrates the quality of the lower state energy values. The obtained data sets allow us to account for most of the temperature dependence of the absorption over the considered region. To illustrate the interest of the WKMC line lists for planetary applications, we perform simulations of Titan spectra at different resolutions taken from the ground with instruments such as the FTS at the CFHT and CRIRES at the VLT or by the DISR instrument on board the Huygens probe. The agreement between the simulations and the observations clearly demonstrates an important improvement with respect to previous works.