An empirical line list for methane in the 1.26-1.71 渭m region for planetary investigations (T = 80-300 K). Application to Titan

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• 作者：Alain Campargue^a ; ^{Alain.Campargue@ujf-grenoble.fr} ; Le Wang^a ; Didier Mondelain^a ; Samir Kassi^a ; Bruno Bé ; zard^b ; Emmanuel Lellouch^b ; Athena Coustenis^b ; Catherine de Bergh^b ; Mathieu Hirtzig^b ; Pierre Drossart^b
• 关键词：Titan ; atmosphere ; Spectroscopy ; Infrared observations
• 刊名：Icarus
• 出版年：2012
• 期刊代码：90_00191035
• 类别：ph
• 出版时间：May, 2012
• 卷：219
• 期：1
• 页码：110-128
• 文件大小：2899 K

摘要

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^鈭? (1.71-1.26 渭m) 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^鈭?) and in the icosad (6717-7589 cm^鈭?) (ii) high sensitivity CW-Cavity Ring Down Spectroscopy (CRDS) in the 1.58 渭m and 1.28 渭m transparency windows (6165-6750 cm^鈭? and 7541-7919 cm^鈭?, respectively). In this work, we construct the global line lists for methane in 鈥渘atural鈥?isotopic abundance, covering the entire spectral region from 5854 to 7919 cm^鈭?. These WKMC (for Wang, Kassi, Mondelain, Campargue) empirical lists include 42,988 and 46,320 lines at 80 卤 3 K and 296 卤 3 K, respectively and are assembled here with some important improvements:

(i)

New spectroscopic parameters for the 5854-6148 cm^鈭? region at 80 K which increase significantly the number of observations and lower state energy determinations.

(ii)

Transitions of ¹³CH₄ and CH₃D were systematically identified by comparison with DAS spectra of highly enriched ¹³CH₄ and CH₃D, recorded at the same temperatures.

(iii)

In the 1.58 渭m 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 鈥渢wo temperature method鈥?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.