Elsevier

Icarus

Volume 303, 15 March 2018, Pages 114-130
Icarus

New accurate theoretical line lists of 12CH4 and 13CH4 in the 0–13400 cm1 range: Application to the modeling of methane absorption in Titan’s atmosphere

https://doi.org/10.1016/j.icarus.2017.12.045Get rights and content

Highlights

  • Ab initio calculations.

  • Methane (12CH4 and 13CH4).

  • Theoretical line lists.

  • TheoReTS database.

  • Titan atmosphere.

Abstract

The spectrum of methane is very important for the analysis and modeling of Titan’s atmosphere but its insufficient knowledge in the near infrared, with the absence of reliable absorption coefficients, is an important limitation. In order to help the astronomer community for analyzing high-quality spectra, we report in the present work the first accurate theoretical methane line lists (T = 50–350 K) of 12CH4 and 13CH4 up to 13400 cm1 ( > 0.75 µm). These lists are built from extensive variational calculations using our recent ab initio potential and dipole moment surfaces and will be freely accessible via the TheoReTS information system (http://theorets.univ-reims.fr, http://theorets.tsu.ru). Validation of these lists is presented throughout the present paper. For the sample of lines where upper energies were available from published analyses of experimental laboratory 12CH4 spectra, small empirical corrections in positions were introduced that could be useful for future high-resolution applications. We finally apply the TheoRetS line list to model Titan spectra as observed by VIMS and by DISR, respectively onboard Cassini and Huygens. These data are used to check that the TheoReTS line lists are able to model observations. We also make comparisons with other experimental or theoretical line lists. It appears that TheoRetS gives very reliable results better than ExoMol and even than HITRAN2012, except around 1.6 µm where it gives very similar results. We conclude that TheoReTS is suitable to be used for the modeling of planetary radiative transfer and photometry. A re-analysis of spectra recorded by the DISR instrument during the descent of the Huygens probe suggests that the CH4 mixing ratio decreases with altitude in Titan’s stratosphere, reaching a value of 102 above the 110 km altitude.

Introduction

Precise knowledge of the methane absorption in the study of planetary objects like Uranus and Neptune, and in particular of Titan (Saturn’s largest satellite, whose atmosphere is mainly composed of nitrogen and methane), is very timely and of great importance because it gives access to the determination of the physical properties of these objects. With the advent of the highly-successful Cassini–Huygens mission, in the Saturnian system from July 2004 to September 2017, a large amount of Titan data has been acquired. However, in the absence of reliable absorption coefficients of methane for Titan conditions in the near infrared (especially between 0.8 and 5 µm), the astronomer community faces the difficulty of analysing the high-quality spectra and images that have been collected at these wavelengths. After the landing of the Huygens probe on Titan (14 Jan. 2005), the Cassini spacecraft has been pursuing its trek in the Saturnian system and continued to do so until September 2017, providing new discoveries often enough while, at the same time, raising new questions. But even if the Cassini–Huygens mission has been on location for more than 13 years, Titan is far from having revealed all its secrets. Among other, we still lack a precise description of the lower atmosphere and surface of the satellite. That is where the methane coefficients play a very important role: the modeling of the troposphere and surface requires a precise understanding of the methane opacity, CH4 being the main atmospheric absorbing constituent in the near-infrared.

In the past years there have been attempts to remedy this problem with several experimental and theoretical studies. The opacity from methane and from its isotopologues has thus been retrieved in some near-IR regions from laboratory measurements, theoretical calculations, and empirical models as described in molecular line databases like HITRAN (Rothman et al., 2013) and GEISA (Jacquinet-Husson et al., 2016) and as reviewed in Brown et al. (2013), Campargue, Wang, Mondelain, Kassi, Bézard, Lellouch, Coustenis, Bergh, Hirtzig, Drossart, 2012, Campargue, Leshchishina, Wang, Mondelain, Kassi, 2013, Campargue, Béguier, Zbiri, Mondelain, Kassi, Karlovets, Nikitin, et al., 2016. Simulations based on these new parameters have produced fits to Titan data from the ground and from space, allowing for the extraction of important new information on Titan’s atmosphere and surface (De Bergh, Courtin, Bézard, Coustenis, Lellouch, Hirtzig, Rannou, et al., 2012, Hirtzig, Bézard, Lellouch, Coustenis, De Bergh, Drossart, Campargue, et al., 2013, Solomonidou, Hirtzig, Coustenis, Bratsolis, Le Mouélic, Rodriguez, Stephan, Drossart, et al., 2014, Solomonidou, Coustenis, Hirtzig, Rodriguez, Stephan, Lopes, Drossart, et al., 2016).

Empirical effective spectroscopic models proved to be efficient in line-by-line analyses for low cold bands in energy regions (say  <  7000 cm1) but fail to provide reliable predictions higher. This is due to numerous resonances and a large number of unknown parameters to be adjusted. Another drawback of the empirical effective approach is that every isotopic species is considered as a separate molecule and no accurate links among line parameters of different isotopologues was established. The spectra of the most abundant isotopic species (‘mother’ molecule) are generally better known. Consequently, the available information on methane isotopic spectra (e.g. those of 13CH4) is much poorer than for 12CH4. Global variational calculations offer another way for computing energies, line positions and intensities from ab initio potential energy surfaces (PESs) and dipole moment surfaces (DMSs). They inherently account for all resonance coupling terms in a wide spectral range. Moreover, they are isotopically self-consistent as the same calculations can be performed for both the mother molecule and the minor isotopologues, with a similar accuracy. In the framework of the TheoReTS project (Rey et al., 2016b), we report here new global calculations of rovibrational spectra and dipole transition intensities for 12CH4 and 13CH4 up to 13400 cm1 using our recent ab initio dipole moment and potential surfaces (Nikitin, Rey, Tyuterev, 2011, Nikitin, Rey, Tyuterev, 2017). The corresponding line lists are used to model Titan’s atmosphere. Atmospheric transmittance ratios at different altitudes of Titan’s stratosphere as derived from Huygens/DISR measurements are studied. The TheoReTS database are compared to the HITRAN2012 (Rothman et al., 2013) and ExoMol 10to10 (Yurchenko and Tennyson, 2014) line lists as well as to the band model of Karkoschka and Tomasko (2010) in a retrieval exercise with VIMS data onboard Cassini.

Section snippets

Modeling of methane spectra

The computation of the entire set of molecular quantum states and corresponding transitions in the infrared (IR) for medium-sized molecules such as methane is an extremely challenging issue. In a general manner, the modeling of rotation-vibration spectra of polyatomic molecules is very demanding and requires the development of efficient theoretical methods for computing both line positions and line intensities. Except for few simple cases, the quantum mechanical problems are not exactly

First step: variational calculations

For the present work, our previous room temperature methane spectrum calculations (Rey et al., 2013a) have been extended and improved. The theoretical line-by-line 12CH4 and 13CH4 lists at temperatures 50–350 K are constructed here in the range 0–13400 cm1 applicable for the terrestrial atmosphere and for cold planetary atmospheres (Titan, Saturn, Jupiter, ...). The lists for higher temperature astrophysical applications specific for exoplanets and cold stars for the same spectral is discussed

Validation versus the HITRAN empirical compilation

The HITRAN database contains a large set of accurate spectroscopic room-temperature data, allowing a validation of our theoretical predictions. Figs. 2 and 3 compare the entire HITRAN spectrum (in black) for 12CH4 and 13CH4 with our theoretical lists (in red). HITRAN2012 contains 337 000 and 72 000 lines of 12CH4 and 13CH4, respectively. It is clearly seen that our lists are more complete for high energy regions, particularly for 13CH4. Apart from some missing lines and/or sparse regions in

Quasi-continuum contribution to the opacity

In all problems of molecular spectroscopy involving millions of lines (or even billions of transitions in hot spectrum predictions, see for example Rey, Nikitin, Tyuterev, 2014b, Yurchenko, Tennyson, 2014, Rey, Nikitin, Tyuterev, 2017) of lines, one key point is to choose an appropriate intensity cut-off for practical applications. In case of methane this is not a real problem for the first five polyads for temperatures up to 296 K since the spectrum is dominated by more or less strong

Comparisons between TheoReTS and other linelists in a retrieval exercise with VIMS onboard Cassini

The spectrum of the outgoing signal on Titan depends on atmosphere (that is, gases, aerosols and condensate particles) scattering and absorption properties and, to a smaller extent, on the surface albedo. With a good knowledge of the atmosphere properties, one can expect to reproduce the observed signal and then to retrieve information about the atmosphere structure and the surface properties with a good accuracy. In this part, we use the intensity observed with VIMS (Cassini Visual Infrared

Comparison with Huygens/DISR measurements

During its descent to Titan’s surface on 14 January 2005, the Descent Imager/Spectral Radiometer (DISR) aboard the Huygens probe recorded spectra of the ambient radiation in the visible (0.48–0.96 µm) and IR (0.85–1.7 µm) ranges (Tomasko et al., 2005). In particular, the Upward-Looking Infrared Spectrometer (ULIS) measured the attenuation of the sunlight due to methane gas and aerosols from 141 km down to the surface. While these measurements can be used to measure the vertical profile of CH4

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