The near-IR spectrum of Titan modeled with an improved methane line list

Abstract

We have obtained spatially resolved spectra of Titan in the near-infrared J, H and K bands at a resolving power of ~5000 using the near-infrared integral field spectrometer (NIFS) on the Gemini North 8m telescope. Using recent data from the Cassini/Huygens mission on the atmospheric composition and surface and aerosol properties, we develop a multiple-scattering radiative transfer model for the Titan atmosphere. The Titan spectrum at these wavelengths is dominated by absorption due to methane with a series of strong absorption band systems separated by window regions where the surface of Titan can be seen. We use a line-by-line approach to derive the methane absorption coefficients. The methane spectrum is only accurately represented in standard line lists down to ~2.1 µm. However, by making use of recent laboratory data and modeling of the methane spectrum we are able to construct a new line list that can be used down to 1.3 µm. The new line list allows us to generate spectra that are a good match to the observations at all wavelengths longer than 1.3 µm and allow us to model regions, such as the 1.55 µm window that could not be studied usefully with previous line lists such as HITRAN 2008. We point out the importance of the far-wing line shape of strong methane lines in determining the shape of the methane windows. Line shapes with Lorentzian, and sub-Lorentzian regions are needed to match the shape of the windows, but different shape parameters are needed for the 1.55 µm and 2 µm windows. After the methane lines are modelled our observations are sensitive to additional absorptions, and we use the data in the 1.55 µm region to determine a D/H ratio of 1.77 ± 0.20 × 10 -4 , and a CO mixing ratio of 50 ± 11 ppmv. In the 2 µm window we detect absorption features that can be identified with the ν 5 +3ν 6 and 2ν 3 +2ν 6 bands of CH 3 D.

Warm ice giant GJ 3470b

“…5.3. All models are calculated following the steps outlined by Barman (2007), with the only exceptions being the use of more recent CH 4 line data (Sromovsky et al 2012;Bailey et al 2011) and new collision-induced opacities (Richard et al 2012). We plot the temperature-pressure profiles and vertical abundance profiles of several molecular species in Fig.…”

Intensities, broadening and narrowing parameters in the ν 3 band of methane

“…Methane is the second most important greenhouse gas in Earth's atmosphere after CO 2 [3] and its abundance with the altitude in our atmosphere, monitored near 3.4 µm, has been recently reported in [4][5][6]. Methane is one of the important constituents detected in the atmosphere of Saturn [7][8][9], Titan [10][11][12][13][14][15][16], Jupiter [17][18], Uranus [19][20], Mars [21][22] and Pluto [23]. Accurate knowledge of the spectroscopic parameters of CH 4 is needed for monitoring its abundance in Earth's atmosphere and for the retrieval of methane abundances in various planetary atmospheres.…”

Titan Science with the James Webb Space Telescope

“…Early measurements of the CO abundance in Titan's stratosphere ranged over a factor two in values (Hidayat et al 1998;Lellouch et al 2003;Gurwell 2004;López-Valverde et al 2005;Bailey et al 2011). Although Cassini CIRS measurements in the lower stratosphere have reduced uncertainties down to ±20% (de Kok et al 2007), the highest possible precision is required because CO is expected to be horizontally uniform due to its extremely long chemical lifetime, and therefore may be used as an additional thermometer in Titan's atmosphere.…”