Applications of a new set of methane line parameters to the modeling of Titan's spectrum in the 1.58 μm window
Résumé
In this paper we apply a recently released set of methane line parameters (Wang et al., 2011) to the modeling of Titan spectra in the 1.58 μm window at both low and high spectral resolution. We first compare the methane absorption based on this new set of methane data to that calculated from the methane absorption coefficients derived in situ from DISR/Huygens (Tomasko et al., 2008a; Karkoschka and Tomasko, 2010) and from the band models of Irwin et al. (2006) and Karkoschka and Tomasko (2010). The Irwin et al. (2006) band model clearly underestimates the absorption in the window at temperature-pressure conditions representative of Titan's troposphere, while the Karkoschka and Tomasko (2010) band model gives an acceptable agreement in the whole window, overestimating the absorption by about 15% in the range 6300-6500 cm-1. We also find that the transmittance of Titan's atmosphere is in excellent agreement with that calculated from the Tomasko et al. (2008a) coefficients after reducing them by about 7%. Synthetic spectra computed with spectral resolutions of 1.2 cm-1 (R˜5400) and 0.35 cm-1 (R˜18000) are then compared with two high-resolution Earth-based measurements of Titan's albedo obtained in 1982 and 1993 (with KPNO/FTS and IRTF/CSHELL). The new set of methane line parameters leads to an excellent match of all the CH3D and CH4 absorption features in these spectra, and permits us to derive a ratio of CH3D/CH4=(4.5±1.0)×10-4 - hence a D/H ratio in methane for Titan of (1.13±0.25)×10-4 - and a CO mole fraction of 40±10 ppm (from the KPNO/FTS dataset) and 51±7 ppm (from the IRTF/CSHELL dataset). We also infer constraints on the far-wing lineshape of methane lines of the 2ν3 band. We finally present two other examples of models of Titan's spectrum using the new line parameters, one potentially useful for future higher-resolution (R=40,000) observations, another one applicable to the ongoing low-resolution (R˜100) observations by Cassini VIMS. We show that the aerosol model of Tomasko et al. (2008b) produces too much intensity at low phase angle compared to a VIMS spectrum recorded near the Huygens site and we propose a slightly revised model that reproduces this observation.