Water vapor in Titan's stratosphere from Cassini CIRS far-infrared spectra

V. Cottini, C. A. Nixon, D. E. Jennings, C. M. Anderson, N. Gorius, G. L. Bjoraker, A. Coustenis, N. A. Teanby, R. K. Achterberg, B. Bezard, R. de Kok, E. Lellouch, P. G. J. Irwin, F. M. Flasar, G. Bampasidis

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Here we report the measurement of water vapor in Titan's stratosphere using the Cassini Composite Infrared Spectrometer (CIRS, Flasar, F.M. et al. [2004]. Space Sci. Rev. 115, 169-297). CIRS senses water emissions in the far infrared spectral region near 50 mu m, which we have modeled using two independent radiative transfer codes (NEMESIS (Irwin, P.G.J. et al. [2008]. J. Quant. Spectrosc. Radiat. Trans. 109, 1136-1150) and ART (Coustenis, A. et al. [2007]. Icarus 189, 35-62; Coustenis, A. et al. [2010]. Icarus 207, 461-476). From the analysis of nadir spectra we have derived a mixing ratio of 0.14 +/- 0.05 ppb at an altitude of 97 km, which corresponds to an integrated (from 0 to 600 km) surface normalized column abundance of 3.7 +/- 1.3 x 10(14) molecules/cm(2). In the latitude range 80 degrees S to 30 degrees N we see no evidence for latitudinal variations in these abundances within the error bars. Using limb observations, we obtained mixing ratios of 0.13 +/- 0.04 ppb at an altitude of 115 km and 0.45 +/- 0.15 ppb at an altitude of 230 km, confirming that the water abundance has a positive vertical gradient as predicted by photochemical models (e.g. Lara, L.M., Lellouch, F., Lopez-Moreno, J.J., Rodrigo, R. [1996]. J. Geophys. Res. 101(23), 261; Wilson, E.H., Atreya, S.K. [2004]. J. Geophys. Res. 109, E6; Horst, S.M., Vuitton, V., Yelle, R.V. [2008]. J. Geophys. Res., 113, E10). We have also fitted our data using scaling factors of similar to 0.1-0.6 to these photochemical model profiles, indicating that the models over-predict the water abundance in Titan's lower stratosphere. (C) 2012 Elsevier Inc. All rights reserved.

Original languageEnglish
Pages (from-to)855-862
Number of pages8
Issue number2
Publication statusPublished - Aug 2012

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