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E. Lellouch (Obs. Paris), M.A. Lopez-Valverde (IAA Granada), A. Coustenis (Obs. Paris), B. Schmitt (LPG Grenoble)
Following our initial (Nov. 2000) observations of Titan's 5-\mum window (Lellouch et al. Icarus, 162, 125, 2003), we have performed a detailed study of Titan's spectrum in the 4.55-5.1 \mum spectrum. Observations were conducted at unit 1 (UT1, Antu) of the VLT, using the ISAAC instrument, at a spectral resolution of about 2500. On Nov. 19-20, 2002, we covered extensively the 4.55-4.85 \mum spectrum. The main result of these observations was the detection of no less than 30-40 emission lines, coinciding with rovibrational lines from the CO(1-0) (P18 to R11) and CO(2-1) (P11 to R11) vibrational bands, confirming and extending the preliminary evidence for such emission lines at 4.75-4.84 \mum from our Nov. 2000 observations. These emissions occur in non-LTE conditions, and are due to radiative de-excitation from the v=1 and v=2 levels at 4.7 and 2.3 \mum excited by solar radiation. A full non-LTE model is being currently developed, and indicates that these emissions probe Titan's stratosphere at 150-500 km, eventually permitting a new determination of CO mixing ratio in Titan's stratosphere.
The second focus of our observations was the search for rotational variations in Titan's 5-\mum continuum (i.e. at 4.95-5.05 \mum). For this purpose, observations of the 4.86-5.10 \mum range were acquired on Nov. 17, Nov. 19, Dec. 24, 2002, and January 14, 2003. The sampled orbital longitudes are L=8, 54, 124 and 239, respectively. These repeated observations clearly indicate that the 5-\mum continuum geometric albedo follows a lightcurve, correlated with the near infrared (0.83-2.0 \mum) lightcurves, and with an amplitude (~ 30 %) similar to the 1.6 and 2 \mum lightcurves. This variation, which is the ultimate proof that Titan's surface is sensed in the 5-\mum window, will be interpreted in the context of models of Titan's surface composition, notably in view of the increasing evidence for the presence of water ice at Titan's surface (Griffith et al. Science, 300, 628, 2003).
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.