Titan's 5-[micro]m window: observations with the Very Large Telescope

We report on mid-resolution (R~2000) spectroscopic observations of Titan, acquired in November 2000 with the Very Large Telescope and covering the range 4.75-5.07 [micro]m. These observations provide a detailed characterization of the CO (1-0) vibrational band, clearly separating for the first time individual CO lines (P10 to P19 lines of [sup.13]CO). They indicate that the CO/[N.sub.2] mixing ratio in Titan's troposphere is 32 [+ or -] 10 ppm. Comparison with photochemical models indicates that CO is not in a steady state in Titan's atmosphere. The observations confirm that Titan's 5-[micro]m continuum geometric albedo is ~0.06, and further indicates a ~20% albedo decrease over 4.98-5.07 [micro]m. Nonzero flux is detected at the 0.01 geometric albedo level in the saturated core of the [sup.12]CO (1-0) band, at 4.75-4.85 [micro]m, providing evidence for backscattering on the stratospheric haze. Finally, emission lines are detected at 4.75-4.835 [micro]m, coinciding in position with lines from the CO(1-0) and/or CO(2-1) bands. Matching them by thermal emission would require Titan's stratosphere to be much warmer (by ~ 25 K at 0.1 mbar) than indicated by the methane 7.7-[micro]m emission and the Voyager radio-occultation. We show instead that a nonthermal mechanism, namely solar-excited fluorescence, is a more plausible source for these emissions. Improved observations and laboratory measurements on the vibrational-translational relaxation of CO are needed for further interpretation of these emissions in terms of a CO stratospheric mixing ratio. Keywords: Infrared Observations; Atmospheres, Composition; Titan