Titan’s 5-μm window: observations with the Very Large Telescope

Abstract

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 μm. 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 13CO). They indicate that the CO/N 2 mixing ratio in Titan’s troposphere is 32±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-μm continuum geometric albedo is ∼0.06, and further indicates a ∼20% albedo decrease over 4.98–5.07 μm. Nonzero flux is detected at the 0.01 geometric albedo level in the saturated core of the 12CO (1–0) band, at 4.75–4.85 μm, providing evidence for backscattering on the stratospheric haze. Finally, emission lines are detected at 4.75–4.835 μm, 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-μm 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.