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

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

Mar 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003icar..162..125l&link_type=abstract

Icarus, Volume 162, Issue 1, p. 125-142.

Computer Science

22

Infrared Observations, Atmospheres, Composition, Titan

Scientific paper

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/N2 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.

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