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E. Lellouch, A. Coustenis, B. Sebag (Observatoire de Paris), J.-G. Cuby (European Southern Observatory ), J. Crovisier (Observatoire de Paris), J.P. Maillard (I.A.P. Paris)
Earlier observations by Noll and Knacke (Icarus, 101, 272, 1993) and Noll et al. (Icarus, 124, 625, 1996) have shown that the 5 micron region of Titan's spectrum is a methane window probing Titan's lower atmosphere and surface. In particular, Noll et al. found that the 4.8-4.95 micron radiation is dominated by absorption in the (1-0) vibrational band of CO in Titan's troposphere, and that the 4.95-5.2 micron flux is reflected at the surface. They inferred a 10 ppm tropospheric CO mixing ratio, but this value was uncertain because of limited S/N of their data, which in particular did not show the individual rovibrational lines of CO.
On November 16, 2000, we observed Titan near Eastern Elongation with the 8-m UT1 (Antu) of the Very Large Telescope, equipped with the Infrared Spectrometer and Array Camera (ISAAC). The insrument was used in long slit spectroscopy mode. The 4.75-5.1 micron range was covered, at a mean resolution of about 2000. Standard reduction included flat-fielding, correction for horizontal distortion and slit curvature, spectrum extraction, correction for telluric absorption, and wavelength and flux calibration.
Preliminary results indicate that (i) the continuum flux at 5.0 micron is about 100 mJy (indicating a geometric albedo of 0.055), consistent with Noll et al. 1996 (ii) the albedo tends to decrease longward of 5.0 micron (iii) residual flux is detected at 4.75-4.83 micron (iv) several rovibrational lines of CO are seen in absorption (P10 to P19 of 13CO) at 4.85-4.95 micron. We will present the analysis of these data and infer a new measurement of the CO abundance in Titan's troposphere, contributing to the long-standing problem of the abundance, vertical distribution and origin of carbon monoxide on Titan. In addition, the spectrum seems to show emission features at 4.75-4.83 micron coinciding with the position of some CO(1-0) lines; however stratospheric thermal emission and solar induced fluorescence seem to be unable to explain quantitatively the observed flux levels.