Combes M., Vapillon L., Gendron E., Coustenis A., Lai O.

Spatially Resolved Images of Titan in the Near-Infrared

Spatially resolved images of Titan in the near-IR atmospheric transparent windows have been recorded at the 3.6 m ESO telescope in Oct. 1993 and Sept. 1994 with the COME-ON PLUS adaptive optics system (developped by DESPA, ESO and French Cos). A first resolved image of Titan was obtained in 1991 (Saint Pe et al. 1993, Icarus 105, 163). The diffraction pattern at 2 micrometers was resolved during the runs at all times (Strehl ratio between 0.1 and 0.4). Images have been recorded close to the Eastern Elongation of Titan with two sets of narrow-band filters around 2, 1.6 and 1.3 micrometers: one set in which the recorded flux is entirely due to light back-scattered by the stratosphere of Titan and another (adjusted to the center of the atmospheric windows), where the flux is mainly due to scattering in the deep atmosphere but also, for about 30%, to reflected light by Titan's surface. Correction of permanent detection defects and flat-field was applied. The deconvolution process was very efficient due to the very high S/N of the PSF (star) images. The stratospheric contribution was deduced from the "stratospheric images" and removed from the "surface images". The center-to-limb variations were modeled, as a function of CH(sub)4 abundances, and limb effects corrected on the images.

The stratospheric images exhibit a strong North-South hemispheric asymmetry with bright enhancement in the South, where a concentration of aerosols is expected. The surface images show a well-defined bright spot associated with smaller and less contrasted features, all rotating over 6 consecutive nights at the expected rotation rate of Titan's solid body. They are attributed to ground features at the surface. Our images are in agreement with HST images (Smith et al. 1995), both on the location and on the shape of the bright features. As expected, the contrast of the surface marks is higher in our images, the spatial resolutions being equivalent. For the investigation of the chemical nature of the surface features, additional CVF imaging (R~60) is available. New observations in Oct. 1995 should allow us to retrieve full coverage of the Titan orbit.

Titan paper