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.