Titan's Near-Infrared Imaging with Adaptive Optics
M. Combes, A. Coustenis, E. Gendron, L. Vapillon, R. Wittemberg, J-P. Veran (DESPA, Observatoire de Paris, F-92195 Meudon, France)
Titan's images have been acquired since 1993 with the ADONIS adaptive optics system installed at the ESO telescope in La Silla (Chile). The near-infrared range 1-2.5 m was covered in the centers and the wings of the methane atmospheric windows at 1.3, 1.6 and 2.0 micron with narrow-band filters and CVF. The low Strehl ratio in the J-band images has prevented their analysis up to now, but the H- and K-band images are diffraction-limited and allow to resolve Titan's disk. The associated Point Spread Functions have very high signal-to-noise. We have corrected for systematic and center-to-limb effects and applied deconvolution processes to our data. The contrast on the reduced images is about 30%.
Both the leading and the trailing hemispheres of Titan have been observed. The 2- m images show a broad equatorial bright region on Titan's leading hemisphere near 114 degrees LCM, in accordance with HST results (Smith et al., 1996, Icarus 119, 336) and spectroscopic measurements (Coustenis et al., 1995, Icarus 118, 87). High-quality flat-fielding of our data has allowed us to resolve this region into several (2 or 3) distinct areas (Combes et al., 1997, Icarus, in press).
The trailing hemisphere of Titan is not completely dark, but shows bright zones near the poles, with a pronounced North-South asymmetry. The high-northern latitudes are uniformly brighter at 2.0 m than the equatorial and southern ones.
We will present new images of Titan taken near 1.6 micron (H band) with narrow-band filters and CVF, that set stringent contraints on the nature of the satellite's surface.