Ground-based glimpses of Titan in the year of the Cassini-Huygens rendez-vous

Titan is currently the only confirmed exobiotic environment known to us. It is also perhaps the most intriguing object in our Solar System. Its uncanny resemblance to our own planet has motivated generations of scientists into studying it from both the space and from the ground, and has initiated the Saturn-bound Cassini/Huygens ESA/NASA mission. In the meantime, Titan is observed from the ground (using large telescopes, such as those in Hawaii and Chile), but also from space (initially with Voyager 1 and 2, with the HST, and recently with ISO). Thus, we know today that the thick atmospheric layer - covering the satellite's mysterious surface - is essentially made of nitrogen, with small amounts of methane and hydrogen. The combination among these mother molecules produces an exciting organic chemistry in Titan's atmosphere, with hydrocarbons and nitriles. As a difference with our own planet we note the absence of significant amounts of oxygen (only traces of CO, CO2 and more recently, H2O have been discovered), as well as the low temperatures prevailing that delay chemical reactions. The general shape of the thermal profile is, however, quite similar to that of the Earth's. Titan's surface is hidden under a veil of a thick aerosol cloud, but recently, spectroscopy and imaging of the satellite in the near-IR have shown that this surface is inhomogeneous, bright on the leading side and darker on the trailing one. We have performed observations with FTS/CFHT and ISAAC/VLT in spectroscopy and used the adaptive optics systems of PUEO/CFHT and ADONIS/ESO, as well as NAOS/VLT to uncover some of the features related with the lower atmosphere and surface of Titan. Thus, we have definite indication today of the presence of significant seasonal and diurnal effects in Titan's atmosphere. In imaging, a large, bright equatorial region - possibly connected with relief - is found on the leading hemisphere, while bright areas are also observed near the poles. The recent view of the trailing hemisphere with large ground-based telescopes has allowed us to infer that also "this darker side of the moon" affords a complex morphology and composition. The exact nature of this ground remains to be discovered, but spectroscopy indicates that it is probably a mixture of topography, ices, hydrocarbon liquid and rocks. How far can we take the analogy between Titan and our own planet? Refs: Coustenis, A. et al. 1995. Titan's surface: composition and variability from the near-infrared albedo. Icarus 118, pp. 87-104. Combes, M., et al. 1997. Spatially resolved images of Titan by means of adaptive optics. Icarus 129, pp. 482-497. Coustenis, A., et al.1998. Evidence for water vapor in Titan's atmosphere from ISO/SWS data. Astron. Astrophys. 336, L85-L89. Coustenis, A., et al. 2001. Images of Titan at 1.3 and 1.6 microns with adaptive optics at the CFHT. Icarus 154, 501-515. Coustenis, A., et al. 2003. Titan's atmosphere from ISO mid-infrared spectroscopy. Icarus, 161, 383-403. Gendron, E., et al. 2003. VLT/NACO adaptive optics imaging of Titan. Astron. Astroph., in press. Lellouch, E., et al. 2003. Titan's 5-micron window: observations with the very large telescope. Icarus 162, 125-142. Lellouch, E. et al. 2004. Titan's 5 micron lightcurve. Icarus, in press.