Since 1994, our team at the LESIA has explored Titan's atmosphere, gathering information before and during the arrival of the Cassini-Huygens mission. The data presented here rely on adaptive optics, harvesting infrared images of Titan between 0.8 and 2 micron with either PUEO at the CFHT or NAOS/CONICA (NACO) at the VLT. The imaging modes we used vary from one observing run to another : (a) narrow-band filter imaging, around each methane window at 1.3, 1.6 and 2.0 micron [Combes et al (1997), Coustenis et al (2001), Gendron et al (2004)], (b) NACO spectroscopy around 2 micron with a 1400 resolving power (Negrao et al., 2006), (c) SDI imaging, probing the core and wings of the 1.6 micron methane window [Hirtzig et al (2006a)], and (d) Fabry-Pérot Imaging (FPI), used as a collection of 2-nm wide filters to scan the 2.0 micron window. We also used (e) Integral Field Spectrometry to probe the 0.8-1 micron range [Hirtzig et al (2006b)].
We will describe here the latest conclusions drawn from these runs, focusing both on the features detected on Titan's surface [Coustenis et al (2005)] and the diagnostic of the atmosphere [Hirtzig et al (2006)] : the latter it displays at the present era variations or new apparitions of atmospheric phenomena, such as seasonal and diurnal effects, or some very interesting features in the Southern polar region. The North-South Asymmetry (NSA) is shown to have changed since 2000 in the near-IR and to be currently organized in a brighter northern than southern pole. We report here on this evolution. From our data, we also have significant statistical evidence for diurnal effects in Titan's stratosphere, with a brighter (by as much as 19%) morning limb appearing in our images in many cases, when the phase effect is on the evening side. The southern bright revolving feature detected by many authors since 2000 cannot be seen any more since January 2005. It was a meteorological (and possibly seasonal) phenomenon, revolving around the Southern pole (confined in its motion within the 80th S parallel) and located somewhere in the upper troposphere (20-40 km of altitude). We will discuss some of its aspects.
Our knowledge of the nature of Titan's surface is based on the accumulation of many images acquired systematically at given wavelengths in the aforementioned "methane absorption windows". First we will show the latest surface maps, before giving new hints regarding the chemical composition of the surface components. Two ways we can consider in this study: direct spectroscopy of Titan's surface albedo spectrum, as returned by our radiative transfer model [Rannou et al (2005), Negrão et al (2005)]; or differential spectroscopy, so as to eliminate as many model-dependent artefacts as possible.
All theses studies can illustrate, if needed, that the remote observation from Earth, in the infrared, is utterly complementary to the in situ measurements by the state-of-the-art Cassini/Huygens mission.