A&A 417, L21–L24 (2004) DOI: 10.1051/0004-6361:20040027 c  ESO 2004 Astronomy & Astrophysics VLT/NACO adaptive optics imaging of Titan ⋆ E. Gendron 1 , A. Coustenis 1 , P. Drossart 1 , M. Combes 1 , M. Hirtzig 1 , F. Lacombe 1 , D. Rouan 1 , C. Collin 1 , S. Pau 1 , A-M. Lagrange 2 , D. Mouillet 2 , P. Rabou 2 , T. Fusco 3 , and G. Zins 4 1 Observatoire de Paris, LESIA, UMR 8109 du CNRS, 5 place Jules Janssen, 92195 Meudon Cedex, France 2 Laboratoire d’Astrophysique, Observatoire de Grenoble, BP 53, 38041 Grenoble, France 3 ONERA – DOTA, 92322 Chˆ atillon, France 4 ESO, Karl-Schwarzschild -Str. 2, 85748 Garching bei M¨ unchen, Germany Received 18 March 2003 / Accepted 17 January 2004 Abstract. The advent of the NAOS/CONICA adaptive optics system at the ESO Very Large Telescope recently gave us the opportunity to map the surface of Titan and to search for atmospheric variations at high spatial resolution and contrast. We report here the first results from a series of observations of Titan performed with this instrument in a number of near-infrared narrow-band filters, covering various altitude regions and three different longitudes (out of the 16 days of Titan’s orbit). We have achieved unequaled contrast on images showing complex topography on Titan’s trailing hemisphere and have found robust evidence for the north-south asymmetry inversion. The presence of other interesting atmospheric features at Titan’s South Pole is described. Key words. instrumentation: adaptive optics – planets and satellites: individual: Titan – infrared: solar system 1. Introduction Despite its close exploration by the Voyager missions, many aspects of the atmosphere and surface of Titan remain cur- rently unknown. Thus, the existence of seasonal or diurnal phenomena, the tropospheric content in condensates (clouds), the surface composition and topography are still under debate, although we have obtained many clues in the past decade, through spectroscopy and radar observations, but also by disk- resolved imaging. Besides the data obtained by the HST (Smith et al. 1996), Earth-based observations have been most valuable especially since the advent of the adaptive optics and related techniques which permit in the near-IR the sounding of the deeper atmosphere and surface of the satellite. For Titan the first results were obtained with the ESO ADONIS adaptive op- tics system in the mid-90s (Combes et al. 1997). Recently, a large set of new data was obtained from the CFHT, Keck and Gemini adaptive optics systems (Gibbard et al. 1999; Coustenis et al. 2001; Roe et al. 2002; Brown et al. 2002) and from the HST again (Meier et al. 2000) showing Titan’s surface with the bright equatorial region (observed near 100 LCM on the lead- ing side) and also darker areas in other parts of the disk. The trailing hemisphere has been under-observed until now and its more subtle structure obviously required higher-contrasted ob- servations. Current surface models suggest all possible combi- nations between ices, liquid hydrocarbons, organics and rocks (e.g. Coustenis et al. 2001). Send offprint requests to: E. Gendron, e-mail: Eric.Gendron@obspm.fr ⋆ Based on observations collected at the European Southern Observatory, Chile (ESO Proposal 70.C-0588). The situation in the lower atmosphere of Titan is also un- clear. Evidence for tropospheric clouds has been found in spec- tra (Griffith et al. 2003) and references therein and in images of Titan (Roe et al. 2002; Brown et al. 2002), while other models and observations indicate that Titan’s troposphere must be supersaturated in methane. Further up, in the stratosphere, adaptive optics images showed until recently the haze-sensitive north-south asymmetry with a brighter South Pole. This asym- metry was recently found to have reversed. Another bright at- mospheric feature, noticed for the first time on Titan’s morning limb, was interpreted as a diurnal effect (Coustenis et al. 2001). Titan is the main target of the NASA/ESA Cassini/Huygens mission, which will investigate the satellite in situ. In the mean- time, ground-based data are vital to optimize the return of the mission. Hence, the advent of the new adaptive optics sys- tem NACO in combination with the Very Large Telescope of ESO in Chile offers a unique opportunity to study the resolved Titan disk with high sensitivity and increased spatial resolution. 2. Data acquisition Adaptive optics compensates for the image degradation due to atmospheric turbulence, thanks to active optical elements driven in real-time by a servo-loop. 2.1. The VLT adaptive optics system NACO NACO (Rousset et al. 2002) is the state-of-the-art adaptive op- tics system of the ESO Very Large Telescope, the four european 8-m telescopes located at Mount Paranal, Chile. NACO equips the Nasmyth-B focus of the fourth telescope unit (Yepun). Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20040027