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Titanic Weather Forecasting
New Detailed VLT Images of Saturn's Largest Moon.
1 avril 2004
Titan, the largest moon of Saturn was discovered by Dutch astronomer Christian Huygens in 1655 and certainly deserves its name. With a diameter of no less than 5,150 km, it is larger than Mercury and twice as large as Pluto. It is unique in having a hazy atmosphere of nitrogen, methane and oily hydrocarbons. Although it was explored in some detail by the NASA Voyager missions, many aspects of the atmosphere and surface still remain unknown. Thus, the existence of seasonal or diurnal phenomena, the presence of clouds, the surface composition and topography are still under debate. There have even been speculations that some kind of primitive life (now possibly extinct) may be found on Titan.
Optimizing space missions
Titan is the main target of the NASA/ESA Cassini/Huygens mission, launched in 1997 and scheduled to arrive at Saturn on July 1, 2004. The ESA Huygens probe is designed to enter the atmosphere of Titan, and to descend by parachute to the surface.
Ground-based observations are essential to optimize the return of this space mission, because they will complement the information gained from space and add confidence to the interpretation of the data. Hence, the advent of the adaptive optics system NAOS-CONICA (NACO)  in combination with ESO's Very Large Telescope (VLT) at the Paranal Observatory in Chile now offers a unique opportunity to study the resolved disc of Titan with high sensitivity and increased spatial resolution.
Adaptive Optics (AO) systems work by means of a computer-controlled deformable mirror that counteracts the image distortion induced by atmospheric turbulence. It is based on real-time optical corrections computed from image data obtained by a special camera at very high speed, many hundreds of times each second.
The southern smile
A team of French astronomers  have recently used the NACO state-of-the-art adaptive optics system on the fourth 8.2-m VLT unit telescope, Yepun, to map the surface of Titan by means of near-infrared images and to search for changes in the dense atmosphere.
These extraordinary images have a nominal resolution of 1/30th arcsec and show details of the order of 200 km on the surface of Titan. To provide the best possible views, the raw data from the instrument were subjected to deconvolution (image sharpening).
Images of Titan were obtained through 9 narrow-band filters, sampling near-infrared wavelengths with large variations in methane opacity. This permits sounding of different altitudes ranging from the stratosphere to the surface.
Titan harbours at 1.24 and 2.12 μm a "southern smile", that is a north-south asymmetry, while the opposite situation is observed with filters probing higher altitudes, such as 1.64, 1.75 and 2.17 μm.
A high-contrast bright feature is observed at the South Pole and is apparently caused by a phenomenon in the atmosphere, at an altitude below 140 km or so. This feature was found to change its location on the images from one side of the south polar axis to the other during the week of observations.
An additional series of NACO observations of Titan is foreseen later this month (April 2004). These will be a great asset in helping optimize the return of the Cassini/Huygens mission. Several of the instruments aboard the spacecraft depend on such ground-based data to better infer the properties of Titan's surface and lower atmosphere.
Although the astronomers have yet to model and interpret the physical and geophysical phenomena now observed and to produce a full cartography of the surface, this first analysis provides a clear demonstration of the marvellous capabilities of the NACO imaging system. More examples of the exciting science possible with this facility will be found in a series of five papers published today in the European research journal Astronomy & Astrophysics (Vol. 417 / No 1 (April I 2004), L1 to L24).
 NACO is an abbreviation of NAOS/CONICA. The NAOS adaptive optics corrector was built, under an ESO contract, by Office National d'Etudes et de Recherches Aérospatiales (ONERA), Laboratoire d'Astrophysique de Grenoble (LAOG) and the LESIA and GEPI laboratories of the Observatoire de Paris in France, in collaboration with ESO. The CONICA infra-red camera was built, under an ESO contract, by the Max-Planck-Institut für Astronomie (MPIA) (Heidelberg) and the Max-Planck Institut für Extraterrestrische Physik (MPE) (Garching) in Germany, in collaboration with ESO.
 The team is composed of Eric Gendron, Athéna Coustenis, Pierre Drossart, Michel Combes, Mathieu Hirtzig, François Lacombe, Daniel Rouan, Claude Collin, and Sylvain Pau (LESIA, Observatoire de Paris, CNRS, France), Anne-Marie Lagrange, David Mouillet, Patrick Rabou (Laboratoire d'Astrophysique, Observatoire de Grenoble, France), Thierry Fusco (ONERA) and Gérard Zins (ESO).
The results presented here are based on an article published in Astronomy & Astrophysics (A&A 417, L21-24, 2004): "VLT/NACO adaptive optics imaging of Titan" by E. Gendron et al.
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