New detailed VLT images of the largest moon in the solar system
EUROPEAN SOUTHERN OBSERVATORY
Posted: April 1, 2004
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.
These extraordinary images have a nominal resolution of 1/30th arcsec and show details of the order of 200 kilometres 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 kilometres 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 April 1st in the European research journal Astronomy & Astrophysics
(Vol. 47, L1 to L24).
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