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Ground-Based AO Observations Of Titan Back Up Huygens Probe Landing

Garching bei München, Germany – February 24, 2005:   On January 14, 2005, the ESA Huygens probe arrived at Saturn's largest satellite, Titan. After a faultless descent through the dense atmosphere, it touched down on the icy surface of this strange world from where it continued to transmit precious data back to the Earth.
 
 
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  Several of the world's large ground-based telescopes were also active during this exciting event, observing Titan before and near the Huygens encounter, within the framework of a dedicated campaign coordinated by the Huygens Project Scientist Team.

Large astronomical telescopes equipped with state-of-the art adaptive optics systems can provide detailed images of Titan's disc. Moreover, ground-based observations are not restricted to the limited period of the fly-by of Cassini and the landing of Huygens. Thus, the ground-based observations provide an ideal complement to the data gathered by this NASA/ESA mission, further optimising the overall scientific return.

A group of astronomers observed Titan with ESO's Very Large Telescope (VLT) at the Paranal Observatory (Chile) during the nights from 14 to 16 January, using the adaptive optics NAOS/CONICA instrument mounted on the 8.2-m Yepun telescope.
Science With Adaptive Optics, Brandner
The observations were carried out in several modes, resulting in a series of high quality images and detailed spectra of this mysterious moon. The new data complement earlier VLT observations of Titan (ESO Press Release 09/04).

The new images show Titan's atmosphere and surface in various near-infrared spectral bands. The surface of Titan's trailing side is visible in images taken through narrow-band filters at wavelengths of 1.28µm, 1.6µm and 2.0µm. These filters correspond to the so-called "methane windows" which provide a view all the way through Titan's lower atmosphere to its surface. On the other hand, Titan's atmosphere is visible through filters centred on the wings of these methane bands, at 2.12µm and 2.17µm.
 
 
Titan
A comparison between the NACO/SDI image and an image taken by the Cassini Imaging Science Subsystem (ISS) while approaching Titan. The Cassini image shows the Huygens landing site map wrapped around Titan, rotated to the same position as the January NACO SDI observations. The yellow "X" marks the landing site of the ESA Huygens probe.
The Cassini/ISS image is courtesy of NASA, JPL, Space Science Institute. The coloured lines delineate the regions that were imaged by Cassini at differing resolutions. The lower-resolution imaging sequences are outlined in blue. Other areas have been specifically targeted for moderate and high resolution mosaicking of surface features. These include the site where the European Space Agency's Huygens probe has touched down in mid-January (marked with the yellow X), part of the bright region named Xanadu (easternmost extent of the area covered), and a boundary between dark and bright regions.
 
Eric Gendron of the Observatoire de Paris in France, and leader of the team, is delighted: "We believe that some of these images are the highest-contrast images of Titan ever taken with any ground-based or earth-orbiting telescope."

The excellent images of Titan's surface show the location of the Huygens landing site in detail. In particular, those centred on 1.6µm and obtained with the Simultaneous Differential Imager (SDI) on NACO provide the highest contrast and best views. There are two reasons for this. Firstly, the filters match the 1.6 micron methane window most accurately. Secondly, it is possible to get an even clearer view of the surface by subtracting accurately the simultaneously recorded images of the atmospheric haze, taken at a wavelength of 1.625 microns.

The images show the great complexity of Titan's trailing side, which was earlier thought to be very dark. However, it is now obvious that both bright and dark regions cover the field of these images.

The best resolution achieved on the surface features is about 0.039 arcsec, corresponding to 200 km on Titan. The image above illustrates the striking agreement between the NACO/SDI image taken with the VLT from the ground and the ISS/Cassini map.

The VLT and NACO were also used to obtain spectroscopic data, providing information about the surface composition of Titan. The Cassini/VIMS spectrometer on Cassini explores Titan's surface in the infrared range and, being so close to this moon, it obtains spectra with a much better spatial resolution than is possible with Earth-based telescopes. However, with NACO at the VLT, astronomers have the advantage of observing Titan with considerably higher spectral resolution, allowing them to gain more detailed spectral information about Titan's composition. The observations from the ground and the Cassini orbiter therefore complement each other.

Once the composition of the surface at the location of the Huygens landing is known from the detailed analysis of the in-situ measurements, it should become possible to learn more about the nature of the surface features elsewhere on Titan by combining the Huygens results with more extended cartography from Cassini, as well as from VLT observations to come.

Technical Note: 
The Simultaneous Differential Imager is a novel optical device that provides four simultaneous high-resolution images at three wavelengths around a near-infrared atmospheric methane absorption feature. The main application of the SDI is high-contrast imaging for the search for substellar companions with methane in their atmosphere, e.g. brown dwarfs and giant exoplanets, near other stars. However, as the present photos demonstrate, it is also superbly suited for Titan imaging.
 

Reference: 
A. Coustenis, M. Hirtzig, E. Gendron, P. Drossart, O. Lai, M. Combes and A. Negrão,
"Maps of Titan's surface from 1 to 2.5 µm," Icarus 177, 89–105 (2005)
 
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