VLT NACO Performs Outstanding Observations of Titan's Atmosphere and Surface

http://www.eso.org/outreach/press-re...hot-04-05.html


ESO Press Photo 04/05
24 February 2005
For immediate release

Another Look at an Enigmatic New World

VLT NACO Performs Outstanding Observations of Titan's Atmosphere
and Surface

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.

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 members of the Huygens Project Scientist Team. Indeed, large
astronomical telescopes with state-of-the art adaptive optics systems
allow scientists to image Titan's disc in quite some detail. Moreover,
ground-based observations are not restricted to the limited period of
the fly-by of Cassini and landing of Huygens. They hence complement
ideally the data gathered by this NASA/ESA mission, further optimising
the overall scientific return.

A group of astronomers [1] observed Titan with ESO's Very Large
Telescope (VLT) at the Paranal Observatory (Chile) during the nights
from 14 to 16 January, by means of the adaptive optics NAOS/CONICA
instrument mounted on the 8.2-m Yepun telescope [2]. The
observations were carried out in several modes, resulting in a series
of
fine images and detailed spectra of this mysterious moon. They
complement earlier VLT observations of Titan, cf. ESO Press Photos
08/04
and ESO Press Release 09/04.


The highest contrast images

ESO PR Photo 04a/05 images/phot-04a-05-preview.jpg

ESO PR Photo 04a/05

Titan's surface (NACO/VLT)

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ESO PR Photo 04b/05 images/phot-04b-05-preview.jpg

ESO PR Photo 04b/05

Map of Titan's Surface (NACO/VLT)

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Caption: ESO PR Photo 04a/05 shows Titan's trailing hemisphere [3].
with the Huygens landing site marked as an "X". The left image
was taken with NACO and a narrow-band filter centred at 2 microns. On
the right is the NACO/SDI image of the same location showing Titan's
surface through the 1.6 micron methane window. A spherical projection
with coordinates on Titan is overplotted. ESO PR Photo 04b/05 is a map
of Titan taken with NACO at 1.28 micron (a methane window allowing it
to
probe down to the surface). On the leading side of Titan, the bright
equatorial feature ("Xanadu") is dominating. On the trailing side, the
landing site of the Huygens probe is indicated.

ESO PR Photo 04c/05 images/phot-04c-05-preview.jpg

ESO PR Photo 04c/05

Titan, the Enigmatic Moon, and Huygens Landing Site (NACO-SDI/VLT and
Cassini/ISS)

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Caption: ESO PR Photo 04c/05 is a comparison between the NACO/SDI image
and an image taken by Cassini/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 (see
http://sci.esa.int/science-e/www/obj...objectid=36222). 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.

ESO PR Photo 04d/05 images/phot-04d-05-preview.jpg

ESO PR Photo 04d/05

Evolution of the Atmosphere of Titan (NACO/VLT)

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Caption: ESO PR Photo 04d/05 is an image of Titan's atmosphere at 2.12
microns as observed with NACO on the VLT at three different epochs from
2002 till now. Titan's atmosphere exhibits seasonal and meteorological
changes which can clearly be seen here : the North-South asymmetry -
indicative of changes in the chemical composition in one pole or the
other, depending on the season - is now clearly in favour of the North
pole. Indeed, the situation has reversed with respect to a few years
ago
when the South pole was brighter. Also visible in these images is a
bright feature in the South pole, found to be presently dimming after
having appeared very bright from 2000 to 2003. The differences in size
are due to the variation in the distance to Earth of Saturn and its
planetary system.

The new images show Titan's atmosphere and surface at various
near-infrared spectral bands. The surface of Titan's trailing side is
visible in images taken through narrow-band filters at wavelengths
1.28,
1.6 and 2.0 microns. They correspond to the so-called "methane windows"
which allow to peer all the way through the lower Titan atmosphere to
the surface. On the other hand, Titan's atmosphere is visible through
filters centred in the wings of these methane bands, e.g. at 2.12 and
2.17 microns.

Eric Gendron of the Paris Observatory in France and leader of the team,
is extremely pleased: "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 much detail. In particular, those centred at wavelength
1.6 micron and obtained with the Simultaneous Differential Imager (SDI)
on NACO [4] provide the highest contrast and best views. This
is firstly because 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 wavelength 1.625 micron.

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 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. ESO PR Photo 04c/04
illustrates the striking agreement between the NACO/SDI image taken
with
the VLT from the ground and the ISS/Cassini map.

The images of Titan's atmosphere at 2.12 microns show a still-bright
south pole with an additional atmospheric bright feature, which may be
clouds or some other meteorological phenomena. The astronomers have
followed it since 2002 with NACO and notice that it seems to be fading
with time. At 2.17 microns, this feature is not visible and the
north-south asymmetry - also known as "Titan's smile" - is clearly in
favour in the north. The two filters probe different altitude levels
and
the images thus provide information about the extent and evolution of
the north-south asymmetry.


Probing the composition of the surface

ESO PR Photo 04e/05 images/phot-04e-05-preview.jpg

ESO PR Photo 04e/05

Spectrum of Two Regions on Titan (NACO/VLT)

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Caption: ESO PR Photo 04e/05 represents two of the many spectra
obtained
on January 16, 2005 with NACO and covering the 2.02 to 2.53 micron
range. The blue spectrum corresponds to the brightest region on Titan's
surface within the slit, while the red spectrum corresponds to the dark
area around the Huygens landing site. In the methane band, the two
spectra are equal, indicating a similar atmospheric content; in the
methane window centred at 2.0 microns, the spectra show differences in
brightness, but are in phase. This suggests that there is no real
variation in the composition beyond different atmospheric mixings.

ESO PR Photo 04f/05 images/phot-04f-05-preview.jpg

ESO PR Photo 04f/05

Imaging Titan with a Tunable Filter (NACO Fabry-Perot/VLT)

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Caption: ESO PR Photo 04f/05 presents a series of images of Titan taken
around the 2.0 micron methane window probing different layers of the
atmosphere and the surface. The images are currently under thorough
processing and analysis so as to reveal any subtle variations in
wavelength that could be indicative of the spectral response of the
various surface components, thus allowing the astronomers to identify
them.

Because the astronomers have also obtained spectroscopic data at
different wavelengths, they will be able to recover useful information
on the surface composition.

The Cassini/VIMS instrument 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 what is possible with Earth-based
telescopes. However, with NACO at the VLT, the astronomers have the
advantage of observing Titan with considerably higher spectral
resolution, and thus to gain more detailed spectral information about
the composition, etc. The observations 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 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.


More information

Results on Titan obtained with data from NACO/VLT are in press in the
journal Icarus ("Maps of Titan's surface from 1 to 2.5 micron" by A.
Coustenis et al.). Previous images of Titan obtained with NACO and with
NACO/SDI are accessible as ESO PR Photos 08/04 and ESO PR Photos 11/04.
See also these Press Releases for additional
scientific references.


Notes

[1]: The team is composed of Eric Gendron, Athena Coustenis, Mathieu
Hirtzig, Michel Combes, Pierre Drossart, and Alberto Negrao (LESIA,
Paris-Meudon Observatory, France), Pascal Rannou (Univ. de Versailles,
France), Markus Hartung (ESO), Tom Herbst (Max-Planck Institute for
Astronomy, Heidelberg, Germany), Tobias Owen (IfA, Hawaii), Laird Close
(University of Arizona, USA), Olivier Witasse and Jean-Pierre Lebreton
(ESA/ESTEC).

[2]: Adaptive Optics (AO) systems work by means of a
computer-controlled
deformable mirror that counteracts the image distortion induced by
atmospheric turbulence. Adaptive Optics 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 (see e.g. ESO Press
Release 25/01 ../pr-2001/pr-25-01.html , ESO PR Photos 04a-c/02
../pr-2002/phot-04-02.html, ESO PR Photos 19a-c/02
../pr-2002/phot-19-02.html, ESO PR Photos 21a-c/02
../pr-2002/phot-21-02.html, ESO Press Release 17/02
../pr-2002/pr-17-02.html, and ESO Press Release 26/03
../pr-2003/pr-26-03.html for earlier NACO images, and ESO Press
Release 11/03 ../pr-2003/pr-11-03.html for MACAO-VLTI results).

[3]: Titan is tidally-locked to Saturn, and hence always presents the
same face towards the planet. To image all sides of Titan (from the
Earth) therefore requires observations during almost one entire orbital
period, 16 days. The trailing hemisphere is the one we see when Titan
moves away from us in its course around Saturn. The leading hemisphere
is the one on the other side.

[4]: 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. (see
ESO PR 09/04 ../pr-2004/pr-09-04.html and PR 02/05 pr-02-05.html
for
more details).


Contacts

Eric Gendron
LESIA (Bat. 18)
Observatoire de Paris
92195 Meudon Cedex, France
Tel: 331 45 07 79 18


Ath?na Coustenis
LESIA (Bat. 18)
Observatoire de Paris
92195 Meudon Cedex, France
Tel: 331 45 07 77 20


Markus Hartung
ESO Santiago
Chile
Tel: +56 2 463 3071