New Detailed Images of Titan

http://www.eso.org/outreach/press-re...hot-08-04.html

ESO Press Photos 08a-c/04
1 April 2004

For immediate release

Titanic Weather Forecasting

New Detailed VLT Images of the Largest Moon in the Solar System

Optimizing space missions

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.

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) [1] #note1 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 (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.)


The southern smile

ESO PR Photo 08a/04 images/phot-08a-04-preview.jpg

ESO PR Photo 08a/04

Images of Titan on November 20, 25 and 26, 2002 Through Five Filters
(VLT YEPUN + NACO)

[Preview - JPEG: 522 x 400 pix - 40k] images/phot-08a-04-preview.jpg
[Normal - JPEG: 1043 x 800 pix - 340k] images/phot-08a-04-normal.jpg
[Hires - JPEG: 2875 x 2205 pix - 1.2M] images/phot-08a-04-hires.jpg

Caption: ESO PR Photo 08a/04 shows Titan (apparent visual magnitude
8.05, apparent diameter 0.87 arcsec) as observed with the NAOS/CONICA
instrument at VLT Yepun (Paranal Observatory, Chile) on November 20, 25
and 26, 2003, between 6.00 UT and 9.00 UT. The median seeing values were
1.1 arcsec and 1.5 arcsec respectively for the 20th and 25th.
Deconvoluted ("sharpened") images of Titan are shown through 5 different
narrow-band filters - they allow to probe in some detail structures at
different altitudes and on the surface. Depending on the filter, the
integration time varies from 10 to 100 seconds. While Titan shows its
leading hemisphere (i.e. the one observed when Titan moves towards us)
on Nov. 20, the trailing side (i.e the one we see when Titan moves away
from us in its course around Saturn) - which displays less bright
surface features - is observed on the last two dates.


ESO PR Photo 08b/04 images/phot-08b-04-preview.jpg

ESO PR Photo 08b/04

Titan Observed Through Nine Different Filters on November 26, 2002

[Preview - JPEG: 480 x 400 pix - 36k] images/phot-08b-04-preview.jpg
[Normal - JPEG: 960 x 800 pix - 284k] images/phot-08b-04-normal.jpg

Caption: ESO PR Photo 08b/04: Images of Titan taken on November 26, 2002
through nine different filters to probe different altitudes, ranging
from the stratosphere to the surface. On this night, a stable "seeing"
(image quality before adaptive optics correction) of 0.9 arcsec allowed
the astronomers to attain the diffraction limit of the telescope (0.032
arcsec resolution). Due to these good observing conditions, Titan's
trailing hemisphere was observed with contrasts of about 40%, allowing
the detection of several bright features on this surface region, once
thought to be quite dark and featureless.


ESO PR Photo 08c/04 images/phot-08c-04-preview.jpg

ESO PR Photo 08c/04

Titan Surface Projections

[Preview - JPEG: 601 x 400 pix - 64k] images/phot-08c-04-preview.jpg
[Normal - JPEG: 1201 x 800 pix - 544k] images/phot-08c-04-normal.jpg

Caption: ESO PR Photo 08c/04 : Titan images obtained with NACO on
November 26th, 2002. Left: Titan's surface projection on the trailing
hemisphere as observed at 1.3 ?m, revealing a complex brightness
structure thanks to the high image contrast of about 40%. Right: a new,
possibly meteorological, phenomenon observed at 2.12 ?m in Titan's
atmosphere, in the form of a bright feature revolving around the South
Pole.


A team of French astronomers [2] #note2 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 ?m 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 ?m.

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.


Outlook

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
http://www.edpsciences.org/articles/aa/abs/2004/13/contents/contents.html
published today in the European research journal Astronomy &
Astrophysics http://www.edpsciences.org/journal/index.cfm?edpsname=aa
(Vol. 47, L1 to L24).


More information

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.

Images of Saturn taken with NACO can be found in ESO PR Photo 04a/02
../pr-2002/phot-04-02.html.


Notes

[1] 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.

[2] 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).


Contact

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


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