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VLT Looks through Narrow Atmospheric Window and Produces Most Detailed
Images Yet

Summary

New images of unsurpassed clarity have been obtained with the ESO Very
Large Telescope (VLT) of formations on the surface of Titan, the
largest moon in the Saturnian system. They were made by an
international research team [22][1] during recent commissioning
observations with the "Simultaneous Differential Imager (SDI)", a
novel optical device, just installed at the NACO Adaptive Optics
instrument [23][2].

With the high-contrast SDI camera, it is possible to obtain extremely
sharp images in three colours simultaneously. Although mainly
conceived for exoplanet imaging, this device is also very useful for
observations of objects with thick atmospheres in the solar system
like Titan. Peering at the same time through a narrow, unobscured
near-infrared spectral window in the dense methane atmosphere and an
adjacent non-transparent waveband, images were obtained that are
virtually uncontaminated by atmospheric components. They map the
reflectivity of a large number of surface features in unprecedented
detail.

The images show a number of surface regions with very different
reflectivity. Of particular interest are several large "dark" areas of
uniformly low reflectivity. One possible interpretation is that they
represent huge surface reservoirs of liquid hydrocarbons.

Whatever the case, these new observations will be most useful for the
planning of the delivery of the Huygens probe - now approaching the
Saturn system on the NASA/ESA Cassini spacecraft and scheduled for
descent to Titan's surface in early 2005.

[24]PR Photo 11a/04: Map of Titan's surface features at 1.575 µm.
[25]PR Photo 11b/04: Four SDI-NACO images.
[26]PR Photo 11c/04: Simultaneous Views of Titan's surface and
atmosphere.
[27]PR Photo 11d/04: Six nightly views of Titan's surface.
[28]PR Photo 11e/04: A comparison of NACO images of Titan's surface.
[29]PR Photo 11f/04: Provisional names of Titan surface features.
_________________________________________________________________

Views of Titan - the quest for the surface

[30]ESO PR Photo 11a/04 

ESO PR Photo 11a/04

Map of Titan's Surface Features

[31][Preview - JPEG: 608 x 400 pix - 40k]
[32][Fullres - JPEG: 1920 x 1260 pix - 672k]

Caption: ESO PR Photo 11a/04 shows the clearest view of Titan's
surface, available so far. It was obtained through a "transparent",
narrow spectral window with the 8.2-m VLT YEPUN telescope and the NACO
adaptive optics instrument operated in the Simultaneous Differential
Imager (SDI) mode [33][2]. It covers about three-quarters of the full
surface and has an image resolution (sharpness) of 0.06 arcsec,
corresponding to 360 km on the surface. One degree of longitude on the
equator corresponds to 45 km on Titan's surface. The brightness is
proportional to the surface reflectivity. The nature of the various
regions is still unknown although it is speculated that the darkest
areas may indicate the extent of reservoirs of liquid hydrocarbons.

Titan, the largest Saturnian moon and the second largest moon of the
solar system (only Jupiter's Ganymede is slightly larger), is the only
satellite known with a substantial atmosphere. It is composed mainly
of nitrogen (like that of the Earth) and also contains significant
amounts of methane. Opaque orange hazes and clouds of complex organic
molecules effectively shield the solid surface from view, cf. e.g. the
[34]Voyager images.

Recent spectroscopic and radar observations suggest that there are
huge surface reservoirs of liquid hydrocarbons and a methane-based
meteorological cycle similar to Earth's hydrological cycle. This makes
Titan the only known object with rainfall and potential surface oceans
other than the Earth and thus a tantalizing research object for the
study of pre-biotic chemistry and the origin of life on Earth.

The [35]Huygens probe launched from the NASA/ESA [36]Cassini-Huygens
mission will enter Titan's atmosphere in early 2005 to make
measurements of the physical and chemical conditions, hopefully
surviving the descent to document the surface as well.

Coordinated ground-based observations will provide essential support
for the scientific return of the Cassini-Huygens encounter. However,
only 8-10 m class telescopes with adaptive optics imaging systems or
space-borne instruments can achieve sufficient image sharpness to
attain a useful level of detail.

The new map of a large part of Titan's surface, shown in PR Photo
11a/04, represents an important contribution in this direction.

A question of atmospheric windows

[37]ESO PR Photo 11b/04 

ESO PR Photo 11b/04

Four SDI-NACO Images

[38][Preview - JPEG: 400 x 501 pix - 23k]
[39][Fullres - JPEG: 1539 x 1926 pix - 568k]

Caption: ESO PR Photo 11b/04 shows four images of Titan, obtained
simultaneously with the NACO adaptive optics instrument in the SDI
observing mode with the corresponding wavebands indicated. The
individual images have a diameter of 0.86 arcsec and have here been
magnified for clarity. As explained in the text, the images obtained
at wavelengths 1.575 and 1.600 µm penetrate right to the surface while
the images at 1.625 µm show the comparatively featureless atmosphere.

The first intriguing views of Titan's surface were obtained by the
[40]Hubble Space Telescope (HST) in the 1990's. From the ground,
images were obtained in 2001-2 with the [41]Keck II and Gemini North
telescopes and more recently with the [42]ESO Very Large Telescope
(VLT), cf. [43]ESO PR Photos 08a-c/04. All of these observations were
made through a single narrow-band filter at a time.

The wavelengths used for such observations are critical for the amount
of surface detail captured on the images. Optimally, one would look
for a spectral band in which the atmosphere is completely transparent;
a number of such "windows" are known to exist. But although the above
observations were made in wavebands roughly matching atmospheric
windows and do show surface features, they also include the light from
different atmospheric layers. In a sense, they therefore correspond to
viewing Titan's surface through a somewhat opaque screen or, more
poetically, the sight by an ancient sailor, catching for the first
time a glimpse of an unknown continent through the coastal haze.

One narrow "window" is available in the near-infrared spectral region
near wavelength 1.575 µm. In February 2004, an international research
team [44][1] working at the ESO VLT at the Paranal Observatory (Chile)
obtained images of Titan's surface through this spectral window with
unprecedented spatial resolution and with the lowest contamination of
atmospheric condensates to date.

They accomplished this during six nights (February 2, 3, 5, 6, 7 and
8, 2004) at the time of the commissioning phase of a novel
high-contrast imaging mode for the [45]NACO adaptive optics instrument
on the 8.2-m VLT YEPUN telescope, using the Simultaneous Differential
Imager (SDI) [46][2]. This novel optical device provides four
simultaneous high-resolution images (PR Photo 11b/04) 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.

Mapping Titan's surface in unprecedented detail

[47]ESO PR Photo 11c/04 

ESO PR Photo 11c/04

Simultaneous Views of Titan's Surface and Atmosphere

[48][Preview - JPEG: 605 x 400 pix - 33k]
[49][Fullres - JPEG: 1914 x 1266 pix - 609k]

[50]ESO PR Photo 11d/04 

ESO PR Photo 11d/04

Six Nightly Views of Titan's Surface

[51][Preview - JPEG: 698 x 400 pix - 44k]
[52][Fullres - JPEG: 1924 x 1103 pix - 668k]

Caption: ESO PR Photo 11c/04 shows simultaneous images of Titan,
obtained on February 7, 2004, with NACO in SDI mode. Left: at 1.575 µm
with a clear view towards the surface. Right: at 1.625 µm, where the
atmosphere appears entirely opaque. PR Photo 11d/04 shows views of
Titan, obtained on six nights in February 2004. At the right, the
image from the first night (Feburary 1-2, 2004) has been enlarged for
clarity and the coordinate grid on Titan is indicated. The images are
false-colour renderings with the three SDI wavebands as red (1.575 µm;
surface), green (1.600 µm; surface) and blue (1.625 µm; atmosphere),
respectively.

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. Still, the present week-long observing campaign
enabled the team to map approximately three-quarters of the surface of
Titan.

A new map of the surface of Titan (in cylindrical projection and
covering most, but not all of the area imaged during these
observations) is shown in PR Photo 11a/04. For this, the simultaneous
"atmospheric" images (at waveband 1.625 µm) were "subtracted" from the
"surface" images (1.575 and 1.600 µm) in order to remove any residual
atmospheric features present in the latter. The ability to subtract
simultaneous images is unique to the SDI camera [53][2].

This truly unique map shows the fraction of sunlight reflected from
the surface - bright areas reflect more light than the darker ones.
The amount of reflection (in astronomical terms: the "albedo") depends
on the composition and structure of the surface layer and it is not
possible with this single-wavelength ("monochromatic") map alone to
elucidate the true nature of those features.

Nevertheless, recent radar observations with the [54]Arecibo antenna
have provided evidence for liquid surfaces on Titan, and the
low-reflection areas (dark on PR Photos 11a/04 and 11f/04) could
indicate the locations of those suspected reservoirs of liquid
hydrocarbons. They also provide a possible source for the
replenishment of methane that is continuously lost in the atmosphere
because of decomposition by the sunlight.

Presumably, the bright, highly reflective regions are ice-covered
highlands.

Provisional names of the new features

[55]ESO PR Photo 11e/04 

ESO PR Photo 11e/04

NACO Images of Titan's Surface

[56][Preview - JPEG: 583 x 400 pix - 39k]
[57][Fullres - JPEG: 1924 x 1320 pix - 749k]

[58]ESO PR Photo 11f/04 

ESO PR Photo 11f/04

Provisional Names of Titan Surface Features

[59][Preview - JPEG: 565 x 400 pix - 46k]
[60][Fullres - JPEG: 1914 x 1356 pix - 776k]

Caption: In PR Photo 11e/04, an earlier image of Titan by NACO,
obtained in a waveband at 1.3 µm that does not perfectly match an
atmospheric window (cf. [61]PR Photo 08c/04) is compared to a new
SDI-NACO image of the same region. The greater clarity and contrast of
the latter is evident; it is due to the smaller degree of "atmospheric
contamination". PR Photo 11f/04 identifies the low-reflection areas
now seen on the surface of Titan and which were given provisional
names by the research team [62][1]; see the text.

A comparison (PR Photo 11e/04) with an earlier NACO image (available
as PR Photo 08c/04) obtained through another filter is useful. It
demonstrates the importance of employing a filter that precisely fits
the atmospheric window and hence the gain of clarity with the present
observations. It also provides independent confirmation of the reality
of the gross features, since the observations are separated by 15
months in time.

Over the range of longitudes which have been mapped during the present
observations (PR Photo 11a/04), it is obvious that the southern
hemisphere of Titan is dominated by a single bright region centered at
approximately 15° longitude. (Note that this is not the so-called
"bright feature" seen in the HST images at longitude 80° - 130°, an
area that was not covered during the present observations).

The equatorial area displays the above mentioned, well-defined dark
(low-reflection) structures, cf. PR Photo 11g/04. In order to
facilitate their identification, the team decided to give these dark
features provisional names - official names will be assigned at a
later moment by the [63]Working Group on Planetary System Nomenclature
of the International Astronomical Union (IAU WGPSN). From left to
right, the SDI team [64][1] has referred to these features informally
as: the "lying H", the "dog" chasing a "ball", and the "dragon's
head".

More observations to come

The team expects to continue imaging and monitoring of Titan in the
coming months, with the goal of assisting the Cassini-Huygens team in
the interpretation and understanding of what will certainly be a rich
and complex flow of information about this enigmatic moon.

More information

The information and images in this press release are based on data
that were obtained during commissioning observations of the SDI device
on NACO [65][2] and are now available in the [66]ESO Science Data
Archive. This work is described in detail in a research paper "First
surface map of Titan at 1.575 microns" by M. Hartung et al., submitted
to the European research journal Astronomy & Astrophysics.

Notes

[1] The team is composed of Markus Hartung (ESO-Chile), Laird M. Close
(Steward Observatory, University of Arizona, Tucson, USA), Rainer
Lenzen, Tom M. Herbst and Wolfgang Brandner (Max-Planck Institut for
Astronomie, Heidelberg, Germany), Eric Nielsen and Beth Biller
(Steward Observatory, University of Arizona, Tucson, USA), and Olivier
Marco and Chris Lidman (ESO-Chile).

[2] The novel Simultaneous Differential Imager (SDI) is a special set
of optics mounted into the near-infrared camera CONICA on VLT YEPUN.
It is comprised of a double calcite Wollaston prism responsible for
the quad beam splitting and a special four-quadrant narrow-band filter
that is located directly in front of the detector. It was developed
and deployed by Laird Close (Steward Observatory, University of
Arizona) and Rainer Lenzen (Max-Planck-Institut für Astronomie in
Heidelberg) in collaboration with ESO. 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.

Contacts

Markus Hartung
ESO Santiago
Chile
Tel: +56 2 463 3071
[67]mhartung at eso.org

Laird Close
University of Arizona
USA
Tel: +1 520 626 5992
[68]lclose at as.arizona.edu

Rainer Lenzen
Max-Planck Institut für Astronomie, Heidelberg
Germany
Tel: +49 6221 528 228
[69]lenzen at mpia.de

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last modified: 2004/04/14
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