A 'Dragon' on the Surface of Titan

http://www.eso.org/outreach/press-re.../pr-09-04.html

ESO Press Release 09/04
European Southern Obvervatory
14 April 2004

For immediate release

A "Dragon" on the Surface of Titan

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 [1] during recent commissioning
observations with the "Simultaneous Differential Imager (SDI)", a
novel optical device, just installed at the NACO Adaptive Optics
instrument [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 hydrocarbonates.

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.

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

---------------------------------------------------------------

Views of Titan - the quest for the surface

[ESO PR Photo 11a/04] ESO PR Photo 11a/04

Map of Titan's Surface Features

[Preview - JPEG: 608 x 400 pix - 40k]
[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
[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 hydrocarbonates.

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
Voyager images.

Recent spectroscopic and radar observations suggest that there
are huge surface reservoirs of liquid hydrocarbonates 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 Huygens probe launched from the NASA/ESA 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

[ESO PR Photo 11b/04] ESO PR Photo 11b/04

Four SDI-NACO Images

[Preview - JPEG: 400 x 501 pix - 23k]
[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 Hubble Space Telescope (HST) in the 1990's. From the
ground, images were obtained in 2001-2 with the Keck II and
Gemini North telescopes and more recently with the ESO Very
Large Telescope (VLT), cf. 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 [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 NACO adaptive optics
instrument on the 8.2-m VLT YEPUN telescope, using the
Simultaneous Differential Imager (SDI) [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

[ESO PR Photo 11c/04] ESO PR [ESO PR Photo 11d/04] ESO PR
Photo Photo
11c/04 11d/04

Simultaneous Views of Titan's Six Nightly Views of Titan's
Surface and Atmosphere Surface

[Preview - JPEG: 605 x 400 pix [Preview - JPEG: 698 x 400 pix
- 33k] - 44k]
[Fullres - JPEG: 1914 x 1266 [Fullres - JPEG: 1924 x 1103
pix - 609k] 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 [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 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 hydrocarbonates. 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

[ESO PR Photo 11e/04] ESO PR [ESO PR Photo 11f/04] ESO PR
Photo Photo
11e/04 11f/04

NACO Images of Titan's Surface Provisional Names of Titan
Surface Features
[Preview - JPEG: 583 x 400 pix
- 39k] [Preview - JPEG: 565 x 400 pix
[Fullres - JPEG: 1924 x 1320 - 46k]
pix - 749k] [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. 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 [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 Working Group on
Planetary System Nomenclature of the International
Astronomical Union (IAU WGPSN). From left to right, the SDI
team [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 [2] and are now available in the 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


Laird Close
University of Arizona
USA
Tel: +1 520 626 5992


Rainer Lenzen
Max-Planck Institut für Astronomie, Heidelberg
Germany
Tel: +49 6221 528 228