Scientists expect to have a much clearer vision of the surface of Titan, the
largest moon of Saturn, when the Huygens probe touches down on its surface
in 2004. In the meantime, both ground-based telescopes and space
observatories are contributing to the growing body of information on the
nature of Titan’s surface.

Titan, a planet-sized moon, is of particular interest because it is
considered to be representative of a pre-biotic state similar to that of
early Earth.

Methane, after nitrogen, is the most abundant compound in Titan’s atmosphere.
Both of these compounds are being continuously broken apart by ultraviolet
solar photons, energetic electrons from Saturn’s magnetosphere, and cosmic
rays.

The fragments of the parent molecules recombine and form new more complex
compounds. Photochemical models predict that ethane should be the main
organic product of these atmospheric reactions.

Ethane and other more complex organics may rain down from the atmosphere
onto Titan’s surface. If this is true, then we would expect to find huge
seas of ethane on Titan’s surface. Up until recently, it was believed that
the surface of Titan was mainly composed of lakes or oceans of liquid
hydrocarbons. The remaining dry parts of the surface should also be
covered with complex organic deposits.

Recent results obtained by ground-based telescopes have confirmed earlier
observations made by the Hubble Space Telescope. But tantalising new
results at infrared wavelengths have stirred up the debate about what the
surface of Titan is really like.

In particular, excellent observing conditions during a recent observation
campaign with the new adaptive optics system, PUEO, at the Canada-France-
Hawaii Telescope (CFHT) produced images of excellent quality. The images
were taken in the middle of the methane windows at 1.3 µm and 1.6 µm.
Methane absorbs light photons. But, in certain wavelength ranges, only a
small fraction of the photons coming from the surface is absorbed. This
allows us to ‘see’ Titan’s surface through these so-called methane windows.
These results were discussed at the General Assembly of the European
Geophysical Society (EGS) in Nice in March 2001.

“We have been obtaining data with adaptive optics since 1994,” says Athena
Coustenis, an astronomer at the Paris-Meudon observatory and one of the
scientists involved in the ADONIS (ESO) and HST observation campaign, “when
both ADONIS at ESO in Chile and HST produced an acceptable image of Titan.
It was the first to show Titan’s surface.”

These observations showed the existence of a bright area, which was highly
contrasted in the ADONIS images, but only recently with PUEO has it been
possible to analyze the details of this spot at shorter wavelengths.

“Having a good instrument is not enough, it is also important to have good
weather, even in Mauna Kea,” continued Coustenis. “Recently, we were lucky
and we obtained diffraction limited images. In addition, another bright
feature at Titan’s Western limb was noticed for the first time. This
feature might be diagnostic of diurnal effects but requires further
investigation before its origin can be firmly identified.”

A map of Titan’s geometrical albedo was also obtained. “From our albedo
maps, it appears that the darker areas are about 3 times darker than the
bright spot and they are compatible with a combination of organic deposits
and ice extents, possibly related to topography,” concluded Coustenis.

Another important feature of the recent CFHT observations was the
acquisition of data at 0.9 µm (another methane window) with the spectrograph
OASIS, which provides information for the first time at more than 70
different locations on Titan’s disk.

Is there lightning in Titan’s atmosphere?

Despite the lack of evidence for lightning on Titan, it is still considered
by many scientists to be a strong possibility. Therefore, ESA’s Huygens
probe has been designed (and tested) to withstand lightning strikes as it
descends through the Titan atmosphere.

Developing models and improving prediction capabilities for lightning
phenomena on Titan are important in light of the impact of lightning strikes
on the Huygens probe. Some of the work in progress in this area of research
was also discussed at the Nice meeting.

There are basically two possible charging mechanisms on Titan which could
lead to lightning strikes: charging by free electrons and ions or charging
by collision.

“Lightning on Titan may be rare because of low solar input, low temperature,
low gravity, etc. but nevertheless it may be possible,” says Tetsuya Tokano,
a scientist at the Institut für Geophysik und Meteorologie, Universität zu
K&oumlln.

“Methane clouds are necessary for charging, and there is evidence for
occasional clouds in the troposphere. Once it is formed a cloud rapidly
attracts a large number of free electrons which are abundant in Titan’s
troposphere. As a consequence, the negative space charge in the cloud may
cause a cloud-to-ground lightning strike in Titan’s lower troposphere. The
collisional charging mechanism, on the other hand, appears to be less
efficient since the charge transfer itself may be limited at Titan’s cold
temperatures and no substantial charge redistribution takes place in the
cloud due to the weak updraft and gravitation,” Tokano concludes.

Are aerosols on Titan sticky?

As part of the Cassini-Huygens mission to Saturn and Titan the Huygens
probe is set to sample aerosols as it descends through Titan’s atmosphere.
During the descent these aerosol particles may cover the surfaces of the
detectors, which would prevent Huygens from sampling the surroundings.
Whether this happens or not depends primarily on the stickiness of the
aerosols, which in turn is related to the age of the aerosol particles.

Dr Vladimir Dimitrov of the University of Tel Aviv presented a very
interesting study in which he described how aging of hydrocarbon aerosols
in Titan’s atmosphere occurs and what this implies for the Huygens Probe.

“Aging and charging of aerosols are very favorable phenomena with respect
to the functioning of the Huygens probe,” said Dimitrov, “because they
essentially weaken the possibility of damaging the detectors on-board the
Probe.”

In the course of time, the aerosol material changes its properties, either
spontaneously or as a result of several external factors. As a consequence,
it becomes much more inert, dense and hard, while becoming less sticky.
Moreover, external irradiation produces charging of the aerosol particles,
so that they have the ability to capture external electrons.

“Altogether,” concluded Dimitrov, “the combined effect of aging and
charging at the altitude range where the Huygens probe will operate
decreases the interference with the measurements by a factor of 50 to
100, and continuous trouble free operation of Huygens is ensured.”

We must still wait for the best close-up view

With still three years to go before Cassini-Huygens reaches Titan, the
puzzle over the nature of Titan’s surface remains. New ground-based
observations, and laboratory work, continue to fuel the debate in the
scientific community about the nature and complexity of its surface and
of its atmosphere.

Future observations with increased spectral resolution and adaptive optics
systems are important in order to prepare well for the Cassini-Huygens
observations. “But we have to be patient,” says Jean-Pierre Lebreton, ESA’s
Huygens project scientist. “We have to wait for Cassini-Huygens to be able
to reveal the secrets of Titan hidden behind the thick orange haze curtain
that shrouds the atmosphere.”

USEFUL LINKS FOR THIS STORY

* Why go to Titan?

http://sci.esa.int/content/doc/a5/1957_.htm

* Some facts about Titan

http://seds.lpl.arizona.edu/nineplanets/nineplanets/titan.html

* Titan images obtained with ADONIS at ESO

http://despa.obspm.fr/planeto/titan_adonis.html

* Images of Titan from PUEO at the CFHT

http://despa.obspm.fr/planeto/titan_pueo.html

* New Titan images obtained with the Keck telescope

http://www.spacedaily.com/news/cassini-99b.html

IMAGE CAPTIONS:

[Image 1:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=12&cid=12&oid=27002&ooid=27046]
Titan’s surface as seen with PUEO on the Canada-France-Hawaii Telescope.
These images were obtained by Athena Coustenis and colleagues. Copyright
© Athena Coustenis et al.

[Image 2:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=12&cid=12&oid=27002&ooid=15605]
HST images of Titan’s surface. Scientists for the first time have made
images of the surface of Saturn’s giant, haze-shrouded moon, Titan. They
mapped light and dark features over the surface of the satellite during
nearly a complete 16-day rotation. One prominent bright area they
discovered is a surface feature 2,500 miles across, about the size of
the continent of Australia. Copyright: © UA Lunar and Planetary Laboratory