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's surface as seen with PUEO on the Canada-France-Hawaii Telescope. Photo: Athena Coustenis et al.

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."

Hubble Space Telescope images of Titan's surface. Photo: UA Lunar and Planetary Laboratory

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 Köln.

"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.

Artist's concept of Huygens probe descending to Titan. Photo: NASA/ESA

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."