Titan's Atmosphere and Surface Titan, Saturn's largest natural satellite, is located 1300 million kilometres from Earth. It is a fascinating object and a real challenge to scientists. Indeed, Titan resembles the very young Earth before the appearance of life, when oxygen probably existed in the form of carbon dioxide and supplies of methane were abundant. Titan lacks the input from intense meteoritic bombardment which occurred on Earth during its accretion phase and delivered the water and organics into the atmosphere. As temperatures on Titan are quite low, the chemical reactions leading to complex prebiotic molecular chains are very slow. There is also a deficiency of the main catalyst, liquid water. In addition, there is very little oxygen on Titan and until very recently it was detected exclusively in the form of traces of carbon monoxide and dioxide. But recently we have discovered water vapour in the atmosphere, see below. Titan's surface may be partly covered with organic deposits falling like rain through the atmosphere. The liquid lakes would be mostly hydrocarbons. The rest of the surface could be composed of ices and rocks. Nevertheless, within the Solar System Titan resembles our planet most closely and it is the best possible natural laboratory to study conditions on the primordial (very young) Earth. ISO discovers water on Titan In 1997, ESA's artificial satellite ISO, the Infrared Space Observatory, in geostationary orbit around the Earth since 1995, offered a wonderful opportunity to study Titan. ISO performed beautifully for two years and gave us fantastic new results on both astronomical and planetary objects allowing astronomers to study the Universe without having to worry about the interference from the Earth's atmosphere. The ISO observations of Titan showed two narrow emission lines at wavelengths around 40 µm (1 µm = 0.001 mm) which were due to water vapour. This provides an explanation for the existence of oxygen compounds on Titan. The water vapour enters Titan's atmosphere in the form of water ice particles sputtered from the near-by Saturnian rings and satellites, which then vaporise and combine with hydrocarbons to produce the CO and CO2 we see today. Yet another similarity was discovered between Titan and our home planet! A very complex atmosphere What about the rest of the atmosphere? Where does the nitrogen come from? It is a challenge for astronomers to understand how two bodies sharing so many features could form at such great distances from each other as the Earth and Titan and in so diverging conditions. Titan's atmosphere is indeed an enigma, because no other satellite has such characteristics. Just as for terrestrial planet atmospheres, there are two controversial theories for the origin of Titan's nitrogen gas envelope. Some scientists believe that the primary atmosphere was created from outgassing from the satellite's interior during its formation. Others say it resulted from cometary impacts on the surface. Recent measurements of the isotopic composition of some elements in comets show differences with those of Titan. So the first theory is probably correct. The mystery will perhaps not be solved before the vital clues that will be provided by the Cassini/Hyugens mission. The surface But what about the surface of this fascinating object? We do not know much about Titan's surface. It lies hidden under thick layers of methane clouds. For a long time, it was thought that with all these organics falling on the surface, and with ethane being the major product in liquid form, Titan's surface must be covered entirely with ethane-methane oceans. It has been shown since that this is not the case. From spectroscopic measurements over almost ten years, we have discovered that the surface of Titan is not the same when you look at different longitudes or latitudes. The leading hemisphere is brighter than the trailing one. We would like to know if Titan's surface is mostly liquid (not water, but liquid hydrocarbon, also known as fuel, falling down through the atmosphere in some sort of rain), or if is it mostly solid, with ices and rocks sharing the landscape. From images that have been made with the Hubble Space Telescope and other advanced techniques (like "adaptive optics") using Earth's biggest telescopes, such as the Canada-France-Hawaii Telescope (CFHT) and the European Southern Observatory (ESO), it has been found that both bright and dark terrain are present on Titan's surface, an indication that it may be covered with combinations of frozen continents and hydrocarbon lakes. Based on observations, it would seem that water ice covers most of Titan, but rocks, in the form of silicates and some minor lake formations can also exist. Starting this year, a group of astronomers plans to use the ESO Very Large Telescope (VLT) in Chile to study Titan's surface more extensively. Great prospects! A fabulous landscape should appear in future images of Titan: snow-covered mountains, dark brown soft-edged lakes and a lot of rocks. However, the reality of this landscape will probably only be fully revealed to us by the Huygens probe, in 2004.

Titans surface with adaptive optics at the CFHT Telescope The definitive proof of the presence of water vapour in Titan/s atmosphere was obtained in 1997 when ESA's Infrared Space Observatory (ISO) observed Titan during several hours with its Short Wavelength Spectrometer. Two clear emission peaks were found at the expected far-infrared wavelengths, 39.37 and 43.89 microns. (Spectrum from ESA; photo from JPL - Voyager). Does the surface of Titan really look like this? We may know more a few years from now, when Cassini/Huygens explore this enigmatic Saturnian moon. This is an artist's concept of the Huygens Probe as it descends toward the surface of Titan (JPL).