Titan is one of Saturn's 30 known moons. It is slightly less than half the volume of Mars. The chemistry going on in its dense atmosphere, which consists largely of nitrogen, methane (a carbon source) and hydrogen, is thought to be similar to the chemistry that went on in Earth's early atmosphere before life transformed it into the air we breathe today.

The Huygens probe descends to the surface of Titan

"When you study Titan, it's a bit like going back in a time machine to Earth 4 billion years ago. The atmosphere is a natural laboratory for studying prebiotic chemistry on early Earth - the chemistry that led to life," says Helmut Lammer from the Space Research Institute of the Austrian Academy of Sciences, Graz, Austria who is associated with a co-investigator team on Huygens. "There's a very low probability of finding real life there (it's far too cold and all the water is deep frozen on or below the surface), but the chemistry in the atmosphere may be very similar to the chemistry that preceded life on Earth."

When ESA's Huygens probe enters Titan's atmosphere in early 2005, one of its main objectives will be to study this chemistry. The probe is hitching a lift aboard NASA's Cassini spacecraft, which will also study Titan from orbit around Saturn. The two spacecraft are already on the final leg of their journey between Jupiter and Saturn.

"Methane in Titan's atmosphere is continuously destroyed by ultraviolet light. To explain the amount of the gas present in the atmosphere, we think there must be a large source either on or under Titan's surface. It could be in the form of lakes or oceans, or subsurface reservoirs," says Jean-Pierre Lebreton, ESA's project scientist for Huygens.

"Of course, we may not find the precise transformation that turned complex organic compounds into living things. But the better we understand the chemistry, the better our chance of working out how it led to life," says Francois Raulin, an astrobiologist from the University of Paris, France and a co-investigator on Huygens.