Titan - Saturn's Largest Moon With a diameter of 5150 km (3,218 miles), half our planet's size, Titan is second only to Jupiter's Ganymede (5260 km or 3,288 miles across) and bigger than Mercury, Pluto or the Moon. It is located nine times farther away from the Sun than the Earth and was discovered on March 25, 1655, by a Dutch amateur astronomer and optician, Christiaan Huygens. It took more than 250 years to discover that Titan had an atmosphere and another 50 years to find out that this atmosphere contained organic material. Titan was, however, mainly revealed to us by the Voyager missions in 1981. Titan's extended atmosphere Despite its somewhat disappointing appearance on the Voyager images, Titan happens to be the only satellite in our Solar system that has a thick and extended atmosphere, with nitrogen as its main gaseous constituent. The atmosphere of Titan is foggy, and besides nitrogen, it consists of small amounts of methane, and a little molecular hydrogen. In this fascinating environment, methane and nitrogen combine and photodissociate to produce saturated and unsaturated hydrocarbons, nitriles and other organics that eventually fall through the atmosphere and are deposited on the surface. Carl Sagan and others tried to reproduce these organics in the laboratory. What they recovered was a sort of brownish sludge they called "tholin", from the Greek word "Tholos", meaning mud. Biologists believe that when our planet was formed, these molecules, some of which, like hydrogen cyanide or cyanoacetylene, are called prebiotic, contributed to the development of life. The laboratory simulations show that molecules of even higher complexity could be expected on Titan. A dark and cold place A day in Titan lasts 16 Earth days while the year lasts about 30 of our years. The pressure at the surface of Titan is 1.5 atmospheres, that is just 1.5 times the Earth's pressure (the same that you would feel at the bottom of a swimming pool). You could easily move about on Titan's surface, except you would be very cold. Titan is so far from the Sun that the highest possible temperature encountered in the atmosphere is about -100°C and the light at noontime never gets brighter than twilight on Earth. In the atmosphere, the temperature varies with altitude in the same way as it does on Earth, with an inversion minimum at around 40 km. Thanks to a small greenhouse effect, as on Earth, the temperature on Titan's surface reaches -180°C! So there is no question - Titan is a cold and dark place! This is where the Huygens probe on the NASA/ESA Cassini/Hyugens mission will descend in 2004.

This Voyager 2 photograph of Titan, taken from a distance of 2.3 million kilometres, shows some detail in the cloud systems on this Saturnian moon. (Photo courtesy of JPL)