Titan, one of the moons of Saturn, photographed by Voyager 2, 1981.

Titan, the 15th and largest of Saturn's moons, is unique amongst the moons of the Solar System in that it has an atmosphere. The atmosphere is mainly nitrogen, with small amounts of argon, methane, other organic compounds such as ethane, hydrogen cyanide and carbon dioxide, and some water. The atmosphere is so thick and opaque that neither of the two Voyager probes was able to see the moon's surface, which is thought to consist of ice, overlaying a core of rock. The Casini probe is due to map Titan with radar in 2004 in order to determine what the surface looks like. NASA's two Voyager spacecraft were launched in 1977 to explore the planets in the outer solar system. Voyager 2 pased Saturn in August 1981, before continuing on to Uranus and Neptune.