Titan is Saturn's biggest moon. It is larger than either of the planets Mercury or Pluto and is the second largest natural satellite in the solar system after Ganymede. (It was originally thought to be slightly larger than Ganymede, but recent observations have shown that its thick atmosphere caused an overestimation of its diameter). Titan was discovered on March 25, 1655 by the Dutch astronomer Christian Huygens, making it one of the first non-terrestrial moons to be observed.
Titan is similar in bulk properties to Ganymede, Callisto, Triton and (probably) Pluto. Titan is about half water ice and half rocky material. It is probably differentiated into several layers with a 3400 km rocky center surrounded by several layers composed of different crystal forms of ice. Its interior may still be hot. Though similar in composition to Rhea and the rest of Saturn's moons, it is denser because it is so large that its gravity compresses its interior.
Titan is the only known moon with a fully developed atmosphere that consists of more than just trace gases. The presence of a significant atmosphere was first discovered by Gerard P. Kuiper in 1944 using a spectroscopic technique that yielded an estimate of a an atmospheric partial pressure of methane of order 100 millibars. Since that time, observations from Voyager program space probes have shown that, in fact, Titan's atmosphere is denser than Earth's, with a surface pressure more than one and a half times that of our planet and supports an opaque cloud layer that obscures Titan's surface features. It is thought that Titan may possess bodies of liquid ethane. Recent radar measurements from Earth suggest that there is no large-scale ocean of ethane on Titan, but it may still be present in smaller lakes.
At the surface, Titan's temperature is about 94 K. At this temperature water ice does not sublimate and thus there is little water vapor in the atmosphere. There are scattered variable clouds in Titan's atmosphere in addition to the overall deep haze. These clouds are probably composed of methane, ethane or other simple organics. Other more complex chemicals in small quantities must be responsible for the orange color as seen from space.
Titan was examined by both Voyager 1 and Voyager 2, with Voyager 1's course being diverted specifically to make a closer pass of Titan. Unfortunately Voyager 1 did not possess any instruments that could penetrate Titan's haze, which had not been known about up until that point in time.
The Cassini-Huygens Mission reached Saturn on July 1, 2004 and will map Titan's surface using radar later in the year; it will also release a probe named Huygens on December 252004 which will then dive into Titan's atmosphere where detailed measurements will be taken during its descent on January 14, 2005. The Huygens probe may even survive impact with Titan's surface for long enough to send back data on the conditions there.
In Arthur C. Clarke's novel Imperial Earth, Titan is home to a human colony with a population of 250 000 and provides an important role in the Solar System's economics; Titan's atmosphere supplies the hydrogen needed to support interplanetary travel.
In Stephen Baxter's novel Titan, a NASA mission to Titan must struggle to survive after a disastrous landing.
In the BBC television show Red Dwarf, the character Lister illegally imports a cat from Titan that goes on to found a well-dressed, but not particular intelligent, species called Felis Sapiens.
In the television show Starhunter, Titan features prominently as the former home of the character Dante, and is the site of a large colony.
In the comic series, 2000 A.D, Titan is used as a penal colony.