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Titan ("TYE tun") is Saturn's biggest moon. It was discovered on March 25, 1655 by the Dutch astronomer Christiaan Huygens, and was the first satellite in the solar system to be discovered after the Galilean moons of Jupiter.

Huygens named his discovery simply Saturni Luna ("moon of Saturn"). Later, Jean-Dominique Cassini named the four moons he discovered (Tethys, Dione, Rhea and Iapetus) Lodicea Sidera ("the stars of Louis") to honour king Louis XIV. Astronomers fell into the habit of referring to them as Saturn 1 through Saturn 5. Other epithets used were the "Huygenian satellite of Saturn" (or "Huyghenian"), or the "sixth satellite of Saturn" (in order of distance from Saturn, once Mimas and Enceladus were also discovered in 1789).

The name "Titan" and the names of all seven satellites of Saturn then known come from John Herschel (son of William Herschel, discoverer of Mimas and Enceladus) in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope, wherein he suggested the names of the Titans, sisters and brothers of Cronos (the Greek Saturn), be used.

Physical characteristics

Titan is larger than the planet Mercury (though less massive) 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. Like several other satellites, Titan is also larger and more massive than Pluto.

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.

Atmosphere

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 an atmospheric partial pressure of methane of order 100 millibars. Since that time, observations from Voyager 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.

The atmosphere is 94% nitrogen — the only nitrogen-rich atmosphere in the solar system aside from our own — with significant traces of various hydrocarbons making up much of the remainder (including methane, ethane, diacetylene, methylacetylene, cyanoacetylene, acetylene, propane, along with carbon dioxide, cyanogen, hydrogen cyanide, and helium). These hydrocarbons are thought to form in Titan's upper atmosphere in reactions resulting from the breakup of methane by the Sun's ultraviolet light, producing a thick orange smog, and Titan's surface may be coated in a tar-like layer of organic precipitate called tholin. Titan has no magnetic field and sometimes orbits outside Saturn's magnetosphere, directly exposing it to the solar wind. This may ionize and carry away some molecules from the top of the atmosphere.

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.

Surface Features

At present, maps of Titan's surface are vague and imprecise, owing to the obscuring atmosphere. However, a large, highly reflective area about the size of Australia has been identified in infra-red images, and is already detectable in visible light images from the Cassini spacecraft. This region has been unofficially named Xanadu; it is not certain what kind of terrain it represents. There is a similarly-sized dark area on the other side of the moon, which some have speculated may be a methane sea. More recent images from Cassini have also descried some enigmatic linear markings, which some scientists have suggested may indicate tectonic activity.

Titan's surface features will be better understood by late 2004, when Cassini will use radar mapping techniques during several close flybys of the moon.

See list of geological features on Titan.

Exploration of Titan

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 25, 2004 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 or splash-down on Titan's surface for long enough to send back data on the conditions there.

Titan in fiction

• 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.

External links

• The Darian Calendar for Titan

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• Saturn's moons