Titan (moon)

Titan is the planet Saturn's largest moon. It is larger than either of the planets Mercury or Pluto and is the second-largest moon 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 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 discovered.

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; in fact, Titan's atmosphere is denser than Earth's, with a surface pressure more than one and a half times that of Earth. Its opaque cloud cover has obscured Titan's surface features from all probes sent so far, but it is thought that Titan may posses 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 with significant traces of various hydrocarbons making up much of the remainder (including methane, ethane, diacetylene, methylacetylene[?], cyanoacetylene[?], acetylene, propane, and also 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. 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.

The Cassini probe, scheduled to reach Saturn in 2004, will map Titan's surface with radar and drop a smaller subprobe named Huygens into Titan's atmosphere at 8.1 degrees North, 208.7 degrees longitude. The Huygens probe may even survive impact with Titan's surface long enough to send back data on conditions there.