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Titan is the largest moon of Saturn. 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.

Name

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 the order of 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.

The October 2004 Cassini flyby photographed bright, high clouds at Titan's south pole, but they do not appear to be methane, as had been expected. This discovery has baffled scientists, and studies are currently underway to determine the composition of the clouds and decide whether our understanding of Titan's atmosphere needs to be revised.

Observations by Cassini of the atmosphere made in 2004 suggest that Titan is a "super rotator", like Venus, with an atmosphere that rotates much faster than its surface.

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 from the Hubble Space Telescope and the Cassini spacecraft. This region has been unofficially named Xanadu Regio; it is not certain what kind of terrain it represents. There are similarly-sized dark areas elsewhere on the moon, observed by Hubble, the Keck telescopes, and the Very Large Telescope, which some speculated may be methane or ethane seas, though Cassini observations seem to indicate otherwise. Cassini has taken higher-resolution pictures of all these features, and has also spotted some enigmatic linear markings, which some scientists have suggested may indicate tectonic activity.

During the October 26, 2004 fly-by of Titan, a smooth surface with few impact craters was observed, marked by strongly differentiated light and dark regions. This suggests that the moon has an active surface that is constantly being resurfaced, possibly by hydrocarbon rain or snow filling in the craters or by volcanic activity. The probe's spectrometer has revealed that the light and dark regions reflect sunlight at the same wavelength, suggesting that they may be composed of (or at least covered by) the same material. What that material is, is still unknown. It had been thought that hydrocarbon lakes or oceans might have been detectable by observing sunlight being reflected from the surface of any bodies of liquid, but no specular reflection was observed. This has led scientists to suggest that Titan's surface may be entirely frozen or slushy.

In order to understand Titan's surface features better, the Cassini spacecraft is currently using radar mapping techniques during its close fly-bys of the moon. The first images have revealed a complex, diverse geology with both rough and smooth areas. There are features that seem volcanic in origin, which probably disgorge water mixed with ammonia. There are also streaky features that appear to be caused by windblown particles. The few objects that seem to be impact craters appeared to have been filled in, perhaps by raining hydrocarbons. The presence or absence of lakes has not yet been confirmed; there are areas that return the signals that might be expected of open bodies of liquid, but there are other explanations. The moon appears to be fairly smooth with no height variation greater than 50 metres.

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. Many years later, heavy digital processing of images taken through Voyager 1's orange filter did reveal hints of the light and dark features known as Xanadu and the Sickle , but by then they had already been observed in the infrared by the Hubble Space Telescope.

The Cassini-Huygens Mission reached Saturn on July 1 2004 and has begun the process of mapping Titan's surface by radar; The Cassini probe flew by Titan on October 26 2004 and took the highest-resolution images ever of the moon's surface, discerning patches of light and dark that would be invisible to the human eye. Cassini released 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.
• Kurt Vonnegut's novel The Sirens of Titan features a journey that climaxes on Titan.
• In the BBC television show Red Dwarf, the character Lister illegally imports a cat from Titan that, through the action of hard radiation over millions of years, becomes the progenitor of a well-dressed, but not particularly 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 2000 AD comic series Judge Dredd, Titan is used as a penal colony.
• In the Marvel Comics Universe, Titan is home to a colony of Eternals, a godlike race of men and women.
• In the movie Gattaca (1997), Titan is the goal for a space mission at the movie's climax.
• In the anime Cowboy Bebop (1998), Titan was once the site of a war. It is unclear whether there was a colony on the moon.
• An Apple II game called Titan Empire had human inhabitants of this moon attempting to take over the solar system.
• In the novel Shattered Faith by Trevor Mark, Titan is the center of a vast extraterrestrial civilization that is angered by an ancient injustice, and aims for Earth to seek revenge.

External links

• SolarViews entry on Titan
• NASA's Cassini page focusing on the exploration of Titan
• The Darian Calendar for Titan

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