This page is about the moon of Saturn. For other meanings, see Titan (disambiguation).

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. Titan is the only moon in our solar system with a significant atmosphere.

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 [1] (http://adsabs.harvard.edu//full/seri/MNRAS/0008//0000042.000.html), wherein he suggested the names of the Titans, sisters and brothers of Cronos (the Greek Saturn), be used.

Visibility from Earth

Titan has a magnitude between +7.9 and +8.7 and reaches an angular distance of about 20 Saturn radii from Saturn. It can be observed through small telescopes (diameter greater than 5cm) or strong binoculars. It subtends a disk of 0.8" in diameter.

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 reflects a large amount of light causing 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 due to gravitational compression.

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 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 dense 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, carbon monoxide, 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. 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, effecting a nearly water-vaporless atmosphere. Scattered variable clouds punctuate an overall haze in Titan's atmosphere. These clouds are probably composed of methane, ethane or other simple organics. Other more complex chemicals in small quantities must produce the orange color as seen from space.

The thick atmosphere blocks most sunlight from reaching Titan's surface. The Huygens probe was unable to detect the direction of the sun during its descent, and although it was able to take images from the surface, scientists say the process was like photographing asphalt at dusk [2] (http://www.space.com/missionlaunches/titan_update_050121.html). It is therefore unlikely that Saturn is ever visible from the surface of Titan.

The findings of the Huygens probe indicate that Titan's atmosphere periodically rains liquid methane and other organic compounds onto the moon's surface [3] (http://www.planetary.org/news/2005/huygens_science-results_0121.html). It is possible that areas of Titan's surface may be coated in a tar-like layer of organic precipitate called tholin, but this has not been confirmed. The presence of argon 40 was also discovered in the atmosphere, evidence of cryovolcanism producing a "lava" of water ice and ammonia [4] (http://www.esa.int/esaCP/SEMHB881Y3E_index_0.html).

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 [5] (http://space.com/scienceastronomy/titan_mysteries_041028.html).

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

Overall topography

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.

In order to understand Titan's surface features better, the Cassini spacecraft is currently using radar altimetry and synthetic aperture radar imaging to map portions of Titan 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 area mapped so far appears to be fairly smooth with no height variation greater than 50 metres [6] (http://www.newscientist.com/news/news.jsp?id=ns99996598); however, radar altimetry has so far only covered part of the north polar region.

During the October 26 2004 fly-by of Titan by Cassini, 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 Huygens probe photographed pale hills with dark 'rivers' running down to a dark plain. Current understanding is that the hills are composed of water ice. Dark organic compounds rain from Titan's atmosphere and flow down the hills to form the dark plains [7] (http://www.esa.int/SPECIALS/Cassini-Huygens/SEMHB881Y3E_0.html).

Liquids on Titan

It has long been believed that lakes or even seas of methane might exist on Titan's surface. When the Cassini probe arrived in the Saturnian system, it was hoped that hydrocarbon lakes or oceans might be detectable by reflected sunlight from the surface of any liquid bodies, but no specular reflections were observed.

The findings of the January 14 2005 landing on Titan by the Huygens probe do not show any open areas of liquid. However, they strongly indicate the presence of liquids in the recent past. The Huygens images show pale hills crisscrossed with dark drainage channels. The channels lead into a wide, flat, darker region. It was initially thought that the dark region might be a lake of a fluid or at least tarry substance. However, it is now clear that Huygens landed on the dark region, and the craft recorded instead a muddy material variously described as being like loose sand, wet clay, or perhaps cr�me br�l�e (that is, a hard crust covering a sticky material). The images taken after the probe's landing show a flat plain covered in pebbles. The pebbles, which may be made of water ice, are somewhat rounded, perhaps indicating the action of fluids on them [8] (http://www.planetary.org/news/2005/huygens_images_0115.html).

The Huygens images have not confirmed the existence of lakes on Titan. It is currently believed that Huygens landed during a dry season on Titan, and that there has been heavy methane rain in the recent past that forms lakes which evaporate. The length of the intervals between rainy periods on Titan are unknown, and scientists stress that Huygens sampled only one tiny site on this planet-sized moon, insufficient for evaluating the entire planet [9] (http://www.planetary.org/news/2005/huygens_science-results_0121.html).

Features of the Huygens landing site

Huygens landed on a dark plain covered in small rocks or pebbles, which are composed of water ice [10] (http://www.esa.int/SPECIALS/Cassini-Huygens/SEMHB881Y3E_0.html). The two rocks just below the middle of the image on the right are smaller than they may appear. The left-hand one is 15 centimeters (about 6 inches) across, and the one in the center is 4 centimeters (about 1.5 inches) across, at a distance of about 85 centimeters (about 33 inches) from Huygens. There is evidence of erosion at the base of the rocks, indicating possible fluvial activity.

The surface is darker than originally expected, consisting of a mixture of water and hydrocarbon ice. It is believed that the 'soil' visible in the images is precipitation from the hydrocarbon haze above.

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 until then. 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 [11] (http://www.lpl.arizona.edu/~jrich/vgertitan.html), but by then they had already been observed in the infrared by the Hubble Space Telescope. Voyager 2 took only a cursory look at Titan. The Voyager 2 team had the option of steering the spacecraft to take a detail look at Titan or to use another trajectory which would allow it to visit Uranus and Neptune. Given the lack of surface features seen by Voyager 1, the latter plan was implemented.

The Cassini-Huygens spacecraft 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. Huygens landed on Titan on January 14 2005, discovering that much of the moon's surface features seem to have been formed by flowing fluids at some point in the past; however, the presence of actual open fluids on Titan today has not been confirmed at present.

Potential for life in Titan's volcanic hotbeds

Scientists have speculated that conditions on Titan resemble those of early Earth, though at a much lower temperature. Evidence of volcanic activity from the latest Cassini mission suggests that temperatures are probably much higher in hotbeds. Argon 40 detection in the atmosphere indicates that volcanoes spew flumes of water and ammonia.

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 Philip K. Dick's post-apocalyptic novel The Game-Players of Titan, a neurotic and suicidal man named Pete Garden must roll a three in Bluff, the game that's become a blinding obsession for the last inhabitants of Earth, against opponents who are from Titan.
• 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, but, due to a writer's error, is in orbit around Jupiter. This was later explained as being due to a scientific experiment in teleportation.
• 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.
• There is a novel by Alan E. Nourse, the American Science Fiction writer, that appeared in English on December 1954 called Trouble on Titan, translated into French as Revolte sur Titan in 1971.
• In the table-top science fiction game Warhammer 40,000 the Grey Knights Space Marine chapter keep their Fortress-Monastery on Titan.
• In the C64 computer game Project Firestart the setting of the story is located on a scientific space vessel which is floating near Titan in the Saturn system.
• Flight on Titan, a short story by Stanley G. Weinbaum
• In James P. Hogan's novel Code of the Lifemaker, Titan is inhabited by a race of Clanking Replicators

External links

• CLOUD WORLD: MISSION to TITAN (http://hammernews.com/cloudworld.htm)- Jan 14-26, the last grand space mission �the 11 year $3 billion Cassini mission to Saturn, successfully dropped a landing probe into the most earthlike atmosphere in the Solar System
• Nature article on Titan's weather (http://www.nature.com/news/2005/050117/full/050117-5.html)
• SolarViews entry on Titan (http://www.solarviews.com/eng/titan.htm)
• NASA's Cassini page focusing on the exploration of Titan (http://www.nasa.gov/mission_pages/cassini/main/index.html)
• The Darian Calendar for Titan (http://www.martiana.org/mars/saturn/Darian_Titan_frm.htm)
• Official ESA mission website (http://www.esa.int/SPECIALS/Cassini-Huygens/index.html)

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