Titan (moon)

Titan (tye'-tun, Greek Τιτάνας) is the largest moon of Saturn and the second largest moon in the solar system[1], after Jupiter's moon Ganymede. It was discovered on March 25, 1655 by the Dutch astronomer Christiaan Huygens[2], 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 to have a dense atmosphere[3]. Until very recently, this atmosphere inhibited understanding of Titan's surface, but the moon is currently undergoing study by the Cassini-Huygens mission, and new information about it is continuously accumulating.

Titan

Discovery
Discoverer	Christiaan Huygens
Date	March 25, 1655
Orbital characteristics
Semimajor axis	1,221,931 km
Eccentricity	0.028880 [4]
Orbital period	15.94542 d
Inclination	0.34854° (to Saturn's equator)
Is a satellite of	Saturn
Physical characteristics
Mean diameter	5150 km (0.404 Earths)
Surface area	83×106km2
Mass	1.345×1023 kg
Mean density	1.88 g/cm3
Equatorial surface gravity	1.35 m/s2, or 0.14 g
Rotation period	(synchronous)
Axial tilt	zero
Albedo	0.21
Surface temp.	min mean max  ?K 94 K  ?K
Atmospheric characteristics
Pressure	160 kPa
Nitrogen	95 percent
Methane	5 percent

Name

Huygens named his discovery simply Saturni Luna (Latin for "Saturn's moon", which can also be written Luna Saturni) (De Saturni Luna observatio nova, 1656; XV). 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 I through Saturn V. Other epithets used were the "Huygenian satellite of Saturn" (or "Huyghenian"), or the "sixth satellite of Saturn" (Saturn VI, still in use) (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[5], 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 5 cm) or strong binoculars. It subtends a disk 0.8" in diameter.

Physical characteristics

Titan is larger than the planet Mercury[6] (though less massive) and is the second largest natural satellite in the solar system after Ganymede[7]. 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 in false colour showing surface details and atmosphere. Photographed on October 26, 2004 by the Cassini spacecraft

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 (10 kPa). Since that time, observations from Voyager space probes have shown that the Titanian 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 95% 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 (−179 °C). 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 [8]. 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 [9]. 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 [10]. Later, a methane-spewing volcano was spotted in close-up images, and Titanian volcanism is now believed to be a significant source of the methane in the atmosphere; previously hypothesized methane oceans now appear to be virtually absent [11]. 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 [12]. 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

"Xanadu" is the bright region at the centre-right of this Cassini image

The Cassini mission has revealed that Titan's surface is relatively smooth. The few objects that seem to be impact craters appeared to have been filled in, perhaps by raining hydrocarbons or volcanoes. The area mapped so far appears to have no height variation greater than 50 metres [13]; however, radar altimetry has so far only covered part of the north polar region.

One of the first radar images of Titan's complex surface

Titan's surface is marked by broad regions of bright and dark terrain. These include a large, highly reflective area about the size of Australia identified in infra-red images from the Hubble Space Telescope and the Cassini spacecraft. This region is named Xanadu and appears to represent an area of relatively high ground. There are dark areas of similar size elsewhere on the moon, observed from the ground and by Cassini; it had been speculated that these are methane or ethane seas, but Cassini observations seem to indicate otherwise (see below). Cassini has also spotted some enigmatic linear markings, which some scientists have suggested may indicate tectonic activity, as well as regions of bright material cross-cut by dark lineaments within the dark terrain.

In order to understand Titanian 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.

RADAR SAR data taken during a flyby on February 15, 2005 revealed even more intriguing surface features, including a 440-km wide multi-ring impact basin (seen by ISS as a bright-dark concentric pattern), a smaller 60-km wide flat-floored crater, and regions of roughly parallel bright and dark lineaments which maybe ice- or hydrocarbon-rich sand dunes.

The Huygens probe landed near a bright region now called Adiri, and photographed pale hills with dark 'rivers' running down to a dark plain. Current understanding is that the hills (also referred to as highlands) are composed mainly of water ice. Dark organic compounds, that are created in the upper atmosphere by the ultraviolet radiation of the Sun, may rain from Titan's atmosphere. They are washed down the hills with the methane rain and are deposited on the plains over geological time scales [14].

Liquids on Titan

It has long been believed that Titanian lakes or even seas of methane might exist on the surface. However, while many of the surface features could be explained as the products of flowing liquids, no conclusive evidence has yet been found for the presence of liquids on Titan's surface at the present time. [15]

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.

Image taken during the descent of Huygens, showing hills riddled with channels, and what appears (deceptively) to be a shoreline

The findings of the January 14, 2005 landing on Titan by the Huygens probe do not show any open areas of liquid, although 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 it is solid.

There is no immediate trace of liquid on the Huygens landing site. A penetrometer studied the composition of the surface as the craft impacted it, and it was initially reported that the surface is similar to loose sand, wet clay, or perhaps crème brûlée (that is, a hard crust covering a sticky material). However, subsequent analysis of the data suggests that this reading was likely caused by Huygens displacing a large pebble as it landed, and that the surface is better described as a 'sand' made of ice grains[16]. 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, which may indicate the action of fluids on them [17].

The existence of lakes on Titan thus remains unconfirmed, and some scientists now believe that many of the moon's features are caused by cryovolcanism rather than running liquids. However, it has been hypothesized that Huygens landed during a dry season on Titan, and that periods of heavy methane rain in the recent past could form lakes that subsequently 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, which is insufficient for evaluating the entire body [18].

Two recent developments have, however, kept the possibility of Titanian lakes alive at Titan's south pole, where clouds have been observed to cluster. An enigmatic dark feature at the pole, named Lacus Ontario has been identified as a possible lake created by precipitation from the clouds that cluster at the pole [19]. A possible shoreline has also been identified at the pole via radar imagery [20]. These identifications remain uncertain at present.

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 plumes of water and ammonia.

Features of the Huygens landing site

Huygens image from Titan's surface, colorized in accordance with measurements taken by the probe

Huygens landed on a dark plain covered in small rocks or pebbles, which are composed of water ice [21]. 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 [22], 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 detailed 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[23] and took the highest-resolution images ever of the moon's surface, at only 1,200 kilometers away from the planet[24], discerning patches of light and dark that would be invisible to the human eye. Huygens landed on Titan on January 14, 2005[25], 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.

Titan in fiction

Titan is one of the most popular settings in science fiction other than Earth's Moon and the planets.

Titan in literature

1935: Flight on Titan, a short story by Stanley G. Weinbaum

1951: Titan is the Sol System home base of the parasitical aliens who attempt to take control of Earth in The Puppet Masters by Robert A. Heinlein.

1954: 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.

1959: Kurt Vonnegut's novel The Sirens of Titan features a journey that climaxes on Titan.

1963: 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.

1976: 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.

1983: In James P. Hogan's novel Code of the Lifemaker, Titan is inhabited by a race of Clanking Replicators

1986: Stanisław Lem's novel Fiasco contains several chapters set on Titan, and a character who ends up frozen on the surface for several hundred years.

1997: In Stephen Baxter's novel Titan, a NASA mission to Titan must struggle to survive after a disastrous landing.

2000: In Michael McCollum's novel, The Clouds of Saturn (ISBN 0740805282), Titan is a homeworld of Envon and Kimber Crawford, and Arvin Taggart.

Titan in cinema and television

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 film Gattaca (1997), Titan is the goal for a space mission, which is seen lifting off in the final sequence.

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 1985 horror film Creature, Titan is where ancient aliens preserved dormant creatures from all over the galaxy.

LV-426, the fictional moon in the scifi horror movie Alien, bears a striking resemblence to Titan. Like Titan, it orbits a ringed gas giant and possesses a substantial, "almost primordial" atmosphere of nitrogen and methane. Its size and surface temperature are also comparable, though it is described as having a rocky, lava base, as opposed to Titan's icy structure. There is also evidence of cryovolcanism on LV-426, just as some scientists suspect occurs on Titan.

In Titan A.E. (2000), a spaceship called Titan is used as an escapepod after the destruction of Earth. 'Titan' as name for the spaceship is an homage to the possibility that Titan can bear life.

In the German pulp science fiction series Perry Rhodan, Titan has a prison and advanced medical facilities.

Titan in comics and anime

In the Marvel Comics Universe, Titan is home to a colony of Eternals, a godlike race of men and women.

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 anime Cowboy Bebop (1998), Titan was once the site of a war. It is unclear whether there was a colony on the moon.

In the DC Comics Universe, Titan is home to a race of telepaths. Depending on what part of the continuity you are reading, they may have been seeded there in the late 20th century as a colony by Lar Gand (variously known as Mon-El, Valor and M'Onel). The most prominent example of a DC Titanian is Saturn Girl, a founding member of the 30th Century Legion of Super Heroes.

Titan in video games

An Apple II game called Titan Empire had human inhabitants of this moon attempting to take over the solar system.

In the table-top science fiction game Warhammer 40,000, the Grey Knights Space Marines 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.

In the Activision game Battlezone (1998), Titan is the site of a Soviet base and several battles between the American, Soviet, and "Fury" forces.

The game Flashback (1992) takes place mostly on Titan.

Interplay's game Hardwar (1998) takes place in a fictionalized city called Misplaced Optimism, which is on Titan.

Some levels of Descent (1994), as well as its second sequel, Descent 3 (2000), take place on Titan. In Descent you are in a mine, while in Descent 3 you travel to a proving ground.

In Starlancer, Titan is the site of a major, decisive battle between Alliance and Coalition forces.

Titan Unveiled: Saturn's Mysterious Moon Explored, Ralph Lorenz, Jacqueline Mitton

Titan from Cassini-Huygens, Robert Brown, Jean Pierre Lebreton, Hunter Waite

Pan | Daphnis | Atlas | Prometheus | S/2004 S 6 | S/2004 S 4 | S/2004 S 3 | Pandora | Epimetheus and Janus | Mimas | Methone | Pallene | Enceladus | Telesto, Tethys, and Calypso | Polydeuces, Dione, and Helene | Rhea | Titan | Hyperion | Iapetus | Kiviuq | Ijiraq | Phoebe | Paaliaq | Skathi | Albiorix | S/2004 S 11 | Erriapo | Siarnaq | S/2004 S 13 | Tarvos | Mundilfari | S/2004 S 17 | Narvi | S/2004 S 15 | S/2004 S 10 | Suttungr | S/2004 S 12 | S/2004 S 18 | S/2004 S 9 | S/2004 S 14 | S/2004 S 7 | Thrymr | S/2004 S 16 | Ymir | S/2004 S 8

see also: Rings of Saturn | Cassini-Huygens | Themis