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Titan is the largest moon of the planet Saturn.

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Titan is the second largest moon in the solar system, after Jupiter's moon Ganymede. Titan is roughly 50% larger than Earth's moon by diameter, and is larger by diameter and mass than all known dwarf planets. Titan is also larger by diameter than the planet Mercury, though Mercury is more than twice as massive.

Titan Moon.
titan moon.
Titan Moon: mysterious and beautiful.
Titan seen from the Cassini-Huygens spacecraft.
Discoverer Christiaan Huygens
Date March 25, 1655
Orbital characteristics
Semimajor axis 1,221,931 km
eccentricity 0.028880
Orbital period 15.94542 days
inclination 0.34854º (to Saturn's equator)
Is a Satellite of Saturn
Physical characteristics
Mean diameter 5,150 km (0.404 Earths)
Surface area 8.3-107 kmÂ
mass 1.345-1023 kg (0.0225 Earths)
Mean density 1.88 g/cmÂ
Equatorial surface
1.35 m/s2,
or 0.14 g
escape velocity 2.63 km/s
rotation period (synchronous)
axial tilt zero
Albedo 0.21
Surface temp.
 ?K 90 K  ?K
Atmospheric characteristics
pressure 146.7 kPa
Nitrogen 98.4 percent
methane 1.6 percent

Titan is the only moon in our solar system to have a dense atmosphere. 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 accumulating.

Titan 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.

On July 27, 2006, NASA confirmed the presence of hydrocarbon lakes in Titan's north polar region.

Name Titan.

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, Giovanni Domenico Cassini named the four moons he discovered (Tethys, Dione, Rhea and Iapetus) Sidera Lodoicea ("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, 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 arcseconds in diameter.

Physical characteristics of the moon Titan.

Titan in false color showing surface details and atmosphere.

At 5,150 km across, Titan is larger than the planet Mercury (though less massive) and the Dwarf planet Pluto and is the second largest natural satellite in the solar system after Ganymede. Prior to the arrival of Voyager 1 in 1980, Titan was thought to be slightly larger than Ganymede, an error resulting from the haze in its atmosphere which extends around 880 km above the surface and is almost opaque to visible light. Hence, visual observations of Titan before discovery of this haze overestimated its diameter.

Internal structure of the moon Titan.

Titan's internal structure.
Titan's internal structure.

Titan's diameter and mass (and thus its density) are similar to Jovian moons Ganymede and Callisto. Based on its bulk density of 1.88 g/cmÂ, Titan bulk composition is half water ice and half rocky material. It is probably differentiated into several layers with a 3400 km (2,040 mi) rocky center surrounded by several layers composed of different crystal forms of ice. Its interior may still be hot and there may be a liquid layer consisting of water and ammonia between the ice crust and the rocky core. Though similar in composition to Rhea and the rest of Saturn's moons, it is denser due to gravitational compression.

Atmosphere of the moon Titan.

Titan's night-side.
Layers of haze seen in a colorized ultraviolet image of Titan's night-side limb.
Titan's atmosphere.
True-color image of layers of haze in Titan's 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 (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 at visible wavelengths. The haze that can be seen in the picture to the right contributes to the moon's anti-greenhouse effect and lowers the temperature by reflecting sunlight away from the satellite. The thick atmosphere blocks most visible wavelength light from the sun and other sources from reaching Titan's surface. It is so thick, in fact, and the gravity is so low, that a human could fly through it by flapping "wings" attached to his arms . 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.

The atmosphere is 98.4% Nitrogen the only dense nitrogen-rich atmosphere in the solar system aside from our own with the remaining 1.6% composed of methane and only trace amounts of other gases such as hydrocarbons (including Ethane, diacetylene, methylacetylene, cyanoacetylene, Acetylene, propane), Argon, carbon dioxide, Carbon monoxide, cyanogen, hydrogen cyanide and helium. The 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.

The nitrogen ratio of14N to15N is 183 compared with the Earth's average of 272. In the methane the isotope ratio of12C/13C is 82.3 compared with the earth standard of 89.9. The isotope ratio of H/D is 3.6 x 103 compared with 3.0 x 103 on Earth. The depletion of the lighter isotope of nitrogen indicates atmospheric escapes where as the carbon and the hydrogen are far less depleted. The ratio of argon to nitrogen is 100 times less than in Earth's atmosphere.

Titan's north pole.
Possible cloud of Ethane (red) over Titan's north pole (marked NP).

From all available data several theoretical models and experiments for the development of the Titan atmosphere have been derived. The high UV-radiation and high energy electrons are an energy source for many chemical reactions in the atmosphere. The hydrogen compounds ammonia and methane undergo dehydrogenation, forming complex organic compounds, nitrogen and hydrogen which is lost over cosmological time. The absence of ammonia and the presence of methane, although they should have a similar half life, indicates a source for methane on Titan. Clathrates (methane incorporated into ice), comets and a Fischer Tropsch like synthesis are suggestions for the abundance of methane .

The most recent flyby has suggested the existence of a large cloud over Titan's north pole, existing at a height of 40km. At this altitude it is cold enough for ethane to freeze, and the detected size of these particles is only 1-3 microns, suggesting again ethane, rather than methane which is also known to condense in the atmosphere of Titan. The downdrafts at high northern latitudes are strong enough to drive these particles towards the surface. A theory is that it is currently raining (or if cool enough - snowing) on the north pole. When the seasons switch, ethane will begin to condense over the south pole.

Climate of the moon Titan.

At the surface, Titan's temperature is about 94 K (-179 ºC, or -290.2 ºF). At this temperature water ice does not sublimate, so the atmosphere is nearly free of water vapor. 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 findings of the Huygens probe indicate that Titan's atmosphere periodically rains liquid methane and other organic compounds onto the moon's surface . 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 . 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 . 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 of the moon Titan.

Cassini image.
"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 meters (165 feet); however, radar altimetry has so far only covered part of the north polar region.

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.

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, and while features that look like lakes have been discovered at the poles, conclusive evidence for the presence of liquids on Titan's surface at the present time remains absent.

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.

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

The photographs taken during the January 14, 2005 landing on Titan by the Huygens probe do not show any open areas of liquid, but they strongly indicate the presence of liquids in the recent past, showing pale hills crisscrossed with dark drainage channels that 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 that it is solid.

There are no indications of liquids at 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 was similar to loose sand, or wet clay. 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. 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.

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.

Two recent developments have 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 Ontario Lacus has been identified as a possible lake created by precipitation from the clouds that cluster at the pole. A possible shoreline has also been identified at the pole via radar imagery.

Radar image of northern latitudes of Titan, showing the dark features identified as lakes.

Following a flyby on July 22, in which the Cassini spacecraft's radar imaged the northern latitudes (which are currently in winter), a number of large, dark (and thus smooth to radar) patches were seen dotting the surface near the pole. Scientists speculate that these features are almost certainly the long sought hydrocarbon lakes of Titan. Some of the lakes appear to have channels running in or out of them, which are just as smooth. Ethane and methane may be liquids near Titan's poles, which are cold enough for these gases to condense. Repeated coverage of these areas should prove whether they are truly liquid, as any changes that correspond with wind blowing on the surface of the liquid, would alter the roughness of the surface and be visible in the radar. NASA recently confirmed that there is ice from hydrocarbon rain at the north polar area.

Impact craters on the moon Titan.

Radar SAR and imaging data from Cassini have revealed a relative paucity of impact craters on Titan's surface, suggesting a youthful surface. To date, only three impact craters have been confirmed, include a 440-km wide multi-ring impact basin named Menrva (seen by ISS as a bright-dark concentric pattern), a smaller 80-km wide flat-floored crater named Sinlap, and a 30-km crater with a central peak and dark floor named Ksa. RADAR and Cassini imaging have also revealed a number of "crateriforms", circular features on the surface of Titan that maybe impact related, but lack certain features that would make identification certain. For example, a 90-km wide ring of bright, rough material known as Guabonito has been observed by Cassini. This feature is thought to be an impact crater filled-in by dark, windblown sediment. Several other similar features have been observed in the dark Shangri-la and Aaru regions. RADAR observed several circular features that may be craters in the bright region Xanadu during Cassini's April 30, 2006 flyby of Titan.

cryovolcanism of the moon Titan.

Titan's complex surface.
One of the first radar images of Titan's complex surface. The circular feature at left, Ganesa Macula, is thought to be a cryovolcanic dome.

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, enough for liquid water to exist. Argon 40 detection in the atmosphere indicates that volcanoes spew plumes of water and ammonia.

Dark terrain of the moon Titan.

Titan's sand dunes.
Titan's sand dunes (below), compared with dunes on Earth's surface (above).

In the first images of Titan's surface taken by Earth-based telescopes in the early 2000s, large regions of dark terrain were revealed straddling Titan's equator. Prior to the arrival of Cassini, these regions were thought to be seas of organic matter like tar or seas of liquid hydrocarbons. However, Radar images captured by the Cassini spacecraft has instead revealed some of these regions to be extensive plains covered in longitudinal sand dunes. The sand dunes are believed to be formed by wind generated as a result of tidal forces from Saturn on Titan's atmosphere, which are 400 times stronger than the tidal forces of the Moon on Earth. The tidal winds cause dunes to build up in long parallel lines, with Titan's zonal winds aligning the dunes west-to-east. The dunes break this pattern around mountains, where the wind direction is shifted.

Widespread evidence has also recently been found to support the claim that lakes of hydrocarbons do in fact exist on Titan's North pole during the second of several planned Cassini spacecraft flybys. (see section Liquids on Titan).

The sand on Titan might have formed when liquid methane rained and eroded the ice bedrock, possibly in the form of flash floods. Alternatively, the sand could also have come from organic solids produced by photochemical reactions in Titan's atmosphere.

Huygens landing site of the moon Titan.

Titan's surface.
Huygens image from Titan's surface.

The Huygens probe landed just off the easternmost tip of 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, 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.

Huygens landed on a dark plain covered in small rocks and pebbles, which are composed of water ice. 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.

List of geological features on Titan: Exploration of Titan.

Epimetheus and Titan.
Cassini image of Epimetheus and 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, an unforseen factor. Many years later, intensive digital processing of images taken through Voyager 1's orange filter did reveal hints of the light and dark features now known as Xanadu and the Sickle, 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 and took the highest-resolution images ever of the moon's surface, at only 1,200 kilometers, 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. Present liquid on the surface may be found near the north pole, in the form of many lakes that were recently discovered by Cassini.

Life on Titan.

Artistic representation of a view from the surface of Titan, with dune fields, lakes, channels and Saturn in the background.

Scientists believe that the atmosphere of early Earth is similar in composition to the atmosphere on Titan. Consequently, many hypotheses have developed that attempt to bridge the step from chemical to biological evolution. The Miller-Urey experiment and several following experiments have shown that with an atmosphere similar to that of Titan and the addition of UV radiation, complex molecules and polymer substances like tholins can be generated. The reaction starts with dissociation of nitrogen and methane forming hydrocyan and ethine. Further reactions have been studied extensively.

All of these experiments have led to the suggestion that enough organic material exists on Titan to start a chemical evolution like on Earth, if liquid water is available for longer periods. Several theories suggest that liquid water from an impact could be preserved under a frozen isolation layer, or even that water ammonia oceans can exist deep below the surface. The limited solar energy would only provide energy for a limited biota. For Titan, the search for life is at an early stage. The Cassini-Huygens mission was not equipped to provided evidence for complex organics or even biology. Due to this lack of experimental data, scientists have found no hint of life so far. Future missions are not planned and after adding the time for planning, construction, and the voyage itself, further scientific results are several decades in the future.

Of course, we must keep in mind that terrestrial life is, so far, the ONLY clear case of life we have".

Titan in fiction.

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

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