Titan (moon)

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

Physical characteristics

Titan is larger than the planet Mercury{{an|larger_than_mercury}} (though less massive) and is the second largest natural satellite in the solar system after Ganymede{{an|second_largest_satellite}}. 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.

Related Topics:
Mercury - Ganymede - Pluto

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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.

Related Topics:
Ganymede - Callisto - Triton - Pluto - 3400 km - Rhea

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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. 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.

Related Topics:
Atmosphere - Gerard P. Kuiper - 1944 - Spectroscopic technique - Partial pressure - Methane - ''Voyager'' - Space probe - Earth - Ethane - Ocean

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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.

Related Topics:
Nitrogen - Methane - Ethane - Diacetylene - Methylacetylene - Cyanoacetylene - Acetylene - Propane - Carbon dioxide - Carbon monoxide - Cyanogen - Hydrogen cyanide - Helium - Sun - Ultraviolet - Magnetic field - Magnetosphere - Solar wind - Ionize

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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.

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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 {{an|Selding}}. It is therefore unlikely that Saturn is ever visible from the surface of Titan.

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The findings of the Huygens probe indicate that Titan's atmosphere periodically rains liquid methane and other organic compounds onto the moon's surface {{an|LakdawallaArizona1}}. 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 {{an|ESANews1}}. 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 {{an|NewScientist}}.

Related Topics:
Tholin - Argon 40 - Cryovolcanism

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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 {{an|Bortman}}.

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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.

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Surface features

Overall topography

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 {{an|Battersby}}; however, radar altimetry has so far only covered part of the north polar region.

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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.

Related Topics:
Reflective - Australia - Infra-red - Hubble Space Telescope - Xanadu - Methane - Ethane - Tectonic

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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.

Related Topics:
Radar - Synthetic aperture radar - Volcanic - Ammonia

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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.

Related Topics:
SAR - February 15 - 2005 - Dunes

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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 {{an|ESANews2}}.

Related Topics:
Adiri - Methane

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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. {{an|LakdawallaCurves}}

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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.

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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.

Related Topics:
January 14 - 2005 - Huygens - Fluid - Tar

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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{{an|BBCBashDown}}. 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 {{an|LakdawallaNewImages}}.

Related Topics:
Penetrometer - Sand - Clay - Crème brûlée

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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 {{an|LakdawallaArizona2}}.

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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 http://www.planetary.org/news/2005/titan_lake_0628.html. A possible shoreline has also been identified at the pole via radar imagery http://www.spaceref.com/news/viewpr.html?pid=17829. These identifications remain uncertain at present.

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Titan's volcanic hotbeds

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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.

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Features of the Huygens landing site

Huygens landed on a dark plain covered in small rocks or pebbles, which are composed of water ice {{an|ESANews3}}. 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.

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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.

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See list of geological features on Titan.

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