Titan (moon)

**Titan**
Discovery
Discoverer	Christiaan Huygens
Date	March 25, 1655
Orbital characteristics
Semimajor axis	1,221,931 km
Eccentricity	0.028880 ^{^*}
Orbital period	15.94542 d
Inclination	0.34854° (to Saturn's equator)
Is a satellite of	Saturn
Physical characteristics
Mean diameter	5,150 km (0.404 Earth)
Surface area	8.3 km²
Mass	1.345 kg (0.0225 Earths)
Mean density	1.88 g/cm³
Equatorial surface gravity	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.

min	mean	max
?K	94 K	?K

Atmospheric characteristics Pressure 146.7 kPa Nitrogen 98.4 percent Methane 1.6 percent

Titan (tye'-tən, IPA , Greek Τιτάν) or Saturn VI is the largest moon of Saturn and the second largest moon in the solar systemNASA page: News-Features-the Story of Saturn "Titan is the second-largest moon in the entire solar system.", after Jupiter's moon Ganymede. It is roughly 50% larger than our own moon, and is larger than the planets Mercury and Pluto. Titan is the only moon in our solar system to have a dense atmosphere NASA page: News-Features-the Story of Saturn "it's the only moon with 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 continuously accumulating.

Titan was discovered on March 25, 1655 by the Dutch astronomer Christiaan HuygensNASA page: The Story of Saturn/The moons "On March 24, 1655, ... The next day, Christiaan Huygens ... discovered its largest moon, Titan." States the date of discovery. verified 22:00, 29 Mar 2005 (UTC), 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 (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) Lodoicea 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 HopeSatellites of Saturn; Observations of Mimas, the closest and most interior satellite of Saturn, Mr Lassell, Monthly Notices of the Royal Astronomical Society, volume 8, page 42, 1847-11-12, verified 2005-03-29, 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

Titan is larger than the planet Mercury*Bill Arnett (2005). Titan. Retrieved April 10, 2005. "Titan is nevertheless larger in diameter than Mercury" (though less massive) and is the second largest natural satellite in the solar system after Ganymedeibid. "It was long thought that Titan was the largest satellite in the solar system but recent observations have shown that Titan's atmosphere is so thick that its solid surface is slightly smaller than Ganymede's.". It was originally thought to be slightly larger than Ganymede, an error resulting from the haze in its atmosphere which extends around 880 kmMori K., Tsunemi H., Katayama H., Burrows D.N., Garmire G.P., Metzger A.E. (2004), An X-Ray Measurement of Titan's Atmospheric Extent from Its Transit of the Crab Nebula, Astrophysical Journal, v. 607, pp. 1065-1069. Chandra images used by Mori et al can be viewed here. above the surface and is almost opaque to visible light. Hence, visual observations of Titan before discovery of this haze overestimated its diameter. Like several other satellites, Titan is also larger and more massive than the planet Pluto.

Titan has similar bulk properties to Jovian moons Ganymede and Callisto, the Neptunian moon Triton, and (probably) the planet Pluto. Titan is about 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

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. 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 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 of ¹⁴N to ¹⁵N is 183 compared with 272 the earth average. In the methane the isotope ratio of ¹²C/¹³C is 82.3 compared with the earth standard of 89.9. The isotope ratio of H/D is 3.6 10³ compared with 3.0 10³ 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 atmosphere.

At the surface, Titan's temperature is about 94 K (−179 °C, or −290.2 °F). 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 Huygens Probe Sheds New Light on Titan, Peter de Selding, Space News, 2005-01-21, verified 2005-03-28. 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 Titan: Arizona in an Icebox?, Emily Lakdawalla, 2004-01-21, verified 2005-03-28. 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 Seeing, touching and smelling the extraordinarily Earth-like world of Titan, ESA News, European Space Agency, 2005-01-21, verified 2005-03-28. 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 Hydrocarbon volcano discovered on Titan, David L. Chandler, NewScientist.com news service, New Scientist, 2005-06-08. 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 New Images of Titan Baffle Astronomers, Henry Bortman, Astrobiology Magazine, 2004-10-28, verified 2005-03-28. 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.

From all available data several theoretical models and experiments for the development of the titan atmosphere have 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 have 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 .

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 meters Titan's complex and strange world revealed, Stephen Battersby, NewScientist.com news service, New Scientist, 2004-10-29, verified 2005-03-28 (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.

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 Seeing, touching and smelling the extraordinarily Earth-like world of Titan, ESA News, European Space Agency, 2005-01-21, verified 2005-03-28.

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. Early Huygens Results: Titan Threw Curves at ESA Probe, Emily Lakdawalla, The Planetary Society, 2005-03-19, verified 2005-03-28

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, 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 grainsTitan probe's pebble 'bash-down', BBC News, Apr. 10, 2005.. 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 New Images from the Huygens Probe: Shorelines and Channels, But an Apparently Dry Surface, Emily Lakdawalla, 2005-01-15, verified 2005-03-28.

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 Titan: Arizona in an Icebox?, Emily Lakdawalla, 2004-01-21, verified 2005-03-28.

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 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 [http://www.spaceref.com/news/viewpr.html?pid=17829. These identifications remain uncertain at present.

Sand Dunes

Radar images captured by the Cassini spacecraft as it flew by Titan in October 2005 show sand dunes at Titan's equator much like those in deserts of Earth.

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.

Scientists had previously thought that the dark regions on Titan's equator were seas of liquid methane.

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. , Sand Dunes Found on Saturn's Moon Titan http://www.space.com/scienceastronomy/060504_sands_titan.html by Sara Goudarzi, space.com, 04 May 2006

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 landed on a dark plain covered in small rocks and pebbles, which are composed of water ice Seeing, touching and smelling the extraordinarily Earth-like world of Titan, ESA News, European Space Agency, 2005-01-21, verified 2005-03-28. 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, 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 Titan's Surface and Rotation: New Results from Voyager 1 Images James Richardson, Ralph Lorenz, & Alfred McEwen, Icarus, July 2004, Vol. 170/1, pp. 113-124 verified 2005-03-28, 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 2004NASA Page: Cassini-Huygens: Operations "Oct. 26, 2004: Cassini makes its first close pass by Titan. Cruising by at a distance of only 1,200 kilometers (750 miles), the spacecrafts radar provides the first detailed glimpses of the moon's mysterious surface." 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 2005ibid. "Jan. 14, 2005: The European Space Agency's Huygens probe descends through Titan's cloudy atmosphere, touching down on the surface about two and half hours later."'', 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 extraterrestrial settings in science fiction other than Earth's Moon and the planets.

External links

Cassini Imaging Central Laboratory for Operations (2005). CICLOPS: Cassini Imaging. Retrieved March 28, 2005.
European Space Agency. (2005). ESA - Cassini-Huygens. Retrieved March 28, 2005.
Gangale, Thomas (2002). The Darian Calendar for Titan. Retrieved March 28, 2005.
Hamilton, Calvin J. (2001). Titan - Saturn IV. Retrieved March 28, 2005.
Hammerschlag, Michael (2005). CLOUD WORLD: MISSION to TITAN. Retrieved March 28, 2005.
NASA (2005). NASA - Cassini-Huygens: Close Encounter With Saturn. Retrieved March 28, 2005.
Perry, Jason (2005). Titan Today. Retrieved March 28, 2005.
The Planetary Society (2005). TPS: Saturn's moon Titan. Retrieved March 28, 2005.
University of Arizona Lunar and Planetary Lab (2005). Lunar and Planetary Lab The Descent Imager-Spectral Radiometer of the Cassini-Huygens Mission to Titan. Retrieved March 28, 2005.
Video of Huygens’ descent from the ESA

References

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Saturn's moons

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