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Titan seen from the Cassini-Huygens spacecraft.
Discovered by: Christiaan Huygens
Discovery date: March 25, 1655
Orbital characteristics
Semi-major axis: 1,221,931 km[1]
Eccentricity: 0.028880 [2]
Orbital period: 15.94542 days
Inclination: 0.34854° (to Saturn's equator)
Satellite of: Saturn
Mean radius: 2575.50 ± 2.00 km (0.404 Earths) [3]
Surface area: 8.3×107 km²
Mass: 1.34520029 ± 0.00020155×1023 kg (0.0225 Earths) [3]
Mean density: 1.8798 ± 0.0044 g/cm³ [3]
Equatorial surface gravity: 1.352 m/s2 (0.14 g)
Escape velocity: 2.639 km/s
Rotation period: (synchronous)
Axial tilt: zero
Albedo: 0.21
Temperature: 90 K (−297°F)
Surface pressure: 146.7 kPa
Composition: 98.4% nitrogen
1.6% methane
Re: Titan
Titan or Saturn VI is the largest moon of Saturn and the second largest moon in the solar system,[4] after Jupiter's moon Ganymede. It is roughly 50% larger than Earth's moon by diameter, and is larger by diameter and mass than all known dwarf planets. It is also larger by diameter than the planet Mercury, though Mercury is more than twice as massive. It was discovered on March 25, 1655, by the Dutch astronomer Christiaan Huygens,[5] and was the first of Saturn's moons to be discovered.
Titan is the only moon in our solar system to have a dense atmosphere.[4] 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, such as the discovery of liquid hydrocarbon lakes near its north pole. These are the only large, stable bodies of surface liquid known to exist anywhere other than Earth.
Titan is never visible to the naked eye, but can be observed through small telescopes (diameter greater than 5 cm) or strong binoculars. It has a maximum magnitude of +7.9, which is outshone by six asteroids (Vesta, Pallas, Iris, Hebe, Juno, Melpomene) and the dwarf planet Ceres. Titan reaches an angular distance of about 20 Saturn radii from Saturn and subtends a disk 0.8 arcseconds in diameter.
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 "Saturn's ordinary satellite",[6] 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,[7] wherein he suggested the names of the Titans, sisters and brothers of Cronos (the Greek Saturn), be used.
« Last Edit: June 19, 2007, 06:50:16 am by Abraxas » Report Spam Logged
Size comparison of the Earth and Titan.
Titan's internal structure.
Internal structure
Titan's diameter and mass (and thus its density) are similar to Jovian moons Ganymede and Callisto.[11] Based on its bulk density of 1.88 g/cm³, Titan's 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.[12] 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.
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).[13] 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 humans could fly through it by flapping "wings" attached to their arms.[14] 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.[15]
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.[16] 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 the Earth's average of 272.[16] In the methane the isotope ratio of 12C/13C is 82.3 compared with the Earth standard of 89.9.[16] 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 whereas the carbon and the hydrogen are far less depleted. The ratio of argon to nitrogen is 100 times less than in Earth's atmosphere.[16]
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.[17]
The most recent flyby has suggested the existence of a large cloud over Titan's north pole, existing at a height of 40 km. 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.[18]
Simulations of global wind patterns based off of wind speed data taken by Huygens during its descent have suggested that Titan's atmosphere circulates in a single enormous Hadley cell. Warm air rises in Titan's southern hemisphere - experiencing summer during Huygens' descent - and sinks in the northern hemisphere, resulting in high-altitude air flow from south to north and low-altitude airflow from north to south. Such a large Hadley cell is only possible on a slowly rotating world such as Titan. There was also a pattern of air circulation flowing in the direction of Titan's rotation, from west to east.[19]
Natural ELF radio waves have been tentatively identified by Cassini in Titan's atmosphere. Titan's surface is thought to be a poor reflector of ELF waves, so they may be reflecting instead off of the liquid-ice boundary of a subsurface ocean of water and ammonia predicted by some theoretical models. Titan's ionosphere is also more complex than Earth's, with the main ionosphere at an altitude of 1200 km but with an additional layer of charged particles at 63 km. This splits Titan's atmosphere to some extent into two separate resonating chambers. The source of natural ELF waves on Titan is unclear as there doesn't appear to be extensive lightning activity
True-color image of layers of haze in Titan's atmosphere.
Layers of haze seen in a colorized ultraviolet image of Titan's night-side limb
Possible cloud of ethane (red) over Titan's north pole (marked NP)
A methane cloud imaged in false colour over Titan's north pole
At the surface, Titan's temperature is about 94 K (−179 °C, or −290.2 °F). 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.
Ground-based observations show that Titan's climate changes with the seasons. Over the course of Saturn's 30-year orbit, Titan's cloud systems appear to manifest for 25 years, and then fade for four to five years, before reappearing again.[21]
The findings of the Huygens probe indicate that Titan's atmosphere periodically rains liquid methane and other organic compounds onto the moon's surface.[22] 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.[23] 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. As of early 2007, liquid methane oceans appear to be present (see section below). This is a departure from the previously accepted theory that such oceans were all but entirely absent.[24]
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.[25] 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.
In December 2006, Cassini imaged a large cloud of methane, ethane and other organics over the moon's north pole. The cloud is over 2400 km in diameter and was still visible during a following flyby a month later. This cloud likely formed from evaporation from methane lakes recently observed at that pole, which were in turn likely formed by methane rains from the cloud. (see below). This is the strongest evidence yet for the long hypothesised "methanological" cycle (analogous to Earth's hydrological cycle) on Titan.[21]
It is currently summer in Titan's southern hemisphere. Southern summer on Titan will last until 2010, when Saturn's orbit, which governs the moon's motion, will tilt the northern hemisphere towards the Sun.[
Titan in false color showing surface details and atmosphere. "Xanadu" is the bright region at the centre-right
Surface features
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);[26] 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 infrared 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
http://en.wikipedia.org/wiki/Titan_%28moon%29
Pseudo-colored radar imaging data from a Cassini flyby of northern lattitudes, published in the journal Nature to illustrate the convincing evidence for large bodies of liquid. The false blue coloring indicates low radar reflectivity areas, likely caused by liquid.
Liquids on Titan
It has long been believed that lakes or even seas of methane might exist on Titan's surface but until recently, conclusive evidence has proven elusive.[27] 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 initially observed. Cassini observed surface features that could be explained as the products of flowing liquids, but again, there were few conclusive observations.
The first indication of the presence of a lake was observed at Titan's south pole, where clouds have been observed to cluster, and where an enigmatic dark feature at the pole, named Ontario Lacus was identified as a possible lake created by precipitation from them.[28] A possible shoreline has also been identified at the pole via radar imagery.[29] Then, on January 3, 2007, it was announced that scientists have "definitive evidence of lakes filled with methane on Saturn's moon Titan."[30][31] The high relative humidity of methane in Titan's lower atmosphere could be maintained by evaporation from lakes covering only 0.002–0.02 of the whole surface [32].
Following a flyby on July 22, 2006, 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.[33] The Cassini-Huygens team have now concluded 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.
The strongest evidence yet of lakes on Titan came during a Cassini flyby in late February 2007, as observations by the radar and camera instruments revealed several large features in the north polar region that may be large expanses of liquid methane and/or ethane, including one sea with an area of over 100,000 square kilometers (larger than Lake Superior), and another (though less definite) region potentially the size of the Caspian Sea.[34]
The discoveries at the poles contrast with the findings of the Huygens probe, which landed near Titan's equator on January 14, 2005. The images taken by the probe during its descent show no open areas of liquid, but 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, but it is now clear that Huygens landed on the dark region, and that it is solid without any indication of liquids. 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, 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.[35] 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
Since the existence of lakes on Titan has only been recently confirmed, and the hypothesized vast methane oceans have not been found, some scientists now believe that many of the moon's features are caused by cryovolcanism rather than running liquids. Alternatively, it has been hypothesized that Huygens landed during a dry season on Titan, and that periods of heavy methane rain 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 small site on this planet-sized moon, which is insufficient for evaluating the entire body.
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