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 atmosphereNASA 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.
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).
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.
At 5,150 km across, Titan is larger than the planets Mercury (though less massive) and Pluto and is the second largest natural satellite in the solar system after Ganymede.*Bill Arnett (2005). Titan. Retrieved April 10, 2005. "Titan is nevertheless larger in diameter than Mercury"; "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." 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.Mori 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. Hence, visual observations of Titan before discovery of this haze overestimated its diameter.
Internal Structure
Titan's diameter and mass (and thus its density) are similar to Jovian moons Ganymede and Callisto.Lunine, J. "Comparing the Triad of Great Moons." Astrobiology Magazine: March 21, 2005. Retrieved July 20, 2006. Based on its bulk density of 1.88 g/cm3, 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.G. Tobie et al. (2005). "Titan's internal structure inferred from a coupled thermal-orbital model". Icarus175 (2): 496-502. 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
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).G. P. Kuiper (1944). "Titan: a Satellite with an Atmosphere". Astrophysical Journal100: 378. Since that time, observations from Voyagerspace 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. 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.
Layers of haze seen in a colorized ultraviolet image of Titan's night-side limb
The nitrogen ratio of 14N to 15N is 183 compared with the Earth's average of 272. 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 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 atmosphere.
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 .
Climate
At the surface, Titan's temperature is about 94 K (âˆ'179 °C, or âˆ'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.
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.
Surface features
"Xanadu" is the bright region at the centre-right of this Cassini image
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.
Impact craters
RADAR SAR and imaging data from Cassini have revelaed a relative paucity of impact craters on Titan's surface. To date, known craters 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 90-km wide ring of bright material thought to be an impact crater named Guabonito.
Cryovolcanism
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 melt. Argon 40 detection in the atmosphere indicates that volcanoes spew plumes of water and ammonia.
Dark Terrain
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.H. G. Roe et al. (2004). "A new 1.6-micron map of Titan's surface". Geophys. Res. Lett.31 (17): CiteID L17S03. 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. [2] (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. [Saharan 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
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.
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.
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 142005 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.
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 [3]. A possible shoreline has also been identified at the pole via radar imagery [4]. These identifications remain uncertain at present.
Radar image of northern latitudes of Titan.
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 in radar) patches were seen dotting the surface near the pole.[5] 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.
Huygens landing site
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, 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.
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.
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 SickleTitan'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 12004 and has begun the process of mapping Titan's surface by radar;The Cassini probe flew by Titan on October 262004NASA 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 142005ibid. "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. Present liquid on the surface may be found near the north pole, in the form of many lakes that were recently discovered by Cassini. [6]
