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This article is about the planet Saturn\'s largest natural satellite. For other uses, see Titan (disambiguation).
 Titan seen from the Cassini–Huygens spacecraft. |
|
Discovery | |
|---|---|
| Discovered by | Christiaan Huygens |
| Discovery date | March 25 1655 |
| Orbital characteristicsUnless otherwise specified: JPL HORIZONS solar system data and ephemeris computation service. Solar System Dynamics. NASA, Jet Propulsion Laboratory. Retrieved on 2007-08-19. | |
| Semi-major axis | 1,221,870 km |
| Eccentricity | 0.0288 |
| Orbital period | 15.945 days |
| Inclination | 0.34854° (to Saturn\'s equator) |
| Satellite of | Saturn |
Physical characteristics | |
| Mean radius | 2576 ± 2.00 km (0.404 Earths) Jacobson, R. A.; Antreasian, P. G.; Bordi, J. J.; Criddle, K. E.; et.al. (December 2006). "The gravity field of the saturnian system from satellite observations and spacecraft tracking data". The Astronomical Journal 132 (6): 2520–2526. doi:10.1086/508812. |
| Surface area | 8.3×107 km² |
| Mass | 1.3452 ± 0.0002×1023 kg (0.0225 Earths) |
| Mean density | 1.8798 ± 0.0044 g/cm³ |
| Equatorial surface gravity | 1.352 m/s2 (0.14 g) |
| Escape velocity | 2.639 km/s |
| Rotation period | (synchronous) |
| Axial tilt | zero |
| Albedo | 0.22Williams, David R.. Saturnian Satellite Fact Sheet. NASA. Retrieved on 2007-09-03. |
| Temperature | 93.7 K (−179.45 °C)Giuseppe, Mitri; et al. (February 2007). "Hydrocarbon Lakes on Titan" (PDF). Icarus 186: 385–394. doi:10.1016/j.icarus.2006.09.004. |
| Apparent magnitude | 7.9 |
| Adjectives | Titanian |
Atmosphere | |
| Surface pressure | 146.7 kPa |
| Composition | 98.4% nitrogen 1.6% methane H. B. Niemann, et al. (2005). "The abundances of constituents of Titan’s atmosphere from the GCMS instrument on the Huygens probe". Nature 438: 779–784. doi:10.1038/nature04122. |
Titan (pronounced /ˈtaɪtən/ TYE-tən, or as Greek: Τῑτάν) or Saturn VI is the largest moon of Saturn, the only moon known to have a dense atmosphere,News Features: The Story of Saturn. Cassini-Huygens Mission to Saturn & Titan. NASA, Jet Propulsion Laboratory. Retrieved on 2007-01-08. and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found.
Titan is the twentieth most distant moon of Saturn and sixth farthest among those large enough to assume a spheroid shape. Frequently described as a satellite with planet-like characteristics, Titan has a diameter roughly 50% larger than Earth\'s moon and is 80% more massive. It is the second-largest moon in the Solar System, after Jupiter\'s moon Ganymede, and it is larger by diameter than the smallest planet, Mercury (although only half as massive). Titan was the first known moon of Saturn, discovered in 1655 by the Dutch astronomer Christiaan Huygens.Huygens Discovers Luna Saturni. Astronomy Picture of the Day. NASA. Retrieved on 2007-08-18.
Titan is primarily composed of water ice and rocky material. The dense atmosphere prevented understanding of Titan\'s surface until new information accumulated with the arrival of the Cassini–Huygens mission in 2004, including the discovery of liquid hydrocarbon lakes in the satellite\'s polar regions. These are the only large, stable bodies of surface liquid known to exist anywhere other than Earth. The surface is geologically young; although mountains and several possible cryovolcanoes have been discovered, it is relatively smooth and few impact craters have been discovered.
The atmosphere of Titan is largely composed of nitrogen and its climate includes methane and ethane clouds. The climate—including wind and rain—creates surface features that are similar to those on Earth, such as sand dunes and shorelines, and, like Earth, is dominated by seasonal weather patterns. With its liquids (both surface and subsurface) and robust nitrogen atmosphere, Titan is viewed as analogous to the early Earth, although at a much lower temperature. The satellite has thus been cited as a possible host for microbial extraterrestrial life or, at least, as a prebiotic environment rich in complex organic chemistry. Researchers have suggested a possible underground liquid ocean might serve as a biotic environment.Grasset, O., Sotin C., Deschamps F., (2000). "On the internal structure and dynamic of Titan". Planetary and Space Science 48: 617–636. doi:10.1016/S0032-0633(00)00039-8. Fortes, A.D. (2000). "Exobiological implications of a possible ammonia-water ocean inside Titan". Icarus 146 (2): 444–452. doi:10.1006/icar.2000.6400.
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Christiaan Huygens, discoverer of Titan
Titan was discovered on March 25, 1655, by the Dutch astronomer Christiaan Huygens. Huygens was inspired by Galileo\'s discovery of Jupiter\'s four largest moons in 1610 and his improvements on telescope technology.Discoverer of Titan: Christiaan Huygens. European Space Agency (April 24, 2007). Retrieved on 2007-08-18. Huygens himself made advances in the technology and his discovery of Titan owed "partly to the quality of his telescope and partly to luck".Huygens Ring, Cassini\'s Division & Saturn\'s Children. Dibner Library Lecture. Smithsonian Institute Libraries (October 27, 2004). Retrieved on 2007-08-19. He named it simply Saturni Luna (or Luna Saturni, Latin for "Saturn\'s moon"), publishing in the 1655 tract De Saturni Luna Observatio Nova. After Giovanni Domenico Cassini published his discoveries of four more moons of Saturn between 1673 and 1686, astronomers fell into the habit of referring to these and Titan as Saturn I through V (with Titan then in fourth position). Other early epithets for Titan include "Saturn\'s ordinary satellite". (1673) "A Discovery of two New Planets about Saturn, made in the Royal Parisian Observatory by Signor Cassini, Fellow of both the Royal Societys, of England and France; English\'t out of French.". Philosophical Transactions 8 (1673): 5178–5185. Titan is officially numbered Saturn VI because after the 1789 discoveries the numbering scheme was frozen to avoid causing any more confusion (Titan having borne the numbers II and IV as well as VI). Numerous small moons have been discovered closer to Saturn since then.
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.Mr Lassell (November 12, 1847). "Satellites of Saturn; Observations of Mimas, the closest and most interior satellite of Saturn". Monthly Notices of the Royal Astronomical Society 8: 42. Retrieved on 2005-03-29. He suggested the names of the mythological Titans, sisters and brothers of Cronos, the Greek Saturn.
Titan orbits Saturn once every 15 days and 22 hours. Like the Earth\'s moon and many of the other gas giant satellites, its orbital period is identical to its rotational period; Titan is thus tidally locked in synchronous rotation with Saturn. Its orbital eccentricity is 0.0288, and it is inclined 0.348 degree relative to the Saturnian equator. Viewed from Earth, the moon reaches an angular distance of about 20 Saturn radii (just over 1.2 million kilometers) from Saturn and subtends a disk 0.8 arcseconds in diameter.
Titan is locked in a 3:4 orbital resonance with the small, irregularly shaped satellite Hyperion. A "slow and smooth" evolution of the resonance—in which Hyperion would have migrated from a chaotic orbit—is considered unlikely, based on models. Hyperion likely formed in a stable orbital island, while massive Titan absorbed or ejected bodies that made close approaches.Bevilacqua, R.; Menchi, O.; Milani, A.; Nobili, A. M.; Farinella, P. (April 1980). "Resonances and close approaches. I. The Titan-Hyperion case". Earth, Moon, and Planets 22 (2): 141–152. Retrieved on 2007-08-27.
Titan compared to Earth.
Titan\'s internal structure.
Titan is 5,150 km across, compared to 4,879 km for the planet Mercury and 3,474 km for Earth\'s moon. Before the arrival of Voyager 1 in 1980, Titan was thought to be slightly larger than Ganymede (diameter 5,262 km) and thus the largest moon in the Solar System; this was an overestimation caused by Titan\'s dense, opaque atmosphere, which extends many miles above its surface and increases its apparent diameter.Bill Arnett (2005). Titan. Nine planets. University of Arizona, Tucson. Retrieved on 2005-04-10. Titan\'s diameter and mass (and thus its density) are similar to Jovian moons Ganymede and Callisto.Lunine, J. (March 21 2005). Comparing the Triad of Great Moons. Astrobiology Magazine. Retrieved on 2006-07-20. Based on its bulk density of 1.88 g/cm³, Titan\'s bulk composition is half water ice and half rocky material. Though similar in composition to Dione and Enceladus, it is denser due to gravitational compression.
Titan is probably differentiated into several layers with a 3,400 km (2,040 mi) rocky center surrounded by several layers composed of different crystal forms of ice.G. Tobie, O. Grasset, J. I. Lunine, A. Mocquet, C. Sotin (2005). "Titan\'s internal structure inferred from a coupled thermal-orbital model". Icarus 175 (2): 496–502. doi:10.1016/j.icarus.2004.12.007. Its interior may still be hot and there may be a liquid layer consisting of water and ammonia between the ice I crust and deeper ice layers made of high-pressure forms of ice. Evidence for such an ocean has recently been uncovered by the Cassini probe in the form of natural extremely low frequency (ELF) radio waves in Titan\'s atmosphere. Titan\'s surface is thought to be a poor reflector of ELF waves, so they may instead be reflecting off the liquid-ice boundary of a subsurface ocean."Titan\'s Mysterious Radio Wave", Jet Propulsion Laboratory, June 1, 2007. Retrieved on 2007-06-02.
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. Atmosphere thickness has been suggested ranging between 200 km Facts about Titan. ESA Cassini-Huygens. Retrieved on 2007-08-07. and 880 km. Mori K. et al. (2004). "An X-Ray Measurement of Titan\'s Atmospheric Extent from Its Transit of the Crab Nebula" (PDF). Astrophysical Journal 607 (2): 1065–1069. doi:10.1086/383521. Retrieved on 2007-08-07. Chandra images used by Mori et al.: Photo Album – Titan The atmosphere is opaque at many wavelengths and a complete reflectance spectrum of the surface is impossible to acquire from the outside; Schröder, S. E.; Tomasko, M. G.; Keller, H. U. (August 2005). "The reflectance spectrum of Titan\'s surface as determined by Huygens". American Astronomical Society, DPS meeting #37, #46.15; Bulletin of the American Astronomical Society 37 (726). Retrieved on 2007-08-20. it was this haziness that led to errors in diameter estimates.
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 Journal 100: 378. doi:10.1086/144679. Observations from the 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. It supports opaque haze layers that block most visible light from the Sun and other sources and renders Titan\'s surface features obscure. The atmosphere is so thick and the gravity so low that humans could fly through it by flapping "wings" attached to their arms. Robert Zubrin (1999). Entering Space: Creating a Spacefaring Civilization. Section: Titan: Tarcher/Putnam, 163–166. ISBN 1-58542-036-0. 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, the Huygens team likened the process to "taking pictures of an asphalt parking lot at dusk". Petre de Selding. "Huygens Probe Sheds New Light on Titan", SPACE.com, January 21 2005. Retrieved on 2005-03-28.
The atmosphere is 98.4% nitrogen—the only dense, nitrogen-rich atmosphere in the solar system aside from the Earth\'s—with the remaining 1.6% composed of methane and trace amounts of other gases such as hydrocarbons (including ethane, diacetylene, methylacetylene, acetylene, propane, cyanoacetylene, hydrogen cyanide), carbon dioxide, carbon monoxide, cyanogen, argon and helium. The orange color as seen from space must be produced by other more complex chemicals in small quantities, possibly tholins, tar-like organic precipitates.Baez, John (January 25, 2005). This Week\'s Finds in Mathematical Physics. University of California, Riverside. Retrieved on 2007-08-22. 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. In November 2007, scientists uncovered evidence of negative ions with roughly 10,000 times the mass of hydrogen in Titan\'s ionosphere, which are believed to fall into the lower regions to form the orange haze which obscures Titan\'s surface. Their structure is not currently known, but they are believed to be tholins, and may form the bases for the formation of more complex molecules, such as polycyclic aromatic hydrocarbons.Coates, A. J., F. J. Crary, G. R. Lewis, D. T. Young, J. H. Waite, and E. C. Sittler (2007). "Discovery of heavy negative ions in Titan\'s ionosphere". Geophys. Res. Lett. 34: L22103. doi:10.1029/2007GL030978.
Energy from the Sun should have converted all traces of methane in Titan\'s atmosphere into hydrocarbons within 50 million years; a relatively short time compared to the age of the Solar System. This suggests that methane must be somehow replenished by a reservoir on or within Titan itself. That Titan\'s atmosphere contains over a thousand times more methane than carbon monoxide would appear to rule out significant contributions from cometary impacts, since comets are composed of more carbon monoxide than methane. That Titan might have accreted an atmosphere from the early Saturnian nebula at the time of formation also seems unlikely; in such a case, it ought to have atmospheric abundances similar to the solar nebula, including hydrogen and neon. A. Coustenis (2005). "Formation and evolution of Titan’s atmosphere". Space Science Reviews 116: 171–184. doi:10.1007/s11214-005-1954-2. A possible biological origin for the methane has not been discounted (see below).
There is also a pattern of air circulation found flowing in the direction of Titan\'s rotation, from west to east."The Way the Wind Blows on Titan", Jet Propulsion Laboratory, June 1, 2007. Retrieved on 2007-06-02. Observations by Cassini of the atmosphere made in 2004 also suggest that Titan is a "super rotator", like Venus, with an atmosphere that rotates much faster than its surface.Wind or Rain or Cold of Titan\'s Night?. Astrobiology Magazine (March 11, 2005). Retrieved on 2007-08-24.
Titan\'s ionosphere is also more complex than Earth\'s, with the main ionosphere at an altitude of 1,200 km but with an additional layer of charged particles at 63 km. This splits Titan\'s atmosphere to some extent into two separate radio-resonating chambers. The source of natural ELF waves (see above) on Titan is unclear as there does not appear to be extensive lightning activity.
Titan in false color showing surface details and atmosphere. "Xanadu" is the bright region at the centre-right
The surface of Titan has been described as "complex, fluid-processed, [and] geologically young".Mahaffy, Paul R. (May 13, 2005). "Intensive Titan Exploration Begins". Science 308 (5724): 969–970. Retrieved on 2007-08-19. The Cassini spacecraft has used radar altimetry and synthetic aperture radar (SAR) imaging to map portions of Titan during its close fly-bys of the moon. The first images revealed a 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, some of them hundreds of kilometers in length, that appear to be caused by windblown particles.Battersby, Stephen (October 29, 2004). Titan\'s complex and strange world revealed. New Scientist. Retrieved on 2007-08-31.Spacecraft: Cassini Orbiter Instruments, RADAR. Cassini-Huygens Mission to Saturn & Titan. NASA, Jet Propulsion Laboratory. Retrieved on 2007-08-31. Examination has also shown the surface to be relatively smooth; the few objects that seem to be impact craters appeared to have been filled in, perhaps by raining hydrocarbons or volcanoes. Radar altimetry suggests height variation is low, typically no more than 150 meters. Occasional elevation changes of 500 meters have been discovered and Titan has mountains that sometimes reach several hundred meters to more than 1 kilometer in height.Lorenz, R. D.; Callahan, P. S.; et al. (March 2007). "Titan\'s Shape, Radius and Landscape from Cassini Radar Altimetry" (PDF). Lunar and Planetary Science Conference 38. Retrieved on 2007-08-27.
Titan\'s surface is marked by broad regions of bright and dark terrain. These include Xanadu, a large, reflective equatorial area about the size of Australia. It was first identified in infrared images from the Hubble Space Telescope in 1994, and later viewed by the Cassini spacecraft. The convoluted region is filled with hills and cut by valleys and chasms.Cassini Reveals Titan\'s Xanadu Region To Be An Earth-Like Land. Science Daily (July 23, 2006). Retrieved on 2007-08-27. It is criss-crossed in places by dark lineaments—sinuous topographical features resembling ridges or crevices. These may represent tectonic activity, which would indicate that Xanadu is geologically young. Alternatively, the lineaments may be liquid-formed channels, suggesting old terrain that has been cut through by stream systems.Barnes, Jason W.; Brown, Robert H.; et al. (January 2006). "Global-scale surface spectral variations on Titan seen from Cassini/VIMS" (PDF). Icarus 186 (1). Retrieved on 2007-08-27. 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).
False-color Cassini synthetic-aperture radar mosaic of Titan\'s north polar region, showing evidence for hydrocarbon seas, lakes and tributary networks. The false blue coloring indicates low radar reflectivity areas, likely caused by bodies of liquid ethane, methane and dissolved nitrogen. Photographs [1] suggest that the large body at lower left has approximately twice the extent of what can be seen here.
The possibility that there were seas of liquid methane on Titan were first suggested based on Voyager 1 and 2 data that showed Titan to have a thick atmosphere of approximately the correct temperature and composition to support them, but direct evidence wasn\'t obtained until 1995 when data from Hubble and other observations had already suggested the existence of liquid methane on Titan, either in disconnected pockets or on the scale of satellite-wide oceans, similar to water on Earth.S. F.Dermott, C. Sagan, (1995). "Tidal effects of disconnected hydrocarbon seas on Titan". Nature 374: 238–240. doi:10.1038/374238a0.
The Cassini mission affirmed the former hypothesis, although not immediately. When the probe arrived in the Saturnian system in 2004, 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.Bortman, Henry (November 02, 2004). Titan: Where\'s the Wet Stuff?. Astrobiology Magazine. Retrieved on 2007-08-28. At Titan\'s south pole, an enigmatic dark feature named Ontario Lacus was the first suspected lake identified, possibly created by clouds that are observed to cluster in the area.Emily Lakdawalla. "Dark Spot Near the South Pole: A Candidate Lake on Titan?", The Planetary Society, June 28, 2005. Retrieved on 2006-10-14. A possible shoreline was also identified at the pole via radar imagery.Jet Propulsion Laboratory (September 16, 2005). "NASA Cassini Radar Images Show Dramatic Shoreline on Titan". Press release. Retrieved on 2006-10-14. 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, smooth (and thus dark to radar) patches were seen dotting the surface near the pole.PIA08630: Lakes on Titan. NASA Planetary Photojournal. NASA/JPL. Retrieved on 2006-10-14. Based on the observations, scientists announced "definitive evidence of lakes filled with methane on Saturn\'s moon Titan" in January 2007.Stofan, E. R.; Elachi, C.; et al. (January 4, 2007). "The lakes of Titan". Nature 445 (1): 61–64. Retrieved on 2007-08-27. Titan Has Liquid Lakes, Scientists Report in Nature. NASA/JPL (January 3, 2007). Retrieved on 2007-01-08. The Cassini–Huygens team concluded that the imaged features are almost certainly the long-sought hydrocarbon lakes, the first stable bodies of surface liquid found off Earth. Some appear to have channels associated with liquid and lie in topographical depressions.
Radar, SAR and imaging data from Cassini have revealed a relative paucity of impact craters on Titan\'s surface, suggesting a youthful surface. The few impact craters discovered include a 440 km wide multi-ring impact basin named Menrva (seen by Cassini\'s ISS as a bright-dark concentric pattern).PIA07365: Circus Maximus. NASA Planetary Photojournal. Retrieved on 2006-05-04. A smaller 80 km wide, flat-floored crater named SinlapPIA07368: Impact Crater with Ejecta Blanket. NASA Planetary Photojournal. Retrieved on 2006-05-04. and a 30 km crater with a central peak and dark floor named Ksa have also been observed.PIA08737: Crater Studies on Titan. NASA Planetary Photojournal. Retrieved on 2006-09-15. Radar and Cassini imaging have also revealed a number of "crateriforms", circular features on the surface of Titan that may be 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.PIA08425: Radar Images the Margin of Xanadu. NASA Planetary Photojournal. Retrieved on 2006-09-26. 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.PIA08429: Impact Craters on Xanadu. NASA Planetary Photojournal. Retrieved on 2006-09-26.
Pre-Cassini models of impact trajectories and angles suggest that where the impactor strikes the water ice crust, a small amount of ejecta remains as liquid water within the crater. It may persist as liquid for centuries or longer, sufficient for "the synthesis of simple precursor molecules to the origin of life".Artemieva, Natalia; Lunine, Jonathan (August 2003). "Cratering on Titan: impact melt, ejecta, and the fate of surface organics". Icarus 164: 471–480. Retrieved on 2007-08-28. While infill from various geological processes is one reason for Titan\'s relative deficiency of craters, atmospheric shielding also plays a role; it is estimated that Titan\'s atmosphere reduces the number of craters on its surface by a factor of two.Ivanov, B. A.; Basilevsky, A. T.; Neukum, G. (August 1997). "Atmospheric entry of large meteoroids: implication to Titan". Planetary and Space Science 45: 993–1007. Retrieved on 2007-08-28.
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 "lava" composed of water and ammonia.Tobias Owen (2005). "Planetary science: Huygens rediscovers Titan". Nature 438: 756–757. doi:10.1038/438756a. Cassini detected methane emissions from one suspected cryovolcano, and volcanism is now believed to be a significant source of the methane in the atmosphere."Seeing, touching and smelling the extraordinarily Earth-like world of Titan", ESA News, European Space Agency, January 21 2005. Retrieved on 2005-03-28. David L. Chandler. "Hydrocarbon volcano discovered on Titan", NewScientist.com news service, New Scientist, June 8 2005. Retrieved on 2007-08-07. One of the first features imaged by Cassini, Ganesa Macula, resembles the geographic features called "pancake domes" found on Venus, and is thus believed to be cryovolcanic in origin.C.D. Neish, R.D. Lorenz, D.P. O\'Brien (2005). Shape and thermal modeling of the possible cryovolcanic dome Ganesa Macula on Titan: Astrobiological implications. Lunar and Planetary Laboratory, University of Arizona, Observatoire de la Cote d\'Azur. Retrieved on 2007-08-27.
The pressure necessary to drive the cryovolcanoes may be caused by ice "underplating" Titan\'s outer shell. The low-pressure ice, overlaying a liquid layer of ammonium sulfate, ascends buoyantly, and the unstable system can produce dramatic plume events. Titan is resurfaced through the process by grain-sized ice and ammonium sulfate ash, which helps produce a wind-shaped landscape and sand dune features.Fortes, A. D.; Grindroda, P.M.; Tricketta, S. K.; Vočadloa, L. (May 2007). "Ammonium sulfate on Titan: Possible origin and role in cryovolcanism". Icarus 188 (1): 139–153. Retrieved on 2007-08-27.
A mountain range measuring 150 km long, 30 km wide and 1.5 km high was discovered by Cassini in 2006. This range lies in the southern hemisphere and is thought to be composed of icy material and covered in methane snow. The movement of tectonic plates, perhaps influenced by a nearby impact basin, could have opened a gap through which the mountain\'s material upwelled."Mountain range spotted on Titan", BBC News, December 12 2006. Retrieved on 2007-08-06.
Sand dunes on Earth (top), compared with dunes on Titan\'s surface.
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 liquid hydrocarbons.R. Lorenz (2003). "The Glitter of Distant Seas". Science 302: 403–404. doi:10.1126/science.1090464. Radar images captured by the Cassini spacecraft have instead revealed some of these regions to be extensive plains covered in longitudinal sand dunes, up to 330 meters high. The longitudinal (or linear) dunes are believed to be formed by moderately variable winds that either follow one mean direction or alternate between two different directions. Dunes of this type are always aligned with average wind direction. In the case of Titan, steady zonal (eastward) winds combine with variable tidal winds (approximately 0.5 meter per second). The tidal winds are the result of tidal forces from Saturn on Titan\'s atmosphere, which are 400 times stronger than the tidal forces of the Moon on Earth and tend to drive wind toward the equator. This wind pattern causes sand dunes to build up in long parallel lines aligned west-to-east. The dunes break up around mountains, where the wind direction shifts.
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.N. Lancaster (2006). "Linear Dunes on Titan". Science 312: 702–703. doi:10.1126/science.1126292. Goudarzi, Sara. "Saharan Sand Dunes Found on Saturn\'s Moon Titan", SPACE.com, May 4, 2006. Retrieved on 2007-08-06. Lorenz, RD; Wall S, Radebaugh J, et.al. (2006). "The sand seas of Titan: Cassini RADAR observations of longitudinal dunes". Science 312: 724–727. doi:10.1126/science.1123257.
Titan\'s surface temperature is about 94 K (−179 °C, or −290 °F). At this temperature water ice does not sublimate from solid to gas, so the atmosphere is nearly free of water vapor. The haze in Titan\'s atmosphere contributes to the moon\'s anti-greenhouse effect by reflecting sunlight away from the satellite, making its surface significantly colder than its upper atmosphere.C. A. Hasenkopf. OPTICAL PROPERTIES OF TITAN HAZE LABORATORY ANALOGS USING CAVITY RING DOWN SPECTROSCOPY. Workshop on Planetary Atmospheres (2007). Retrieved on 2007-10-16. The clouds on Titan, probably composed of methane, ethane or other simple organics, are scattered and variable, punctuating the overall haze. This atmospheric methane conversely creates a greenhouse effect on Titan\'s surface, without which Titan would be far colder.Titan Has More Oil Than Earth (February 13, [2008]]). Retrieved on 2008-02-13. The findings of the Huygens probe indicate that Titan\'s atmosphere periodically rains liquid methane and other organic compounds onto the moon\'s surface.Emily Lakdawalla. "Titan: Arizona in an Icebox?", The Planetary Society, January 21 2004. Retrieved on 2005-03-28. In October 2007, observers noted an increase in apparent opacity in the clouds above the equatorial Xanadu region, suggestive of "methane drizzle", though this was not direct evidence for rain.Ádámkovics, Máté; Michael H. Wong, Conor Laver, Imke de Pater (October 11 2007). "Widespread Morning Drizzle on Titan". Science. doi:10.1126/science.1146244. It is possible that areas of Titan\'s surface may be coated in a layer of tholins, but this has not been confirmed.Arpad Somogyi, MA Smith (2006). Mass Spectral Investigation of Laboratory Made Tholins and Their Reaction Products: Implications to Tholin Surface Chemistry on Titan. University of Arizona. Retrieved on 2007-08-28.
Cassini image of Epimetheus and Titan
Titan is never visible to the naked eye, but can be observed through small telescopes or strong binoculars. Amateur observation is difficult because of the proximity of the satellite to Saturn\'s brilliant globe and ring system; an occulting bar, covering part of the eyepiece and used to block the bright planet, greatly improves viewing.Benton, Julius L. Jr. (2005). Saturn and How to Observe It. Springer London, 141-146. ISBN 978-1-84628-045-0. Titan has a maximum apparent magnitude of +7.9. This compares to +4.6 for the similarly sized Ganymede, in the Jovian system.
Observations of Titan prior to the space age were limited. In 1907 Spanish astronomer Josep Comas Solá suggested that he had observed darkening near the edges of Titan\'s disk and two round, white patches in its center. The deduction of an atmosphere by Kuiper in the 1940s was the next major observational event.Näränen, Jyri. The Atmosphere of Titan. University of Helsinki, Department of Astronomy. Retrieved on 2007-08-19.
The first probe to visit the Saturnian system was Pioneer 11 in 1979, which determined that Titan was likely too cold to support life.The Pioneer Missions. Pioneer Project. NASA, Jet Propulsion Laboratory (March 26, 2007). Retrieved on 2007-08-19. The craft took the first images of the moon (including some of it and Saturn together), but these were of low quality; the first-ever close-up of Titan was taken on September 2, 1979.Pioneer XI. Photo Index. NASA. Retrieved on 2007-08-19.
Titan was examined by both Voyager 1 and Voyager 2 in 1980 and 1981, respectively. Voyager 1\'s course was diverted specifically to make a closer pass of Titan. Unfortunately, the craft did not possess any instruments that could penetrate Titan\'s haze, an unforeseen 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 Shangri-la,James Richardson, Ralph Lorenz, & Alfred McEwen (July 2004). "Titan\'s Surface and Rotation: New Results from Voyager 1 Images". Icarus 170 (1): 113–124. doi:10.1016/j.icarus.2004.03.010. 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.
Even with the data provided by the Voyagers, Titan remained a body of mystery—a planet-like satellite shrouded in an atmosphere that makes detailed observation difficult. The intrigue that had surrounded Titan since the 17th-century observations of Christiaan Huygens and Giovanni Cassini was finally gratified by the spacecraft named in their honor.
The Cassini–Huygens spacecraft reached Saturn on July 1, 2004 and has begun the process of mapping Titan\'s surface by radar. A joint project of the European Space Agency (ESA) and NASA, Cassini–Huygens, has proved a very successful mission. 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 km, discerning patches of light and dark that would be invisible to the human eye from the Earth. Huygens landed on Titan on January 14, 2005, discovering that many of the moon\'s surface features seem to have been formed by flowing fluids at some point in the past. Cassini at Saturn: Introduction. NASA, Jet Propulsion Laboratory. Retrieved on 2007-09-06. On July 22, 2006, Cassini made the first of a series of 21 planned, targeted, close fly-bys, each at only 950 km from Titan; the last is scheduled for May 12, 2008.CASSINI AT SATURN - Saturn Tour Dates. NASA/JPL. Retrieved on 2007-10-31. Present liquid on the surface may have been found near the north pole, in the form of many lakes that were recently discovered by Cassini. Titan is the most distant body from Earth that has seen a space probe landing.Huygens Exposes Titan\'s Surface. Spacetoday. Retrieved on 2007-08-19. Titan is also the second moon in the solar system to have a man-made object land on its surface.
Huygens image from Titan\'s surface
The Huygens probe landed just off the easternmost tip of a bright region now called Adiri, where it 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.
After landing, Huygens photographed 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 left are smaller than they may appear: the left-hand one is 15 centimeters (6 in) across, and the one in the center is 4 centimeters (about 1.5 in) across, at a distance of about 85 centimeters (about 33 in) 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.
In March 2007, NASA, ESA, and COSPAR decided to name the Huygens landing site the Hubert Curien Memorial Station in memory of the former president of the ESA.Huygens landing site to be named after Hubert Curien. ESA (March 5, 2007). Retrieved on 2007-08-06.
Scientists believe that the atmosphere of early Earth was similar in composition to the current atmosphere on Titan. 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 ethyne. Further reactions have been studied extensively.Raulin F., Owen T. (2002). "Organic chemistry and exobiology on Titan". Space Science Review 104 (1–2): 377–394. doi:10.1023/A:1023636623006.
All of these experiments have led to the suggestion that enough organic material exists on Titan to start a chemical evolution analogous to what is thought to have started life on Earth. While the analogy assumes the presence of liquid water for longer periods than is currently observable, several theories suggest that liquid water from an impact could be preserved under a frozen isolation layer.Artemivia N., Lunine J, (2003). "Createring on Titan: Impact melt ejecta and the fate of surface organics". Icarus 164: 471–480. doi:10.1016/S0019-1035(03)00148-9. It has also been observed that liquid ammonia oceans could exist deep below the surface; one model suggests an ammonia–water solution as much as 200 km deep beneath a water ice crust, conditions that, "while extreme by terrestrial standards, are such that life could indeed survive". Heat transfer between the interior and upper layers would be critical in sustaining any sub-surface oceanic life.
Detection of microbial life on Titan would depend on its biogenic effects. That the atmospheric methane and nitrogen are of biological origin has been examined, for example. Hydrogen has been cited as one molecule suitable to test for life on Titan: if methanogenic life is consuming atmospheric hydrogen in sufficient volume, it will have a measurable effect on the mixing ratio in the troposphere.McKay, C. P.; Smith, H. D. (2005). "Possibilities for methanogenic life in liquid methane on the surface of Titan". Icarus 178 (1): 274–276. doi:10.1016/j.icarus.2005.05.018.
Despite these biological possibilities, there are formidable obstacles to life on Titan, and any analogy to Earth is inexact. At a vast distance from the Sun, Titan is frigid (a fact exacerbated by the anti-greenhouse effect of its cloud cover), and its atmosphere lacks CO2. Given these difficulties, the topic of life on Titan may be best described as an experiment for examining theories on conditions necessary prior to flourishing life on Earth.Saturn\'s Moon Titan: Prebiotic Laboratory. Astrobiology Magazine (August 11, 2004). Retrieved on 2004-08-11. While life itself may not exist, the prebiotic conditions of the Titanian environment, and the possible presence of organic chemistry, remain of great interest in understanding the early history of the terrestrial biosphere.Raulin, F. (2005). "Exo-astrobiological aspects of Europa and Titan: From observations to speculations". Space Science Review 116 (1–2): 471–487. doi:10.1007/s11214-005-1967-x. Using Titan as a prebiotic experiment involves not only observation through spacecraft, but laboratory experiment, and chemical and photochemical modellin
