Titan (moon) 47402 224232820 2008-07-07T22:31:37Z Fletcher 1190808 /* Cassini–Huygens */ added image from Cassini flyby {{pp-semi-vandalism|small=yes}} <!-- Scroll down to edit the contents of this page. Additional parameters for this template are available at [[Template:Infobox Planet]]. --> {{featuredarticle}} {{otheruses4|the planet Saturn's largest natural satellite||Titan (disambiguation)}} {{Infobox Planet |name = Titan |image = [[Image:Titan in natural color Cassini.jpg|250px|Titan in natural color]] |caption = Titan seen from the [[Cassini–Huygens]] spacecraft. |bgcolour = #ffa812 |orbit_ref = <ref name=horizons>Unless otherwise specified: {{cite web |url=http://ssd.jpl.nasa.gov/horizons.cgi#top |title=JPL HORIZONS solar system data and ephemeris computation service |work=Solar System Dynamics|publisher=[[NASA]], Jet Propulsion Laboratory |accessdate=2007-08-19}}</ref> |discovery = yes |discoverer = [[Christiaan Huygens]] |discovered = [[March 25]] [[1655]] |semimajor = 1,221,870 [[Kilometer|km]] |eccentricity = 0.0288 |period = 15.945 days |inclination = 0.34854° (to Saturn's equator) |satellite_of = [[Saturn (planet)|Saturn]] |physical_characteristics = yes |mean_radius = 2576 ± 2.00&nbsp;km (0.404 Earths) <ref name="Jacobson 2006">{{cite journal |last=Jacobson |first=R. A. |coauthors=Antreasian, P. G.; Bordi, J. J.; Criddle, K. E.; et.al. |title=The gravity field of the saturnian system from satellite observations and spacecraft tracking data |journal=The Astronomical Journal |month=December |year=2006 |volume=132 |issue=6 |pages=2520–2526 |doi=10.1086/508812}}</ref> |surface_area = 8.3{{e|7}}&nbsp;[[Square kilometre|km²]] |mass = 1.3452 ± 0.0002{{e|23}}&nbsp;[[Kilogram|kg]] (0.0225 Earths)<ref name="Jacobson 2006"/> |density = 1.8798 ± 0.0044 [[Gram|g]]/[[Cubic centimeter|cm³]]<ref name="Jacobson 2006"/> |surface_grav = 1.352 [[Acceleration|m/s<sup>2</sup>]] (0.14 [[G-force|''g'']]) |escape_velocity = 2.639 km/s |rotation = ([[Synchronous rotation|synchronous]]) |axial_tilt = zero |albedo = 0.22<ref>{{cite web|first=David R.|last=Williams|title=Saturnian Satellite Fact Sheet|work=NASA|url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturniansatfact.html|accessdate=2007-09-03}}</ref> |magnitude = 7.9 |single_temperature = 93.7 [[Kelvin|K]] (&minus;179.45 °C)<ref name=Mitri>{{cite journal| author=Giuseppe, Mitri; ''et al.'' |month=February|year=2007| title=Hydrocarbon Lakes on Titan |journal=Icarus |volume=186 | pages=385–394 |doi=10.1016/j.icarus.2006.09.004 |url=http://www.lpl.arizona.edu/~showman/publications/mitri-etal-2007-lakes.pdf |format=PDF}}</ref> |atmosphere = yes |surface_pressure = 146.7 [[Pascal (unit)|kPa]] |atmosphere_composition = 98.4% [[nitrogen]]<br/>1.6% [[methane]]<ref name=Niemann> {{cite journal | title= The abundances of constituents of Titan’s atmosphere from the GCMS instrument on the Huygens probe |author= H. B. Niemann, et al. |journal= [[Nature]] |volume=438 |pages=779–784 |year=2005 |doi=10.1038/nature04122 }}</ref> | adjectives = Titanian }} [[Image:Christiaan Huygens-painting.jpeg|thumb|upright|Christiaan Huygens, discoverer of Titan]] '''Titan''' ({{pronEng|ˈtaɪtən}} ''<small>TYE</small>-tən,'' or as {{lang-gr|''Τῑτάν}})'' or '''Saturn VI''' is the largest [[natural satellite|moon]] of [[Saturn]], the only moon known to have a dense [[celestial body atmosphere|atmosphere]],<ref Name=NasaNews>{{cite web |url=http://saturn.jpl.nasa.gov/news/features/saturn-story/moons.cfm |title=News Features: The Story of Saturn |work=Cassini-Huygens Mission to Saturn & Titan|publisher=NASA, Jet Propulsion Laboratory |accessdate=2007-01-08}}</ref> and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found.<ref name=NatureDefinitive/> 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 [[Moon|Earth's moon]] and is 80% more [[mass]]ive. It is the [[List of natural satellites by diameter|second-largest moon]] in the [[Solar System]], after [[Jupiter]]'s moon [[Ganymede (moon)|Ganymede]], and it is larger by diameter than the smallest planet, [[Mercury (planet)|Mercury]] (although only half as massive). Titan was the first known moon of Saturn, discovered in 1655 by the [[Netherlands|Dutch]] astronomer [[Christiaan Huygens]].<ref>{{cite web |url=http://apod.nasa.gov/apod/ap050325.html |title=Huygens Discovers Luna Saturni |accessdate=2007-08-18 |publisher=[[NASA]] |work=Astronomy Picture of the Day}}</ref> 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 [[cryovolcano]]es have been discovered, it is relatively smooth and few [[impact crater]]s 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 [[Microorganism| 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.<ref name=Grasset2000>{{cite journal | author = Grasset, O., Sotin C., Deschamps F., | title = On the internal structure and dynamic of Titan|year = 2000 | journal = Planetary and Space Science | volume = 48| pages = 617–636|doi=10.1016/S0032-0633(00)00039-8}}</ref><ref name=Fortes2000>{{cite journal | journal = Icarus | volume= 146 | issue = 2 | pages = 444–452 | year= 2000 | doi = 10.1006/icar.2000.6400 | title = Exobiological implications of a possible ammonia-water ocean inside Titan | author = Fortes, A.D.}}</ref> ==Discovery and naming== Titan was discovered on [[March 25]], [[1655]], by the [[Netherlands|Dutch]] astronomer [[Christiaan Huygens]]. Huygens was inspired by [[Galileo]]'s discovery of Jupiter's four [[Galilean moons|largest moons]] in 1610 and his improvements on [[telescope]] technology.<ref>{{cite web |url=http://www.esa.int/esaSC/SEMJRT57ESD_index_0.html |title=Discoverer of Titan: Christiaan Huygens |accessdate=2007-08-18 |date= April 24, 2007 |publisher=[[European Space Agency]] }}</ref> Huygens himself made advances in the technology and his discovery of Titan owed "partly to the quality of his telescope and partly to luck".<ref>{{cite web |url=http://www.sil.si.edu/silpublications/dibner-library-lectures/2004-VanHelden/2004_VanHelden.pdf |title=Huygens Ring, Cassini's Division & Saturn's Children |accessdate=2007-08-19 |last= |first= |authorlink= |coauthors= |date=October 27, 2004 |format= |work= Dibner Library Lecture|publisher=Smithsonian Institute Libraries }}</ref> 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".<ref>{{cite journal |url=http://links.jstor.org/sici?sici=0370-2316%281673%298%3C5178%3AADOTNP%3E2.0.CO%3B2-Z |title=''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.'' |journal=Philosophical Transactions |volume=8 |issue=1673 |pages=5178–5185 |year=1673 |doi=10.1098/rstl.1673.0003 |author=Cassini, Signor}}</ref> 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''.<ref>{{cite journal |url=http://adsabs.harvard.edu//full/seri/MNRAS/0008//0000042.000.html |title=Satellites of Saturn; Observations of Mimas, the closest and most interior satellite of Saturn |author=Mr Lassell |journal=Monthly Notices of the [[Royal Astronomical Society]] |volume=8 |pages=42 |date=November 12, 1847 |accessdate=2005-03-29}}</ref> He suggested the names of the mythological [[Titan (mythology)|Titans]], sisters and brothers of [[Cronus|Cronos]], the Greek Saturn. ==Orbit and rotation== [[Image:Titan's orbit.jpg|thumb|left|Titan's orbit (highlighted in red) among the other large inner moons of Saturn. The moons outside its orbit are (l-r) Iapetus and Hyperion; those inside are Dione, Tethys, Enceladus and Mimas]] 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.<ref name=horizons/> 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 [[arcsecond]]s in diameter. Titan is locked in a 3:4 [[orbital resonance]] with the small, irregularly shaped satellite [[Hyperion (moon)|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.<ref>{{cite journal |last=Bevilacqua |first= R. |coauthors=Menchi, O.; Milani, A.; Nobili, A. M.; Farinella, P. |year=1980 |month=April |title= Resonances and close approaches. I. The Titan-Hyperion case|journal=Earth, Moon, and Planets |volume=22 |issue=2 |pages=141–152 |url=http://www.springerlink.com/content/g627852062714784/ |accessdate= 2007-08-27 |quote= |doi= 10.1007/BF00898423 }}</ref> ==Bulk characteristics== [[Image:Titan Earth Comparison at 29 km per px.png|thumb|Titan compared to Earth.]] [[Image:Titan cutaway.svg|thumb|Titan's internal structure.]] Titan is 5,150&nbsp;km across, compared to 4,879&nbsp;km for the planet Mercury and 3,474&nbsp;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&nbsp;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.<ref name=nineplanets>{{cite web |author=Bill Arnett |year=2005 |url=http://seds.lpl.arizona.edu/nineplanets/nineplanets/titan.html |title=Titan |publisher=University of Arizona, Tucson |work=Nine planets |accessdate=2005-04-10}}</ref> Titan's diameter and mass (and thus its density) are similar to Jovian moons [[Ganymede (moon)|Ganymede]] and [[Callisto (moon)|Callisto]].<ref name=LunineAstro>{{cite web |author=Lunine, J. |url=http://www.astrobio.net/news/article1493.html |title=Comparing the Triad of Great Moons |publisher=Astrobiology Magazine |date=[[March 21]] [[2005]] |accessdate=2006-07-20}}</ref> Based on its bulk density of 1.88&nbsp;g/cm³, Titan's bulk composition is half water ice and half rocky material. Though similar in composition to [[Dione (moon)|Dione]] and [[Enceladus (moon)|Enceladus]], it is denser due to [[gravitational compression]]. Titan is probably differentiated into several layers with a 3,400&nbsp;km rocky center surrounded by several layers composed of different crystal forms of ice.<ref name=Tobie>{{cite journal | author = G. Tobie, O. Grasset, J. I. Lunine, A. Mocquet, C. Sotin | year = 2005| url = http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005Icar..175..496| title = Titan's internal structure inferred from a coupled thermal-orbital model| journal = Icarus |volume =175| issue =2| pages = 496–502|doi =10.1016/j.icarus.2004.12.007}}</ref> 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.<ref name="Titan ELF">{{cite news| url=http://saturn.jpl.nasa.gov/news/features/feature20070601c.cfm| title=Titan's Mysterious Radio Wave| date=June 1, 2007| publisher=Jet Propulsion Laboratory| accessdate=2007-06-02}}</ref> Surface features were observed by the ''Cassini'' spacecraft to systematically shift by up to 30&nbsp;km between October 2005 and May 2007, which suggests that the crust is decoupled from the interior, and provides additional evidence for an interior liquid layer.<ref name=NS2008>David Shiga, [http://space.newscientist.com/article/dn13516-titans-changing-spin-hints-at-hidden-ocean.html Titan's changing spin hints at hidden ocean], New Scientist, 20 March 2008</ref> ==Atmosphere== [[Image:Titan-Complex 'Anti-greenhouse'.jpg|thumb|left|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 gas]]es. Atmosphere thickness has been suggested ranging between 200&nbsp;km<ref> {{cite web |publisher=ESA Cassini-Huygens |url=http://www.esa.int/SPECIALS/Cassini-Huygens/SEMMF2HHZTD_0.html |title=Facts about Titan |accessdate=2007-08-07}} </ref> and 880&nbsp;km.<ref> {{cite journal |author=Mori K. et al. |year=2004 |title=An X-Ray Measurement of Titan's Atmospheric Extent from Its Transit of the Crab Nebula |journal=Astrophysical Journal |volume=607 |issue=2 |pages=1065–1069 |url=http://arxiv.org/abs/astro-ph/0403283v1 |accessdate=2007-08-07 |doi=10.1086/383521}} Chandra images used by Mori ''et al.'': [http://chandra.harvard.edu/photo/2004/titan/ Photo Album – Titan] </ref> The atmosphere of Titan is opaque at many [[wavelengths]] and a complete reflectance spectrum of the surface is impossible to acquire from the outside;<ref> {{cite journal |author=Schröder, S. E. |coauthors= Tomasko, M. G.; Keller, H. U. |year=2005 |month=August |title= The reflectance spectrum of Titan's surface as determined by Huygens |journal= American Astronomical Society, DPS meeting #37, #46.15; Bulletin of the American Astronomical Society |volume=37 |issue=726 |pages= |id= |url=http://adsabs.harvard.edu/abs/2005DPS....37.4615S |accessdate= 2007-08-20 |quote= }} </ref> it was this haziness that led to errors in diameter estimates. The presence of a significant atmosphere was first suspected by Spanish astronomer [[Jose Comes Sola]], who observed distinct [[limb darkening]] on Titan in 1903,<ref>{{cite book|title=The Atlas of the Solar System|author=P. Moore, G. Hunt, I. Nicolson, P. Cattermole|year=1990|publishers=Mitchell Beazley |isbn=0-517-00192-6}}</ref> and confirmed by [[Gerard P. Kuiper]] in 1944 using a [[spectroscopy|spectroscopic technique]] that yielded an estimate of an atmospheric [[partial pressure]] of [[methane]] of the order of 100 millibars (10 kPa).<ref name=Kuiper> {{cite journal|author= G. P. Kuiper|year= 1944 | url= http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1944ApJ...100..378|title = Titan: a Satellite with an Atmosphere | journal =Astrophysical Journal|volume = 100 |pages = 378| doi =10.1086/144679}} </ref> Observations from the [[Voyager program|''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.<ref name=Zubrin> {{cite book |author=Robert Zubrin |title=Entering Space: Creating a Spacefaring Civilization |location=Section: Titan |pages=163–166 |publisher=Tarcher/Putnam |year=1999 |isbn=1-58542-036-0}} </ref> The [[Huygens probe|''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".<ref> {{cite news |url=http://www.space.com/missionlaunches/titan_update_050121.html |title=Huygens Probe Sheds New Light on Titan |author=Petre de Selding |publisher=SPACE.com |date=[[January 21]] [[2005]] |accessdate=2005-03-28}} </ref> The atmosphere is 98.4% [[nitrogen]]&mdash;the only dense, nitrogen-rich atmosphere in the solar system aside from the Earth's&mdash;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]].<ref name=Niemann /> The orange color as seen from space must be produced by other more complex chemicals in small quantities, possibly [[tholin]]s, tar-like organic precipitates.<ref>{{cite web |first=John|last=Baez |url= http://www.math.ucr.edu/home/baez/week210.html|title=This Week's Finds in Mathematical Physics |accessdate=2007-08-22 |publisher=[[University of California]], Riverside |date=January 25, 2005 }}</ref> 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 [[ion]]ize 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 basis for the formation of more complex molecules, such as [[polycyclic aromatic hydrocarbons]].<ref>{{cite journal | author = Coates, A. J., F. J. Crary, G. R. Lewis, D. T. Young, J. H. Waite, and E. C. Sittler | year = 2007 | title = Discovery of heavy negative ions in Titan's ionosphere | journal = Geophys. Res. Lett. | volume = 34 | pages = L22103 | doi = 10.1029/2007GL030978}}</ref> 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]].<ref> {{cite journal| title= Formation and evolution of Titan’s atmosphere |author= A. Coustenis |journal= Space Science Reviews |volume= 116 |pages= 171–184 |year= 2005 |doi= 10.1007/s11214-005-1954-2 }} </ref> Many astronomers have suggested that the ultimate origin for the methane in Titan's atmosphere is from within Titan itself, released via eruptions from [[cryovolcanoes]].<ref> *{{cite journal|title=Titan's methane cycle|author=Sushil K. Atreyaa, Elena Y. Adamsa, Hasso B. Niemann et al.|year=2006|doi=10.1016/j.pss.2006.05.028|journal=Planetary and Space Science| volume= 54| issue= 12|url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V6T-4KGPPJ1-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=f858293b47d9ba6c75dc57f7cd9ef266 |accessdate=2008-06-13|pages=1177}} *{{cite journal|title=Letters|journal=Nature|author=E. R. Stofan, C. Elachi, J. I. Lunine et. al.|volume= 445|date=4 January 2007 |doi=10.1038/nature05438|year=2007|url=http://64.233.179.104/scholar?hl=en&lr=&safe=off&q=cache:IKxVFlPcz_sJ:www.geosc.psu.edu/~kasting/Meteo_466/Readings/Stofan_etal_Nature_07.pdf+origin+OR+source+methane+titan+atmosphere |accessdate=2008=-06-13|pages=1177}} *{{cite journal|title=Episodic outgassing as the origin of atmospheric methane on Titan|author=Gabriel Tobie, Jonathan Lunine, Cristophe Sotin|journal=Nature| volume=440|issue=7080|pages=61–64 |year=2006|url=http://adsabs.harvard.edu/abs/2006Natur.440...61T|accessdate=2008-06-13|doi=10.1038/nature04497}}</ref> A possible biological origin for the methane has not been discounted (see [[#Prebiotic conditions and possible life|below]]).<ref name=Fortes2000/> There is also a pattern of air circulation found flowing in the direction of Titan's rotation, from west to east.<ref name="Titan wind">{{cite news| title=The Way the Wind Blows on Titan|date=June 1, 2007| url=http://saturn.jpl.nasa.gov/news/features/feature20070601f.cfm| accessdate=2007-06-02| publisher=Jet Propulsion Laboratory}}</ref> Observations by ''Cassini'' of the atmosphere made in 2004 also suggest that Titan is a "super rotator", like [[Venus (planet)|Venus]], with an atmosphere that rotates much faster than its surface.<ref>{{cite web |url=http://www.astrobio.net/news/article1480.html |title=Wind or Rain or Cold of Titan's Night?|accessdate=2007-08-24 |date=March 11, 2005 |publisher=Astrobiology Magazine}}</ref> Titan's ionosphere is also more complex than Earth's, with the main ionosphere at an altitude of 1,200&nbsp;km but with an additional layer of charged particles at 63&nbsp;km. This splits Titan's atmosphere to some extent into two separate radio-resonating chambers. The source of natural ELF waves ([[#Bulk characteristics|see above]]) on Titan is unclear as there does not appear to be extensive lightning activity.<ref name="Titan ELF" /> ==Surface features== {{seealso|List of geological features on Titan}} [[Image:Titan multi spectral overlay.jpg|thumb|left|Titan in false color showing surface details and atmosphere. "Xanadu" is the bright region at the centre-right]] [[Image:Titan 2007 Oct PIA08399.jpg|thumb|left|200 px|A composite image of Titan's surface.]] The surface of Titan has been described as "complex, fluid-processed, [and] geologically young".<ref>{{cite journal |last=Mahaffy |first=Paul R. |date= May 13, 2005 |title=Intensive Titan Exploration Begins |journal=[[Science (magazine)|Science]] |volume=308|issue= 5724|pages=969–970 |doi= 10.1126/science.1113205 |accessdate= 2007-08-19 |pmid=15890870}}</ref> 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 [[Cryovolcanism|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.<ref>{{cite web |last=Battersby |first=Stephen |date= October 29, 2004 |title=Titan's complex and strange world revealed |publisher=[[New Scientist]] |url=http://www.newscientist.com/article.ns?id=dn6598 |accessdate= 2007-08-31}}</ref><ref>{{cite web |url=http://saturn.jpl.nasa.gov/spacecraft/instruments-cassini-radar.cfm |title=Spacecraft: Cassini Orbiter Instruments, RADAR |work=Cassini-Huygens Mission to Saturn & Titan|publisher=NASA, Jet Propulsion Laboratory |accessdate=2007-08-31}}</ref> Examination has also shown the surface to be relatively smooth; the few objects that seem to be [[impact crater]]s appeared to have been filled in, perhaps by raining hydrocarbons or volcanoes. Radar altimetry suggests height variation is low, typically no more than 150&nbsp;meters. Occasional elevation changes of 500&nbsp;meters have been discovered and Titan has mountains that sometimes reach several hundred meters to more than 1 kilometer in height.<ref>{{cite journal |last=Lorenz |first=R. D. |coauthors=Callahan, P. S.; et al. |year=2007 |month=March |title=Titan's Shape, Radius and Landscape from Cassini Radar Altimetry |journal=Lunar and Planetary Science Conference |volume=38 |format=PDF |url= http://www.lpi.usra.edu/meetings/lpsc2007/pdf/1329.pdf|accessdate= 2007-08-27 }}</ref> Titan's surface is marked by broad regions of bright and dark terrain. These include [[Xanadu (Titan)|Xanadu]], a large, [[Reflection (physics)|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.<ref>{{cite web |url= http://www.sciencedaily.com/releases/2006/07/060721202957.htm|title= Cassini Reveals Titan's Xanadu Region To Be An Earth-Like Land|accessdate=2007-08-27 |date=July 23, 2006 |publisher=Science Daily }}</ref> It is criss-crossed in places by dark lineaments&mdash;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.<ref>{{cite journal |last=Barnes |first=Jason W. |coauthors=Brown, Robert H.; et al. |year=2006 |month=January |title=Global-scale surface spectral variations on Titan seen from Cassini/VIMS |journal=Icarus |issue=1|volume=186 |url= http://c3po.barnesos.net/publications/papers/Titan.spectral.diversity.pdf |format=PDF|accessdate= 2007-08-27 }}</ref> 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). ===Liquids=== {{main|Lakes of Titan}} [[Image:PIA10008 Seas and Lakes on Titan.jpg|thumb|right|300px|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. [http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=2539 Photographs] suggest that the large body at lower left, [[Kraken Mare]], has about twice the extent visible here.]] The possibility that there were seas of liquid methane on Titan were first suggested based on [[Voyager 1]] and [[Voyager 2|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.<ref>{{cite journal| author= S. F.Dermott, [[Carl Sagan|C. Sagan]], |year=1995| title= Tidal effects of disconnected hydrocarbon seas on Titan |journal=Nature |volume=374 | pages=238–240 |doi=10.1038/374238a0}}</ref> 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 reflection]]s were initially observed.<ref>{{cite web |first=Henry|last=Bortman|url= http://saturn.astrobio.net/news/article81.html|title=Titan: Where's the Wet Stuff? |accessdate=2007-08-28 |date=November 02, 2004 |publisher=Astrobiology Magazine}}</ref> 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.<ref>{{cite news| url=http://www.planetary.org/news/2005/0628_Dark_Spot_Near_the_South_Pole_A.html| title=Dark Spot Near the South Pole: A Candidate Lake on Titan?| publisher=The Planetary Society| |author=Emily Lakdawalla |date=June 28, 2005 |accessdate=2006-10-14}}</ref> A possible shoreline was also identified at the pole via radar imagery.<ref>{{cite press release| url=http://www.spaceref.com/news/viewpr.html?pid=17829| title=NASA Cassini Radar Images Show Dramatic Shoreline on Titan|date=September 16, 2005| accessdate=2006-10-14| publisher=Jet Propulsion Laboratory}}</ref> 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.<ref name="PIA08630">{{cite web| url=http://photojournal.jpl.nasa.gov/catalog/PIA08630| title=PIA08630: Lakes on Titan| publisher=NASA/JPL |work=NASA Planetary Photojournal | accessdate=2006-10-14}}</ref> Based on the observations, scientists announced "definitive evidence of lakes filled with methane on Saturn's moon Titan" in January 2007.<ref name=NatureDefinitive>{{cite journal|title=The lakes of Titan|author=Stofan, E. R.|coauthors= Elachi, C.; et al.|issue=1 |volume=445|pages=61–64|journal=Nature|date=January 4, 2007|doi= 10.1038/nature05438|accessdate=2007-08-27}}</ref><ref>{{cite web |url=http://saturn.jpl.nasa.gov/news/features/feature20070103.cfm |title=Titan Has Liquid Lakes, Scientists Report in Nature |publisher=NASA/JPL |date=[[January 3]], [[2007]] |accessdate=2007-01-08}}</ref> 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.<ref name=NatureDefinitive/> ===Impact craters=== [[Image:Titancrater.jpg|thumb|250 px|Impact crater on Titan's surface]] 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&nbsp;km wide multi-ring impact basin named Menrva (seen by ''Cassini''<nowiki>'</nowiki>s ISS as a bright-dark concentric pattern).<ref>{{cite web| url=http://photojournal.jpl.nasa.gov/catalog/PIA07365| title=PIA07365: Circus Maximus| publisher=NASA Planetary Photojournal| accessdate=2006-05-04}}</ref> A smaller 80&nbsp;km wide, flat-floored crater named Sinlap<ref>{{cite web| url=http://photojournal.jpl.nasa.gov/catalog/PIA07368| title=PIA07368: Impact Crater with Ejecta Blanket| publisher=NASA Planetary Photojournal| accessdate=2006-05-04}}</ref> and a 30&nbsp;km crater with a central peak and dark floor named [[Ksa (crater)|Ksa]] have also been observed.<ref>{{cite web| url=http://photojournal.jpl.nasa.gov/catalog/PIA08737| title=PIA08737: Crater Studies on Titan| publisher=NASA Planetary Photojournal| accessdate=2006-09-15}}</ref> 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&nbsp;km wide ring of bright, rough material known as [[Guabonito (Titan)|Guabonito]] has been observed by ''Cassini''.<ref name=Guabonito>{{cite web| url=http://photojournal.jpl.nasa.gov/catalog/PIA08425| title=PIA08425: Radar Images the Margin of Xanadu| publisher=NASA Planetary Photojournal| accessdate=2006-09-26}}</ref> 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''<nowiki>'</nowiki>s [[April 30]], [[2006]] flyby of Titan.<ref name=Xcraters>{{cite web| url=http://photojournal.jpl.nasa.gov/catalog/PIA08429| title=PIA08429: Impact Craters on Xanadu| publisher=NASA Planetary Photojournal| accessdate=2006-09-26}}</ref> 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".<ref>{{cite journal |last= Artemieva |first=Natalia |coauthors=Lunine, Jonathan |year=2003 |month=August |title=Cratering on Titan: impact melt, ejecta, and the fate of surface organics |journal=Icarus |volume=164 |pages=471–480 |url=http://adsabs.harvard.edu/abs/2003Icar..164..471A |accessdate= 2007-08-28 |doi=10.1016/S0019-1035(03)00148-9 }}</ref> 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.<ref>{{cite journal |last= Ivanov |first=B. A. |coauthors=Basilevsky, A. T.; Neukum, G. |year=1997 |month=August |title=Atmospheric entry of large meteoroids: implication to Titan |journal=Planetary and Space Science |volume=45 |pages=993–1007|url=http://adsabs.harvard.edu/abs/1997P&SS...45..993I|accessdate= 2007-08-28 |doi=10.1016/S0032-0633(97)00044-5 }}</ref> ===Cryovolcanism and mountains=== {{See also|Cryovolcano}} [[Image:Genesa.jpg|thumb|left|Near-infrared image of [[Tortola Facula]], thought to be a possible cryovolcano.]] 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|Argon 40]] detection in the atmosphere indicates that volcanoes spew plumes of "lava" composed of water and ammonia.<ref>{{cite journal|author=Tobias Owen |title= Planetary science: Huygens rediscovers Titan| journal=Nature| volume=438| pages=756–757 |year=2005 |doi=10.1038/438756a}}</ref> ''Cassini'' detected methane emissions from one suspected cryovolcano, and volcanism is now believed to be a significant source of the methane in the atmosphere.<ref name="Seeing_Touching_Titan-ESA">{{cite news| url=http://www.esa.int/SPECIALS/Cassini-Huygens/SEMHB881Y3E_0.html| title=Seeing, touching and smelling the extraordinarily Earth-like world of Titan| publisher=ESA News, [[European Space Agency]]| date=[[January 21]] [[2005]]| accessdate=2005-03-28}}</ref><ref>{{cite news |url=http://www.newscientist.com/article.ns?id=dn7489 |title=Hydrocarbon volcano discovered on Titan |author=David L. Chandler |publisher=NewScientist.com news service, [[New Scientist]] |date=[[June 8]] [[2005]] |accessdate=2007-08-07}}</ref> One of the first features imaged by ''Cassini'', [[Ganesa Macula]], resembles the geographic features called "[[pancake dome]]s" found on [[Venus]], and is thus believed to be cryovolcanic in origin.<ref>{{cite web|title=Shape and thermal modeling of the possible cryovolcanic dome Ganesa Macula on Titan: Astrobiological implications|author=C.D. Neish, R.D. Lorenz, D.P. O'Brien|work=Lunar and Planetary Laboratory, University of Arizona, Observatoire de la Cote d'Azur|url=http://www.aas.org/publications/baas/v37n3/dps2005/257.htm|year=2005|accessdate=2007-08-27}}</ref> 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 [[Eolian processes|wind-shaped]] landscape and sand dune features.<ref>{{cite journal |last=Fortes |first=A. D. |coauthors= Grindroda, P.M.; Tricketta, S. K.; Vočadloa, L.|year=2007 |month=May |title=Ammonium sulfate on Titan: Possible origin and role in cryovolcanism |journal=Icarus |volume=188 |issue=1 |pages=139–153 |url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-4MM2660-2&_user=1790654&_coverDate=05%2F31%2F2007&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000054312&_version=1&_urlVersion=0&_userid=1790654&md5=e12d369a2df7c4ef1a56c690abc9e398 |accessdate= 2007-08-27 |quote= |doi=10.1016/j.icarus.2006.11.002 }}</ref> A mountain range measuring 150&nbsp;km long, 30&nbsp;km wide and 1.5&nbsp;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.<ref>{{cite news|url=http://news.bbc.co.uk/2/hi/science/nature/6174501.stm|title=Mountain range spotted on Titan |publisher=BBC News|date=[[December 12]] [[2006]] |accessdate=2007-08-06}}</ref> Prior to Cassini, scientists assumed that most of the topography on Titan would be impact structures, yet these findings reveal that similar to Earth, the mountains were formed through geological processes.<ref>[http://newswise.com/articles/view/536441/ Mountains Discovered on Saturn’s Largest Moon] Newswise, Retrieved on July 2, 2008.</ref> ===Dark terrain=== [[Image:Titan dunes.jpg|thumb|300px|right|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.<ref name=Roe>H. G. Roe ''et al.'' (2004). "[http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2004GeoRL..3117S03 A new 1.6-micron map of Titan's surface]". ''Geophys. Res. Lett.'' '''31''' (17): CiteID L17S03.</ref> Prior to the arrival of ''Cassini'', these regions were thought to be seas of organic matter like tar or liquid hydrocarbons.<ref>{{cite journal | title=The Glitter of Distant Seas | author= R. Lorenz|journal= Science | year=2003 | volume= 302 | pages= 403–404 |url=http://www.sciencemag.org/cgi/content/summary/312/5774/702 |doi=10.1126/science.1090464 | pmid=16675686 }}</ref> Radar images captured by the ''Cassini'' spacecraft have instead revealed some of these regions to be extensive plains covered in longitudinal sand [[dune]]s, up to 330 meters high.<ref name=Saharan/> 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 and meridional|zonal]] (eastward) winds combine with variable tidal winds (approximately 0.5 meter per second).<ref name=Lorenz2006/> The tidal winds are the result of [[tidal force]]s 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.<ref>{{cite journal | title=Linear Dunes on Titan | author= N. Lancaster |journal= Science | year=2006 | volume= 312 | pages= 702–703 |url=http://www.sciencemag.org/cgi/content/summary/312/5774/702 |doi=10.1126/science.1126292 | pmid=16675686 }}</ref><ref name=Saharan>{{cite news| title=Saharan Sand Dunes Found on Saturn's Moon Titan| url=http://www.space.com/scienceastronomy/060504_sands_titan.html|first= Sara| last=Goudarzi| publisher=[[SPACE.com]]|date=May 4, 2006 |accessdate=2007-08-06}}</ref><ref name=Lorenz2006>{{cite journal|title=The sand seas of Titan: Cassini RADAR observations of longitudinal dunes |last=Lorenz|first= RD|coauthors=Wall S, Radebaugh J, et.al. |journal= Science | year=2006 | volume= 312 | pages= 724–727 |doi=10.1126/science.1123257 }}</ref> Studies of dunes' composition in May, 2008, revealed that they possessed less water than the rest of Titan, and are most likely to derive from organic material clumping together after raining onto the surface.<ref>{{cite web|title=Titan's Smoggy Sand Grains|work=JPL|year=2008|url=http://www.jpl.nasa.gov/news/features.cfm?feature=1679|accessdate=2008-05-06}}</ref> ==Climate== [[Image:TitanAtmDetail.jpg|thumb|right|300px|A graph detailing temperature, pressure, and other aspects of Titan's climate. The atmospheric haze lowers the temperature in the lower atmosphere, while methane raises the temperature at the surface. Cryovolcanoes erupt methane into the atmosphere, which then rains down onto the surface, forming lakes.]] Titan's surface temperature is about 94&nbsp;K (−179&nbsp;°C, or −290&nbsp;°F). At this temperature water ice does not [[Sublimation (chemistry)|sublimate]] or [[Evaporation|evaporate]], 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.<ref>{{cite web |title=OPTICAL PROPERTIES OF TITAN HAZE LABORATORY ANALOGS USING CAVITY RING DOWN SPECTROSCOPY |author= C. A. Hasenkopf |work=Workshop on Planetary Atmospheres (2007) |url=http://64.233.183.104/search?q=cache:H1UhAFtLEpEJ:www.lpi.usra.edu/meetings/patm2007/pdf/9034.pdf+titan+anti-greenhouse-effect+2006+OR+2007&hl=en&ct=clnk&cd=3&gl=uk |accessdate=2007-10-16}} </ref> The clouds on Titan, probably composed of methane, ethane or other simple organics, are scattered and variable, punctuating the overall haze.<ref name=nineplanets>{{cite web |author=Bill Arnett |year=2005 |url=http://seds.lpl.arizona.edu/nineplanets/nineplanets/titan.html |title=Titan |publisher=University of Arizona, Tucson |work=Nine planets |accessdate=2005-04-10}}</ref> This atmospheric methane conversely creates a [[greenhouse effect]] on Titan's surface, without which Titan would be far colder.<ref name=oil>{{cite web|url=http://www.space.com/scienceastronomy/080213-titan-oil.html |title=Titan Has More Oil Than Earth |date=[[February 13]], [2008]] |accessdate=2008-02-13}}</ref> The findings of the ''Huygens'' probe indicate that Titan's atmosphere periodically rains liquid methane and other organic compounds onto the moon's surface.<ref name="planetary-Arizona_Icebox">{{cite news |url=http://www.planetary.org/news/2005/huygens_science-results_0121.html |title=Titan: Arizona in an Icebox? |author=Emily Lakdawalla |date=[[January 21]] [[2004]] |publisher=The Planetary Society |accessdate=2005-03-28}}</ref> In October 2007, observers noted an increase in apparent opacity in the clouds above the equatorial [[Xanadu (Titan)|Xanadu]] region, suggestive of "methane drizzle", though this was not direct evidence for rain.<ref>{{cite journal| url=http://www.sciencemag.org/cgi/content/abstract/1146244| journal=Science| month= October 11| year= 2007| doi=10.1126/science.1146244 | title=Widespread Morning Drizzle on Titan| first=Máté| last= Ádámkovics| coauthors= Michael H. Wong, Conor Laver, Imke de Pater | volume=318 | pages=962 | pmid=17932256}}</ref> It is possible that areas of Titan's surface may be coated in a layer of tholins, but this has not been confirmed.<ref>{{cite web|title=Mass Spectral Investigation of Laboratory Made Tholins and Their Reaction Products: Implications to Tholin Surface Chemistry on Titan|author=Arpad Somogyi, MA Smith|work=University of Arizona|url=http://adsabs.harvard.edu/abs/2006DPS....38.2730S|year=2006|accessdate=2007-08-28}}</ref> Simulations of global wind patterns based on 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—which was 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.<ref name="Titan wind"/> The pole-to-pole wind circulation cell appears to be centered on the stratosphere; simulations suggest it ought to change every twelve years, with a three-year transition period, over the course of Titan's year (30 terrestrial years).<ref name=Rannou2006>{{cite journal |last=Rannou |first=R.; ''et al.'' |year=2006 |month=January |title= The Latitudinal Distribution of Clouds on Titan|journal= [[Science Magazine|Science]] |volume=311 |issue=5758 |pages=201–205 |url= http://www.sciencemag.org/cgi/content/abstract/311/5758/201 |accessdate=2007-09-01 |doi=10.1126/science.1118424 |pmid= 16410519}}</ref> This cell creates a global band of low pressure—what is in effect a variation of Earth's [[Intertropical Convergence Zone]]. Unlike on Earth, however, where the oceans confine the ITCZ to the tropics, on Titan, the zone wanders from one pole to the other, taking methane rainclouds with it. This means that Titan, despite its frigid temperatures, can be said to have a tropical climate.<ref name=pierrehumbert>{{cite web|title=Tropical Titan|work=astrobio.net|year=2007|url=http://www.astrobio.net/news/article2485.html|accessdate=2007-10-16}}</ref> The number of methane lakes visible near Titan's southern pole is decidedly smaller than the number observed near the north pole. As the south pole is currently in summer and the north in winter, an emerging hypothesis is that methane rains onto the poles in winter and evaporates in summer.<ref>{{cite web|title=NASA Cassini Image: Radar Images Titan's South Pole|work=JPL|year=2008|url=http://www.spaceref.com/news/viewsr.html?pid=26627|accessdate=2008-01-11}}</ref> ===Clouds=== [[Image:Titancloud.jpg|thumb|left|A cloud imaged in false colour over Titan's north pole]] In September 2006, ''Cassini'' imaged a large cloud at a height of 40&nbsp;km over Titan's north pole. Although methane is known to condense in Titan's atmosphere, the cloud was more likely to be ethane, as the detected size of the particles was only 1–3 [[micrometre|micrometers]] and ethane can also freeze at these altitudes. In December, ''Cassini'' again observed cloud cover and detected methane, ethane and other organics. The cloud was over 2,400&nbsp;km in diameter and was still visible during a following flyby a month later. One hypothesis is that it is currently raining (or, if cool enough, snowing) on the north pole; the downdrafts at high northern latitudes are strong enough to drive organic particles towards the surface. These were the strongest evidence yet for the long-hypothesised "methanological" cycle (analogous to Earth's [[hydrological cycle]]) on Titan.<ref name=nasagov /> Clouds have also been found over the south pole. While typically covering 1% of Titan's disk, outburst events have been observed in which the cloud cover rapidly expands to as much as 8%. One hypothesis asserts that the southern clouds are formed when heightened [[insolation|levels of sunlight]] during the Titanian summer generate uplift in the atmosphere, resulting in [[convection]]. This explanation is complicated by the fact that cloud formation has been observed not only post–summer solstice but also at mid-spring. Increased methane humidity at the south pole possibly contributes to the rapid increases in cloud size.<ref>{{cite journal |last=Emily L. |first=Schaller |coauthors=Brouwn, Michael E.; Roe, Henry G. Roe; Bouchez, Antonin H. |year=2006 |month=February |title=A large cloud outburst at Titan’s south pole |journal=Icarus |issue= 182 |pages=224–229 |format=PDF |url=http://www.gps.caltech.edu/~mbrown/papers/ps/largecloud.pdf |accessdate= 2007-08-23 }}</ref> It is currently summer in Titan's southern hemisphere and will remain so until 2010, when Saturn's orbit, which governs the moon's motion, will tilt the northern hemisphere towards the Sun.<ref name="Titan wind">{{cite news| title=The Way the Wind Blows on Titan|date=June 1, 2007| url=http://saturn.jpl.nasa.gov/news/features/feature20070601f.cfm| accessdate=2007-06-02| publisher=Jet Propulsion Laboratory}}</ref> When the seasons switch, ethane will begin to condense over the south pole.<ref>{{cite journal| title= Huge ethane cloud discovered on Titan |author= David Shiga |journal= New Scientist |volume= 313 |pages= 1620 |year= 2006 |url=http://space.newscientist.com/channel/solar-system/cassini-huygens/dn10073-huge-ethane-cloud-discovered-on-titan.html |accessdate=2007-08-07}}</ref> Research models that match well with observations suggest that clouds on Titan cluster at preferred coordinates and that cloud cover varies by distance from the surface on different parts of the satellite. In the polar regions (above 60 degrees [[latitude]]), widespread and permanent ethane clouds appear in and above the [[troposphere]]; at lower latitudes, mainly methane clouds are found between 15 and 18&nbsp;km, and are more sporadic and localized. In the summer hemisphere, frequent, thick but sporadic methane clouds seem to cluster around 40°.<ref name=Rannou2006/> Ground-based observations also reveal seasonal variations in cloud cover. 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.<ref name=nasagov>{{cite web|title=Cassini Images Mammoth Cloud Engulfing Titan's North Pole|work=NASA|url=http://www.nasa.gov/mission_pages/cassini/media/cassini-20070201.html|year=2007|accessdate=2007-04-14}}</ref> ==Observation and exploration== [[Image:PIA08391 Epimetheus, Rings and Titan.jpg|thumb|left|240px|''Cassini'' image of [[Epimetheus (moon)|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.<ref>{{cite book |last=Benton |first=Julius L. Jr. |pages=141-146|url=http://www.springerlink.com/content/t7887977563172w5/|title=Saturn and How to Observe It |year=2005 |publisher=Springer London |location= |isbn= 978-1-84628-045-0}}</ref> 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.<ref>{{cite web |first=Jyri|last=Näränen|url=http://www.astro.helsinki.fi/~naranen/titan/titan.html |title=The Atmosphere of Titan |accessdate=2007-08-19 |publisher=[[University of Helsinki]], Department of Astronomy |work= }}</ref> The first probe to visit the Saturnian system was ''[[Pioneer 11]]'' in 1979, which determined that Titan was likely too cold to support life.<ref>{{cite web |date=March 26, 2007 |title=The Pioneer Missions |publisher=NASA, Jet Propulsion Laboratory |work=Pioneer Project |url= http://www.nasa.gov/centers/ames/missions/archive/pioneer.html |accessdate= 2007-08-19 |quote= }}</ref> 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]].<ref>{{cite web |title=Pioneer XI |publisher=NASA |work=Photo Index |url=ftp://ftp.hq.nasa.gov/pub/pao/images/index/photoindex/pioneer11.htm|accessdate= 2007-08-19 }}</ref> Titan was examined by both ''[[Voyager 1]]'' and ''[[Voyager 2]]'' in 1980 and 1981, respectively. ''Voyager 1''<nowiki>'</nowiki>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'''<nowiki>'</nowiki>s orange filter did reveal hints of the light and dark features now known as [[Xanadu (Titan)|Xanadu]] and [[Shangri-la (Titan)|Shangri-la]],<ref>{{cite journal |url=http://www.lpl.arizona.edu/~jrich/vgertitan.html |title=Titan's Surface and Rotation: New Results from Voyager 1 Images |author=James Richardson, Ralph Lorenz, & Alfred McEwen |journal=Icarus |date=July 2004 |volume=170 |issue=1 |pages=113–124 |doi=10.1016/j.icarus.2004.03.010 }} verified [[2005-03-28]].</ref> 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. ===''Cassini–Huygens''=== {{main|Cassini–Huygens|Huygens probe}} [[Image:Titan globe.jpg|thumb|left|A mosaic of nine processed images acquired during Cassini's first very close flyby of Titan on Oct. 26, 2004]] 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&nbsp;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.<ref name="huygens_picture_saturn"> {{cite web |url=http://saturn.jpl.nasa.gov/operations/index.cfm |title=Cassini at Saturn: Introduction |accessdate=2007-09-06 |format= |publisher=NASA, Jet Propulsion Laboratory }}</ref> On [[July 22]], [[2006]], ''Cassini'' made the first of a series of 21 planned, targeted, close fly-bys, each at only 950&nbsp;km from Titan; the last is scheduled for [[May 12]], [[2008]].<ref>{{cite web|title=CASSINI AT SATURN - Saturn Tour Dates|work=NASA/JPL |url=http://saturn.jpl.nasa.gov/operations/cassini-calendar-ALL.cfm |accessdate=2007-10-31}}</ref> 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''.<ref name="PIA08630"/> Titan is the most distant body from Earth that has seen a space probe landing.<ref>{{cite web |url= http://www.spacetoday.org/SolSys/Saturn/SaturnHuygens.html|title=Huygens Exposes Titan's Surface |accessdate=2007-08-19 |publisher=Spacetoday}}</ref> Titan is also the second moon in the solar system to have a man-made object land on its surface. ====''Huygens'' landing site==== [[Image:Huygens surface color.jpg|thumb|upright|''Huygens'' image from Titan's surface]] The ''[[Huygens probe|Huygens]]'' probe landed just off the easternmost tip of a bright region now called [[Adiri (Titan)|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.<ref name="Seeing_Touching_Titan-ESA"/> After landing, ''Huygens'' photographed a dark plain covered in small rocks and pebbles, which are composed of water ice.<ref name="Seeing_Touching_Titan-ESA"/> 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&nbsp;centimeters across, and the one in the center is 4&nbsp;centimeters across, at a distance of about 85&nbsp;centimeters 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.<ref>{{cite web |url=http://www.esa.int/esaCP/SEM9GNN0LYE_index_0.html |title=Huygens landing site to be named after Hubert Curien |publisher=ESA |date=March 5, 2007 |accessdate=2007-08-06}}</ref> ==Prebiotic conditions and possible life== {{see also|Planetary habitability}} 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 [[tholin]]s can be generated. The reaction starts with [[dissociation (chemistry)|dissociation]] of nitrogen and methane, forming [[Hydrogen Cyanide|hydrocyan]] and [[ethyne]]. Further reactions have been studied extensively.<ref name=Raulin2002>{{cite journal | journal = Space Science Review | volume= 104 | issue = 1–2 | pages = 377–394 | year= 2002 | doi = 10.1023/A:1023636623006 | title = Organic chemistry and exobiology on Titan | author = Raulin F., Owen T.}}</ref> 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.<ref>{{cite journal | author = Artemivia N., Lunine J, | title = Createring on Titan: Impact melt ejecta and the fate of surface organics | year = 2003 | journal = Icarus | volume = 164| pages = 471–480|doi=10.1016/S0019-1035(03)00148-9}}</ref> It has also been observed that liquid ammonia oceans could exist deep below the surface;<ref name=Grasset2000/><ref>[http://www.news.nationalgeographic.com/news/2008/03/080320-titan-ocean_2.html news.nationalgeographic.com]</ref> one model suggests an ammonia–water solution as much as 200&nbsp;km deep beneath a water ice crust, conditions that, "while extreme by terrestrial standards, are such that life could indeed survive".<ref name=Fortes2000/> [[Heat transfer]] between the interior and upper layers would be critical in sustaining any sub-surface oceanic life.<ref name=Grasset2000/> 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.<ref name=Fortes2000/> 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]].<ref>{{cite journal | journal = Icarus | volume= 178 | issue = 1 | pages = 274–276 | year= 2005 | doi = 10.1016/j.icarus.2005.05.018 | title = Possibilities for methanogenic life in liquid methane on the surface of Titan | author = McKay, C. P.; Smith, H. D.}}</ref> 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 CO<sub>2</sub>. 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.<ref>{{cite web| url=http://www.astrobio.net/news/article1130.html| title=Saturn's Moon Titan: Prebiotic Laboratory | publisher=Astrobiology Magazine |date=August 11, 2004 | accessdate=2004-08-11}}</ref> 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.<ref name=Raulin2005>{{cite journal | journal = Space Science Review | volume= 116 | issue = 1–2 | pages = 471–487 | year= 2005 | doi = 10.1007/s11214-005-1967-x | title = Exo-astrobiological aspects of Europa and Titan: From observations to speculations | author = Raulin, F.}}</ref> Using Titan as a prebiotic experiment involves not only observation through spacecraft, but laboratory experiment, and chemical and photochemical modelling on Earth.<ref name=Raulin2002/> An alternate explanation for life's hypothetical existence on Titan has been proposed: if life were to be found on Titan, it would be statistically more likely to have originated from Earth than to have appeared independently, a process known as [[panspermia]]. It is theorized that large asteroid and cometary impacts on Earth's surface have caused hundreds of millions of fragments of microbe-laden rock to escape Earth's gravity. Calculations indicate that a number of these would encounter many of the bodies in the solar system, including Titan.<ref>{{cite news| url=http://news.bbc.co.uk/1/hi/sci/tech/4819370.stm |title=Earth could seed Titan with life |publisher=BBC News |date=March 18, 2006 |accessdate=2007-03-10}}</ref><ref>{{cite journal | author = Gladman, Brett; Dones, Luke; Levinson, Harold F.; Burns, Joseph A. | title = Impact Seeding and Reseeding in the Inner Solar System | year = 2005 | journal = Astrobiology | volume = 5 | pages = 483–496 |doi=10.1089/ast.2005.5.483}}</ref> Conditions on Titan could become far more habitable in future. Six billion years from now, as the Sun becomes a [[red giant]], surface temperatures could rise to ~200K, high enough for stable oceans of water/ammonia mixture to exist on the surface. As the Sun's ultraviolet output decreases, the haze in Titan's upper atmosphere will deplete, lessening the anti-greenhouse effect on the surface and enabling the greenhouse created by atmospheric methane to play a far greater role. These conditions together could create an environment agreeable to exotic forms of life, and will subsist for several hundred million years, long enough for at least primitive life to form.<ref>{{cite web|title=Titan under a red giant sun: A new kind of "habitable" moon|author=Ralph D. Lorenz, Jonathan I. Lunine, Christopher P. McKay|work=NASA Ames Research Center, Lunar and Planetary Laboratory, Department of Planetary Sciences, University of Arizona|year=1997|url=http://www.lpl.arizona.edu/~rlorenz/redgiant.pdf|accessdate=2008-03-21}}</ref> While the ''[[Cassini–Huygens]]'' mission was not equipped to provide evidence for biology or complex organics, it did support the theory of an environment on Titan that is similar, in some ways, to that of the primordial Earth.<ref name=Raulin2005/> There are a wide range of options for future missions to Titan that might address these and other questions,<ref>{{cite journal | author = Lorenz, Ralph | title = Post-Cassini Exploration of Titan : Science Rationale and Mission Concepts | year = 2000 | journal = Journal of the British Interplanetary Society | volume = 53 | pages = 218–234 }}</ref> including orbiters, landers, balloons etc. == See also == {{portal|Solar System|Solar system.jpg}} * [[Colonization of Titan]] * [[Huygens probe]] * [[List of planetary bodies]] * [[List of Solar System objects by radius]] * [[Saturn's moons in fiction]] * [[Timeline of discovery of Solar System planets and their moons]] * [[Titan in fiction]] ==References== <!-- ---------------------------------------------------------- See http://en.wikipedia.org/wiki/Wikipedia:Footnotes for a discussion of different citation methods and how to generate footnotes using the <ref>, </ref> and <reference /> tags ----------------------------------------------------------- --> {{reflist|2}} <!-- Dead note "nasa_book_luna_saturni": [http://antwrp.gsfc.nasa.gov/apod/ap050325.html NASA Astronomy Picture of the Day of Titan], [http://assets.cambridge.org/052179/3483/sample/0521793483ws.pdf Book: Lifting Titan's Veil] NASA: ''"Huygens discovered Luna Saturni - now known as...Titan"'' Book: ''Huygens discovered Luna Saturni - now known as Saturn's moon Titan'' --> ==Further reading== * {{cite book| title=Lifting Titan's Veil: Exploring the Giant Moon of Saturn| first=Ralph| last= Lorenz| coauthors=Jacqueline Mitton| publisher=Cambridge University Press| year=May 2002| id= ISBN 0-521-79348-3}} ==External links== {{Commons|Titan (moon)}} * [http://esamultimedia.esa.int/images/huygens_alien_winds_descent.mp3 The Alien Noise]. This recording is a laboratory reconstruction of the sounds heard by Huygens' microphones. * [http://saturn.jpl.nasa.gov Cassini-Huygens Mission To Saturn & Titan]. Multimedia Feature [http://saturn.jpl.nasa.gov/news/features/feature20070129.cfm Titan Virtual Tour] * [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sat_Titan Titan Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration] * Cassini Imaging Central Laboratory for Operations (2005). [http://ciclops.org/ CICLOPS: Cassini Imaging]. Retrieved March 28, 2005. * [[European Space Agency]]. (2005). [http://www.esa.int/SPECIALS/Cassini-Huygens/index.html ESA - Cassini-Huygens]. Retrieved March 28, 2005. * Gangale, Thomas (2002). [http://www.martiana.org/mars/saturn/Darian_Titan_frm.htm The Darian Calendar for Titan]. Retrieved March 28, 2005. * Hamilton, Calvin J. (2001). [http://www.solarviews.com/eng/titan.htm Titan - Saturn IV]. Retrieved March 28, 2005. * Hammerschlag, Michael (2005). [http://hammernews.com/cloudworld.htm CLOUD WORLD: MISSION to TITAN]. Retrieved March 28, 2005. * [[NASA]] (2005). [http://www.nasa.gov/mission_pages/cassini/main/index.html NASA - Cassini-Huygens: Close Encounter With Saturn]. Retrieved March 28, 2005. * Perry, Jason (2005). [http://volcanopele.blogspot.com/ Titan Today]. Retrieved March 28, 2005. * The [[Planetary Society]] (2005). [http://www.planetary.org/explore/topics/saturn/titan.html TPS: Saturn's moon Titan]. Retrieved March 28, 2005. * [[University of Arizona]] Lunar and Planetary Lab (2005). [http://www.lpl.arizona.edu/~kholso/ Lunar and Planetary Lab The Descent Imager-Spectral Radiometer of the Cassini-Huygens Mission to Titan]. Retrieved March 28, 2005. * [http://www.esa.int/SPECIALS/Cassini-Huygens/SEMKVQOFGLE_0.html Video of ''Huygens’'' descent] from the [[ESA]] <br/> {{Moons of Saturn|state=uncollapsed}} {{Solar System moons (compact)}} <!--As featured article--> <!--Categories--> [[Category:Titan|Titan]] [[Category:Saturn's moons]] {{Link FA|pt}} <!--Other languages--> [[af:Titan (maan)]] [[als:Titan (Mond)]] [[ar:تايتان (قمر)]] [[frp:Titan (satèlite)]] [[zh-min-nan:Titan (oē-chheⁿ)]] [[bs:Titan (mjesec)]] [[br:Titan (loarenn)]] [[bg:Титан (спътник)]] [[ca:Tità (satèl·lit)]] [[cs:Titan (měsíc)]] [[co:Titanu (astrunumia)]] [[cy:Titan (lloeren)]] [[da:Titan (måne)]] [[de:Titan (Mond)]] [[et:Titan]] [[el:Τιτάνας (δορυφόρος)]] [[es:Titán (luna)]] [[eo:Titano (luno)]] [[eu:Titan (ilargia)]] [[fa:تیتان]] [[fr:Titan (lune)]] [[gv:Titan (fo-phlannad)]] [[gl:Titán (satélite)]] [[zh-classical:土衛六]] [[ko:티탄 (위성)]] [[hr:Titan (mjesec)]] [[io:Titano]] [[id:Titan]] [[it:Titano (astronomia)]] [[he:טיטאן (ירח)]] [[la:Titan (satelles)]] [[lv:Titāns (pavadonis)]] [[lb:Titan (Mound)]] [[lt:Titanas (palydovas)]] [[hu:Titán (hold)]] [[ms:Titan (bulan)]] [[nl:Titan (maan)]] [[ja:タイタン (衛星)]] [[no:Titan (måne)]] [[nn:Saturnmånen Titan]] [[pl:Tytan (księżyc)]] [[pt:Titã]] [[ro:Titan (satelit)]] [[ru:Титан (спутник)]] [[scn:Titanu]] [[simple:Titan (moon)]] [[sk:Titan (mesiac)]] [[sl:Titan (luna)]] [[sr:Титан (месец)]] [[fi:Titan (kuu)]] [[sv:Titan (måne)]] [[th:ไททัน]] [[vi:Titan (vệ tinh)]] [[tr:Titan (uydu)]] [[uk:Титан (супутник)]] [[zh:土卫六]]