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Titan (moon)
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{{short description|Largest moon of Saturn}}{{Distinguish|Titania (moon)|Triton (moon)}}{{Use American English|date=February 2017}}{{Use mdy dates|date=March 2019}}







factoids
Titania (moon)>Titania. The latter may be pronounced with an ''{{respelltˈɑːin}}), but the form for Titan is only pronounced with an ''{{respelltˈeɪin}}. The less common "Titanean" {{IPAc-enaɪənə|n}} refers only to Titan.| image = Titan in true color.jpgorganonitrogen) haze.| background = #f8f9fa| orbit_ref =| discoverer = Christiaan Huygens| discovered = March 25, 16551186680|u=km}}1257060|u=km}}1221870|u=km}}0.0288}}15.945|u=days}}| avg_speed = 5.57 km/s (calculated)0.34854|u=°}} (to Saturn's equator)| satellite_of = Saturn2574.73u=km}} (0.404 Earths)10.1126/SCIENCE.1168905> PMID=19342551 LAST1=ZEBKER1 LAST2=STILES LAST3=HENSLEY LAST4=LORENZ LAST5=KIRK LAST6=LUNINE DATE=MAY 15, 2009 VOLUME=324 PAGES=921–923, (1.480 Moons)8.3u=km2}} (0.163 Earths) (2.188 Moons)7.16u=km3}} (0.066 Earths) (3.3 Moons)1.3452e=23 FIRST1=R. A. FIRST2=P. G. FIRST3=J. J. FIRST4=K. E. FIRST5=R. FIRST6=J. B. FIRST7=R. A. FIRST8=M. C. FIRST9=D. LAST10=PELLETIER LAST11=OWEN, JR. LAST12=ROTH LAST13=ROUNDHILL LAST14=STAUCH TITLE=THE GRAVITY FIELD OF THE SATURNIAN SYSTEM FROM SATELLITE OBSERVATIONS AND SPACECRAFT TRACKING DATA VOLUME=132 PAGES=2520–2526 BIBCODE=2006AJ....132.2520JJACOBSON ANTREASIAN ET AL., 2006, }} (1.829 Moons)1.8798u=g/cm3}}1.3520.138G-force>g}}) (0.835 Moons)0.3414LAST2= RAPPAPORTLAST3= JACOBSONLAST4= RACIOPPALAST5= STEVENSONLAST6= TORTORALAST7= ARMSTRONGLAST8= ASMARTITLE= GRAVITY FIELD, SHAPE, AND MOMENT OF INERTIA OF TITANVOLUME= 327DATE= MARCH 12, 2010DOI= 10.1126/SCIENCE.1182583BIBCODE = 2010SCI...327.1367I, (estimate)2.639|u=km/s}} (0.236 Earths) (1.11 Moons)| rotation = Synchronous| axial_tilt = ZeroDATE=FEBRUARY 22, 2011 PUBLISHER=NASA ACCESSDATE=APRIL 22, 2015 ARCHIVEURL=HTTPS://WEB.ARCHIVE.ORG/WEB/20100430122034/HTTP://NSSDC.GSFC.NASA.GOV/PLANETARY/FACTSHEET/SATURNIANSATFACT.HTML, April 30, 2010, | magnitude = 8.2 to 9.093.7°C}}MITRI >FIRST=G. FIRST2=ADAM P. FIRST3=JONATHAN I. FIRST4=RALPH D. TITLE=HYDROCARBON LAKES ON TITAN JOURNAL=ICARUS ISSUE=2 DOI=10.1016/J.ICARUS.2006.09.004 DEADURL=NO ARCHIVEDATE=FEBRUARY 27, 2008, | atmosphere = yes146.71.45|ul=atm}})| atmosphere_composition = VariableStratosphere:98.4% nitrogen ({{chem|N|2}}),1.4% methane ({{chem|CH|4}}),0.2% hydrogen ({{chem|H|2}});Lower troposphere:95.0% {{chem|N|2}}, 4.9% {{chem|CH|4}};{{nowrap|97% {{chem|N|2}},}} {{nowrap|2.7±0.1% {{chem|CH|4}},}} {{nowrap|0.1–0.2% {{chem|H|2}}{{sfnp |Coustenis |Taylor |2008 |pp=154–155}}}}}}Titan is the largest moon of Saturn and the second-largest natural satellite in the Solar System. It is the only moon known to have a dense atmosphere.Titan is the sixth gravitationally rounded moon from Saturn. Frequently described as a planet-like moon, Titan is 50% larger than Earth's moon and 80% more massive. It is the second-largest moon in the Solar System after Jupiter's moon Ganymede, and is larger than the planet Mercury, but only 40% as massive. Discovered in 1655 by the Dutch astronomer Christiaan Huygens, Titan was the first known moon of Saturn, and the sixth known planetary satellite (after Earth's moon and the four Galilean moons of Jupiter). Titan orbits Saturn at 20 Saturn radii. From Titan's surface, Saturn subtends an arc of 5.09 degrees and would appear 11.4 times larger in the sky than the Moon from Earth.Titan is primarily composed of ice and rocky material, which is likely differentiated into a rocky core surrounded by various layers of ice, including a crust of ice Ih and a subsurface layer of ammonia-rich liquid water.BOOK, Titan from Cassini-Huygens, Robert Brown, Jean Pierre Lebreton, Hunter Waite, Springer Science & Business Media, 2009, 69, 9781402092152, Much as with Venus before the Space Age, the dense opaque atmosphere prevented understanding of Titan's surface until the Cassini–Huygens mission in 2004 provided new information, including the discovery of liquid hydrocarbon lakes in Titan's polar regions. The geologically young surface is generally smooth, with few impact craters, although mountains and several possible cryovolcanoes have been found.The atmosphere of Titan is largely nitrogen; minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog. The climate—including wind and rain—creates surface features similar to those of Earth, such as dunes, rivers, lakes, seas (probably of liquid methane and ethane), and deltas, and is dominated by seasonal weather patterns as on Earth. With its liquids (both surface and subsurface) and robust nitrogen atmosphere, Titan's methane cycle is analogous to Earth's water cycle, at the much lower temperature of about {{convert|94|K|°C F}}.

History

Discovery

File:Christiaan Huygens-painting.jpeg|thumb|left|upright|Christiaan HuygensChristiaan HuygensTitan was discovered on March 25, 1655, by the Dutch astronomer Christiaan Huygens.WEB,weblink Lifting Titan's Veil, Cambridge,weblink" title="web.archive.org/web/20050222073209weblink">weblink February 22, 2005, 4, pdf, WEB,weblink NASA, Astronomy Picture of the Day, Titan,weblink" title="web.archive.org/web/20050327011129weblink">weblink March 27, 2005, Huygens was inspired by Galileo's discovery of Jupiter's four largest moons in 1610 and his improvements in telescope technology. Christiaan, with the help of his older brother Constantijn Huygens, Jr., began building telescopes around 1650 and discovered the first observed moon orbiting Saturn with one of the telescopes they built.WEB, September 4, 2008, Discoverer of Titan: Christiaan Huygens,weblink European Space Agency, April 18, 2009, no,weblink" title="web.archive.org/web/20110809144038weblink">weblink August 9, 2011, It was the sixth moon ever discovered, after Earth's Moon and the Galilean moons of Jupiter.APOD, March 25, 2005, Huygens Discovers Luna Saturni, August 18, 2007,

Naming

Huygens named his discovery Saturni Luna (or Luna Saturni, Latin for "Saturn's moon"), publishing in the 1655 tract De Saturni Luna Observatio Nova (A New Observation of Saturn's Moon).BOOK, Huygens, Christiaan, Société hollandaise des sciences, Oeuvres complètes de Christiaan Huygens, 1888, Martinus Nijhoff, The Hague, Netherlands, vol. 1, 387–388,weblink Latin, 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".JOURNAL, Cassini, G. D., 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, 1673, 10.1098/rstl.1673.0003, 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, came from John Herschel (son of William Herschel, discoverer of two other Saturnian moons, Mimas and Enceladus), in his 1847 publication Results of Astronomical Observations Made during the Years 1834, 5, 6, 7, 8, at the Cape of Good Hope.JOURNAL, Lassell, November 12, 1847, Observations of Mimas, the closest and most interior satellite of Saturn,weblinkfull/seri/MNRAS/00080000042.000.html, Monthly Notices of the Royal Astronomical Society, 8, 3, 42–43, March 29, 2005, 10.1093/mnras/8.3.42, 1848MNRAS...8...42L, BOOK,weblink Results of astronomical observations made during the years 1834, 5, 6, 7, 8, at the Cape of Good Hope : being the completion of a telescopic survey of the whole surface of the visible heavens, commenced in 1825, Herschel, Sir John F. W., Smith, Elder & Co., 1847, London, 415, He suggested the names of the mythological Titans (), brothers and sisters of Cronus, the Greek Saturn. In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, that ruled during the legendary Golden Age.{{clearleft}}

Orbit and rotation

(File:Titan's orbit.svg|thumb|Titan's orbit (highlighted in red) among the other large inner moons of Saturn. The moons outside its orbit are (from the outside to the inside) Iapetus and Hyperion; those inside are Rhea, Dione, Tethys, Enceladus, and Mimas.)Titan orbits Saturn once every 15 days and 22 hours. Like the Moon and many of the satellites of the giant planets, its rotational period (its day) is identical to its orbital period; Titan is tidally locked in synchronous rotation with Saturn, and permanently shows one face to the planet, so Titan's "day" is equal to its orbit period. Because of this, there is a sub-Saturnian point on its surface, from which the planet would always appear to hang directly overhead. Longitudes on Titan are measured westward, starting from the meridian passing through this point.WEB,weblink EVS-Islands: Titan's Unnamed Methane Sea, October 22, 2009, no,weblink" title="web.archive.org/web/20110809201152weblink">weblink August 9, 2011, Its orbital eccentricity is 0.0288, and the orbital plane is inclined 0.348 degrees relative to the Saturnian equator.Unless otherwise specified: WEB,weblink JPL HORIZONS solar system data and ephemeris computation service, Solar System Dynamics, NASA, Jet Propulsion Laboratory, August 19, 2007, no,weblink" title="web.archive.org/web/20121007034731weblink">weblink October 7, 2012, Viewed from Earth, Titan reaches an angular distance of about 20 Saturn radii (just over {{convert|1200000|km|mi|sp=us}}) from Saturn and subtends a disk 0.8 arcseconds in diameter.The small, irregularly shaped satellite Hyperion is locked in a 3:4 orbital resonance with Titan. A "slow and smooth" evolution of the resonance—in which Hyperion migrated from a chaotic orbit—is considered unlikely, based on models. Hyperion probably formed in a stable orbital island, whereas the massive Titan absorbed or ejected bodies that made close approaches.JOURNAL, Bevilacqua, R., Menchi, O., Milani, A., Nobili, A. M., Farinella, P., 1980, Resonances and close approaches. I. The Titan-Hyperion case, Earth, Moon, and Planets, 22, 2, 141–152, 10.1007/BF00898423, 1980M&P....22..141B,

Bulk characteristics

{{multiple image
| align = left
| direction = vertical
| image1 = Titan, Earth & Moon size comparison.jpg
| width1 = 300
| caption1 = Size comparison: Titan (lower left) with the Moon and Earth (top and right)
| image2 = Titan poster.svg
| width2 = 300
| caption2 = A model of Titan's internal structure showing ice-six layer
}}Titan is {{convert|5149.46|km|mi|sp=us}} in diameter, 1.06 times that of the planet Mercury, 1.48 that of the Moon, and 0.40 that of Earth. Before the arrival of Voyager 1 in 1980, Titan was thought to be slightly larger than Ganymede (diameter {{convert|5262|km|mi|sp=us}}) and thus the largest moon in the Solar System; this was an overestimation caused by Titan's dense, opaque atmosphere, which extends many kilometres above its surface and increases its apparent diameter.WEB, Arnett, Bill, 2005,weblink Titan, University of Arizona, Tucson, Nine planets, April 10, 2005,weblink" title="web.archive.org/web/20051121130738weblink">weblink November 21, 2005, yes, Titan's diameter and mass (and thus its density) are similar to those of the Jovian moons Ganymede and Callisto.WEB, Lunine, J.,weblink Comparing the Triad of Great Moons, Astrobiology Magazine, March 21, 2005, July 20, 2006, no,weblink" title="web.archive.org/web/20190707213259weblink">weblink July 7, 2019, Based on its bulk density of 1.88 g/cm3, Titan's composition is half water ice and half rocky material. Though similar in composition to Dione and Enceladus, it is denser due to gravitational compression. It has a mass 1/4226 that of Saturn, making it the largest moon of the gas giants relative to the mass of its primary. It is second in terms of relative diameter of moons to a gas giant; Titan being 1/22.609 of Saturn's diameter, Triton is larger in diameter relative to Neptune at 1/18.092.Titan is probably partially differentiated into distinct layers with a {{convert|3400|km|mi|adj=on|sp=us}} rocky center.JOURNAL, Mitri, G., Pappalardo, R. T., Stevenson, D. J., 2009-12-01, Is Titan Partially Differentiated?,weblink AGU Fall Meeting Abstracts, 43, P43F–07, This rocky center is surrounded by several layers composed of different crystalline forms of ice.TOBIE >FIRST=G. FIRST2=OLIVIER FIRST3=JONATHAN I. FIRST4=ANTOINE FIRST5=CHRISTOPHE BIBCODE=2005ICAR..175..496T JOURNAL=ICARUS ISSUE=2, 496–502 magma" composed of water and ammonia between the Ice Ih>ice Ih crust and deeper ice layers made of high-pressure forms of ice. The presence of ammonia allows water to remain liquid even at a temperature as low as {{convertKeutectic mixture with water). The Cassini probe discovered the evidence for the layered structure in the form of natural extremely low frequency>extremely-low-frequency radio waves in Titan's atmosphere. Titan's surface is thought to be a poor reflector of extremely-low-frequency radio waves, so they may instead be reflecting off the liquid–ice boundary of a subsurface ocean.HTTP://WWW.ESA.INT/ESAMI/CASSINI-HUYGENS/SEM17F9RR1F_0.HTML >TITLE=TITAN'S MYSTERIOUS RADIO WAVE PUBLISHER=ESA CASSINI-HUYGENS WEB SITE DEADURL=NO ARCHIVEDATE=JUNE 5, 2011, Surface features were observed by the Cassini spacecraft to systematically shift by up to {{convertkmsp=us}} 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.SHIGA >FIRST=DAVID URL=HTTPS://WWW.NEWSCIENTIST.COM/ARTICLE/DN13516 WORK=NEW SCIENTIST ARCHIVEURL=HTTPS://WEB.ARCHIVE.ORG/WEB/20141021144006/HTTP://WWW.NEWSCIENTIST.COM/ARTICLE/DN13516, October 21, 2014, Further supporting evidence for a liquid layer and ice shell decoupled from the solid core comes from the way the gravity field varies as Titan orbits Saturn.JOURNAL
, 10.1126/science.1219631, 22745254, The Tides of Titan, Science
, 337, 6093, 457–9, 2012
, Iess, L.
, Jacobson, R. A.
, Ducci, M.
, Stevenson, D. J.
, Lunine, J. I.
, Armstrong, J. W.
, Asmar, S. W.
, Racioppa, P.
, Rappaport, N. J.
, Tortora, P.
, 2012Sci...337..457I, Comparison of the gravity field with the RADAR-based topography observationsJOURNAL, 10.1126/science.1168905, 19342551
, Size and Shape of Saturn's Moon Titan, Science, 324
, 5929, 921–3, 2009
, Zebker, H. A.
, Stiles, B.
, Hensley, S.
, Lorenz, R.
, Kirk, R. L.
, Lunine, J.
, 2009Sci...324..921Z, also suggests that the ice shell may be substantially rigid.JOURNAL, 10.1038/nature12400, 23985871, A rigid and weathered ice shell on Titan, Nature, 500, 7464, 550–2, 2013, Hemingway, D., Nimmo, F., Zebker, H., Iess, L., 2013Natur.500..550H, WEB,weblink Cassini Data: Saturn Moon May Have Rigid Ice Shell, JPL, no,weblink" title="web.archive.org/web/20141020103435weblink">weblink October 20, 2014,

Formation

The moons of Jupiter and Saturn are thought to have formed through co-accretion, a similar process to that believed to have formed the planets in the Solar System. As the young gas giants formed, they were surrounded by discs of material that gradually coalesced into moons. Whereas Jupiter possesses four large satellites in highly regular, planet-like orbits, Titan overwhelmingly dominates Saturn's system and possesses a high orbital eccentricity not immediately explained by co-accretion alone. A proposed model for the formation of Titan is that Saturn's system began with a group of moons similar to Jupiter's Galilean satellites, but that they were disrupted by a series of giant impacts, which would go on to form Titan. Saturn's mid-sized moons, such as Iapetus and Rhea, were formed from the debris of these collisions. Such a violent beginning would also explain Titan's orbital eccentricity.WEB, Giant impact scenario may explain the unusual moons of Saturn, Space Daily, 2012,weblink October 19, 2012, A 2014 analysis of Titan's atmospheric nitrogen suggested that it has possibly been sourced from material similar to that found in the Oort cloud and not from sources present during co-accretion of materials around Saturn.PRESS RELEASE, Dyches, Preston, Clavin, Whitney, Titan's Building Blocks Might Pre-date Saturn,weblink June 23, 2014, Jet Propulsion Laboratory, June 28, 2014, no,weblink" title="web.archive.org/web/20140627165816weblink">weblink June 27, 2014,

Atmosphere

(File:Titan-Complex 'Anti-greenhouse'.jpg|thumb|right|upright|True-color image of layers of haze in Titan's atmosphere)Titan is the only known moon with a significant atmosphere,WEB, News Features: The Story of Saturn,weblinkweblink" title="web.archive.org/web/20051202030828weblink">weblink December 2, 2005, Cassini–Huygens Mission to Saturn & Titan, NASA & Jet Propulsion Lab, JPL, January 8, 2007, and its atmosphere is the only nitrogen-rich dense atmosphere in the Solar System aside from Earth's. Observations of it made in 2004 by Cassini suggest that Titan is a "super rotator", like Venus, with an atmosphere that rotates much faster than its surface.WEB,weblink Wind or Rain or Cold of Titan's Night?, March 11, 2005, Astrobiology Magazine, July 17, 2007,weblink" title="web.archive.org/web/20070717081303weblink">weblink August 24, 2007, Observations from the Voyager space probes have shown that Titan's atmosphere is denser than Earth's, with a surface pressure about 1.45 atm. It is also about 1.19 times as massive as Earth's overall,{{sfnp |Coustenis |Taylor |2008 |p=130}} or about 7.3 times more massive on a per surface area basis. Opaque haze layers block most visible light from the Sun and other sources and obscures Titan's surface features.BOOK, Zubrin, Robert, Entering Space: Creating a Spacefaring Civilization, Section: Titan, 163–166, Tarcher/Putnam, 1999, 978-1-58542-036-0, Titan's lower gravity means that its atmosphere is far more extended than Earth's.MAGAZINE, Exploring the Surface of Titan with Cassini–Huygens, Turtle, Elizabeth P., 2007, Smithsonian,weblink April 18, 2009, no,weblink" title="web.archive.org/web/20130720123340weblink">weblink July 20, 2013, The atmosphere of Titan is opaque at many wavelengths and as a result, a complete reflectance spectrum of the surface is impossible to acquire from orbit.JOURNAL, Schröder, S. E., Tomasko, M. G., Keller, H. U., August 2005, The reflectance spectrum of Titan's surface as determined by Huygens, 726, American Astronomical Society, DPS Meeting No. 37, #46.15; Bulletin of the American Astronomical Society, 37, 726, 2005DPS....37.4615S, It was not until the arrival of the Cassini–Huygens spacecraft in 2004 that the first direct images of Titan's surface were obtained.NEWS,weblink Huygens Probe Sheds New Light on Titan, de Selding, Petre, Space.com, January 21, 2005, March 28, 2005, no,weblink" title="web.archive.org/web/20121019133741weblink">weblink October 19, 2012, File:PIA18431-SaturnMoon-Titan-SouthPoleVortex-Cloud-20121129.jpg|thumb|left|Titan's South Pole Vortex—a swirling HCN gas cloud (November 29, 2012).]]Titan's atmospheric composition is nitrogen (97%), methane (2.7±0.1%), hydrogen (0.1–0.2%) with trace amounts of other gases.{{sfnp |Coustenis |Taylor |2008 |pp=154–155}} There are trace amounts of other hydrocarbons, such as ethane, diacetylene, methylacetylene, acetylene and propane, and of other gases, such as cyanoacetylene, hydrogen cyanide, carbon dioxide, carbon monoxide, cyanogen, argon and helium.JOURNAL, The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe, H. B., Niemann, S. K., Atreya, S. J., Bauer, G. R., Carignan, J. E., Demick, R. L., Frost, D., Gautier, J. A., Haberman, D. N., Harpold, D. M., Hunten, G., Israel, J. I., Lunine, W. T., Kasprzak, T. C., Owen, M., Paulkovich, F., Raulin, E., Raaen, S. H., Way, 1, Nature, 438, 779–784, 2005, 10.1038/nature04122, 16319830, 7069, 2005Natur.438..779N,weblink 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.JOURNAL, Waite, J. H., Cravens, T. E., Coates, A. J., Crary, F. J., Magee, B., Westlake, J., 2007, The Process of Tholin Formation in Titan's Upper Atmosphere, Science, 316, 10.1126/science.1139727, 17495166, 5826, 2007Sci...316..870W, 870–5, Titan spends 95% of its time within Saturn's magnetosphere, which may help shield it from the solar wind.NEWS, Courtland, Rachel,weblink Saturn magnetises its moon Titan, New Scientist, September 11, 2008, no,weblink" title="web.archive.org/web/20150531045100weblink">weblink May 31, 2015, Energy from the Sun should have converted all traces of methane in Titan's atmosphere into more complex hydrocarbons within 50 million years—a short time compared to the age of the Solar System. This suggests that methane must be replenished by a reservoir on or within Titan itself.JOURNAL, Formation and evolution of Titan's atmosphere, Coustenis, A., Space Science Reviews, 116, 1–2, 171–184, 2005, 10.1007/s11214-005-1954-2, 2005SSRv..116..171C, The ultimate origin of the methane in its atmosphere may be its interior, released via eruptions from cryovolcanoes.WEB, NASA Titan – Surface, NASA,weblink February 14, 2013, no,weblink" title="web.archive.org/web/20130217182727weblink">weblink February 17, 2013, WEB, Hydrocarbon lakes on Titan, 2007, G., Mitri,weblink February 14, 2013, no,weblink" title="web.archive.org/web/20131004212910weblink">weblink October 4, 2013, JOURNAL, Titan's methane cycle, Sushil K., Atreyaa, Elena Y., Adamsa, Hasso B., Niemann, Demick-Montelar, Jaime E. a, Owen, Tobias C., Fulchignoni, Marcello, Ferri, Francesca, Wilson, Eric H., 2006, 10.1016/j.pss.2006.05.028, Planetary and Space Science, 54, 12, 1177–1187, 2006P&SS...54.1177A, JOURNAL, Nature, 8, Stofan, E. R., Elachi, C., Lunine, J. I., Lorenz, R. D., Stiles, B., Mitchell, K. L., Ostro, S., Soderblom, L., Wood, C., Zebker, H., Wall, S., Janssen, M., Kirk, R., Lopes, R., Paganelli, F., Radebaugh, J., Wye, L., Anderson, Y., Allison, M., Boehmer, R., Callahan, P., Encrenaz, P., Flamini, E., Franscetti, G., Gim, Y., Hamilton, G., Hensley, S., Johnson, W. T. K., Kelleher, K., Muhleman, D., Paillou, P., Picardi, G., Posa, F., Roth, L., Seul, R., Shaffer, S., Vetrella, S., West, R., 445, 2007, 10.1038/nature05438, 17203056, 7123, The lakes of Titan, 61–64, 2007Natur.445...61S, JOURNAL, Episodic outgassing as the origin of atmospheric methane on Titan, Tobie, Gabriel, Lunine, Jonathan, Sotin, Cristophe, Nature, 440, 7080, 61–64, 2006, 10.1038/nature04497, 16511489, 2006Natur.440...61T, atmospheric methane, File:PIA18410-TitanSunsetStudies-CassiniSpacecraft-20140527.jpg|thumb|left|Sunset studies on Titan by Cassini help to better understand exoplanet atmosphereatmosphereFile:Titan-SaturnMoon-Maps-TraceGases-20141022.jpg|thumb|right|Trace organic gases in Titan's atmosphere—HNC (left) and HC3N (right).]]On April 3, 2013, NASA reported that complex organic chemicals, collectively called tholins, likely arise on Titan, based on studies simulating the atmosphere of Titan.On June 6, 2013, scientists at the IAA-CSIC reported the detection of polycyclic aromatic hydrocarbons in the upper atmosphere of Titan.NEWS, López-Puertas, Manuel,weblink PAH's in Titan's Upper Atmosphere, June 6, 2013, Spanish National Research Council, CSIC, June 6, 2013, no,weblink" title="web.archive.org/web/20131203030521weblink">weblink December 3, 2013, On September 30, 2013, propene was detected in the atmosphere of Titan by NASA's Cassini spacecraft, using its composite infrared spectrometer (CIRS).WEB, Brown, Dwayne, Neal-Jones, Nancy, Zubritsky, Elizabeth, Cook, Jia-Rui, NASA's Cassini Spacecraft Finds Ingredient of Household Plastic in Space,weblink September 30, 2013, NASA, December 2, 2013, no,weblink" title="web.archive.org/web/20131127041925weblink">weblink November 27, 2013, This is the first time propene has been found on any moon or planet other than Earth and is the first chemical found by the CIRS. The detection of propene fills a mysterious gap in observations that date back to NASA's Voyager 1 spacecraft's first close planetary flyby of Titan in 1980, during which it was discovered that many of the gases that make up Titan's brown haze were hydrocarbons, theoretically formed via the recombination of radicals created by the Sun's ultraviolet photolysis of methane.On October 24, 2014, methane was found in polar clouds on Titan.WEB, Dyches, Preston, Zubritsky, Elizabeth, NASA Finds Methane Ice Cloud in Titan's Stratosphere,weblink October 24, 2014, NASA, October 31, 2014, no,weblink" title="web.archive.org/web/20141028193849weblink">weblink October 28, 2014, WEB, Zubritsky, Elizabeth, Dyches, Preston, NASA Identifies Ice Cloud Above Cruising Altitude on Titan,weblink October 24, 2014, NASA, October 31, 2014, no,weblink" title="web.archive.org/web/20141031133440weblink">weblink October 31, 2014, {{wide image|Titan-Earth-PolarClouds-20141024.jpg|600px|align-cap=center|Polar clouds, made of methane, on Titan (left) compared with polar clouds on Earth (right), which are made of water or water ice.}}

Climate

File:Vortex on saturn's moon titan.png|thumb|left|Atmospheric text-topTitan's surface temperature is about {{convert|94|K|°C}}. At this temperature, water ice has an extremely low vapor pressure, so the little water vapor present appears limited to the stratosphere.JOURNAL, 8, Cottini, V., Nixon, C.A., Jennings, D.E., Anderson, C.M., Gorius, N., Bjoraker, G.L., Coustenis, A., Teanby, N.A., Achterberg, R.K., Bézard, B., de Kok, R., Lellouch, E., Irwin, P.G.J., Flasar, F.M., Bampasidis, G., Water vapor in Titan's stratosphere from Cassini CIRS far-infrared spectra, Icarus, 220, 2, 2012, 855–862, 0019-1035, 10.1016/j.icarus.2012.06.014, 2012Icar..220..855C, Titan receives about 1% as much sunlight as Earth.WEB,weblink Titan: A World Much Like Earth, Space.com, August 6, 2009, April 2, 2012, no,weblink" title="web.archive.org/web/20121012005030weblink">weblink October 12, 2012, Before sunlight reaches the surface, about 90% has been absorbed by the thick atmosphere, leaving only 0.1% of the amount of light Earth receives.Faint sunlight enough to drive weather, clouds on Saturn’s moon Titan {{webarchive|url=https://web.archive.org/web/20170403194843weblink |date=April 3, 2017 }} Between the large distance from the Sun and the thick atmosphere, Titan's surface receives about 0.1 percent of the solar energy that Earth does.Atmospheric methane creates a greenhouse effect on Titan's surface, without which Titan would be far colder.WEB,weblink Titan Has More Oil Than Earth, February 13, 2008, February 13, 2008, no,weblink" title="web.archive.org/web/20120708055735weblink">weblink July 8, 2012, Conversely, haze in Titan's atmosphere contributes to an anti-greenhouse effect by reflecting sunlight back into space, cancelling a portion of the greenhouse effect and making its surface significantly colder than its upper atmosphere.JOURNAL, 10.1126/science.11538492, C.P., McKay, J. B., Pollack, R., Courtin, 1991, The greenhouse and antigreenhouse effects on Titan, Science, 253, 5024, 1118–1121, 11538492, File:PIA18420-Titan-MethaneClouds-20140722.gif|thumb|right|upright|Methane clouds (animated; July 2014).WEB, Dyches, Preston, Cassini Tracks Clouds Developing Over a Titan Sea,weblink August 12, 2014, NASANASATitan's clouds, probably composed of methane, ethane or other simple organics, are scattered and variable, punctuating the overall haze. The findings of the Huygens probe indicate that Titan's atmosphere periodically rains liquid methane and other organic compounds onto its surface.NEWS,weblink Titan: Arizona in an Icebox?, Lakdawalla, Emily, Emily Lakdawalla, The Planetary Society, January 21, 2004, February 12, 2010,weblink" title="web.archive.org/web/20100212062535weblink">weblink March 28, 2005, Clouds typically cover 1% of Titan's disk, though 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 levels of sunlight during the southern summer generate uplift in the atmosphere, resulting in convection. This explanation is complicated by the fact that cloud formation has been observed not only after the southern summer solstice but also during mid-spring. Increased methane humidity at the south pole possibly contributes to the rapid increases in cloud size.JOURNAL, Emily L., Schaller, Brouwn, Michael E., Roe, Henry G., Bouchez, Antonin H., 2006, A large cloud outburst at Titan's south pole, 10.1016/j.icarus.2005.12.021, Icarus, 1, 224–229,weblink 182, 2006Icar..182..224S, August 23, 2007, no,weblink" title="web.archive.org/web/20070926102541weblink">weblink September 26, 2007, It was summer in Titan's southern hemisphere until 2010, when Saturn's orbit, which governs Titan's motion, moved Titan's northern hemisphere into the sunlight.NEWS, The Way the Wind Blows on Titan, June 1, 2007,weblink June 2, 2007, Jet Propulsion Laboratory, yes,weblink" title="web.archive.org/web/20090427110242weblink">weblink April 27, 2009, When the seasons switch, it is expected that ethane will begin to condense over the south pole.JOURNAL, Huge ethane cloud discovered on Titan, Shiga, David, New Scientist, 313, 1620, 2006,weblink August 7, 2007, no,weblink" title="web.archive.org/web/20081220030748weblink">weblink December 20, 2008,

Surface features

{{See also|List of geological features on Titan}}(File:PIA22770-SaturnMoon-Titan-Surface-20181206.jpg|thumb|center|600px|Titan − the surface under the haze (December 2018))PIA20713-Titan-SaturnMoon-LabeledFeaturesIAU-June2015.jpg|Global map of Titan – with IAU labels (August 2016).PIA21923-Titan-SaturnMoon-InfraredViews-20180718.jpg|Titan – infrared views (2004–2017)PIA19657-SaturnMoon-Titan-NorthPole-20140407.jpg|Titan's North Pole (2014)PIA19657-SaturnMoon-Titan-SouthPole-20140407.jpg|Titan's South Pole (2014)The surface of Titan has been described as "complex, fluid-processed, [and] geologically young".JOURNAL, Mahaffy, Paul R., May 13, 2005, Intensive Titan Exploration Begins, Science (magazine), Science, 308, 5724, 969–970, 10.1126/science.1113205, 15890870, 2005Sci...308..969M, 10.1.1.668.2877, Titan has been around since the Solar System's formation, but its surface is much younger, between 100 million and 1 billion years old. Geological processes may have reshaped Titan's surface.WEB,weblink River networks on Titan point to a puzzling geologic history, Jennifer, Chu, July 2012, MIT Research, July 24, 2012, no,weblink" title="web.archive.org/web/20121030175726weblink">weblink October 30, 2012, Titan's atmosphere is twice as thick as Earth's, making it difficult for astronomical instruments to image its surface in the visible light spectrum.NEWS, Taimoor, Tariq, Titan, Saturn's largest moon is finally unravelled in detail, March 12, 2012,weblink News Pakistan, March 12, 2012, no,weblink" title="web.archive.org/web/20140811000259weblink">weblink August 11, 2014, The Cassini spacecraft used infrared instruments, radar altimetry and synthetic aperture radar (SAR) imaging to map portions of Titan during its close fly-bys. The first images revealed a diverse geology, with both rough and smooth areas. There are features that may be volcanic in origin, disgorging water mixed with ammonia onto the surface. There is also evidence that Titan's ice shell may be substantially rigid, which would suggest little geologic activity.JOURNAL, 10.1016/j.icarus.2011.01.019, Titan: An exogenic world?, Icarus, 212, 2, 790–806, 2011, Moore, J. M., Pappalardo, R. T., 2011Icar..212..790M, There are also streaky features, some of them hundreds of kilometers in length, that appear to be caused by windblown particles.MAGAZINE, Battersby, Stephen, October 29, 2004, Titan's complex and strange world revealed, New Scientist,weblink August 31, 2007, no,weblink" title="web.archive.org/web/20081221052236weblink">weblink December 21, 2008, WEB,weblink Spacecraft: Cassini Orbiter Instruments, RADAR, Cassini–Huygens Mission to Saturn & Titan, NASA, Jet Propulsion Laboratory, August 31, 2007, no,weblink" title="web.archive.org/web/20110807011916weblink">weblink August 7, 2011, 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.JOURNAL, Lorenz, R. D., Callahan, P. S., Gim, Y., Alberti, G., Flamini, E., Seu, R., Picardi, G., Orosei, R., Zebker, H., Lunine, J., Hamilton, G., Hensley, S., Johnson, W. T. K., Schaffer, S., Wall, S., West, R., Francescetti, G., 1, 2007, Titan's Shape, Radius and Landscape from Cassini Radar Altimetry, Lunar and Planetary Science Conference, 38, 1338,weblink 2007LPI....38.1329L, 1329, August 27, 2007, no,weblink" title="web.archive.org/web/20070926102540weblink">weblink September 26, 2007, 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.WEB,weblink Cassini Reveals Titan's Xanadu Region To Be An Earth-Like Land, July 23, 2006, Science Daily, August 27, 2007, no,weblink" title="web.archive.org/web/20110629211805weblink">weblink June 29, 2011, 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.JOURNAL, 8, Barnes, Jason W., Brown, Robert H., Soderblom, Laurence, Buratti, Bonnie J., Sotin, Christophe, Rodriguez, Sebastien, Le Mouèlic, Stephane, Baines, Kevin H., Clark, Roger, Nicholson, Phil, 2006, Global-scale surface spectral variations on Titan seen from Cassini/VIMS, 10.1016/j.icarus.2006.08.021, Icarus, 1, 186,weblink July 25, 2011,weblink" title="web.archive.org/web/20110725155935weblink">weblink August 27, 2007, 242–258, 2007Icar..186..242B, There are dark areas of similar size elsewhere on Titan, observed from the ground and by Cassini; at least one of these, Ligeia Mare, Titan's second-largest sea, is almost a pure methane sea.NEWS, Klotz, Irene,weblink One of Titan, Discovery News, Space.com, April 28, 2016, May 1, 2016, no,weblink" title="web.archive.org/web/20160430113237weblink">weblink April 30, 2016, JOURNAL, Composition, seasonal change, and bathymetry of Ligeia Mare, Titan, derived from its microwave thermal emission, Journal of Geophysical Research, February 25, 2016, Le Gall, A., Malaska, M. J., 10.1002/2015JE004920, 2016JGRE..121..233L, etal, 121, 2, 233–251, {|class="wikitable" style="margin-left: auto; margin-right: auto; border: none;" style="text-align:center; vertical-align: top;"centerTitan mosaic from a Cassini flyby. The large dark region is Shangri-La.]]centerTitan in false color showing surface details and atmosphere. Xanadu is the bright region at the bottom-center.]]centerTitan globe, a mosaic of infrared images with nomenclature.)centerTitan composite image in infrared. It features the dark, dune-filled regions Fensal (north) and Aztlan (south).)

Lakes

(File:PIA22481-SaturnMoon-Titan-Lakes-20170911.jpg|thumb|left|400px|Titan lakes (September 11, 2017))File:PIA10008 Seas and Lakes on Titan.jpg|thumb|False-color Cassini radar mosaic of Titan's north polar region. Blue coloring indicates low radar reflectivity, caused by hydrocarbon seas, lakes and tributary networks filled with liquid ethane, methane and dissolved {{chem|N|2}}.{{sfnp |Coustenis |Taylor |2008 |pp=154–155}} About half of the large body at lower left, Kraken Mare, is shown. Ligeia MareLigeia Mare(File:HRICoastLineMoasic H.jpg|thumb|Mosaic of three Huygens images of channel system on Titan)The possibility of hydrocarbon seas on Titan was 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 was not obtained until 1995 when data from Hubble and other observations 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.JOURNAL, Dermott, S. F., Carl Sagan, Sagan, C., 1995, Tidal effects of disconnected hydrocarbon seas on Titan, Nature, 374, 238–240, 10.1038/374238a0, 7885443, 6519, 1995Natur.374..238D, The Cassini mission confirmed the former hypothesis. When the probe arrived in the Saturnian system in 2004, it was hoped that hydrocarbon lakes or oceans would be detected from the sunlight reflected off their surface, but no specular reflections were initially observed.WEB, Henry, Bortman,weblink Titan: Where's the Wet Stuff?, November 2, 2004, Astrobiology Magazine, November 3, 2006,weblink" title="web.archive.org/web/20061103235110weblink">weblink August 28, 2007, Near Titan's south pole, an enigmatic dark feature named Ontario Lacus was identifiedNEWS,weblink Dark Spot Near the South Pole: A Candidate Lake on Titan?, Lakdawalla, Emily, Emily Lakdawalla, The Planetary Society, June 28, 2005, June 5, 2011,weblink" title="web.archive.org/web/20110605101239weblink">weblink October 14, 2006, (and later confirmed to be a lake).WEB, NASA Confirms Liquid Lake On Saturn Moon, 2008, NASA,weblink December 20, 2009, no,weblink" title="web.archive.org/web/20110629175214weblink">weblink June 29, 2011, A possible shoreline was also identified near the pole via radar imagery.PRESS RELEASE,weblink NASA Cassini Radar Images Show Dramatic Shoreline on Titan, September 16, 2005, Jet Propulsion Laboratory, October 14, 2006, no,weblink" title="archive.today/20120530031240weblink">weblink May 30, 2012, Following a flyby on July 22, 2006, in which the Cassini spacecraft's radar imaged the northern latitudes (that were then in winter), several large, smooth (and thus dark to radar) patches were seen dotting the surface near the pole.WEB,weblink PIA08630: Lakes on Titan, NASA/JPL, Planetary Photojournal, October 14, 2006, no,weblink" title="web.archive.org/web/20110718143841weblink">weblink July 18, 2011, Based on the observations, scientists announced "definitive evidence of lakes filled with methane on Saturn's moon Titan" in January 2007.JOURNAL, The lakes of Titan, Stofan, E. R., Elachi, C., Lunine, J. I., Lorenz, R. D., Stiles, B., Mitchell, K. L., Ostro, S., Soderblom, L., Wood, C., 8, 1, 445, 61–64, Nature, 2007, 10.1038/nature05438, 17203056, 2007Natur.445...61S, WEB,weblink Titan Has Liquid Lakes, Scientists Report in Nature, NASA/JPL, January 3, 2007, January 8, 2007, no,weblink" title="web.archive.org/web/20130523192017weblink">weblink May 23, 2013, 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 outside Earth. Some appear to have channels associated with liquid and lie in topographical depressions. The liquid erosion features appear to be a very recent occurrence: channels in some regions have created surprisingly little erosion, suggesting erosion on Titan is extremely slow, or some other recent phenomena may have wiped out older riverbeds and landforms. Overall, the Cassini radar observations have shown that lakes cover only a small percentage of the surface, making Titan much drier than Earth.NEWS,weblink Ethane lakes in a red haze: Titan's uncanny moonscape, New Scientist, Hecht, Jeff, July 11, 2011, July 25, 2011, no,weblink" title="web.archive.org/web/20110713220529weblink">weblink July 13, 2011, Most of the lakes are concentrated near the poles (where the relative lack of sunlight prevents evaporation), but several long-standing hydrocarbon lakes in the equatorial desert regions have also been discovered, including one near the Huygens landing site in the Shangri-La region, which is about half the size of the Great Salt Lake in Utah, USA. The equatorial lakes are probably "oases", i.e. the likely supplier is underground aquifers.PRESS RELEASE, Tropical Methane Lakes on Saturn's Moon Titan, Jet Propulsion Laboratory, SpaceRef, 2012,weblink March 2, 2014, {{multiple image|caption_align=center
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}}In June 2008, the Visual and Infrared Mapping Spectrometer on Cassini confirmed the presence of liquid ethane beyond doubt in Ontario Lacus.WEB, Scientists Confirm Liquid Lake, Beach on Saturn's Moon Titan, Hadhazy, Adam,weblink Scientific American, 2008, July 30, 2008, no,weblink" title="web.archive.org/web/20120905111129weblink">weblink September 5, 2012, On December 21, 2008, Cassini passed directly over Ontario Lacus and observed specular reflection in radar. The strength of the reflection saturated the probe's receiver, indicating that the lake level did not vary by more than 3 mm (implying either that surface winds were minimal, or the lake's hydrocarbon fluid is viscous).NEWS, Grossman, Lisa, Saturn moon's mirror-smooth lake 'good for skipping rocks', New Scientist, August 21, 2009,weblink November 25, 2009, no,weblink January 10, 2016, JOURNAL, Wye, L. C., Zebker, H. A., Lorenz, R. D., Smoothness of Titan's Ontario Lacus: Constraints from Cassini RADAR specular reflection data, Geophysical Research Letters, 36, 16, L16201, 2009, 10.1029/2009GL039588, 2009GeoRL..3616201W, (File:Specular Spectacular (PIA18432).jpg|thumb|right|Near-infrared radiation from the Sun reflecting off Titan's hydrocarbon seas)On July 8, 2009, Cassini's VIMS observed a specular reflection indicative of a smooth, mirror-like surface, off what today is called Jingpo Lacus, a lake in the north polar region shortly after the area emerged from 15 years of winter darkness. Specular reflections are indicative of a smooth, mirror-like surface, so the observation corroborated the inference of the presence of a large liquid body drawn from radar imaging.WEB, Cook, J.-R. C., Glint of Sunlight Confirms Liquid in Northern Lake District of Titan, Cassini mission page, NASA, December 17, 2009,weblink December 18, 2009, no,weblink" title="web.archive.org/web/20110605131154weblink">weblink June 5, 2011, WEB, Lakdawalla, Emily, Emily Lakdawalla, Cassini VIMS sees the long-awaited glint off a Titan lake, The Planetary Society Blog, Planetary Society, December 17, 2009,weblink December 17, 2009, no,weblink" title="web.archive.org/web/20120630011024weblink">weblink June 30, 2012, Early radar measurements made in July 2009 and January 2010 indicated that Ontario Lacus was extremely shallow, with an average depth of 0.4–3 m, and a maximum depth of {{convert|3|to|7|m|sp=us|abbr=on}}. In contrast, the northern hemisphere's Ligeia Mare was initially mapped to depths exceeding 8 m, the maximum discernable by the radar instrument and the analysis techniques of the time.WEB, Wall, Mike, Saturn Moon's 'Lake Ontario': Shallow and Virtually Wave-free, Space.Com web site, December 17, 2010,weblink December 19, 2010, no,weblink" title="web.archive.org/web/20121020044023weblink">weblink October 20, 2012, Later science analysis, released in 2014, more fully mapped the depths of Titan's three methane seas and showed depths of more than {{convert|200|m|sp=us}}. Ligeia Mare averages from {{convert|20|to|40|m|sp=us|abbr=on}} in depth, while other parts of Ligeia did not register any radar reflection at all, indicating a depth of more than {{convert|200|m|sp=us|abbr=on}}. While only the second largest of Titan's methane seas, Ligeia "contains enough liquid methane to fill three Lake Michigans".NEWS, Crockett, Christopher, Cassini maps depths of Titan’s seas,weblink November 18, 2014, ScienceNews, November 17, 2014, no,weblink April 3, 2015, In May 2013, Cassini's radar altimeter observed Titan's Vid Flumina channels, defined as a drainage network connected to Titan's second largest hydrocarbon sea, Ligeia Mare. Analysis of the received altimeter echoes showed that the channels are located in deep (up to ~570 m), steep-sided, canyons and have strong specular surface reflections that indicate they are currently liquid filled. Elevations of the liquid in these channels are at the same level as Ligeia Mare to within a vertical precision of about 0.7 m, consistent with the interpretation of drowned river valleys. Specular reflections are also observed in lower order tributaries elevated above the level of Ligeia Mare, consistent with drainage feeding into the main channel system. This is likely the first direct evidence of the presence of liquid channels on Titan and the first observation of hundred-meter deep canyons on Titan. Vid Flumina canyons are thus drowned by the sea but there are a few isolated observations to attest to the presence of surface liquids standing at higher elevations.Valerio Poggiali, Marco Mastrogiuseppe, Alexander G. Hayes, Roberto Seu, Samuel P. D. Birch, Ralph Lorenz, Cyril Grima, Jason D. Hofgartner, "Liquid-filled Canyons on Titan", August 9, 2016, JOURNAL, Liquid-filled canyons on Titan, Geophysical Research Letters, 43, 15, 7887–7894, 10.1002/2016GL069679, 2016, Poggiali, V., Mastrogiuseppe, M., Hayes, A. G., Seu, R., Birch, S. P. D., Lorenz, R., Grima, C., Hofgartner, J. D., During six flybys of Titan from 2006 to 2011, Cassini gathered radiometric tracking and optical navigation data from which investigators could roughly infer Titan's changing shape. The density of Titan is consistent with a body that is about 60% rock and 40% water. The team's analyses suggest that Titan's surface can rise and fall by up to 10 metres during each orbit. That degree of warping suggests that Titan's interior is relatively deformable, and that the most likely model of Titan is one in which an icy shell dozens of kilometres thick floats atop a global ocean.NEWS, Sid, Perkins, Tides turn on Titan, June 28, 2012,weblink Nature, June 29, 2012, no,weblink" title="web.archive.org/web/20121007082709weblink">weblink October 7, 2012, The team's findings, together with the results of previous studies, hint that Titan's ocean may lie no more than {{convert|100|km|mi|sp=us}} below its surface.WEB, Puiu, Tibi, Saturn's moon Titan most likely harbors a subsurface ocean of water, zmescience.com web site, June 29, 2012,weblink June 29, 2012, no,weblink" title="web.archive.org/web/20120903075517weblink">weblink September 3, 2012, On July 2, 2014, NASA reported the ocean inside Titan may be as salty as the Dead Sea.WEB, Dyches, Preston, Brown, Dwayne, Ocean on Saturn Moon Could be as Salty as the Dead Sea,weblink July 2, 2014, NASA, July 2, 2014, no,weblink" title="web.archive.org/web/20140709165628weblink">weblink July 9, 2014, JOURNAL, Mitri, Giuseppe, Meriggiola, Rachele, Hayes, Alex, Lefevree, Axel, Tobie, Gabriel, Genovad, Antonio, Lunine, Jonathan I., Zebker, Howard, Shape, topography, gravity anomalies and tidal deformation of Titan, 2014, Icarus (journal), Icarus, 10.1016/j.icarus.2014.03.018, 236, 169–177, 2014Icar..236..169M, On September 3, 2014, NASA reported studies suggesting methane rainfall on Titan may interact with a layer of icy materials underground, called an "alkanofer", to produce ethane and propane that may eventually feed into rivers and lakes.WEB, Dyches, Preston, Mousis, Olivier, Altobelli, Nicolas, Icy Aquifers on Titan Transform Methane Rainfall,weblink September 3, 2014, NASA, September 4, 2014, no,weblink" title="web.archive.org/web/20140905000214weblink">weblink September 5, 2014, In 2016, Cassini found the first evidence of fluid-filled channels on Titan, in a series of deep, steep-sided canyons flowing into Ligeia Mare. This network of canyons, dubbed Vid Flumina, range in depth from 240 to 570 m and have sides as steep as 40°. They are believed to have formed either by crustal uplifting, like Earth's Grand Canyon, or a lowering of sea level, or perhaps a combination of the two. The depth of erosion suggests that liquid flows in this part of Titan are long-term features that persist for thousands of years.WEB, Cassini Finds Flooded Canyons on Titan,weblink NASA, 2016, August 12, 2016, no,weblink" title="web.archive.org/web/20160811140908weblink">weblink August 11, 2016, {|class="wikitable"centerupright=0.75)centerupright=0.625)||Photo of infrared specular reflection off Jingpo Lacus, a lake in the north polar region|Perspective radar view of Bolsena Lacus (lower right) and other northern hemisphere hydrocarbon lakescenterupright=1.5)centerupright=1.5)|Contrasting images of the number of lakes in Titan's northern hemisphere (left) and southern hemisphere (right)|Two images of Titan's southern hemisphere acquired one year apart, showing changes in south polar lakes

Impact craters

File:Titancrater.jpg|thumb|left|Radar image of a 139 km-diameter impact crater on Titan's surface, showing a smooth floor, rugged rim, and possibly a central peak.]]Radar, SAR and imaging data from Cassini have revealed few impact craters on Titan's surface. These impacts appear to be relatively young, compared to Titan's age. The few impact craters discovered include a {{convert|440|km|mi|adj=mid|-wide|sp=us}} two-ring impact basin named Menrva seen by Cassini's ISS as a bright-dark concentric pattern.WEB,weblink PIA07365: Circus Maximus, NASA, Planetary Photojournal, May 4, 2006, no,weblink" title="web.archive.org/web/20110718144030weblink">weblink July 18, 2011, A smaller, {{convert|60|km|mi|adj=mid|-wide|sp=us}}, flat-floored crater named SinlapWEB,weblink PIA07368: Impact Crater with Ejecta Blanket, NASA, Planetary Photojournal, May 4, 2006, no,weblink" title="web.archive.org/web/20121105210015weblink">weblink November 5, 2012, and a {{convert|30|km|mi|abbr=on}} crater with a central peak and dark floor named Ksa have also been observed.WEB,weblink PIA08737: Crater Studies on Titan, NASA, Planetary Photojournal, September 15, 2006, no,weblink" title="web.archive.org/web/20120531003954weblink">weblink May 31, 2012, Radar and Cassini imaging have also revealed "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 {{convert|90|km|mi|adj=mid|-wide|sp=us}} ring of bright, rough material known as Guabonito has been observed by Cassini.WEB,weblink PIA08425: Radar Images the Margin of Xanadu, NASA, Planetary Photojournal, September 26, 2006, no,weblink" title="web.archive.org/web/20110608095543weblink">weblink June 8, 2011, 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.WEB,weblink PIA08429: Impact Craters on Xanadu, NASA, Planetary Photojournal, September 26, 2006, no,weblink" title="web.archive.org/web/20120716051338weblink">weblink July 16, 2012, File:PIA19052-SaturnMoon-Titan-LigeiaMare-SAR&DespeckledViews-20150212.jpg|thumb|right|Ligeia Mare – SAR and clearer despeckled views.JOURNAL, Lucas, etal, 2014, Insights into Titan's geology and hydrology based on enhanced image processing of Cassini RADAR data, Journal of Geophysical Research, 119, 10, 2149–2166, 10.1002/2013JE004584, 2014JGRE..119.2149L, ]]Many of Titan's craters or probable craters display evidence of extensive erosion, and all show some indication of modification. Most large craters have breached or incomplete rims, despite the fact that some craters on Titan have relatively more massive rims than those anywhere else in the Solar System. There is little evidence of formation of palimpsests through viscoelastic crustal relaxation, unlike on other large icy moons. Most craters lack central peaks and have smooth floors, possibly due to impact-generation or later eruption of cryovolcanic lava. 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.JOURNAL, Ivanov, B. A., Basilevsky, A. T., Neukum, G., 1997, Atmospheric entry of large meteoroids: implication to Titan, Planetary and Space Science, 45, 8, 993–1007, 10.1016/S0032-0633(97)00044-5, 1997P&SS...45..993I, The limited high-resolution radar coverage of Titan obtained through 2007 (22%) suggested the existence of nonuniformities in its crater distribution. Xanadu has 2–9 times more craters than elsewhere. The leading hemisphere has a 30% higher density than the trailing hemisphere. There are lower crater densities in areas of equatorial dunes and in the north polar region (where hydrocarbon lakes and seas are most common).JOURNAL, Wood, C. A., Lorenz, R., Kirk, R., Lopes, R., Mitchell, K., Stofan, E., The Cassini RADAR Team, Impact craters on Titan, Icarus (journal), Icarus, 206, 1, 334–344, September 6, 2009, 10.1016/j.icarus.2009.08.021, 2010Icar..206..334L, 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".JOURNAL, Artemieva, Natalia, Lunine, Jonathan, 2003, Cratering on Titan: impact melt, ejecta, and the fate of surface organics, Icarus, 164, 2, 471–480, 10.1016/S0019-1035(03)00148-9, 2003Icar..164..471A,

Cryovolcanism and mountains

{{See also|Cryovolcano}}(File:Tortola Facula in infrared.jpg|thumb|left|Near-infrared image of Tortola Facula, thought to be a possible cryovolcano)Scientists have long speculated that conditions on Titan resemble those of early Earth, though at a much lower temperature. The detection of argon-40 in the atmosphere in 2004 indicated that volcanoes had spawned plumes of "lava" composed of water and ammonia.JOURNAL, Owen, Tobias, Planetary science: Huygens rediscovers Titan, Nature, 438, 756–757, 2005, 10.1038/438756a, 16363022, 7069, 2005Natur.438..756O, Global maps of the lake distribution on Titan's surface revealed that there is not enough surface methane to account for its continued presence in its atmosphere, and thus that a significant portion must be added through volcanic processes.WEB, Cassini Finds Hydrocarbon Rains May Fill The Lakes, Media Relations Office: Cassini Imaging Central Laboratory For Operations, Space Science Institute, Boulder, Colorado, 2009,weblink January 29, 2009, no,weblink" title="web.archive.org/web/20110725171249weblink">weblink July 25, 2011, Still, there is a paucity of surface features that can be unambiguously interpreted as cryovolcanoes.JOURNAL, Moore, J.M., Pappalardo, R.T., Titan: Callisto With Weather?, American Geophysical Union, Fall Meeting, 2008, 11, P11D–06, 2008AGUFM.P11D..06M, One of the first of such features revealed by Cassini radar observations in 2004, called Ganesa Macula, resembles the geographic features called "pancake domes" found on Venus, and was thus initially thought to be cryovolcanic in origin, until Kirk et al. refuted this hypothesis at the American Geophysical Union annual meeting in December 2008. The feature was found to be not a dome at all, but appeared to result from accidental combination of light and dark patches.WEB, Shape and thermal modeling of the possible cryovolcanic dome Ganesa Macula on Titan: Astrobiological implications, Neish, C.D., Lorenz, R.D., O'Brien, D.P., Lunar and Planetary Laboratory, University of Arizona, Observatoire de la Cote d'Azur,weblink 2005, August 27, 2007,weblink" title="web.archive.org/web/20070814122704weblink">weblink August 14, 2007, WEB, Genesa Macula Isn't A Dome, Lakdawalla, Emily, Emily Lakdawalla, The Planetary Society, 2008,weblink January 30, 2009, no,weblink" title="web.archive.org/web/20130618084806weblink">weblink June 18, 2013, In 2004 Cassini also detected an unusually bright feature (called Tortola Facula), which was interpreted as a cryovolcanic dome.JOURNAL, Sotin, C., Jaumann, R., Buratti, B., Brown, R., Clark, R., Soderblom, L., Baines, K., Bellucci, G., Bibring, J., Capaccioni, F., Cerroni, P., Combes, M., Coradini, A., Cruikshank, D. P., Drossart, P., Formisano, V., Langevin, Y., Matson, D. L., McCord, T. B., Nelson, R. M., Nicholson, P. D., Sicardy, B., Lemouelic, S., Rodriguez, S., Stephan, K., Scholz, C. K., Release of volatiles from a possible cryovolcano from near-infrared imaging of Titan, Nature, 435, 7043, 786–789, 2005, 15944697, 10.1038/nature03596, 2005Natur.435..786S, No similar features have been identified as of 2010.JOURNAL, LeCorre, L., LeMouélic, S., Sotin, C., Cassini/VIMS observations of cryo-volcanic features on Titan, 2008,weblink Lunar and Planetary Science, XXXIX, no,weblink" title="web.archive.org/web/20121025054055weblink">weblink October 25, 2012, In December 2008, astronomers announced the discovery of two transient but unusually long-lived "bright spots" in Titan's atmosphere, which appear too persistent to be explained by mere weather patterns, suggesting they were the result of extended cryovolcanic episodes.JOURNAL, Is Titan (cryo)volcanically active?, Longstaff, Alan, Royal Observatory, Greenwich (Astronomy Now), February 2009, 19, In March 2009, structures resembling lava flows were announced in a region of Titan called Hotei Arcus, which appears to fluctuate in brightness over several months. Though many phenomena were suggested to explain this fluctuation, the lava flows were found to rise {{convert|200|m|ft|sp=us}} above Titan's surface, consistent with it having been erupted from beneath the surface.JOURNAL, Giant 'ice flows' bolster case for Titan's volcanoes, Shiga, David, New Scientist, March 28, 2009, A mountain range measuring {{convert|150|km|mi|sp=us}} long, {{convert|30|km|mi|sp=us}} wide and {{convert|1.5|km|mi|sp=us}} high was also 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.NEWS,weblink Mountain range spotted on Titan, BBC News, December 12, 2006, August 6, 2007, no,weblink" title="web.archive.org/web/20121031002110weblink">weblink October 31, 2012, 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.NEWS,weblink Mountains Discovered on Saturn's Largest Moon, Newswise, July 2, 2008, no,weblink" title="web.archive.org/web/20130531075903weblink">weblink May 31, 2013, In December 2010, the Cassini mission team announced the most compelling possible cryovolcano yet found. Named Sotra Patera, it is one in a chain of at least three mountains, each between 1000 and 1500 m in height, several of which are topped by large craters. The ground around their bases appears to be overlaid by frozen lava flows.WEB, Saturn Moon Has Ice Volcano—And Maybe Life?, Richard A., Lovett, National Geographic,weblink 2010, December 19, 2010, no,weblink" title="web.archive.org/web/20121019073955weblink">weblink October 19, 2012, Most of Titan's highest peaks occur near its equator in so-called "ridge belts". They are believed to be analogous to Earth's fold mountains such as the Rockies or the Himalayas, formed by the collision and buckling of tectonic plates, or to subduction zones like the Andes, where upwelling lava (or cryolava) from a melting descending plate rises to the surface. One possible mechanism for their formation is tidal forces from Saturn. Because Titan's icy mantle is less viscous than Earth's magma mantle, and because its icy bedrock is softer than Earth's granite bedrock, mountains are unlikely to reach heights as great as those on Earth. In 2016, the Cassini team announced what they believe to be the tallest mountain on Titan. Located in the Mithrim Montes range, it is 3,337 m tall.WEB, Cassini Spies Titan's Tallest Peaks,weblink NASA, 2016, August 12, 2016, no,weblink" title="web.archive.org/web/20160819154049weblink">weblink August 19, 2016, File:Sotra Facula.jpg|thumb|right|False-color VIMS image of the possible cryovolcano Sotra PateraSotra PateraIf volcanism on Titan really exists, the hypothesis is that it is driven by energy released from the decay of radioactive elements within the mantle, as it is on Earth. Magma on Earth is made of liquid rock, which is less dense than the solid rocky crust through which it erupts. Because ice is less dense than water, Titan's watery magma would be denser than its solid icy crust. This means that cryovolcanism on Titan would require a large amount of additional energy to operate, possibly via tidal flexing from nearby Saturn. 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.JOURNAL, 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, 10.1016/j.icarus.2006.11.002, 2007Icar..188..139F, In 2008 Jeffrey Moore (planetary geologist of Ames Research Center) proposed an alternate view of Titan's geology. Noting that no volcanic features had been unambiguously identified on Titan so far, he asserted that Titan is a geologically dead world, whose surface is shaped only by impact cratering, fluvial and eolian erosion, mass wasting and other exogenic processes. According to this hypothesis, methane is not emitted by volcanoes but slowly diffuses out of Titan's cold and stiff interior. Ganesa Macula may be an eroded impact crater with a dark dune in the center. The mountainous ridges observed in some regions can be explained as heavily degraded scarps of large multi-ring impact structures or as a result of the global contraction due to the slow cooling of the interior. Even in this case, Titan may still have an internal ocean made of the eutectic water–ammonia mixture with a temperature of {{convert|176|K|C}}, which is low enough to be explained by the decay of radioactive elements in the core. The bright Xanadu terrain may be a degraded heavily cratered terrain similar to that observed on the surface of Callisto. Indeed, were it not for its lack of an atmosphere, Callisto could serve as a model for Titan's geology in this scenario. Jeffrey Moore even called Titan Callisto with weather.WEB, Lakdawalla, Emily, Emily Lakdawalla, AGU: Titan: Volcanically active world, or "Callisto with weather?,weblink October 11, 2010, The Planetary Society, December 17, 2008, no,weblink" title="web.archive.org/web/20130618091010weblink">weblink June 18, 2013, Many of the more prominent mountains and hills have been given official names by the International Astronomical Union. According to JPL, "By convention, mountains on Titan are named for mountains from Middle-earth, the fictional setting in fantasy novels by J. R. R. Tolkien." Colles (collections of hills) are named for characters from the same Tolkien works.{{citation |title=Mountains of Titan Map – 2016 Update |url=https://saturn.jpl.nasa.gov/resources/71 |publisher=NASA JPL |date=March 23, 2016 |accessdate=October 31, 2016 |deadurl=no |archiveurl=https://web.archive.org/web/20161101102622weblink |archivedate=November 1, 2016 }}

Dark equatorial terrain

File:Titan dunes crop.png|thumb|left|upright|Sand dunes in the Namib DesertNamib DesertIn 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.JOURNAL, Roe, H. G., 2004, 2004GeoRL..3117S03R, A new 1.6-micron map of Titan's surface, Geophys. Res. Lett., 31, 17, L17S03, 10.1029/2004GL019871, Prior to the arrival of Cassini, these regions were thought to be seas of liquid hydrocarbons.JOURNAL, The Glitter of Distant Seas, Lorenz, R., Science, 2003, 302, 403–404, 10.1126/science.1090464, 14526089, 5644, Radar images captured by the Cassini spacecraft have instead revealed some of these regions to be extensive plains covered in longitudinal dunes, up to {{convert|330|ft|m|abbr=on}} high about a kilometer wide, and tens to hundreds of kilometers long.JOURNAL, Lorenz, R. D., Winds of Change on Titan, Science, 329, 5991, 519–20, July 30, 2010, 20671175, 10.1126/science.1192840, 2010Sci...329..519L, 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 meters 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, it was hypothesized, causes granular material on the surface to gradually build up in long parallel dunes aligned west-to-east. The dunes break up around mountains, where the wind direction shifts.The longitudinal (or linear) dunes were initially presumed to be formed by moderately variable winds that either follow one mean direction or alternate between two different directions. Subsequent observations indicate that the dunes point to the east although climate simulations indicate Titan's surface winds blow toward the west. At less than 1 meter per second, they are not powerful enough to lift and transport surface material. Recent computer simulations indicate that the dunes may be the result of rare storm winds that happen only every fifteen years when Titan is in equinox.WEB, Violent Methane Storms on Titan May Explain Dune Direction, Spaceref, 2015,weblink April 19, 2015, These storms produce strong downdrafts, flowing eastward at up to 10 meters per second when they reach the surface.The "sand" on Titan is likely not made up of small grains of silicates like the sand on Earth,WEB, Cassini Sees the Two Faces of Titan's Dunes,weblink JPL, NASA, no,weblink" title="web.archive.org/web/20130502160658weblink">weblink May 2, 2013, but rather might have formed when liquid methane rained and eroded the water-ice bedrock, possibly in the form of flash floods. Alternatively, the sand could also have come from organic solids called tholins, produced by photochemical reactions in Titan's atmosphere.NEWS, Saharan Sand Dunes Found on Saturn's Moon Titan,weblink Sara, Goudarzi, SPACE.com, May 4, 2006, August 6, 2007, no,weblink" title="web.archive.org/web/20110804233044weblink">weblink August 4, 2011, JOURNAL, The sand seas of Titan: Cassini RADAR observations of longitudinal dunes, Lorenz, RD, Wall, S, Radebaugh, J, Boubin, G, Reffet, E, Janssen, M, Stofan, E, Lopes, R, Kirk, R, 8, Science, 2006, 312, 724–727, 10.1126/science.1123257, 16675695, 5774, 2006Sci...312..724L, JOURNAL, Linear Dunes on Titan, Lancaster, N., Science, 2006, 312, 702–703, 10.1126/science.1126292, 16675686, 5774, Studies of dunes' composition in May 2008 revealed that they possessed less water than the rest of Titan, and are thus most likely derived from organic soot like hydrocarbon polymers clumping together after raining onto the surface.WEB, Titan's Smoggy Sand Grains, JPL, NASA, 2008,weblink May 6, 2008, no,weblink" title="web.archive.org/web/20130523190919weblink">weblink May 23, 2013, Calculations indicate the sand on Titan has a density of one-third that of terrestrial sand.WEB, Dunes on Titan need firm winds to move, Spaceref, 2015,weblink April 23, 2015, The low density combined with the dryness of Titan's atmosphere might cause the grains to clump together because of static electricity buildup. The "stickiness" might make it difficult for the generally mild breeze close to Titan's surface to move the dunes although more powerful winds from seasonal storms could still blow them eastward.JOURNAL, Electrified sand could explain Titan's backward dunes, New Scientist, April 1, 2017, 18, Around equinox, strong downburst winds can lift micron-sized solid organic particles up from the dunes to create Titanian dust storms, observed as intense and short-lived brightenings in the infrared.JOURNAL, Rodriguez, S., Le Mouélic, S., Barnes, J. W., Kok, J. F., Rafkin, S. C. R., Lorenz, R. D., Charnay, B., Radebaugh, J., Narteau, C., Cornet, T., Bourgeois, O., Lucas, A., Rannou, P., Griffith, C. A., Coustenis, A., Appéré, T., Hirtzig, M., Sotin, C., Soderblom, J. M., Brown, R. H., Bow, J., Vixie, G., Maltagliati, L., Courrech du Pont, S., Jaumann, R., Stephan, K., Baines, K. H., Buratti, B. J., Clark, R. N., Nicholson, P. D., Observational evidence for active dust storms on Titan at equinox, Nature Geoscience, 11, 10, 727–732, 2018, 10.1038/s41561-018-0233-2, 3, File:PIA22484-SaturnMoon-Titan-3DustStorms-20180924.jpg|thumb|400px|center|Titan - three dust storms detected in 2009-2010.WEB, McCartney, Gretchen, Brown, Dwayne, Wendel, JoAnna, Bauer, Markus, Dust Storms on Titan Spotted for the First Time,weblink September 24, 2018, NASANASA

Observation and exploration

File:Titan's thick haze layer-picture from voyager1.jpg|thumb|right|upright|Voyager 1Voyager 1Titan 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 Titan 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.BOOK, Benton, Julius L. Jr., 141–146,weblink Saturn and How to Observe It, 2005, Springer, London, 978-1-84628-045-0, Titan has a maximum apparent magnitude of +8.2,WEB, Classic Satellites of the Solar System,weblink Observatorio ARVAL, June 28, 2010, yes,weblink" title="web.archive.org/web/20110709203915weblink">weblink July 9, 2011, and mean opposition magnitude 8.4.WEB, Planetary Satellite Physical Parameters, JPL (Solar System Dynamics),weblink April 3, 2009, June 29, 2010, no,weblink" title="web.archive.org/web/20090522002331weblink">weblink May 22, 2009, 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 i Solà observed limb darkening of Titan, the first evidence that the body has an atmosphere. In 1944 Gerard P. Kuiper used a spectroscopic technique to detect an atmosphere of methane.JOURNAL, Kuiper, G. P., 1944, Titan: a Satellite with an Atmosphere, Astrophysical Journal, 100, 10.1086/144679, 378, 1944ApJ...100..378K, File:NASA-Cassini-Saturn-TitanFlybyTests-20140617.jpg|thumb|left|Cassini's Titan flyby radio signal studies (artist's concept)]]The first probe to visit the Saturnian system was Pioneer 11 in 1979, which revealed that Titan was probably too cold to support life.WEB, March 26, 2007, The Pioneer Missions, NASA, Jet Propulsion Laboratory, Pioneer Project,weblink August 19, 2007, no,weblink" title="web.archive.org/web/20110629033952weblink">weblink June 29, 2011, It took images of Titan, including Titan and Saturn together in mid to late 1979.WEB, Pioneer XI, NASA, Photo Index,weblink August 19, 2007, The quality was soon surpassed by the two Voyagers.Titan was examined by both Voyager 1 and 2 in 1980 and 1981, respectively. Voyager 1s trajectory was designed to provide an optimized Titan flyby, during which the spacecraft was able to determine the density, composition, and temperature of the atmosphere, and obtain a precise measurement of Titan's mass.BOOK, Bell, Jim, The Interstellar Age: Inside the Forty-Year Voyager Mission,weblink February 24, 2015, Penguin Publishing Group, 978-0-698-18615-6, 93, no,weblink September 4, 2016, Atmospheric haze prevented direct imaging of the surface, though in 2004 intensive digital processing of images taken through Voyager 1s orange filter did reveal hints of the light and dark features now known as Xanadu and Shangri-la,JOURNAL, Richardson, J., Lorenz, Ralph D., McEwen, Alfred, Titan's Surface and Rotation: New Results from Voyager 1 Images, Icarus, 2004, 170, 1, 113–124, 10.1016/j.icarus.2004.03.010, 2004Icar..170..113R, which had been observed in the infrared by the Hubble Space Telescope. Voyager 2, which would have been diverted to perform the Titan flyby if Voyager 1 had been unable to, did not pass near Titan and continued on to Uranus and Neptune.{{rp|94}}

Cassini–Huygens

{{multiple image| align = right| image1 = Titan and rings PIA14909.jpg| width1 = 125| alt1 = | caption1 = Cassini image of Titan in front of the rings of Saturn| image2 = PIA08391 Epimetheus, Rings and Titan.jpg| width2 = 175| alt2 = Cassini
image of Titan, behind Epimetheus (moon)>Epimetheus and the rings| footer = }}Even with the data provided by the Voyagers, Titan remained a body of mystery—a large satellite shrouded in an atmosphere that makes detailed observation difficult. The mystery that had surrounded Titan since the 17th-century observations of Christiaan Huygens and Giovanni Cassini was revealed by a spacecraft named in their honor.The Cassini–Huygens spacecraft reached Saturn on July 1, 2004, and began the process of mapping Titan's surface by radar. A joint project of the European Space Agency (ESA) and NASA, Cassini–Huygens proved a very successful mission. The Cassini probe flew by Titan on October 26, 2004, and took the highest-resolution images ever of Titan's surface, at only {{convert|1200|km|mi|sp=us}}, discerning patches of light and dark that would be invisible to the human eye.On July 22, 2006, Cassini made its first targeted, close fly-by at {{convert|950|km|mi|sp=us}} from Titan; the closest flyby was at {{convert|880|km|mi|sp=us}} on June 21, 2010.WEB, Cassini Equinox Mission: Titan Flyby (T-70) – June 21, 2010, NASA/JPL,weblink July 8, 2010, yes,weblink" title="web.archive.org/web/20120318223434weblink">weblink March 18, 2012, Liquid has been found in abundance on the surface in the north polar region, in the form of many lakes and seas discovered by Cassini.

Huygens landing

{{multiple image| align = left| image1 = Huygens surface color.jpg| width1 = 150| alt1 = | caption1 = Huygens in situ image from Titan's surface—the only image from the surface of a body farther away than Mars| image2 = Huygens_surface_color_sr.jpg| width2 = 150| alt2 = | caption2 = Same image with contrast enhanced| footer = }}Huygens was an atmospheric probe that touched down on Titan on January 14, 2005,JOURNAL, Steve, Lingard, Norris, Pat,weblink How To Land on Titan, 23, June 2005, Ingenia, January 11, 2009, no,weblink" title="web.archive.org/web/20110721163014weblink">weblink July 21, 2011, discovering that many of its surface features seem to have been formed by fluids at some point in the past.WEB,weblink Cassini at Saturn: Introduction, September 6, 2007, NASA, Jet Propulsion Laboratory, yes,weblink" title="web.archive.org/web/20090403131313weblink">weblink April 3, 2009, Titan is the most distant body from Earth to have a space probe land on its surface.WEB,weblink Huygens Exposes Titan's Surface, August 19, 2007, Space Today, no,weblink" title="web.archive.org/web/20110807190437weblink">weblink August 7, 2011, The Huygens probe landed just off the easternmost tip of a bright region now called Adiri. The probe 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.NEWS,weblink Seeing, touching and smelling the extraordinarily Earth-like world of Titan, ESA News, European Space Agency, January 21, 2005, March 28, 2005, no,weblink" title="web.archive.org/web/20111007111129weblink">weblink October 7, 2011, 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 right are smaller than they may appear: the left-hand one is 15 centimeters across, and the one in the center is 4 centimeters across, at a distance of about 85 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. The "soil" visible in the images is interpreted to be 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.WEB,weblink Huygens landing site to be named after Hubert Curien, ESA, March 5, 2007, August 6, 2007, no,weblink" title="web.archive.org/web/20120303201334weblink">weblink March 3, 2012,

Planned: Dragonfly

The Dragonfly mission will launch in 2026 consisting of a large drone powered by an RTG to fly in the atmosphere of Titan as the New Frontiers program mission #4.WEB,weblink New Science Mission to Explore Our Solar System, Bridenstine, Jim, June 27, 2019, Twitter, June 27, 2019, NEWS, Brown, David W., NASA Announces New Dragonfly Drone Mission to Explore Titan - The quadcopter was selected to study the moon of Saturn after a “Shark Tank”-like competition that lasted two and a half years.,weblink June 27, 2019, The New York Times, June 27, 2019, Its instruments will study how far prebiotic chemistry may have progressed.Dragonfly: A Rotorcraft Lander Concept for Scientific Exploration at Titan {{Webarchive|url=https://web.archive.org/web/20171222053305weblink |date=December 22, 2017 }} (PDF). Ralph D. Lorenz, Elizabeth P. Turtle, Jason W. Barnes, Melissa G. Trainer, Douglas S. Adams, Kenneth E. Hibbard, Colin Z. Sheldon, Kris Zacny, Patrick N. Peplowski, David J. Lawrence, Michael A. Ravine, Timothy G. McGee, Kristin S. Sotzen, Shannon M. MacKenzie, Jack W. Langelaan, Sven Schmitz, Larry S. Wolfarth, and Peter D. Bedini. Johns Hopkins APL Technical Digest, Pre-publication draft (2017).

Proposed or conceptual missions

(File:TSSM-TandEM-Montgolfiere.jpg|thumb|right|upright|The balloon proposed for the Titan Saturn System Mission (artistic rendition))There have been several conceptual missions proposed in recent years for returning a robotic space probe to Titan. Initial conceptual work has been completed for such missions by NASA, the ESA and JPL. At present, none of these proposals have become funded missions.The Titan Saturn System Mission (TSSM) was a joint NASA/ESA proposal for exploration of Saturn's moons.WEB, Mission Summary: TANDEM/TSSM Titan and Enceladus Mission, ESA, 2009,weblink January 30, 2009, no,weblink" title="web.archive.org/web/20110523081148weblink">weblink May 23, 2011, It envisions a hot-air balloon floating in Titan's atmosphere for six months. It was competing against the Europa Jupiter System Mission (EJSM) proposal for funding. In February 2009 it was announced that ESA/NASA had given the EJSM mission priority ahead of the TSSM.NEWS, Rincon, Paul, February 18, 2009, Jupiter in space agencies' sights, BBC News,weblink no,weblink" title="web.archive.org/web/20101024015124weblink">weblink October 24, 2010, The proposed Titan Mare Explorer (TiME) was a low-cost lander that would splash down in a lake in Titan's northern hemisphere and float on the surface of the lake for three to six months.WEB,weblink TiME: Titan Mare Explorer, August 17, 2011, Stofan, Ellen, 2010, Caltech, yes,weblink" title="web.archive.org/web/20120330205710weblink">weblink March 30, 2012, NEWS, Kate, Taylor, NASA picks project shortlist for next Discovery mission, May 9, 2011,weblink TG Daily, May 20, 2011, no,weblink" title="web.archive.org/web/20120904204847weblink">weblink September 4, 2012, NEWS, Nell, Greenfieldboyce, Exploring A Moon By Boat, September 16, 2009, National Public Radio (NPR),weblink November 8, 2009, no,weblink" title="web.archive.org/web/20120825092930weblink">weblink August 25, 2012, It was selected for a Phase-A design study in 2011 as a candidate mission for the 12th NASA Discovery Program opportunity,WEB, NASA Discovery Program,weblink NASA Announces Three New Mission Candidates,weblink November 18, 2016, May 5, 2011, June 13, 2017, but was not selected for flight.WEB,weblink Let's go sailing on lakes of Titan!,weblink" title="web.archive.org/web/20121010200010weblink">weblink October 10, 2012, November 1, 2009, Another mission to Titan proposed in early 2012 by Jason Barnes, a scientist at the University of Idaho, is the Aerial Vehicle for In-situ and Airborne Titan Reconnaissance (AVIATR): an unmanned plane (or drone) that would fly through Titan's atmosphere and take high-definition images of the surface of Titan. NASA did not approve the requested $715 million, and the future of the project is uncertain.NEWS,weblink AVIATR: An Airplane Mission for Titan, Universetoday.com, January 2, 2012, February 26, 2013, no,weblink" title="web.archive.org/web/20130328084232weblink">weblink March 28, 2013, WEB,weblink Soaring on Titan: Drone designed to scout Saturn's moon, NBC News, January 10, 2012, February 26, 2013, no,weblink" title="web.archive.org/web/20140413125210weblink">weblink April 13, 2014, A conceptual design for another lake lander was proposed in late 2012 by the Spanish-based private engineering firm SENER and the Centro de Astrobiología in Madrid. The concept probe is called Titan Lake In-situ Sampling Propelled Explorer (TALISE).BOOK, TALISE: Titan Lake In-situ Sampling Propelled Explorer, European Planetary Science Congress 2012, I., Urdampilleta, O., Prieto-Ballesteros, R., Rebolo, J., Sancho, EPSC Abstracts, 2012, 7, EPSC2012-64 2012,weblink October 10, 2012, no,weblink October 12, 2012, NEWS, Elizabeth, Landau, Probe would set sail on a Saturn moon, October 9, 2012,weblink CNN – Light Years, October 10, 2012, no,weblink" title="web.archive.org/web/20130619112735weblink">weblink June 19, 2013, The major difference compared to the TiME probe would be that TALISE is envisioned with its own propulsion system and would therefore not be limited to simply drifting on the lake when it splashes down.A Discovery Program contestant for its mission #13 is Journey to Enceladus and Titan (JET), an astrobiology Saturn orbiter that would assess the habitability potential of Enceladus and Titan.CONFERENCE, Sotin, C., Altwegg, K., Brown, R. H., etal, JET: Journey to Enceladus and Titan,weblink PDF, 42nd Lunar and Planetary Science Conference, Lunar and Planetary Institute, 2011, no,weblink" title="web.archive.org/web/20150415113634weblink">weblink April 15, 2015, CONFERENCE, Matousek, Steve, Sotin, Christophe, Goebel, Dan, Lang, Jared, JET: Journey to Enceladus and Titan,weblink PDF, Low Cost Planetary Missions Conference, California Institute of Technology, June 18–21, 2013, no,weblink" title="web.archive.org/web/20160304072017weblink">weblink March 4, 2016, NEWS, Kane, Van, Discovery Missions for an Icy Moon with Active Plumes,weblink The Planetary Society, April 3, 2014, April 9, 2015, no,weblink" title="web.archive.org/web/20150416094147weblink">weblink April 16, 2015, {{Clear}}In 2015, the NASA Innovative Advanced Concepts program (NIAC) awarded a Phase II grantWEB,weblink Titan Submarine: Exploring the Depths of Kraken, Loura, Hall, May 30, 2014, no,weblink" title="web.archive.org/web/20150730000928weblink">weblink July 30, 2015, to a design study of a submarine to explore the seas of Titan.WEB, Lewin, Sarah, July 15, 2015,weblink NASA Funds Titan Submarine, Other Far-Out Space Exploration Ideas, Space.com, no,weblink" title="web.archive.org/web/20150804042315weblink">weblink August 4, 2015, Lorenz, R. D.; Oleson, S.; Woytach, J.; Jones, R.; Colozza, A.; Schmitz, P.; Landis, G.; Paul, M.; and Walsh, J. (March 16–20, 2015). "Titan Submarine: Vehicle Design and Operations Concept for the Exploration of the Hydrocarbon Seas of Saturn's Giant Moon", 46th Lunar and Planetary Science Conference, The Woodlands, Texas. LPI Contribution No. 1832, p.1259Hartwig, J., et al., (June 24–26, 2015). "Titan Submarine: Exploring the Depths of Kraken Mare", 26th Space Cryogenics Workshop, Phoenix, Arizona. link to NASA Report. Retrieved June 13, 2017.

Prebiotic conditions and life

{{See also|Planetary habitability}}Titan is thought to be a prebiotic environment rich in complex organic chemistryWEB, Staff, NASA team investigates complex chemistry at Titan,weblink April 3, 2013, Phys.Org, April 11, 2013, no,weblink" title="web.archive.org/web/20130421003912weblink">weblink April 21, 2013, NEWS,weblink Saturn's moon Titan may harbour simple life forms – and reveal how organisms first formed on Earth, August 30, 2017, July 27, 2017, The Conversation, no,weblink August 30, 2017, with a possible subsurface liquid ocean serving as a biotic environment.Titan is thought by some scientists to be a possible host for microbial extraterrestrial life.JOURNAL, Grasset, O., Sotin, C., Deschamps, F., On the internal structure and dynamic of Titan, 2000, Planetary and Space Science, 48, 7–8, 617–636, 10.1016/S0032-0633(00)00039-8, 2000P&SS...48..617G, JOURNAL, Fortes, A. D., 2000, Exobiological implications of a possible ammonia-water ocean inside Titan, Icarus (journal), Icarus, 146, 2, 444–452, 10.1006/icar.2000.6400, 2000Icar..146..444F, WEB, Have We Discovered Evidence For Life On Titan, Mckay, Chris, 2010,weblink New Mexico State University, College of Arts and Sciences, Department of Astronomy, May 15, 2014, yes,weblink" title="web.archive.org/web/20160309224810weblink">weblink March 9, 2016, The Cassini–Huygens mission was not equipped to provide evidence for biosignatures or complex organic compounds; it showed an environment on Titan that is similar, in some ways, to ones hypothesized for the primordial Earth. Scientists surmise that the atmosphere of early Earth was similar in composition to the current atmosphere on Titan, with the important exception of a lack of water vapor on Titan.NEWS, Staff, October 4, 2010, Lakes on Saturn's Moon Titan Filled With Liquid Hydrocarbons Like Ethane and Methane, Not Water, ScienceDaily,weblink October 5, 2010, no,weblink" title="web.archive.org/web/20121020100943weblink">weblink October 20, 2012,

Formation of complex molecules

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 hydrogen cyanide and acetylene. Further reactions have been studied extensively.JOURNAL, Space Science Reviews, 104, 1–2, 377–394, 2002, 10.1023/A:1023636623006, Organic chemistry and exobiology on Titan, Raulin, F., Owen, T., 2002SSRv..104..377R, It has been reported that when energy was applied to a combination of gases like those in Titan's atmosphere, five nucleotide bases, the building blocks of DNA and RNA, were among the many compounds produced. In addition, amino acids, the building blocks of protein were found. It was the first time nucleotide bases and amino acids had been found in such an experiment without liquid water being present.NEWS, Staff, October 8, 2010, Titan's haze may hold ingredients for life, Astronomy,weblink October 14, 2010, no,weblink" title="web.archive.org/web/20150923175854weblink">weblink September 23, 2015, On April 3, 2013, NASA reported that complex organic chemicals could arise on Titan based on studies simulating the atmosphere of Titan.On June 6, 2013, scientists at the IAA-CSIC reported the detection of polycyclic aromatic hydrocarbons (PAH) in the upper atmosphere of Titan.On July 26, 2017, Cassini scientists positively identified the presence of carbon chain anions in Titan's upper atmosphere which appeared to be involved in the production of large complex organics.JOURNAL, Desai, R. T., A. J. Coates, A. Wellbrock, V. Vuitton, D. González-Caniulef, etal, yes, 2017, Carbon Chain Anions and the Growth of Complex Organic Molecules in Titan's Ionosphere, Astrophys. J. Lett., 844, 2, 10.3847/2041-8213/aa7851, L18arxiv = 1706.01610, These highly reactive molecules were previously known to contribute to building complex organics in the Interstellar Medium, therefore highlighting a possibly universal stepping stone to producing complex organic material.WEB,weblink Has Cassini found a universal driver for prebiotic chemistry at Titan?, August 12, 2017, July 26, 2017, European Space Agency, On July 28, 2017, scientists reported that acrylonitrile, or vinyl cyanide, (C2H3CN), possibly essential for life by being related to cell membrane and vesicle structure formation, had been found on Titan.WEB, Wall, Mike, Saturn Moon Titan Has Molecules That Could Help Make Cell Membranes,weblink July 28, 2017, Space.com, July 29, 2017, no,weblink July 29, 2017, JOURNAL, Palmer, Maureen Y., etal, ALMA detection and astrobiological potential of vinyl cyanide on Titan, July 28, 2017, Science Advances, 3, e1700022, 7, 10.1126/sciadv.1700022, 28782019, 5533535, 2017SciA....3E0022P, NEWS, Kaplan, Sarah, This weird moon of Saturn has some essential ingredients for life,weblink August 8, 2017, Washington Post, August 8, 2017, no,weblink August 8, 2017, In October 2018, researchers reported low-temperature chemical pathways from simple organic compounds to complex polycyclic aromatic hydrocarbon (PAH) chemicals. Such chemical pathways may help explain the presence of PAHs in the low-temperature atmosphere of Titan, and may be significant pathways, in terms of the PAH world hypothesis, in producing precursors to biochemicals related to life as we know it.WEB, Staff, "A Prebiotic Earth" – Missing Link Found on Saturn's Moon Titan,weblink October 11, 2018, DailyGalaxy.com, October 11, 2018, JOURNAL, Zhao, Long, etal, Low-temperature formation of polycyclic aromatic hydrocarbons in Titan's atmosphere,weblink October 8, 2018, Nature Astronomy, October 11, 2018,

Possible subsurface habitats

Laboratory simulations 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. The analogy assumes the presence of liquid water for longer periods than is currently observable; several hypotheses postulate that liquid water from an impact could be preserved under a frozen isolation layer.JOURNAL, Artemivia, N., Lunine, J, Cratering on Titan: impact melt, ejecta, and the fate of surface organics, 2003, Icarus, 164, 2, 471–480, 10.1016/S0019-1035(03)00148-9, 2003Icar..164..471A, It has also been hypothesized that liquid-ammonia oceans could exist deep below the surface.JOURNAL, Lovett, Richard A., March 20, 2008,weblink Saturn Moon Titan May Have Underground Ocean, National Geographic, no,weblink" title="web.archive.org/web/20121018035136weblink">weblink October 18, 2012, Another model suggests an ammonia–water solution as much as {{convert|200|km|mi|sp=us}} deep beneath a water-ice crust with conditions that, although extreme by terrestrial standards, are such that life could survive. Heat transfer between the interior and upper layers would be critical in sustaining any subsurface oceanic life. Detection of microbial life on Titan would depend on its biogenic effects, with the atmospheric methane and nitrogen examined.

Methane and life at the surface

{{see also|Hypothetical types of biochemistry}}It has been speculated that life could exist in the lakes of liquid methane on Titan, just as organisms on Earth live in water. Such organisms would inhale H2 in place of O2, metabolize it with acetylene instead of glucose, and exhale methane instead of carbon dioxide.All life forms on Earth (including methanogens) use liquid water as a solvent; it is speculated that life on Titan might instead use a liquid hydrocarbon, such as methane or ethane,WEB, Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Council,weblink The Limits of Organic Life in Planetary Systems, The National Academies Press, 2007, 74, although water is a stronger solvent than methane. Water is also more chemically reactive, and can break down large organic molecules through hydrolysis. A life form whose solvent was a hydrocarbon would not face the risk of its biomolecules being destroyed in this way.In 2005, astrobiologist Chris McKay argued that if methanogenic life did exist on the surface of Titan, it would likely have a measurable effect on the mixing ratio in the Titan troposphere: levels of hydrogen and acetylene would be measurably lower than otherwise expected.JOURNAL, Icarus, 178, 1, 274–276, 2005, 10.1016/j.icarus.2005.05.018, Possibilities for methanogenic life in liquid methane on the surface of Titan, McKay, C. P., Smith, H. D., 2005Icar..178..274M, In 2010, Darrell Strobel, from Johns Hopkins University, identified a greater abundance of molecular hydrogen in the upper atmospheric layers of Titan compared to the lower layers, arguing for a downward flow at a rate of roughly 1028 molecules per second and disappearance of hydrogen near Titan's surface; as Strobel noted, his findings were in line with the effects McKay had predicted if methanogenic life-forms were present.WEB, What is Consuming Hydrogen and Acetylene on Titan?, NASA/JPL, 2010, June 6, 2010,weblink yes,weblink" title="web.archive.org/web/20110629185640weblink">weblink June 29, 2011, JOURNAL, Molecular hydrogen in Titan's atmosphere: Implications of the measured tropospheric and thermospheric mole fractions, Strobel, Darrell F., Icarus, 208, 2, 878–886, 2010, 10.1016/j.icarus.2010.03.003,weblink 2010Icar..208..878S, yes,weblink" title="web.archive.org/web/20120824195338weblink">weblink August 24, 2012, The same year, another study showed low levels of acetylene on Titan's surface, which were interpreted by McKay as consistent with the hypothesis of organisms consuming hydrocarbons. Although restating the biological hypothesis, he cautioned that other explanations for the hydrogen and acetylene findings are more likely: the possibilities of yet unidentified physical or chemical processes (e.g. a surface catalyst accepting hydrocarbons or hydrogen), or flaws in the current models of material flow. Composition data and transport models need to be substantiated, etc. Even so, despite saying that a non-biological catalytic explanation would be less startling than a biological one, McKay noted that the discovery of a catalyst effective at {{convert|95|K|°C|-1|abbr=on}} would still be significant.As NASA notes in its news article on the June 2010 findings: "To date, methane-based life forms are only hypothetical. Scientists have not yet detected this form of life anywhere." As the NASA statement also says: "some scientists believe these chemical signatures bolster the argument for a primitive, exotic form of life or precursor to life on Titan's surface."In February 2015, a hypothetical cell membrane capable of functioning in liquid methane in Titan conditions was modeled. Composed of small molecules containing carbon, hydrogen, and nitrogen, it would have the same stability and flexibility as cell membranes on Earth, which are composed of phospholipids, compounds of carbon, hydrogen, oxygen, and phosphorus. This hypothetical cell membrane was termed an "azotosome", a combination of "azote", French for nitrogen, and "liposome".WEB,weblink Life 'not as we know it' possible on Saturn's moon Titan, no,weblink" title="web.archive.org/web/20150317002959weblink">weblink March 17, 2015, JOURNAL, Stevenson, James, Lunine, Jonathan, Clancy, Paulette, Membrane alternatives in worlds without oxygen: Creation of an azotosome, Science Advances, February 27, 2015, 1, 1, e1400067, 10.1126/sciadv.1400067, 26601130, 2015SciA....1E0067S, 4644080,

Obstacles

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, and its atmosphere lacks CO2. At Titan's surface, water exists only in solid form. Because of these difficulties, scientists such as Jonathan Lunine have viewed Titan less as a likely habitat for life, than as an experiment for examining hypotheses on the conditions that prevailed prior to the appearance of life on Earth.WEB,weblink Saturn's Moon Titan: Prebiotic Laboratory, Astrobiology Magazine, August 11, 2004, August 28, 2004,weblink" title="web.archive.org/web/20040828233135weblink">weblink August 11, 2004, Although life itself may not exist, the prebiotic conditions on Titan and the associated organic chemistry remain of great interest in understanding the early history of the terrestrial biosphere.JOURNAL, Space Science Reviews, 116, 1–2, 471–487, 2005, 10.1007/s11214-005-1967-x, Exo-astrobiological aspects of Europa and Titan: From observations to speculations, Raulin, F., 2005SSRv..116..471R, Using Titan as a prebiotic experiment involves not only observation through spacecraft, but laboratory experiments, and chemical and photochemical modeling on Earth.

Panspermia hypothesis

It is hypothesized that large asteroid and cometary impacts on Earth's surface may have caused fragments of microbe-laden rock to escape Earth's gravity, suggesting the possibility of transpermia. Calculations indicate that these would encounter many of the bodies in the Solar System, including Titan.NEWS,weblink Earth could seed Titan with life, BBC News, March 18, 2006, March 10, 2007, no,weblink" title="web.archive.org/web/20121031002211weblink">weblink October 31, 2012, JOURNAL, Gladman, Brett, Dones, Luke, Levinson, Harold F., Burns, Joseph A., Impact Seeding and Reseeding in the Inner Solar System, 2005, Astrobiology, 5, 483–496, 10.1089/ast.2005.5.483, 16078867, 4, 2005AsBio...5..483G, On the other hand, Jonathan Lunine has argued that any living things in Titan's cryogenic hydrocarbon lakes would need to be so different chemically from Earth life that it would not be possible for one to be the ancestor of the other.JOURNAL, 0908.0762, Lunine, Jonathan, Saturn's Titan: A Strict Test for Life's Cosmic Ubiquity,weblinkweblink" title="web.archive.org/web/20130512013831weblink">weblink May 12, 2013, 153, 4, 403, 2008, Proceedings of the American Philosophical Society, 2009arXiv0908.0762L, yes, copy at archive.org

Future conditions

Conditions on Titan could become far more habitable in the far future. Five billion years from now, as the Sun becomes a red giant, its surface temperature could rise enough for Titan to support liquid water on its surface, making it habitable.WEB, Climate Change in the Solar System, The National Air and Space Museum, 2012,weblink January 14, 2012, yes,weblink" title="web.archive.org/web/20120311101403weblink">weblink March 11, 2012, As the Sun's ultraviolet output decreases, the haze in Titan's upper atmosphere will be depleted, 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 a habitable environment, and could persist for several hundred million years. This is proposed to have been sufficient time for simple life to spawn on Earth, though the presence of ammonia on Titan would cause chemical reactions to proceed more slowly.WEB, Titan under a red giant sun: A new kind of "habitable" moon, Lorenz, Ralph D., Lunine, Jonathan I., McKay, Christopher P., NASA Ames Research Center, Lunar and Planetary Laboratory, Department of Planetary Sciences, University of Arizona, 1997,weblink March 21, 2008, no,weblink" title="web.archive.org/web/20110724173621weblink">weblink July 24, 2011,

See also

References

{{Reflist|30em}}

Bibliography

  • BOOK, Titan: Exploring an Earthlike World, Coustenis, Athéna, Taylor, F. W., World Scientific, 2008,weblink 978-981-270-501-3, harv,

Further reading

  • BOOK, Lifting Titan's Veil: Exploring the Giant Moon of Saturn, Ralph, Lorenz, Jacqueline, Mitton, Jacqueline Mitton, Cambridge University Press, 2002, 978-0-521-79348-3,
  • BOOK, Titan Unveiled, Ralph, Lorenz, Jacqueline, Mitton, Princeton University Press, 2008, 978-0691146331,
  • BOOK, NASA/ESA/ASI Cassini-Huygens: 1997 onwards (Cassini orbiter, Huygens probe and future exploration concepts) (Owners' Workshop Manual), Ralph, Lorenz, Haynes Manuals, UK, 2017, 978-1785211119,
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External links

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