View source for Astronomy:Titan (moon)

{{short description|Largest moon of Saturn and second-largest moon in Solar System}}
{{Infobox planet
| name = Titan
| mpc_name = Saturn VI
| pronounced = {{IPAc-en|audio=En-us-Titan.ogg|ˈ|t|aɪ|t|ən}}<ref>{{OED|Titan}}</ref>
| adjectives = Titanian<ref>{{cite web|work=JPL |url=https://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20050118-2/ |date=January 18, 2005 |title=Cassini Equinox Mission: Huygens Landed with a Splat |access-date=May 26, 2010 |url-status=dead |archive-url=https://web.archive.org/web/20100620002838/https://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20050118-2/ |archive-date=June 20, 2010 }}</ref> or Titanean<ref>{{cite journal | last1 = Luz | display-authors = etal | year = 2003 | title = Latitudinal transport by barotropic waves in Titan's stratosphere | journal = Icarus | volume = 166 | issue = 2 | pages = 343–358 | doi = 10.1016/j.icarus.2003.08.014 }}</ref> (both {{IPAc-en|t|aɪ|ˈ|t|eɪ|n|i|ə|n}})<ref>{{OED|Titanian}}</ref><ref>"Titanian" is the written adjectival form of both Titan and Uranus's moon [[Astronomy:Titania (moon)|Titania]]. However, Uranus's moon has a Shakespearean pronunciation with a short "i" vowel and the "a" of ''spa'': {{IPAc-en|t|ᵻ|ˈ|t|ɑː|n|i|ə|n}}, while either spelling for Titan is pronounced with those two vowels long: {{IPAc-en|t|aɪ|ˈ|t|eɪ|n|i|ə|n}}.</ref>
| named_after = Τῑτάν ''Tītan''
| image = Titan in true color by Kevin M. Gill.jpg
| background = PaleGoldenrod
| caption = Titan pictured in 2011 in natural color. The thick atmosphere is yellow due to a dense [[:Category:Organonitrogen compounds|organonitrogen]] haze.
| orbit_ref = <ref name="horizons" />
| discoverer = [[Biography:Christiaan Huygens|Christiaan Huygens]]
| discovered = March 25, 1655
| satellite_of = [[Astronomy:Saturn|Saturn]]
| periapsis = {{val|1186680|u=km}}
| apoapsis = {{val|1257060|u=km}}
| semimajor = {{val|1221870|u=km}}
| eccentricity = {{val|0.0288}}
| period = {{val|15.945|u=days}}
| avg_speed = 5.57 km/s (calculated)
| inclination = {{val|0.34854|u=°}} (to Saturn's equator)
| mean_radius = {{val|2574.73|0.09|u=km}} (0.404 [[Astronomy:Earth|Earth]]'s)<ref name="Zebker 2018">{{cite journal |doi=10.1126/science.1168905 |pmid=19342551 |first1=Howard A. |last1=Zebker |first2=Bryan |last2=Stiles |first3=Scott |last3=Hensley |first4=Ralph |last4=Lorenz |first5=Randolph L. |last5=Kirk |first6=Jonathan I. |last6=Lunine |title=Size and Shape of Saturn's Moon Titan |date=May 15, 2009 |journal=Science |volume=324 |issue=5929 |pages=921–923 |url=https://pdfs.semanticscholar.org/7ec3/29458f5dabfa6c370476df8236779941f93f.pdf |archive-url=https://web.archive.org/web/20200212021254/https://pdfs.semanticscholar.org/7ec3/29458f5dabfa6c370476df8236779941f93f.pdf |url-status=dead |archive-date=February 12, 2020 |bibcode=2009Sci...324..921Z |s2cid=23911201 }}</ref>
| surface_area = {{val|8.3|e=7|u=km2}} (0.163 Earth's)
| volume = {{val|7.16|e=10|u=km3}} (0.066 Earth's)
| mass = {{val|1.3452|0.0002|e=23|u=kg}} <br />(0.0225 Earth's)<ref name="Jacobson 2006">{{cite journal| doi=10.1086/508812| last1=Jacobson | first1=R. A.| last2=Antreasian | first2=P. G.| last3=Bordi | first3=J. J.| last4=Criddle | first4=K. E.| last5=Ionasescu | first5=R.| last6=Jones | first6=J. B.| last7=Mackenzie | first7=R. A.| last8=Meek | first8=M. C.| last9=Parcher | first9=D.| first10=F. J. | last10=Pelletier| first11=W. M. | last11=Owen, Jr.| first12=D. C. | last12=Roth| first13=I. M. | last13=Roundhill| first14=J. R. | last14=Stauch| date=December 2006| title=The Gravity Field of the Saturnian System from Satellite Observations and Spacecraft Tracking Data| journal=The Astronomical Journal| volume=132 | issue=6 | pages=2520–2526| bibcode=2006AJ....132.2520J| ref={{sfnRef|Jacobson Antreasian et al.|2006}}| doi-access=free}}</ref>
| density = {{val|1.8798|0.0044|u=g/cm3}}<ref name="Jacobson 2006" />
| surface_grav = {{val|1.352|u=m/s2}} ({{val|0.138|u=[[Physics:G-force|''g'']]}})
| moment_of_inertia_factor = {{val|0.3414|0.0005}}<ref name="Iess2010">{{cite journal|last1= Iess|first1= L.|last2= Rappaport|first2= N. J.|last3= Jacobson|first3= R. A.|last4= Racioppa|first4= P.|last5= Stevenson|first5= D. J.|last6= Tortora|first6= P.|last7= Armstrong|first7= J. W.|last8= Asmar|first8= S. W.|title= Gravity Field, Shape, and Moment of Inertia of Titan|journal= Science|volume= 327|issue= 5971|date= March 12, 2010|pages= 1367–1369|doi= 10.1126/science.1182583|pmid= 20223984|bibcode= 2010Sci...327.1367I|s2cid= 44496742}}</ref> (estimate)
| escape_velocity = {{val|2.641|u=km/s}}
| rotation = Synchronous
| axial_tilt = Zero (to the orbital plane);<br />{{val|27|u=°}} (to the sun)
| albedo = 0.22 ([[Physics:Geometric albedo|geometric]]) <ref>{{cite web |last=Williams |first=D. R. |date=February 22, 2011 |title=Saturnian Satellite Fact Sheet |publisher=NASA |url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/saturniansatfact.html |access-date=April 22, 2015 |url-status=live |archive-url=https://web.archive.org/web/20100430122034/https://nssdc.gsfc.nasa.gov/planetary/factsheet/saturniansatfact.html |archive-date=April 30, 2010  }}</ref>
{{val|0.265|0.03}} ([[Physics:Bond albedo|Bond]])<ref>{{cite journal|last1=Li|first1=Liming|last2=Nixon|first2=Conor A.|last3=Achterberg|first3=Richard K.|last4=Smith|first4=Mark A.|last5=Gorius|first5=Nicolas J. P.|last6=Jiang|first6=Xun|last7=Conrath|first7=Barney J.|last8=Gierasch|first8=Peter J.|last9=Simon-Miller|first9=Amy A.|last10=Flasar|first10=F. Michael|last11=Baines|first11=Kevin H.|last12=Ingersoll|first12=Andrew P.|last13=West|first13=Robert A.|last14=Vasavada|first14=Ashwin R.|last15=Ewald|first15=Shawn P.|display-authors=1|date=December 2011|title=The global energy balance of Titan|url=https://ntrs.nasa.gov/api/citations/20110023012/downloads/20110023012.pdf|journal=Geophysical Research Letters|volume=38|issue=23|doi=10.1029/2011GL050053|bibcode=2011GeoRL..3823201L|access-date=20 August 2023}}</ref>
| magnitude = 8.2<ref name="arval" /> to 9.0
| single_temperature = {{convert|93.7|K|°C}}<ref name="Mitri">{{cite journal |last1=Mitri |first1=G. |last2=Showman |first2=Adam P. |last3=Lunine |first3=Jonathan I. |last4=Lorenz |first4=Ralph D. |date=2007 |title=Hydrocarbon Lakes on Titan |url=https://www.lpl.arizona.edu/~showman/publications/mitri-etal-2007-lakes.pdf |journal=Icarus |volume=186 |issue=2 |pages=385–394 |doi=10.1016/j.icarus.2006.09.004 |bibcode=2007Icar..186..385M |url-status=live |archive-url=https://web.archive.org/web/20080227112028/https://www.lpl.arizona.edu/~showman/publications/mitri-etal-2007-lakes.pdf |archive-date=February 27, 2008  }}</ref>
| atmosphere = yes
| surface_pressure = {{val|146.7|ul=kPa}} ({{val|1.45|ul=atm}})
| atmosphere_composition = Variable

[[Earth:Stratosphere|Stratosphere]]:<br />98.4% [[Chemistry:Nitrogen|nitrogen]] ({{chem|N|2}}),<br />1.4% [[Chemistry:Methane|methane]] ({{chem|CH|4}}),<br />0.2% [[Chemistry:Hydrogen|hydrogen]] ({{chem|H|2}});

Lower [[Earth:Troposphere|troposphere]]:<br />95.0% {{chem|N|2}}, 4.9% {{chem|CH|4}};<ref name="Niemann" /><br />{{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}}}}
}}
<!-- Per WP:LEADCITE items in the lead are sourced in the body of the article -->

'''Titan''' is the largest [[Astronomy:Moons of Saturn|moon of Saturn]] and the [[Astronomy:List of Solar System objects by size|second-largest]] in the [[Astronomy:Solar System|Solar System]], larger than any of the [[Astronomy:Dwarf planet|dwarf planet]]s of the Solar System. It is the only [[Astronomy:Natural satellite|moon]] known to have a dense [[Astronomy:Atmosphere|atmosphere]], and is the only known object in space other than [[Astronomy:Earth|Earth]] on which clear evidence of stable bodies of surface liquid has been found.<ref name="NYT-20191203">{{cite news |last=Overbye |first=Dennis |title=Go Ahead, Take a Spin on Titan – Saturn's biggest moon has gasoline for rain, soot for snow, and a subsurface ocean of ammonia. Now there's a map to help guide the search for possible life there. |url=https://www.nytimes.com/2019/12/03/science/titan-saturn-map.html |date=December 3, 2019 |work={{wipe|The New York Times}} |access-date=December 5, 2019 |archive-date=December 5, 2019 |archive-url=https://web.archive.org/web/20191205015625/https://www.nytimes.com/2019/12/03/science/titan-saturn-map.html |url-status=live }}</ref>

Titan is one of the seven gravitationally rounded moons in orbit around [[Astronomy:Saturn|Saturn]], and the second most distant from Saturn of those seven. Frequently described as a [[Astronomy:Planetary-mass moon|planet-like moon]], Titan is 50% larger (in diameter) than Earth's [[Astronomy:Moon|Moon]] and 80% more massive. It is the second-largest moon in the [[Astronomy:Solar System|Solar System]] after Jupiter's moon [[Astronomy:Ganymede (moon)|Ganymede]], and is larger than the planet [[Astronomy:Mercury (planet)|Mercury]], but only 40% as [[Physics:Mass|mass]]ive.

Discovered in 1655 by the Dutch astronomer [[Biography:Christiaan Huygens|Christiaan Huygens]], Titan was the first known moon of Saturn, and the sixth known planetary satellite (after [[Astronomy:Moon|Earth's moon]] and the four [[Astronomy:Galilean moons|Galilean moons]] of Jupiter). Titan orbits Saturn at 20 Saturn radii. From Titan's surface, Saturn subtends an arc of 5.09 degrees, and if it were visible through the moon's thick atmosphere, it would appear 11.4 times larger in the sky, in diameter, than the Moon from Earth, which subtends 0.48° of arc.

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 [[Earth:Ice Ih|ice I<sub>h</sub>]] and a subsurface layer of ammonia-rich liquid water.<ref>{{cite book |title=Titan from Cassini-Huygens |url=https://archive.org/details/titanfromcassini00brow |url-access=limited |editor=Robert Brown |editor2=Jean Pierre Lebreton |editor3=Hunter Waite |publisher= Springer Science & Business Media |year=2009 |page=[https://archive.org/details/titanfromcassini00brow/page/n73 69] |isbn=978-1-4020-9215-2}}</ref> Much as with [[Astronomy:Venus|Venus]] before the [[Engineering:Space Age|Space Age]], the dense opaque atmosphere prevented understanding of Titan's surface until the ''[[Astronomy:Cassini–Huygens|Cassini–Huygens]]'' mission in 2004 provided new information, including the discovery of liquid hydrocarbon lakes in Titan's polar regions and the discovery of its [[Earth:Atmospheric super-rotation|atmospheric super-rotation]]. The geologically young surface is generally smooth, with few [[Astronomy:Impact crater|impact crater]]s, although mountains and several possible [[Astronomy:Cryovolcano|cryovolcano]]es have been found.


== History ==
=== Discovery ===
[[File:Christiaan Huygens-painting.jpeg|thumb|left|upright|Christiaan Huygens discovered Titan in 1655.]]

Titan was discovered on March 25, 1655, by the Dutch astronomer [[Biography:Christiaan Huygens|Christiaan Huygens]].<ref>{{cite web|url=https://assets.cambridge.org/052179/3483/sample/0521793483ws.pdf |title=Lifting Titan's Veil |publisher=Cambridge |archive-url=https://web.archive.org/web/20050222073209/https://assets.cambridge.org/052179/3483/sample/0521793483ws.pdf |archive-date=February 22, 2005|page=4}}</ref><ref>{{cite web |url=https://antwrp.gsfc.nasa.gov/apod/ap050325.html |publisher=NASA |work=Astronomy Picture of the Day |title=Titan |archive-url=https://web.archive.org/web/20050327011129/https://antwrp.gsfc.nasa.gov/apod/ap050325.html |archive-date=March 27, 2005 }}</ref> Huygens was inspired by Galileo's discovery of Jupiter's four [[Astronomy:Galilean moons|largest moons]] in 1610 and his improvements in [[Astronomy:Telescope|telescope]] technology. Christiaan, with the help of his elder brother [[Biography:Constantijn Huygens Jr.|Constantijn Huygens Jr.]], began building telescopes around 1650 and discovered the first observed moon orbiting Saturn with one of the telescopes they built.<ref>{{cite web |date=September 4, 2008 |title=Discoverer of Titan: Christiaan Huygens |url=https://www.esa.int/esaSC/SEMJRT57ESD_index_0.html |publisher=European Space Agency |access-date=April 18, 2009 |url-status=live |archive-url=https://web.archive.org/web/20110809144038/https://www.esa.int/esaSC/SEMJRT57ESD_index_0.html |archive-date=August 9, 2011  }}</ref> It was the sixth moon ever discovered, after Earth's [[Astronomy:Moon|Moon]] and the [[Astronomy:Galilean moons|Galilean moons]] of [[Astronomy:Jupiter|Jupiter]].<ref>{{Cite APOD |date=March 25, 2005 |title=Huygens Discovers Luna Saturni |access-date=August 18, 2007 }}</ref>

Titan is the largest and brightest moon of Saturn, and so is the easiest to observe of Saturn's moons with a standard optical telescope from Earth.

=== Naming ===
Huygens named his discovery ''Saturni Luna'' (or ''Luna Saturni'', Latin for "moon of Saturn"), publishing in the 1655 tract ''De Saturni Luna Observatio Nova'' (''A New Observation of Saturn's Moon'').<ref>{{cite book |last1=Huygens |first1=Christiaan |last2=Société hollandaise des sciences |title=Oeuvres complètes de Christiaan Huygens |date=1888 |publisher=Martinus Nijhoff |location=The Hague, Netherlands |volume=1 |pages=387–388 |url=https://www.biodiversitylibrary.org/item/61160#page/415/mode/1up |language=la |access-date=January 31, 2019 |archive-date=January 31, 2019 |archive-url=https://web.archive.org/web/20190131145231/https://www.biodiversitylibrary.org/item/61160#page/415/mode/1up |url-status=live }}</ref> After Giovanni Domenico Cassini published his discoveries of four more moons of Saturn between 1673 and 1686, astronomers fell into the habit of referring to these and Titan as Saturn I through V (with Titan then in fourth position). Other early epithets for Titan include "Saturn's ordinary satellite".<ref>{{cite journal |last=Cassini |first=G. D. |title=A Discovery of two New Planets about Saturn, made in the Royal Parisian Observatory by Signor Cassini, Fellow of both the Royal Societys, of England and France; English't out of French |journal=Philosophical Transactions |volume=8 |issue=1673 |pages=5178–5185 |date=1673 |doi=10.1098/rstl.1673.0003 |bibcode=1673RSPT....8.5178C |doi-access=free }}</ref> The International Astronomical Union officially numbers Titan as '''Saturn VI'''.<ref name=usgs>{{cite web |url=https://planetarynames.wr.usgs.gov/Page/Planets |title=Planet and Satellite Names and Discoverers |publisher=USGS |access-date=2021-03-06 |archive-date=November 28, 2017 |archive-url=https://web.archive.org/web/20171128093419/https://planetarynames.wr.usgs.gov/Page/Planets |url-status=live }}</ref>

The name ''Titan'', and the names of all seven satellites of Saturn then known, came from [[Biography:John Herschel|John Herschel]] (son of William Herschel, discoverer of two other Saturnian moons, Mimas and [[Astronomy:Enceladus|Enceladus]]), in his 1847 publication ''Results of Astronomical Observations Made during the Years 1834, 5, 6, 7, 8, at the Cape of Good Hope''.<ref>{{cite journal |last=Lassell |date=November 12, 1847 |title=Observations of Mimas, the closest and most interior satellite of Saturn |url=https://adsabs.harvard.edu//full/seri/MNRAS/0008//0000042.000.html |journal=[[Organization:Monthly Notices of the Royal Astronomical Society|Monthly Notices of the Royal Astronomical Society]] |volume=8 |issue=3 |pages=42–43 |access-date=March 29, 2005 |doi=10.1093/mnras/8.3.42 |bibcode=1848MNRAS...8...42L |doi-access=free |archive-date=September 11, 2006 |archive-url=https://web.archive.org/web/20060911204009/http://adsabs.harvard.edu//full/seri/MNRAS/0008//0000042.000.html |url-status=live }}</ref><ref>{{Cite book |url=https://archive.org/stream/Resultsastronom00Hers#page/414/mode/2up/search/Titan|title=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 |last=Herschel |first=Sir John F. W. |publisher=Smith, Elder & Co. |year=1847 |location=London |pages=415}}</ref> Numerous small moons have been discovered around Saturn since then.<ref>{{Cite web |title=Overview {{!}} Saturn Moons |url=https://solarsystem.nasa.gov/moons/saturn-moons/overview/?page=0&per_page=40&order=name+asc&search=&placeholder=Enter+moon+name&condition_1=38%3Aparent_id&condition_2=moon%3Abody_type%3Ailike&condition_3=moon%3Abody_type |access-date=1 March 2021 |website=solarsystem.nasa.gov |publisher=[[Organization:NASA|NASA]] |archive-date=November 29, 2021 |archive-url=https://web.archive.org/web/20211129110147/https://solarsystem.nasa.gov/moons/saturn-moons/overview/?page=0&per_page=40&order=name+asc&search=&placeholder=Enter+moon+name&condition_1=38:parent_id&condition_2=moon:body_type:ilike&condition_3=moon:body_type |url-status=live }}</ref> Saturnian moons are named after mythological giants. The name Titan comes from the Titans, a race of immortals in [[Religion:Greek mythology|Greek mythology]].<ref name="usgs" />
{{Clear|left}}

== Orbit and rotation ==
{{Main|Astronomy:Moons of Saturn}}
[[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 Earth's [[Astronomy:Moon|Moon]] and many of the satellites of the [[Astronomy:Giant planet|giant planet]]s, its rotational period (its day) is identical to its orbital period; Titan is [[Astronomy:Tidal locking|tidally locked]] in synchronous rotation with Saturn, and permanently shows one face to the planet. Longitudes on Titan are measured westward, starting from the meridian passing through this point.<ref>{{cite web |url=https://www.evs-islands.com/2008/02/titans-unnamed-methane-sea.html |title=EVS-Islands: Titan's Unnamed Methane Sea |access-date=October 22, 2009 |url-status=live |archive-url=https://web.archive.org/web/20110809201152/https://www.evs-islands.com/2008/02/titans-unnamed-methane-sea.html |archive-date=August 9, 2011  }}</ref> Its orbital eccentricity is 0.0288, and the orbital plane is inclined 0.348 degrees relative to the Saturnian equator,<ref name="horizons">Unless otherwise specified: {{cite web |url=https://ssd.jpl.nasa.gov/horizons.cgi#top |title=JPL HORIZONS solar system data and ephemeris computation service |work=Solar System Dynamics |publisher=NASA, Jet Propulsion Laboratory |access-date=August 19, 2007 |url-status=live |archive-url=https://web.archive.org/web/20121007034731/https://ssd.jpl.nasa.gov/horizons.cgi#top |archive-date=October 7, 2012  }}</ref><!-- As https://ssd.jpl.nasa.gov/?sat_elem shows, that's 0.28° to the local Laplace plane, itself inclined 0.6° to Saturn's equator; an example of the calculation required to get the number shown here is given at Talk:Phoebe (moon) --> and hence also about a third of a degree off of the equatorial ring plane. 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 [[Arcsecond|arcsecond]]s in diameter.{{Citation needed|date=February 2021}}

The small and irregularly shaped satellite [[Astronomy:Hyperion (moon)|Hyperion]] is locked in a 3:4 orbital resonance with Titan. Hyperion probably formed in a stable orbital island, whereas the massive Titan absorbed or ejected any other bodies that made close approaches.<ref>{{cite journal |last1=Bevilacqua |first1=R. |last2=Menchi |first2=O. |last3=Milani |first3=A. |last4=Nobili |first4=A. M. |last5=Farinella |first5=P. |date=1980 |title=Resonances and close approaches. I. The Titan-Hyperion case |journal=Earth, Moon, and Planets |volume=22 |issue=2 |pages=141–152 |doi=10.1007/BF00898423 |bibcode=1980M&P....22..141B |s2cid=119442634 }}</ref>

== 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 probably partially differentiated into distinct layers with a {{convert|3400|km|mi|adj=on|sp=us}} rocky center.<ref>{{Cite journal |last1=Mitri |first1=G. |last2=Pappalardo |first2=R. T. |last3=Stevenson |first3=D. J. |date=December 1, 2009 |title=Is Titan Partially Differentiated? |journal=AGU Fall Meeting Abstracts |volume=43 |pages=P43F–07 |bibcode=2009AGUFM.P43F..07M}}</ref> This rocky center is believed to be surrounded by several layers composed of different crystalline forms of ice, and/or water.<ref name="Tobie">{{cite journal
|last1=Tobie |first1=G. |last2=Grasset |first2=Olivier |last3=Lunine |first3=Jonathan I. |last4=Mocquet |first4=Antoine |last5=Sotin |first5=Christophe
|date=2005 |bibcode=2005Icar..175..496T
|title=Titan's internal structure inferred from a coupled thermal-orbital model |journal=Icarus
|volume=175 |issue=2 |pages=496–502
|doi=10.1016/j.icarus.2004.12.007 }}</ref> The exact structure depends heavily on the heat flux from within Titan itself, which is poorly constrained. The interior may still be hot enough for a liquid layer consisting of a "[[Biology:Magma|magma]]" composed of water and [[Chemistry:Ammonia|ammonia]] between the [[Earth:Ice Ih|ice I<sub>h</sub>]] crust and deeper ice layers made of high-pressure forms of ice. The heat flow from inside Titan may even be too high for high pressure ices to form, with the outermost layers instead consisting primarily of liquid water underneath a surface crust.<ref>{{Cite journal |last1=Sohl |first1=F. |last2=Solomonidou |first2=A. |last3=Wagner |first3=F. W. |last4=Coustenis |first4=A. |last5=Hussmann |first5=H. |last6=Schulze-Makuch |first6=D. |date=2014-05-23 |title=Structural and tidal models of Titan and inferences on cryovolcanism |journal=Journal of Geophysical Research: Planets |language=en |volume=119 |issue=5 |pages=1013–1036 |doi=10.1002/2013JE004512|doi-access=free }}</ref> The presence of ammonia allows water to remain liquid even at a temperature as low as {{convert|176|K|°C}} (for [[Physics:Eutectic|eutectic]] mixture with water).<ref name="longstaff" /> The ''Cassini'' probe discovered evidence for the layered structure in the form of natural [[Physics: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.<ref name="Titan ELF">{{cite news |url=https://www.esa.int/esaMI/Cassini-Huygens/SEM17F9RR1F_0.html |title=Titan's Mysterious Radio Wave |date=June 1, 2007 |publisher=ESA Cassini-Huygens web site |access-date=March 25, 2010 |url-status=live |archive-url=https://web.archive.org/web/20110605000409/https://www.esa.int/esaMI/Cassini-Huygens/SEM17F9RR1F_0.html |archive-date=June 5, 2011  }}</ref> Surface features were observed by the ''Cassini'' spacecraft to systematically shift by up to {{convert|30|km|mi|sp=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.<ref name="NS2008">{{cite web |last=Shiga |first=David |date=March 20, 2008 |url=https://www.newscientist.com/article/dn13516 |title=Titan's changing spin hints at hidden ocean |work=New Scientist |url-status=live |archive-url=https://web.archive.org/web/20141021144006/https://www.newscientist.com/article/dn13516 |archive-date=October 21, 2014  }}</ref> 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.<ref name="Iess et al. 2012">{{Cite journal |doi=10.1126/science.1219631 |pmid=22745254 |title=The Tides of Titan |journal=Science |volume=337 |issue=6093 |pages=457–9 |year=2012 |last1=Iess |first1=L. |last2=Jacobson |first2=R. A. |last3=Ducci |first3=M. |last4=Stevenson |first4=D. J. |last5=Lunine |first5=Jonathan I. |last6=Armstrong |first6=J. W. |last7=Asmar |first7=S. W. |last8=Racioppa |first8=P. |last9=Rappaport |first9=N. J. |last10=Tortora |first10=P. |bibcode=2012Sci...337..457I |hdl=11573/477190 |s2cid=10966007 |hdl-access=free }}</ref> Comparison of the gravity field with the RADAR-based topography observations<ref>{{Cite journal |doi = 10.1126/science.1168905| pmid = 19342551 |title = Size and Shape of Saturn's Moon Titan |journal = Science |volume = 324 |issue = 5929 |pages = 921–3 |year = 2009 |last1 = Zebker |first1=H. A. |last2 = Stiles |first2 = B. |last3 = Hensley |first3 = S. |last4 = Lorenz |first4 = R. |last5 = Kirk |first5=R. L. |last6=Lunine |first6=Jonathan I. |bibcode =2009Sci...324..921Z |s2cid = 23911201 |url = https://pdfs.semanticscholar.org/7ec3/29458f5dabfa6c370476df8236779941f93f.pdf |archive-url = https://web.archive.org/web/20200212021254/https://pdfs.semanticscholar.org/7ec3/29458f5dabfa6c370476df8236779941f93f.pdf |url-status = dead |archive-date = 2020-02-12}}</ref> also suggests that the ice shell may be substantially rigid.<ref name="Hemingway et al. 2013">{{Cite journal |doi = 10.1038/nature12400| pmid = 23985871 |title = A rigid and weathered ice shell on Titan |journal = Nature |volume = 500 |issue = 7464 |pages = 550–2 |year = 2013| last1 = Hemingway |first1 = D. |last2 = Nimmo |first2 = F. |last3 = Zebker |first3 = H. |last4 = Iess |first4 = L. |bibcode = 2013Natur.500..550H | hdl = 11573/563592 |s2cid = 4428328}}</ref><ref name="jpl.nasa.gov">{{cite web |url=https://www.jpl.nasa.gov/news/news.php?release=2013-261 |title=Cassini Data: Saturn Moon May Have Rigid Ice Shell |publisher=JPL |url-status=live |archive-url=https://web.archive.org/web/20141020103435/https://www.jpl.nasa.gov/news/news.php?release=2013-261 |archive-date=October 20, 2014}}</ref>

== Formation ==
The moons of Jupiter and Saturn are thought to have formed through [[Physics:Accretion (astrophysics)|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 [[Astronomy:Rhea (moon)|Rhea]], were formed from the debris of these collisions. Such a violent beginning would also explain Titan's orbital eccentricity.<ref>{{cite web |title=Giant impact scenario may explain the unusual moons of Saturn |work=Space Daily |date=2012 |url=https://www.spacedaily.com/reports/Giant_impact_scenario_may_explain_the_unusual_moons_of_Saturn_999.html |access-date=October 19, 2012 |archive-date=March 28, 2016 |archive-url=https://web.archive.org/web/20160328232204/http://www.spacedaily.com/reports/Giant_impact_scenario_may_explain_the_unusual_moons_of_Saturn_999.html |url-status=live }}</ref>

A 2014 analysis of Titan's atmospheric nitrogen suggested that it was possibly sourced from material similar to that found in the [[Astronomy:Oort cloud|Oort cloud]] and not from sources present during the co-accretion of materials around Saturn.<ref name="NASA-201450623">{{cite press release |last1=Dyches |first1=Preston |last2=Clavin |first2=Whitney |title=Titan's Building Blocks Might Pre-date Saturn |url=https://www.jpl.nasa.gov/news/news.php?release=2014-200 |date=June 23, 2014 |publisher=[[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]] |access-date=June 28, 2014 |url-status=live |archive-url=https://web.archive.org/web/20140627165816/https://www.jpl.nasa.gov/news/news.php?release=2014-200 |archive-date=June 27, 2014  }}</ref>

== Atmosphere ==
{{Main|Astronomy:Atmosphere of Titan}}
[[File:Titan - June 18 2014.png|thumb|True-color image of layers of haze in Titan's atmosphere]]
Titan is the only known moon with a significant [[Astronomy:Atmosphere|atmosphere]],<ref name="NasaNews.moons">{{cite web |title=News Features: The Story of Saturn |url=https://saturn.jpl.nasa.gov/news/features/saturn-story/moons.cfm |archive-url=https://web.archive.org/web/20051202030828/https://saturn.jpl.nasa.gov/news/features/saturn-story/moons.cfm |archive-date=December 2, 2005 |work=Cassini–Huygens Mission to Saturn & Titan |publisher=NASA & JPL |url-status=dead |access-date=January 8, 2007 }}</ref> 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.<ref>{{cite web |url=https://www.astrobio.net/news/article1480.html |title=Wind or Rain or Cold of Titan's Night? |date=March 11, 2005 |work=Astrobiology Magazine |archive-date=July 17, 2007 |archive-url=https://web.archive.org/web/20070717081303/https://www.astrobio.net/news/article1480.html |access-date=August 24, 2007  |url-status=dead}}</ref> Observations from the [[Astronomy:Voyager program|''Voyager'']] space probes have shown that Titan's atmosphere is denser than Earth's, with a surface pressure about 1.45 [[Physics:Atmosphere (unit)|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. [[Physics:Opacity (optics)|Opaque]] haze layers block most visible light from the Sun and other sources and obscure Titan's surface features.<ref name="Zubrin">{{cite book |last=Zubrin |first=Robert |title=Entering Space: Creating a Spacefaring Civilization |url=https://archive.org/details/enteringspacecre00zubr |url-access=registration |location=Section: Titan |pages=[https://archive.org/details/enteringspacecre00zubr/page/163 163–166] |publisher=Tarcher/Putnam |date=1999 |isbn=978-1-58542-036-0 }}</ref> Titan's lower gravity means that its atmosphere is far more extended than Earth's.<ref>{{cite magazine |title=Exploring the Surface of Titan with Cassini–Huygens |last=Turtle |first=Elizabeth P. |date=2007 |magazine=Smithsonian |url=https://www.youtube.com/watch?v=cfCTmv-9GkE |access-date=April 18, 2009 |url-status=live |archive-url=https://web.archive.org/web/20130720123340/https://www.youtube.com/watch?v=cfCTmv-9GkE |archive-date=July 20, 2013  }}</ref> The atmosphere of Titan is [[Physics:Opacity (optics)|opaque]] at many wavelengths and as a result, a complete reflectance spectrum of the surface is impossible to acquire from orbit.<ref>{{cite journal |last1=Schröder |first1=S. E. |last2=Tomasko |first2=M. G. |last3=Keller |first3=H. U. |date=August 2005 |title=The reflectance spectrum of Titan's surface as determined by Huygens |page=726 |journal=American Astronomical Society, DPS Meeting No. 37, #46.15; Bulletin of the American Astronomical Society |volume=37 |issue=726 |bibcode=2005DPS....37.4615S  }}</ref> It was not until the arrival of the ''[[Astronomy:Cassini–Huygens|Cassini–Huygens]]'' spacecraft in 2004 that the first direct images of Titan's surface were obtained.<ref>{{cite news |url=https://www.space.com/710-huygens-probe-sheds-light-titan.html |title=Huygens Probe Sheds New Light on Titan |last=de Selding |first=Petre |publisher=Space.com |date=January 21, 2005 |access-date=March 28, 2005 |url-status=live |archive-url=https://web.archive.org/web/20121019133741/https://www.space.com/710-huygens-probe-sheds-light-titan.html |archive-date=October 19, 2012  }}</ref>

{{multiple image
|header=Titan Clouds
|align=left
|direction=vertical
|width=240
|image1=Titan clouds by NIRCam (2022-11-04 annotated).png
|caption1=Clouds (Nov 4, 2022)
|image2=Titan clouds by NIRCam and Keck NIRC-2 (2022-11-04,06 annotated).png
|caption2=Clouds (Nov 6, 2022)
}}
Titan's atmospheric composition is nitrogen (97%), methane (2.7±0.1%), and hydrogen (0.1–0.2%), with trace amounts of other gases.{{sfnp |Coustenis |Taylor |2008 |pp=154–155}} There are trace amounts of other [[Chemistry:Hydrocarbon|hydrocarbon]]s, such as [[Chemistry:Ethane|ethane]], [[Chemistry:Diacetylene|diacetylene]], methylacetylene, [[Chemistry:Acetylene|acetylene]] and [[Chemistry:Propane|propane]], and of other gases, such as [[Chemistry:Cyanoacetylene|cyanoacetylene]], [[Chemistry:Hydrogen cyanide|hydrogen cyanide]], [[Chemistry:Carbon dioxide|carbon dioxide]], [[Chemistry:Carbon monoxide|carbon monoxide]], [[Chemistry:Cyanogen|cyanogen]], [[Chemistry:Argon|argon]] and [[Chemistry:Helium|helium]].<ref name="Niemann">{{cite journal |title=The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe |first1=H. B. |last1=Niemann |first2=S. K. |last2=Atreya |first3=S. J. |last3=Bauer |first4=G. R. |last4=Carignan |first5=J. E. |last5=Demick |first6=R. L. |last6=Frost |first7=D. |last7=Gautier |first8=J. A. |last8=Haberman |first9=D. N. |last9=Harpold |first10=D. M. |last10=Hunten |first11=G. |last11=Israel |first12=Jonathan I. |last12=Lunine |first13=W. T. |last13=Kasprzak |first14=T. C. |last14=Owen |first15=M. |last15=Paulkovich |first16=F. |last16=Raulin |first17=E. |last17=Raaen |first18=S. H. |last18=Way |display-authors=1 |journal=Nature |volume=438 |pages=779–784 |date=2005 |doi=10.1038/nature04122 |pmid=16319830 |issue=7069 |bibcode=2005Natur.438..779N |url=https://deepblue.lib.umich.edu/bitstream/2027.42/62703/1/nature04122.pdf |hdl=2027.42/62703 |s2cid=4344046 |hdl-access=free |access-date=April 17, 2018 |archive-date=April 14, 2020 |archive-url=https://web.archive.org/web/20200414035805/https://deepblue.lib.umich.edu/bitstream/handle/2027.42/62703/nature04122.pdf;jsessionid=31739AEF2B6261988585EC558A2F898B?sequence=1 |url-status=live }}</ref> The hydrocarbons are thought to form in Titan's upper atmosphere in reactions resulting from the breakup of [[Chemistry:Methane|methane]] by the Sun's [[Physics:Ultraviolet|ultraviolet]] light, producing a thick orange smog.<ref name=waite>{{cite journal |last1=Waite |first1=J. H. |last2=Cravens |first2=T. E. |last3=Coates |first3=A. J. |last4=Crary |first4=F. J. |last5=Magee |first5=B. |last6=Westlake|first6=J. |date=2007 |title=The Process of Tholin Formation in Titan's Upper Atmosphere |journal=Science |volume=316 |doi=10.1126/science.1139727 |pmid=17495166 |issue=5826 |bibcode=2007Sci...316..870W |pages=870–5 |s2cid=25984655 }}</ref> Titan spends 95% of its time within Saturn's magnetosphere, which may help shield it from the [[Astronomy:Solar wind|solar wind]].<ref>{{cite news |last=Courtland |first=Rachel |url=https://www.newscientist.com/article/dn14717-saturn-magnetises-its-moon-titan.html |title=Saturn magnetises its moon Titan |work=New Scientist |date=September 11, 2008 |url-status=live |archive-url=https://web.archive.org/web/20150531045100/https://www.newscientist.com/article/dn14717-saturn-magnetises-its-moon-titan.html |archive-date=May 31, 2015  }}</ref>

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.<ref>{{cite journal |title=Formation and evolution of Titan's atmosphere |last=Coustenis |first=A. |journal=Space Science Reviews |volume=116 |issue=1–2 |pages=171–184 |date=2005 |doi=10.1007/s11214-005-1954-2 |bibcode=2005SSRv..116..171C |s2cid=121298964 }}</ref> The ultimate origin of the methane in its atmosphere may be its interior, released via eruptions from cryovolcanoes.<ref name="NASA.sci.76">{{cite web |title=NASA Titan – Surface |publisher=NASA |url=https://saturn.jpl.nasa.gov/science/index.cfm?SciencePageID=76 |access-date=February 14, 2013 |url-status=live |archive-url=https://web.archive.org/web/20130217182727/https://saturn.jpl.nasa.gov/science/index.cfm?SciencePageID=76 |archive-date=February 17, 2013  }}</ref><ref>{{cite journal |title=Titan's methane cycle |first1=Sushil K. |last1=Atreyaa |first2=Elena Y. |last2=Adamsa |first3=Hasso B. |last3=Niemann |author4=Demick-Montelar, Jaime E. a|author5=Owen, Tobias C. |author6=Fulchignoni, Marcello |author7=Ferri, Francesca |author8=Wilson, Eric H. |date=2006 |doi=10.1016/j.pss.2006.05.028 |journal=Planetary and Space Science |volume=54 |issue=12 |pages=1177–1187 |bibcode=2006P&SS...54.1177A }}</ref><ref>{{cite journal |journal=Nature |display-authors=8 |last=Stofan |first=E. R. |author2=Elachi, C.|author3=Lunine, Jonathan I. |author4=Lorenz, R. D.|author5=Stiles, B.|author6=Mitchell, K. L.|author7=Ostro, S.|author8=Soderblom, L.|author9=Wood, C.|author10=Zebker, H.|author11=Wall, S.|author12=Janssen, M.|author13=Kirk, R.|author14=Lopes, R.|author15=Paganelli, F.|author16=Radebaugh, J.|author17=Wye, L.|author18=Anderson, Y.|author19=Allison, M.|author20=Boehmer, R.|author21=Callahan, P.|author22=Encrenaz, P.|author23=Flamini, E.|author24=Franscetti, G.|author25=Gim, Y.|author26=Hamilton, G.|author27=Hensley, S.|author28=Johnson, W. T. K.|author29=Kelleher, K.|author30=Muhleman, D.|author31=Paillou, P.|author32=Picardi, G.|author33=Posa, F.|author34=Roth, L.|author35=Seul, R.|author36=Shaffer, S.|author37=Vetrella, S.|author38=West, R. |volume=445 |date=2007 |doi=10.1038/nature05438 |pmid=17203056 |issue=7123 |title=The lakes of Titan |pages=61–64 |bibcode=2007Natur.445...61S |s2cid=4370622 }}</ref><ref>{{cite journal |title=Episodic outgassing as the origin of atmospheric methane on Titan |last1=Tobie |first1=Gabriel |last2=Lunine |first2=Jonathan I. |last3=Sotin |first3=Christophe |journal=Nature |volume=440 |issue=7080 |pages=61–64 |date=2006 |doi=10.1038/nature04497 |pmid=16511489 |bibcode=2006Natur.440...61T |title-link=atmospheric methane |s2cid=4335141 }}</ref>
[[File:Titan-SaturnMoon-Maps-TraceGases-20141022.jpg|thumb|[[Biology:Trace element|Trace]] organic gases in Titan's atmosphere—HNC (left) and HC<sub>3</sub>N (right).]]
On April 3, 2013, NASA reported that complex organic chemicals, collectively called [[Astronomy:Tholin|tholin]]s, likely arise on Titan, based on studies simulating the atmosphere of Titan.<ref name="PhysOrg-20130403" />
<!---
{{external media
|video1 = [https://www.youtube.com/watch?v=tCk9u41A2q8 Occultation of the star SAO 164648 by Titan on 2022 July 9. The changes in brightness are gradual due to the Titan's atmosphere.]
}}
--->
On June 6, 2013, scientists at the [[Organization:Instituto de Astrofísica de Andalucía|IAA-CSIC]] reported the detection of [[Astronomy:Polycyclic aromatic hydrocarbon|polycyclic aromatic hydrocarbon]]s in the upper atmosphere of Titan.<ref name="IAA-20130606">{{cite news |last=López-Puertas |first=Manuel |url=https://www.iaa.es/content/pahs-titans-upper-atmosphere |title=PAH's in Titan's Upper Atmosphere |date=June 6, 2013 |work=[[Organization:Spanish National Research Council|CSIC]] |access-date=June 6, 2013 |url-status=live |archive-url=https://web.archive.org/web/20131203030521/https://www.iaa.es/content/pahs-titans-upper-atmosphere |archive-date=December 3, 2013  }}</ref><ref name="journalcours">{{cite journal|arxiv=2001.02791|doi=10.1016/j.icarus.2019.113571|title=The 3.4μm absorption in Titan's stratosphere: Contribution of ethane, propane, butane and complex hydrogenated organics|journal=Icarus|volume=339|pages=113571|year=2020|last1=Cours|first1=T.|last2=Cordier|first2=D.|last3=Seignovert|first3=B.|last4=Maltagliati|first4=L.|last5=Biennier|first5=L.|bibcode=2020Icar..33913571C|s2cid=210116807}}</ref>

On September 30, 2013, [[Chemistry:Propene|propene]] was detected in the atmosphere of Titan by [[Organization:NASA|NASA]]'s ''Cassini'' spacecraft, using its composite infrared spectrometer (CIRS).<ref name="NASA-20130930">{{cite web |last1=Brown |first1=Dwayne |last2=Neal-Jones |first2=Nancy |last3=Zubritsky |first3=Elizabeth |last4=Cook |first4=Jia-Rui |title=NASA's Cassini Spacecraft Finds Ingredient of Household Plastic in Space |url=https://www.nasa.gov/content/goddard/nasas-cassini-spacecraft-finds-ingredient-of-household-plastic-in-space/ |date=September 30, 2013 |work=[[Organization:NASA|NASA]] |access-date=December 2, 2013 |url-status=live |archive-url=https://web.archive.org/web/20131127041925/https://www.nasa.gov/content/goddard/nasas-cassini-spacecraft-finds-ingredient-of-household-plastic-in-space/ |archive-date=November 27, 2013  }}</ref> 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 ''[[Astronomy:Voyager 1|Voyager 1]]'' spacecraft's first close [[Engineering:Planetary flyby|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.<ref name="waite" />

On October 24, 2014, [[Chemistry:Methane|methane]] was found in polar clouds on Titan.<ref name="NASA-20141024-PD">{{cite web |last1=Dyches |first1=Preston |last2=Zubritsky |first2=Elizabeth |title=NASA Finds Methane Ice Cloud in Titan's Stratosphere |url=https://www.jpl.nasa.gov/news/news.php?feature=4354 |date=October 24, 2014 |work=[[Organization:NASA|NASA]] |access-date=October 31, 2014 |url-status=live |archive-url=https://web.archive.org/web/20141028193849/https://www.jpl.nasa.gov/news/news.php?feature=4354 |archive-date=October 28, 2014  }}</ref><ref name="NASA-20141024-EZ">{{cite web |last1=Zubritsky |first1=Elizabeth |last2=Dyches |first2=Preston |title=NASA Identifies Ice Cloud Above Cruising Altitude on Titan |url=https://www.nasa.gov/content/goddard/nasa-identifies-ice-cloud-above-cruising-altitude-on-titan |date=October 24, 2014 |work=[[Organization:NASA|NASA]] |access-date=October 31, 2014 |url-status=live |archive-url=https://web.archive.org/web/20141031133440/https://www.nasa.gov/content/goddard/nasa-identifies-ice-cloud-above-cruising-altitude-on-titan/ |archive-date=October 31, 2014  }}</ref> On December 1, 2022, astronomers reported viewing clouds, likely made of [[Chemistry:Methane|methane]], moving across Titan, using the [[Astronomy:James Webb Space Telescope|James Webb Space Telescope]].<ref name="SPC-20221201">{{cite news |last=Bartels |first=Meghan |title=James Webb Space Telescope view of Saturn's weirdest moon Titan thrills scientists |url=https://www.space.com/james-webb-space-telescope-saturn-moon-titan |date=December 1, 2022 |work=Space.com |accessdate=December 2, 2022 }}</ref><ref name="NYT-20221205">{{cite news |last=Overbye |first=Dennis |title=Telescopes Team Up to Forecast an Alien Storm on Titan - Saturn's largest moon came under the gaze of NASA's powerful Webb space observatory, allowing it and another telescope to capture clouds drifting through Titan's methane-rich atmosphere. |url=https://www.nytimes.com/2022/12/05/science/titan-webb-telescope-pictures.html |date=December 5, 2022 |work={{wipe|The New York Times}} |accessdate=December 6, 2022 }}</ref>

{{wide image|Titan-Earth-PolarClouds-20141024.jpg|600px|align-cap=center|Polar clouds, made of methane, on Titan (left) compared with [[Earth:Polar stratospheric cloud|polar clouds]] on [[Astronomy:Earth|Earth]] (right), which are made of water or water ice.}}

== Climate ==
{{Main|Astronomy:Climate of Titan}}
[[File:Vortex on saturn's moon titan.png|thumb|left|Atmospheric [[Physics:Polar vortex|polar vortex]] over Titan's south pole|text-top]]
Titan's surface temperature is about {{convert|94|K|°C}}. At this temperature, water ice has an extremely low [[Physics:Vapor pressure|vapor pressure]], so the little [[Physics:Water vapor|water vapor]] present appears limited to the stratosphere.<ref name="CottiniNixon2012">{{cite journal |display-authors=8 |last1=Cottini |first1=V. |last2=Nixon |first2=C.A. |last3=Jennings |first3=D.E. |last4=Anderson |first4=C.M. |last5=Gorius |first5=N. |last6=Bjoraker |first6=G.L. |last7=Coustenis |first7=A. |last8=Teanby |first8=N.A. |last9=Achterberg |first9=R.K. |last10=Bézard |first10=B. |last11=de Kok |first11=R. |last12=Lellouch |first12=E. |last13=Irwin |first13=P.G.J. |last14=Flasar |first14=F.M. |last15=Bampasidis |first15=G. |title=Water vapor in Titan's stratosphere from Cassini CIRS far-infrared spectra |journal=Icarus |volume=220 |issue=2 |date=2012 |pages=855–862 |issn=0019-1035 |doi=10.1016/j.icarus.2012.06.014 |bibcode = 2012Icar..220..855C |hdl=2060/20120013575|s2cid=46722419 |hdl-access=free }}</ref> Titan receives about 1% as much sunlight as Earth.<ref>{{cite web |url=https://www.space.com/7103-titan-world-earth.html |title=Titan: A World Much Like Earth |publisher=Space.com |date=August 6, 2009 |access-date=April 2, 2012 |url-status=live |archive-url=https://web.archive.org/web/20121012005030/https://www.space.com/7103-titan-world-earth.html |archive-date=October 12, 2012  }}</ref> Before sunlight reaches the surface, about 90% has been absorbed by the thick atmosphere, leaving only 0.1% of the amount of light [[Astronomy:Earth|Earth]] receives.<ref>[https://arstechnica.com/science/2012/01/like-ogres-titans-atmosphere-may-have-layers/ Faint sunlight enough to drive weather, clouds on Saturn's moon Titan] {{webarchive|url=https://web.archive.org/web/20170403194843/https://arstechnica.com/science/2012/01/like-ogres-titans-atmosphere-may-have-layers/ |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.</ref>

Atmospheric methane creates a [[Earth:Greenhouse effect|greenhouse effect]] on Titan's surface, without which Titan would be much colder.<ref name="oil">{{cite web |url=https://www.space.com/4968-titan-oil-earth.html |title=Titan Has More Oil Than Earth |website=Space.com |date=February 13, 2008 |access-date=February 13, 2008 |url-status=live |archive-url=https://web.archive.org/web/20120708055735/https://www.space.com/4968-titan-oil-earth.html |archive-date=July 8, 2012  }}</ref> Conversely, [[Earth:Haze|haze]] in Titan's atmosphere contributes to an [[Earth:Anti-greenhouse effect|anti-greenhouse effect]] by absorbing sunlight, cancelling a portion of the greenhouse effect and making its surface significantly colder than its upper atmosphere.<ref>{{cite journal |doi=10.1126/science.11538492 |first1=C.P. |last1=McKay |first2=J. B. |last2=Pollack |first3=R. |last3=Courtin |date=1991 |title=The greenhouse and antigreenhouse effects on Titan |journal=Science |volume=253 |issue=5024 |pages=1118–1121 |pmid=11538492 |url=https://pdfs.semanticscholar.org/55b1/69bd4d31772da8c5e7ad17586baff4389481.pdf |archive-url=https://web.archive.org/web/20200412143944/https://pdfs.semanticscholar.org/55b1/69bd4d31772da8c5e7ad17586baff4389481.pdf |url-status=dead |archive-date=2020-04-12 |bibcode=1991Sci...253.1118M |s2cid=10384331 }}</ref>

[[File:PIA18420-Titan-MethaneClouds-20140722.gif|thumb|upright|Methane clouds (animated; July 2014).<ref name="NASA-20140812">{{cite web |last=Dyches |first=Preston |title=Cassini Tracks Clouds Developing Over a Titan Sea |url=https://www.jpl.nasa.gov/news/news.php?release=2014-274 |date=August 12, 2014 |work=NASA |access-date=August 13, 2014 |url-status=live |archive-url=https://web.archive.org/web/20140813004912/https://www.jpl.nasa.gov/news/news.php?release=2014-274 |archive-date=August 13, 2014  }}</ref>]]
Titan's clouds, probably composed of methane, ethane or other simple organics, are scattered and variable, punctuating the overall haze.<ref name="nineplanets" /> The findings of the ''Huygens'' probe indicate that Titan's atmosphere periodically rains liquid methane and other organic compounds onto its surface.<ref name="planetary-Arizona_Icebox">{{cite news |url=https://planetary.org/news/2005/0121_Titan_Arizona_in_an_Icebox.html |title=Titan: Arizona in an Icebox? |last=Lakdawalla |first=Emily |publisher=The Planetary Society |date=January 21, 2004 |archive-date=February 12, 2010 |archive-url=https://web.archive.org/web/20100212062535/https://planetary.org/news/2005/0121_Titan_Arizona_in_an_Icebox.html |access-date=March 28, 2005 }}</ref>

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 [[Astronomy:Convection|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.<ref>{{cite journal |last1=Emily L. |first1=Schaller |last2=Brouwn |first2=Michael E. |last3=Roe |first3=Henry G. |last4=Bouchez |first4=Antonin H. |date=2006 |title=A large cloud outburst at Titan's south pole |doi=10.1016/j.icarus.2005.12.021 |journal=Icarus |issue=1 |pages=224–229 |url=https://www.gps.caltech.edu/~mbrown/papers/ps/largecloud.pdf |volume=182 |bibcode=2006Icar..182..224S |access-date=August 23, 2007 |url-status=live |archive-url=https://web.archive.org/web/20070926102541/https://www.gps.caltech.edu/~mbrown/papers/ps/largecloud.pdf |archive-date=September 26, 2007  }}</ref> 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.<ref name="Titan wind">{{cite news|title=The Way the Wind Blows on Titan |date=June 1, 2007 |url=https://saturn.jpl.nasa.gov/news/cassinifeatures/feature20070601f/ |access-date=June 2, 2007 |publisher=Jet Propulsion Laboratory |url-status=dead |archive-url=https://web.archive.org/web/20090427110242/https://saturn.jpl.nasa.gov/news/cassinifeatures/feature20070601f/ |archive-date=April 27, 2009 }}</ref> When the seasons switch, it is expected that ethane will begin to condense over the south pole.<ref>{{cite journal |title=Huge ethane cloud discovered on Titan |last=Shiga |first=David |journal=New Scientist |volume=313 |page=1620 |date=2006 |url=https://www.newscientist.com/article/dn10073-huge-ethane-cloud-discovered-on-titan.html |access-date=August 7, 2007 |url-status=live |archive-url=https://web.archive.org/web/20081220030748/https://www.newscientist.com/article/dn10073-huge-ethane-cloud-discovered-on-titan.html |archive-date=December 20, 2008  }}</ref>

== Surface features ==
[[File:First global geologic map of Titan (PIA23174).jpg|thumb|center|600px|{{center|Global geologic map
 of Titan (2019)<ref name="NYT-20191203" />}}]]

<gallery mode="packed" heights="140px">
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)
</gallery>
<!---[[File:PIA19658-SaturnMoon-TitanGlobalMap-June2015.jpg|thumb|left|250px|{{center|Global map of Titan}}]]--->

The surface of Titan has been described as "complex, fluid-processed, [and] geologically young".<ref>{{cite journal |last=Mahaffy |first=Paul R. |date=May 13, 2005 |title=Intensive Titan Exploration Begins |journal=Science |volume=308 |issue=5724 |pages=969–970 |doi=10.1126/science.1113205 |pmid=15890870 |bibcode=2005Sci...308..969M |citeseerx=10.1.1.668.2877 |s2cid=41758337 }}</ref> 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.<ref name="puzzling geologic">{{cite web |url=https://web.mit.edu/newsoffice/2012/river-networks-on-titan-0720.html |title=River networks on Titan point to a puzzling geologic history |first=Jennifer |last=Chu |date=July 2012 |publisher=MIT Research |access-date=July 24, 2012 |url-status=live |archive-url=https://web.archive.org/web/20121030175726/https://web.mit.edu/newsoffice/2012/river-networks-on-titan-0720.html |archive-date=October 30, 2012  }}</ref> Titan's atmosphere is four times as thick as Earth's,<ref>{{cite web |url=https://www.nasa.gov/feature/goddard/2020/nasa-scientists-discover-a-weird-molecule-in-titan-s-atmosphere |title='Weird' Molecule Discovered in Titan's Atmosphere |work=nasa.gov |date=October 20, 2020 |access-date=February 25, 2021 |archive-date=July 15, 2021 |archive-url=https://web.archive.org/web/20210715113442/https://www.nasa.gov/feature/goddard/2020/nasa-scientists-discover-a-weird-molecule-in-titan-s-atmosphere/ |url-status=live }}</ref> making it difficult for astronomical instruments to image its surface in the visible light spectrum.<ref>{{cite news |first=Taimoor |last=Tariq |title=Titan, Saturn's largest moon is finally unravelled in detail |date=March 12, 2012 |url=https://www.newspakistan.pk/2012/03/12/Titan-Saturn-s-largest-moon-is-finally-unravelled-in-detail/ |work=News Pakistan |access-date=March 12, 2012 |url-status=live |archive-url=https://web.archive.org/web/20140811000259/https://www.newspakistan.pk/2012/03/12/Titan-Saturn-s-largest-moon-is-finally-unravelled-in-detail/ |archive-date=August 11, 2014  }}</ref> 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,<ref name="Hemingway et al. 2013" /><ref name="jpl.nasa.gov" /> which would suggest little geologic activity.<ref name="Moore and Pappalardo 2011">{{Cite journal| doi = 10.1016/j.icarus.2011.01.019| title = Titan: An exogenic world?| journal = Icarus| volume = 212| issue = 2| pages = 790–806| year = 2011| last1 = Moore| first1 = J. M.| last2 = Pappalardo| first2 = R. T.| bibcode = 2011Icar..212..790M| url = https://zenodo.org/record/1259045| access-date = March 18, 2020| archive-date = July 26, 2021| archive-url = https://web.archive.org/web/20210726123209/https://zenodo.org/record/1259045| url-status = live}}</ref>
<!---[[File:PIA18309-SaturnMoonTitan-SenkyoRegion-20150108.jpg|thumb|Senkyo region]]--->
There are also streaky features, some of them hundreds of kilometers in length, that appear to be caused by windblown particles.<ref>{{cite magazine |last=Battersby |first=Stephen |date=October 29, 2004 |title=Titan's complex and strange world revealed |magazine=New Scientist |url=https://www.newscientist.com/article/dn6598 |access-date=August 31, 2007 |url-status=live |archive-url=https://web.archive.org/web/20081221052236/https://www.newscientist.com/article/dn6598 |archive-date=December 21, 2008  }}</ref><ref>{{cite web |url=https://saturn.jpl.nasa.gov/spacecraft/overview/ |title=Spacecraft: Cassini Orbiter Instruments, RADAR |work=Cassini–Huygens Mission to Saturn & Titan |publisher=NASA, Jet Propulsion Laboratory |access-date=August 31, 2007 |url-status=live |archive-url=https://web.archive.org/web/20110807011916/https://saturn.jpl.nasa.gov/spacecraft/overview/ |archive-date=August 7, 2011  }}</ref> 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&nbsp;meters. Occasional elevation changes of 500&nbsp;meters have been discovered and Titan has mountains that sometimes reach several hundred meters to more than 1 kilometer in height.<ref>{{cite journal |last1=Lorenz |first1=R. D. |last2=Callahan |first2=P. S. |last3=Gim |first3=Y. |last4=Alberti |first4=G. |last5=Flamini |first5=E. |last6=Seu |first6=R. |last7=Picardi |first7=G. |last8=Orosei |first8=R. |last9=Zebker |first9=H. |last10=Lunine |first10=Jonathan I. |last11=Hamilton |first11=G. |last12=Hensley |first12=S. |last13=Johnson |first13=W. T. K. |last14=Schaffer |first14=S. |last15=Wall |first15=S. |last16=West |first16=R. |last17=Francescetti |first17=G. |display-authors=1 |date=2007 |title=Titan's Shape, Radius and Landscape from Cassini Radar Altimetry |journal=Lunar and Planetary Science Conference |volume=38 |issue=1338 |url=https://www.lpi.usra.edu/meetings/lpsc2007/pdf/1329.pdf |bibcode=2007LPI....38.1329L |page=1329 |access-date=August 27, 2007 |url-status=live |archive-url=https://web.archive.org/web/20070926102540/https://www.lpi.usra.edu/meetings/lpsc2007/pdf/1329.pdf |archive-date=September 26, 2007  }}</ref>

Titan's surface is marked by broad regions of bright and dark terrain. These include [[Astronomy:Xanadu (Titan)|Xanadu]], a large, [[Physics:Reflection|reflective]]<!--to what wavelengths?--> equatorial area about the size of Australia. It was first identified in [[Physics:Infrared|infrared]] images from the [[Astronomy:Hubble Space Telescope|Hubble Space Telescope]] in 1994, and later viewed by the ''Cassini'' spacecraft. The convoluted region is filled with hills and cut by valleys and chasms.<ref>{{cite web |url=https://www.sciencedaily.com/releases/2006/07/060721202957.htm |title=Cassini Reveals Titan's Xanadu Region To Be An Earth-Like Land |date=July 23, 2006 |website=Science Daily |access-date=August 27, 2007 |url-status=live |archive-url=https://web.archive.org/web/20110629211805/https://www.sciencedaily.com/releases/2006/07/060721202957.htm |archive-date=June 29, 2011  }}</ref> 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.<ref>{{cite journal |display-authors=8 |last1=Barnes |first1=Jason W. |last2=Brown |first2=Robert H. |last3=Soderblom |first3=Laurence |last4=Buratti |first4=Bonnie J. |last5=Sotin |first5=Christophe |last6=Rodriguez |first6=Sebastien |last7=Le Mouèlic |first7=Stephane |last8=Baines |first8=Kevin H. |last9=Clark |first9=Roger |last10=Nicholson |first10=Phil |date=2006 |title=Global-scale surface spectral variations on Titan seen from Cassini/VIMS |doi=10.1016/j.icarus.2006.08.021 |journal=Icarus |issue=1 |volume=186 |url=https://c3po.barnesos.net/publications/papers/Titan.spectral.diversity.pdf |archive-date=July 25, 2011 |archive-url=https://web.archive.org/web/20110725155935/https://c3po.barnesos.net/publications/papers/Titan.spectral.diversity.pdf |access-date=August 27, 2007 |pages=242–258 |bibcode=2007Icar..186..242B }}</ref> 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.<ref name="methane sea">{{cite news |last=Klotz |first=Irene |url=https://www.space.com/32741-one-of-titans-strange-seas-is-nearly-pure-methane.html |title=One of Titan |work=Discovery News |publisher=Space.com |date=April 28, 2016 |access-date=May 1, 2016 |url-status=live |archive-url=https://web.archive.org/web/20160430113237/https://www.space.com/32741-one-of-titans-strange-seas-is-nearly-pure-methane.html |archive-date=April 30, 2016  }}</ref><ref name="Le Gall 2016">{{cite journal |last1=Le Gall |first1=A. |last2=Malaska |first2=M. J. |last3=Lorenz |first3=Ralph D. |last4=Janssen |first4=M. A. |last5=Tokano |first5=T. |last6=Hayes |first6=Alexander G. |last7=Mastrogiuseppe |first7=Marco |last8=Lunine |first8=Jonathan I. |last9=Veyssière |first9=G. |last10=Encrenaz |first10=P. |last11=Karatekin |first11=O. |title=Composition, seasonal change, and bathymetry of Ligeia Mare, Titan, derived from its microwave thermal emission |journal=Journal of Geophysical Research: Planets |volume=121 |issue=2 |date=2016-02-25 |pages=233–251 |doi=10.1002/2015JE004920 |bibcode=2016JGRE..121..233L |url=https://hal.archives-ouvertes.fr/hal-01259869/document |doi-access=free |access-date=August 12, 2021 |archive-date=August 12, 2021 |archive-url=https://web.archive.org/web/20210812002657/https://hal.archives-ouvertes.fr/hal-01259869/document |url-status=live |hdl=11573/1560395 |hdl-access=free }}</ref>

{|class="wikitable" style="margin-left: auto; margin-right: auto; border: none;"
|- style="text-align:center; vertical-align: top;"
|[[File:Titan2005.jpg|center|thumb|Titan mosaic from a ''Cassini'' flyby. The large dark region is Shangri-La.]]
|[[File:Titan multi spectral overlay.jpg|center|thumb|Titan in false color showing surface details and atmosphere. Xanadu is the bright region at the bottom-center.]]
|
|[[File:PIA20016-SaturnMoon-Titan-20151113.jpg|center|thumb|Titan 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|{{center|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 Mare is at lower right.]]
[[File:HRICoastLineMoasic H.jpg|thumb|Mosaic of three ''Huygens'' images of channel system on Titan]]
[[File:PIA23172-SaturnMoon-Titan-RimmedLakes-ArtistConcept-20190906.jpg|thumb|Rimmed lakes of Titan<br />(artist concept)]]

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.<ref>{{cite journal |last1=Dermott |first1=S. F. |last2=Sagan |first2=C. |date=1995 |title=Tidal effects of disconnected hydrocarbon seas on Titan |journal=Nature |volume=374 |pages=238–240 |doi=10.1038/374238a0 |pmid=7885443 |issue=6519 |bibcode=1995Natur.374..238D |s2cid=4317897 }}</ref>

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 [[Physics:Specular reflection|specular reflection]]s were initially observed.<ref>{{cite web |first=Henry |last=Bortman |url=https://saturn.astrobio.net/news/article81.html |title=Titan: Where's the Wet Stuff? |date=November 2, 2004 |work=Astrobiology Magazine |archive-date=November 3, 2006 |archive-url=https://web.archive.org/web/20061103235110/https://saturn.astrobio.net/news/article81.html |access-date=August 28, 2007 }}</ref> Near Titan's south pole, an enigmatic dark feature named Ontario Lacus was identified<ref>{{cite news |url=https://www.planetary.org/news/2005/0628_Dark_Spot_Near_the_South_Pole_A.html |title=Dark Spot Near the South Pole: A Candidate Lake on Titan? |last=Lakdawalla |first=Emily |publisher=The Planetary Society |date=June 28, 2005 |archive-date=June 5, 2011 |archive-url=https://web.archive.org/web/20110605101239/https://www.planetary.org/news/2005/0628_Dark_Spot_Near_the_South_Pole_A.html |access-date=October 14, 2006 }}</ref> (and later confirmed to be a lake).<ref name="NASA.20080730">{{cite web |title=NASA Confirms Liquid Lake On Saturn Moon |date=2008 |publisher=NASA |url=https://www.nasa.gov/mission_pages/cassini/media/cassini-20080730.html |access-date=December 20, 2009 |url-status=live |archive-url=https://web.archive.org/web/20110629175214/https://www.nasa.gov/mission_pages/cassini/media/cassini-20080730.html |archive-date=June 29, 2011  }}</ref> A possible shoreline was also identified near the pole via radar imagery.<ref>{{cite press release |url=https://www.spaceref.com/news/viewpr.html?pid=17829 |title=NASA Cassini Radar Images Show Dramatic Shoreline on Titan |date=September 16, 2005 |publisher=Jet Propulsion Laboratory |access-date=October 14, 2006 |url-status=live |archive-url=https://archive.today/20120530031240/http://www.spaceref.com/news/viewpr.html?pid=17829 |archive-date=May 30, 2012  }}</ref> 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.<ref name="PIA08630">{{cite web |url=https://photojournal.jpl.nasa.gov/catalog/PIA08630 |title=PIA08630: Lakes on Titan |publisher=NASA/JPL |work=Planetary Photojournal |access-date=October 14, 2006 |url-status=live |archive-url=https://web.archive.org/web/20110718143841/https://photojournal.jpl.nasa.gov/catalog/PIA08630 |archive-date=July 18, 2011  }}</ref> Based on the observations, scientists announced "definitive evidence of lakes filled with methane on Saturn's moon Titan" in January 2007.<ref name="NatureDefinitive">{{cite journal |title=The lakes of Titan |last1=Stofan |first1=E. R. |last2=Elachi |first2=C. |last3=Lunine |first3=Jonathan I. |last4=Lorenz |first4=R. D. |last5=Stiles |first5=B. |last6=Mitchell |first6=K. L. |last7=Ostro |first7=S. |last8=Soderblom |first8=L. |last9=Wood |first9=C. | display-authors=8 |issue=1 |volume=445 |pages=61–64 |journal=Nature |date=2007 |doi=10.1038/nature05438 |pmid=17203056 |bibcode=2007Natur.445...61S  |s2cid=4370622 }}</ref><ref>{{cite web |url=https://www.jpl.nasa.gov/news/features.cfm?feature=1258 |title=Titan Has Liquid Lakes, Scientists Report in Nature |publisher=NASA/JPL |date=January 3, 2007 |access-date=January 8, 2007 |url-status=live |archive-url=https://web.archive.org/web/20130523192017/https://www.jpl.nasa.gov/news/news.php?feature=1258 |archive-date=May 23, 2013  }}</ref> The ''Cassini–Huygens'' team concluded that the imaged features are almost certainly the long-sought hydrocarbon lakes, the first stable bodies of surface liquid found outside Earth.<ref name="NatureDefinitive" /> Some appear to have channels associated with liquid and lie in topographical depressions.<ref name="NatureDefinitive" /> 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.<!--<ref>{{cite web |url=https://www.saturntoday.com/news/viewpr.html?pid=37865 |title=River networks on Titan point to a puzzling geologic history |publisher=MIT |date=July 20, 2012 |access-date=July 23, 2012 }}</ref>--><ref name="puzzling geologic" /> Overall, the ''Cassini'' radar observations have shown that lakes cover only a small percentage of the surface, making Titan much drier than Earth.<ref>{{cite news |url=https://www.newscientist.com/article/mg21128201.600-ethane-lakes-in-a-red-haze-titans-uncanny-moonscape.html?page=2 |title=Ethane lakes in a red haze: Titan's uncanny moonscape |work=New Scientist |last=Hecht |first=Jeff |date=July 11, 2011 |access-date=July 25, 2011 |url-status=live |archive-url=https://web.archive.org/web/20110713220529/https://www.newscientist.com/article/mg21128201.600-ethane-lakes-in-a-red-haze-titans-uncanny-moonscape.html?page=2 |archive-date=July 13, 2011  }}</ref> 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 [[Earth:Aquifer|aquifer]]s.<ref>{{cite press release |title=Tropical Methane Lakes on Saturn's Moon Titan |author=Jet Propulsion Laboratory |publisher=SpaceRef |date=2012 |url=https://spaceref.com/news/viewpr.html?pid=37429 |archive-url=https://archive.today/20140303103242/http://spaceref.com/news/viewpr.html?pid=37429 |url-status=dead |archive-date=March 3, 2014 |access-date=March 2, 2014 }}</ref>
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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.<ref>{{cite web |title=Scientists Confirm Liquid Lake, Beach on Saturn's Moon Titan |last=Hadhazy |first=Adam |url=https://www.scientificamerican.com/article.cfm?id=liquid-lake-on-titan |work=Scientific American |date=2008 |access-date=July 30, 2008 |url-status=live |archive-url=https://web.archive.org/web/20120905111129/https://www.scientificamerican.com/article.cfm?id=liquid-lake-on-titan |archive-date=September 5, 2012  }}</ref> 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&nbsp;mm (implying either that surface winds were minimal, or the lake's hydrocarbon fluid is viscous).<ref name="New Scientist">{{cite news |last=Grossman |first=Lisa |title=Saturn moon's mirror-smooth lake 'good for skipping rocks' |work=New Scientist |date=August 21, 2009 |url=https://www.newscientist.com/article/dn17665-saturn-moons-mirror-smooth-lake-good-for-skipping-rocks/ |access-date=November 25, 2009 |url-status=live |archive-url=https://web.archive.org/web/20160110105315/https://www.newscientist.com/article/dn17665-saturn-moons-mirror-smooth-lake-good-for-skipping-rocks/ |archive-date=January 10, 2016  }}</ref><ref name="Wye">{{cite journal |last1=Wye |first1=L. C. |last2=Zebker |first2=H. A. |last3=Lorenz |first3=R. D. |title=Smoothness of Titan's Ontario Lacus: Constraints from Cassini RADAR specular reflection data |journal=Geophysical Research Letters |volume=36 |issue=16 |pages=L16201 |date=2009 |doi=10.1029/2009GL039588 |bibcode=2009GeoRL..3616201W |doi-access=free }}</ref>

[[File:Specular Spectacular (PIA18432).jpg|thumb|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.<ref name="press">{{cite web |last=Cook |first=J.-R. C. |title=Glint of Sunlight Confirms Liquid in Northern Lake District of Titan |work=Cassini mission page |publisher=NASA |date=December 17, 2009 |url=https://www.nasa.gov/mission_pages/cassini/whycassini/cassini20091217.html |access-date=December 18, 2009 |url-status=live |archive-url=https://web.archive.org/web/20110605131154/https://www.nasa.gov/mission_pages/cassini/whycassini/cassini20091217.html |archive-date=June 5, 2011  }}</ref><ref name="VIMS_specular">{{cite web |last=Lakdawalla |first=Emily |title=Cassini VIMS sees the long-awaited glint off a Titan lake |work=The Planetary Society Blog |publisher=Planetary Society |date=December 17, 2009 |url=https://planetary.org/blogs/emily-lakdawalla/2009/2267.html |access-date=December 17, 2009 |url-status=live |archive-url=https://web.archive.org/web/20120630011024/https://www.planetary.org/blogs/emily-lakdawalla/2009/2267.html |archive-date=June 30, 2012  }}</ref>

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&nbsp;m, and a maximum depth of {{convert|3|to|7|m|sp=us|abbr=on}}.<ref name="sdc20101217" /> In contrast, the northern hemisphere's Ligeia Mare was initially mapped to depths exceeding 8&nbsp;m, the maximum discernable by the radar instrument and the analysis techniques of the time.<ref name=sdc20101217>{{cite web |last=Wall |first=Mike |title=Saturn Moon's 'Lake Ontario': Shallow and Virtually Wave-free |work=Space.Com web site |date=December 17, 2010 |url=https://www.space.com/10512-saturn-moon-lake-ontario-shallow-virtually-wave-free.html |access-date=December 19, 2010 |url-status=live |archive-url=https://web.archive.org/web/20121020044023/https://www.space.com/10512-saturn-moon-lake-ontario-shallow-virtually-wave-free.html |archive-date=October 20, 2012  }}</ref>
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".<ref name=sn20141117>{{cite news |last1=Crockett |first1=Christopher |title=Cassini maps depths of Titan's seas |url=https://www.sciencenews.org/article/cassini-maps-depths-titan%E2%80%99s-seas |access-date=November 18, 2014 |work=ScienceNews |date=November 17, 2014 |url-status=live |archive-url=https://web.archive.org/web/20150403062748/https://www.sciencenews.org/article/cassini-maps-depths-titan%E2%80%99s-seas |archive-date=April 3, 2015  }}</ref>

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 filled with liquid. 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.<ref>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, {{cite journal |title=Liquid-filled canyons on Titan |journal=Geophysical Research Letters |volume=43 |issue=15 |pages=7887–7894  |doi=10.1002/2016GL069679 |year = 2016|last1 = Poggiali|first1 = V.|last2=Mastrogiuseppe |first2=M. |last3=Hayes |first3=A. G. |last4=Seu |first4=R. |last5=Birch |first5=S. P. D. |last6=Lorenz |first6=R. |last7=Grima |first7=C. |last8=Hofgartner |first8=J. D. |hdl=11573/932488 |hdl-access=free |bibcode=2016GeoRL..43.7887P |s2cid=132445293 }}</ref>

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.<ref name="Tides">{{cite news |first=Sid |last=Perkins |title=Tides turn on Titan |date=June 28, 2012 |url=https://www.nature.com/news/tides-turn-on-titan-1.10917 |work=Nature |access-date=June 29, 2012 |url-status=live |archive-url=https://web.archive.org/web/20121007082709/https://www.nature.com/news/tides-turn-on-titan-1.10917 |archive-date=October 7, 2012  }}</ref> 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.<ref name="Tides" /><ref name="Titan ocean of water">{{cite web |last=Puiu |first=Tibi |title=Saturn's moon Titan most likely harbors a subsurface ocean of water |work=zmescience.com web site |date=June 29, 2012 |url=https://www.zmescience.com/research/studies/titan-subsurface-ocean-of-water-23323/ |access-date=June 29, 2012 |url-status=live |archive-url=https://web.archive.org/web/20120903075517/https://www.zmescience.com/research/studies/titan-subsurface-ocean-of-water-23323/ |archive-date=September 3, 2012  }}</ref> On July 2, 2014, NASA reported the ocean inside Titan may be as salty as the [[Earth:Dead Sea|Dead Sea]].<ref name="NASA-20140702">{{cite web |last1=Dyches |first1=Preston |last2=Brown |first2=Dwayne |title=Ocean on Saturn Moon Could be as Salty as the Dead Sea |url=https://www.jpl.nasa.gov/news/news.php?release=2014-217 |date=July 2, 2014 |work=[[Organization:NASA|NASA]] |access-date=July 2, 2014 |url-status=live |archive-url=https://web.archive.org/web/20140709165628/https://www.jpl.nasa.gov/news/news.php?release=2014-217 |archive-date=July 9, 2014  }}</ref><ref name="ICARUS-2014">{{cite journal |last1=Mitri |first1=Giuseppe |last2=Meriggiola |first2=Rachele |last3=Hayes |first3=Alex |last4=Lefevree |first4=Axel |last5=Tobie | first5=Gabriel |last6=Genovad |first6=Antonio |last7=Lunine |first7=Jonathan I. |last8=Zebker |first8=Howard |title=Shape, topography, gravity anomalies and tidal deformation of Titan |date= 2014 |journal=Icarus |doi=10.1016/j.icarus.2014.03.018 |volume=236 |pages=169–177 |bibcode = 2014Icar..236..169M }}</ref> On September 3, 2014, NASA reported studies suggesting [[Chemistry:Methane|methane]] rainfall on Titan may interact with a layer of icy materials underground, called an "alkanofer", to produce [[Chemistry:Ethane|ethane]] and [[Chemistry:Propane|propane]] that may eventually feed into rivers and lakes.<ref name="NASA-20140903">{{cite web |last1=Dyches |first1=Preston |last2=Mousis |first2=Olivier |last3=Altobelli |first3=Nicolas |title=Icy Aquifers on Titan Transform Methane Rainfall |url=https://www.jpl.nasa.gov/news/news.php?release=2014-294 |date=September 3, 2014 |work=[[Organization:NASA|NASA]] |access-date=September 4, 2014 |url-status=live |archive-url=https://web.archive.org/web/20140905000214/https://www.jpl.nasa.gov/news/news.php?release=2014-294 |archive-date=September 5, 2014  }}</ref>

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, ranges in depth from 240 to 570&nbsp;m and has sides as steep as 40°. They are believed to have formed either by crustal uplifting, like Earth's [[Earth:Grand Canyon|Grand Canyon]], 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.<ref>{{cite web|title=Cassini Finds Flooded Canyons on Titan|url=https://www.jpl.nasa.gov/news/news.php?release=2016-207|publisher=NASA|year=2016|access-date=August 12, 2016|url-status=live|archive-url=https://web.archive.org/web/20160811140908/https://www.jpl.nasa.gov/news/news.php?release=2016-207|archive-date=August 11, 2016}}</ref>

{|class="wikitable"
|[[File:PIA12481 Titan specular reflection.jpg|center|frameless|upright=0.75]]
|[[File:Liquid lakes on titan.jpg|center|frameless|upright=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 lakes
|-
|[[File:Titan 2009-01 ISS polar maps.jpg|center|frameless|upright=1.5]]
|[[File:Titan S. polar lake changes 2004-5.jpg|center|frameless|upright=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
|}
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=== Impact craters ===
[[File:Titancrater.jpg|thumb|left|Radar image of a 139&nbsp;km-diameter<ref name="Wood2009" /> impact crater on Titan's surface, showing a smooth floor, rugged rim, and possibly a [[Astronomy:Complex crater|central peak]].]]
Radar, SAR and imaging data from ''Cassini'' have revealed few impact craters on Titan's surface.<ref name="puzzling geologic" /> These impacts appear to be relatively young, compared to Titan's age.<ref name="puzzling geologic" /> The few impact craters discovered include a {{convert|392|km|mi|adj=mid|-wide|sp=us}} two-ring impact basin named [[Astronomy:Menrva (crater)|Menrva]] seen by ''Cassini's'' ISS as a bright-dark concentric pattern.<ref>{{cite web |url=https://photojournal.jpl.nasa.gov/catalog/PIA07365 |title=PIA07365: Circus Maximus |publisher=NASA |work=Planetary Photojournal |access-date=May 4, 2006 |url-status=live |archive-url=https://web.archive.org/web/20110718144030/https://photojournal.jpl.nasa.gov/catalog/PIA07365 |archive-date=July 18, 2011  }}</ref> A smaller, {{convert|80|km|mi|adj=mid|-wide|sp=us}}, flat-floored crater named [[Astronomy:Sinlap|Sinlap]]<ref>{{cite web |url=https://photojournal.jpl.nasa.gov/catalog/PIA07368 |title=PIA07368: Impact Crater with Ejecta Blanket |publisher=NASA |work=Planetary Photojournal |access-date=May 4, 2006 |url-status=live |archive-url=https://web.archive.org/web/20121105210015/https://photojournal.jpl.nasa.gov/catalog/PIA07368 |archive-date=November 5, 2012  }}</ref> and a {{convert|30|km|mi|abbr=on}} crater with a central peak and dark floor named Ksa have also been observed.<ref>{{cite web |url=https://photojournal.jpl.nasa.gov/catalog/PIA08737 |title=PIA08737: Crater Studies on Titan |publisher=NASA |work=Planetary Photojournal |access-date=September 15, 2006 |url-status=live |archive-url=https://web.archive.org/web/20120531003954/https://photojournal.jpl.nasa.gov/catalog/PIA08737 |archive-date=May 31, 2012  }}</ref> 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''.<ref name="Guabonito">{{cite web |url=https://photojournal.jpl.nasa.gov/catalog/PIA08425 |title=PIA08425: Radar Images the Margin of Xanadu |publisher=NASA |work=Planetary Photojournal |access-date=September 26, 2006 |url-status=live |archive-url=https://web.archive.org/web/20110608095543/https://photojournal.jpl.nasa.gov/catalog/PIA08425 |archive-date=June 8, 2011}}</ref> This feature is thought to be an impact crater filled in by dark, windblown sediment. Several other similar features have been observed in the dark [[Astronomy:Shangri-La (Titan)|Shangri-La]] and Aaru regions. Radar observed several circular features that may be craters in the bright region [[Astronomy:Xanadu (Titan)|Xanadu]] during ''Cassini's'' April 30, 2006 flyby of Titan.<ref name="Xcraters">{{cite web |url=https://photojournal.jpl.nasa.gov/catalog/PIA08429 |title=PIA08429: Impact Craters on Xanadu |publisher=NASA |work=Planetary Photojournal |access-date=September 26, 2006 |url-status=live |archive-url=https://web.archive.org/web/20120716051338/https://photojournal.jpl.nasa.gov/catalog/PIA08429 |archive-date=July 16, 2012  }}</ref>
[[File:PIA19052-SaturnMoon-Titan-LigeiaMare-SAR&DespeckledViews-20150212.jpg|thumb|Ligeia Mare – SAR and clearer despeckled views.<ref name="Lucas2014">{{cite journal | last1 = Lucas | display-authors = etal | year = 2014 | title = Insights into Titan's geology and hydrology based on enhanced image processing of Cassini RADAR data | url = https://authors.library.caltech.edu/53259/1/jgre20314.pdf | journal = Journal of Geophysical Research | volume = 119 | issue = 10 | pages = 2149–2166 | doi = 10.1002/2013JE004584 | bibcode = 2014JGRE..119.2149L | doi-access = free | access-date = December 7, 2019 | archive-date = July 1, 2021 | archive-url = https://web.archive.org/web/20210701174111/https://authors.library.caltech.edu/53259/1/jgre20314.pdf | url-status = live }}</ref>]]
Many of Titan's craters or probable craters display evidence of extensive erosion, and all show some indication of modification.<ref name="Wood2009" /> 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 [[Astronomy:Palimpsest (planetary astronomy)|palimpsests]] through viscoelastic crustal relaxation, unlike on other large [[Astronomy:Icy moon|icy moon]]s.<ref name="Wood2009" /> Most craters lack central peaks and have smooth floors, possibly due to impact-generation or later eruption of [[Astronomy:Cryovolcano|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.<ref>{{cite journal |last1=Ivanov |first1=B. A. |last2=Basilevsky |first2=A. T. |last3=Neukum |first3=G. |date=1997 |title=Atmospheric entry of large meteoroids: implication to Titan |journal=Planetary and Space Science |volume=45 |issue=8 |pages=993–1007 |doi=10.1016/S0032-0633(97)00044-5 |bibcode=1997P&SS...45..993I }}</ref>

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).<ref name="Wood2009">{{cite journal |last1=Wood |first1=C. A. |last2=Lorenz |first2=R. |last3=Kirk |first3=R. |last4=Lopes |first4=R. |last5=Mitchell |first5=K. |last6=Stofan |first6=E. |author7=The Cassini RADAR Team |title=Impact craters on Titan |journal=Icarus |volume=206 |issue=1 |pages=334–344 |date=September 6, 2009 |doi=10.1016/j.icarus.2009.08.021 |bibcode=2010Icar..206..334L }}</ref>

Pre-''Cassini'' models of impact trajectories and angles suggest that where the impactor strikes the water ice crust, a small amount of [[Astronomy:Ejecta|ejecta]] remains as liquid water within the crater. It may persist as liquid for centuries or longer, sufficient for "the synthesis of simple precursor molecules to the origin of life".<ref>{{cite journal |last1=Artemieva |first1=Natalia |last2=Lunine |first2=Jonathan I. |date=2003 |title=Cratering on Titan: impact melt, ejecta, and the fate of surface organics |journal=Icarus |volume=164 |issue=2 |pages=471–480 |doi=10.1016/S0019-1035(03)00148-9 |bibcode=2003Icar..164..471A }}</ref>

=== Cryovolcanism and mountains ===
[[File:Tortola Facula in infrared.jpg|thumb|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.<ref>{{cite journal |last=Owen |first=Tobias |title=Planetary science: Huygens rediscovers Titan |journal=Nature |volume=438 |pages=756–757 |date=2005 |doi=10.1038/438756a |pmid=16363022 |issue=7069 |bibcode=2005Natur.438..756O |s2cid=4421251 }}</ref> 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.<ref name="rain">{{cite web |title=Cassini Finds Hydrocarbon Rains May Fill The Lakes |author=Media Relations Office: Cassini Imaging Central Laboratory For Operations |publisher=Space Science Institute, Boulder, Colorado |date=2009 |url=https://ciclops.org/view.php?id=5471&js=1 |access-date=January 29, 2009 |url-status=live |archive-url=https://web.archive.org/web/20110725171249/https://ciclops.org/view.php?id=5471&js=1 |archive-date=July 25, 2011  }}</ref>

Still, there is a paucity of surface features that can be unambiguously interpreted as cryovolcanoes.<ref name="Moore2008">{{cite journal |last1=Moore |first1=J.M. |last2=Pappalardo |first2=R.T. |title=Titan: Callisto With Weather? |journal=American Geophysical Union, Fall Meeting |year=2008 |volume=11 |pages=P11D–06 |bibcode=2008AGUFM.P11D..06M}}</ref> One of the first of such features revealed by ''Cassini'' radar observations in 2004, called [[Astronomy:Ganesa Macula|Ganesa Macula]], resembles the geographic features called "[[Astronomy:Pancake dome|pancake dome]]s" found on Venus, and was thus initially thought to be cryovolcanic in origin, until Kirk et al. refuted this hypothesis at the [[Organization:American Geophysical Union|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.<ref>{{cite web |title=Shape and thermal modeling of the possible cryovolcanic dome Ganesa Macula on Titan: Astrobiological implications |last1=Neish |first1=C.D. |last2=Lorenz |first2=R.D. |last3=O'Brien |first3=D.P. |work=Lunar and Planetary Laboratory, University of Arizona, Observatoire de la Cote d'Azur |url=https://www.aas.org/publications/baas/v37n3/dps2005/257.htm |date=2005 |access-date=August 27, 2007 |archive-url=https://web.archive.org/web/20070814122704/https://www.aas.org/publications/baas/v37n3/dps2005/257.htm |archive-date=August 14, 2007 }}</ref><ref>{{cite web |title=Genesa Macula Isn't A Dome |last=Lakdawalla |first=Emily |publisher=The Planetary Society |date=2008 |url=https://www.planetary.org/blog/article/00001771/ |access-date=January 30, 2009 |url-status=live |archive-url=https://web.archive.org/web/20130618084806/https://www.planetary.org/blogs/emily-lakdawalla/2008/1771.html |archive-date=June 18, 2013  }}</ref> In 2004 ''Cassini'' also detected an unusually bright feature (called Tortola Facula), which was interpreted as a cryovolcanic dome.<ref name="Sotin2005">{{Cite journal| last1 = Sotin | first1 = C.| last2 = Jaumann | first2 = R.| last3 = Buratti | first3 = B.| last4 = Brown | first4 = R.| last5 = Clark | first5 = R.| last6 = Soderblom | first6 = L.| last7 = Baines | first7 = K.| last8 = Bellucci | first8 = G.| last9 = Bibring | first9 = J.| last10 = Capaccioni | first10 = F.| last11 = Cerroni | first11 = P.| last12 = Combes | first12 = M.| last13 = Coradini | first13 = A.| last14 = Cruikshank | first14 = D. P.| last15 = Drossart | first15 = P.| last16 = Formisano | first16 = V.| last17 = Langevin | first17 = Y.| last18 = Matson | first18 = D. L.| last19 = McCord | first19 = T. B.| last20 = Nelson | first20 = R. M.| last21 = Nicholson | first21 = P. D.| last22 = Sicardy | first22 = B.| last23 = Lemouelic | first23 = S.| last24 = Rodriguez | first24 = S.| last25 = Stephan | first25 = K.| last26 = Scholz | first26 = C. K.| title = Release of volatiles from a possible cryovolcano from near-infrared imaging of Titan| journal = Nature| volume = 435| issue = 7043| pages = 786–789| year = 2005| pmid = 15944697| doi = 10.1038/nature03596|bibcode = 2005Natur.435..786S | s2cid = 4339531| url = https://hal-univ-paris.archives-ouvertes.fr/hal-03657592/file/Nature_VIMS_final%20%281%29.pdf}}</ref> No similar features have been identified as of 2010.<ref name="LeCorre">{{cite journal |last1=LeCorre |first1=L. |last2=LeMouélic |first2=S. |last3=Sotin |first3=C. |title=Cassini/VIMS observations of cryo-volcanic features on Titan |date=2008 |url=https://www.lpi.usra.edu/meetings/lpsc2008/pdf/1932.pdf |journal=Lunar and Planetary Science |volume=XXXIX |issue=1391 |page=1932 |bibcode=2008LPI....39.1932L |url-status=live |archive-url=https://web.archive.org/web/20121025054055/https://www.lpi.usra.edu/meetings/lpsc2008/pdf/1932.pdf |archive-date=October 25, 2012  }}</ref> 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.<ref name="longstaff">{{cite journal |title=Is Titan (cryo)volcanically active? |last=Longstaff |first=Alan |journal=Royal Observatory, Greenwich (Astronomy Now) |date=February 2009 |page=19 }}</ref>

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.<ref>{{cite news |url=https://news.bbc.co.uk/2/hi/science/nature/6174501.stm |title=Mountain range spotted on Titan |work=BBC News |date=December 12, 2006 |access-date=August 6, 2007 |url-status=live |archive-url=https://web.archive.org/web/20121031002110/https://news.bbc.co.uk/2/hi/science/nature/6174501.stm |archive-date=October 31, 2012  }}</ref> Prior to ''Cassini'', scientists assumed that most of the topography on Titan would be impact structures, yet these findings reveal that similar to Earth, the mountains were formed through geological processes.<ref>{{cite news |url=https://newswise.com/articles/view/536441/ |title=Mountains Discovered on Saturn's Largest Moon |work=Newswise |access-date=July 2, 2008 |url-status=live |archive-url=https://web.archive.org/web/20130531075903/https://newswise.com/articles/view/536441/ |archive-date=May 31, 2013  }}</ref>

In 2008 Jeffrey Moore (planetary geologist of [[Organization:Ames Research Center|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, [[Earth:Mass wasting|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 [[Earth:Escarpment|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''.<ref name="Moore2008" /><ref name="Lakdawalla2008">{{cite web |last=Lakdawalla |first=Emily |title=AGU: Titan: Volcanically active world, or "Callisto with weather? |url=https://www.planetary.org/blog/article/00001775/ |access-date=October 11, 2010 |publisher=The Planetary Society |date=December 17, 2008 |url-status=live |archive-url=https://web.archive.org/web/20130618091010/https://www.planetary.org/blogs/emily-lakdawalla/2008/1775.html |archive-date=June 18, 2013  }}</ref>

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 erupted from beneath the surface.<ref>{{cite journal |title=Giant 'ice flows' bolster case for Titan's volcanoes |last=Shiga |first=David |journal=New Scientist |date=March 28, 2009 }}</ref>

In December 2010, the ''Cassini'' mission team announced the most compelling possible cryovolcano yet found. Named [[Astronomy:Sotra Patera|Sotra Patera]], it is one in a chain of at least three mountains, each between 1000 and 1500&nbsp;m in height, several of which are topped by large craters. The ground around their bases appears to be overlaid by frozen lava flows.<ref>{{cite web |title=Saturn Moon Has Ice Volcano—And Maybe Life? |first=Richard A. |last=Lovett |work=National Geographic |url=https://news.nationalgeographic.com/news/2010/12/101215-saturn-moon-titan-ice-volcano-nasa-science-pictures/ |date=2010 |access-date=December 19, 2010 |url-status=dead |archive-url=https://web.archive.org/web/20121019073955/https://news.nationalgeographic.com/news/2010/12/101215-saturn-moon-titan-ice-volcano-nasa-science-pictures/ |archive-date=October 19, 2012  }}</ref>

Crater-like landforms possibly formed via explosive, [[Earth:Maar|maar]]-like or [[Earth:Caldera|caldera]]-forming cryovolcanic eruptions have been identified in Titan's polar regions.<ref name="Wood2020">{{cite journal|last1= Wood|first1= C.A.|last2= Radebaugh|first2= J.|title= Morphologic Evidence for Volcanic Craters near Titan's North Polar Region|journal= Journal of Geophysical Research: Planets|year= 2020|volume= 125|issue= 8|pages= e06036|doi= 10.1029/2019JE006036|bibcode= 2020JGRE..12506036W|s2cid= 225752345}}</ref> These formations are sometimes nested or overlapping and have features suggestive of explosions and collapses, such as elevated rims, halos, and internal hills or mountains.<ref name="Wood2020" /> The polar location of these features and their colocalization with Titan's lakes and seas suggests volatiles such as methane may help power them. Some of these features appear quite fresh, suggesting that such volcanic activity continues to the present.<ref name="Wood2020" />

Most of Titan's highest peaks occur near its equator in so-called "ridge belts". They are believed to be analogous to Earth's [[Earth:Fold mountains|fold mountains]] such as the Rockies or the Himalayas, formed by the collision and buckling of tectonic plates, or to subduction zones like the [[Place:Andes|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&nbsp;m tall.<ref>{{cite web|title=Cassini Spies Titan's Tallest Peaks|url=https://www.nasa.gov/feature/jpl/cassini-spies-titans-tallest-peaks|publisher=NASA|year=2016|access-date=August 12, 2016|url-status=live|archive-url=https://web.archive.org/web/20160819154049/https://www.nasa.gov/feature/jpl/cassini-spies-titans-tallest-peaks/|archive-date=August 19, 2016}}</ref>

[[File:Sotra Facula.jpg|thumb|False-color VIMS image of the possible cryovolcano Sotra Patera, combined with a 3D map based on radar data, showing 1000-meter-high peaks and a 1500-meter-deep crater.]]
If 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.<ref name="longstaff" /> 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.<ref name="longstaff" /> <!--Alternatively, the pressure necessary to drive the cryovolcanoes may be caused by the weight of the ice shell.{{cn}}--> The low-pressure ice, overlaying a liquid layer of [[Chemistry:Ammonium sulfate|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.<ref>{{cite journal |last1=Fortes |first1=A. D. |last2=Grindroda |first2=P.M. |last3=Tricketta |first3=S. K. |last4=Vočadloa |first4=L. |date=May 2007 |title=Ammonium sulfate on Titan: Possible origin and role in cryovolcanism |journal=Icarus |volume=188 |issue=1 |pages=139–153 |doi=10.1016/j.icarus.2006.11.002 |bibcode=2007Icar..188..139F }}</ref> Titan may have been much more geologically active in the past; models of Titan's internal evolution suggest that Titan's crust was only 10 kilometers thick until about 500 million years ago, allowing vigorous cryovolcanism with low viscosity water magmas to erase all surface features formed before that time. Titan's modern geology would have formed only after the crust thickened to 50 kilometers and thus impeded constant cryovolcanic resurfacing, with any cryovolcanism occurring since that time producing much more viscous water magma with larger fractions of ammonia and methanol; this would also suggest that Titan's methane is no longer being actively added to its atmosphere and could be depleted entirely within a few tens of millions of years.<ref>{{Cite web|last=Wood|first=C.A|title=Titan's Global Crustal Thickening Event|url=https://www.hou.usra.edu/meetings/lpsc2019/pdf/3032.pdf|access-date=26 February 2021|website=[[Organization:Universities Space Research Association|Universities Space Research Association]]|archive-date=July 1, 2021|archive-url=https://web.archive.org/web/20210701174116/https://www.hou.usra.edu/meetings/lpsc2019/pdf/3032.pdf|url-status=live}}</ref>

Many of the more prominent mountains and hills have been given official names by the [[Organization:International Astronomical Union|International Astronomical Union]]. According to [[Organization:Jet Propulsion Laboratory|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.<ref name="jrr">{{citation |title=Mountains of Titan Map – 2016 Update |url=https://saturn.jpl.nasa.gov/resources/71 |publisher=[[Organization:Jet Propulsion Laboratory|NASA JPL]] |date=March 23, 2016 |access-date=October 31, 2016 |url-status=live |archive-url=https://web.archive.org/web/20161101102622/https://saturn.jpl.nasa.gov/resources/71/ |archive-date=November 1, 2016  }}</ref>

=== Dark equatorial terrain ===
[[File:Titan dunes crop.png|thumb|upright|Sand dunes in the Namib Desert on Earth (top), compared with dunes in Belet on Titan]]
In the first images of Titan's surface taken by Earth-based telescopes in the early 2000s, large regions of dark terrain were revealed straddling Titan's equator.<ref name="Roe">{{cite journal |last=Roe |first=H. G. |date=2004 |bibcode=2004GeoRL..3117S03R |title=A new 1.6-micron map of Titan's surface |journal=Geophys. Res. Lett. |volume=31 |issue=17 |page=L17S03 |doi=10.1029/2004GL019871 |citeseerx=10.1.1.67.3736 |s2cid=13877191 |url=https://authors.library.caltech.edu/34661/1/2004GL019871.pdf |access-date=December 7, 2019 |archive-date=July 1, 2021 |archive-url=https://web.archive.org/web/20210701174121/https://authors.library.caltech.edu/34661/1/2004GL019871.pdf |url-status=live }}</ref> Prior to the arrival of ''Cassini'', these regions were thought to be seas of liquid hydrocarbons.<ref>{{cite journal |title=The Glitter of Distant Seas |last=Lorenz |first=R. |journal=Science |date=2003 |volume=302 |pages=403–404 |doi=10.1126/science.1090464 |pmid=14526089 |issue=5644 |s2cid=140157179 |url=https://pdfs.semanticscholar.org/8196/7172a61bfadcf4e1d7b631dce88be7760a2a.pdf |archive-url=https://web.archive.org/web/20200215215055/https://pdfs.semanticscholar.org/8196/7172a61bfadcf4e1d7b631dce88be7760a2a.pdf |url-status=dead |archive-date=2020-02-15 }}</ref> Radar images captured by the ''Cassini'' spacecraft have instead revealed some of these regions to be extensive plains covered in longitudinal [[Earth:Dune|dune]]s, up to {{convert|330|ft|m|abbr=on}} high,<ref name="Saharan" /> about a kilometer wide, and tens to hundreds of kilometers long.<ref>{{cite journal |last=Lorenz |first=R. D. |title=Winds of Change on Titan |journal=Science |volume=329 |issue=5991 |pages=519–20 |date=July 30, 2010 |pmid=20671175 |doi=10.1126/science.1192840 |bibcode=2010Sci...329..519L |s2cid=41624889 }}</ref> Dunes of this type are always aligned with average wind direction. In the case of Titan, steady [[Zonal and meridional|zonal]] (eastward) winds combine with variable tidal winds (approximately 0.5 meters per second).<ref name="Lorenz2006" /> The tidal winds are the result of [[Physics:Tidal force|tidal force]]s from Saturn on Titan's atmosphere, which are 400 times stronger than the tidal forces of the Moon on Earth and tend to drive wind toward the equator. This wind pattern, 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.<ref>{{cite web |date=2006-05-10 |title=Study of Saturn's moon finds Titan's liquid oceans are likely solid seas of sand |url=http://news.stanford.edu/news/2006/may10/titan-051006.html |access-date=2022-06-09 |website=Stanford University |language=en | archive-url = https://web.archive.org/web/20110801021958/http://news.stanford.edu/news/2006/may10/titan-051006.html | archive-date = 1 August 2011}}</ref>

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 [[Astronomy:Equinox|equinox]]. These storms produce strong downdrafts, flowing eastward at up to 10 meters per second when they reach the surface.<ref>{{cite web|date=2015|title=Violent Methane Storms on Titan May Explain Dune Direction|url=https://spaceref.com/saturn/violent-methane-storms-on-titan-may-explain-dune-direction.html|archive-url=https://archive.today/20150419103020/http://spaceref.com/saturn/violent-methane-storms-on-titan-may-explain-dune-direction.html|url-status=dead|archive-date=April 19, 2015|access-date=April 19, 2015|publisher=Spaceref}}</ref>

The "sand" on Titan is likely not made up of small grains of [[Chemistry:Silicate|silicate]]s like the sand on Earth,<ref>{{cite web |title=Cassini Sees the Two Faces of Titan's Dunes |url=https://www.jpl.nasa.gov/news/news.php?release=2012-021 |publisher=JPL, NASA |url-status=live |archive-url=https://web.archive.org/web/20130502160658/https://www.jpl.nasa.gov/news/news.php?release=2012-021 |archive-date=May 2, 2013  }}</ref> 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 [[Astronomy:Tholin|tholin]]s, produced by photochemical reactions in Titan's atmosphere.<ref name="Saharan">{{cite news |title=Saharan Sand Dunes Found on Saturn's Moon Titan |url=https://www.space.com/2371-saharan-sand-dunes-saturn-moon-titan.html |first=Sara |last=Goudarzi |publisher=SPACE.com |date=May 4, 2006 |access-date=August 6, 2007 |url-status=live |archive-url=https://web.archive.org/web/20110804233044/https://www.space.com/2371-saharan-sand-dunes-saturn-moon-titan.html |archive-date=August 4, 2011  }}</ref><ref name="Lorenz2006">{{cite journal |title=The sand seas of Titan: Cassini RADAR observations of longitudinal dunes |last1=Lorenz |first1=RD |last2=Wall |first2=S |last3=Radebaugh |first3=J |last4=Boubin |first4=G |last5=Reffet |first5=E |last6=Janssen |first6=M |last7=Stofan |first7=E |last8=Lopes |first8=R |last9=Kirk |first9=R |display-authors=8 |journal=Science |date=2006 |volume=312 |pages=724–727 |doi=10.1126/science.1123257 |pmid=16675695 |issue=5774 |bibcode=2006Sci...312..724L |s2cid=39367926 |url=https://authors.library.caltech.edu/51871/7/Lorenz.SOM.pdf |access-date=April 12, 2020 |archive-date=July 23, 2018 |archive-url=https://web.archive.org/web/20180723202819/https://authors.library.caltech.edu/51871/7/Lorenz.SOM.pdf |url-status=live }}</ref><ref>{{cite journal |title=Linear Dunes on Titan |last=Lancaster |first=N. |journal=Science |date=2006 |volume=312 |pages=702–703 |doi=10.1126/science.1126292 |pmid=16675686 |issue=5774 |s2cid=126567530 }}</ref> 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 [[Earth:Soot|soot]] like hydrocarbon polymers clumping together after raining onto the surface.<ref>{{cite web |title=Titan's Smoggy Sand Grains |publisher=JPL, NASA |date=2008 |url=https://www.jpl.nasa.gov/news/features.cfm?feature=1679 |access-date=May 6, 2008 |url-status=live |archive-url=https://web.archive.org/web/20130523190919/https://www.jpl.nasa.gov/news/news.php?feature=1679 |archive-date=May 23, 2013  }}</ref> Calculations indicate the sand on Titan has a density of one-third that of terrestrial sand.<ref>{{cite web |title=Dunes on Titan need firm winds to move |publisher=Spaceref |date=2015 |url=https://spaceref.com/saturn/dunes-on-titan-need-firm-winds-to-move.html |archive-url=https://archive.today/20150423163836/http://spaceref.com/saturn/dunes-on-titan-need-firm-winds-to-move.html |url-status=dead |archive-date=April 23, 2015 |access-date=April 23, 2015 }}</ref> 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.<ref>{{cite journal |title=Electrified sand could explain Titan's backward dunes |journal=New Scientist |date=March 27, 2017 |pages=18 |first=Leah |last=Crane |url=https://www.newscientist.com/article/2125780-electrified-sand-could-explain-titans-odd-backward-facing-dunes/ |access-date=February 4, 2021 |archive-date=November 12, 2020 |archive-url=https://web.archive.org/web/20201112020535/https://www.newscientist.com/article/2125780-electrified-sand-could-explain-titans-odd-backward-facing-dunes/ |url-status=live }}</ref>

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.<ref name="Rodriguez2018">{{cite journal|last1= Rodriguez|first1= S.|last2= Le Mouélic|first2= S.|last3= Barnes|first3= J. W.|last4= Kok|first4= J. F.|last5= Rafkin|first5= S. C. R.|last6= Lorenz|first6= R. D.|last7= Charnay|first7= B.|last8= Radebaugh|first8= J.|last9= Narteau|first9= C.|last10= Cornet|first10= T.|last11= Bourgeois|first11= O.|last12= Lucas|first12= A.|last13= Rannou|first13= P.|last14= Griffith|first14= C. A.|last15= Coustenis|first15= A.|last16= Appéré|first16= T.|last17= Hirtzig|first17= M.|last18= Sotin|first18= C.|last19= Soderblom|first19= J. M.|last20= Brown|first20= R. H.|last21= Bow|first21= J.|last22= Vixie|first22= G.|last23= Maltagliati|first23= L.|last24= Courrech du Pont|first24= S.|last25= Jaumann|first25= R.|last26= Stephan|first26= K.|last27= Baines|first27= K. H.|last28= Buratti|first28= B. J.|last29= Clark|first29= R. N.|last30= Nicholson|first30= P. D.|title= Observational evidence for active dust storms on Titan at equinox|journal= Nature Geoscience|volume= 11|issue= 10|pages= 727–732|year= 2018|doi= 10.1038/s41561-018-0233-2|bibcode= 2018NatGe..11..727R|s2cid= 134006536|display-authors= 3|url= https://elib.dlr.de/124618/1/ill-req2.pdf|access-date= December 7, 2019|archive-date= July 1, 2021|archive-url= https://web.archive.org/web/20210701174117/https://elib.dlr.de/124618/1/ill-req2.pdf|url-status= live}}</ref>

[[File:PIA22484-SaturnMoon-Titan-3DustStorms-20180924.jpg|thumb|400px|center|{{center|1=Titan – three dust storms detected in 2009–2010.<ref name="NASA-20180924">{{cite web |last1=McCartney |first1=Gretchen |last2=Brown |first2=Dwayne |last3=Wendel |first3=JoAnna |last4=Bauer |first4=Markus |title=Dust Storms on Titan Spotted for the First Time |url=https://www.jpl.nasa.gov/news/news.php?feature=7243 |date=September 24, 2018 |work=NASA |access-date=September 24, 2018 |archive-date=January 11, 2021 |archive-url=https://web.archive.org/web/20210111223938/https://www.jpl.nasa.gov/news/news.php?feature=7243 |url-status=live }}</ref>}}]]

== Observation and exploration ==
[[File:Titan's thick haze layer-picture from voyager1.jpg|thumb|upright|''Voyager 1'' view of haze on Titan's limb (1980)]]

Titan is never visible to the naked eye, but can be observed through small telescopes or strong binoculars. Amateur observation is difficult because of the proximity of 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.<ref>{{cite book |last=Benton |first=Julius L. Jr. |pages=[https://archive.org/details/saturnhowtoobser00bent/page/n144 141]–146 |title=Saturn and How to Observe It |url=https://archive.org/details/saturnhowtoobser00bent |url-access=limited |date=2005 |publisher=Springer |location=London |doi=10.1007/1-84628-045-1_9 |isbn=978-1-84628-045-0 }}</ref> Titan has a maximum [[Astronomy:Apparent magnitude|apparent magnitude]] of +8.2,<ref name="arval">{{cite web |title=Classic Satellites of the Solar System |url=https://www.oarval.org/ClasSaten.htm |publisher=Observatorio ARVAL |access-date=June 28, 2010 |url-status=dead |archive-url=https://web.archive.org/web/20110709203915/https://www.oarval.org/ClasSaten.htm |archive-date=July 9, 2011  }}</ref> and mean opposition magnitude 8.4.<ref name="jpl-sat">{{cite web |title=Planetary Satellite Physical Parameters |publisher=[[Organization:JPL|JPL]] (Solar System Dynamics) |url=https://ssd.jpl.nasa.gov/?sat_phys_par |date=April 3, 2009 |access-date=June 29, 2010 |url-status=live |archive-url=https://web.archive.org/web/20090522002331/https://ssd.jpl.nasa.gov/?sat_phys_par |archive-date=May 22, 2009  }}</ref> This compares to +4.6 for the similarly sized Ganymede, in the Jovian system.<ref name="jpl-sat" />

Observations of Titan prior to the space age were limited. In 1907 Spanish astronomer [[Biography:Josep Comas i Solà|Josep Comas i Solà]] observed [[Astronomy:Limb darkening|limb darkening]] of Titan, the first evidence that the body has an atmosphere. In 1944 Gerard P. Kuiper used a [[Physics:Spectroscopy|spectroscopic technique]] to detect an atmosphere of methane.<ref name="Kuiper">{{cite journal |last=Kuiper |first=G. P. |date=1944 |title=Titan: a Satellite with an Atmosphere |journal=Astrophysical Journal |volume=100 |doi=10.1086/144679 |page=378 |bibcode=1944ApJ...100..378K }}</ref>

=== Fly-by missions: Pioneer and Voyager ===
The first probe to visit the Saturnian system was ''[[Astronomy:Pioneer 11|Pioneer 11]]'' in 1979, which revealed that Titan was probably too cold to support life.<ref>{{cite web |date=March 26, 2007 |title=The Pioneer Missions |publisher=NASA, Jet Propulsion Laboratory |work=Pioneer Project |url=https://www.nasa.gov/centers/ames/missions/archive/pioneer.html |access-date=August 19, 2007 |url-status=live |archive-url=https://web.archive.org/web/20110629033952/https://www.nasa.gov/centers/ames/missions/archive/pioneer.html |archive-date=June 29, 2011  }}</ref> It took images of Titan, including Titan and Saturn together in mid to late 1979.<ref>{{cite web |title=40 Years Ago: Pioneer 11 First to Explore Saturn |publisher=NASA |date=September 3, 2019 |url=https://www.nasa.gov/feature/40-years-ago-pioneer-11-first-to-explore-saturn |access-date=February 22, 2020 |archive-date=August 24, 2021 |archive-url=https://web.archive.org/web/20210824131858/https://www.nasa.gov/feature/40-years-ago-pioneer-11-first-to-explore-saturn/ |url-status=live }}</ref> The quality was soon surpassed by the two ''Voyagers''.<ref>{{cite web|date=November 21, 2021|title=Voyager Camera Desc|publisher=Planetary Data System|url=https://pds-rings.seti.org/voyager/iss/inst_cat_wa1.html|access-date=November 21, 2021|archive-date=November 7, 2021|archive-url=https://web.archive.org/web/20211107025433/https://pds-rings.seti.org/voyager/iss/inst_cat_wa1.html|url-status=live}}</ref>

Titan was examined by both ''[[Astronomy:Voyager 1|Voyager 1]]'' and ''[[Astronomy:Voyager 2|2]]'' in 1980 and 1981, respectively. ''Voyager 1''<nowiki>'</nowiki>s 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.<ref name="Bell2015">{{cite book|author=Bell, Jim|title=The Interstellar Age: Inside the Forty-Year Voyager Mission|url=https://books.google.com/books?id=KXPoAwAAQBAJ&pg=PT93|date=February 24, 2015|publisher=Penguin Publishing Group|isbn=978-0-698-18615-6|page=93|url-status=live|archive-url=https://web.archive.org/web/20160904191620/https://books.google.com/books?id=KXPoAwAAQBAJ&pg=PT93|archive-date=September 4, 2016}}</ref> Atmospheric haze prevented direct imaging of the surface, though in 2004 intensive digital processing of images taken through ''Voyager 1''<nowiki>'</nowiki>s orange filter did reveal hints of the light and dark features now known as [[Astronomy:Xanadu (Titan)|Xanadu]] and Shangri-la,<ref>{{cite journal |last1=Richardson |first1=J. |last2=Lorenz |first2=Ralph D. |last3=McEwen |first3=Alfred |title=Titan's Surface and Rotation: New Results from Voyager 1 Images |journal=Icarus |date=2004 |volume=170 |issue=1 |pages=113–124 |doi=10.1016/j.icarus.2004.03.010 |bibcode=2004Icar..170..113R }}</ref> 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.<ref name="Bell2015" />{{rp|94}}

=== ''Cassini–Huygens'' ===
{{Main|Astronomy:Cassini–Huygens|Engineering:Huygens (spacecraft)}}
{{multiple image
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| caption1 = ''Cassini'' image of Titan in front of the [[Astronomy:Rings of Saturn|rings of Saturn]]
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| caption2 = ''Cassini'' image of Titan, behind [[Astronomy:Epimetheus (moon)|Epimetheus]] and the rings
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[[File:NASA-Cassini-Saturn-TitanFlybyTests-20140617.jpg|thumb|Cassini's Titan flyby radio signal studies (artist's concept)]]

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 ''Cassini–Huygens'' spacecraft reached Saturn on July 1, 2004, and began the process of mapping Titan's surface by [[Engineering:Radar|radar]]. A joint project of the [[Organization:European Space Agency|European Space Agency]] (ESA) and [[Organization:NASA|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.{{Citation needed|date=February 2021}}

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.<ref>{{cite web |title=Cassini Equinox Mission: Titan Flyby (T-70) – June 21, 2010 |publisher=NASA/JPL |url=https://saturn.jpl.nasa.gov/mission/flybys/titan20100621/ |access-date=July 8, 2010 |url-status=dead |archive-url=https://web.archive.org/web/20120318223434/https://saturn.jpl.nasa.gov/mission/flybys/titan20100621/ |archive-date=March 18, 2012  }}</ref> 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''.<ref name="PIA08630" />

==== ''Huygens'' landing ====
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| caption1          = ''Huygens'' ''[[Biology:In situ|in situ]]'' image from Titan's surface—the only image from the surface of a body permanently farther away than Mars
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<!-- [[File:Huygens surface color.jpg|thumb|upright|''Huygens'' image ''in situ'' from Titan's surface—the only image from the surface of a planetary body outside the inner Solar system]]
[[File:Huygens surface color sr.jpg|thumb|upright|Same with different data processing]] -->

''Huygens'' was an atmospheric probe that touched down on Titan on January 14, 2005,<ref>{{cite journal |first1=Steve |last1=Lingard |last2=Norris |first2=Pat |url=https://www.ingenia.org.uk/Ingenia/Articles/7cf998f7-b886-42b7-8f1e-4596af1354e8 |title=How To Land on Titan |issue=23 |date=June 2005 |journal=Ingenia Magazine|access-date=January 11, 2009 |url-status=live |archive-url=https://web.archive.org/web/20110721163014/https://www.ingenia.org.uk/ingenia/articles.aspx?Index=317 |archive-date=July 21, 2011  }}</ref> discovering that many of its surface features seem to have been formed by fluids at some point in the past.<ref name="huygens_picture_saturn">{{cite web |url=https://saturn.jpl.nasa.gov/operations/index.cfm |title=Cassini at Saturn: Introduction |access-date=September 6, 2007 |publisher=NASA, Jet Propulsion Laboratory |url-status=dead |archive-url=https://web.archive.org/web/20090403131313/https://saturn.jpl.nasa.gov/operations/index.cfm |archive-date=April 3, 2009  }}</ref> Titan is the most distant body from Earth to have a space probe land on its surface.<ref>{{cite web |url=https://www.spacetoday.org/SolSys/Saturn/SaturnHuygens.html |title=Huygens Exposes Titan's Surface |access-date=August 19, 2007 |work=Space Today |url-status=live |archive-url=https://web.archive.org/web/20110807190437/https://www.spacetoday.org/SolSys/Saturn/SaturnHuygens.html |archive-date=August 7, 2011  }}</ref>
[[File:Huygens descent.ogv|thumb|The ''[[Engineering:Huygens (spacecraft)|Huygens]]'' probe descends by parachute and lands on Titan on January 14, 2005]]
The ''Huygens'' probe landed just off the easternmost tip of a bright region now called [[Astronomy:Adiri (Titan)|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.<ref name="Seeing_Touching_Titan-ESA">{{cite news |url=https://www.esa.int/SPECIALS/Cassini-Huygens/SEMHB881Y3E_0.html |title=Seeing, touching and smelling the extraordinarily Earth-like world of Titan |publisher=ESA News, European Space Agency |date=January 21, 2005 |access-date=March 28, 2005 |url-status=live |archive-url=https://web.archive.org/web/20111007111129/https://www.esa.int/SPECIALS/Cassini-Huygens/SEMHB881Y3E_0.html |archive-date=October 7, 2011  }}</ref>

After landing, ''Huygens'' photographed a dark plain covered in small rocks and pebbles, which are composed of water ice.<ref name="Seeing_Touching_Titan-ESA" /> The two rocks just below the middle of the image on the right are smaller than they may appear: the left-hand one is 15&nbsp;centimeters across, and the one in the center is 4&nbsp;centimeters across, at a distance of about 85&nbsp;centimeters from ''Huygens''. There is evidence of [[Earth:Erosion|erosion]] at the base of the rocks, indicating possible [[Earth:Fluvial processes|fluvial]] activity. The ground surface is darker than originally expected, consisting of a mixture of water and hydrocarbon ice.<ref>{{cite web|url=https://photojournal.jpl.nasa.gov/catalog/PIA07232|title=PIA07232: First Color View of Titan's Surface|publisher=NASA/JPL/ESA/University of Arizona|date=January 15, 2005|access-date=February 13, 2021|archive-date=May 6, 2021|archive-url=https://web.archive.org/web/20210506114620/https://photojournal.jpl.nasa.gov/catalog/PIA07232|url-status=live}}</ref>

In March 2007, NASA, ESA, and COSPAR decided to name the ''Huygens'' landing site the ''[[Biography:Hubert Curien|Hubert Curien]] Memorial Station'' in memory of the former president of the ESA.<ref>{{cite web |url=https://www.esa.int/esaCP/SEM9GNN0LYE_index_0.html |title=Huygens landing site to be named after Hubert Curien |publisher=ESA |date=March 5, 2007 |access-date=August 6, 2007 |url-status=live |archive-url=https://web.archive.org/web/20120303201334/https://www.esa.int/esaCP/SEM9GNN0LYE_index_0.html |archive-date=March 3, 2012  }}</ref>

=== ''Dragonfly'' ===
{{Main|Engineering:Dragonfly (spacecraft)}}

The [[Engineering:Dragonfly (spacecraft)|''Dragonfly'']] mission, developed and operated by the [[Organization:Applied Physics Laboratory|Johns Hopkins Applied Physics Laboratory]], will launch in July 2028.<ref name="sn-20231128">{{cite web |last=Foust |first=Jeff |url=https://spacenews.com/nasa-postpones-dragonfly-review-launch-date/ |title=NASA postpones Dragonfly review, launch date |work=SpaceNews |date=28 November 2023 |access-date=28 November 2023}}</ref> It consists of a large drone powered by an [[Physics:Radioisotope thermoelectric generator|RTG]] to fly in the atmosphere of Titan as [[Astronomy:New Frontiers program|New Frontiers]] 4.<ref>{{Cite web|url=https://twitter.com/JimBridenstine/status/1144334797101260800|title=New Science Mission to Explore Our Solar System|last=Bridenstine|first=Jim|date=June 27, 2019|website=Twitter|access-date=June 27, 2019|archive-date=January 27, 2020|archive-url=https://web.archive.org/web/20200127140150/https://twitter.com/JimBridenstine/status/1144334797101260800|url-status=live}}</ref><ref name="NYT-20190627">{{cite news |last=Brown |first=David W. |title=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. |url=https://www.nytimes.com/2019/06/27/science/nasa-titan-dragonfly-caesar.html |date=June 27, 2019 |work={{wipe|The New York Times}} |access-date=June 27, 2019 |archive-date=May 20, 2020 |archive-url=https://web.archive.org/web/20200520124851/https://www.nytimes.com/2019/06/27/science/nasa-titan-dragonfly-caesar.html |url-status=live }}</ref> Its instruments will study how far [[Biology:Abiogenesis|prebiotic chemistry]] may have progressed.<ref name="dragonfly">[https://dragonfly.jhuapl.edu/docs/DragonflyTechDigestAPL.pdf Dragonfly: A Rotorcraft Lander Concept for Scientific Exploration at Titan]. {{Webarchive|url=https://web.archive.org/web/20171222053305/https://dragonfly.jhuapl.edu/docs/DragonflyTechDigestAPL.pdf |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).</ref> The mission is planned to arrive at Titan in the mid-2030s.<ref name="NYT-20190627" />

=== Proposed or conceptual missions ===
[[File:TSSM-TandEM-Montgolfiere.jpg|thumb|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 [[Engineering:Space probe|space probe]] to Titan. Initial conceptual work has been completed for such missions by NASA (and [[Organization:Jet Propulsion Laboratory|JPL]]), and [[Organization:European Space Agency|ESA]]. At present, none of these proposals have become funded missions.{{Citation needed|date=February 2021}}

The [[Astronomy:Titan Saturn System Mission|Titan Saturn System Mission]] (TSSM) was a joint NASA/[[Organization:European Space Agency|ESA]] proposal for exploration of [[Astronomy:Saturn|Saturn]]'s moons.<ref>{{cite web |title=Mission Summary: TANDEM/TSSM Titan and Enceladus Mission |publisher=ESA |date=2009 |url=https://sci.esa.int/science-e/www/area/index.cfm?fareaid=106 |access-date=January 30, 2009 |url-status=live |archive-url=https://web.archive.org/web/20110523081148/https://sci.esa.int/science-e/www/area/index.cfm?fareaid=106 |archive-date=May 23, 2011  }}</ref> 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.<ref>{{cite news |last=Rincon |first=Paul |date=February 18, 2009 |title=Jupiter in space agencies' sights |work=BBC News |url=https://news.bbc.co.uk/2/hi/science/nature/7897585.stm |url-status=live |archive-url=https://web.archive.org/web/20101024015124/https://news.bbc.co.uk/2/hi/science/nature/7897585.stm |archive-date=October 24, 2010  }}</ref>

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.<ref name="talk 2010">{{cite web |url=https://www.kiss.caltech.edu/workshops/titan2010/presentations/aharonson.pdf |title=TiME: Titan Mare Explorer |access-date=August 17, 2011 |last=Stofan |first=Ellen |date=2010 |publisher=Caltech |url-status=dead |archive-url=https://web.archive.org/web/20120330205710/https://www.kiss.caltech.edu/workshops/titan2010/presentations/aharonson.pdf |archive-date=March 30, 2012  }}</ref><ref name="Taylor">{{cite news |first=Kate |last=Taylor |title=NASA picks project shortlist for next Discovery mission |date=May 9, 2011 |url=https://www.tgdaily.com/space-features/55816-nasa-picks-project-shortlist-for-next-discovery-mission |work=TG Daily |access-date=May 20, 2011 |url-status=live |archive-url=https://web.archive.org/web/20120904204847/https://www.tgdaily.com/space-features/55816-nasa-picks-project-shortlist-for-next-discovery-mission |archive-date=September 4, 2012  }}</ref><ref name="NPR">{{cite news |first=Nell |last=Greenfieldboyce |title=Exploring A Moon By Boat |date=September 16, 2009 |publisher=National Public Radio (NPR) |url=https://www.npr.org/templates/story/story.php?storyId=112835248 |access-date=November 8, 2009 |url-status=live |archive-url=https://web.archive.org/web/20120825092930/https://www.npr.org/templates/story/story.php?storyId=112835248 |archive-date=August 25, 2012  }}</ref> It was selected for a Phase-A design study in 2011 as a candidate mission for the 12th NASA [[Astronomy:Discovery Program|Discovery Program]] opportunity,<ref>{{cite web |website=NASA Discovery Program |url=https://discovery.msfc.nasa.gov/news/index.cfml?ID=1034 |title=NASA Announces Three New Mission Candidates |archive-url=https://web.archive.org/web/20161118221519/https://discovery.msfc.nasa.gov/news/index.cfml?ID=1034 |archive-date=November 18, 2016 |date=May 5, 2011 |url-status=dead |access-date=June 13, 2017 }}</ref> but was not selected for flight.<ref>{{cite web |url=https://www.scientificamerican.com/article.cfm?id=lets-go-sailing-on-lakes-of-titan-2009-11 |title=Let's go sailing on lakes of Titan! |website=Scientific American |archive-url=https://web.archive.org/web/20121010200010/https://www.scientificamerican.com/article.cfm?id=lets-go-sailing-on-lakes-of-titan-2009-11 |archive-date=October 10, 2012 |date=November 1, 2009 }}</ref>

Another mission to Titan proposed in early 2012 by Jason Barnes, a scientist at the [[Organization:University of Idaho|University of Idaho]], is the Aerial Vehicle for In-situ and Airborne Titan Reconnaissance (AVIATR): an uncrewed plane (or [[Engineering:Unmanned aerial vehicle|drone]]) that would fly through Titan's atmosphere and take [[Engineering:High-definition video|high-definition]] images of the surface of Titan. NASA did not approve the requested $715 million, and the future of the project is uncertain.<ref>{{cite news |url=https://www.universetoday.com/92286/aviatr-an-airplane-mission-for-titan/ |title=AVIATR: An Airplane Mission for Titan |work=Universetoday.com |date=January 2, 2012 |access-date=February 26, 2013 |url-status=live |archive-url=https://web.archive.org/web/20130328084232/https://www.universetoday.com/92286/aviatr-an-airplane-mission-for-titan/ |archive-date=March 28, 2013  }}</ref><ref>{{cite web |url=https://www.nbcnews.com/id/45946425/ns/technology_and_science-innovation/t/soaring-titan-drone-designed-scout-saturns-moon/#.USyd4VF3_qI |title=Soaring on Titan: Drone designed to scout Saturn's moon |work=NBC News |date=January 10, 2012 |access-date=February 26, 2013 |url-status=live |archive-url=https://web.archive.org/web/20140413125210/https://www.nbcnews.com/id/45946425/ns/technology_and_science-innovation/t/soaring-titan-drone-designed-scout-saturns-moon/#.USyd4VF3_qI |archive-date=April 13, 2014  }}</ref>

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 [[Place:Madrid|Madrid]]. The concept probe is called [[Astronomy:Titan Lake In-situ Sampling Propelled Explorer|Titan Lake In-situ Sampling Propelled Explorer]] (TALISE).<ref name="TALISE">{{cite book |chapter=TALISE: Titan Lake In-situ Sampling Propelled Explorer |title=European Planetary Science Congress 2012 |editor-first=I. |editor-last=Urdampilleta |editor2-first=O. |editor2-last=Prieto-Ballesteros |editor3-first=R. |editor3-last=Rebolo |editor4-first=J. |editor4-last=Sancho |publisher=EPSC Abstracts |date=2012 |volume=7, EPSC2012-64 2012 |chapter-url=https://meetingorganizer.copernicus.org/EPSC2012/EPSC2012-64.pdf |access-date=October 10, 2012 |url-status=live |archive-url=https://www.webcitation.org/6BMM5Agzm?url=https://meetingorganizer.copernicus.org/EPSC2012/EPSC2012-64.pdf |archive-date=October 12, 2012  }}</ref><ref>{{cite news |first=Elizabeth |last=Landau |title=Probe would set sail on a Saturn moon |date=October 9, 2012 |url=https://lightyears.blogs.cnn.com/2012/10/09/probe-would-set-sail-on-a-saturn-moon/?hpt=hp_mid |work=CNN – Light Years |access-date=October 10, 2012 |url-status=live |archive-url=https://web.archive.org/web/20130619112735/https://lightyears.blogs.cnn.com/2012/10/09/probe-would-set-sail-on-a-saturn-moon/?hpt=hp_mid |archive-date=June 19, 2013  }}</ref> 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.<ref name="TALISE" />

A [[Astronomy:Discovery Program|Discovery Program]] contestant for its mission #13 is [[Astronomy:Journey to Enceladus and Titan|Journey to Enceladus and Titan]] (JET), an [[Astronomy:Astrobiology|astrobiology]] Saturn orbiter that would assess the [[Astronomy:Planetary habitability|habitability potential]] of [[Astronomy:Enceladus|Enceladus]] and Titan.<ref name="Sotin2011">{{cite conference |last1=Sotin |first1=C. |last2=Altwegg |first2=K.|last3=Brown |first3=R. H. |display-authors=0 |title=JET: Journey to Enceladus and Titan |url=https://www.lpi.usra.edu/meetings/lpsc2011/pdf/1326.pdf |conference=42nd Lunar and Planetary Science Conference |publisher=Lunar and Planetary Institute |date=2011 |url-status=live |archive-url=https://web.archive.org/web/20150415113634/https://www.lpi.usra.edu/meetings/lpsc2011/pdf/1326.pdf |archive-date=April 15, 2015  }}</ref><ref name="LCPM-2013">{{cite conference |last1=Matousek |first1=Steve |last2=Sotin |first2=Christophe |last3=Goebel |first3=Dan |last4=Lang |first4=Jared |title=JET: Journey to Enceladus and Titan |url=https://lcpm10.caltech.edu/pdf/session-5/3_JET-LCPM-130618-Matousek-final.pdf |conference=Low Cost Planetary Missions Conference |publisher=California Institute of Technology |date=June 18–21, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20160304072017/https://lcpm10.caltech.edu/pdf/session-5/3_JET-LCPM-130618-Matousek-final.pdf |archive-date=March 4, 2016 |access-date=April 10, 2015 }}</ref><ref name="JET 204">{{cite news |last=Kane |first=Van |title=Discovery Missions for an Icy Moon with Active Plumes |url=https://www.planetary.org/blogs/guest-blogs/van-kane/20140402-discovery-missions-for-an-icy-moon-with-plumes.html |work=The Planetary Society |date=April 3, 2014 |access-date=April 9, 2015 |url-status=live |archive-url=https://web.archive.org/web/20150416094147/https://www.planetary.org/blogs/guest-blogs/van-kane/20140402-discovery-missions-for-an-icy-moon-with-plumes.html |archive-date=April 16, 2015  }}</ref>

In 2015, the [[Organization:NASA Institute for Advanced Concepts|NASA Innovative Advanced Concepts]] program (NIAC) awarded a Phase II grant<ref>{{cite web |url=https://www.nasa.gov/content/titan-submarine-exploring-the-depths-of-kraken/ |title=Titan Submarine: Exploring the Depths of Kraken |first=Loura |last=Hall |date=May 30, 2014 |url-status=live |archive-url=https://web.archive.org/web/20150730000928/https://www.nasa.gov/content/titan-submarine-exploring-the-depths-of-kraken/ |archive-date=July 30, 2015 }}</ref> to a design study of a Titan Submarine to explore the seas of Titan.<ref name="NYT-20210221">{{cite news |last=Overbye |first=Dennis |title=Seven Hundred Leagues Beneath Titan's Methane Seas – Mars, Shmars; this voyager is looking forward to a submarine ride under the icebergs on Saturn's strange moon. |url=https://www.nytimes.com/2021/02/21/science/saturn-titan-moon-exploration.html |archive-url=https://ghostarchive.org/archive/20211228/https://www.nytimes.com/2021/02/21/science/saturn-titan-moon-exploration.html |archive-date=2021-12-28 |url-access=limited |date=21 February 2021 |work={{wipe|The New York Times}} |access-date=21 February 2021 }}{{cbignore}}</ref><ref name="NASA-20150701">{{cite news |last1=Oleson |first1=Steven R. |last2=Lorenz |first2=Ralph D. |last3=Paul |first3=Michael V. |title=Phase I Final Report: Titan Submarine |url=https://ntrs.nasa.gov/citations/20150014581 |date=1 July 2015 |work=[[Organization:NASA|NASA]] |access-date=21 February 2021 |archive-date=July 24, 2021 |archive-url=https://web.archive.org/web/20210724054637/https://ntrs.nasa.gov/citations/20150014581 |url-status=live }}</ref><ref>{{cite web |last=Lewin |first=Sarah |date=July 15, 2015 |url=https://www.space.com/29953-titan-submarine-nasa-niac-proposals.html |title=NASA Funds Titan Submarine, Other Far-Out Space Exploration Ideas |website=Space.com |url-status=live |archive-url=https://web.archive.org/web/20150804042315/https://www.space.com/29953-titan-submarine-nasa-niac-proposals.html |archive-date=August 4, 2015 }}</ref><ref>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.1259</ref><ref>Hartwig, J., ''et al.'', (June 24–26, 2015). "Titan Submarine: Exploring the Depths of Kraken Mare", 26th Space Cryogenics Workshop, Phoenix, Arizona. [https://ntrs.nasa.gov/search.jsp?R=20150023076 link to NASA Report] {{Webarchive|url=https://web.archive.org/web/20201123225350/https://ntrs.nasa.gov/search.jsp?R=20150023076 |date=November 23, 2020 }}. Retrieved June 13, 2017.</ref>

== Prebiotic conditions and life ==
{{Main|Astronomy:Life on Titan}}

Titan is thought to be a [[Biology:Abiogenesis|prebiotic environment]] rich in complex [[Chemistry:Organic compound|organic compound]]s,<ref name="PhysOrg-20130403">{{cite web |author=Staff |title=NASA team investigates complex chemistry at Titan |url=https://phys.org/news/2013-04-nasa-team-complex-chemistry-titan.html |date=April 3, 2013 |work=Phys.Org |access-date=April 11, 2013 |url-status=live |archive-url=https://web.archive.org/web/20130421003912/https://phys.org/news/2013-04-nasa-team-complex-chemistry-titan.html |archive-date=April 21, 2013}}</ref><ref name="The Conversation">{{cite news |url=https://theconversation.com/saturns-moon-titan-may-harbour-simple-life-forms-and-reveal-how-organisms-first-formed-on-earth-81527 |title=Saturn's moon Titan may harbour simple life forms – and reveal how organisms first formed on Earth |access-date=August 30, 2017 |date=July 27, 2017 |work=The Conversation |url-status=live |archive-url=https://web.archive.org/web/20170830150224/https://theconversation.com/saturns-moon-titan-may-harbour-simple-life-forms-and-reveal-how-organisms-first-formed-on-earth-81527 |archive-date=August 30, 2017  }}</ref> but its surface is in a deep freeze at {{cvt|-179|C|F K}} so it is currently understood that life cannot exist on the moon's frigid surface.<ref name='A Mag Cooper'>[https://www.astrobio.net/news-exclusive/the-habitability-of-titan-and-its-ocean/ The Habitability of Titan and its Ocean.] {{Webarchive|url=https://web.archive.org/web/20210603092552/https://www.astrobio.net/news-exclusive/the-habitability-of-titan-and-its-ocean/ |date=June 3, 2021 }} Keith Cooper, ''Astrobiology Magazine''. July 12, 2019.</ref> However, Titan seems to contain a global ocean beneath its ice shell, and within this ocean, conditions are potentially suitable for microbial life.<ref name="Grasset2000">{{cite journal |last1=Grasset |first1=O. |last2=Sotin |first2=C. |last3=Deschamps |first3=F. |title=On the internal structure and dynamic of Titan |date=2000 |journal=Planetary and Space Science |volume=48 |issue=7–8 |pages=617–636 |doi=10.1016/S0032-0633(00)00039-8 |bibcode=2000P&SS...48..617G }}</ref><ref name="Fortes2000">{{cite journal |last=Fortes |first=A. D. |date=2000 |title=Exobiological implications of a possible ammonia-water ocean inside Titan |journal=Icarus |volume=146 |issue=2 |pages=444–452 |doi=10.1006/icar.2000.6400 |bibcode=2000Icar..146..444F }}</ref><ref name="life?">{{cite web|title=Have We Discovered Evidence For Life On Titan|author=Mckay, Chris|date=2010|url=https://astronomy.nmsu.edu/tharriso/ast105/making_sense.php.html|publisher=[[Organization:New Mexico State University|New Mexico State University]], College of Arts and Sciences, Department of Astronomy|access-date=May 15, 2014|url-status=dead|archive-url=https://web.archive.org/web/20160309224810/https://astronomy.nmsu.edu/tharriso/ast105/making_sense.php.html|archive-date=March 9, 2016}}</ref>

The ''Cassini–Huygens'' mission was not equipped to provide evidence for [[Astronomy:Biosignature|biosignature]]s or complex organic compounds; it showed an environment on Titan that is similar, in some ways, to ones hypothesized for the primordial Earth.<ref name="Raulin2005" /> 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.<ref>{{cite news |author=Staff |date=October 4, 2010 |title=Lakes on Saturn's Moon Titan Filled With Liquid Hydrocarbons Like Ethane and Methane, Not Water |work=ScienceDaily |url=https://www.sciencedaily.com/releases/2010/09/100921144133.htm |access-date=October 5, 2010 |url-status=live |archive-url=https://web.archive.org/web/20121020100943/https://www.sciencedaily.com/releases/2010/09/100921144133.htm |archive-date=October 20, 2012  }}</ref><ref name="The Conversation" />

=== Formation of complex molecules ===
The [[Biology:Miller–Urey experiment|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 [[Astronomy:Tholin|tholin]]s can be generated. The reaction starts with [[Chemistry:Dissociation|dissociation]] of nitrogen and methane, forming hydrogen cyanide and acetylene. Further reactions have been studied extensively.<ref name="Raulin2002">{{cite journal |journal=Space Science Reviews |volume=104 |issue=1–2 |pages=377–394 |date=2002 |doi=10.1023/A:1023636623006 |title=Organic chemistry and exobiology on Titan |last1=Raulin |first1=F. |last2=Owen |first2=T. |bibcode=2002SSRv..104..377R |s2cid=49262430 }}</ref>

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 [[Biology:DNA|DNA]] and [[Biology:RNA|RNA]], were among the many compounds produced. In addition, amino acids, the building blocks of [[Biology:Protein|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.<ref>{{cite news |author=Staff |date=October 8, 2010 |title=Titan's haze may hold ingredients for life |work=Astronomy |url=https://www.astronomy.com/news-observing/news/2010/10/titans%20haze%20may%20hold%20ingredients%20for%20life |access-date=October 14, 2010 |url-status=live |archive-url=https://web.archive.org/web/20150923175854/https://www.astronomy.com/news-observing/news/2010/10/titans%20haze%20may%20hold%20ingredients%20for%20life |archive-date=September 23, 2015  }}</ref>

On April 3, 2013, NASA reported that complex organic chemicals could arise on Titan based on studies simulating the atmosphere of Titan.<ref name="PhysOrg-20130403" />

On June 6, 2013, scientists at the [[Organization:Instituto de Astrofísica de Andalucía|IAA-CSIC]] reported the detection of [[Astronomy:Polycyclic aromatic hydrocarbon|polycyclic aromatic hydrocarbon]]s (PAH) in the upper atmosphere of Titan.<ref name="IAA-20130606" /><ref name="journalcours" />

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.<ref>{{cite journal
|author = Desai, R. T.
|author2 = A. J. Coates
|author3 = A. Wellbrock
|author4 = V. Vuitton
|author5 = D. González-Caniulef
|display-authors = etal
|name-list-style = amp
|year = 2017
|title = Carbon Chain Anions and the Growth of Complex Organic Molecules in Titan's Ionosphere
|journal = Astrophys. J. Lett.
|volume = 844
|issue = 2
|doi = 10.3847/2041-8213/aa7851
|pages =L18
|bibcode=2017ApJ...844L..18D|arxiv = 1706.01610 |s2cid = 32281365
|doi-access = free
}}</ref>
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.<ref>{{cite web |url=https://sci.esa.int/cassini-huygens/59350-has-cassini-found-a-universal-driver-for-prebiotic-chemistry-at-titan/ |title=Has Cassini found a universal driver for prebiotic chemistry at Titan? |access-date=August 12, 2017 |date=July 26, 2017 |publisher=European Space Agency |archive-date=August 13, 2017 |archive-url=https://web.archive.org/web/20170813145822/http://sci.esa.int/cassini-huygens/59350-has-cassini-found-a-universal-driver-for-prebiotic-chemistry-at-titan/ |url-status=live }}</ref>

On July 28, 2017, scientists reported that [[Chemistry:Acrylonitrile|acrylonitrile]], or vinyl cyanide, (C<sub>2</sub>H<sub>3</sub>CN), possibly essential for life by being related to [[Biology:Cell membrane|cell membrane]] and [[Biology:Vesicle (biology and chemistry)|vesicle structure]] formation, had been found on Titan.<ref name="SP-20170728">{{cite web |last=Wall |first=Mike |title=Saturn Moon Titan Has Molecules That Could Help Make Cell Membranes |url=https://www.space.com/37653-saturn-moon-titan-cell-membrane-molecules.html |date=July 28, 2017 |work=Space.com |access-date=July 29, 2017 |url-status=live |archive-url=https://web.archive.org/web/20170729011842/https://www.space.com/37653-saturn-moon-titan-cell-membrane-molecules.html |archive-date=July 29, 2017  }}</ref><ref name="SA-20170728">{{cite journal |author=Palmer, Maureen Y. |display-authors=0 |title=ALMA detection and astrobiological potential of vinyl cyanide on Titan |date=July 28, 2017 |journal=Science Advances |volume=3 |pages=e1700022 |number=7 |doi=10.1126/sciadv.1700022 |pmid=28782019 |pmc=5533535 |bibcode=2017SciA....3E0022P  }}</ref><ref name="WP-20170808">{{cite news |last=Kaplan |first=Sarah |title=This weird moon of Saturn has some essential ingredients for life |url=https://www.washingtonpost.com/news/speaking-of-science/wp/2017/08/08/this-weird-moon-of-saturn-has-some-essential-ingredients-for-life/ |date=August 8, 2017 |newspaper=Washington Post |access-date=August 8, 2017 |url-status=live |archive-url=https://web.archive.org/web/20170808154437/https://www.washingtonpost.com/news/speaking-of-science/wp/2017/08/08/this-weird-moon-of-saturn-has-some-essential-ingredients-for-life/ |archive-date=August 8, 2017  }}</ref>

In October 2018, researchers reported low-temperature chemical pathways from simple [[Chemistry:Organic compound|organic compound]]s to complex [[Astronomy:Polycyclic aromatic hydrocarbon|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 [[Biology:PAH world hypothesis|PAH world hypothesis]], in producing precursors to biochemicals related to life as we know it.<ref name="DG-20181008">{{cite web |author=Staff |title="A Prebiotic Earth" – Missing Link Found on Saturn's Moon Titan |url=https://dailygalaxy.com/2018/10/a-prebiotic-earth-missing-link-found-on-saturns-moon-titan/ |date=October 11, 2018 |work=DailyGalaxy.com |access-date=October 11, 2018 |archive-date=August 14, 2021 |archive-url=https://web.archive.org/web/20210814144308/https://dailygalaxy.com/2018/10/a-prebiotic-earth-missing-link-found-on-saturns-moon-titan/ |url-status=live }}</ref><ref name="NA-20181008">{{cite journal |author=Zhao, Long |display-authors=0 |title=Low-temperature formation of polycyclic aromatic hydrocarbons in Titan's atmosphere |date=October 8, 2018 |journal=[[Astronomy:Nature Astronomy|Nature Astronomy]] |volume=2 |issue=12 |pages=973–979 |doi=10.1038/s41550-018-0585-y |url=https://escholarship.org/content/qt1pj0q014/qt1pj0q014.pdf?t=q87uvg |bibcode=2018NatAs...2..973Z |s2cid=105480354 |access-date=April 12, 2020 |archive-date=July 2, 2021 |archive-url=https://web.archive.org/web/20210702011100/https://escholarship.org/content/qt1pj0q014/qt1pj0q014.pdf?t=q87uvg |url-status=live }}</ref>

=== 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.<ref>{{cite journal |last1=Artemivia |first1=N. |last2=Lunine |first2=Jonathan I. |title=Cratering on Titan: impact melt, ejecta, and the fate of surface organics |date=2003 |journal=Icarus |volume=164 |issue=2 |pages=471–480 |doi=10.1016/S0019-1035(03)00148-9 |bibcode=2003Icar..164..471A }}</ref> It has also been hypothesized that liquid-ammonia oceans could exist deep below the surface.<ref name="Grasset2000" /><ref>{{cite journal |last=Lovett |first=Richard A. |date=March 20, 2008 |url=https://news.nationalgeographic.com/news/2008/03/080320-titan-ocean_2.html |title=Saturn Moon Titan May Have Underground Ocean |journal=National Geographic |url-status=dead |archive-url=https://web.archive.org/web/20121018035136/https://news.nationalgeographic.com/news/2008/03/080320-titan-ocean_2.html |archive-date=October 18, 2012  }}</ref> 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.<ref name="Fortes2000" /> [[Physics:Heat transfer|Heat transfer]] between the interior and upper layers would be critical in sustaining any subsurface oceanic life.<ref name="Grasset2000" /> Detection of microbial life on Titan would depend on its biogenic effects, with the atmospheric methane and nitrogen examined.<ref name="Fortes2000" />

=== Methane and life at the surface ===

It has been speculated that life could exist in the lakes of liquid methane on Titan, just as organisms on Earth live in water.<ref name="mckay" /> Such organisms would inhale H<sub>2</sub> in place of O<sub>2</sub>, metabolize it with [[Chemistry:Acetylene|acetylene]] instead of [[Chemistry:Glucose|glucose]], and exhale methane instead of carbon dioxide.<ref name="life?" /><ref name="mckay" /> However, such hypothetical organisms would be required to metabolize at a deep freeze temperature of {{cvt|-179.2|C|F K|abbr=}}.<ref name='A Mag Cooper' />

All life forms on Earth (including [[Biology:Methanogen|methanogen]]s) use liquid water as a solvent; it is speculated that life on Titan might instead use a liquid hydrocarbon, such as methane or ethane,<ref name="methanesolvent">{{cite book |title=Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Council |chapter-url=https://books.nap.edu/openbook.php?record_id=11919&page=74 |chapter=The Limits of Organic Life in Planetary Systems |publisher=The National Academies Press |date=2007 |page=74 |doi=10.17226/11919 |isbn=978-0-309-10484-5 |access-date=February 20, 2022 |archive-date=August 20, 2015 |archive-url=https://web.archive.org/web/20150820025541/http://books.nap.edu/openbook.php?record_id=11919&page=74 |url-status=live }}</ref> although water is a stronger solvent than methane.<ref name="methlife" /> Water is also more chemically reactive, and can break down large organic molecules through [[Chemistry:Hydrolysis|hydrolysis]].<ref name="methanesolvent" /> A life form whose solvent was a hydrocarbon would not face the risk of its biomolecules being destroyed in this way.<ref name="methanesolvent" />

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. Assuming metabolic rates similar to those of methanogenic organisms on Earth, the concentration of molecular hydrogen would drop by a factor of 1000 on the Titanian surface solely due to a hypothetical biological sink. McKay noted that, if life is indeed present, the low temperatures on Titan would result in very slow metabolic processes, which could conceivably be hastened by the use of catalysts similar to enzymes. He also noted that the low solubility of organic compounds in methane presents a more significant challenge to any possible form of life. Forms of [[Biology:Active transport|active transport]], and organisms with large [[Biology:Surface-area-to-volume ratio|surface-to-volume ratios]] could theoretically lessen the disadvantages posed by this fact.<ref name="mckay">{{cite journal |journal=Icarus |volume=178 |issue=1 |pages=274–276 |date=2005 |doi=10.1016/j.icarus.2005.05.018 |title=Possibilities for methanogenic life in liquid methane on the surface of Titan |last1=McKay |first1=C. P. |last2=Smith |first2=H. D. |bibcode=2005Icar..178..274M |url=https://zenodo.org/record/1259025 |access-date=March 18, 2020 |archive-date=March 9, 2021 |archive-url=https://web.archive.org/web/20210309044958/https://zenodo.org/record/1259025 |url-status=live }}</ref>

In 2010, Darrell Strobel, from [[Organization:Johns Hopkins University|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 10<sup>28</sup> 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.<ref name="mckay" /><ref name="methlife">{{cite web|title=What is Consuming Hydrogen and Acetylene on Titan? |publisher=NASA/JPL |date=2010 |access-date=June 6, 2010 |url=https://www.jpl.nasa.gov/news/news.cfm?release=2010-190 |url-status=dead |archive-url=https://web.archive.org/web/20110629185640/https://www.jpl.nasa.gov/news/news.cfm?release=2010-190 |archive-date=June 29, 2011 }}</ref><ref>{{cite journal|title=Molecular hydrogen in Titan's atmosphere: Implications of the measured tropospheric and thermospheric mole fractions |last=Strobel |first=Darrell F. |journal=Icarus |volume=208 |issue=2 |pages=878–886 |date=2010 |doi=10.1016/j.icarus.2010.03.003 |url=https://astrobiology.jhu.edu/wp-content/uploads/2010/06/Icarus-2010-Strobel.pdf |bibcode=2010Icar..208..878S |url-status=dead |archive-url=https://web.archive.org/web/20120824195338/https://astrobiology.jhu.edu/wp-content/uploads/2010/06/Icarus-2010-Strobel.pdf |archive-date=August 24, 2012 }}</ref> 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.<ref name="methlife" /> 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 [[Chemistry:Catalyst|catalyst]] accepting hydrocarbons or hydrogen), or flaws in the current models of material flow.<ref name="life?" /> 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.<ref name="life?" /> With regards to the acetylene findings, Mark Allen, the principal investigator with the NASA Astrobiology Institute Titan team, provided a speculative, non-biological explanation: sunlight or cosmic rays could transform the acetylene in icy aerosols in the atmosphere into more complex molecules that would fall to the ground with no acetylene signature.<ref>{{Cite web|url=https://www.sciencedaily.com/releases/2010/06/100606103125.htm|title=Life on Titan? New clues to what's consuming hydrogen, acetylene on Saturn's moon|website=ScienceDaily}}</ref>

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."<ref name="methlife" /> 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."<ref name="methlife" />

In February 2015, a hypothetical [[Biology:Cell membrane|cell membrane]] capable of functioning in liquid [[Chemistry:Methane|methane]] at cryogenic temperatures (deep freeze) 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 [[Chemistry:Phospholipid|phospholipid]]s, compounds of carbon, hydrogen, oxygen, and [[Chemistry:Phosphorus|phosphorus]]. This hypothetical cell membrane was termed an "azotosome", a combination of "azote", French for nitrogen, and "[[Biology:Liposome|liposome]]".<ref name=azotosomemodel>{{cite web|url=https://phys.org/news/2015-02-life-saturn-moon-titan.html|title=Life 'not as we know it' possible on Saturn's moon Titan|url-status=live|archive-url=https://web.archive.org/web/20150317002959/https://phys.org/news/2015-02-life-saturn-moon-titan.html|archive-date=March 17, 2015}}</ref><ref>{{cite journal |last1=Stevenson |first1=James |last2=Lunine|first2=Jonathan I. |last3=Clancy |first3=Paulette |title=Membrane alternatives in worlds without oxygen: Creation of an azotosome |journal=Science Advances |date=February 27, 2015 |volume=1 |issue=1 |pages=e1400067 |doi=10.1126/sciadv.1400067 |pmid=26601130 |bibcode=2015SciA....1E0067S |pmc=4644080 }}</ref>

=== 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 [[Astronomy:Sun|Sun]], Titan is frigid, and its atmosphere lacks CO<sub>2</sub>. At Titan's surface, water exists only in solid form. Because of these difficulties, scientists such as [[Biography:Jonathan Lunine|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.<ref>{{cite web |url=https://www.astrobio.net/news/article1130.html |title=Saturn's Moon Titan: Prebiotic Laboratory—Interview with Jonathan Lunine |first=Henry |last=Bortman |work=Astrobiology Magazine |date=August 11, 2004 |archive-date=August 28, 2004 |archive-url=https://web.archive.org/web/20040828233135/https://www.astrobio.net/news/article1130.html |access-date=August 11, 2004 |url-status=dead}}</ref> 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.<ref name="Raulin2005">{{cite journal |journal=Space Science Reviews |volume=116 |issue=1–2 |pages=471–487 |date=2005 |doi=10.1007/s11214-005-1967-x |title=Exo-astrobiological aspects of Europa and Titan: From observations to speculations |last=Raulin |first=F. |bibcode=2005SSRv..116..471R |s2cid=121543884 }}</ref> Using Titan as a prebiotic experiment involves not only observation through spacecraft, but laboratory experiments, and chemical and photochemical modeling on Earth.<ref name="Raulin2002" />

=== Panspermia hypothesis ===
{{Main|Biology:Panspermia}}

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 [[Biology:Panspermia|panspermia]]. Calculations indicate that these would encounter many of the bodies in the Solar System, including Titan.<ref>{{cite news |url=https://news.bbc.co.uk/2/hi/science/nature/4819370.stm |title=Earth could seed Titan with life |work=BBC News |date=March 18, 2006 |access-date=March 10, 2007 |url-status=live |archive-url=https://web.archive.org/web/20121031002211/https://news.bbc.co.uk/2/hi/science/nature/4819370.stm |archive-date=October 31, 2012  }}</ref><ref>{{cite journal |last1=Gladman |first1=Brett |last2=Dones |first2=Luke |last3=Levinson |first3=Harold F. |last4=Burns |first4=Joseph A. |title=Impact Seeding and Reseeding in the Inner Solar System |date=2005 |journal=Astrobiology |volume=5 |pages=483–496 |doi=10.1089/ast.2005.5.483 |pmid=16078867 |issue=4 |bibcode=2005AsBio...5..483G }}</ref> On the other hand, [[Biography:Jonathan Lunine|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.<ref>{{cite journal|arxiv=0908.0762 |last=Lunine |first=Jonathan I. |title=Saturn's Titan: A Strict Test for Life's Cosmic Ubiquity |url=https://www.amphilsoc.org/sites/default/files/CCLunine1530402.pdf |archive-url=https://web.archive.org/web/20130512013831/https://www.amphilsoc.org/sites/default/files/CCLunine1530402.pdf |archive-date=May 12, 2013 |volume=153 |issue=4 |page=403 |date=2008 |journal=Proceedings of the American Philosophical Society |bibcode=2009arXiv0908.0762L |url-status=dead }} [https://archive.org/details/SaturnsTitan copy at archive.org]</ref>

=== Future conditions ===
Conditions on Titan could become far more [[Astronomy:Planetary habitability|habitable]] in the far future. Five billion years from now, as the Sun becomes a sub-[[Astronomy:Red giant|red giant]], its surface temperature could rise enough for Titan to support liquid water on its surface, making it habitable.<ref>{{cite web |title=Climate Change in the Solar System |author=The National Air and Space Museum |date=2012 |url=https://blog.nasm.si.edu/2012/03/07/climate-change-in-the-solar-system/ |access-date=January 14, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120311101403/https://blog.nasm.si.edu/2012/03/07/climate-change-in-the-solar-system/ |archive-date=March 11, 2012  }}</ref> 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 higher [[Physics:Viscosity|viscosity]] of ammonia-water solutions coupled with low temperatures would cause chemical reactions to proceed more slowly on Titan.<ref>{{cite journal |title=Titan under a red giant sun: A new kind of "habitable" moon |last1=Lorenz |first1=Ralph D. |last2=Lunine |first2=Jonathan I. |last3=McKay |first3=Christopher P. |journal=NASA Ames Research Center, Lunar and Planetary Laboratory, Department of Planetary Sciences, University of Arizona |date=1997 |volume=24 |issue=22 |pages=2905–8 |doi=10.1029/97gl52843 |pmid=11542268 |citeseerx=10.1.1.683.8827 |url=https://www.lpl.arizona.edu/~rlorenz/redgiant.pdf |access-date=March 21, 2008 |url-status=live |archive-url=https://web.archive.org/web/20110724173621/https://www.lpl.arizona.edu/~rlorenz/redgiant.pdf |archive-date=July 24, 2011 |bibcode=1997GeoRL..24.2905L |s2cid=14172341 }}</ref>

== See also ==
{{div col|colwidth=20em}}
* [[Astronomy:Colonization of Titan|Colonization of Titan]]
* Lakes of Titan
* [[Astronomy:Atmosphere of Titan|Atmosphere of Titan]]
* [[Astronomy:Life on Titan|Life on Titan]]
* [[Astronomy:List of natural satellites|List of natural satellites]]
* Saturn's moons in fiction
* [[Astronomy:Extraterrestrial skies#The sky of Titan|The sky of Titan]]
* Titan in fiction
* Titan Winged Aerobot
* [[Astronomy:Vid Flumina|Vid Flumina]] a river of methane and ethane on Titan
{{div col end}}
{{Clear}}

== References ==
{{Reflist|30em}}

== Bibliography ==
* {{cite book |title=Titan: Exploring an Earthlike World |last1=Coustenis |first1=Athéna |last2=Taylor |first2=F. W. |publisher=World Scientific |date=2008 |url=https://books.google.com/books?id=j3O47dxrDAQC |isbn=978-981-270-501-3 }}

== Further reading ==
* {{cite book|title=Lifting Titan's Veil: Exploring the Giant Moon of Saturn |first1=Ralph |last1=Lorenz |first2=Jacqueline |last2=Mitton |publisher=Cambridge University Press |date=2002 |isbn=978-0-521-79348-3 }}
* {{cite book|title=Titan Unveiled |url=https://archive.org/details/titanunveiledsat00lore |url-access=registration |first1=Ralph |last1=Lorenz |first2=Jacqueline |last2=Mitton |publisher=Princeton University Press |date=2008 |isbn=978-0-691-14633-1}}
* {{Cite book|title=NASA/ESA/ASI Cassini-Huygens: 1997 onwards (Cassini orbiter, Huygens probe and future exploration concepts) (Owners' Workshop Manual) |first=Ralph |last=Lorenz |publisher=Haynes Manuals, UK |year=2017 |isbn=978-1-78521-111-9}}
* {{Cite journal |last=O'Callaghan |first=Jonathan |date=21 November 2019 |title=A Map of Saturn's Largest Moon |url=https://media.nature.com/original/magazine-assets/d41586-019-03539-8/d41586-019-03539-8.pdf |journal=Nature |volume=575 |issue=7783 |pages=426–427|doi=10.1038/d41586-019-03539-8 |pmid=31745360 |bibcode=2019Natur.575..426O |s2cid=208171884 }}

== External links ==
* [https://web.archive.org/web/20060428051758/https://saturn.jpl.nasa.gov/ ''Cassini–Huygens'' Mission To Saturn and Titan]. Multimedia Feature [https://web.archive.org/web/20070429204420/https://saturn.jpl.nasa.gov/news/features/feature20070129.cfm Titan Virtual Tour]
* [https://www.esa.int/SPECIALS/Cassini-Huygens/SEMKVQOFGLE_0.html Video of ''Huygens’'' descent] from the ESA
* [https://ciclops.org/search.php?x=19&y=3&search=Titan Cassini Imaging Central Laboratory for Operations (CICLOPS) site Titan image search] {{Webarchive|url=https://web.archive.org/web/20220220093938/http://ciclops.org/search.php?x=19&y=3&search=Titan |date=February 20, 2022 }}
* The Planetary Society (2005). [https://web.archive.org/web/20110605100801/https://www.planetary.org/explore/topics/saturn/titan.html TPS: Saturn's moon Titan]. Retrieved March 28, 2005.
* [https://esamultimedia.esa.int/images/huygens_alien_winds_descent.mp3 The Alien Noise]. This recording is a laboratory reconstruction of the sounds heard by ''Huygens''{{'}} microphones.
* [https://www.astronomycast.com/astronomy/planets/our-solar-system/ep-201-titan/ AstronomyCast: Titan] {{Webarchive|url=https://web.archive.org/web/20111012055210/http://www.astronomycast.com/astronomy/planets/our-solar-system/ep-201-titan/ |date=October 12, 2011 }} Fraser Cain and Pamela Gay, 2010.
* [https://planetarynames.wr.usgs.gov/Page/TITAN/target Titan nomenclature] and [https://planetarynames.wr.usgs.gov/images/Titan_comp.pdf Titan map with feature names] from the [https://planetarynames.wr.usgs.gov/ USGS planetary nomenclature page]
* [https://www.google.com/maps/space/titan/@39.2264184,-165.5899535,12980453m/data=!3m1!1e3 Google Titan 3D], interactive map of the moon
* [https://www.flickr.com/photos/kevinmgill/sets/72157674247824052 Image album] by Kevin M. Gill

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