Changes

== Physical characteristics ==

== Physical characteristics ==

Titan is slightly larger in radius than [[Mercury]] (and [[Earth]]'s [[Moon]]), and is more massive than any of the [[dwarf planet]]s [[Eris]], [[Pluto]], and [[Ceres]] by one to two orders of magnitude. Titan was once thought to be the broadest of all satellites, but the [[Voyager 1]] probe showed that Titan's large apparent diameter was due to its atmosphere, and in fact [[Ganymede]] is slightly larger in radius than is Titan.<ref name=Arnett>Arnett, Bill. "[http://www.nineplanets.org/titan.html Entry for Titan]." ''The <s>Nine</s> 8 Planets'', May 27, 2005. Accessed February 16, 2008</ref>.

Titan is slightly larger in radius than [[Mercury]] (and [[Earth]]'s [[Moon]]), and is more massive than any of the [[dwarf planet]]s [[Eris]], [[Pluto]], and [[Ceres]] by one to two orders of magnitude. Titan was once thought to be the broadest of all satellites, but the [[Voyager 1]] probe showed that Titan's large apparent diameter was due to its atmosphere, and in fact [[Ganymede]] is slightly larger in radius than is Titan.<ref name=Arnett>Arnett, Bill. "[http://www.nineplanets.org/titan.html Entry for Titan]." ''The <s>Nine</s> 8 Planets'', May 27, 2005. Accessed February 16, 2008</ref>

Titan's bulk is probably half water ice and half rock, in several layers with a rock center and several layers of various crystalline forms of ice.<ref name=Arnett/>

Titan's bulk is probably half water ice and half rock, in several layers with a rock center and several layers of various crystalline forms of ice.<ref name=Arnett/>

The argon finding is significant because much of it is ⁴⁰Ar, an isotope usually associated with volcanism. The currently favored interpretation is that Titan has multiple "cryovolcanos" that erupt with water ice and ammonia.<ref name=HuygensFindings>"[http://www.esa.int/SPECIALS/Cassini-Huygens/SEMHB881Y3E_0.html Seeing, touching and smelling the extraordinarily Earth-like world of Titan]." ''Cassini-Huygens project'', [[European Space Agency]], January 21, 2005. Accessed February 16, 2008.</ref>

The argon finding is significant because much of it is ⁴⁰Ar, an isotope usually associated with volcanism. The currently favored interpretation is that Titan has multiple "cryovolcanos" that erupt with water ice and ammonia.<ref name=HuygensFindings>"[http://www.esa.int/SPECIALS/Cassini-Huygens/SEMHB881Y3E_0.html Seeing, touching and smelling the extraordinarily Earth-like world of Titan]." ''Cassini-Huygens project'', [[European Space Agency]], January 21, 2005. Accessed February 16, 2008.</ref>

The atmosphere is not of uniform composition. Clouds, presumably of methane, do form. Furthermore, Huygens' images of the surface show clear signs of erosion and formations that look like dry riverbeds, a sign that rain occurs--though the rain is of methane, not water.<ref name=Arnett/> However, the data do ''not'' indicate the looked-for "ocean" of methane, but rather indicate something akin to the seasonal rivers and lakes seen on earth.<ref name=Arnett/><ref name=HuygensFindings/>

The atmosphere is not of uniform composition. Clouds, presumably of methane, do form. Furthermore, Huygens' images of the surface show clear signs of erosion and formations that look like dry riverbeds, a sign that rain occurs—though the rain is of methane, not water.<ref name=Arnett/> However, the data do ''not'' indicate the looked-for "ocean" of methane, but rather indicate something akin to the seasonal rivers and lakes seen on earth.<ref name=Arnett/><ref name=HuygensFindings/>

Until 2005, Titan was regarded as the only natural satellite in the [[solar system]] to have a true [[atmosphere]]. But the [[Cassini-Huygens Mission]] has already demonstrated that Saturn's inner moon [[Enceladus]] ''also'' has an atmosphere, believed composed of ionized [[water]] vapor from repeated eruptions from [[volcano]]es and/or [[geyser]]s on its surface. This "atmosphere" is constantly escaping from Enceladus, perhaps to persist as the E ring of Saturn's ring system. Thus it requires a continuous source of replenishment. The theories of Enceladus' volcanic and/or geothermal activity are an attempt to explain this replenishment.<ref name=encelatm>"[http://www.esa.int/SPECIALS/Cassini-Huygens/SEM9RC2IU7E_0.html Cassini finds atmosphere on Enceladus]." ''ESA Cassini-Huygens,'' March 16, 2005. Accessed February 16, 2008.</ref>

Until 2005, Titan was regarded as the only natural satellite in the [[solar system]] to have a true [[atmosphere]]. But the [[Cassini-Huygens Mission]] has already demonstrated that Saturn's inner moon [[Enceladus]] ''also'' has an atmosphere, believed composed of ionized [[water]] vapor from repeated eruptions from [[volcano]]es and/or [[geyser]]s on its surface. This "atmosphere" is constantly escaping from Enceladus, perhaps to persist as the E ring of Saturn's ring system. Thus it requires a continuous source of replenishment. The theories of Enceladus' volcanic and/or geothermal activity are an attempt to explain this replenishment.<ref name=encelatm>"[http://www.esa.int/SPECIALS/Cassini-Huygens/SEM9RC2IU7E_0.html Cassini finds atmosphere on Enceladus]." ''ESA Cassini-Huygens,'' March 16, 2005. Accessed February 16, 2008.</ref>

Before the launch of the [[Cassini-Huygens Mission]] (see below), speculations on whether Titan had the required elements for [[abiogenesis]] were rife.<ref name=solarviews/> Specifically, scientists believed that the [[methane]] in Titan's atmosphere would combine photochemically to yield [[ethane]], [[ethylene]] (ethene), [[acetylene]] (ethyne), and hydrogen [[cyanide]], and thus form [[amino acid]]s.<ref name=solarviews/><ref name=nasa2/>

Before the launch of the [[Cassini-Huygens Mission]] (see below), speculations on whether Titan had the required elements for [[abiogenesis]] were rife.<ref name=solarviews/> Specifically, scientists believed that the [[methane]] in Titan's atmosphere would combine photochemically to yield [[ethane]], [[ethylene]] (ethene), [[acetylene]] (ethyne), and hydrogen [[cyanide]], and thus form [[amino acid]]s.<ref name=solarviews/><ref name=nasa2/>

The Huygens lander has thus far found no biological process, though it has found the suspected constituents, and more. But gas-chromatograph mass spectrophotometric findings have also shown that the bulk of the methane on Titan is actually ¹³C methane--not the ¹²C methane that one would normally expect from biological processes.<ref name=Goddard>"[http://www.spaceref.com/news/viewpr.html?pid=18410 Titan's Mysterious Methane Comes From Inside, Not The Surface]." Press release, [[Goddard Space Flight Center]], [[NASA]], November 30, 2005. Accessed February 16, 2008, from <http://www.spaceref.com/></ref> Scientists at the Goddard Space Flight Center conclude that this methane is vented to the surface from a supply that was somehow trapped deep beneath the surface when Titan was formed. Furthermore, this lends further support to Thomas Gold's 1998 hypothesis that Earth's own supplies of oil and natural gas are in fact abiotic in origin.<ref name=Corsi2>"[http://www.worldnetdaily.com/news/article.asp?ARTICLE_ID=47675 'Fossil fuel' theory takes hit with NASA finding]." ''WorldNetDaily'', December 1, 2005. Accessed February 16, 2008.</ref>

The Huygens lander has thus far found no biological process, though it has found the suspected constituents, and more. But gas-chromatograph mass spectrophotometric findings have also shown that the bulk of the methane on Titan is actually ¹³C methane—not the ¹²C methane that one would normally expect from biological processes.<ref name=Goddard>"[http://www.spaceref.com/news/viewpr.html?pid=18410 Titan's Mysterious Methane Comes From Inside, Not The Surface]." Press release, [[Goddard Space Flight Center]], [[NASA]], November 30, 2005. Accessed February 16, 2008, from <http://www.spaceref.com/></ref> Scientists at the Goddard Space Flight Center conclude that this methane is vented to the surface from a supply that was somehow trapped deep beneath the surface when Titan was formed. Furthermore, this lends further support to Thomas Gold's 1998 hypothesis that Earth's own supplies of oil and natural gas are in fact abiotic in origin.<ref name=Corsi2>"[http://www.worldnetdaily.com/news/article.asp?ARTICLE_ID=47675 'Fossil fuel' theory takes hit with NASA finding]." ''WorldNetDaily'', December 1, 2005. Accessed February 16, 2008.</ref>

This latest finding militates against the speculation that life might exist on Titan. Many scientists believe that Titan's chemistry and physical features are of a type that could have given rise to the earliest chemicals required for [[life]], had Titan been much closer to the [[Sun]] than it actually is. That speculation predates the Cassini-Huygens mission and includes a model suggesting that Titan harbored a vast underground reservoir of [[ammonia]] solution.<ref name=Fortes>Fortes, A. B. "[http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-45FC0TP-26&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=aa73d6642eb13527397bbec9ff9ca535 Exobiological Implications of a Possible Water-Ammonia Ocean Inside Titan]." ''Icarus'', 146(2):444-452, August 2000. {{doi|10.1006/icar.2000.6400}} Accessed February 16, 2008. [http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6WGF-45FC0TP-26-1&_cdi=6821&_user=10&_orig=search&_coverDate=08%2F31%2F2000&_sk=998539997&view=c&wchp=dGLbVlb-zSkWz&md5=644f3640a5d78afc50f8e4e50a4a09d2&ie=/sdarticle.pdf Full text] available to subscribers or for a fee.</ref> That particular model depended on the methane and the cyanide being biogenic, but as has been noted above, the methane certainly is ''not'' biogenic.

This latest finding militates against the speculation that life might exist on Titan. Many scientists believe that Titan's chemistry and physical features are of a type that could have given rise to the earliest chemicals required for [[life]], had Titan been much closer to the [[Sun]] than it actually is. That speculation predates the Cassini-Huygens mission and includes a model suggesting that Titan harbored a vast underground reservoir of [[ammonia]] solution.<ref name=Fortes>Fortes, A. B. "[http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-45FC0TP-26&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=aa73d6642eb13527397bbec9ff9ca535 Exobiological Implications of a Possible Water-Ammonia Ocean Inside Titan]." ''Icarus'', 146(2):444-452, August 2000. {{doi|10.1006/icar.2000.6400}} Accessed February 16, 2008. [http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6WGF-45FC0TP-26-1&_cdi=6821&_user=10&_orig=search&_coverDate=08%2F31%2F2000&_sk=998539997&view=c&wchp=dGLbVlb-zSkWz&md5=644f3640a5d78afc50f8e4e50a4a09d2&ie=/sdarticle.pdf Full text] available to subscribers or for a fee.</ref> That particular model depended on the methane and the cyanide being biogenic, but as has been noted above, the methane certainly is ''not'' biogenic.

== Possible Subsurface Ammonia Ocean ==

== Possible Subsurface Ammonia Ocean ==

The ammonia-water "[[aquifer]]" might yet exist, however, as a vast subsurface ocean. The ⁴⁰Ar findings certainly suggest this.<ref name=HuygensFindings/> But recently Cassini's controllers carefully mapped fifty of Titan's landmarks and then discovered an apparent shift of all fifty landmarks by as much as 30 km (less than 20 miles). This finding suggests that the moon's crust is somehow decoupled from its core and is floating on top of a liquid layer. Astronomers believe that this layer lies 100 km (over 60 miles) deep to the rocky surface.<ref name=NASANH3>Brown, Dwayne, and Martinez, Carolina. "[http://www.nasa.gov/home/hqnews/2008/mar/HQ_08085_cassini_ocean.html Cassini Spacecraft Finds Ocean May Exist Beneath Titan's Crust]." [[National Aeronautics and Space Administration|NASA]], March 20, 2008. Accessed May 16, 2008.</ref>

The ammonia-water "[[aquifer]]" might yet exist, however, as a vast subsurface ocean. The ⁴⁰Ar findings certainly suggest this.<ref name=HuygensFindings/> But recently Cassini's controllers carefully mapped fifty of Titan's landmarks and then discovered an apparent shift of all fifty landmarks by as much as 30&nbsp;km (less than 20 miles). This finding suggests that the moon's crust is somehow decoupled from its core and is floating on top of a liquid layer. Astronomers believe that this layer lies 100&nbsp;km (over 60 miles) deep to the rocky surface.<ref name=NASANH3>Brown, Dwayne, and Martinez, Carolina. "[http://www.nasa.gov/home/hqnews/2008/mar/HQ_08085_cassini_ocean.html Cassini Spacecraft Finds Ocean May Exist Beneath Titan's Crust]." [[National Aeronautics and Space Administration|NASA]], March 20, 2008. Accessed May 16, 2008.</ref>

== Problems for Uniformitarian Theories ==

== Problems for Uniformitarian Theories ==

The most serious problem that Titan poses for [[uniformitarianism|uniformitarian]] theories is that astronomers still have not found the vast reserves of methane that would be necessary to maintain the current level of methane in the atmosphere over [[deep time]]. Lorenz and his colleagues state flatly that Titan must be subject to drastic climate change unless an as-yet unaccounted-for internal source of methane is replenishing the atmosphere ''"at exactly the right rate"'' (emphasis added).<ref name=Lorenz/>

The most serious problem that Titan poses for [[uniformitarianism|uniformitarian]] theories is that astronomers still have not found the vast reserves of methane that would be necessary to maintain the current level of methane in the atmosphere over [[deep time]]. Lorenz and his colleagues state flatly that Titan must be subject to drastic climate change unless an as-yet unaccounted-for internal source of methane is replenishing the atmosphere ''"at exactly the right rate"'' (emphasis added).<ref name=Lorenz/>

Yet the findings suggesting an ammonia aquifer 100 km deep to the surface make such an internal source much more difficult to explain. This is especially so if, as suspected, the entire surface of Titan floats on the ammonia ocean beneath it with no connection to the core.

Yet the findings suggesting an ammonia aquifer 100&nbsp;km deep to the surface make such an internal source much more difficult to explain. This is especially so if, as suspected, the entire surface of Titan floats on the ammonia ocean beneath it with no connection to the core.

== Observation and Exploration ==

== Observation and Exploration ==

Image:Titan haze 2007.jpg|Titan's hazy atmosphere in ultraviolet light

Image:Titan haze 2007.jpg|Titan's hazy atmosphere in ultraviolet light

Image:Titan mosaics 2006.jpg|Three false-color mosaics of Titan showing changes in its atmosphere within three months

Image:Titan mosaics 2006.jpg|Three false-color mosaics of Titan showing changes in its atmosphere within three months

Image:Titan surface approach 1.jpg|Surface of Titan from 17 to 8 km up, from Huygens lander as it descended

Image:Titan surface approach 1.jpg|Surface of Titan from 17 to 8&nbsp;km up, from Huygens lander as it descended

Image:Titan surface approach 2.jpg|Surface of Titan from 7 to 0.5 km up, from Huygens lander as it descended

Image:Titan surface approach 2.jpg|Surface of Titan from 7 to 0.5&nbsp;km up, from Huygens lander as it descended

Image:Titan landing.jpg|Surface of Titan, from Huygens lander after it landed

Image:Titan landing.jpg|Surface of Titan, from Huygens lander after it landed

Image:Titan landing color.jpg|Surface of Titan, in color, from Huygens lander

Image:Titan landing color.jpg|Surface of Titan, in color, from Huygens lander