Changes

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>

[[Image:TitanNorth.jpg|thumb|400px|left|Radar map of Titan's north polar region]]

[[Image:TitanNorth.jpg|thumb|400px|left|Radar map of Titan's north polar region]]

== Liquid Hydrocarbons on Titan ==

== Liquid Hydrocarbons on Titan ==

Until the Cassinin spacecraft arrived at Saturn, most astronomers understood that Titan's atmosphere had methane as a significant component, and speculated that this methane must derive from a surface ocean of methane. Yet initial observations of Titan's south pole disclosed no such ocean,<ref name=physorg>"[http://www.physorg.com/news5626.html No oceans on Titan, say researchers]." <http://physorg.com/>, August 4, 2005. Accessesd May 16, 2008.</ref> and indeed not more than two small lakes.<ref name=esach>"[http://www.esa.int/esaSC/SEMCSUUHJCF_index_0.html Titan's surface organics surpass oil reserves on Earth]." [[European Space Agency]], February 13, 2008. Accessed May 16, 2008.</ref> But observations of the north pole revealed vast liquid hydrocarbon deposits in the form of multiple lakes and streams, rather than a world-covering ocean.<ref name=esach/><ref name=Lorenz>Lorenz, Ralph D., Mitchell, Karl L., Kirk, Randolph L., ''et al''. "[http://www.agu.org/pubs/crossref/2008/2007GL032118.shtml Titan's inventory of organic surface materials]." ''Geophysical Research Letters'', vol. 35, L02206, January 29, 2008. <doi:10.1029/2007GL032118> Accessed May 16, 2008.</ref> These findings easily surpass all the known hydrocarbon reserves on Earth. Yet Lorenz ''et al.'' note that while the liquid lakes appear adequate in the short term to replenish methane lost through photolysis, they could ''not'' keep the atmosphere supplied with methane over [[deep time]].

Until the Cassinin spacecraft arrived at Saturn, most astronomers understood that Titan's atmosphere had methane as a significant component, and speculated that this methane must derive from a surface ocean of methane. Yet initial observations of Titan's south pole disclosed no such ocean,<ref name=physorg>"[http://www.physorg.com/news5626.html No oceans on Titan, say researchers]." <http://physorg.com/>, August 4, 2005. Accessesd May 16, 2008.</ref> and indeed not more than two small lakes.<ref name=esach>"[http://www.esa.int/esaSC/SEMCSUUHJCF_index_0.html Titan's surface organics surpass oil reserves on Earth]." [[European Space Agency]], February 13, 2008. Accessed May 16, 2008.</ref> But observations of the north pole revealed vast liquid hydrocarbon deposits in the form of multiple lakes and streams, rather than a world-covering ocean.<ref name=esach/><ref name=Lorenz>Lorenz, Ralph D., Mitchell, Karl L., Kirk, Randolph L., ''et al''. "[http://www.agu.org/pubs/crossref/2008/2007GL032118.shtml Titan's inventory of organic surface materials]." ''Geophysical Research Letters'', vol. 35, L02206, January 29, 2008. <doi:10.1029/2007GL032118> Accessed May 16, 2008.</ref> These findings easily surpass all the known hydrocarbon reserves on Earth. Yet Lorenz ''et al.'' note that while the liquid lakes appear adequate in the short term to replenish methane lost through photolysis, they could ''not'' keep the atmosphere supplied with methane over [[deep time]].