Editing Titan (moon) (section)

== Prebiotic conditions and life ==
{{Main|Life on Titan}}
{{See also|Planetary habitability}}
[[File:AtmosphericComparison Titan Earth.svg|thumb|alt=Profile of Titan's atmosphere compared to Earth's|Profile of Titan's atmosphere compared to Earth's]]
Titan is thought to be a [[Abiogenesis|prebiotic environment]] rich in complex [[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 (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=[[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 [[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 [[Miller–Urey experiment]] and several following experiments have shown that with an atmosphere similar to that of Titan and the addition of [[UV radiation]], complex molecules and polymer substances like [[tholin]]s can be generated. The reaction starts with [[dissociation (chemistry)|dissociation]] of nitrogen and methane, forming hydrogen cyanide 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 [[DNA]] and [[RNA]], were among the many compounds produced. In addition, [[amino acids]]—the building blocks of [[protein]]—were found. It was the first time nucleotide bases and amino acids had been found in such an experiment without liquid water being present.<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>

=== 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 200 km (120) deep beneath a water-ice crust with conditions that, although extreme by terrestrial standards, are such that life could survive.<ref name="Fortes2000" /> [[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 ===
{{See also|Hypothetical types of biochemistry}}

It has been speculated that life could exist in the lakes of liquid methane on Titan, just as organisms on Earth live in water.<ref name="mckay" /> Such organisms would inhale H<sub>2</sub> in place of O<sub>2</sub>, metabolize it with [[acetylene]] instead of [[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 [[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 [[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]] [[Christopher McKay|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 [[active transport]], and organisms with large [[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 [[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 [[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 [[cell membrane]] capable of functioning in liquid [[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 [[phospholipid]]s, compounds of carbon, hydrogen, oxygen, and [[phosphorus]]. This hypothetical cell membrane was termed an "[[azotosome]]", a combination of "azote", French for nitrogen, and "[[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 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 [[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|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 [[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, [[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 [[Planetary habitability|habitable]] in the far future. Five billion years from now, as the Sun becomes a sub-[[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 [[viscosity]] of ammonia-water solutions coupled with low temperatures would cause chemical reactions to proceed more slowly on Titan.<ref>{{cite journal  |last1=Lorenz |first1=Ralph D. |last2=Lunine |first2=Jonathan I. |last3=McKay |first3=Christopher P. |title=Titan under a red giant sun: A new kind of "habitable" moon |journal=Geophysical Research Letters |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>