Editing Titan (moon) (section)

=== 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>