Editing Habitable zone (section)

==Habitability outside the HZ==
[[Image:Liquid lakes on titan.jpg|thumb|upright|The discovery of hydrocarbon lakes on Saturn's moon Titan has begun to call into question the [[carbon chauvinism]] that underpins HZ concept.]]
Liquid-water environments have been found to exist in the absence of atmospheric pressure and at temperatures outside the HZ temperature range. For example, [[Saturn]]'s moons [[Titan (moon)|Titan]] and [[Enceladus]] and [[Jupiter]]'s moons [[Europa (moon)|Europa]] and [[Ganymede (moon)|Ganymede]], all of which are outside the habitable zone, may hold large volumes of liquid water in [[subsurface ocean]]s.<ref>{{cite web |url = http://phl.upr.edu/library/media/liquidwaterinthesolarsystem |title = Liquid Water in the Solar System |access-date = 2013-12-15 |last = Torres |first = Abel |date = 2012-06-12 |archive-date = 2013-11-18 |archive-url = https://web.archive.org/web/20131118034218/http://phl.upr.edu/library/media/liquidwaterinthesolarsystem |url-status = dead }}</ref>

Outside the HZ, [[tidal heating]] and [[radioactive decay]] are two possible heat sources that could contribute to the existence of liquid water.<ref name="Cowen2008"/><ref name="Bryner, Jeanna"/> Abbot and Switzer (2011) put forward the possibility that subsurface water could exist on [[rogue planet]]s as a result of radioactive decay-based heating and insulation by a thick surface layer of ice.<ref name="physcisarxivlab-2011"/>

With some theorising that life on Earth may have actually originated in stable, subsurface habitats,<ref name=Munro2013>{{Citation
 |title = Miners deep underground in northern Ontario find the oldest water ever known
 |url = https://nationalpost.com/news/canada/worlds-oldest-water-bubbling-into-northern-ontario-mine
 |date = 2013
 |author = Munro, Margaret
 |work = National Post
 |access-date = 2013-10-06
}}</ref><ref name=Davies2013>{{cite journal
 |title        = The Origin of Life II: How did it begin?
 |journal      = Science Progress
 |issn         = 2047-7163
 |volume       = 84
 |issue        = 1
 |pages        = 17–29
 |date         = February 2001
 |doi          = 10.3184/003685001783239096
 |doi-access   = free
 |pmid         = 11382135
 |pmc          = 10367499
 |last         = Davies
 |first        = Paul
 }}</ref> it has been suggested that it may be common for wet subsurface extraterrestrial habitats such as these to 'teem with life'.<ref name=Taylor1996>{{Citation
 |title = Life Underground
 |url = http://www.psrd.hawaii.edu/Dec96/PSRD-LifeUnderground.pdf
 |date = 1996
 |author = Taylor, Geoffrey
 |journal = Planetary Science Research Discoveries
 |page = 4
 |bibcode = 1996psrd.reptE...4T
 |access-date = 2013-10-06
}}</ref> On Earth itself, living organisms may be found more than {{convert|6|km|mi|abbr=on}} below the surface.<ref name=Doyle2013>{{Citation
 |title = Deep underground, worms and "zombie microbes" rule
 |url = https://www.reuters.com/article/us-life-idUSBRE9230WM20130304
 |author = Doyle, Alister
 |access-date = 2013-10-06
 |work=Reuters
 |date=4 March 2013
}}</ref>

Another possibility is that outside the HZ organisms may use [[alternative biochemistry|alternative biochemistries]] that do not require water at all. Astrobiologist [[Christopher McKay]], has suggested that [[methane]] ({{chem|C|H|4}}) may be a solvent conducive to the development of "cryolife", with the Sun's "methane habitable zone" being centered on {{convert|1610000000|km|mi AU|sigfig=2|abbr=on}} from the star.<ref name=villard-2011 /> This distance is coincident with the location of Titan, whose lakes and rain of methane make it an ideal location to find McKay's proposed cryolife.<ref name=villard-2011 /> In addition, [[list of microorganisms tested in outer space|testing of a number of organisms]] has found some are capable of surviving in extra-HZ conditions.<ref>
{{cite journal
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