Editing Supercritical fluid (section)

==Natural occurrence==

===Hydrothermal circulation===
[[File:Blacksmoker in Atlantic Ocean.jpg|thumb|right|A [[black smoker]], a type of hydrothermal vent]]
{{See also|Hydrothermal circulation}}
Hydrothermal circulation occurs within the Earth's crust wherever fluid becomes heated and begins to [[Convection|convect]]. These fluids are thought to reach supercritical conditions under a number of different settings, such as in the formation of porphyry copper deposits or high temperature circulation of seawater in the sea floor. At mid-ocean ridges, this circulation is most evident by the appearance of hydrothermal vents known as "black smokers". These are large (metres high) chimneys of sulfide and sulfate minerals which vent fluids up to 400&nbsp;°C. The fluids appear like great black billowing clouds of smoke due to the precipitation of dissolved metals in the fluid. It is likely that at that depth many of these vent sites reach supercritical conditions, but most cool sufficiently by the time they reach the sea floor to be subcritical. One particular vent site, Turtle Pits, has displayed a brief period of supercriticality at the vent site. A further site, [[Beebe Hydrothermal Vent Field|Beebe]], in the Cayman Trough, is thought to display sustained supercriticality at the vent orifice.<ref>{{cite web|last1=Webber|first1=A.P.|last2=Murton|first2=B.|last3=Roberts|first3=S.|last4=Hodgkinson|first4=M.|title=Supercritical Venting and VMS Formation at the Beebe Hydrothermal Field, Cayman Spreading Centre|url=http://goldschmidt.info/2014/abstracts/abstractView?abstractId=2504|website=Goldschmidt Conference Abstracts 2014|publisher=Geochemical Society|access-date=29 July 2014|archive-url=https://web.archive.org/web/20140729183137/http://goldschmidt.info/2014/abstracts/abstractView?abstractId=2504|archive-date=29 July 2014|url-status=dead}}</ref>

===Planetary atmospheres===
The [[atmosphere of Venus]] is 96.5% carbon dioxide and 3.5% nitrogen. The surface pressure is {{convert|9.3| MPa }} and the surface temperature is {{convert |735| K}}, above the critical points of both major constituents and making the surface atmosphere a supercritical fluid.<ref name="u3r1a">{{cite journal | last1=Lebonnois | first1=Sebastien | last2=Schubert | first2=Gerald | title=The deep atmosphere of Venus and the possible role of density-driven separation of CO2 and N2 | journal=Nature Geoscience | publisher=Springer Science and Business Media LLC | volume=10 | issue=7 | date=2017-06-26 | issn=1752-0894 | doi=10.1038/ngeo2971 | pages=473–477| bibcode=2017NatGe..10..473L | s2cid=133864520 | url=https://hal.archives-ouvertes.fr/hal-01635402/file/deepatm_persp_rev2.pdf }}</ref>

The interior atmospheres of the Solar System's four [[giant planet]]s are composed mainly of hydrogen and helium at temperatures well above their critical points. The gaseous outer atmospheres of the [[gas giant]]s [[Jupiter]] and [[Saturn]] transition smoothly into the dense liquid interior, while the nature of the transition zones of the [[ice giant]]s [[Neptune]] and [[Uranus]] is unknown.{{cn|date=May 2024}} Theoretical models of [[extrasolar planet]] [[Gliese 876 d]] have posited an ocean of pressurized, supercritical fluid water with a sheet of solid high pressure water ice at the bottom.{{cn|date=May 2024}}