Editing Triple point (section)

=== Gas–liquid–solid triple point ===
{{see also|Properties of water#Triple point}}
[[File:Water-triple-point-20210210.gif|thumb|Water boiling at 0°C using a vacuum pump.]]Following the 2019 revision of the SI, the value of the triple point of water is no longer used as a defining point. However, its empirical value remains important: the unique combination of pressure and temperature at which liquid [[water]], solid [[ice]], and [[water vapour]] coexist in a stable equilibrium is approximately {{val|273.16|0.0001|u=K}}<ref name="SI Brochure 9">{{cite web |title=SI Brochure: The International System of Units (SI) – 9th edition |url=https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-EN.pdf/2d2b50bf-f2b4-9661-f402-5f9d66e4b507 |publisher=BIPM |access-date=21 February 2022}}</ref> and a vapour pressure of {{convert|611.657|Pa|mbar atm}}.<ref name="Wagner">[https://www.nist.gov/srd/upload/jpcrd477.pdf International Equations for the Pressure along the Melting and along the Sublimation Curve of Ordinary Water Substance]. W. Wagner, A. Saul and A. Pruss (1994), J. Phys. Chem. Ref. Data, '''23''', 515.</ref><ref name="Murphy">{{cite journal |doi=10.1256/qj.04.94 | volume=131 | issue=608 | title=Review of the vapour pressures of ice and supercooled water for atmospheric applications | year=2005 | journal=Quarterly Journal of the Royal Meteorological Society | pages=1539–1565 | last1 = Murphy | first1 = D. M.| bibcode=2005QJRMS.131.1539M| s2cid=122365938 | url=https://zenodo.org/record/1236243 | doi-access=free }}</ref>

Liquid water can only exist at pressures equal to or greater than the triple point. Below this, in the vacuum of [[outer space]], solid ice [[Sublimation (phase transition)|sublimates]], transitioning directly into water vapor when heated at a constant pressure. Conversely, above the triple point, solid ice first melts into liquid water upon heating at a constant pressure, then evaporates or boils to form vapor at a higher temperature.

For most substances, the gas–liquid–solid triple point is the minimum temperature where the liquid can exist. For water, this is not the case. The melting point of ordinary ice decreases with pressure, as shown by the [[phase diagram]]'s dashed green line. Just below the triple point, compression at a constant temperature transforms water vapor first to solid and then to liquid.

Historically, during the [[Mariner 9]] mission to [[Mars]], the triple point pressure of water was used to define "sea level". Now, [[laser altimetry]] and gravitational measurements are preferred to define Martian elevation.<ref>{{cite book |first=Michael H. |last=Carr |title=The Surface of Mars |url=https://archive.org/details/surfacemars00carr |url-access=limited |publisher=Cambridge University Press |year=2007 |page=[https://archive.org/details/surfacemars00carr/page/n19 5] |isbn=978-0-521-87201-0 }}</ref>