Editing Europa (moon) (section)

==Physical characteristics==
[[File:Europa, Earth & Moon size comparison.jpg|thumb|Size comparison of Europa (''lower left'') with the Moon (''top left'') and Earth (''right'')]]
Europa is slightly smaller than the [[Moon|Earth's moon]]. At just over {{Convert|3100|km|mi}} in [[diameter]], it is the [[List of moons by diameter|sixth-largest moon]] and [[List of Solar System objects by size|fifteenth-largest object]] in the [[Solar System]]. Though by a wide margin the least massive of the Galilean satellites, it is nonetheless more massive than all known moons in the Solar System smaller than itself combined.<ref name="Masses">Mass of Europa: 48{{e|21}} kg. Mass of Triton plus all smaller moons: 39.5{{e|21}} kg (see note ''k'' [[Triton (moon)#cite ref-nMassOthers 44-0|here]])</ref> Its bulk density suggests that it is similar in composition to [[terrestrial planet]]s, being primarily composed of [[silicate]] [[rock (geology)|rock]].<ref>{{cite journal |last1=Kargel |first1=Jeffrey S. |last2=Kaye |first2=Jonathan Z. |last3=Head |first3=James W. |last4=Marion |first4=Giles M. |last5=Sassen |first5=Roger |last6=Crowley |first6=James K. |last7=Ballesteros |first7=Olga Prieto |last8=Grant |first8=Steven A. |last9=Hogenboom |first9=David L. |title=Europa's Crust and Ocean: Origin, Composition, and the Prospects for Life |journal=Icarus |date=November 2000 |volume=148 |issue=1 |pages=226–265 |doi=10.1006/icar.2000.6471 |bibcode=2000Icar..148..226K |url=https://zenodo.org/record/1229838 |access-date=10 January 2020 |archive-date=31 July 2020 |archive-url=https://web.archive.org/web/20200731043909/https://zenodo.org/record/1229838 |url-status=live }}</ref>

===Internal structure===
It is estimated that Europa has an outer layer of [[water]] around {{Convert|100|km|mi|-0|abbr=on}} thick – a part frozen as its crust and a part as a liquid ocean underneath the ice. Recent [[magnetic field|magnetic-field]] data from the [[Galileo (spacecraft)|''Galileo'']] orbiter showed that Europa has an induced magnetic field through interaction with Jupiter's, which suggests the presence of a subsurface conductive layer.<ref name="Phillips 2014">{{cite journal |title=Europa Clipper Mission Concept |journal=Eos, Transactions American Geophysical Union |date=20 May 2014 |last1=Phillips |first1=Cynthia B. |author1-link=Cynthia B. Phillips|last2=Pappalardo |first2=Robert T. |volume=95 |issue=20 |pages=165–167 |doi=10.1002/2014EO200002 |bibcode = 2014EOSTr..95..165P |doi-access=free }}</ref> This layer is likely to be a salty liquid-water ocean. Portions of the crust are estimated to have undergone a rotation of nearly 80°, nearly flipping over (see [[true polar wander]]), which would be unlikely if the ice were solidly attached to the mantle.<ref name="Cowen2008">{{cite news |first=Ron |last=Cowen |title=A Shifty Moon |url=http://www.sciencenews.org/view/generic/id/32135/title/A_shifty_moon |work=Science News |date=7 June 2008 |access-date=29 May 2008 |archive-date=4 November 2011 |archive-url=https://web.archive.org/web/20111104175610/http://www.sciencenews.org/view/generic/id/32135/title/A_shifty_moon |url-status=live }}</ref> Europa probably contains a [[metal]]lic [[iron]] core.<ref name="Kivelson">{{cite journal |last=Kivelson |first=Margaret G. |author2=Khurana, Krishan K. |author3=Russell, Christopher T. |author4=Volwerk, Martin |author5=Walker, Raymond J. |author6= Zimmer, Christophe |s2cid=44381312 |date=2000 |title=Galileo Magnetometer Measurements: A Stronger Case for a Subsurface Ocean at Europa |journal=[[Science (journal)|Science]] |volume=289 |issue=5483 |pages=1340–1343 |doi=10.1126/science.289.5483.1340 |pmid=10958778 |bibcode = 2000Sci...289.1340K }}</ref><ref name="Bhatia2017">{{cite journal|last1=Bhatia|first1= G.K.|last2=Sahijpal|first2= S.|title=Thermal evolution of trans-Neptunian objects, icy satellites, and minor icy planets in the early solar system |journal=Meteoritics & Planetary Science |doi=10.1111/maps.12952|volume=52|issue= 12|year=2017|pages=2470–2490|bibcode=2017M&PS...52.2470B|s2cid= 133957919|doi-access=free}}</ref>

===Surface features===
[[File:Europa_nomenclature_map_(USGS_August_2023).png|thumb|443x443px|Map of Europa, by the [[United States Geological Survey]]]]
Europa is the smoothest known object in the Solar System, lacking large-scale features such as mountains and craters.<ref name="waterworld">{{cite web |url=http://teachspacescience.org/cgi-bin/search.plex?catid=10000304&mode=full |archive-url=https://web.archive.org/web/20110721210346/http://teachspacescience.org/cgi-bin/search.plex?catid=10000304&mode=full |archive-date=21 July 2011 |title=Europa: Another Water World? |date=2001 |access-date=9 August 2007 |publisher=[[NASA]], Jet Propulsion Laboratory |work=Project Galileo: Moons and Rings of Jupiter }}</ref> The prominent markings crisscrossing Europa appear to be mainly [[albedo feature]]s that emphasize low topography. There are few craters on Europa, because its surface is tectonically too active and therefore young.<ref name="Arnett1996">Arnett, Bill (7 November 1996) [http://www.astro.auth.gr/ANTIKATOPTRISMOI/nineplanets/nineplanets/europa.html ''Europa''] {{Webarchive|url=https://web.archive.org/web/20110904142701/http://www.astro.auth.gr/ANTIKATOPTRISMOI/nineplanets/nineplanets/europa.html |date=4 September 2011 }}. astro.auth.gr</ref><ref name="EuropaAlbedo">{{cite web |url=http://www.solarviews.com/eng/europa.htm |author=Hamilton, Calvin J. |title=Jupiter's Moon Europa |work=solarviews.com |access-date=27 February 2007 |archive-date=24 January 2012 |archive-url=https://web.archive.org/web/20120124135804/http://www.solarviews.com/eng/europa.htm |url-status=live }}</ref> The craters show the presence of hydrated salts dredged from the subsurface, but little sulfuric acid, indicating the impacts that formed them were very recent<ref>{{Cite journal |last1=Davis |first1=M. Ryleigh |last2=Brown |first2=Michael E. |date=2024-05-01 |title=Pwyll and Manannán Craters as a Laboratory for Constraining Irradiation Timescales on Europa |journal=The Planetary Science Journal |volume=5 |issue=5 |pages=107 |doi=10.3847/PSJ/ad3944 |doi-access=free |arxiv=2404.15474 |bibcode=2024PSJ.....5..107D |issn=2632-3338}}</ref>. Its icy crust has an [[albedo]] (light reflectivity) of 0.64, one of the highest of any moon.<ref name="datasheet" /><ref name="EuropaAlbedo" /> This indicates a young and active surface: based on estimates of the frequency of [[comet]]ary bombardment that Europa experiences, the surface is about 20 to 180&nbsp;million years old.<ref name="Schenk">Schenk, Paul M.; Chapman, Clark R.; Zahnle, Kevin; and Moore, Jeffrey M. (2004) [https://books.google.com/books?id=8GcGRXlmxWsC&pg=PA427 "Chapter 18: Ages and Interiors: the Cratering Record of the Galilean Satellites"] {{Webarchive|url=https://web.archive.org/web/20161224153428/https://books.google.com/books?id=8GcGRXlmxWsC&pg=PA427 |date=24 December 2016 }}, pp. 427 ff. in Bagenal, Fran; Dowling, Timothy E.; and McKinnon, William B., editors; ''Jupiter: The Planet, Satellites and Magnetosphere'', Cambridge University Press, {{ISBN|0-521-81808-7}}.</ref> There is no scientific consensus about the explanation for Europa's surface features.<ref name="Astrobio2007">{{cite web |url=http://www.astrobio.net/exclusive/603/high-tide-on-europa |title=High Tide on Europa |date=2007 |access-date=20 October 2007 |publisher=astrobio.net |work=Astrobiology Magazine |archive-date=29 September 2007 |archive-url=https://web.archive.org/web/20070929092639/http://www.astrobio.net/news/article603.html |url-status=usurped}}</ref>

It has been postulated Europa's equator may be covered in icy spikes called [[Penitente (snow formation)|penitentes]], which may be up to 15&nbsp;meters high. Their formation is due to direct overhead sunlight near the equator causing the ice to [[Sublimation (phase transition)|sublime]], forming vertical cracks.<ref>{{cite news|url=https://www.bbc.co.uk/news/science-environment-21341176|title=Ice blades threaten Europa landing|newspaper=BBC News|date=20 March 2013|last1=Rincon|first1=Paul|access-date=21 June 2018|archive-date=7 November 2018|archive-url=https://web.archive.org/web/20181107151101/https://www.bbc.co.uk/news/science-environment-21341176|url-status=live}}</ref><ref>[https://earthsky.org/space/jupiters-moon-europa-penitentes-ice-spikes Europa may have towering ice spikes on its surface] {{Webarchive|url=https://web.archive.org/web/20210121030101/https://earthsky.org/space/jupiters-moon-europa-penitentes-ice-spikes |date=21 January 2021 }}. Paul Scott Anderson, ''Earth and Sky.'' 20 October 2018.</ref><ref name ='Hobley 2018'>{{cite journal |last1=Hobley |first1=Daniel E. J. |last2=Moore |first2=Jeffrey M. |last3=Howard |first3=Alan D. |last4=Umurhan |first4=Orkan M. |title=Formation of metre-scale bladed roughness on Europa's surface by ablation of ice |journal=Nature Geoscience |date=8 October 2018 |volume=11 |issue=12 |pages=901–904 |doi=10.1038/s41561-018-0235-0 |bibcode=2018NatGe..11..901H |s2cid=134294079 |url=http://orca.cf.ac.uk/115808/1/D%20Hobley%20Nature_PDF_15128_2_merged_1533721936.pdf |access-date=11 January 2020 |archive-date=31 July 2020 |archive-url=https://web.archive.org/web/20200731024053/http://orca.cf.ac.uk/115808/1/D%20Hobley%20Nature_PDF_15128_2_merged_1533721936.pdf |url-status=live }}</ref> Although the imaging available from the ''Galileo'' orbiter does not have the resolution for confirmation, radar and thermal data are consistent with this speculation.<ref name ='Hobley 2018'/>

The [[ionizing radiation]] level at Europa's surface is equivalent to a daily dose of about 5.4&nbsp;[[sievert|Sv]] (540&nbsp;[[Röntgen equivalent man|rem]]),<ref name="ringwald">{{cite web|date=29 February 2000 |title=SPS 1020 (Introduction to Space Sciences) |publisher=California State University, Fresno |author=Frederick A. Ringwald |url=http://zimmer.csufresno.edu/~fringwal/w08a.jup.txt |access-date=4 July 2009 |url-status=dead |archive-url=https://web.archive.org/web/20080725050708/http://zimmer.csufresno.edu/~fringwal/w08a.jup.txt |archive-date=25 July 2008 }}</ref> an amount that would cause severe illness or death in human beings exposed for a single Earth day (24 hours).<ref name="remeffects">[https://archive.org/details/TheEffectsOfNuclearWeapons ''The Effects of Nuclear Weapons''], Revised ed., US DOD 1962, pp. 592–593</ref> A Europan day is about 3.5 times as long as an Earth day.<ref>{{Cite web|title=Europa: Facts about Jupiter's Moon, Europa • The Planets|url=https://theplanets.org/europa/|access-date=9 January 2021|website=The Planets|language=en-US|archive-date=11 January 2021|archive-url=https://web.archive.org/web/20210111150628/https://theplanets.org/europa/|url-status=live}}</ref>

==== Lineae ====
{{See also|List of lineae on Europa}}
[[File:Europa - PJ45-1.png|alt=|thumb|300x300px|True color mosaic of Europa's numerous [[lineae]]. The region of lineae at the center of this image is the [[List of geological features on Europa#Regiones|Annwn Regio]].<ref>{{cite web | url=https://planetarynames.wr.usgs.gov/Feature/14291 | title=Planetary Names }}</ref><ref>{{cite web|url=https://asc-planetarynames-data.s3.us-west-2.amazonaws.com/europa_15m_100ppi.pdf|title=Europa Nomenclature|website=asc-planetarynames|access-date=25 February 2024}}</ref> ]]
Europa's most striking surface features are a series of dark streaks crisscrossing the entire globe, called [[lineae]] ({{langx|en|lines}}). Close examination shows that the edges of Europa's crust on either side of the cracks have moved relative to each other. The larger bands are more than {{Convert|20|km|mi|0|abbr=on}} across, often with dark, diffuse outer edges, regular striations, and a central band of lighter material.<ref name="Geissler1998">{{cite journal |last1=Geissler |first1=P.E. |last2=Greenberg |first2=R. |last3=Hoppa |first3=G. |last4=McEwen |first4=A. |last5=Tufts |first5=R. |last6=Phillips |first6=C. |author6-link=Cynthia B. Phillips|last7=Clark |first7=B. |last8=Ockert-Bell |first8=M. |last9=Helfenstein |first9=P. |last10=Burns |first10=J. |last11=Veverka |first11=J. |last12=Sullivan |first12=R. |last13=Greeley |first13=R. |last14=Pappalardo |first14=R.T. |last15=Head |first15=J.W. |last16=Belton |first16=M.J.S. |last17=Denk |first17=T. |s2cid=15375333 |title=Evolution of Lineaments on Europa: Clues from Galileo Multispectral Imaging Observations |journal=Icarus |date=September 1998 |volume=135 |issue=1 |pages=107–126 |doi=10.1006/icar.1998.5980 |bibcode=1998Icar..135..107G |doi-access=free }}</ref>

The most likely hypothesis is that the lineae on Europa were produced by a series of eruptions of warm ice as Europa's crust slowly spreads open to expose warmer layers beneath.<ref name="Figueredo2003">{{cite journal |last1=Figueredo |first1=Patricio H. |last2=Greeley |first2=Ronald |title=Resurfacing history of Europa from pole-to-pole geological mapping |journal=Icarus |date=February 2004 |volume=167 |issue=2 |pages=287–312 |doi=10.1016/j.icarus.2003.09.016 |bibcode=2004Icar..167..287F }}</ref> The effect would have been similar to that seen on Earth's [[oceanic ridge]]s. These various fractures are thought to have been caused in large part by the tidal flexing exerted by Jupiter. Because Europa is tidally locked to Jupiter, and therefore always maintains approximately the same orientation towards Jupiter, the stress patterns should form a distinctive and predictable pattern. However, only the youngest of Europa's fractures conform to the predicted pattern; other fractures appear to occur at increasingly different orientations the older they are. This could be explained if Europa's surface rotates slightly faster than its interior, an effect that is possible due to the subsurface ocean mechanically decoupling Europa's surface from its rocky mantle and the effects of Jupiter's gravity tugging on Europa's outer ice crust.<ref name="Hurford2006">{{cite journal |last1=Hurford |first1=T.A. |last2=Sarid |first2=A.R. |last3=Greenberg |first3=R. |title=Cycloidal cracks on Europa: Improved modeling and non-synchronous rotation implications |journal=Icarus |date=January 2007 |volume=186 |issue=1 |pages=218–233 |doi=10.1016/j.icarus.2006.08.026 |bibcode=2007Icar..186..218H }}</ref> Comparisons of ''[[Voyager program|Voyager]]'' and ''Galileo'' spacecraft photos serve to put an upper limit on this hypothetical slippage. A full revolution of the outer rigid shell relative to the interior of Europa takes at least 12,000 years.<ref name="Kattenhorn1">{{cite journal |last=Kattenhorn |first=Simon A. |title=Nonsynchronous Rotation Evidence and Fracture History in the Bright Plains Region, Europa |journal=Icarus |volume=157 |issue=2 |pages=490–506 |date=2002 |doi=10.1006/icar.2002.6825 |bibcode=2002Icar..157..490K }}</ref> Studies of ''Voyager'' and ''Galileo'' images have revealed evidence of [[subduction]] on Europa's surface, suggesting that, just as the cracks are analogous to ocean ridges,<ref name="Schenk1989">{{cite journal |last1=Schenk |first1=Paul M. |last2=McKinnon |first2=William B. |title=Fault offsets and lateral crustal movement on Europa: Evidence for a mobile ice shell |journal=Icarus |date=May 1989 |volume=79 |issue=1 |pages=75–100 |doi=10.1016/0019-1035(89)90109-7 |bibcode=1989Icar...79...75S }}</ref><ref name="Katternhorn2">{{cite journal |last1=Kattenhorn |first1=Simon A. |last2=Prockter |first2=Louise M. |title=Evidence for subduction in the ice shell of Europa |journal=Nature Geoscience |date=7 September 2014 |volume=7 |issue=10 |pages=762–767 |doi=10.1038/ngeo2245 |bibcode=2014NatGe...7..762K }}</ref> so plates of icy crust analogous to [[tectonic plate]]s on Earth are recycled into the molten interior. This evidence of both crustal spreading at bands<ref name="Schenk1989" /> and convergence at other sites<ref name="Katternhorn2" /> suggests that Europa may have active [[plate tectonics]], similar to Earth.<ref name="NASA-20140908" /> However, the physics driving these plate tectonics are not likely to resemble those driving terrestrial plate tectonics, as the forces resisting potential Earth-like plate motions in Europa's crust are significantly stronger than the forces that could drive them.<ref>{{cite journal |last1=Howell |first1=Samuel M. |last2=Pappalardo |first2=Robert T. |title=Can Earth-like plate tectonics occur in ocean world ice shells? |journal=Icarus |date=1 April 2019 |volume=322 |pages=69–79 |doi=10.1016/j.icarus.2019.01.011 |bibcode=2019Icar..322...69H |s2cid=127545679 }}</ref>

==== Chaos and lenticulae ====
{{See also|List of geological features on Europa}} {{multiple image
 |direction = horizontal
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 |image1 = PIA01092 - Evidence of Internal Activity on Europa.jpg
 |image2 = Europa chaotic terrain.jpg
 |footer = Left: surface features indicative of [[tidal flexing]]: lineae, lenticulae and the [[Conamara Chaos]] region (close-up, right) where craggy, 250&nbsp;m high peaks and smooth plates are jumbled together
}}

Other features present on Europa are circular and elliptical {{Lang|la|lenticulae}} ([[Latin]] for "freckles"). Many are domes, some are pits and some are smooth, dark spots. Others have a jumbled or rough texture. The dome tops look like pieces of the older plains around them, suggesting that the domes formed when the plains were pushed up from below.<ref name="diapir">{{cite journal |last1=Sotin |first1=Christophe |last2=Head |first2=James W. |last3=Tobie |first3=Gabriel |title=Europa: Tidal heating of upwelling thermal plumes and the origin of lenticulae and chaos melting |journal=Geophysical Research Letters |date=April 2002 |volume=29 |issue=8 |pages=74-1–74-4 |doi=10.1029/2001GL013844 |bibcode=2002GeoRL..29.1233S |s2cid=14413348 |url=http://planetary.brown.edu/pdfs/2685.pdf |access-date=12 April 2020 |archive-date=31 July 2020 |archive-url=https://web.archive.org/web/20200731021343/http://planetary.brown.edu/pdfs/2685.pdf |url-status=live }}</ref>

One hypothesis states that these lenticulae were formed by [[diapir]]s of warm ice rising up through the colder ice of the outer crust, much like [[magma chamber]]s in Earth's crust.<ref name="diapir" /> The smooth, dark spots could be formed by [[meltwater]] released when the warm ice breaks through the surface. The rough, jumbled lenticulae (called regions of "chaos"; for example, [[Conamara Chaos]]) would then be formed from many small fragments of crust, embedded in hummocky, dark material, appearing like [[iceberg]]s in a frozen sea.<ref name="Goodman">{{cite journal |last1=Goodman |first1=Jason C. |title=Hydrothermal plume dynamics on Europa: Implications for chaos formation |journal=Journal of Geophysical Research |date=2004 |volume=109 |issue=E3 |pages=E03008 |doi=10.1029/2003JE002073 |bibcode=2004JGRE..109.3008G |hdl=1912/3570 |hdl-access=free }}
</ref>

An alternative hypothesis suggests that lenticulae are actually small areas of chaos and that the claimed pits, spots and domes are artefacts resulting from the over-interpretation of early, low-resolution Galileo images. The implication is that the ice is too thin to support the convective diapir model of feature formation.<ref name="thinice">{{cite journal |title=Tidal Heat in Europa: Ice Thickness and the Plausibility of Melt-Through |last1=O'Brien |first1=David P. |last2=Geissler |first2=Paul |last3=Greenberg |first3=Richard |journal=Bulletin of the American Astronomical Society |date=October 2000 |volume=30 |page=1066 |bibcode=2000DPS....32.3802O}}</ref><ref>{{cite book |title=Unmasking Europa |author=Greenberg, Richard |date=2008 |url=https://www.springer.com/astronomy/book/978-0-387-47936-1 |isbn=978-0-387-09676-6 |publisher=Springer + Praxis Publishing |series=Copernicus |pages=205–215, 236 |access-date=28 August 2017 |archive-date=22 January 2010 |archive-url=https://web.archive.org/web/20100122060908/http://www.springer.com/astronomy/book/978-0-387-47936-1 |url-status=live }}</ref>

In November 2011, a team of researchers, including researchers at [[University of Texas at Austin]], presented evidence suggesting that many "[[chaos terrain]]" features on Europa sit atop vast lakes of liquid water.<ref>{{Cite journal |last1=Schmidt |first1=B. E. |last2=Blankenship |first2=D. D. |last3=Patterson |first3=G. W. |last4=Schenk |first4=P. M. |date=November 2011 |title=Active formation of 'chaos terrain' over shallow subsurface water on Europa |url=https://www.nature.com/articles/nature10608 |journal=Nature |language=en |volume=479 |issue=7374 |pages=502–505 |doi=10.1038/nature10608 |pmid=22089135 |bibcode=2011Natur.479..502S |issn=1476-4687}}</ref><ref name="europagreatlakeairhart">{{cite web| title=Scientists Find Evidence for "Great Lake" on Europa and Potential New Habitat for Life| author=Airhart, Marc| date=2011| publisher=Jackson School of Geosciences| url=http://www.jsg.utexas.edu/news/2011/11/scientists-find-evidence-for-great-lake-on-europa/| access-date=16 November 2011| archive-date=18 December 2013| archive-url=https://web.archive.org/web/20131218161411/http://www.jsg.utexas.edu/news/2011/11/scientists-find-evidence-for-great-lake-on-europa/| url-status=live}}</ref> These lakes would be entirely encased in Europa's icy outer shell and distinct from a liquid ocean thought to exist farther down beneath the ice shell. Full confirmation of the lakes' existence will require a space mission designed to probe the ice shell either physically or indirectly, e.g. using radar.<ref name="europagreatlakeairhart"/> Chaos features may also be a result of increased melting of the ice shell and deposition of marine ice at low latitudes as a result of heterogeneous heating.<ref>{{Cite journal |last1=Soderlund |first1=K. M. |last2=Schmidt |first2=B. E. |last3=Wicht |first3=J. |last4=Blankenship |first4=D. D. |date=January 2014 |title=Ocean-driven heating of Europa's icy shell at low latitudes |url=https://www.nature.com/articles/ngeo2021 |journal=Nature Geoscience |language=en |volume=7 |issue=1 |pages=16–19 |doi=10.1038/ngeo2021 |bibcode=2014NatGe...7...16S |issn=1752-0894}}</ref>

Work published by researchers from [[Williams College]] suggests that chaos terrain may represent sites where impacting comets penetrated through the ice crust and into an underlying ocean.<ref name=":0">{{Cite journal|last1=Cox|first1=Rónadh|last2=Bauer|first2=Aaron W.|date=October 2015|title=Impact breaching of Europa's ice: Constraints from numerical modeling: IMPACT BREACHING OF EUROPA'S ICE|journal=Journal of Geophysical Research: Planets|language=en|volume=120|issue=10|pages=1708–1719|doi=10.1002/2015JE004877|s2cid=17563282|doi-access=free}}</ref><ref name=":1">{{Cite journal|last1=Cox|first1=Rónadh|last2=Ong|first2=Lissa C. F.|last3=Arakawa|first3=Masahiko|last4=Scheider|first4=Kate C.|date=December 2008|title=Impact penetration of Europa's ice crust as a mechanism for formation of chaos terrain|url=http://doi.wiley.com/10.1111/j.1945-5100.2008.tb00659.x|journal=Meteoritics & Planetary Science|language=en|volume=43|issue=12|pages=2027–2048|doi=10.1111/j.1945-5100.2008.tb00659.x|bibcode=2008M&PS...43.2027C|s2cid=129700548|access-date=12 January 2021|archive-date=1 October 2021|archive-url=https://web.archive.org/web/20211001064153/https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1945-5100.2008.tb00659.x|url-status=live}}</ref>

=== Subsurface ocean ===
[[Image:Europa_poster.svg|thumb|upright=2|Model of Europa's possible interior structure, with a thin ice crust and a [[subsurface ocean]] atop a rocky mantle and metallic core]]
The scientific consensus is that a layer of liquid water exists beneath Europa's surface, and that heat from tidal flexing allows the [[subsurface ocean]] to remain liquid.<ref name="geology" /><ref name="greenberg" /> Europa's surface temperature averages about {{Convert|110|K|C F|-1|lk=on}} at the equator and only {{Convert|50|K|C F|-1|}} at the poles, keeping Europa's icy crust as hard as granite.<ref name="cyclo">{{cite book |title=The Encyclopedia of the Solar System |author1=McFadden, Lucy-Ann |author2=Weissman, Paul |author3=Johnson, Torrence |publisher=Elsevier |date=2007 |page=[https://archive.org/details/encyclopediaofso0000unse_m0r6/page/432 432] |isbn=978-0-12-226805-2 |url=https://archive.org/details/encyclopediaofso0000unse_m0r6/page/432 }}</ref> The first hints of a subsurface ocean came from theoretical considerations of tidal heating (a consequence of Europa's slightly eccentric orbit and orbital resonance with the other Galilean moons). ''Galileo'' imaging team members argue for the existence of a subsurface ocean from analysis of ''Voyager'' and ''Galileo'' images.<ref name="greenberg">{{cite book|last=Greenberg|first=Richard|year=2005|title=Europa: The Ocean Moon: Search for an Alien Biosphere|publisher=Springer + Praxis|isbn=978-3-540-27053-9|pages=7 ff|doi=10.1007/b138547|series=Springer Praxis Books}}</ref> The most dramatic example is "chaos terrain", a common feature on Europa's surface that some interpret as a region where the subsurface ocean has melted through the icy crust. This interpretation is controversial. Most geologists who have studied Europa favor what is commonly called the "thick ice" model, in which the ocean has rarely, if ever, directly interacted with the present surface.<ref name="greeley">Greeley, Ronald; ''et al.'' (2004) "Chapter 15: Geology of Europa", pp. 329 ff. in Bagenal, Fran; Dowling, Timothy E.; and McKinnon, William B., editors; ''Jupiter: The Planet, Satellites and Magnetosphere'', Cambridge University Press, {{ISBN|0-521-81808-7}}.</ref> The best evidence for the thick-ice model is a study of Europa's large craters. The largest impact structures are surrounded by concentric rings and appear to be filled with relatively flat, fresh ice; based on this and on the calculated amount of heat generated by Europan tides, it is estimated that the outer crust of solid ice is approximately {{cvt|10|to|30|km|mi|sigfig=1}} thick,<ref>{{cite journal |title=Improved detection of tides at Europa with radiometric and optical tracking during flybys |journal=Planetary and Space Science |date=July 2015 |last1=Park |first1=Ryan S. |last2=Bills |first2=Bruce |last3=Buffington |first3=Brent B. |volume=112 |pages=10–14 |doi=10.1016/j.pss.2015.04.005 |bibcode = 2015P&SS..112...10P }}</ref> including a ductile "warm ice" layer, which could mean that the liquid ocean underneath may be about {{Convert|100|km|mi|-1|abbr=on}} deep.<ref>{{cite news |first=Zaina |last=Adamu |title=Water near surface of a Jupiter moon only temporary |date=1 October 2012 |url=http://lightyears.blogs.cnn.com/2012/10/01/a-moon-of-jupiter-may-have-water-temporarily/?hpt=us_bn4 |work=CNN News |access-date=2 October 2012 |archive-date=5 October 2012 |archive-url=https://web.archive.org/web/20121005011205/http://lightyears.blogs.cnn.com/2012/10/01/a-moon-of-jupiter-may-have-water-temporarily/?hpt=us_bn4 |url-status=dead }}</ref> This leads to a volume of Europa's oceans of 3×10<sup>18</sup>m<sup>3</sup>, between two or three times the volume of Earth's oceans.<ref>{{Cite APOD|title=All the Water on Europa|date=24 May 2012|access-date=8 March 2016}}</ref><ref>{{cite web|author=Williams, Matt|url=http://www.universetoday.com/15201/jupiters-moon-europa/|title=Jupiter's Moon Europa|work=Universe Today|date=15 September 2015|access-date=9 March 2016|archive-date=10 March 2016|archive-url=https://web.archive.org/web/20160310022713/http://www.universetoday.com/15201/jupiters-moon-europa/|url-status=live}}</ref>

The thin-ice model suggests that Europa's ice shell may be only a few kilometers thick. However, most planetary scientists conclude that this model considers only those topmost layers of Europa's crust that behave elastically when affected by Jupiter's tides.<ref name="Billings"/> One example is flexure analysis, in which Europa's crust is modeled as a plane or sphere weighted and flexed by a heavy load. Models such as this suggest the outer elastic portion of the ice crust could be as thin as {{Convert|200|m|ft}}. If the ice shell of Europa is really only a few kilometers thick, this "thin ice" model would mean that regular contact of the liquid interior with the surface could occur through open ridges, causing the formation of areas of chaotic terrain.<ref name="Billings">{{cite journal |title=The great thickness debate: Ice shell thickness models for Europa and comparisons with estimates based on flexure at ridges |author=Billings, Sandra E. |author2=Kattenhorn, Simon A. |journal=Icarus |volume=177 |issue=2 | pages=397–412 |date=2005 |doi=10.1016/j.icarus.2005.03.013 |bibcode=2005Icar..177..397B}}</ref> Large impacts going fully through the ice crust would also be a way that the subsurface ocean could be exposed.<ref name=":0" /><ref name=":1" />

====Composition====
[[File:Europa PIA2387x - Chaos Transition, Crisscrossing Bands & Chaos Near Agenor Linea.jpg|thumb|300px|Closeup views of Europa obtained on 26 September 1998; images clockwise from upper left show locations from north to south as indicated at lower left.]]
The ''Galileo'' orbiter found that Europa has a weak [[magnetic moment]], which is induced by the varying part of the Jovian magnetic field. The field strength at the magnetic equator (about 120 [[Tesla (unit)|nT]]) created by this magnetic moment is about one-sixth the strength of Ganymede's field and six times the value of Callisto's.<ref name="Zimmer">{{cite journal |last1=Zimmer |first1=C |title=Subsurface Oceans on Europa and Callisto: Constraints from Galileo Magnetometer Observations |journal=Icarus |date=October 2000 |volume=147 |issue=2 |pages=329–347 |doi=10.1006/icar.2000.6456 |bibcode=2000Icar..147..329Z |citeseerx=10.1.1.366.7700 }}</ref> The existence of the induced moment requires a layer of a highly electrically conductive material in Europa's interior. The most plausible candidate for this role is a large subsurface ocean of liquid saltwater.<ref name="Kivelson" />
{{Multiple image |header=Europa Closeups |caption_align=center |align=left |width=150 |direction=vertical |image1=PIA26332-JupiterMoonEuropaCloseupA.jpg |caption1=29 September 2022 |width1= |image2=PIA25696-Europa-JupiterMoon-20220929.jpg |caption2=9 September 2022 |width2= |footer= }}
Since the ''Voyager'' spacecraft flew past Europa in 1979, scientists have worked to understand the composition of the reddish-brown material that coats fractures and other geologically youthful features on Europa's surface.<ref>{{cite news |url=http://www.jpl.nasa.gov/news/news.php?feature=4602 |title=Europa Mission to Probe Magnetic Field and Chemistry |work=Jet Propulsion Laboratory |date=27 May 2015 |access-date=29 May 2015 |archive-date=2 December 2020 |archive-url=https://web.archive.org/web/20201202163713/https://www.jpl.nasa.gov/news/news.php?feature=4602 |url-status=live }}</ref> Spectrographic evidence suggests that the darker, reddish streaks and features on Europa's surface may be rich in salts such as [[magnesium sulfate]], deposited by evaporating water that emerged from within.<ref name="McCord1998">{{cite journal |title=Salts on Europa's Surface Detected by Galileo's Near Infrared Mapping Spectrometer |author1=McCord, Thomas B. |author2=Hansen, Gary B. |display-authors=etal |date=1998 |doi=10.1126/science.280.5367.1242 |volume=280 |issue=5367 |journal=Science |pages=1242–1245|pmid=9596573 |bibcode=1998Sci...280.1242M }}</ref> [[Sulfuric acid]] hydrate is another possible explanation for the contaminant observed spectroscopically.<ref name="Carlson2005">{{Cite journal | last1 = Carlson | first1 = R. W. | last2 = Anderson | first2 = M. S. | last3 = Mehlman | first3 = R. | last4 = Johnson | first4 = R. E. | title = Distribution of hydrate on Europa: Further evidence for sulfuric acid hydrate | doi = 10.1016/j.icarus.2005.03.026 | journal = Icarus | volume = 177 | issue = 2 | page = 461 | year = 2005 |bibcode = 2005Icar..177..461C }}</ref> In either case, because these materials are colorless or white when pure, some other material must also be present to account for the reddish color, and [[sulfur]] compounds are suspected.<ref name="Calvin">{{cite journal |last=Calvin |first=Wendy M. |author2=Clark, Roger N. |author3=Brown, Robert H. |author4= Spencer, John R. |title=Spectra of the ice Galilean satellites from 0.2 to 5 μm: A compilation, new observations, and a recent summary |journal=Journal of Geophysical Research |date=1995 |volume=100 |issue=E9 |pages=19,041–19,048 |bibcode=1995JGR...10019041C|doi=10.1029/94JE03349 }}</ref>

[[File:Europa_%28NIRCam_image,_cropped%29_%28weic2323b%29.jpg|right|thumb|upright|[[Near-infrared spectroscopy|NIR]] image of Europa by the [[James Webb Space Telescope]], confirming the presence of carbon dioxide on the moon<ref>{{cite web | url=https://www.nasa.gov/solar-system/nasas-webb-finds-carbon-source-on-surface-of-jupiters-moon-europa/#:~:text=life%2C%20particularly%20carbon.-,Astronomers%20using%20data%20from%20NASA%27s%20James%20Webb%20Space%20Telescope%20have,meteorites%20or%20other%20external%20sources | title=NASA's Webb Finds Carbon Source on Surface of Jupiter's Moon Europa - NASA | date=21 September 2023 }}</ref>]]
Another hypothesis for the colored regions is that they are composed of abiotic [[organic compounds]] collectively called [[tholins]].<ref name='Tholins Europa'>{{cite journal|doi=10.1029/2002JE001841 | bibcode=2002JGRE..107.5114B | volume=107 | issue=E11 | title=A new energy source for organic synthesis in Europa's surface ice | year=2002 | journal=Journal of Geophysical Research: Planets | pages=24–1–24–5 | last1 = Borucki | first1 = Jerome G. | last2 = Khare | first2 = Bishun | last3 = Cruikshank | first3 = Dale P.| doi-access=free }}</ref><ref name='amino acids'>{{cite conference |bibcode=2017AAS...22913804W |title=MISE: A Search for Organics on Europa |conference=American Astronomical Society Meeting Abstracts #229 |volume=229 |pages=138.04 |last1=Whalen |first1=Kelly |last2=Lunine |first2=Jonathan I. |last3=Blaney |first3=Diana L.|author3-link= Diana Blaney |year=2017 }}</ref><ref name='JPL2015'>{{cite news |url=http://www.jpl.nasa.gov/news/news.php?feature=4602 |title=Europa Mission to Probe Magnetic Field and Chemistry |work=Jet Propulsion Laboratory |date=27 May 2015 |access-date=23 October 2017 |archive-date=2 December 2020 |archive-url=https://web.archive.org/web/20201202163713/https://www.jpl.nasa.gov/news/news.php?feature=4602 |url-status=live }}</ref> The morphology of Europa's impact craters and ridges is suggestive of fluidized material welling up from the fractures where [[pyrolysis]] and [[radiolysis]] take place. In order to generate colored tholins on Europa, there must be a source of materials (carbon, nitrogen, and water) and a source of energy to make the reactions occur. Impurities in the water ice crust of Europa are presumed both to emerge from the interior as [[Cryovolcano|cryovolcanic]] events that resurface the body, and to accumulate from space as interplanetary dust.<ref name='Tholins Europa'/> Tholins bring important [[Astrobiology|astrobiological]] implications, as they may play a role in prebiotic chemistry and [[abiogenesis]].<ref name='prebiotic chem'>{{cite journal| pmc=3796891 | pmid=24143126 | doi=10.2174/13852728113179990078 | volume=17 | issue=16 | title=Atmospheric Prebiotic Chemistry and Organic Hazes | year=2013 | journal=Curr Org Chem | pages=1710–1723 | last1 = Trainer | first1 = MG}}</ref><ref name='Coll 2010'>{{cite conference |bibcode=2010cosp...38..777C |title=Prebiotic chemistry on Titan ? The nature of Titan's aerosols and their potential evolution at the satellite surface |conference=38th Cospar Scientific Assembly |volume=38 |pages=11 |last1=Coll |first1=Patrice |last2=Szopa |first2=Cyril |last3=Buch |first3=Arnaud |last4=Carrasco |first4=Nathalie |last5=Ramirez |first5=Sandra I. |last6=Quirico |first6=Eric |last7=Sternberg |first7=Robert |last8=Cabane |first8=Michel |last9=Navarro-Gonzalez |first9=Rafael |last10=Raulin |first10=Francois |last11=Israel |first11=G. |last12=Poch |first12=O. |last13=Brasse |first13=C. |year=2010 }}</ref><ref>{{cite journal |last1=Ruiz-Bermejo |first1=Marta |last2=Rivas |first2=Luis A. |last3=Palacín |first3=Arantxa |last4=Menor-Salván |first4=César |last5=Osuna-Esteban |first5=Susana |title=Prebiotic Synthesis of Protobiopolymers Under Alkaline Ocean Conditions |journal=Origins of Life and Evolution of Biospheres |date=16 December 2010 |volume=41 |issue=4 |pages=331–345 |doi=10.1007/s11084-010-9232-z |pmid=21161385 |bibcode=2011OLEB...41..331R |s2cid=19283373 }}</ref>

The presence of [[sodium chloride]] in the internal ocean has been suggested by a 450&nbsp;nm absorption feature, characteristic of irradiated NaCl crystals, that has been spotted in [[Hubble Space Telescope|HST]] observations of the chaos regions, presumed to be areas of recent subsurface upwelling.<ref name="Trumbo2019">{{cite journal |last1=Trumbo |first1=Samantha K. |last2=Brown |first2=Michael E. |last3=Hand |first3=Kevin P. |title=Sodium chloride on the surface of Europa |journal=Science Advances |date=12 June 2019 |volume=5 |issue=6 |pages=eaaw7123 |doi=10.1126/sciadv.aaw7123 |pmid=31206026 |pmc=6561749 |bibcode=2019SciA....5.7123T }}</ref> The subterranean ocean of Europa contains carbon<ref>{{Cite news |last=Devlin |first=Hannah |date=21 September 2023 |title=Scientists excited to find ocean of one of Jupiter's moons contains carbon |newspaper=The Guardian |url=https://www.theguardian.com/science/2023/sep/21/scientists-excited-to-find-ocean-of-one-of-jupiters-moons-contains-carbon }}</ref> and was observed on the surface ice as a concentration of [[carbon dioxide]] within Tara Regio, a geologically recently resurfaced terrain.<ref>{{Cite journal |last=Trumbo |first=Samantha |date=September 2023 |title=The distribution of CO2 on Europa indicates an internal source of carbon |journal=Science |volume=381 |issue=6664 |pages=1308–1311|doi=10.1126/science.adg4155 |pmid=37733851 |arxiv=2309.11684 }}</ref> JWST [[NIRSpec]] observations show that the northern hemisphere show crystalline water [[ice]] beneath the surface and amorphous ice dominating the surface. In the southern hemisphere Regiones Tara and Powys crystalline water ice dominates both the surface and the deeper layers. These two regiones likely experience ongoing thermal (re)crystallization, as the radiation near Jupiter cause particle amorphization at the top 10 microns over a period of less than 15 days.<ref name="Cartwright2025">{{cite arXiv|eprint=2504.05283 |last1=Cartwright |first1=Richard J. |last2=Hibbits |first2=Charles A. |last3=Holler |first3=Bryan J. |last4=Raut |first4=Ujjwal |last5=Nordheim |first5=Tom A. |last6=Neveu |first6=Marc |last7=Protopapa |first7=Silvia |last8=Glein |first8=Christopher R. |last9=Leonard |first9=Erin J. |last10=Roth |first10=Lorenz |last11=Beddingfield |first11=Chloe B. |last12=Villanueva |first12=Geronimo L. |title=JWST Reveals Spectral Tracers of Recent Surface Modification on Europa |date=2025 |class=astro-ph.EP }}</ref>
[[File:CO2_Europa.png|thumb|upright=4|center|A series of images of Europa in different wavelengths by the James Webb Space Telescope. The different wavelengths show the presence of different forms of carbon dioxide on Europa.]]

====Sources of heat====

Europa receives thermal energy from [[tidal heating]], which occurs through the tidal friction and tidal flexing processes caused by [[tidal acceleration]]: orbital and rotational energy are dissipated as heat in the [[Planetary core|core]] of the moon, the internal ocean, and the ice crust.<ref name="Europa FAQ 2012">{{cite news |url=http://solarsystem.nasa.gov/europa/faq.cfm |title=Frequently Asked Questions about Europa |work=NASA |date=2012 |access-date=18 April 2016 |archive-url=https://web.archive.org/web/20160428110229/http://solarsystem.nasa.gov/europa/faq.cfm |archive-date=28 April 2016 |url-status=dead }}</ref>

=====Tidal friction=====
Ocean tides are converted to heat by frictional losses in the oceans and their interaction with the solid bottom and with the top ice crust. In late 2008, it was suggested Jupiter may keep Europa's oceans warm by generating large planetary tidal waves on Europa because of its small but non-zero obliquity. This generates so-called [[Rossby wave]]s that travel quite slowly, at just a few kilometers per day, but can generate significant kinetic energy. For the current axial tilt estimate of 0.1 degree<!-- and assuming a linear dependency -->, the resonance from Rossby waves would contain 7.3{{E|18}} J of kinetic energy, which is two thousand times larger than that of the flow excited by the dominant tidal forces.<ref name="Zyga2008">{{cite web |title=Scientist Explains Why Jupiter's Moon Europa Could Have Energetic Liquid Oceans |url=http://www.physorg.com/news148278114.html |first=Lisa |last=Zyga |publisher=PhysOrg.com |date=12 December 2008 |access-date=28 July 2009 |archive-date=17 February 2009 |archive-url=https://web.archive.org/web/20090217213351/http://www.physorg.com/news148278114.html |url-status=live }}</ref><ref name="Tyler2008">{{cite journal |last=Tyler |first=Robert H. |title=Strong ocean tidal flow and heating on moons of the outer planets |journal=Nature |date=11 December 2008|volume=456 | pages=770–772 |doi=10.1038/nature07571 |pmid=19079055 |issue=7223 |bibcode =2008Natur.456..770T |s2cid=205215528 }}</ref> Dissipation of this energy could be the principal heat source of Europa's ocean.<ref name="Zyga2008"/><ref name="Tyler2008"/>

=====Tidal flexing=====
Tidal flexing kneads Europa's interior and ice shell, which becomes a source of heat.<ref>{{cite news |url=http://solarsystem.nasa.gov/europa/energy.cfm |title=Europa: Energy |work=NASA |date=2012 |access-date=18 April 2016 |quote=Tidal flexing of the ice shell could create slightly warmer pockets of ice that rise slowly upward to the surface, carrying material from the ocean below. |archive-url=https://web.archive.org/web/20160428191605/https://solarsystem.nasa.gov/europa/energy.cfm |archive-date=28 April 2016 |url-status=dead }}</ref> Depending on the amount of tilt, the heat generated by the ocean flow could be 100 to thousands of times greater than the heat generated by the flexing of Europa's rocky core in response to the gravitational pull from Jupiter and the other moons circling that planet.<ref>{{cite news |last=Tyler |first=Robert |url=https://www.sciencedaily.com/releases/2008/12/081212092056.htm |title=Jupiter's Moon Europa Does The Wave To Generate Heat |work=University of Washington |publisher=Science Daily |date=15 December 2008 |access-date=18 April 2016 |archive-date=14 May 2016 |archive-url=https://web.archive.org/web/20160514044845/https://www.sciencedaily.com/releases/2008/12/081212092056.htm |url-status=live }}</ref> Europa's seafloor could be heated by the moon's constant flexing, driving hydrothermal activity similar to undersea volcanoes in Earth's oceans.<ref name="Europa FAQ 2012"/>

Experiments and ice modeling published in 2016, indicate that tidal flexing dissipation can generate one order of magnitude more heat in Europa's ice than scientists had previously assumed.<ref name="Stacey 2016">{{cite news |last=Stacey |first=Kevin |url=https://news.brown.edu/articles/2016/04/europa |title=Europa's heaving ice might make more heat than scientists thought |work=Brown University |date=14 April 2016 |access-date=18 April 2016 |archive-date=21 April 2016 |archive-url=https://web.archive.org/web/20160421003232/https://news.brown.edu/articles/2016/04/europa |url-status=live }}</ref><ref name="McCarthy 2016">{{cite journal |title=Tidal dissipation in creeping ice and the thermal evolution of Europa |journal=Earth and Planetary Science Letters |date=1 June 2016 |last1=McCarthy |first1=Christine |last2=Cooper |first2=Reid F. |volume=443 |pages=185–194 |doi=10.1016/j.epsl.2016.03.006 |bibcode = 2016E&PSL.443..185M |doi-access=free }}</ref> Their results indicate that most of the heat generated by the ice actually comes from the ice's [[Crystal structure|crystalline structure]] (lattice) as a result of deformation, and not friction between the ice grains.<ref name="Stacey 2016"/><ref name="McCarthy 2016"/> The greater the deformation of the ice sheet, the more heat is generated.

=====Radioactive decay=====
In addition to tidal heating, the interior of Europa could also be heated by the decay of radioactive material ([[radiogenic heating]]) within the rocky mantle.<ref name="Europa FAQ 2012"/><ref>{{cite book |last1=Barr |first1=Amy C. |last2=Showman |first2=Adam P. |chapter=Heat transfer in Europa's icy shell |pages=405–430 |bibcode=2009euro.book..405B |citeseerx=10.1.1.499.6279 |editor-last=Pappalardo |editor-first=Robert T. |editor2-last=McKinnon |editor2-first=William B. |editor3-last=Khurana |editor3-first=Krishan |title=Europa |publisher=University of Arizona Press |year=2009 |isbn=978-0-8165-2844-8 }}</ref> But the models and values observed are one hundred times higher than those that could be produced by radiogenic heating alone,<ref>{{cite journal |title=Hydrothermal systems on Europa |journal=Geophysical Research Letters |date=9 March 2005 |last1=Lowell |first1=Robert P. |last2=DuBosse |first2=Myesha |volume=32 |issue=5 |pages=L05202 |doi=10.1029/2005GL022375 |bibcode = 2005GeoRL..32.5202L |s2cid=129270513 }}</ref> thus implying that tidal heating has a leading role in Europa.<ref>{{cite journal |last1=Ruiz |first1=Javier |title=The heat flow of Europa |journal=Icarus |date=October 2005 |volume=177 |issue=2 |pages=438–446 |doi=10.1016/j.icarus.2005.03.021 |bibcode=2005Icar..177..438R |url=https://eprints.ucm.es/id/eprint/10490/1/15-Europa_5.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://eprints.ucm.es/id/eprint/10490/1/15-Europa_5.pdf |archive-date=9 October 2022 |url-status=live }}</ref>

==== Plumes ====
<!---[[File:Water vapour plumes on Jupiter's moon Europa (artist's impression).jpg|thumb|Water vapor plumes on Jupiter's moon Europa (artist's impression)<ref name="Plumes2013" />]] --->[[File:Photo composite of suspected water plumes on Europa.jpg|thumb|Photo composite of suspected water plumes on Europa<ref name="Plumes2013">{{cite web|title=Photo composite of suspected water plumes on Europa|url=https://www.spacetelescope.org/images/opo1633a/|website=www.spacetelescope.org|access-date=6 October 2016|archive-date=9 October 2016|archive-url=https://web.archive.org/web/20161009104726/https://www.spacetelescope.org/images/opo1633a/|url-status=live}}</ref>]]
The [[Hubble Space Telescope]] acquired an image of Europa in 2012 that was interpreted to be a plume of water vapour erupting from near its south pole.<ref>{{Cite web|url=https://www.spacetelescope.org/news/heic1322/|title=Hubble discovers water vapour venting from Jupiter's moon Europa|date=12 December 2013|website=www.spacetelescope.org|publisher=Hubble Space Telescope/European Space Agency|language=en|access-date=16 April 2019|archive-date=16 April 2019|archive-url=https://web.archive.org/web/20190416012613/https://www.spacetelescope.org/news/heic1322/|url-status=live}}</ref><ref name="Plumes2013"/> The image suggests the plume may be {{convert|200|km|mi|abbr=on}} high, or more than 20 times the height of Mt. Everest.,<ref name="NASA-20131212-EU" /><ref>{{cite news | first = Leigh | last = Fletcher | title = The Plumes of Europa | date = 12 December 2013 | url = http://www.planetary.org/blogs/guest-blogs/2013/1212-fletcher-the-plumes-of-europa.html | work = The Planetary Society | access-date = 17 December 2013 | archive-date = 15 December 2013 | archive-url = https://web.archive.org/web/20131215041956/http://www.planetary.org/blogs/guest-blogs/2013/1212-fletcher-the-plumes-of-europa.html | url-status = live }}</ref><ref>{{cite news | first = Charles Q. | last = Choi | title = Jupiter Moon Europa May Have Water Geysers Taller Than Everest | date = 12 December 2013 | url = http://www.space.com/23923-europa-water-geyers-taller-than-everest.html | work = Space.com | access-date = 17 December 2013 | archive-date = 15 December 2013 | archive-url = https://web.archive.org/web/20131215173940/http://www.space.com/23923-europa-water-geyers-taller-than-everest.html | url-status = live }}</ref> though recent observations and modeling suggest that typical Europan plumes may be much smaller.<ref>{{Cite journal|last1=Fagents|first1=Sarah A.|last2=Greeley|first2=Ronald|last3=Sullivan|first3=Robert J.|last4=Pappalardo|first4=Robert T.|last5=Prockter|first5=Louise M.|date=30 June 1999|title=Cryomagmatic Mechanisms for the Formation of Rhadamanthys Linea, Triple Band Margins, and Other Low-Albedo Features on Europa|url=https://www.sciencedirect.com/science/article/abs/pii/S0019103599962541|journal=Icarus|volume=144|issue=1 |pages=54–88|doi=10.1006/icar.1999.6254|access-date=16 June 2022|archive-date=16 June 2022|archive-url=https://web.archive.org/web/20220616235235/https://www.sciencedirect.com/science/article/abs/pii/S0019103599962541|url-status=live}}</ref><ref>{{Cite journal|last1=Quick|first1=Lynnae C.|author-link=Lynnae Quick|last2=Barnouin|first2=Olivier S.|last3=Prockter|first3=Louise|author-link3=Louise Prockter|last4=Patterson|first4=G. Wesley|date=15 September 2013|title=Constraints on the Detection of Cryovolcanic Plumes on Europa|url=https://www.sciencedirect.com/science/article/abs/pii/S0032063313001803|journal=Planetary and Space Science|volume=86|issue=1 |pages=1–9|doi=10.1006/icar.1999.6254|access-date=16 June 2022|archive-date=16 June 2022|archive-url=https://web.archive.org/web/20220616235322/https://www.sciencedirect.com/science/article/abs/pii/S0032063313001803|url-status=live}}</ref><ref>{{Cite journal|last1=Paganini|first1=L.|last2=Villanueva|first2=G.L.|last3=Mandell|first3=A.M.|last4=Hurford|first4=T.A.|last5=Retherford|first5=K.D.|last6=Mumma|first6=M.A.|title=CA measurement of water vapour amid a largely quiescent environment on Europa|date=18 November 2019|url=https://www.nature.com/articles/s41550-019-0933-6|journal=Nature Astronomy|volume=4|issue=3|pages=266–272|doi=10.1038/s41550-019-0933-6|s2cid=210278335|access-date=16 June 2022|archive-date=18 June 2022|archive-url=https://web.archive.org/web/20220618112026/https://www.nature.com/articles/s41550-019-0933-6|url-status=live}}</ref> It has been suggested that if plumes exist, they are episodic<ref>{{cite news |last=Dyches |first=Preston |url=http://www.nasa.gov/jpl/signs-of-europa-plumes-remain-elusive-in-search-of-cassini-data |title=Signs of Europa Plumes Remain Elusive in Search of Cassini Data |work=NASA |date=30 July 2015 |access-date=18 April 2016 |archive-date=16 April 2016 |archive-url=https://web.archive.org/web/20160416214519/http://www.nasa.gov/jpl/signs-of-europa-plumes-remain-elusive-in-search-of-cassini-data/ |url-status=live }}</ref> and likely to appear when Europa is at its farthest point from Jupiter, in agreement with [[tidal force]] modeling predictions.<ref name='Europa tidal forces 2013'>{{cite journal |last1=Roth |first1=L. |last2=Saur |first2=J. |last3=Retherford |first3=K. D. |last4=Strobel |first4=D. F. |last5=Feldman |first5=P. D. |last6=McGrath |first6=M. A. |last7=Nimmo |first7=F. |title=Transient Water Vapor at Europa's South Pole |journal=Science |date=12 December 2013 |volume=343 |issue=6167 |pages=171–174 |doi=10.1126/science.1247051 |pmid=24336567 |bibcode=2014Sci...343..171R |s2cid=27428538 }}</ref> Additional imaging evidence from the Hubble Space Telescope was presented in September 2016.<ref name="plumes 2016">{{cite news |last=Berger |first=Eric |url=https://arstechnica.com/science/2016/09/hubble-finds-additional-evidence-of-water-vapor-plumes-on-europa/ |title=Hubble finds additional evidence of water vapor plumes on Europa |work=NASA |publisher=ARS Technica |date=26 September 2016 |access-date=26 September 2016 |archive-date=26 September 2016 |archive-url=https://web.archive.org/web/20160926202346/http://arstechnica.com/science/2016/09/hubble-finds-additional-evidence-of-water-vapor-plumes-on-europa/ |url-status=live }}</ref><ref>{{cite news |last=Amos |first=Jonathan |url=https://www.bbc.com/news/science-environment-37473617 |title=Europa moon 'spewing water jets' |work=BBC News |date=26 September 2016 |access-date=26 September 2016 |archive-date=26 September 2016 |archive-url=https://web.archive.org/web/20160926204050/http://www.bbc.com/news/science-environment-37473617 |url-status=live }}</ref>

In May 2018, astronomers provided supporting evidence of water plume activity on Europa, based on an updated critical analysis of data obtained from the ''Galileo'' space probe, which orbited Jupiter between 1995 and 2003. ''Galileo'' flew by Europa in 1997 within {{convert|206|km|mi|abbr=on}} of the moon's surface and the researchers suggest it may have flown through a water plume.<ref name="NA-20180514">{{cite journal |last1=Jia |first1=Xianzhe |last2=Kivelson |first2=Margaret G. |last3=Khurana |first3=Krishan K. |last4=Kurth |first4=William S. |title=Evidence of a plume on Europa from Galileo magnetic and plasma wave signatures |date=14 May 2018 |journal=[[Nature Astronomy]] |volume=2 |issue=6 |pages=459–464 |doi=10.1038/s41550-018-0450-z |bibcode=2018NatAs...2..459J |s2cid=134370392 }}</ref><ref name="NASA-20180514">{{cite web |last1=McCartney |first1=Gretchen |last2=Brown |first2=Dwayne |last3=Wendel |first3=JoAnna |title=Old Data Reveal New Evidence of Europa Plumes |website=[[Jet Propulsion Laboratory]] |url=https://www.jpl.nasa.gov/news/news.php?feature=7122 |date=14 May 2018 |access-date=14 May 2018 |archive-date=17 June 2019 |archive-url=https://web.archive.org/web/20190617213109/https://www.jpl.nasa.gov/news/news.php?feature=7122 |url-status=live }}</ref><ref name="NYT-20180514">{{cite news |last=Chang |first=Kenneth |title=NASA Finds Signs of Plumes From Europa, Jupiter's Ocean Moon |url=https://www.nytimes.com/2018/05/14/science/europa-plumes-water.html |date=14 May 2018 |work=[[The New York Times]] |access-date=14 May 2018 |archive-date=14 May 2018 |archive-url=https://web.archive.org/web/20180514165834/https://www.nytimes.com/2018/05/14/science/europa-plumes-water.html |url-status=live }}</ref><ref name="SP-20180514">{{cite web |last=Wall |first=Mike |title=This May Be the Best Evidence Yet of a Water Plume on Jupiter's Moon Europa |url=https://www.space.com/40575-jupiter-moon-europa-plume-galileo-spacecraft.html |date=14 May 2018 |work=[[Space.com]] |access-date=14 May 2018 |archive-date=14 May 2018 |archive-url=https://web.archive.org/web/20180514183300/https://www.space.com/40575-jupiter-moon-europa-plume-galileo-spacecraft.html |url-status=live }}</ref> Such plume activity could help researchers in a [[Life|search for life]] from the subsurface Europan ocean without having to land on the moon.<ref name="NA-20180514"/>

The tidal forces are about 1,000 times stronger than the Moon's effect on [[Earth]]. The only other moon in the Solar System exhibiting water vapor plumes is [[Enceladus]].<ref name="NASA-20131212-EU" /> The estimated eruption rate at Europa is about 7000&nbsp;kg/s<ref name='Europa tidal forces 2013'/> compared to about 200&nbsp;kg/s for the plumes of Enceladus.<ref name="Hansen2006">{{Cite journal | last1 = Hansen | first1 = C. J. | last2 = Esposito | first2 = L. | last3 = Stewart | first3 = A. I. | last4 = Colwell | first4 = J. | last5 = Hendrix | first5 = A. | last6 = Pryor | first6 = W. | last7 = Shemansky | first7 = D. | last8 = West | first8 = R. | s2cid = 2954801 | doi = 10.1126/science.1121254 | title = Enceladus' Water Vapor Plume | journal = Science | volume = 311 | issue = 5766 | pages = 1422–1425 | date = 10 March 2006| pmid = 16527971|bibcode = 2006Sci...311.1422H }}</ref><ref name="Spencer2013">{{Cite journal | last1 = Spencer | first1 = J. R. | last2 = Nimmo | first2 = F. | s2cid = 140646028 | doi = 10.1146/annurev-earth-050212-124025 | title = Enceladus: An Active Ice World in the Saturn System | journal = [[Annual Review of Earth and Planetary Sciences]]| volume = 41 | page = 693 | date = May 2013| issue = 1 |bibcode = 2013AREPS..41..693S }}</ref> If confirmed, it would open the possibility of a flyby through the plume and obtain a sample to analyze ''in situ'' without having to use a lander and drill through kilometres of ice.<ref name="plumes 2016"/><ref>{{cite news |last=O'Neill |first=Ian |url=http://www.space.com/34151-nasa-activity-spied-on-europa-but-its-not-aliens.html |title=NASA: Activity Spied on Europa, But It's 'NOT Aliens' |work=Discovery News |publisher=Space |date=22 September 2016 |access-date=23 September 2016 |archive-date=23 September 2016 |archive-url=https://web.archive.org/web/20160923011825/http://www.space.com/34151-nasa-activity-spied-on-europa-but-its-not-aliens.html |url-status=live }}</ref><ref name="europa_plume_in_situ">{{cite journal |title=On the in-situ detectability of Europa's water vapour plumes from a flyby mission |author1=Huybrighs, Hans |author2=Futaana, Yoshifumi |author3=Barabash, Stas |author4=Wieser, Martin |author5=Wurz, Peter|author6=Krupp, Norbert|author7=Glassmeier, Karl-Heinz|author8=Vermeersen, Bert |journal=Icarus |date=June 2017 |volume=289 |pages=270–280 |doi=10.1016/j.icarus.2016.10.026|arxiv=1704.00912|bibcode=2017Icar..289..270H|s2cid=119470009 }}</ref>

In November 2020, a study was published in the peer-reviewed scientific journal ''[[Geophysical Research Letters]]'' suggesting that the plumes may originate from water within the crust of Europa as opposed to its subsurface ocean. The study's model, using images from the Galileo space probe, proposed that a combination of freezing and pressurization may result in at least some of the cryovolcanic activity. The pressure generated by migrating briny water pockets would thus, eventually, burst through the crust, thereby creating these plumes. The hypothesis that cryovolcanism on Europa could be triggered by freezing and pressurization of liquid pockets in the icy crust was first proposed by Sarah Fagents at the University of Hawai'i at Mānoa, who in 2003, was the first to model and publish work on this process.<ref>{{Cite journal|last1=Fagents|first1=Sarah A.|date=27 December 2003|title=Considerations for effusive cryovolcanism on Europa:The post-Galileo perspective|url=https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2003JE002128|journal=Icarus|language=en|volume=108|issue=E12|pages=5139|doi=10.1029/2003JE002128|bibcode=2003JGRE..108.5139F|access-date=16 June 2022|archive-date=16 June 2022|archive-url=https://web.archive.org/web/20220616235245/https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2003JE002128|url-status=live}}</ref> A press release from NASA's Jet Propulsion Laboratory referencing the November 2020 study suggested that plumes sourced from migrating liquid pockets could potentially be less hospitable to life. This is due to a lack of substantial energy for organisms to thrive off, unlike proposed hydrothermal vents on the subsurface ocean floor.<ref name="JPL20201113">{{cite web |last1=McCartney |first1=Gretchen |last2=Hautaluoma |first2=Grey |last3=Johnson |first3=Alana |last4=Tucker |first4=Danielle |title=Potential Plumes on Europa Could Come From Water in the Crust |website=[[Jet Propulsion Laboratory]] |url=https://www.jpl.nasa.gov/news/news.php?feature=7785 |date=13 November 2020 |access-date=13 November 2020 |archive-date=13 November 2020 |archive-url=https://web.archive.org/web/20201113191732/https://www.jpl.nasa.gov/news/news.php?feature=7785 |url-status=live }}</ref><ref name="GRL20201105">{{cite journal |last1=Steinbrügge |first1=G. |last2=Voigt |first2=J. R. C. |last3=Wolfenbarger |first3=N. S. |last4=Hamilton |first4=C. W. |last5=Soderlund |first5=K. M. |last6=Young D. |first6=D. A. |last7=Blankenship |first7=D. |last8=Vance D. |first8=S. D. |last9=Schroeder |first9=M. |title=Brine Migration and Impact-Induced Cryovolcanism on Europa |date=5 November 2020 |journal=[[Geophysical Research Letters]] |volume=47 |issue=21 |pages={e2020GL090797} |doi=10.1029/2020GL090797|bibcode=2020GeoRL..4790797S |s2cid=228890686 }}</ref>

=== Atmosphere ===
[[File:PIA26239-EuropaMoon-OxygenProduction-20240304.jpg|thumb|right|270px|Diagram of how Europa's atmosphere is created by bombardment from ionized particles]]
The atmosphere of Europa can be categorized as thin and tenuous (often called an exosphere), primarily composed of oxygen and trace amounts of water vapor.<ref name=":2">{{Cite web |title=Life Beyond Earth - The Habitable Zone - Europa |url=https://www.pbs.org/lifebeyondearth/alone/europa.html#:~:text=Europa%20does%20have%20an%20atmosphere,surface%20and%20produce%20water%20vapor. |access-date=13 May 2022 |website=www.pbs.org |archive-date=13 May 2022 |archive-url=https://web.archive.org/web/20220513202621/https://www.pbs.org/lifebeyondearth/alone/europa.html#:~:text=Europa%20does%20have%20an%20atmosphere,surface%20and%20produce%20water%20vapor. |url-status=live }}</ref> However, this quantity of oxygen is produced in a non-biological manner. Given that Europa's surface is icy, and subsequently very cold; as solar ultraviolet radiation and charged particles (ions and electrons) from the Jovian magnetospheric environment collide with Europa's surface, water vapor is created and instantaneously separated into oxygen and hydrogen constituents. As it continues to move, the hydrogen is light enough to pass through the surface gravity of the atmosphere leaving behind only oxygen.<ref name=":02">{{Cite web |title=Hubble Finds Oxygen Atmosphere on Jupiter's Moon, Europa |url=http://hubblesite.org/contents/news-releases/1995/news-1995-12 |access-date=13 May 2022 |website=HubbleSite.org |language=en |archive-date=16 April 2023 |archive-url=https://web.archive.org/web/20230416151919/https://hubblesite.org/contents/news-releases/1995/news-1995-12.html |url-status=live }}</ref> The surface-bounded atmosphere forms through radiolysis, the [[Dissociation (chemistry)|dissociation]] of molecules through radiation.<ref name="Johnson1982">{{cite journal |last1=Johnson |first1=Robert E. |last2=Lanzerotti |first2=Louis J. |last3=Brown |first3=Walter L. |date=1982 |title=Planetary applications of ion induced erosion of condensed-gas frosts |journal=Nuclear Instruments and Methods in Physics Research |volume=198 |issue=1 |page=147 |doi=10.1016/0167-5087(82)90066-7|bibcode=1982NIMPR.198..147J }}</ref> This accumulated oxygen atmosphere can get to a height of {{convert|190|km|mi|abbr=on}} above the surface of Europa. Molecular oxygen is the densest component of the atmosphere because it has a long lifetime; after returning to the surface, it does not stick (freeze) like a water or [[hydrogen peroxide]] molecule but rather desorbs from the surface and starts another [[External ballistics|ballistic]] arc. Molecular hydrogen never reaches the surface, as it is light enough to escape Europa's surface gravity.<ref name="Liang">{{cite journal |last1=Liang |first1=Mao-Chang |date=2005 |title=Atmosphere of Callisto |journal=Journal of Geophysical Research |volume=110 |issue=E2 |pages=E02003 |bibcode=2005JGRE..110.2003L |doi=10.1029/2004JE002322 |s2cid=8162816 |url=https://resolver.caltech.edu/CaltechAUTHORS:20140825-144257305 |access-date=15 July 2022 |archive-date=16 April 2023 |archive-url=https://web.archive.org/web/20230416151918/https://authors.library.caltech.edu/48861/ |url-status=live |doi-access=free }}</ref><ref name="Smyth">{{cite conference |last1=Smyth |first1=W. H. |last2=Marconi |first2=M. L. |year=2007 |title=Processes Shaping Galilean Satellite Atmospheres from the Surface to the Magnetosphere |conference=Workshop on Ices |volume=1357 |pages=131 |bibcode=2007LPICo1357..131S}}</ref> Europa is one of the few moons in the [[Solar System]] with a quantifiable atmosphere, along with [[Titan (moon)|Titan]], [[Io (moon)#Atmosphere|Io]], [[Triton (moon)|Triton]], [[Ganymede (moon)#Atmosphere and ionosphere|Ganymede]] and [[Callisto (moon)#Atmosphere and ionosphere|Callisto]].<ref name=":3">{{Cite web |title=Hubble Finds Oxygen Atmosphere On Jupiter's Moon Europa |url=https://solarviews.com/eng/europapr.htm |access-date=13 May 2022 |website=solarviews.com |archive-date=2 October 2022 |archive-url=https://web.archive.org/web/20221002191429/https://solarviews.com/eng/europapr.htm |url-status=live }}</ref> Europa is also one of several moons in the Solar System with very large quantities of [[Volatile (astrogeology)|ice (volatiles)]], otherwise known as "icy moons".<ref name=":4">{{Cite web |last=Cartier |first=Kimberly M. S. |date=14 December 2020 |title=Do Uranus's Moons Have Subsurface Oceans? |url=http://eos.org/articles/do-uranuss-moons-have-subsurface-oceans |access-date=13 May 2022 |website=Eos |language=en-US |archive-date=16 May 2022 |archive-url=https://web.archive.org/web/20220516065456/https://eos.org/articles/do-uranuss-moons-have-subsurface-oceans |url-status=live }}</ref>[[Image:Europa field.png|thumb|left|upright|Magnetic field around Europa. The red line shows a trajectory of the ''Galileo'' spacecraft during a typical flyby (E4 or E14).]]Europa is also considered to be geologically active due to the constant release of hydrogen-oxygen mixtures into space. As a result of the moon's particle venting, the atmosphere requires continuous replenishment.<ref name=":02" /> Europa also contains a small magnetosphere (approximately 25% of Ganymede's). However, this magnetosphere varies in size as Europa orbits through Jupiter's magnetic field. This confirms that a conductive element, such as a large ocean, likely lies below its icy surface.<ref>{{Cite web |title=Europa |url=https://solarsystem.nasa.gov/moons/jupiter-moons/europa/in-depth |access-date=13 May 2022 |website=NASA Solar System Exploration |archive-date=14 May 2022 |archive-url=https://web.archive.org/web/20220514053708/https://solarsystem.nasa.gov/moons/jupiter-moons/europa/in-depth/ |url-status=live }}</ref> As multiple studies have been conducted over Europa's atmosphere, several findings conclude that not all oxygen molecules are released into the atmosphere. This unknown percentage of oxygen may be absorbed into the surface and sink into the subsurface. Because the surface may interact with the subsurface ocean (considering the geological discussion above), this molecular oxygen may make its way to the ocean, where it could aid in biological processes.<ref name="Chyba">{{Cite journal |last1=Chyba |first1=C. F. |last2=Hand |first2=K. P. |year=2001 |title=PLANETARY SCIENCE: Enhanced: Life Without Photosynthesis |journal=Science |volume=292 |issue=5524 |pages=2026–2027 |doi=10.1126/science.1060081 |pmid=11408649 |s2cid=30589825}}</ref><ref>{{Cite journal |last1=Chyba |first1=Christopher F. |last2=Hand |first2=Kevin P. |date=15 June 2001 |title=Life Without Photosynthesis |url=https://www.science.org/doi/10.1126/science.1060081 |journal=Science |language=en |volume=292 |issue=5524 |pages=2026–2027 |doi=10.1126/science.1060081 |pmid=11408649 |s2cid=30589825 |issn=0036-8075 |access-date=13 May 2022 |archive-date=13 May 2022 |archive-url=https://web.archive.org/web/20220513202621/https://www.science.org/doi/10.1126/science.1060081 |url-status=live }}</ref> One estimate suggests that, given the turnover rate inferred from the apparent ~0.5&nbsp;Gyr maximum age of Europa's surface ice, subduction of radiolytically generated oxidizing species might well lead to oceanic free oxygen concentrations that are comparable to those in terrestrial deep oceans.<ref name="ChemDisequilib">{{cite journal |author1=Hand, Kevin P. |author2=Carlson, Robert W. |author3=Chyba, Christopher F. |date=December 2007 |title=Energy, Chemical Disequilibrium, and Geological Constraints on Europa |journal=Astrobiology |volume=7 |issue=6 |pages=1006–1022 |bibcode=2007AsBio...7.1006H |citeseerx=10.1.1.606.9956 |doi=10.1089/ast.2007.0156 |pmid=18163875}}</ref>

Through the slow release of oxygen and hydrogen, a neutral torus around Europa's orbital plane is formed. This "neutral cloud" has been detected by both the ''[[Cassini–Huygens|Cassini]]'' and ''Galileo'' spacecraft, and has a greater content (number of atoms and molecules) than the neutral cloud surrounding Jupiter's inner moon Io.<ref name="Smyth2006">{{cite journal |last=Smyth |first=William H. |author2=Marconi, Max L. |date=2006 |title=Europa's atmosphere, gas tori, and magnetospheric implications |journal=[[Icarus (journal)|Icarus]] |volume=181 |issue=2 |page=510 |bibcode=2006Icar..181..510S |doi=10.1016/j.icarus.2005.10.019}}</ref> This torus was officially confirmed using Energetic Neutral Atom (ENA) imaging. Europa's torus ionizes through the process of neutral particles exchanging electrons with its charged particles. Since Europa's magnetic field rotates faster than its orbital velocity, these ions are left in the path of its magnetic field trajectory, forming a plasma. It has been hypothesized that these ions are responsible for the plasma within Jupiter's magnetosphere.<ref>{{Cite journal |last1=Smith |first1=Howard Todd |last2=Mitchell |first2=Donald G. |last3=Johnson |first3=Robert E. |last4=Mauk |first4=Barry H. |last5=Smith |first5=Jacob E. |date=22 January 2019 |title=Europa Neutral Torus Confirmation and Characterization Based on Observations and Modeling |journal=The Astrophysical Journal |language=en |volume=871 |issue=1 |pages=69 |doi=10.3847/1538-4357/aaed38 |bibcode=2019ApJ...871...69S |s2cid=126922049 |issn=1538-4357 |doi-access=free }}</ref>

On 4 March 2024, astronomers reported that the surface of Europa may have much less [[oxygen]] than previously inferred.<ref name="NYT-20240304km">{{cite news |last=Miller |first=Katrina |title=An Ocean Moon Thought to Be Habitable May Be Oxygen-Starved - A new study suggests that the amount of the element on the moon of Jupiter is on the lower end of previous estimates. |url=https://www.nytimes.com/2024/03/04/science/europa-moon-oxygen.html |date=4 March 2024 |work=[[The New York Times]] |url-status=live |archiveurl=https://archive.today/20240305000839/https://www.nytimes.com/2024/03/04/science/europa-moon-oxygen.html |archivedate=5 March 2024 |accessdate=5 March 2024 }}</ref><ref name="NYT-20240304jrs">{{cite journal |author=Szalay, J.R. |display-authors=et al |title=Oxygen production from dissociation of Europa's water-ice surface |date=4 March 2024 |journal=[[Nature Astronomy]] |volume=8 |issue=5 |pages=567–576 |doi=10.1038/s41550-024-02206-x |doi-access=free |pmid=38798715 |pmc=11111413 |bibcode=2024NatAs...8..567S }}</ref>

==== Discovery of atmosphere ====
The atmosphere of Europa was first discovered in 1995 by astronomers D. T. Hall and collaborators using the [[Goddard High Resolution Spectrograph]] instrument of the [[Hubble Space Telescope]].<ref name="Hall1995">{{cite journal |last1=Hall |first1=D. T. |last2=Strobel |first2=D. F. |last3=Feldman |first3=P. D. |last4=McGrath |first4=M. A. |last5=Weaver |first5=H. A. |title=Detection of an oxygen atmosphere on Jupiter's moon Europa |date=23 February 1995 |journal=Nature |volume=373 |issue=6516 |pages=677–679 |doi=10.1038/373677a0 |pmid=7854447 |bibcode=1995Natur.373..677H }}</ref> This observation was further supported in 1997 by the ''Galileo'' orbiter during its mission within the Jovian system. The ''Galileo'' orbiter performed three radio occultation events of Europa, where the probe's radio contact with Earth was temporarily blocked by passing behind Europa. By analyzing the effects Europa's sparse atmosphere had on the radio signal just before and after the occultation, for a total of six events, a team of astronomers led by A. J. Kliore established the presence of an [[ionosphere|ionized layer]] in Europa's atmosphere.<ref name="Kliore1997">{{cite journal |last1=Kliore |first1=A. J. |last2=Hinson |first2=D. P. |last3=Flaser |first3=F. M. |last4=Nagy |first4=A. F. |last5=Cravens |first5=T. E. |title=The Ionosphere of Europa from Galileo Radio Occultations |date=18 July 1997 |journal=Science |volume=277 |issue=5324 |pages=355–358 |doi=10.1126/science.277.5324.355 |doi-access=free |pmid=9219689 |bibcode=1997Sci...277..355K }}</ref>

==== Climate and weather ====
Despite the presence of a [[gas torus]], Europa has no weather producing clouds. As a whole, Europa has no wind, precipitation, or presence of sky color as its gravity is too low to hold an atmosphere substantial enough for those features. Europa's gravity is approximately 13% of Earth's. The temperature on Europa varies from −160&nbsp;°C at the equator, to −220&nbsp;°C at either of its poles.<ref>{{Cite web |author1=Elizabeth Howell |date=22 March 2018 |title=Europa: Facts About Jupiter's Icy Moon and Its Ocean |url=https://www.space.com/15498-europa-sdcmp.html |access-date=13 May 2022 |website=Space.com |language=en |archive-date=13 May 2022 |archive-url=https://web.archive.org/web/20220513202621/https://www.space.com/15498-europa-sdcmp.html |url-status=live }}</ref> Europa's subsurface ocean is thought to be significantly{{clarify|date=September 2023}} warmer however. It is hypothesized that because of radioactive and tidal heating (as mentioned in the sections above), there are points in the depths of Europa's ocean that may be only slightly cooler than Earth's oceans. Studies have also concluded that Europa's ocean would have been rather acidic at first, with large concentrations of sulfate, calcium, and carbon dioxide. But over the course of 4.5&nbsp;billion years, it became full{{clarify|date=September 2023}} of [[chloride]], thus resembling our 1.94% chloride oceans on Earth.