View source for Astronomy:Europa (moon)

{{short description|Smallest Galilean moon of Jupiter}}
{{Infobox planet
| name = Europa
| alt_names = Jupiter {{rn|II}}
| pronounced = {{IPAc-en|j|ʊ|ˈ|r|oʊ|p|ə}}<ref>{{Cite encyclopedia |url=http://www.lexico.com/definition/Europa |archive-url=https://web.archive.org/web/20200322182116/https://www.lexico.com/definition/europa |url-status=dead |archive-date=22 March 2020 |title=Europa |dictionary=Lexico UK English Dictionary |publisher=[[Organization:Oxford University Press|Oxford University Press]]}}<br>{{MW|Europa}}</ref>
| adjectives = Europan {{IPAc-en|j|ʊ|ˈ|r|oʊ|p|ən}}<ref>G.G. Schaber (1982) "Geology of Europa", in David Morrison, ed., ''Satellites of Jupiter'', vol. 3, International Astronomical Union, p 556 ff.</ref><ref name="Greenberg 2005">Greenberg (2005) ''Europa: the ocean moon''</ref>
| named_after = Ευρώπη ''Eurōpē''
| image = Europa in natural color.png
| image_scale =
| caption = Europa in true colour, with its surface predominantly white. Imaged on 29 September 2022 by the ''[[Engineering:Juno (spacecraft)|Juno]]'' spacecraft. The brown oval on the lower right is the [[Astronomy:List of geological features on Europa#Large ringed features|Callanish]] crater.<ref>{{cite web | url=https://www.jpl.nasa.gov/images/pia25334-junocam-image-of-europa-from-flyby | title=JunoCam Image of Europa from Flyby | website=[[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]] }}</ref> 
| discoverer = [[Biography:Galileo Galilei|Galileo Galilei]]<br />Simon Marius
| discovered = 8 January 1610<ref name="IAUMoonDiscoveries"/>
| orbit_ref = <ref name="horizons">{{cite web|url=http://ssd.jpl.nasa.gov/horizons.cgi|title=JPL HORIZONS solar system data and ephemeris computation service|work=Solar System Dynamics|publisher=[[Organization:NASA|NASA]], Jet Propulsion Laboratory|access-date=10 August 2007|archive-date=7 October 2012|archive-url=https://web.archive.org/web/20121007034731/http://ssd.jpl.nasa.gov/horizons.cgi|url-status=live}}</ref>
| epoch = 8 January 2004
| mean_orbit_radius = {{val|670900|u=km}}<ref name="factsheet">{{cite web|title=Overview of Europa Facts|work=NASA|url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jup_Europa|access-date=27 December 2007|url-status=dead|archive-url=https://web.archive.org/web/20140326221921/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jup_Europa|archive-date=26 March 2014 }}</ref>
| eccentricity = 0.009<ref name="factsheet"/>
| periapsis = {{val|664862|u=km}}<ref group=lower-alpha>Periapsis is derived from the semimajor axis (''a'') and eccentricity (''e''): ''a''(1&nbsp;−&nbsp;''e'').</ref>
| apoapsis = {{val|676938|u=km}}<ref group=lower-alpha>Apoapsis is derived from the semimajor axis (''a'') and eccentricity (''e''): ''a''(1&nbsp;+&nbsp;''e'').</ref>
| period = {{val|3.551181|u=d}}<ref name="factsheet"/>
| avg_speed = {{val| 13743.36|u=m/s}}<ref>{{Cite web|url=https://solarsystem.nasa.gov/moons/jupiter-moons/europa/by-the-numbers|title=By the Numbers &#124; Europa|website=NASA Solar System Exploration|access-date=6 May 2021|archive-date=6 May 2021|archive-url=https://web.archive.org/web/20210506170939/https://solarsystem.nasa.gov/moons/jupiter-moons/europa/by-the-numbers/|url-status=live}}</ref>
| inclination = 0.470° (to Jupiter's equator)<br />1.791° (to the [[Astronomy:Ecliptic|ecliptic]])<ref name="factsheet"/>
| satellite_of = [[Astronomy:Jupiter|Jupiter]]
| group = Galilean moon
| mean_radius = {{val|1560.8|.5|u=km}} (0.245 [[Astronomy:Earth|Earth]]s)<ref name="jplfact"/>
| surface_area = {{val|3.09|e=7|u=km2}} (0.061 Earths)<ref group=lower-alpha>Surface area derived from the radius (''r''): 4{{pi}}''r''<sup> 2</sup>.</ref>
| volume = {{val|1.593|e=10|u=km3}} (0.015 Earths)<ref group=lower-alpha>Volume derived from the radius (''r''): <sup>4</sup>/<sub>3</sub>{{pi}}''r''<sup> 3</sup>.</ref>
| mass = {{val|4.799844|.000013|e=22|u=kg}} (0.008 Earths)<ref name="jplfact"/>
| density = {{val|3.013|.005|u=g/cm3}} (0.546 Earths)<ref name="jplfact"/>
| surface_grav = {{val|1.314|ul=m/s2}} (0.134 g)<ref group=lower-alpha>Surface gravity derived from the mass (''m''), the [[Physics:Gravitational constant|gravitational constant]] (''G'') and the radius (''r''): G''m''/''r''<sup>2</sup>.</ref>
| moment_of_inertia_factor = {{val|0.346|0.005}}<ref name="Showman1999">{{cite journal|last1= Showman|first1=A. P.|last2= Malhotra|first2= R.|s2cid=9492520|title=The Galilean Satellites|journal= Science|volume= 286|issue= 5437|date= 1 October 1999|pages =77–84|doi= 10.1126/science.286.5437.77|pmid=10506564}}</ref> (estimate)
| escape_velocity = {{val|2.025|u=km/s}}<ref group=lower-alpha>Escape velocity derived from the mass (''m''), the [[Physics:Gravitational constant|gravitational constant]] (''G'') and the radius (''r''): <math>\textstyle\sqrt{2Gm/r}</math>.</ref>
| rotation = Synchronous<ref name=Geissler/>
| axial_tilt = 0.1°<ref name="Bills2005">{{cite journal|last=Bills|first=Bruce G.|title=Free and forced obliquities of the Galilean satellites of Jupiter|date=2005|volume=175|issue=1|pages=233–247|doi=10.1016/j.icarus.2004.10.028|bibcode=2005Icar..175..233B|journal=Icarus|url=https://zenodo.org/record/1259023|access-date=29 June 2019|archive-date=27 July 2020|archive-url=https://web.archive.org/web/20200727063125/https://zenodo.org/record/1259023|url-status=live}}</ref>
| albedo = 0.67&nbsp;±&nbsp;0.03<ref name="jplfact">{{cite web|last=Yeomans|first=Donald K.|date=13 July 2006|title=Planetary Satellite Physical Parameters|publisher=JPL Solar System Dynamics|url=http://ssd.jpl.nasa.gov/?sat_phys_par|access-date=5 November 2007|archive-date=14 August 2009|archive-url=https://web.archive.org/web/20090814111752/http://ssd.jpl.nasa.gov/?sat_phys_par|url-status=live}}</ref>
| magnitude = 5.29 (opposition)<ref name="jplfact"/>
| temp_name1 = Surface
| min_temp_1 = ≈&thinsp;50 [[Kelvin|K]]<ref name="cyclo"/>
| mean_temp_1 = 102&nbsp;K (−171&nbsp;°C)
| max_temp_1 = 125&nbsp;K
| atmosphere = yes
| surface_pressure = 0.1 [[Pascal (unit)|µPa]] (10<sup>−12</sup> [[Bar (unit)|bar]])<ref name="McGrathChapter">{{cite book|last=McGrath|editor1=Pappalardo, Robert T.|editor2=McKinnon, William B.|editor3=Khurana, Krishan K.|title=Europa|date=2009|publisher=University of Arizona Press|isbn=978-0-8165-2844-8|chapter=Atmosphere of Europa }}</ref>
}}

'''Europa''' {{IPAc-en|audio=en-Europa.ogg|j|ʊ|ˈ|r|oʊ|p|ə}}, or '''Jupiter II''', is the smallest of the four [[Astronomy:Galilean moons|Galilean moons]] orbiting [[Astronomy:Jupiter|Jupiter]], and the sixth-closest to the planet of all [[Astronomy:Moons of Jupiter#List|the 95 known moons of Jupiter]]. It is also the [[Astronomy:List of natural satellites|sixth-largest moon]] in the [[Astronomy:Solar System|Solar System]]. Europa was discovered independently by Simon Marius and [[Biography:Galileo Galilei|Galileo Galilei]]<ref name="IAUMoonDiscoveries"/> and was named (by Marius) after Europa, the [[History:Phoenicia|Phoenicia]]n mother of King [[Religion:Minos|Minos]] of Crete and lover of [[Religion:Zeus|Zeus]] (the Greek equivalent of the Roman god [[Religion:Jupiter (mythology)|Jupiter]]).

Slightly smaller than [[Astronomy:Moon|Earth's Moon]], Europa is made of [[Chemistry:Silicate|silicate]] rock and has a water-ice crust<ref name="NYT-20150315">{{cite news |last=Chang |first=Kenneth |title=Suddenly, It Seems, Water Is Everywhere in Solar System |url=https://www.nytimes.com/2015/03/13/science/space/suddenly-it-seems-water-is-everywhere-in-solar-system.html |date=12 March 2015 |work={{wipe|The New York Times}} |access-date=13 March 2015 |archive-date=9 May 2020 |archive-url=https://web.archive.org/web/20200509080640/https://www.nytimes.com/2015/03/13/science/space/suddenly-it-seems-water-is-everywhere-in-solar-system.html |url-status=live }}</ref> and probably an [[Astronomy:Iron–nickel alloy|iron–nickel]] core. It has a very thin atmosphere, composed primarily of oxygen. Its white-beige surface is [[Earth:Glacial striation|striated]] by light [[Tan (color)|tan]] cracks and streaks, but craters are relatively few. In addition to Earth-bound telescope observations, Europa has been examined by a succession of space-probe flybys, the first occurring in the early 1970s. In September 2022, the [[Engineering:Juno (spacecraft)|''Juno'']] spacecraft flew within about 320&nbsp;km (200 miles) of Europa for a more recent close-up view.<ref name="NYT-20220930">{{cite news |last=Chang |first=Kenneth |title=New Europa Pictures Beamed Home by NASA's Juno Spacecraft - The space probe has been studying Jupiter since 2016 and just flew within about 200 miles of the surface of the ice-covered ocean moon. |url=https://www.nytimes.com/2022/09/30/science/europa-nasa-juno-photos.html |date=30 September 2022 |work={{wipe|The New York Times}} |accessdate=30 September 2022 |archive-date=30 September 2022 |archive-url=https://web.archive.org/web/20220930091805/https://www.nytimes.com/2022/09/30/science/europa-nasa-juno-photos.html |url-status=live }}</ref>

Europa has the smoothest surface of any known solid object in the Solar System. The apparent youth and smoothness of the surface have led to the [[Hypothesis|hypothesis]] that a water ocean exists beneath the surface, which could conceivably harbor [[Astronomy:Extraterrestrial life|extraterrestrial life]].<ref name="Tritt2002">{{cite web |url=http://people.msoe.edu/~tritt/sf/europa.life.html |title=Possibility of Life on Europa |last=Tritt |first=Charles S. |access-date=10 August 2007 |publisher=Milwaukee School of Engineering |date=2002 |url-status=dead |archive-url=https://web.archive.org/web/20070609150109/http://people.msoe.edu/~tritt/sf/europa.life.html |archive-date=9 June 2007 }}</ref> The predominant model suggests that heat from tidal flexing causes the ocean to remain liquid and drives ice movement similar to [[Earth:Plate tectonics|plate tectonics]], absorbing chemicals from the surface into the ocean below.<ref name="geology">{{cite web |url=http://geology.asu.edu/~glg_intro/planetary/p8.htm |title=Tidal Heating |work=geology.asu.edu |archive-url=https://web.archive.org/web/20060329000051/http://geology.asu.edu/~glg_intro/planetary/p8.htm |archive-date=29 March 2006 |url-status=dead }}</ref><ref name="NASA-20140908">{{cite web |last1=Dyches |first1=Preston |last2=Brown |first2=Dwayne |last3=Buckley |first3=Michael |title=Scientists Find Evidence of 'Diving' Tectonic Plates on Europa |url=http://www.jpl.nasa.gov/news/news.php?release=2014-300 |date=8 September 2014 |work=[[Organization:NASA|NASA]] |access-date=8 September 2014 |archive-date=4 April 2019 |archive-url=https://web.archive.org/web/20190404030841/https://www.jpl.nasa.gov/news/news.php?release=2014-300 |url-status=live }}</ref> [[Earth:Sea salt|Sea salt]] from a subsurface ocean may be coating some geological features on Europa, suggesting that the ocean is interacting with the sea floor. This may be important in determining whether Europa could be habitable.<ref name="NASA-20150512">{{cite web |last1=Dyches |first1=Preston |last2=Brown |first2=Dwayne |title=NASA Research Reveals Europa's Mystery Dark Material Could Be Sea Salt |url=http://www.jpl.nasa.gov/news/news.php?feature=4586 |date=12 May 2015 |work=[[Organization:NASA|NASA]] |access-date=12 May 2015 |archive-date=15 May 2015 |archive-url=https://web.archive.org/web/20150515013057/http://www.jpl.nasa.gov/news/news.php?feature=4586 |url-status=live }}</ref> In addition, the [[Astronomy:Hubble Space Telescope|Hubble Space Telescope]] detected [[Physics:Water vapor|water vapor plumes]] similar to those observed on Saturn's moon [[Astronomy:Enceladus|Enceladus]], which are thought to be caused by erupting cryogeysers.<ref name="NASA-20131212-EU">{{cite web |last1=Cook |first1=Jia-Rui C. |last2=Gutro |first2=Rob |last3=Brown |first3=Dwayne |last4=Harrington |first4=J. D. |last5=Fohn |first5=Joe |title=Hubble Sees Evidence of Water Vapor at Jupiter Moon |url=http://www.jpl.nasa.gov/news/news.php?release=2013-363 |date=12 December 2013 |work=NASA |access-date=12 December 2013 |archive-date=15 December 2013 |archive-url=https://web.archive.org/web/20131215053143/http://www.jpl.nasa.gov/news/news.php?release=2013-363 |url-status=live }}</ref> In May 2018, astronomers provided supporting evidence of water plume activity on Europa, based on an updated analysis of data obtained from the ''[[Engineering:Galileo (spacecraft)|Galileo]]'' space probe, which orbited Jupiter from 1995 to 2003. Such plume activity could help researchers in a search for life from the subsurface Europan ocean without having to land on the moon.<ref name="NA-20180514" /><ref name="NASA-20180514" /><ref name="NYT-20180514" /><ref name="SP-20180514" />

The ''Galileo'' mission, launched in 1989, provides the bulk of current data on Europa. No spacecraft has yet landed on Europa, although there have been several proposed exploration missions. The [[Organization:European Space Agency|European Space Agency]]'s Jupiter Icy Moon Explorer (JUICE) is a mission to [[Astronomy:Ganymede (moon)|Ganymede]] launched on April 14, 2023, that will include two flybys of Europa.<ref>{{cite web |url=https://sci.esa.int/web/juice |title=ESA Science & Technology - JUICE |publisher=ESA |date=8 November 2021 |access-date=10 November 2021 |archive-date=21 September 2019 |archive-url=https://web.archive.org/web/20190921101151/https://sci.esa.int/web/juice |url-status=live }}</ref><ref name="selection">{{cite news |first = Jonathan |last = Amos |url = https://www.bbc.co.uk/news/science-environment-17917102 |title = Esa selects 1bn-euro Juice probe to Jupiter |access-date = 2 May 2012 |date = 2 May 2012 |work = BBC News Online |archive-date = 11 May 2020 |archive-url = https://web.archive.org/web/20200511181342/https://www.bbc.co.uk/news/science-environment-17917102 |url-status = live }}</ref> NASA's [[Astronomy:Europa Clipper|Europa Clipper]] is expected to be launched in October 2024,<ref>{{cite web |url=https://europa.nasa.gov/ |title=NASA's Europa Clipper |publisher=[[Organization:NASA|NASA]] |date=9 April 2023 |access-date=9 April 2023 |archive-date=4 April 2023 |archive-url=https://web.archive.org/web/20230404000842/https://europa.nasa.gov/ |url-status=live }}</ref><ref name="AP-20140304">{{cite news |last=Borenstein |first=Seth |title=NASA plots daring flight to Jupiter's watery moon |url=http://apnews.excite.com/article/20140304/DACB5P9O0.html |date=4 March 2014 |agency=Associated Press |access-date=5 March 2014 |archive-url=https://web.archive.org/web/20140305180141/http://apnews.excite.com/article/20140304/DACB5P9O0.html |archive-date=5 March 2014 |url-status=dead }}</ref> with a complementary lander possible based on its findings.

==Discovery and naming==
Europa, along with Jupiter's three other large moons, [[Astronomy:Io (moon)|Io]], [[Astronomy:Ganymede (moon)|Ganymede]], and [[Astronomy:Callisto (moon)|Callisto]], was discovered by [[Biography:Galileo Galilei|Galileo Galilei]] on 8 January 1610,<ref name="IAUMoonDiscoveries"/> and possibly independently by Simon Marius. The first reported observation of Io and Europa was made by Galileo on 7 January 1610 using a 20×-magnification refracting telescope at the [[Organization:University of Padua|University of Padua]]. However, in that observation, Galileo could not separate Io and Europa due to the low magnification of his telescope, so that the two were recorded as a single point of light. The following day, 8 January 1610 (used as the discovery date for Europa by the [[Organization:International Astronomical Union|IAU]]), Io and Europa were seen for the first time as separate bodies during Galileo's observations of the Jupiter system.<ref name="IAUMoonDiscoveries">{{cite web |last=Blue |first=Jennifer |date=9 November 2009 |url=http://planetarynames.wr.usgs.gov/append7.html |title=Planet and Satellite Names and Discoverers |publisher=USGS |access-date=14 January 2010 |archive-date=25 August 2009 |archive-url=https://web.archive.org/web/20090825153846/http://planetarynames.wr.usgs.gov/append7.html |url-status=live }}</ref>

Europa is the namesake of Europa, daughter of the king of [[Place:Tyre, Lebanon|Tyre]], a [[History:Phoenicia|Phoenicia]]n noblewoman in [[Religion:Greek mythology|Greek mythology]]. Like all the Galilean satellites, Europa is named after a lover of [[Religion:Zeus|Zeus]], the Greek counterpart of [[Religion:Jupiter (mythology)|Jupiter]]. Europa was courted by Zeus and became the queen of Crete.<ref>{{cite web |url=http://nineplanets.org/europa.html |title=Europa |last=Arnett |first=Bill |work=Nine Planets |date=October 2005 |access-date=27 April 2014 |archive-date=28 March 2014 |archive-url=https://web.archive.org/web/20140328014150/http://nineplanets.org/europa.html |url-status=live }}</ref> The naming scheme was suggested by Simon Marius,<ref name="Marius1614"/> who attributed the proposal to [[Biography:Johannes Kepler|Johannes Kepler]]:<ref name="Marius1614">Marius, S.; (1614) ''Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici'' [http://galileo.rice.edu/sci/marius.html] {{Webarchive|url=https://web.archive.org/web/20190929023619/http://galileo.rice.edu/sci/marius.html|date=29 September 2019}}, where he [http://galileo.rice.edu/sci/observations/jupiter_satellites.html attributes the suggestion] {{Webarchive|url=https://web.archive.org/web/20191101192948/http://galileo.rice.edu/sci/observations/jupiter_satellites.html|date=1 November 2019}} to [[Biography:Johannes Kepler|Johannes Kepler]]</ref><ref name="SEDS">{{cite web|url=http://seds.lpl.arizona.edu/messier/xtra/Bios/marius.html |title=Simon Marius (January 20, 1573 – December 26, 1624) |access-date=9 August 2007 |publisher=[[Organization:University of Arizona|University of Arizona]] |work=Students for the Exploration and Development of Space |url-status=dead |archive-url=https://web.archive.org/web/20070713221843/http://seds.lpl.arizona.edu/messier/xtra/Bios/marius.html |archive-date=13 July 2007 }}</ref>

{{blockquote|Jupiter is much blamed by the poets on account of his irregular loves. Three maidens are especially mentioned as having been clandestinely courted by Jupiter with success. Io, daughter of the River Inachus, Callisto of Lycaon, Europa of Agenor. Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell... I think, therefore, that I shall not have done amiss if the First is called by me Io, the Second Europa, the Third, on account of its majesty of light, Ganymede, the Fourth Callisto...<ref name=Helden>{{cite journal |last1=Van Helden |first1=Albert |title=Naming the Satellites of Jupiter and Saturn |journal=The Newsletter of the Historical Astronomy Division of the American Astronomical Society |date=August 1994 |issue=32 |url=https://had.aas.org/sites/had.aas.org/files/HADN32.pdf |access-date=10 March 2023 |archive-date=7 December 2022 |archive-url=https://web.archive.org/web/20221207151902/https://had.aas.org/sites/had.aas.org/files/HADN32.pdf |url-status=live }}</ref><ref>{{cite book |last1=Marius |first1=SImon |title=Mundus Iovialis: anno MDCIX detectus ope perspicilli Belgici, hoc est, quatuor Jovialium planetarum, cum theoria, tum tabulæ |date=1614 |publisher=Sumptibus & Typis Iohannis Lauri |location=Nuremberg |page=B2, recto and verso (images 35 and 36), with erratum on last page (image 78) |url=https://repository.ou.edu/uuid/748b6fe7-62da-5877-ae84-885372b3030c |access-date=30 June 2020 |archive-date=2 July 2020 |archive-url=https://web.archive.org/web/20200702154450/https://repository.ou.edu/uuid/748b6fe7-62da-5877-ae84-885372b3030c |url-status=live }}</ref>}}

The names fell out of favor for a considerable time and were not revived in general use until the mid-20th century.<ref name="marazzini">{{cite journal |last1=Marazzini |first1=Claudio |title=I nomi dei satelliti di Giove: da Galileo a Simon Marius |trans-title=The names of Jupiter's satellites: from Galileo to Simon Marius |language=it |journal=Lettere Italiane |date=2005 |volume=57 |issue=3 |pages=391–407 |jstor=26267017 }}</ref> In much of the earlier [[Astronomy:Astronomy|astronomical]] literature, Europa is simply referred to by its [[Roman numerals|Roman numeral]] designation as ''{{Nowrap|Jupiter II}}'' (a system also introduced by Galileo) or as the "second satellite of Jupiter". In 1892, the discovery of [[Astronomy:Amalthea (moon)|Amalthea]], whose orbit lay closer to Jupiter than those of the Galilean moons, pushed Europa to the third position. The ''Voyager'' probes discovered three more inner satellites in 1979, so Europa is now counted as Jupiter's sixth satellite, though it is still referred to as {{Nowrap|Jupiter II}}.<ref name="marazzini"/>
The adjectival form has stabilized as ''Europan''.<ref name="Greenberg 2005"/><ref>US National Research Council (2000) ''A Science Strategy for the Exploration of Europa''</ref>

== Orbit and rotation ==
[[File:Galilean moon Laplace resonance animation 2.gif|thumb|right|Animation of the Laplace resonance of Io, Europa and Ganymede (conjunctions are highlighted by color changes)]]

Europa orbits Jupiter in just over three and a half days, with an orbital radius of about 670,900&nbsp;km. With an [[Astronomy:Orbital eccentricity|orbital eccentricity]] of only 0.009, the orbit itself is nearly circular, and the [[Astronomy:Orbital inclination|orbital inclination]] relative to Jupiter's equatorial plane is small, at 0.470°.<ref name="datasheet">{{cite web|url=http://www2.jpl.nasa.gov/galileo/europa/#overview|archive-url=https://web.archive.org/web/19970105180851/http://www.jpl.nasa.gov/galileo/europa/|url-status=dead|archive-date=5 January 1997|title=Europa, a Continuing Story of Discovery|access-date=9 August 2007|work=Project Galileo|publisher=NASA, [[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]]}}</ref> Like its fellow Galilean satellites, Europa is [[Astronomy:Tidal locking|tidally locked]] to Jupiter, with one hemisphere of Europa constantly facing Jupiter. Because of this, there is a [[Astronomy:Poles of astronomical bodies|sub-Jovian point]] on Europa's surface, from which Jupiter would appear to hang directly overhead. Europa's [[Earth:Prime meridian|prime meridian]] is a line passing through this point.<ref>{{cite web|title=Planetographic Coordinates|publisher=Wolfram Research|url=http://documents.wolfram.com/applications/astronomer/AdditionalInformation/PlanetographicCoordinates.html|date=2010|access-date=29 March 2010|url-status=dead|archive-url=https://web.archive.org/web/20090301191456/http://documents.wolfram.com/applications/astronomer/AdditionalInformation/PlanetographicCoordinates.html|archive-date=1 March 2009}}</ref> Research suggests that tidal locking may not be full, as a non-synchronous rotation has been proposed: Europa spins faster than it orbits, or at least did so in the past. This suggests an asymmetry in internal mass distribution and that a layer of subsurface liquid separates the icy crust from the rocky interior.<ref name=Geissler>{{cite journal|last1=Geissler|first1=P. E.|last2=Greenberg|first2=R.|last3=Hoppa|first3=G.|last4=Helfenstein|first4=P.|last5=McEwen|first5=A.|last6=Pappalardo|first6=R.|last7=Tufts|first7=R.|last8=Ockert-Bell|first8=M.|last9=Sullivan|first9=R.|last10=Greeley|first10=R.|last11=Belton|first11=M. J. S.|last12=Denk|first12=T.|last13=Clark|first13=B. E.|last14=Burns|first14=J.|last15=Veverka|first15=J.|date=1998|title=Evidence for non-synchronous rotation of Europa|journal=Nature|volume=391|pages=368–70|bibcode=1998Natur.391..368G|doi=10.1038/34869|pmid=9450751|issue=6665|s2cid=4426840}}</ref>

The slight eccentricity of Europa's orbit, maintained by gravitational disturbances from the other Galileans, causes Europa's sub-Jovian point to oscillate around a mean position. As Europa comes slightly nearer to Jupiter, Jupiter's gravitational attraction increases, causing Europa to elongate towards and away from it. As Europa moves slightly away from Jupiter, Jupiter's gravitational force decreases, causing Europa to relax back into a more spherical shape, and creating tides in its ocean. The orbital eccentricity of Europa is continuously pumped by its mean-motion resonance with Io.<ref name="Showman1997">{{cite journal |last1=Showman |first1=Adam P. |last2=Malhotra |first2=Renu |s2cid=55790129 |title=Tidal Evolution into the Laplace Resonance and the Resurfacing of Ganymede |journal=Icarus |date=May 1997 |volume=127 |issue=1 |pages=93–111 |doi=10.1006/icar.1996.5669 |bibcode=1997Icar..127...93S }}</ref> Thus, the tidal flexing kneads Europa's interior and gives it a source of heat, possibly allowing its ocean to stay liquid while driving subsurface geological processes.<ref name="geology" /><ref name="Showman1997" /> The ultimate source of this energy is Jupiter's rotation, which is tapped by Io through the tides it raises on Jupiter and is transferred to Europa and Ganymede by the orbital resonance.<ref name="Showman1997" /><ref name="Moore2003">{{cite journal |last1=Moore |first1=W. B. |title=Tidal heating and convection in Io |journal=Journal of Geophysical Research |date=2003 |volume=108 |issue=E8 |pages=5096 |doi=10.1029/2002JE001943 |bibcode=2003JGRE..108.5096M |citeseerx=10.1.1.558.6924 }}</ref>

Analysis of the unique cracks lining Europa yielded evidence that it likely spun around a tilted axis at some point in time. If correct, this would explain many of Europa's features. Europa's immense network of crisscrossing cracks serves as a record of the stresses caused by massive tides in its global ocean. Europa's tilt could influence calculations of how much of its history is recorded in its frozen shell, how much heat is generated by tides in its ocean, and even how long the ocean has been liquid. Its ice layer must stretch to accommodate these changes. When there is too much stress, it cracks. A tilt in Europa's axis could suggest that its cracks may be much more recent than previously thought. The reason for this is that the direction of the spin pole may change by as much as a few degrees per day, completing one precession period over several months. A tilt could also affect estimates of the age of Europa's ocean. Tidal forces are thought to generate the heat that keeps Europa's ocean liquid, and a tilt in the spin axis would cause more heat to be generated by tidal forces. Such additional heat would have allowed the ocean to remain liquid for a longer time. However, it has not yet been determined when this hypothesized shift in the spin axis might have occurred.<ref>Cook, Jia-Rui C. (18 September 2013) [http://www.jpl.nasa.gov/news/news.php?release=2013-283 Long-stressed Europa Likely Off-kilter at One Time] {{Webarchive|url=https://web.archive.org/web/20140817113356/http://www.jpl.nasa.gov/news/news.php?release=2013-283 |date=17 August 2014 }}. jpl.nasa.gov</ref>

==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 [[Astronomy:Moon|Earth's moon]]. At just over {{Convert|3100|km|mi}} in [[Diameter|diameter]], it is the sixth-largest moon and [[Astronomy:List of Solar System objects by size|fifteenth-largest object]] in the [[Astronomy:Solar System|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'' [[Astronomy:Triton (moon)#cite ref-nMassOthers 44-0|here]])</ref> Its bulk density suggests that it is similar in composition to [[Astronomy:Terrestrial planet|terrestrial planet]]s, being primarily composed of [[Chemistry:Silicate|silicate]] [[Earth: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===
[[File:Europa_nomenclature_map_(USGS_August_2023).png|thumb|443x443px|Map of Europa, by the United States Geological Survey]]
It is estimated that Europa has an outer layer of [[Chemistry:Water|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 [[Physics:Magnetic field|magnetic-field]] data from the [[Engineering: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. |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 [[Earth:True polar wander|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 [[Chemistry:Metal|metal]]lic [[Chemistry:Iron|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 |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===
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=[[Organization:NASA|NASA]], Jet Propulsion Laboratory |work=Project Galileo: Moons and Rings of Jupiter }}</ref> But according to one study, Europa's equator may be covered in icy spikes called [[Earth:Penitente (snow formation)|penitentes]], which may be up to 15&nbsp;meters high, due to direct overhead sunlight on 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 prominent markings crisscrossing Europa appear to be mainly [[Astronomy:Albedo feature|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> Its icy crust has an [[Earth:Albedo|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 [[Astronomy:Comet|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>

The [[Physics:Ionizing radiation|ionizing radiation]] level at Europa's surface is equivalent to a daily dose of about 5.4&nbsp;[[Physics:Sievert|Sv]] (540&nbsp;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=2021-01-09|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 ====
[[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 Annwn Regio<ref>{{cite web | url=https://planetarynames.wr.usgs.gov/Feature/14291 | title=Planetary Names }}</ref><ref>https://asc-planetarynames-data.s3.us-west-2.amazonaws.com/europa_15m_100ppi.pdf</ref>]]
Europa's most striking surface features are a series of dark streaks crisscrossing the entire globe, called [[Astronomy:Lineae|lineae]] ({{lang-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 ridges. 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 ''[[Astronomy: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 [[Earth:Subduction|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 plates 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 [[Earth:Plate tectonics|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
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 |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
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Other features present on Europa are circular and elliptical {{Lang|la|lenticulae}} ([[Social:Latin|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 [[Earth:Diapir|diapir]]s of warm ice rising up through the colder ice of the outer crust, much like [[Earth:Magma chamber|magma chamber]]s in Earth's crust.<ref name="diapir" /> The smooth, dark spots could be formed by [[Earth:Meltwater|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 [[Earth:Iceberg|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 from the [[Organization:University of Texas at Austin|University of Texas at Austin]] and elsewhere presented evidence in the journal ''Nature'' suggesting that many "[[Astronomy:Chaos terrain|chaos terrain]]" features on Europa sit atop vast lakes of liquid water.<ref name="europagreatlake">{{cite journal |title=Active formation of 'chaos terrain' over shallow subsurface water on Europa |author1=Schmidt, Britney |author2=Blankenship, Don |author3=Patterson, Wes |author4=Schenk, Paul |journal=Nature |date=24 November 2011 |volume=479 |pages=502–505 |doi=10.1038/nature10608|bibcode =2011Natur.479..502S |issue=7374 |pmid=22089135 |s2cid=4405195 }}</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, for example, using radar.<ref name="europagreatlakeairhart"/>

Work published by researchers from [[Organization:Williams College|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:EuropaInterior1.jpg|thumb|Two competing models of Europa prior to the confirmation of a subsurface ocean]]
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=live }}</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.{{citation needed|date=April 2021}} 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 [[Physics:Magnetic moment|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" />
[[File:PIA25696-Europa-JupiterMoon-20220929.jpg|thumb|left|200px|<div align="center">Europa closeup<br />(9 September 2022)</div>]]
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 [[Chemistry:Magnesium sulfate|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=0 |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> [[Chemistry:Sulfuric acid|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 [[Chemistry:Sulfur|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>

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. |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 [[Chemistry:Pyrolysis|pyrolysis]] and [[Physics:Radiolysis|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 [[Astronomy:Cryovolcano|cryovolcanic]] events that resurface the body, and to accumulate from space as interplanetary dust.<ref name='Tholins Europa'/> Tholins bring important [[Astronomy:Astrobiology|astrobiological]] implications, as they may play a role in prebiotic chemistry and [[Biology:Abiogenesis|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 [[Chemistry:Sodium chloride|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 [[Astronomy: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 [https://www.theguardian.com/science/2023/sep/21/scientists-excited-to-find-ocean-of-one-of-jupiters-moons-contains-carbon Scientists excited to find ocean of one of Jupiter’s moons contains carbon] and was observed on the surface ice as a concentration of [[Chemistry:Carbon dioxide|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 |arxiv=2309.11684 }}</ref>

====Sources of heat====

Europa receives thermal energy from [[Astronomy:Tidal heating|tidal heating]], which occurs through the tidal friction and tidal flexing processes caused by [[Astronomy:Tidal acceleration|tidal acceleration]]: orbital and rotational energy are dissipated as heat in the [[Astronomy: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 [[Chemistry: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=2022-10-09 |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 [[Astronomy:Hubble Space Telescope|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=1999-06-30|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|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.|last2=Barnouin|first2=Olivier S.|last3=Prockter|first3=Louise|last4=Patterson|first4=G. Wesley|date=2013-09-15|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|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=2019-11-18|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 [[Physics:Tidal force|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=[[Astronomy:Nature Astronomy|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=[[Organization:Jet Propulsion Laboratory|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={{wipe|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 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 [[Astronomy:Earth|Earth]]. The only other moon in the Solar System exhibiting water vapor plumes is [[Astronomy:Enceladus|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 = [[Earth:Annual Review of Earth and Planetary Sciences|Annual Review of Earth and Planetary Sciences]]| volume = 41 | page = 693 | date = May 2013|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 ''[[Earth:Geophysical Research Letters|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 theory 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=2003-12-27|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=[[Organization:Jet Propulsion Laboratory|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=[[Earth:Geophysical Research Letters|Geophysical Research Letters]] |volume=47 |issue=21 |pages={e2020GL090797} |doi=10.1029/2020GL090797|bibcode=2020GeoRL..4790797S |s2cid=228890686 }}</ref>

=== Atmosphere ===

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=2022-05-13 |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=2022-05-13 |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 [[Chemistry:Dissociation|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 |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 [[Chemistry:Hydrogen peroxide|hydrogen peroxide]] molecule but rather desorbs from the surface and starts another [[Physics: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 our solar system with a quantifiable atmosphere, along with [[Astronomy:Titan (moon)|Titan]], [[Astronomy:Io (moon)#Atmosphere|Io]], [[Astronomy:Triton (moon)|Triton]], [[Astronomy:Ganymede (moon)#Atmosphere and ionosphere|Ganymede]] and [[Astronomy:Callisto (moon)#Atmosphere and ionosphere|Callisto]]. Europa is also one of the three formations, among planets and moons, to contain oxygen within its atmosphere.<ref name=":3">{{Cite web |title=Hubble Finds Oxygen Atmosphere On Jupiter's Moon Europa |url=https://solarviews.com/eng/europapr.htm |access-date=2022-05-13 |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 our solar system with very large quantities of [[Earth:Volatile (astrogeology)|ice (volatiles)]], otherwise known as "icy moons."<ref name=":4">{{Cite web |last=Cartier |first=Kimberly M. S. |date=2020-12-14 |title=Do Uranus's Moons Have Subsurface Oceans? |url=http://eos.org/articles/do-uranuss-moons-have-subsurface-oceans |access-date=2022-05-13 |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=2022-05-13 |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=2001-06-15 |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 ''[[Astronomy: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 |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 theorized 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=2019-01-22 |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>

==== Discovery of atmosphere ====
The atmosphere of Europa was first discovered in 1995 by Hall Al and the [[Engineering:Goddard High Resolution Spectrograph|Goddard High Resolution Spectrograph]] of the [[Astronomy:Hubble Space Telescope|Hubble]] telescope. This observation was then confirmed in 1997 by the Galileo probe, built by Hughes Aircraft Company and operated by NASA. The Galileo probe flew only three miles above the estimated maximum atmospheric line (190&nbsp;km from Europa's surface). Still, it then changed course to collide with Jupiter's atmosphere to prevent unwanted impact on Europa's surface. It has been speculated that there will be several more future missions to Europa in hopes of further studying the atmosphere, chemical composition, and possibility of extraterrestrial life below the icy surface.{{Citation needed|date=January 2023}}

==== Climate and weather ====
Despite the presence of a [[Astronomy:Gas torus|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=2018-03-22 |title=Europa: Facts About Jupiter's Icy Moon and Its Ocean |url=https://www.space.com/15498-europa-sdcmp.html |access-date=2022-05-13 |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 subsequently{{clarify|date=September 2023}} warm however. It is theorized 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 [[Chemistry:Chloride|chloride]], thus resembling our 1.94% chloride oceans on Earth.

== Exploration ==
{{multiple image
 |direction = vertical
 |align = right
 |width = 220
 |image1 = Pioneer_10_-_p102b.jpg
 |image2 = Europa_-_July_9_1979_(18267960842).jpg
 |caption1 = In 1973 ''[[Astronomy:Pioneer 10|Pioneer 10]]'' made the first closeup images of Europa – however the probe was too far away to obtain more detailed images
 |caption2 = Europa seen in detail in 1979 by ''[[Astronomy:Voyager 2|Voyager 2]]''
}}

Exploration of Europa began with the Jupiter flybys of ''[[Astronomy:Pioneer 10|Pioneer 10]]'' and ''[[Astronomy:Pioneer 11|11]]'' in 1973 and 1974, respectively. The first closeup photos were of low resolution compared to later missions. The two Voyager probes traveled through the Jovian system in 1979, providing more-detailed images of Europa's icy surface. The images caused many scientists to speculate about the possibility of a liquid ocean underneath. 
Starting in 1995, the [[Engineering:Galileo (spacecraft)|''Galileo'' space probe]] orbited Jupiter for eight years, until 2003, and provided the most detailed examination of the Galilean moons to date. It included the "Galileo Europa Mission" and "Galileo Millennium Mission", with numerous close flybys of Europa.<ref>[https://web.archive.org/web/20041104092641/http://solarsystem.nasa.gov/galileo/mission/journey-extended.cfm The Journey to Jupiter: Extended Tours – GEM and the Millennium Mission]. Solarsystem.nasa.gov. Retrieved on 23 July 2013.</ref> In 2007, ''[[Astronomy:New Horizons|New Horizons]]'' imaged Europa, as it flew by the Jovian system while on its way to [[Astronomy:Pluto|Pluto]].<ref>{{cite web|url=http://photojournal.jpl.nasa.gov/catalog/PIA09246|title=PIA09246: Europa|date=2 April 2007|work=NASA photojournal|access-date=9 March 2016|archive-date=6 March 2016|archive-url=https://web.archive.org/web/20160306211111/http://photojournal.jpl.nasa.gov/catalog/PIA09246|url-status=live}}</ref> In 2022, the ''[[Engineering:Juno (spacecraft)|Juno]]'' orbiter flew by Europa at a distance of 352&nbsp;km (219&nbsp;mi).<ref name="NYT-20220930"/><ref name="nasa-20220929">{{cite web |url=https://www.nasa.gov/feature/jpl/nasa-s-juno-shares-first-image-from-flyby-of-jupiter-s-moon-europa |title=NASA's Juno Shares First Image From Flyby of Jupiter's Moon Europa |work=[[Organization:NASA|NASA]] |date=29 September 2022 |access-date=30 September 2022 |archive-date=1 October 2022 |archive-url=https://web.archive.org/web/20221001003306/https://www.nasa.gov/feature/jpl/nasa-s-juno-shares-first-image-from-flyby-of-jupiter-s-moon-europa/ |url-status=live }}</ref>

=== Future missions ===

Conjectures regarding [[Astronomy:Extraterrestrial life|extraterrestrial life]] have ensured a high profile for Europa and have led to steady lobbying for future missions.<ref name="Europabudget">{{cite web |last=David |first=Leonard |url=http://www.space.com/news/060207_europa_budget.html |title=Europa Mission: Lost In NASA Budget |date=7 February 2006 |publisher=Space.com |access-date=25 February 2007 |archive-date=24 December 2010 |archive-url=https://web.archive.org/web/20101224030016/http://www.space.com/news/060207_europa_budget.html |url-status=live }}</ref><ref name="PlanetarySocEuropa">{{cite web|title=Projects: Europa Mission Campaign; Campaign Update: 2007 Budget Proposal |last=Friedman |first=Louis |date=14 December 2005 |url=http://www.planetary.org/programs/projects/explore_europa/update_12142005.html |archive-date=11 August 2011 |url-status=dead |publisher=The Planetary Society |archive-url=https://web.archive.org/web/20110811002508/http://www.planetary.org/programs/projects/explore_europa/update_12142005.html }}</ref> The aims of these missions have ranged from examining Europa's chemical composition to searching for extraterrestrial life in its hypothesized subsurface oceans.<ref name="EuropaLife">{{cite web |title=Thin ice opens lead for life on Europa |first=David L. |last=Chandler |date=20 October 2002 |url=https://www.newscientist.com/article.ns?id=dn2929 |work=New Scientist |access-date=27 August 2017 |archive-date=14 May 2008 |archive-url=https://web.archive.org/web/20080514182430/http://www.newscientist.com/article.ns?id=dn2929 |url-status=live }}</ref><ref name="Muir2002">Muir, Hazel (22 May 2002) [https://www.newscientist.com/article.ns?id=dn2313 ''Europa has raw materials for life''] {{Webarchive|url=https://web.archive.org/web/20080416015647/http://www.newscientist.com/article.ns?id=dn2313 |date=16 April 2008 }}, ''New Scientist''.</ref> Robotic missions to Europa need to endure the high-radiation environment around Jupiter.<ref name="PlanetarySocEuropa" /> Because it is deeply embedded within Jupiter's magnetosphere, Europa receives about 5.40&nbsp;[[Physics:Sievert|Sv]] of radiation per day.<ref name="Ringwald2000">Ringwald, Frederick A. (29 February 2000) [http://zimmer.csufresno.edu/~fringwal/w08a.jup.txt ''SPS 1020 (Introduction to Space Sciences) Course Notes''] {{webarchive|url=https://web.archive.org/web/20080725050708/http://zimmer.csufresno.edu/~fringwal/w08a.jup.txt |date=25 July 2008 }}, California State University, csufresno.edu.</ref>

In 2011, a Europa mission was recommended by the U.S. [[Astronomy:Planetary Science Decadal Survey|Planetary Science Decadal Survey]].<ref name="zab">{{cite news | title=Lean U.S. missions to Mars, Jupiter moon recommended | url=https://www.reuters.com/article/us-space-usa-future-idUSTRE7266XJ20110308 | work=Reuters | date=7 March 2011 | last=Zabarenko | first=Deborah | access-date=5 July 2021 | archive-date=7 September 2020 | archive-url=https://web.archive.org/web/20200907012354/https://www.reuters.com/article/us-space-usa-future-idUSTRE7266XJ20110308 | url-status=live }}</ref> In response, NASA commissioned Europa lander concept studies in 2011, along with concepts for a Europa flyby (''Europa Clipper''), and a Europa orbiter.<ref>{{cite web | url=http://solarsystem.nasa.gov/missions/profile.cfm?MCode=EAL | title=Europa Lander | access-date=15 January 2014 | work=NASA | url-status=dead | archive-url=https://web.archive.org/web/20140116103022/http://solarsystem.nasa.gov/missions/profile.cfm?MCode=EAL | archive-date=16 January 2014 | df=dmy-all }}</ref><ref>[http://www.lpi.usra.edu/opag/mar2012/presentations/ March 2012 OPAG Meeting] {{Webarchive|url=https://web.archive.org/web/20160303174000/http://www.lpi.usra.edu/opag/mar2012/presentations/ |date=3 March 2016 }}. Lunar and Planetary Institute, NASA. Retrieved on 23 July 2013.</ref> The orbiter element option concentrates on the "ocean" science, while the multiple-flyby element (''Clipper'') concentrates on the chemistry and energy science. On 13 January 2014, the House Appropriations Committee announced a new bipartisan bill that includes $80&nbsp;million in funding to continue the Europa mission concept studies.<ref>{{cite news | first=Amina | last=Khan | title=NASA gets some funding for Mars 2020 rover in federal spending bill | date=15 January 2014 | url=http://www.latimes.com/science/sciencenow/la-sci-sn-federal-spending-omnibus-bill-nasa-congress-65-million-mars-2020-mission-20140115,0,7107985.story | work=Los Angeles Times | access-date=16 January 2014 | archive-date=21 April 2014 | archive-url=https://web.archive.org/web/20140421095615/http://www.latimes.com/science/sciencenow/la-sci-sn-federal-spending-omnibus-bill-nasa-congress-65-million-mars-2020-mission-20140115,0,7107985.story | url-status=live }}</ref><ref>{{cite news | first=Frank C. | last=Girardot | title=JPL's Mars 2020 rover benefits from spending bill | date=14 January 2014 | url=http://www.pasadenastarnews.com/science/20140114/jpls-mars-2020-rover-benefits-from-spending-bill | work=Pasadena Star-News | access-date=15 January 2014 | archive-date=31 July 2017 | archive-url=https://web.archive.org/web/20170731025001/http://www.pasadenastarnews.com/science/20140114/jpls-mars-2020-rover-benefits-from-spending-bill | url-status=live }}</ref>
* In 2012, ''Jupiter Icy Moon Explorer'' (JUICE) was selected by the European Space Agency ([[Organization:European Space Agency|ESA]]) as a planned mission.<ref name="selection" /><ref name="l1">[http://planetary.s3.amazonaws.com/assets/resources/ESA/ESA-SPC_20120417_selection-L1-mission.pdf Selection of the L1 mission] {{Webarchive|url=https://web.archive.org/web/20151016065456/http://planetary.s3.amazonaws.com/assets/resources/ESA/ESA-SPC_20120417_selection-L1-mission.pdf |date=16 October 2015 }}. ESA, 17 April 2012. (PDF). Retrieved on 23 July 2013.</ref> That mission includes two flybys of Europa, but is more focused on [[Astronomy:Ganymede (moon)|Ganymede]].<ref name='juice-objectives'>{{cite web | url = http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=50068 | title = JUICE – Science objectives | access-date = 20 April 2012 | date = 16 March 2012 | work = [[Organization:European Space Agency|European Space Agency]] | archive-date = 8 June 2013 | archive-url = https://web.archive.org/web/20130608082915/http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=50068 | url-status = live }}</ref>
* ''[[Astronomy:Europa Clipper|Europa Clipper]]'' – In July 2013 an updated concept for a flyby Europa mission called ''Europa Clipper'' was presented by the [[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]] (JPL) and the [[Organization:Applied Physics Laboratory|Applied Physics Laboratory]] (APL).<ref>{{cite web |url=http://www.lpi.usra.edu/opag/jul2013/presentations/Clipper_Summary.pdf |title=The Europa Clipper – OPAG Update |date=2013 |first1=Robert |last1=Pappalardo |first2=Brian |last2=Cooke |first3=Barry |last3=Goldstein |first4=Louise |last4=Prockter |first5=Dave |last5=Senske |first6=Tom |last6=Magner |publisher=[[Organization:Jet Propulsion Laboratory|JPL]]/[[Organization:Applied Physics Laboratory|APL]] |access-date=13 December 2013 |archive-date=25 January 2021 |archive-url=https://web.archive.org/web/20210125054349/https://www.lpi.usra.edu/opag/jul2013/presentations/Clipper_Summary.pdf |url-status=live }}</ref> In May 2015, NASA announced that it had accepted development of the ''Europa Clipper'' mission, and revealed the instruments it would use.<ref name="EuropaClipperAnnounce">{{cite web| url=http://www.nasa.gov/press-release/nasa-s-europa-mission-begins-with-selection-of-science-instruments| title=NASA's Europa Mission Begins with Selection of Science Instruments| work=NASA| date=26 May 2015| access-date=3 July 2015| archive-date=5 July 2015| archive-url=https://web.archive.org/web/20150705103500/http://www.nasa.gov/press-release/nasa-s-europa-mission-begins-with-selection-of-science-instruments/| url-status=live}}</ref> The aim of ''Europa Clipper'' is to explore Europa in order to investigate its [[Astronomy:Planetary habitability|habitability]], and to aid in selecting sites for a future lander. The ''Europa Clipper'' would not orbit Europa, but instead orbit Jupiter and conduct 45 low-altitude [[Engineering:Planetary flyby|flybys]] of Europa during its envisioned mission. The probe would carry an ice-penetrating radar, short-wave infrared spectrometer, topographical imager, and an ion- and neutral-mass spectrometer.
*[[Astronomy:Europa Lander (NASA)|Europa Lander (NASA)]] is a recent concept mission under study. 2018 research suggests Europa may be covered in tall, jagged ice spikes, presenting a problem for any potential landing on its surface.<ref>{{Cite web|url=https://www.theverge.com/2018/10/8/17948510/jupiter-moon-europa-nasa-clipper-lander-penitentes|title=Future spacecraft landing on Jupiter's moon Europa may have to navigate jagged blades of ice|last=Grush|first=Loren|date=8 October 2018|website=The Verge|access-date=16 April 2019|archive-date=28 March 2019|archive-url=https://web.archive.org/web/20190328070428/https://www.theverge.com/2018/10/8/17948510/jupiter-moon-europa-nasa-clipper-lander-penitentes|url-status=live}}</ref><ref>{{Cite news|url=https://www.washingtonpost.com/science/2018/10/08/jagged-ice-spikes-cover-jupiters-moon-europa-study-suggests/|title=Jagged ice spikes cover Jupiter's moon Europa, study suggests|last=Guarino|first=Ben|date=8 October 2018|newspaper=The Washington Post|access-date=15 April 2019|archive-date=16 April 2019|archive-url=https://web.archive.org/web/20190416012612/https://www.washingtonpost.com/science/2018/10/08/jagged-ice-spikes-cover-jupiters-moon-europa-study-suggests/|url-status=live}}</ref>

=== Old proposals ===
{{multiple image
 |direction = horizontal
 |align = right
 |width1 = 182
 |width2 = 216
 |image1 = Cryobot.jpg
 |image2 = JIMO Europa Lander Mission.jpg
 |footer = ''Left:'' artist's concept of the cryobot and its deployed "hydrobot" submersible. ''Right:'' Europa Lander Mission concept, NASA 2005.<ref>{{cite web |url=http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/37545/1/05-0403.pdf |title=Small RPS-Enabled Europa Lander Mission |publisher=NASA–JPL |date=13 February 2005 |url-status=dead |archive-url=https://web.archive.org/web/20111008230224/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/37545/1/05-0403.pdf |archive-date=8 October 2011 }}</ref>
}}

In the early 2000s, ''[[Astronomy:Jupiter Europa Orbiter|Jupiter Europa Orbiter]]'' led by NASA and the ''[[Astronomy:Jupiter Ganymede Orbiter|Jupiter Ganymede Orbiter]]'' led by the ESA were proposed together as an [[Astronomy:Outer Planet Flagship Mission|Outer Planet Flagship Mission]] to Jupiter's icy moons called Europa Jupiter System Mission, with a planned launch in 2020.<ref name="NASA2009">{{cite web |url=http://www.nasa.gov/topics/solarsystem/features/20090218.html |title=NASA and ESA Prioritize Outer Planet Missions |access-date=26 July 2009 |publisher=NASA |date=2009 |archive-date=25 August 2011 |archive-url=https://web.archive.org/web/20110825211633/http://www.nasa.gov/topics/solarsystem/features/20090218.html |url-status=live }}</ref> In 2009 it was given priority over ''[[Astronomy:Titan Saturn System Mission|Titan Saturn System Mission]]''.<ref name="Rincon2009">{{cite news |url=http://news.bbc.co.uk/1/hi/sci/tech/7897585.stm |title=Jupiter in space agencies' sights |first=Paul |last=Rincon |work=BBC News |access-date=20 February 2009 |date=20 February 2009 |archive-date=21 February 2009 |archive-url=https://web.archive.org/web/20090221185643/http://news.bbc.co.uk/1/hi/sci/tech/7897585.stm |url-status=live }}</ref> At that time, there was competition from other proposals.<ref name="ESA2007">{{cite web |url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=41177 |title=Cosmic Vision 2015–2025 Proposals |date=21 July 2007 |publisher=ESA |access-date=20 February 2009 |archive-date=2 September 2011 |archive-url=https://web.archive.org/web/20110902033453/http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=41177 |url-status=live }}</ref> [[Organization:JAXA|Japan]] proposed ''[[Astronomy:Jupiter Magnetospheric Orbiter|Jupiter Magnetospheric Orbiter]]''.

''[[Astronomy:Jovian Europa Orbiter|Jovian Europa Orbiter]]'' was an ESA Cosmic Vision concept study from 2007. Another concept was ''Ice Clipper'',<ref name="McKay2002">{{cite journal | title = Planetary protection for a Europa surface sample return: The Ice Clipper mission | journal = Advances in Space Research | date = 2002 | first1 = C. P. | volume = 30 | issue = 6 | pages = 1601–1605 | doi = 10.1016/S0273-1177(02)00480-5 | bibcode = 2002AdSpR..30.1601M | last1 = McKay | url = https://zenodo.org/record/1259969 | access-date = 29 June 2019 | archive-date = 31 July 2020 | archive-url = https://web.archive.org/web/20200731020206/https://zenodo.org/record/1259969 | url-status = live }}</ref> which would have used an impactor similar to the ''[[Engineering:Deep Impact (spacecraft)|Deep Impact]]'' mission—it would make a controlled crash into the surface of Europa, generating a plume of debris that would then be collected by a small spacecraft flying through the plume.<ref name="McKay2002"/><ref name="Goodman1998">Goodman, Jason C. (9 September 1998) [http://www.madsci.org/posts/archives/oct98/905357947.As.r.html ''Re: Galileo at Europa''] {{Webarchive|url=https://web.archive.org/web/20120301191547/http://www.madsci.org/posts/archives/oct98/905357947.As.r.html |date=1 March 2012 }}, MadSci Network forums.</ref>

''[[Astronomy:Jupiter Icy Moons Orbiter|Jupiter Icy Moons Orbiter]]'' (JIMO) was a partially developed fission-powered spacecraft with ion thrusters that was cancelled in 2006.<ref name="PlanetarySocEuropa" /><ref name="Budget" /> It was part of [[Astronomy:Project Prometheus|Project Prometheus]].<ref name="Budget">Berger, Brian; [http://www.space.com/news/nasa_budget_050207.html ''NASA 2006 Budget Presented: Hubble, Nuclear Initiative Suffer''] {{Webarchive|url=https://web.archive.org/web/20090602123151/http://www.space.com/news/nasa_budget_050207.html |date=2 June 2009 }} Space.com (7 February 2005)</ref> The ''Europa Lander Mission'' proposed a small nuclear-powered Europa lander for JIMO.<ref name="elm">[https://web.archive.org/web/20061006071438/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/37545/1/05-0403.pdf Abelson & Shirley – Small RPS-Enabled Europa Lander Mission (2005)]. . (PDF). Retrieved on 23 July 2013.</ref> It would travel with the orbiter, which would also function as a communication relay to Earth.<ref name="elm" />

''[[Astronomy:Europa Orbiter|Europa Orbiter]]'' – Its objective would be to characterize the extent of the ocean and its relation to the deeper interior. Instrument payload could include a radio subsystem, [[Engineering:Lidar|laser altimeter]], [[Physics:Magnetometer|magnetometer]], [[Physics:Langmuir probe|Langmuir probe]], and a mapping camera.<ref>[http://www.lpi.usra.edu/opag/mar2012/presentations/Europa_Reports/1_Europa_Mission_Studies.pdf 2012 Europa Mission Studies] {{Webarchive|url=https://web.archive.org/web/20130603140459/http://www.lpi.usra.edu/opag/mar2012/presentations/Europa_Reports/1_Europa_Mission_Studies.pdf |date=3 June 2013 }}. OPAG 29 March 2012 (PDF). Lunar and Planetary Institute, NASA. Retrieved on 23 July 2013.</ref><ref>{{citation | contribution=Europa Study 2012 Report | title=Europa Orbiter Mission| author=Europa Study Team | publisher=JPL – NASA | date=1 May 2012 | contribution-url=http://solarsystem.nasa.gov/europa/docs/ES%202012%20Report%20B%20Orbiter%20-%20Final%20-%2020120501.pdf | url=http://solarsystem.nasa.gov/europa/docs/ES%202012%20Report%20B%20Orbiter%20-%20Final%20-%2020120501.pdf | access-date=17 January 2014 | archive-url=https://web.archive.org/web/20140202105451/http://solarsystem.nasa.gov/europa/docs/ES%202012%20Report%20B%20Orbiter%20-%20Final%20-%2020120501.pdf | archive-date=2 February 2014 | url-status=dead | df=dmy-all }}</ref> The ''Europa Orbiter'' received the go-ahead in 1999 but was canceled in 2002. This orbiter featured a special ice-penetrating radar that would allow it to scan below the surface.<ref name="waterworld" />

More ambitious ideas have been put forward including an impactor in combination with a thermal drill to search for [[Astronomy:Biosignature|biosignature]]s that might be frozen in the shallow subsurface.<ref name="Weiss2010">{{Cite journal | last1 = Weiss | first1 = P. | last2 = Yung | first2 = K. L. | last3 = Kömle | first3 = N. | last4 = Ko | first4 = S. M. | last5 = Kaufmann | first5 = E. | last6 = Kargl | first6 = G. | doi = 10.1016/j.asr.2010.01.015 | title = Thermal drill sampling system onboard high-velocity impactors for exploring the subsurface of Europa | journal = Advances in Space Research | volume = 48 | issue = 4 | page = 743 | year = 2011 |bibcode = 2011AdSpR..48..743W | hdl = 10397/12621 | hdl-access = free }}</ref><ref name="Thermal">{{cite web
 |title=Dual Drill Designed for Europa's Ice 
 |last=Hsu 
 |first=J. 
 |url=http://www.astrobio.net/exclusive/3463/dual-drill-designed-for-europa’s-ice 
 |publisher=Astrobiology Magazine 
 |date=15 April 2010 
 |archive-url=https://web.archive.org/web/20100418204646/http://www.astrobio.net/exclusive/3463/dual-drill-designed-for-europa%E2%80%99s-ice 
 |archive-date=18 April 2010 |url-status=usurped 
}}</ref>

Another proposal put forward in 2001 calls for a large [[Physics:Nuclear power|nuclear-powered]] "melt probe" (cryobot) that would melt through the ice until it reached an ocean below.<ref name="PlanetarySocEuropa" /><ref name="Knight2002">{{cite web | last=Knight | first=Will | date=14 January 2002 | url=https://www.newscientist.com/article.ns?id=dn1786 | title=Ice-melting robot passes Arctic test | work=New Scientist | access-date=27 August 2017 | archive-date=17 March 2008 | archive-url=https://web.archive.org/web/20080317083303/http://www.newscientist.com/article.ns?id=dn1786 | url-status=live }}</ref> Once it reached the water, it would deploy an autonomous underwater vehicle (hydrobot) that would gather information and send it back to Earth.<ref name="Bridges2000">{{cite web | last=Bridges | first=Andrew | date=10 January 2000 | archive-url=https://web.archive.org/web/20090208150400/http://www.space.com/searchforlife/europa_ocean_update_000110.html | archive-date=8 February 2009 | url-status=live | url=http://www.space.com/searchforlife/europa_ocean_update_000110.html | title=Latest Galileo Data Further Suggest Europa Has Liquid Ocean | publisher=Space.com }}</ref> Both the cryobot and the hydrobot would have to undergo some form of extreme sterilization to prevent detection of Earth organisms instead of native life and to prevent contamination of the subsurface ocean.<ref name="NASSSB">{{cite book | work=National Academy of Sciences Space Studies Board | archive-url=https://web.archive.org/web/20080213194509/http://www7.nationalacademies.org/ssb/europamenu.html | archive-date=13 February 2008 | url-status=live | url=http://www7.nationalacademies.org/ssb/europamenu.html |title=Preventing the Forward Contamination of Europa | publisher=National Academy Press | location=Washington (DC) | date=2000 | isbn=978-0-309-57554-6 }}</ref> This suggested approach has not yet reached a formal conceptual planning stage.<ref name="Powell2005">{{cite journal |title=NEMO: A mission to search for and return to Earth possible life forms on Europa |last1=Powell |first1=Jesse |last2=Powell |first2=James |last3=Maise |first3=George |last4=Paniagua |first4=John |journal=Acta Astronautica |volume=57 |issue=2–8 |pages=579–593 |date=2005|doi=10.1016/j.actaastro.2005.04.003 |bibcode=2005AcAau..57..579P }}</ref>

==Habitability==
[[File:PIA22479-Europa-JupiterMoon-ArtistConcept-20180723.jpg|thumb|400px|Europa – possible effect of radiation on biosignature chemicals]]

So far, there is no evidence that life exists on Europa, but Europa has emerged as one of the most likely locations in the Solar System for potential habitability.<ref name=ChemDisequilib/><ref name=Schulze-Makuch2001>{{cite journal |last1=Schulze-Makuch |first1=Dirk |last2=Irwin |first2=Louis N. |title=Alternative energy sources could support life on Europa |journal=Eos, Transactions American Geophysical Union |date=2001 |volume=82 |issue=13 |pages=150 |doi=10.1029/EO082i013p00150 |bibcode=2001EOSTr..82..150S |s2cid=140714995 |doi-access=free }}</ref> Life could exist in its under-ice ocean, perhaps in an environment similar to Earth's deep-ocean [[Earth:Hydrothermal vent|hydrothermal vent]]s.<ref name=EuropaLife/><ref name=Jones2001>{{cite web|first=Nicola|last=Jones|date=11 December 2001|url=https://www.newscientist.com/article/dn1647-bacterial-explanation-for-europas-rosy-glow/|title=Bacterial explanation for Europa's rosy glow|work=New Scientist|access-date=26 September 2016|archive-date=27 February 2015|archive-url=https://web.archive.org/web/20150227111850/http://www.newscientist.com/article/dn1647|url-status=live}}</ref> Even if Europa lacks volcanic hydrothermal activity, a 2016 NASA study found that Earth-like levels of hydrogen and oxygen could be produced through processes related to [[Earth:Serpentinite|serpentinization]] and ice-derived oxidants, which do not directly involve volcanism.<ref name=jpl_2016-05-18>{{Citation| url=http://www.jpl.nasa.gov/news/news.php?feature=6514| title=Europa's Ocean May Have An Earthlike Chemical Balance| newspaper=Jpl.nasa.gov| access-date=18 May 2016| archive-date=18 May 2016| archive-url=https://web.archive.org/web/20160518103003/http://www.jpl.nasa.gov/news/news.php?feature=6514| url-status=live}}</ref> In 2015, scientists announced that salt from a [[Astronomy:Extraterrestrial liquid water|subsurface ocean]] may likely be coating some geological features on Europa, suggesting that the ocean is interacting with the seafloor. This may be important in determining if Europa could be habitable.<ref name=NASA-20150512/><ref>{{cite news| last=Wall| first=Mike| url=http://www.space.com/29604-nasa-jupiter-moon-europa-surface-mission.html| title=NASA Aiming for Multiple Missions to Jupiter Moon Europa| work=Space.com| date=9 June 2015| access-date=10 June 2015| archive-date=11 June 2015| archive-url=https://web.archive.org/web/20150611133540/http://www.space.com/29604-nasa-jupiter-moon-europa-surface-mission.html| url-status=live}}</ref> The likely presence of liquid water in contact with Europa's rocky [[Earth:Mantle (geology)|mantle]] has spurred calls to send a probe there.<ref name=Phillips2006>[[Biography:Cynthia B. Phillips|Phillips, Cynthia]] (28 September 2006) [http://www.space.com/searchforlife/seti_europa_060928.html ''Time for Europa''] {{Webarchive|url=https://web.archive.org/web/20061125004853/http://www.space.com/searchforlife/seti_europa_060928.html |date=25 November 2006 }}, Space.com.</ref>

The energy provided by tidal forces drives active geological processes within Europa's interior, just as they do to a far more obvious degree on its sister moon Io. Although Europa, like the Earth, may possess an internal energy source from radioactive decay, the energy generated by tidal flexing would be several orders of magnitude greater than any radiological source.<ref name=Wilson2007>{{cite conference |title=Tidal Heating on Io and Europa and its Implications for Planetary Geophysics |last1=Wilson |first1=Colin P. |conference=Northeastern Section – 42nd Annual Meeting |url=http://gsa.confex.com/gsa/2007NE/finalprogram/abstract_117688.htm |date=March 2007 |access-date=21 December 2007 |archive-url=https://web.archive.org/web/20080905090524/http://gsa.confex.com/gsa/2007NE/finalprogram/abstract_117688.htm |archive-date=5 September 2008 }}</ref> Life on Europa could exist clustered around hydrothermal vents on the ocean floor, or below the ocean floor, where [[Biology:Endolith|endolith]]s are known to inhabit on Earth. Alternatively, it could exist clinging to the lower surface of Europa's ice layer, much like algae and bacteria in Earth's polar regions, or float freely in Europa's ocean.<ref name=limit>{{cite journal |title=The Search for Life on Europa: Limiting Environmental Factors, Potential Habitats, and Earth Analogues |author1=Marion, Giles M. |author2=Fritsen, Christian H. |author3=Eicken, Hajo |author4=Payne, Meredith C. |s2cid=23880085 |journal=Astrobiology |date=2003 |doi=10.1089/153110703322736105 |pmid=14987483 |volume=3 |issue=4 |pages=785–811 |bibcode=2003AsBio...3..785M}}</ref> If Europa's oceans were too cold, biological processes similar to those known on Earth could not take place. If it is too salty, only extreme [[Biology:Halophile|halophile]]s could survive in that environment.<ref name=limit/> In 2010, a model proposed by Richard Greenberg of the University of Arizona proposed that irradiation of ice on Europa's surface could saturate its crust with oxygen and peroxide, which could then be transported by tectonic processes into the interior ocean. Such a process could render Europa's ocean as oxygenated as our own within just 12&nbsp;million years, allowing the existence of complex, multicellular lifeforms.<ref>{{cite journal |title=Transport Rates of Radiolytic Substances into Europa's Ocean: Implications for the Potential Origin and Maintenance of Life |author=Richard Greenberg |journal=Astrobiology |date=May 2010 |volume=10 |issue=3 |pages=275–283 |doi=10.1089/ast.2009.0386 |pmid=20446868 |bibcode = 2010AsBio..10..275G }}</ref>

Evidence suggests the existence of lakes of liquid water entirely encased in Europa's icy outer shell and distinct from a liquid ocean thought to exist farther down beneath the ice shell,<ref name=europagreatlake/><ref name=europagreatlakeairhart/> as well as pockets of water that form M-shaped ice ridges when the water freezes on the surface – as in Greenland.<ref>[https://www.space.com/europa-moon-double-ridges-subsurface-water Icy Europa's mysterious double ridges may hint at hidden pockets of water] {{Webarchive|url=https://web.archive.org/web/20220422020629/https://www.space.com/europa-moon-double-ridges-subsurface-water |date=22 April 2022 }} Rahul Rao, Space.com. April 21, 2022</ref> If confirmed, the lakes and pockets of water could be yet another potential habitat for life. Evidence suggests that hydrogen peroxide is abundant across much of the surface of Europa.<ref>[http://www.nasa.gov/topics/solarsystem/features/europa20130404.html NASA – Mapping the Chemistry Needed for Life at Europa]. {{webarchive|url=https://web.archive.org/web/20130408082530/http://www.nasa.gov/topics/solarsystem/features/europa20130404.html |date=8 April 2013 }}. Nasa.gov (4 April 2013). Retrieved on 23 July 2013.</ref> Because hydrogen peroxide decays into oxygen and water when combined with liquid water, the authors argue that it could be an important energy supply for simple life forms.

[[Chemistry:Clay minerals|Clay-like minerals]] (specifically, phyllosilicates), often associated with [[Chemistry:Organic matter|organic matter]] on Earth, have been detected on the icy crust of Europa.<ref name=NASA-20131211>{{cite web |last=Cook |first=Jia-Rui C. |title=Clay-Like Minerals Found on Icy Crust of Europa |url=http://www.jpl.nasa.gov/news/news.php?release=2013-362 |date=11 December 2013 |work=[[Organization:NASA|NASA]] |access-date=11 December 2013 |archive-date=30 January 2020 |archive-url=https://web.archive.org/web/20200130044515/https://www.jpl.nasa.gov/news/news.php?release=2013-362 |url-status=live }}</ref> The presence of the minerals may have been the result of a collision with an [[Astronomy:Asteroid|asteroid]] or comet.<ref name=NASA-20131211/> Some scientists have speculated that life on Earth could have been blasted into space by asteroid collisions and arrived on the moons of Jupiter in a process called [[Biology:Panspermia#Lithopanspermia|lithopanspermia]].<ref>{{cite news |first=Charles Q. |last=Choi |url=http://astrobiology.com/2013/12/life-could-have-hitched-a-ride-to-outer-planet-moons.html |title=Life Could Have Hitched a Ride to Outer Planet Moons |work=Astrobiology Magazine |publisher=Astrobiology Web |date=8 December 2013 |access-date=12 December 2013 |archive-date=12 December 2013 |archive-url=https://archive.today/20131212223353/http://astrobiology.com/2013/12/life-could-have-hitched-a-ride-to-outer-planet-moons.html |url-status=live }}</ref>

==See also==
* [[Astronomy:Moons of Jupiter|Moons of Jupiter]]
* [[Astronomy:Galilean moons|Galilean moons]] (the four biggest moons of Jupiter)
* Jupiter's moons in fiction
* [[Astronomy:List of craters on Europa|List of craters on Europa]]
* [[Astronomy:List of geological features on Europa|List of geological features on Europa]]
* [[Astronomy:List of lineae on Europa|List of lineae on Europa]]
* [[Earth:Snowball Earth|Snowball Earth hypothesis]]

==Notes==
{{reflist|group=lower-alpha}}

==References==
{{Reflist}}

===Further reading===
* {{cite book
 | first=David M. | last=Harland | date=2000
 | title=Jupiter Odyssey: The Story of NASA's Galileo Mission
 | publisher=Springer | isbn=978-1-85233-301-0
}}
* {{cite book
 | first=David A. | last=Rothery | date=1999
 | title=Satellites of the Outer Planets: Worlds in Their Own Right
 | publisher=Oxford University Press US
 | isbn=978-0-19-512555-9
}}

==External links==
{{Commons}}
* [https://web.archive.org/web/20140326221921/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jup_Europa Europa Profile] at NASA
* [http://www.nineplanets.org/europa.html Europa Facts] at ''The Nine Planets''
* [http://www.solarviews.com/eng/europa.htm Europa Facts] at ''Views of the Solar System''
* [http://www.nap.edu/openbook.php?record_id=9895&page=R1 Preventing Forward Contamination of Europa] – USA Space Studies Board (2000)
* [http://photojournal.jpl.nasa.gov/target/Europa Images of Europa at JPL's Planetary Photojournal]
* Movie of [http://sos.noaa.gov/videos/Europa.mp4 Europa's rotation] from the National Oceanic and Atmospheric Administration
* [http://photojournal.jpl.nasa.gov/catalog/PIA03526 Europa map with feature names] from [http://photojournal.jpl.nasa.gov/ Planetary Photojournal]
* [http://planetarynames.wr.usgs.gov/Page/EUROPA/target Europa nomenclature] and [https://planetarynames.wr.usgs.gov/images/europa_comp.pdf Europa map with feature names] from the [http://planetarynames.wr.usgs.gov USGS planetary nomenclature page]
* [http://stereomoons.blogspot.com/2009/09/broken-land-touring-conamara-chaos.html Paul Schenk's 3D images and flyover videos of Europa and other outer Solar System satellites]; [http://stereomoons.blogspot.com/2009/10/galileo-4-moons-at-400-years.html see also]
* Large, high-resolution ''Galileo'' image mosaics of Europan terrain from [http://www.gishbartimes.org/2010/03/and-now-for-something-completely.html Jason Perry] at JPL: [http://pirlwww.lpl.arizona.edu/~perry/io_images/17ESREGMAP01.png 1], [http://pirlwww.lpl.arizona.edu/~perry/io_images/17ESAGENOR03.png 2], [http://pirlwww.lpl.arizona.edu/~perry/io_images/17ESSOUTHP01.png 3], [http://pirlwww.lpl.arizona.edu/~perry/io_images/17ESREGMAP02.png 4], [http://pirlwww.lpl.arizona.edu/~perry/io_images/19ESRHADAM01.png 5], [http://pirlwww.lpl.arizona.edu/~perry/io_images/15ESREGMAP01.png 6], [http://pirlwww.lpl.arizona.edu/~perry/io_images/15ESREGMAP02.png 7]
* [http://apod.nasa.gov/apod/image/0903/europa_galileo_big.jpg Europa image montage from ''Galileo'' spacecraft] NASA
* [http://www.nasa.gov/topics/solarsystem/features/pia16827.html View of Europa from ''Galileo'' flybys]
* [https://www.google.com/maps/space/europa/@-0.6093333,127.5239384,8527522m/data=!3m1!1e3 Google Europa 3D], interactive map of the moon
* [https://www.youtube.com/watch?v=SYcUk-2SJCE High-resolution animation] by Kevin M. Gill of a flyover of Europa; see [https://www.flickr.com/photos/kevinmgill/albums/72157677986931756 album] for more

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