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| {{redirect|Jupiter II|the spaceship in the 1960s television series Lost in Space|Jupiter 2}}
| | Collin is what persons get in touch with me and I consider it appears really superior when you say it. My family lives in New Hampshire. 1 of the issues I like most is badge amassing and I am going to be starting a thing else along with it. The [http://search.about.com/?q=occupation+I%27ve occupation I've] been occupying for a long time is a library assistant. If you want to find out a lot more check out my web-site: http://boinc.med.usherbrooke.ca/nrg/view_profile.php?userid=2472535<br><br>Here is my web blog: [http://boinc.med.usherbrooke.ca/nrg/view_profile.php?userid=2472535 Zapatillas Asics Baratas] |
| {{Other uses|Europa (disambiguation){{!}}Europa}}
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| {{pp-vandalism|small=yes}}
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| {{Use dmy dates|date=July 2013}}
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| {{Infobox Planet
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| | name = Europa
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| | alt_names = Jupiter II
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| | adjectives = Europan
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| | image = [[Image:Europa-moon.jpg|275px]]
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| | caption =
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| | bgcolour = #a0ffa0
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| | discovery = yes
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| | discoverer = [[Galileo Galilei]]<br>[[Simon Marius]]
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| | discovered = 8 January 1610<ref name="IAUMoonDiscoveries"/>
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| | 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=[[NASA]], Jet Propulsion Laboratory |accessdate=10 August 2007 }}</ref>
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| | epoch = 8 January 2004
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| | 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 |accessdate=27 December 2007 }}</ref>
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| | eccentricity = 0.009<ref name="factsheet" />
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| | periapsis = {{val|664862|u=km}}<ref group=lower-alpha>Periapsis is derived from the semimajor axis (''a'') and eccentricity (''e''): ''a''(1−''e'').</ref>
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| | apoapsis = {{val|676938|u=km}}<ref group=lower-alpha>Apoapsis is derived from the semimajor axis (''a'') and eccentricity (''e''): ''a''(1+''e'').</ref>
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| | period = {{val|3.551181|u=d}}<ref name="factsheet" />
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| | orbit_circ = {{val|4216100|u=km}}
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| | avg_speed = {{val|13.740|u=km/s}}<ref name="factsheet" />
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| | inclination = 0.470° (to Jupiter's equator)<ref name="factsheet" />
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| | satellite_of = [[Jupiter]]
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| | physical_characteristics = yes
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| | mean_radius = {{val|1560.8|u=km}} (0.245 [[Earth]]s)<ref name="factsheet" />
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| | surface_area = {{val|3.09|e=7|u=km2}} (0.061 Earths)<ref group=lower-alpha>Surface area derived from the radius (''r''): 4π''r''<sup> 2</sup>.</ref>
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| | 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>π''r''<sup> 3</sup>.</ref>
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| | mass = {{val|4.7998|e=22|u=kg}} (0.008 Earths)<ref name="factsheet" />
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| | density = {{val|3.01|u=g/cm3}}<ref name="factsheet" />
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| | surface_grav = {{val|1.314|ul=m/s2}} (0.134 g)<ref group=lower-alpha>Surface gravity derived from the mass (''m''), the [[gravitational constant]] (''G'') and the radius (''r''): G''m''/''r''<sup> 2</sup>.</ref>
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| | escape_velocity = {{val|2.025|u=km/s}}<ref group=lower-alpha>Escape velocity derived from the mass (''m''), the [[gravitational constant]] (''G'') and the radius (''r''): <math>\textstyle\sqrt{\frac{2Gm}{r}}</math>.</ref>
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| | rotation = [[synchronous rotation|Synchronous]]<ref name=Geissler/>
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| | 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 |year=2005 |volume=175 |issue=1 |pages=233–247 |doi=10.1016/j.icarus.2004.10.028 |bibcode=2005Icar..175..233B |journal=Icarus }}</ref>
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| | albedo = 0.67 ± 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 |accessdate=5 November 2007 }}</ref>
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| | magnitude = 5.29 ([[Opposition (astronomy and astrology)|opposition]])<ref name="jplfact" />
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| | temperatures = yes
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| | temp_name1 = Surface
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| | min_temp_1 = ≈ 50 [[kelvin|K]]<ref name="cyclo" />
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| | mean_temp_1 = 102 K
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| | max_temp_1 = 125 K
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| | atmosphere = yes
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| | surface_pressure = 0.1 [[Pascal (unit)|µPa]] (10<sup>−12</sup> [[bar (unit)|bar]])<ref name="McGrathChapter">{{cite book |last=McGrath |editor=Pappalardo, Robert T.; McKinnon, William B.; and Khurana, Krishan K. |title=Europa |year=2009 |publisher=University of Arizona Press |isbn=0-8165-2844-6 |chapter=Atmosphere of Europa }}</ref>
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| }}
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| '''Europa''' {{IPAc-en|audio=en-Europa.ogg|j|ʊ|ˈ|r|oʊ|p|ə}}<ref name="USdict">{{respell|ew|ROH|pə}}, or as {{lang-el|Ευρώπη}}</ref> ('''Jupiter II'''), is the [[Moons of Jupiter#Table|sixth]]-closest [[moon of the planet Jupiter]], and the smallest of its four [[Galilean satellites]], but still the sixth-largest moon in the [[Solar System]]. Europa was discovered in 1610 by [[Galileo Galilei]]<ref name="IAUMoonDiscoveries" /> and possibly independently by [[Simon Marius]] around the same time. Progressively better observations of Europa have occurred over the centuries by Earth-bound telescopes, and by space probe flybys starting in the 1970s.
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| Slightly smaller than the [[Moon]], Europa is primarily made of [[silicate]] rock and probably has an [[iron]] core. It has a tenuous atmosphere composed primarily of [[oxygen]]. Its surface is composed of water ice and is one of the smoothest in the Solar System.<ref name="waterworld" /> This surface is striated by cracks and streaks, whereas craters are relatively rare. The apparent youth and smoothness of the surface have led to the hypothesis that a [[water]] ocean exists beneath it, which could conceivably serve as an abode for [[extraterrestrial life]].<ref name="Tritt2002">{{cite web |url=http://web.archive.org/web/20070217120400/http://people.msoe.edu/~tritt/sf/europa.life.html |title=Possibility of Life on Europa |last=Tritt |first=Charles S. |accessdate=10 August 2007 |publisher=Milwaukee School of Engineering |year=2002 }}</ref> This hypothesis proposes that heat from [[tidal flexing]] causes the ocean to remain liquid and drives geological activity similar to [[plate tectonics]].<ref name="geology">{{cite web |url=http://geology.asu.edu/~glg_intro/planetary/p8.htm |title=Tidal Heating |accessdate=20 October 2007 |work=geology.asu.edu |archiveurl = http://web.archive.org/web/20060329000051/http://geology.asu.edu/~glg_intro/planetary/p8.htm |archivedate = 29 March 2006}}</ref>
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| The ''[[Galileo (spacecraft)|Galileo]]'' mission, launched in 1989, provided the bulk of current data on Europa. No spacecraft has yet landed on Europa, but its intriguing characteristics have led to several ambitious exploration proposals. The next mission to Europa is the [[European Space Agency]]'s [[Jupiter Icy Moon Explorer]] (JUICE), due to launch in 2022.<ref name="selection">{{cite news|first = Jonathan Amos|url = http://www.bbc.co.uk/news/science-environment-17917102 |title = Esa selects 1bn-euro Juice probe to Jupiter |accessdate = 2 May 2012|date = 2 May 2012|work = [[BBC News Online]]}}</ref>
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| In December 2013, NASA reported the detection of "[[Clay minerals|clay-like minerals]]" (specifically, [[phyllosilicates]]), often associated with "[[organic material]]" on the icy crust of Europa.<ref name="NASA-20131211" /> In addition, NASA announced, based on studies with the [[Hubble Space Telescope]], that [[water vapor|water vapor plumes]] were detected on Europa and were similar to water vapor plumes detected on [[Enceladus (moon)|Enceladus]], moon of [[Saturn (planet)|Saturn]].<ref name="NASA-20131212-EU" />
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| ==Discovery and naming==
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| Europa was discovered on 8 January 1610 by [[Galileo Galilei]],<ref name="IAUMoonDiscoveries" /> and possibly independently by [[Simon Marius]]. It is named after a [[Phoenicia]]n noblewoman in [[Greek mythology]], [[Europa (mythology)|Europa]], who was courted by [[Zeus]] and became the queen of [[Crete]].
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| Europa, along with Jupiter's three other largest moons, [[Io (moon)|Io]], [[Ganymede (moon)|Ganymede]], and [[Callisto (moon)|Callisto]], was discovered by [[Galileo Galilei]] in January 1610. The first reported observation of Io was made by [[Galileo Galilei]] on 7 January 1610 using a [[Galileo Telescope|20x-power, refracting telescope]] at the [[University of Padua]]. However, in that observation, Galileo could not separate Io and Europa due to the low power of his telescope, so the two were recorded as a single point of light. Io and Europa were seen for the first time as separate bodies during Galileo's observations of the Jupiter system the following day, 8 January 1610 (used as the discovery date for Europa by the [[International Astronomical Union|IAU]]).<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 |accessdate= 13 January 2010}}</ref>
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| Like all the Galilean satellites, Europa is named after a lover of [[Zeus]], the Greek counterpart of [[Jupiter (mythology)|Jupiter]], in this case [[Europa (mythology)|Europa]], daughter of the king of [[Tyre, Lebanon|Tyre]]. The naming scheme was suggested by [[Simon Marius]], who apparently discovered the four satellites independently, though Galileo alleged that Marius had plagiarized him. Marius attributed the proposal to [[Johannes Kepler]].<ref name="SEDS">{{cite web |url=http://web.archive.org/web/20060821060833/http://seds.lpl.arizona.edu/messier/xtra/Bios/marius.html |title=Simon Marius (January 20, 1573 – December 26, 1624) |accessdate=9 August 2007 |publisher=[[University of Arizona]] |work=Students for the Exploration and Development of Space }}</ref><ref name="Marius1614">[[Simon Marius|Marius, S.]]; (1614) ''[[Mundus Iovialis]] anno M.DC.IX Detectus Ope Perspicilli Belgici'' [http://galileo.rice.edu/sci/marius.html], where he [http://galileo.rice.edu/sci/observations/jupiter_satellites.html attributes the suggestion] to [[Johannes Kepler]]</ref>
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| 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 |last=Marazzini |first=Claudio |year=2005 |title=I nomi dei satelliti di Giove: da Galileo a Simon Marius (The names of the satellites of Jupiter: from Galileo to Simon Marius) |journal=Lettere Italiane |volume=57 |issue=3 |pages=391–407 }}</ref> In much of the earlier [[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 [[Amalthea (moon)|Amalthea]], whose orbit lay closer to Jupiter than those of the Galilean moons, pushed Europa to the third position. The [[Voyager probes|''Voyager'' probes]] discovered three more [[Inner satellites of Jupiter|inner satellites]] in 1979, so Europa is now considered Jupiter's sixth satellite, though it is still sometimes referred to as {{Nowrap|Jupiter II}}.<ref name="marazzini"/>
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| ==Orbit and rotation==
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| [[File:Galilean moon Laplace resonance animation.gif|thumb|365px|left|Animation showing Io's [[Laplace resonance]] with Europa and Ganymede]]
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| Europa orbits Jupiter in just over three and a half days, with an orbital radius of about 670,900 km. With an eccentricity of only 0.009, the orbit itself is nearly circular, and the [[orbital inclination]] relative to the Jovian [[equatorial plane]] is small, at 0.470°.<ref name="datasheet">{{cite web | url=http://www2.jpl.nasa.gov/galileo/europa/#overview |title=Europa, a Continuing Story of Discovery|accessdate=9 August 2007 |work=Project Galileo|publisher=[[NASA]], Jet Propulsion Laboratory}} {{dead link|date=July 2013}}</ref> Like its fellow [[Galilean satellites]], Europa is [[tidal locking|tidally locked]] to Jupiter, with one hemisphere of Europa constantly facing Jupiter. Because of this, there is a sub-Jovian point on Europa's surface, from which Jupiter would appear to hang directly overhead. Europa's [[prime meridian]] is the line intersecting this point.<ref>{{cite web|title=Planetographic Coordinates|publisher=Wolfram Research|url=http://web.archive.org/web/20090301191456/http://documents.wolfram.com/applications/astronomer/AdditionalInformation/PlanetographicCoordinates.html|year=2010|accessdate=2010-03-29}}</ref> Research suggests the 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 |last= Geissler |first=P. E. |coauthors=Greenberg, R.; Hoppa, G.; Helfenstein, P.; McEwen, A.; Pappalardo, R.; Tufts, R.; Ockert-Bell, M.; Sullivan, R.; Greeley, R.; Belton, M. J. S.; Denk, T.; Clark, B. E.; Burns, J.; Veverka, J. |year=1998 |title=Evidence for non-synchronous rotation of Europa |journal=[[Nature (magazine)|Nature]] |volume=391 |pages=368–70 |bibcode=1998Natur.391..368G |doi= 10.1038/34869 |pmid= 9450751 |issue= 6665}}</ref>
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| The slight eccentricity of Europa's orbit, maintained by the gravitational disturbances from the other Galileans, causes Europa's sub-Jovian point to oscillate about 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 [[orbital resonance|mean-motion resonance]] with [[Io (moon)|Io]].<ref name="Showman1997">{{cite journal|doi=10.1006/icar.1996.5669|last=Showman|first=Adam P.|coauthors=Malhotra, Renu|title=Tidal Evolution into the Laplace Resonance and the Resurfacing of Ganymede|journal=Icarus|volume=127|year=1997|issue=1|pages=93–111|
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| url=http://www.lpl.arizona.edu/~showman/publications/showman-malhotra-1997.pdf|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|volume=108|issue=E8|year=2003|issn=0148-0227|doi=10.1029/2002JE001943|url=http://adsabs.harvard.edu/full/1982MNRAS.201..415G|accessdate=2008-01-02|bibcode = 2003JGRE..108.5096M }}</ref>
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| Scientists analyzing the unique cracks lining the icy face of Europa found evidence showing that this moon of Jupiter likely spun around a tilted axis at some point in time. If this hypothesis is true, this tilt would be an explanation for many of Europa’s features. Europa's immense network of crisscrossing cracks serves as a record of the stresses caused by massive tides in the moon's global ocean. Europa's tilt could influence calculations of how much of the moon's 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. The moon's ice layer must stretch to accommodate these changes. When there is too much stress, it cracks. A tilt in the moon’s axis could suggest that Europa's cracks may be much more recent than previously thought. The reason 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 also could affect the 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 might suggest that more heat is generated by tidal forces. This heat might help the ocean to remain liquid longer. Scientists did not specify when the tilt would have occurred and measurements have not been made of the tilt of Europa's axis.<ref>Cook, Jia-Rui C. (September 18, 2013) [http://www.jpl.nasa.gov/news/news.php?release=2013-283 Long-stressed Europa Likely Off-kilter at One Time]. jpl.nasa.gov</ref>
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| ==Physical characteristics==
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| [[File:Europa Earth Moon Comparison.png|thumb|right|275px|Europa (''lower left'') compared to the Moon (''top left'') and Earth (''right'') to scale approximately. (montage)]]
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| Europa is slightly smaller than [[Earth]]'s [[Moon]]. At just over {{Convert|3100|km|mi}} in [[diameter]], it is the [[List of moons by diameter|sixth-largest moon]] and [[List of Solar System objects by size|fifteenth largest object]] in the [[Solar System]]. Though by a wide margin the least massive of the Galilean satellites, it is nonetheless more massive than all known moons in the Solar System smaller than itself combined.<ref name="Masses">Mass of Europa: 48{{e|21}} kg. Mass of Triton plus all smaller moons: 39.5{{e|21}} kg (see note ''g'' [[Triton (moon)#Notes|here]])</ref> Its bulk density suggests that it is similar in composition to the [[terrestrial planet]]s, being primarily composed of [[silicate]] [[rock (geology)|rock]].<ref>{{cite journal|title=Europa's Crust and Ocean: Origin, Composition, and the Prospects for Life|author=Jeffrey S. Kargel, Jonathan Z. Kaye, James W. Head, III, et al.|url=http://www.planetary.brown.edu/pdfs/2440.pdf|place=Planetary Sciences Group, Brown University|journal=Icarus|volume= 148|issue=1|pages=226–265|year=2000|doi=10.1006/icar.2000.6471|bibcode=2000Icar..148..226K}}</ref>
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| ===Internal structure===
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| [[Image:Europa poster.svg|left|thumb|300px|Model of Europa's interior showing a solid ice crust over a layer of liquid water or soft ice, a silicate mantle and a metallic core.]]
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| It is believed that Europa has an outer layer of [[water]] around {{Convert|100|km|mi|-0|abbr=on}} thick; some as frozen-ice upper crust, some as liquid ocean underneath the ice. Recent [[magnetic field]] data from the [[Galileo (spacecraft)|''Galileo'']] orbiter showed that Europa has an induced magnetic field through interaction with Jupiter's, which suggests the presence of a subsurface conductive layer. The layer is likely a salty liquid water ocean. The crust is estimated to have undergone a shift of 80°{{vague|date=August 2012}}, nearly flipping over (see [[true polar wander]]), which would be unlikely if the ice were solidly attached to the mantle.<ref name="Cowen2008">{{cite news |first=Ron |last=Cowen |title=A Shifty Moon |url=http://www.sciencenews.org/view/generic/id/32135/title/A_shifty_moon |work=Science News |date=7 June 2008 }}</ref> Europa probably contains a [[metal]]lic [[iron]] core.<ref name="Kivelson">{{cite journal |last=Kivelson |first=Margaret G. |coauthors=Khurana, Krishan K.; Russell, Christopher T.; Volwerk, Martin; Walker, Raymond J.; and Zimmer, Christophe |year=2000 |title=Galileo Magnetometer Measurements: A Stronger Case for a Subsurface Ocean at Europa |journal=[[Science (journal)|Science]] |volume=289 |issue=5483 |pages=1340–1343 |doi=10.1126/science.289.5483.1340 |pmid=10958778 |bibcode = 2000Sci...289.1340K }}</ref>
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| ===Surface features===
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| [[File:PIA01295 modest.jpg|275px|thumb|right|Approximate natural color (left) and enhanced color (right) ''Galileo'' view of leading hemisphere]] [[Image:PIA01092 - Evidence of Internal Activity on Europa.jpg|thumb|260px|Mosaic of ''Galileo'' images showing features indicative of [[tidal flexing]]: lineae, lenticulae (domes, pits) and [[Conamara Chaos]].]]
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| Europa is one of the smoothest objects in the Solar System when considering the lack of large scale features such as mountains or craters,<ref name="waterworld">{{cite web |url=http://teachspacescience.org/cgi-bin/search.plex?catid=10000304&mode=full |title=Europa: Another Water World? |year=2001 |accessdate=9 August 2007 |publisher=[[NASA]], Jet Propulsion Laboratory |work=Project Galileo: Moons and Rings of Jupiter }}</ref> however on a smaller scale Europa's equator has been theorised to be covered in 10 metre tall icy spikes called [[penitentes]] caused by the effect of direct overhead sunlight on the equator melting vertical cracks.<ref>[http://www.bbc.co.uk/news/science-environment-21341176 Ice blades threaten Europa landing]</ref> The prominent markings crisscrossing Europa seem to be mainly [[albedo feature]]s, which emphasize low topography. There are few [[Impact crater|craters]] on Europa because its surface is tectonically active and young.<ref name="Arnett1996">Arnett, Bill (7 November 1996) [http://www.astro.auth.gr/ANTIKATOPTRISMOI/nineplanets/nineplanets/europa.html ''Europa'']. 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 }}</ref> Europa's icy crust gives it an [[albedo]] (light reflectivity) of 0.64, one of the highest of all moons.<ref name="datasheet" /><ref name="EuropaAlbedo" /> This would seem to indicate a young and active surface; based on estimates of the frequency of [[comet]]ary bombardment that Europa probably endures, the surface is about 20 to 180 million years old.<ref name="Schenk">Schenk, Paul M.; Chapman, Clark R.; Zahnle, Kevin; and Moore, Jeffrey M. (2004) [http://books.google.com/books?id=8GcGRXlmxWsC&pg=PA427 "Chapter 18: Ages and Interiors: the Cratering Record of the Galilean Satellites"], pp. 427 ff. in ''Jupiter: The Planet, Satellites and Magnetosphere'', Cambridge University Press, ISBN 0521818087.</ref> There is currently no full scientific consensus among the sometimes contradictory explanations for the surface features of Europa.<ref name="Astrobio2007">{{cite web |url=http://www.astrobio.net/exclusive/603/high-tide-on-europa |title=High Tide on Europa |year=2007 |accessdate=20 October 2007 |publisher=astrobio.net |work=Astrobiology Magazine }}</ref>
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| The radiation level at the surface of Europa is equivalent to a dose of about 5400[[millisievert|mSv]] (540 [[Röntgen equivalent man|rem]]) per day,<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 |accessdate=4 July 2009}} [http://www.webcitation.org/5jwBSgPuV (Webcite from 20 September 2009)]</ref> an amount of radiation that would cause severe illness or death in human beings exposed for a single day.<ref name="remeffects">[http://archive.org/details/TheEffectsOfNuclearWeapons ''The Effects of Nuclear Weapons''], Revised ed., US DOD 1962, pp. 592–593</ref>
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| ====Lineae====
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| [[Image:europa g1 true.jpg|left|thumb|260px|Lineae in an image of Europa in approximately natural color by the ''Galileo'' spacecraft]]
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| {{See also|List of lineae on Europa}}
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| Europa's most striking surface features are a series of dark streaks crisscrossing the entire globe, called ''{{lang|la|[[linea]]e}}'' ({{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 doi|10.1006/icar.1998.5980}}</ref>
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| The most likely hypothesis states that these lineae may have been produced by a series of eruptions of warm ice as the Europan crust spread open to expose warmer layers beneath.<ref name="Figueredo2003">{{cite doi|10.1016/j.icarus.2003.09.016}}</ref> The effect would have been similar to that seen in Earth's [[oceanic ridge]]s. These various fractures are thought to have been caused in large part by the [[tidal flexing]] exerted by Jupiter. Because Europa is tidally locked to Jupiter, and therefore always maintains the same approximate 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 doi|10.1016/j.icarus.2006.08.026}}</ref> Comparisons of ''[[Voyager program|Voyager]]'' and ''Galileo'' spacecraft photos serve to put an upper limit on this hypothetical slippage. The full revolution of the outer rigid shell relative to the interior of Europa occurs over a minimum of 12,000 years.<ref name="Kattenhorn">{{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 |year=2002 |doi=10.1006/icar.2002.6825 |bibcode=2002Icar..157..490K }}</ref>
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| ====Other geological features====
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| {{See also|List of geological features on Europa}}
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| [[Image:Europa Chaos.jpg|thumb|260px|Enhanced-color view of part of [[Conamara Chaos]], showing ice rafts up to {{Convert|10|km|mi|0|abbr=on}} across. White areas are ejecta from the crater [[Pwyll (crater)|Pwyll]].]]
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| [[Image:Europa chaotic terrain.jpg|thumb|260px|Craggy, 250 m high peaks and smooth plates are jumbled together in a close-up of [[Conamara Chaos]].]]
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| Other features present on Europa are circular and elliptical ''lenticulae'' ([[Latin]] for "freckles"). Many are domes, some are pits and some are smooth, dark spots. Others have a jumbled or rough texture. The dome tops look like pieces of the older plains around them, suggesting that the domes formed when the plains were pushed up from below.<ref name="diapir">{{cite web |title=Europa: Tidal heating of upwelling thermal plumes and the origin of lenticulae and chaos melting |author=Sotin, Christophe; Head III, James W.; and Tobie, Gabriel |year=2001 |url=http://planetary.brown.edu/planetary/documents/2685.pdf |accessdate=20 December 2007 |format=PDF }}</ref>
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| One hypothesis states that these lenticulae were formed by [[diapir]]s of warm ice rising up through the colder ice of the outer crust, much like [[magma chamber]]s in Earth's crust.<ref name="diapir" /> The smooth, dark spots could be formed by [[meltwater]] released when the warm ice breaks through the surface. The rough, jumbled lenticulae (called regions of "chaos"; for example, [[Conamara Chaos]]) would then be formed from many small fragments of crust embedded in hummocky, dark material, appearing like [[iceberg]]s in a frozen sea.<ref name="Goodman">{{cite web |title=Hydrothermal Plume Dynamics on Europa: Implications for Chaos Formation |author=Goodman, Jason C.; Collins, Geoffrey C.; Marshall, John; and Pierrehumbert, Raymond T. |url=http://www-paoc.mit.edu/paoc/papers/europa_plume.pdf |accessdate=20 December 2007 |format=PDF }}</ref>
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| An alternative hypothesis suggest that lenticulae are actually small areas of chaos and that the claimed pits, spots and domes are artefacts resulting from 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.
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| <ref name="thinice">{{cite journal |title=Tidal Heat in Europa: Ice Thickness and the Plausibility of Melt-Through |author=O'Brien, David P.; Geissler, Paul; and Greenberg, Richard |journal=Bulletin of the American Astronomical Society |date=October 2000 |volume=30 |page=1066 |bibcode=2000DPS....32.3802O |last2=Geissler |last3=Greenberg }}</ref>
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| <ref name="Greenberg2008">{{cite book |title=Unmasking Europa |author=Greenberg, Richard |year=2008 |url=http://www.springer.com/astronomy/book/978-0-387-47936-1|isbn=978-0-387-09676-6|publisher=Springer + Praxis Publishing }}</ref>
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| In November 2011, a team of researchers from the [[University of Texas at Austin]] and elsewhere presented evidence in the journal ''Nature'' suggesting that many "[[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 |author=Schmidt, Britney; Blankenship, Don; Patterson, Wes; 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 }}</ref><ref name="europagreatlakeairhart">{{cite web|title=Scientists Find Evidence for "Great Lake" on Europa and Potential New Habitat for Life|author=Marc Airhart|year=2011|publisher=Jackson School of Geosciences|url=http://www.jsg.utexas.edu/news/2011/11/scientists-find-evidence-for-great-lake-on-europa/|accessdate=16 November 2011}}</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.
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| ===Subsurface ocean===
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| Most planetary scientists believe that a layer of liquid water exists beneath Europa's surface, and that heat energy 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 |author=McFadden, Lucy-Ann; Weissman, Paul; and Johnson, Torrence |publisher=Elsevier |year=2007 |page=432 |isbn=0-12-226805-9 }}</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 (spacecraft)|Galileo]]'' imaging team members argue for the existence of a subsurface ocean from analysis of ''[[Voyager program|Voyager]]'' and ''Galileo'' images.<ref name="greenberg">Greenberg, Richard (2005) ''Europa: The Ocean Moon: Search for an Alien Biosphere'', Springer + Praxis Books, ISBN 978-3-540-27053-9.</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 extremely 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 ''Jupiter: The Planet, Satellites and Magnetosphere'', Cambridge University Press, ISBN 0521818087.</ref> The different models for the estimation of the ice shell thickness give values between a few kilometers and tens of kilometers.<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. |coauthor=and Kattenhorn, Simon A. |journal=Icarus |volume=177 |issue=2 | pages=397–412 |year=2005 |doi=10.1016/j.icarus.2005.03.013 |bibcode=2005Icar..177..397B}}</ref>
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| [[Image:EuropaInterior1.jpg|thumb|left|260px|Two possible models of Europa]]
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| 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 predicted that the outer crust of solid ice is approximately 10–30 km (6–19 mi) thick, 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 name="Schenk" /><ref>{{cite news | first =Zaina 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 | accessdate =2 October 2012}}</ref> This leads to a volume of Europa's oceans of 3 × 10<sup>18</sup> m<sup>3</sup>, slightly more than two times the volume of Earth's oceans.
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| 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. 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" />
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| 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 previously unconsidered kind of tidal force 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 store 7.3{{E|17}} 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|accessdate=28 July 2009 }}</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 }}</ref> Dissipation of this energy could be the principal heat source of Europa's ocean.<!-- no wattage figure given -->
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| The ''Galileo'' orbiter found that Europa has a weak [[magnetic moment]], which is induced by the varying part of the Jovian magnetic field. The field strength at the magnetic equator (about 120 [[Tesla (unit)|nT]]) created by this magnetic moment is about one-sixth the strength of Ganymede's field and six times the value of Callisto's.<ref name="Zimmer">{{cite journal |last=Zimmer |first=Christophe |coauthors=and Khurana, Krishan K. |title=Subsurface Oceans on Europa and Callisto: Constraints from Galileo Magnetometer Observations |journal=Icarus |year=2000 |volume=147 |issue=2 |pages=329–347 |doi=10.1006/icar.2000.6456 |url=http://www.igpp.ucla.edu/people/mkivelson/Publications/ICRUS147329.pdf |format=PDF |bibcode=2000Icar..147..329Z}}</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" /> Spectrographic evidence suggests that the dark, reddish streaks and features on Europa's surface may be rich in salts such as [[magnesium sulfate]], deposited by evaporating water that emerged from within.<ref name="McCord1998">{{cite web |title=Salts on Europa's Surface Detected by Galileo's Near Infrared Mapping Spectrometer |url=http://www.sciencemag.org/cgi/content/abstract/280/5367/1242 |author=McCord, Thomas B.; Hansen, Gary B.; ''et al.'' |year=1998 |accessdate=20 December 2007 }}</ref> [[Sulfuric acid]] hydrate is another possible explanation for the contaminant observed spectroscopically.<ref name="Carlson2005">{{cite doi|10.1016/j.icarus.2005.03.026}}</ref> In either case, because these materials are colorless or white when pure, some other material must also be present to account for the reddish color, and [[sulfur]] compounds are suspected.<ref name="Calvin">{{cite journal |last=Calvin |first=Wendy M. |coauthors=Clark, Roger N.; Brown, Robert H.; and 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 |year=1995 |volume=100 |issue=E9 |pages=19,041–19,048 |bibcode=1995JGR...10019041C|doi=10.1029/94JE03349 }}</ref>
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| ===Plumes===
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| [[File:Water vapour plumes on Jupiter's moon Europa (artist's impression).jpg|thumbnail|Water vapor plumes on Jupiter's moon Europa (artist's impression).<ref>{{cite news|title=Hubble discovers water vapour venting from Jupiter’s moon Europa|url=http://www.spacetelescope.org/news/heic1322/|accessdate=16 December 2013|newspaper=ESA/Hubble Press Release}}</ref>]]
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| Europa may have periodically occurring plumes of water {{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 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 | accessdate = 2013-12-17}}</ref><ref>{{cite news | first = Charles Q. 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 | accessdate = 2013-12-17}}</ref> These plumes appear when Europa is at its farthest point from Jupiter, and are not seen when Europa is at its closest point to Jupiter, in agreement with [[tidal force]] modeling predictions.<ref name='Europa tidal forces 2013'>{{Cite doi|10.1126/science.1247051}}</ref> The tidal forces are about 1,000 times stronger than the [[Moon]]'s effect on [[Earth]]. The only other moon in the Solar System exhibiting water vapor plumes is [[Enceladus]].<ref name="NASA-20131212-EU" /> The estimated eruption rate at Europa is about 7000 kg/sec (see supplementary material)<ref name='Europa tidal forces 2013'/> compared to about 200 kg/sec for the plumes of Enceladus.<ref name="Hansen2006">{{Cite doi|10.1126/science.1121254}}</ref><ref name="Spencer2013">{{Cite doi|10.1146/annurev-earth-050212-124025}}</ref>
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| ===Atmosphere===
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| Observations with the [[Goddard High Resolution Spectrograph]] of the [[Hubble Space Telescope]], first described in 1995, revealed that Europa has a tenuous [[celestial body's atmosphere|atmosphere]] composed mostly of [[oxygen|molecular oxygen]] (O<sub>2</sub>).<ref name="Hall1995">{{cite doi|10.1038/373677a0}}</ref><ref name="EuropaOxygenJPL">{{cite web |first=Donald |last=Savage |coauthors=Jones, Tammy; and Villard, Ray |url=http://hubblesite.org/newscenter/archive/releases/1995/12/text/ |title=Hubble Finds Oxygen Atmosphere on Europa |accessdate=17 August 2007 |publisher=NASA, Jet Propulsion Laboratory |date=23 February 1995 |work=Project Galileo }}</ref> The surface pressure of Europa's atmosphere is 0.1 [[pascal (unit)|μPa]], or 10<sup>−12</sup> times that of the Earth.<ref name="McGrathChapter" /> In 1997, the ''[[Galileo (spacecraft)|Galileo]]'' spacecraft confirmed the presence of a tenuous [[ionosphere]] (an upper-atmospheric layer of charged particles) around Europa created by solar radiation and energetic particles from Jupiter's [[magnetosphere]],<ref name="Kliore1997">{{cite journal |last=Kliore |first=Arvydas J. |coauthors=Hinson, D. P.; Flasar, F. Michael; Nagy, Andrew F.; Cravens, Thomas E. |date=July 1997 |title=The Ionosphere of Europa from Galileo Radio Occultations |journal=[[Science (magazine)|Science]] |volume=277 |issue=5324 |pages=355–358 |doi=10.1126/science.277.5324.355 |pmid=9219689 |bibcode =1997Sci...277..355K }}</ref><ref name="NASA1997">{{cite web|year=1997|title=Galileo Spacecraft Finds Europa has Atmosphere |publisher=[[NASA]], Jet Propulsion Laboratory |url=http://www.jpl.nasa.gov/news/releases/97/europion.html|accessdate=10 August 2007 |work=Project Galileo }}</ref> providing evidence of an atmosphere.
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| [[Image:Europa field.png|thumb|left|200px|Magnetic field around Europa. The red line shows a trajectory of the ''Galileo'' spacecraft during a typical flyby (E4 or E14).]]
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| Unlike the oxygen in [[atmosphere of Earth|Earth's atmosphere]], Europa's is not of biological origin. The surface-bounded atmosphere forms through [[radiolysis]], the [[Dissociation (chemistry)|dissociation]] of molecules through radiation.<ref name="Johnson1982">{{cite journal |title=Planetary applications of ion induced erosion of condensed-gas frosts |author=Johnson, Robert E.; Lanzerotti, Louis J.; and Brown, Walter L. |year=1982 |bibcode=1982NucIM.198..147J |last2=Lanzerotti |last3=Brown |volume=198 |page=147 |journal=Nuclear Instruments and Methods in Physics Research |doi=10.1016/0167-5087(82)90066-7 }}</ref> Solar ultraviolet radiation and charged particles (ions and electrons) from the Jovian magnetospheric environment collide with Europa's icy surface, splitting water into oxygen and hydrogen constituents. These chemical components are then [[adsorption|adsorbed]] and "[[sputtering|sputtered]]" into the atmosphere. The same radiation also creates collisional ejections of these products from the surface, and the balance of these two processes forms an atmosphere.<ref name="Shematovich2003">{{cite journal |last=Shematovich |first=Valery I. |coauthors=Cooper, John F.; and Johnson, Robert E. |date=April 2003 |page=13094 |title=Surface-bounded oxygen atmosphere of Europa |journal=EGS – AGU – EUG Joint Assembly |issue=Abstracts from the meeting held in Nice, France |bibcode=2003EAEJA....13094S |last2=Cooper |last3=Johnson }}</ref> Molecular oxygen is the densest component of the atmosphere because it has a long lifetime; after returning to the surface, it does not stick (freeze) like a water or [[hydrogen peroxide]] molecule but rather desorbs from the surface and starts another ballistic arc. Molecular hydrogen never reaches the surface, as it is light enough to escape Europa's surface gravity.<ref name="Liang">{{cite journal |last=Liang |first=Mao-Chang |coauthors=Lane, Benjamin F.; Pappalardo, Robert T.; Allen, Mark; and Yung, Yuk L. |title=Atmosphere of Callisto |journal=Journal of Geophysical Research |year=2005 |volume=110 |issue=E2 |pages=E02003 |doi=10.1029/2004JE002322 |url=http://web.archive.org/web/20090225131107/http://yly-mac.gps.caltech.edu/ReprintsYLY/N164Liang_Callisto%2005/Liang_callisto_05.pdf |format=PDF |bibcode=2005JGRE..11002003L}}</ref><ref name="Smyth">{{cite conference |last=Smyth |first=William H. |coauthors=Marconi, Max L. |url=http://www.lpi.usra.edu/meetings/icysat2007/pdf/6039.pdf |title=Processes Shaping Galilean Satellite Atmospheres from the Surface to the Magnetosphere |format=PDF |conference=Workshop on Ices, Oceans, and Fire: Satellites of the Outer Solar System, Boulder, Colorado |booktitle=Abstracts |date=15 August 2007 |pages=131–132 }}</ref>
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| Observations of the surface have revealed that some of the molecular oxygen produced by radiolysis is not ejected from the surface. 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 doi|10.1126/science.1060081 }}</ref> One estimate suggests that, given the turnover rate inferred from the apparent ~0.5 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 |title=Energy, Chemical Disequilibrium, and Geological Constraints on Europa |author=Hand, Kevin P.; Carlson, Robert W.; Chyba, Christopher F. |journal=Astrobiology |date=December 2007 |volume=7 |issue=6 |pages=1006–1022 |doi=10.1089/ast.2007.0156 |pmid=18163875 |bibcode=2007AsBio...7.1006H}}</ref>
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| The molecular hydrogen that escapes Europa's gravity, along with atomic and molecular oxygen, forms a torus (ring) of gas in the vicinity of Europa's orbit around Jupiter. This "neutral cloud" has been detected by both the ''[[Cassini–Huygens|Cassini]]'' and ''Galileo'' spacecraft, and has a greater content (number of atoms and molecules) than the neutral cloud surrounding Jupiter's inner moon [[Io (moon)|Io]]. Models predict that almost every atom or molecule in Europa's torus is eventually ionized, thus providing a source to Jupiter's magnetospheric plasma.
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| <ref name="Smyth2006">{{cite journal |last=Smyth |first=William H. |coauthors=Marconi, Max L. |year=2006 |title=Europa's atmosphere, gas tori, and magnetospheric implications |journal=[[Icarus (journal)|Icarus]] |bibcode=2006Icar..181..510S |doi=10.1016/j.icarus.2005.10.019 |volume=181 |issue=2 |page=510 }}</ref>
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| ==Potential for extraterrestrial life==
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| [[Image:Blacksmoker in Atlantic Ocean.jpg|thumb|A [[black smoker]] in the Atlantic Ocean. Driven by geothermal energy, this and [[Lost City (hydrothermal field)|other types]] of hydrothermal vents create [[Chemical equilibrium|chemical disequilibria]] that can provide energy sources for life.]]
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| Europa has emerged as one of the top locations in the Solar System in terms of [[Planetary habitability|potential habitability]] and the possibility of hosting [[extraterrestrial life]].<ref name="Schulze-Makuch2001">{{cite web |title=Alternative Energy Sources Could Support Life on Europa |url=http://www.geo.utep.edu/pub/dirksm/geobiowater/pdf/EOS27March2001.pdf |author=Schulze-Makuch, Dirk; and Irwin, Louis N. |work=Departments of Geological and Biological Sciences, University of Texas at El Paso |year=2001 |accessdate=21 December 2007 |format=PDF | archiveurl =http://web.archive.org/web/20060703033956/http://www.geo.utep.edu/pub/dirksm/geobiowater/pdf/EOS27March2001.pdf| archivedate =3 July 2006}}</ref> Life could exist in its under-ice ocean, perhaps subsisting in an environment similar to Earth's deep-ocean [[hydrothermal vent]]s. Life in such an ocean could possibly be similar to [[Microorganism|microbial]] life on Earth in the [[deep ocean]].<ref name="EuropaLife" /><ref name="Jones2001">Jones, Nicola (11 December 2001) [http://www.newscientist.com/article.ns?id=dn1647 ''Bacterial explanation for Europa's rosy glow''], ''New Scientist''</ref> So far, there is no evidence that life exists on Europa, but the likely presence of liquid water has spurred calls to send a probe there.<ref name="Phillips2006">Phillips, Cynthia (28 September 2006) [http://www.space.com/searchforlife/seti_europa_060928.html ''Time for Europa''], Space.com.</ref>
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| Until the 1970s, life, at least as the concept is generally understood, was believed to be entirely dependent on energy from the Sun. Plants on Earth's surface capture energy from sunlight to [[photosynthesis|photosynthesize]] sugars from carbon dioxide and water, releasing oxygen in the process, and are then consumed by oxygen-respiring animals, passing their energy up the [[food chain]]. Even life in the [[benthic zone|deep ocean]], far below the [[photic zone|reach of sunlight]], was believed to obtain its nourishment either from the organic detritus raining down from the surface, or by eating animals that in turn depend on that stream of nutrients.<ref name="smoker">{{cite web |title=Creatures Of The Abyss: Black Smokers and Giant Worms |author=Chamberlin, Sean |year=1999 |work=Fullerton College |url=http://earthscape.org/t2/chs01/chs01i/chs01ib.html |accessdate=21 December 2007 }} {{Dead link|date=September 2010|bot=H3llBot}}</ref> An environment's ability to support life was thus thought to depend on its access to sunlight.
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| [[Image:Nur04505.jpg|thumb|left|This [[giant tube worm]] colony dwells beside a [[Pacific Ocean]] vent. Although the worms require [[oxygen]] (hence their [[hemoglobin|blood-red color]]), [[methanogen]]s and some other microbes in the vent communities [[Anaerobic organism|do not]].]]
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| However, in 1977, during an exploratory dive to the [[Galapagos Rift]] in the deep-sea exploration submersible ''[[DSV Alvin|Alvin]]'', scientists discovered colonies of [[giant tube worm]]s, [[clam]]s, [[crustacean]]s, [[mussel]]s, and other assorted creatures clustered around undersea volcanic features known as [[black smoker]]s.<ref name="smoker" /> These creatures thrive despite having no access to sunlight, and it was soon discovered that they comprise an entirely independent food chain. Instead of plants, the basis for this food chain was a form of [[bacterium]] that derived its energy from oxidization of reactive chemicals, such as [[hydrogen]] or [[hydrogen sulfide]], that bubbled up from Earth's interior. This [[chemosynthesis]] revolutionized the study of biology by revealing that life need not be sun-dependent; it only requires water and an energy gradient in order to exist. It opened up a new avenue in [[astrobiology]] by massively expanding the number of possible extraterrestrial habitats.
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| Although the tube worms and other multicellular [[eukaryote|eukaryotic]] organisms around these hydrothermal vents [[Cellular respiration|respire]] [[oxygen]] and thus are indirectly dependent on photosynthesis, [[anaerobic organism|anaerobic]] chemosynthetic bacteria and [[archaea]] that inhabit these ecosystems provide a possible model for life in Europa's ocean.<ref name="ChemDisequilib" /> The energy provided by [[Tidal force|tidal flexing]] drives active geological processes within Europa's interior, just as they do to a far more obvious degree on its sister moon [[Io (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 web |title=Tidal Heating on Io and Europa and its Implications for Planetary Geophysics |author=Wilson, Colin P. |work=Geology and Geography Dept., Vassar College |url=http://gsa.confex.com/gsa/2007NE/finalprogram/abstract_117688.htm |year=2007 |accessdate=21 December 2007 }}</ref> However, such an energy source could never support an ecosystem as large and diverse as the photosynthesis-based ecosystem on Earth's surface.<ref name="McCollom1999">{{cite journal |title=Methanogenesis as a potential source of chemical energy for primary biomass production by autotrophic organisms in hydrothermal systems on Europa |author=McCollom, Thomas M. |place=Woods Hole Oceanographic Institute |year=1999 |bibcode=1999JGR...10430729M |volume=104 |page=30729 |journal=Journal of Geophysical Research |doi=10.1029/1999JE001126 }}</ref> Life on Europa could exist clustered around hydrothermal vents on the ocean floor, or below the ocean floor, where [[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 |author=Marion, Giles M.; Fritsen, Christian H.; Eicken, Hajo; and Payne, Meredith C. |journal=Astrobiology |year=2003 |doi=10.1089/153110703322736105|pmid=14987483|volume=3|issue=4|pages=785–811|bibcode = 2003AsBio...3..785M }}</ref> However, if Europa's ocean were too cold, biological processes similar to those known on Earth could not take place. Similarly, if it were too salty, only extreme [[halophile]]s could survive in its environment.<ref name="limit" />
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| In September 2009, planetary scientist Richard Greenberg calculated that [[cosmic rays]] impacting on Europa's surface convert some water ice into free [[oxygen]] (O<sub>2</sub>), which could then be absorbed into the ocean below as water wells up to fill cracks. Via this process, Greenberg estimates that Europa's ocean could eventually achieve an oxygen concentration greater than that of Earth's oceans within just a few million years. This would enable Europa to support not merely anaerobic microbial life but potentially larger, aerobic organisms such as fish.<ref>{{cite web|title=Europa Capable of Supporting Life, Scientist Says|author=Atkinson, Nancy |date=8 October 2009|publisher=Universe Today|url=http://www.universetoday.com/2009/10/08/europa-capable-of-supporting-life-scientist-says/|accessdate=11 October 2009}}</ref>
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| [[File:PIA17659-Europa-WaterPlume-ArtistConcept-20131212.jpg|thumb|250px|right|[[Water vapor|Water vapor plume]] on Europa (artist concept) (December 12, 2013).<ref name="NASA-20131212-EU" />]]
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| In 2006, Robert T. Pappalardo, an assistant professor in the [[LASP|Laboratory for Atmospheric and Space Physics]] at the [[University of Colorado at Boulder|University of Colorado]] in [[Boulder, Colorado|Boulder]] said,
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| {{quote|1=We’ve spent quite a bit of time and effort trying to understand if Mars was once a habitable environment. Europa today, probably, is a habitable environment. We need to confirm this … but Europa, potentially, has all the ingredients for life … and not just four billion years ago … but today.<ref name="Europabudget"/>}}
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| In November 2011, a team of researchers presented evidence in the journal ''Nature'' suggesting the existence of vast 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" /> If confirmed, the lakes could be yet another potential habitat for life.
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| A paper published in March 2013 suggests that [[hydrogen peroxide]] is abundant across much of the surface of Jupiter's moon Europa.<ref>[http://www.nasa.gov/topics/solarsystem/features/europa20130404.html NASA – Mapping the Chemistry Needed for Life at Europa]. Nasa.gov (4 April 2013). Retrieved on 2013-07-23.</ref> The authors argue that if the peroxide on the surface of Europa mixes into the ocean below, it could be an important energy supply for simple forms of life, if life were to exist there. The scientists think hydrogen peroxide is an important factor for the habitability of the global liquid water ocean under Europa's icy crust because hydrogen peroxide decays to oxygen when mixed into liquid water.
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| On December 11, 2013, NASA reported the detection of "[[Clay minerals|clay-like minerals]]" (specifically, [[phyllosilicates]]), often associated with [[organic materials]], 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=December 11, 2013 |work=[[NASA]] |accessdate=December 11, 2013 }}</ref> The presence of the minerals may have been the result of a collision with an [[asteroid]] or [[comet]] according to the scientists.<ref name="NASA-20131211" />
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| On December 12, 2013, NASA announced, based on studies with the [[Hubble Space Telescope]], that [[water vapor|water vapor plumes]] were detected on Europa and were similar to water vapor plumes detected on [[Enceladus (moon)|Enceladus]], moon of [[Saturn (planet)|Saturn]].<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=December 12, 2013 |work=[[NASA]] |accessdate=December 12, 2013 }}</ref>
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| Life on Earth could have been blasted into space by asteroid collisions and arrived on the moons of Jupiter in a process called [[panspermia]].<ref>http://astrobiology.com/2013/12/life-could-have-hitched-a-ride-to-outer-planet-moons.html</ref>
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| ==Exploration==
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| Exploration of Europa began in the 1973 with the Jupiter flybys of [[Pioneer 10]] and [[Pioneer 11|11]] in 1973 and 1974 respectively. The first closeup photos of were low resolution compared to later missions.
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| The two [[Voyager program|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.
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| Starting in 1995, [[Galileo (spacecraft)|Galileo (probe)]] began a Jupiter orbiting mission that lasted 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>[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>
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| [[New Horizons]] imaged Europa in 2007, as it flew by the Jovian system while on its way to [[Pluto]].
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| {{gallery| width=200px|align=center |title= Selected flyby images
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| |File:Pioneer 10 - p102b.jpg|Europa in 1973 by Pioneer 10
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| |File:PIA01970.jpg|Europa in 1979 by Voyager 1
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| |File:NewHorizonsEuropa.svg|Europa in 2006 by New Horizons
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| }}
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| === Future missions ===
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| Conjectures on [[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 |accessdate=10 August 2007 |date=7 February 2006 |publisher=Space.com }}</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://web.archive.org/web/20110811002508/http://www.planetary.org/programs/projects/explore_europa/update_12142005.html |publisher=The Planetary Society |accessdate= 10 August 2007 }}</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 |author=David L. Chandler |date=20 October 2002 |url=http://www.newscientist.com/article.ns?id=dn2929 |work=New Scientist}}</ref><ref name="Muir2002">Muir, Hazel (22 May 2002) [http://www.newscientist.com/article.ns?id=dn2313 ''Europa has raw materials for life''], ''New Scientist''.</ref> Robotic missions to Europa need to endure the high radiation environment around itself and Jupiter.<ref name="PlanetarySocEuropa" /> Europa receives about 540 [[Röntgen equivalent man|rem]] 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''], California State University, csufresno.edu.</ref>
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| In 2011, a Europa mission was recommended by the U.S. [[Planetary Science Decadal Survey]].<ref name="zab">{{cite news|title=Lean U.S. missions to Mars, Jupiter moon recommended|url=http://www.reuters.com/article/2011/03/08/us-space-usa-future-idUSTRE7266XJ20110308|publisher=Reuters|date=7 March 2011|author=Zabarenko, Deborah }}</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 | accessdate = 2014-01-15 | work = NASA}}</ref><ref>[http://www.lpi.usra.edu/opag/mar2012/presentations/ March 2012 OPAG Meeting]. 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 million 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 = January 15, 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#axzz2qZe6ersS | work = Los Angeles Times | accessdate = 2014-01-16}}</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 | accessdate = 2014-01-15}}</ref>
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| *[[Europa Clipper]] — In July 2013 an updated concept for a flyby Europa mission called Europa Clipper was presented by the [[Jet Propulsion Laboratory]] (JPL) and the [[Applied Physics Laboratory]] (APL).<ref>[http://www.lpi.usra.edu/opag/jul2013/presentations/Clipper_Summary.pdf The Europa Clipper – OPAG Update] [[Jet Propulsion Laboratory|JPL]]/[[Applied Physics Laboratory|APL]]</ref> The aim of Europa Clipper is to explore Europa in order to investigate its habitability, and to aid selecting sites for a future lander. The Europa Clipper would not orbit Europa, but instead orbit Jupiter and conduct 45 low-altitude [[flyby]]s of Europa during its envisioned mission. The probe would carry an ice penetrating radar, short wave infra red spectrometer, topographical imager, and an ion and neutral mass spectrometer.
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| *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, [[Lidar|laser altimeter]], [[magnetometer]], [[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]. 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 | editor-first = Europa Study Team | publisher = JPL - NASA | date = 1 May 2012| id = | contribution-url = http://solarsystem.nasa.gov/europa/docs/ES%202012%20Report%20B%20Orbiter%20-%20Final%20-%2020120501.pdf | format = PDF | accessdate = 2014-01-17}}</ref>
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| *Europa Lander — It would investigate the moon's [[Planetary habitability|habitability]] and assess its [[Astrobiology|astrobiological]] potential by confirming the existence and determining the characteristics of water within and below Europa's icy shell.<ref name='Lander 2012 report'>{{citation | contribution = EUROPA STUDY 2012 REPORT | title = EUROPA LANDER MISSION | editor-first = Europa Study Team | publisher = Jet Propulsion Laboratory - NASA | pages = 287 | date = 1 May 2012| id = | contribution-url = https://solarsystem.nasa.gov/europa/docs/ES%202012%20Report%20D%20Lander%20-%20Final%20-%2020120501.pdf | format = PDF | accessdate = 2014-01-15}}</ref>
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| In 2012, [[Jupiter Icy Moon Explorer]] was selected by the European Space Agency ([[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]. ESA, 17 April 2012. (PDF) . Retrieved on 23 July 2013.</ref> That mission includes some flybys of Europa, but is more focused on Ganymede.
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| ===Old proposals===
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| [[File:JIMO Europa Lander Mission.jpg|thumb|right|350px|Europa Lander Mission concept circa 2005 ([http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/37545/1/05-0403.pdf NASA]).]]
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| In the early 2000s, ''[[Jupiter Europa Orbiter]]'' led by NASA and the ''[[Jupiter Ganymede Orbiter]]'' led by the ESA were proposed together as an [[Outer Planet Flagship Mission]] to Jupiter's icy moons, and 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 |accessdate=26 July 2009 |publisher=NASA |year=2009 }}</ref> In 2009 it was given priority over ''[[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 |publisher=BBC News |accessdate=20 February 2009 |date=20 February 2009 }}</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 |accessdate=20 February 2009 }}</ref> [[JAXA|Japan]] proposed ''[[Jupiter Magnetospheric Orbiter]]''. Russia expressed interest in sending ''[[Europa Lander]]'' as part of the international effort.<ref>[http://futureplanets.blogspot.com/2009/04/russian-europa-lander-concept.html Russia Europa Lander – FPE (2009)]. Futureplanets.blogspot.com (5 April 2009). Retrieved on 2013-07-23.</ref> The overall plan collapsed in the early 2010s.<ref name="l1" />
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| ''[[Jovian Europa Orbiter]]'' was an ESA Cosmic Vision concept study from 2007. Another concept was ''Ice Clipper'',<ref>{{cite journal | title = Planetary protection for a Europa surface sample return: The Ice Clipper mission | journal = Advances in Space Research | year = 2002 | first = C.P. McKay | volume = 30 | issue = 6 | pages = 1601–1605| id = | url = http://www.sciencedirect.com/science/article/pii/S0273117702004805 | accessdate = 2013-12-16 | doi=10.1016/S0273-1177(02)00480-5 | bibcode=2002AdSpR..30.1601M}}</ref> which would have used an impactor similar to the ''[[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="Goodman1998">Goodman, Jason C. (9 September 1998) [http://www.madsci.org/posts/archives/oct98/905357947.As.r.html ''Re: Galileo at Europa''], MadSci Network forums.</ref><ref name="McKay2002">{{cite journal |title=Planetary protection for a Europa surface sample return: The ice clipper mission |author=McKay, Christopher P. |journal=Advances in Space Research |volume=30 |issue=6 |pages=1601–1605 |year=2002 |doi=10.1016/S0273-1177(02)00480-5 |bibcode =2002AdSpR..30.1601M }}</ref>
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| [[Image:Cryobot.jpg|thumb|200px|left|Artist's concept of the [[cryobot]] (a thermal drill, seen upper left) and its deployed 'hydrobot' submersible]]
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| ''[[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 [[Project Prometheus]].<ref name="Budget">Berger, Brian; [http://www.space.com/news/nasa_budget_050207.html ''NASA 2006 Budget Presented: Hubble, Nuclear Initiative Suffer''] [[Space.com]] (7 February 2005)</ref> The ''Europa Lander Mission'' proposed a small nuclear-powered Europa lander for JIMO.<ref name="elm">[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" />
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| The [[Europa Orbiter]] received a go-ahead in 1999 but was canceled in 2002. This orbiter featured a special radar that would allow it to scan below the surface.<ref name="waterworld" />
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| More ambitious ideas have been put forward including an impactor in combination with a thermal drill to search for [[biosignature]]s that might be frozen in the shallow subsurface.<ref name="Weiss2010">{{cite doi|10.1016/j.asr.2010.01.015}}</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
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| |publisher=Astrobiology Magazine|date=15 April 2010}}</ref>
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| Another proposal put forward in 2001 calls for a large [[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">Knight, Will (14 January 2002) [http://www.newscientist.com/article.ns?id=dn1786 ''Ice-melting robot passes Arctic test''], ''New Scientist''.</ref> Once it reached the water, it would deploy an autonomous underwater vehicle ([[AUV|hydrobot]]) that would gather information and send it back to Earth.<ref name="Bridges2000">Bridges, Andrew (10 January 2000) [http://web.archive.org/web/20090208150400/http://www.space.com/searchforlife/europa_ocean_update_000110.html ''Latest Galileo Data Further Suggest Europa Has Liquid Ocean''], 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 [[Forward-contamination|contamination]] of the subsurface ocean.<ref name="NASSSB">National Academy of Sciences Space Studies Board, [http://web.archive.org/web/20080213194509/http://www7.nationalacademies.org/ssb/europamenu.html ''Preventing the Forward Contamination of Europa''], National Academy Press, Washington (DC), 2000, ISBN 0309575540.</ref> This proposed mission has not yet reached a serious planning stage.<ref name="Powell2005">{{cite journal |title=NEMO: A mission to search for and return to Earth possible life forms on Europa |author=Powell, Jesse |coauthors=Powell, James; Maise, George; and Paniagua, John |journal=Acta Astronautica |volume=57 |issue=2–8 |pages= 579–593 |year=2005|doi=10.1016/j.actaastro.2005.04.003 |bibcode = 2005AcAau..57..579P }}</ref>
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| ==See also==
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| {{Portal|Solar System|Astrobiology}}
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| * [[Colonization of Europa]]
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| * [[Jupiter's moons in fiction]]
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| * [[List of craters on Europa]]
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| * [[List of geological features on Europa]]
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| * [[List of lineae on Europa]]
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| * [[Moons of Jupiter]]
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| * [[Snowball Earth hypothesis]]
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| * [[Terraforming of Europa]]
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| ==Notes==
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| {{reflist|group=lower-alpha}}
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| ==References==
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| {{Reflist|colwidth=30em}}
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| ===Further reading===
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| *{{cite book
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| | first=David A. | last=Rothery | year=1999
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| | title=Satellites of the Outer Planets: Worlds in Their Own Right
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| | publisher=Oxford University Press US
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| | isbn=0-19-512555-X }}
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| *{{cite book
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| | first=David M. | last=Harland | year=2000
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| | title=Jupiter Odyssey: The Story of NASA's Galileo Mission
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| | publisher=Springer | isbn=1-85233-301-4 }}
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| *{{cite book
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| | first=Richard | last=Greenberg | year=2005
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| | title=EUROPA The Ocean Moon
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| | publisher=Springer | isbn=3-540-22450-5 }}
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| ==External links==
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| {{Commons category|Europa (moon)}}
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| * [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jup_Europa Europa Profile] at [http://solarsystem.nasa.gov NASA's Solar System Exploration site]
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| * [http://www.astronomycast.com/astronomy/planets/our-solar-system/ep-203-europa/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+astronomycast+%28Astronomy+Cast%29 Astronomy Cast: Europa]. Frasier Cain, 2010
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| * [http://www.nineplanets.org/europa.html Europa page] at ''The Nine Planets''
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| * [http://www.solarviews.com/eng/europa.htm Europa page] at ''Views of the Solar System''
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| * [http://web.archive.org/web/20060924231443/http://www.d.lane.btinternet.co.uk/Essay.htm Are our nearest living neighbours on one of Jupiter's Moons?]
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| * [http://www.nap.edu/openbook.php?record_id=9895&page=R1 Preventing Forward Contamination of Europa] – SSB Study of Planetary
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| * [http://www2.jpl.nasa.gov/galileo/europa/ Protection policies for Europa]
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| * [http://photojournal.jpl.nasa.gov/target/Europa Images of Europa at JPL's Planetary Photojournal]
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| *Movie of [http://sos.noaa.gov/videos/Europa.mov Europa's rotation] from the National Oceanic and Atmospheric Administration
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| * [http://photojournal.jpl.nasa.gov/catalog/PIA03526 Europa map with feature names] from [http://photojournal.jpl.nasa.gov/ Planetary Photojournal]
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| *[http://planetarynames.wr.usgs.gov/Page/EUROPA/target Europa nomenclature] and [http://planetarynames.wr.usgs.gov/images/europa_comp.pdf Europa map with feature names] from the [http://planetarynames.wr.usgs.gov USGS planetary nomenclature page]
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| * [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]
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| * Large, high-resolution ''Galileo'' image mosaics of Europan terrain from [http://www.gishbartimes.org/2010/03/and-now-for-something-completely.html Jason] [http://www.gishbartimes.org/2010/03/19esrhadam01-galileo-mosaic-of-europa.html Perry's] (normally Io-related) [http://www.gishbartimes.org/2010/03/two-more-europa-mosaics.html blog]: [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]
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| *[http://apod.nasa.gov/apod/image/0903/europa_galileo_big.jpg Europa image montage from Galileo spacecraft], [http://apod.nasa.gov/apod/ap090308.html NASA APOD]
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| *[http://www.nasa.gov/topics/solarsystem/features/pia16827.html View of Europa from Galileo flybys]
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| [[Category:Europa (moon)|*]]
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