Editing Europa (moon) (section)

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

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

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

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

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

==== Chaos and lenticulae ====
{{See also|List of geological features on Europa}} {{multiple image
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 |image1 = PIA01092 - Evidence of Internal Activity on Europa.jpg
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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}} ([[Latin]] for "freckles"). Many are domes, some are pits and some are smooth, dark spots. Others have a jumbled or rough texture. The dome tops look like pieces of the older plains around them, suggesting that the domes formed when the plains were pushed up from below.<ref name="diapir">{{cite journal |last1=Sotin |first1=Christophe |last2=Head |first2=James W. |last3=Tobie |first3=Gabriel |title=Europa: Tidal heating of upwelling thermal plumes and the origin of lenticulae and chaos melting |journal=Geophysical Research Letters |date=April 2002 |volume=29 |issue=8 |pages=74-1–74-4 |doi=10.1029/2001GL013844 |bibcode=2002GeoRL..29.1233S |s2cid=14413348 |url=http://planetary.brown.edu/pdfs/2685.pdf |access-date=12 April 2020 |archive-date=31 July 2020 |archive-url=https://web.archive.org/web/20200731021343/http://planetary.brown.edu/pdfs/2685.pdf |url-status=live }}</ref>

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

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

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

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