View source for Astronomy:Enceladus

{{Short description|Natural satellite orbiting Saturn}}
{{Infobox planet
| name             = Enceladus
| pronounced       = {{IPAc-en|ɛ|n|ˈ|s|ɛ|l|ə|d|ə|s}}<ref>{{Cite dictionary |url=http://www.lexico.com/definition/Enceladus |archive-url=https://web.archive.org/web/20200731003737/https://www.lexico.com/definition/enceladus |url-status=dead |archive-date=July 31, 2020 |title=Enceladus |dictionary=Lexico UK English Dictionary |publisher=[[Organization:Oxford University Press|Oxford University Press]]}}<br />{{MW|Enceladus}}</ref>
| mpc_name         = Saturn II
| named_after      = Ἐγκέλαδος ''Egkelados''
| adjectives       = Enceladean {{IPAc-en|ɛ|n|s|ə|ˈ|l|eɪ|d|i|ə|n}}<ref>{{cite journal |url=http://www.rfreitas.com/Astro/TerraformSRS1983.htm |bibcode=1983JBIS...36..139F |title=Terraforming Mars and Venus Using Machine Self-Replicating Systems (SRS) |last1=Freitas |first1=R. A. |journal=Journal of the British Interplanetary Society |year=1983 |volume=36 |page=139 }}</ref><ref>Postberg et al. "Plume and surface composition of Enceladus", p. 129–130, 148, 156; Lunine et al. "Future Exploration of Enceladus and Other Saturnian Moons", p. 454; in Schenk et al., eds. (2018) ''Enceladus and the Icy Moons of Saturn''</ref>
| image            = PIA17202 - Approaching Enceladus.jpg
| image_scale       =
| caption          = View of trailing hemisphere in natural color<ref name="caption" group="lower-alpha"/>
| note             = yes
| discoverer       = William Herschel
| discovered       = August 28, 1789<ref name="Discovery"/>
| semimajor        = {{val|237948|u=km}}<!-- Computed using the http://cfa-www.harvard.edu/iau/NatSats/NaturalSatellites.html µ value --><ref name="Facts" />
| eccentricity     = {{val|0.0047}}<ref name="Facts"/><ref name="Porco Helfenstein et al. 2006"/>
| period           = {{val|1.370218|u=days}}<ref name="Facts"/>
| inclination      = 0.009° (to Saturn's equator)<ref name="Facts"/>
| satellite_of     = [[Astronomy:Saturn|Saturn]]
| dimensions       = 513.2&thinsp;×&thinsp;502.8&thinsp;×&thinsp;496.6&nbsp;km<ref name="Facts" /><ref name="Roatsch et al. 2009" />
| mean_radius      = {{val|252.1|0.2|u=km}}<ref name="Roatsch et al. 2009" /><ref name="Jacobson2022"/> ({{val|0.0395}} Earths, {{val|0.1451}} Moons)
| mass             = {{val|1.080318|0.00028|e=20|u=kg}}<ref name="Jacobson2022">{{cite journal |last1=Jacobson |first1=Robert. A. |title=The Orbits of the Main Saturnian Satellites, the Saturnian System Gravity Field, and the Orientation of Saturn's Pole |journal=The Astronomical Journal |date=1 November 2022 |volume=164 |issue=5 |pages=199 |doi=10.3847/1538-3881/ac90c9 |doi-access=free |bibcode=2022AJ....164..199J |s2cid=252992162 }}</ref> (1.8{{e|-5}} Earths)
| density          = {{val|1.6097|0.0038|u=g/cm<sup>3</sup>}}<ref name="Jacobson2022"/>
| surface_grav     = {{Gr|0.108|252.1|3}} [[Physics:Acceleration|m/s<sup>2</sup>]] ({{val|0.0116}} [[Physics:G-force|g]]) <!-- Calculated from mass figure above using mean equatorial radius -->
| moment_of_inertia_factor = {{val|0.3305|0.0025}}<ref name="McKinnon2015">{{cite journal|last1= McKinnon|first1=W. B.|title= Effect of Enceladus's rapid synchronous spin on interpretation of Cassini gravity|journal= Geophysical Research Letters|volume= 42|issue= 7|year= 2015|pages= 2137–2143|doi= 10.1002/2015GL063384|bibcode= 2015GeoRL..42.2137M|doi-access= free}}</ref>
| escape_velocity  = {{V2|0.108|252.1|3}}&nbsp;km/s (860.4&nbsp;km/h)<!-- Calculated from mass figure above. --><ref name="Facts" />
| rotation         = Synchronous
| axial_tilt       = 0
| albedo           = {{val|1.375|0.008}} ([[Physics:Geometric albedo|geometric]] at 550 nm)<ref name="Verbiscer et al. 2007"/> or {{val|0.81|0.04}}&nbsp;([[Physics:Bond albedo|Bond]])<ref name="Howett_2010" />
| magnitude        = 11.7<ref name="Observatorio ARVAL"/>
| temp_name1       = [[Kelvin]]<ref name="Spencer">{{cite journal |doi=10.1126/science.1121661 |title=Cassini Encounters Enceladus: Background and the Discovery of a South Polar Hot Spot |date=2006 |journal=Science |volume=311 |pages=1401–5 |pmid=16527965 |issue=5766 |bibcode=2006Sci...311.1401S |last1=Spencer |first1=John R. |last2=Pearl |first2=J. C. |s2cid=44788825 |display-authors=0 }}</ref>
| min_temp_1       = 32.9&nbsp;[[Kelvin|K]]
| mean_temp_1      = 75&nbsp;K
| max_temp_1       = 145&nbsp;K
| temp_name2       = [[Celsius]]
| min_temp_2       = −240&nbsp;°C
| mean_temp_2      = −198&nbsp;°C
| max_temp_2       = −128&nbsp;°C
| atmosphere       = Trace (water vapor)
| surface_pressure = Trace, significant spatial variability<ref name="Dougherty">{{cite journal|doi=10.1126/science.1120985|title=Identification of a Dynamic Atmosphere at Enceladus with the Cassini Magnetometer|date=2006|journal=Science|volume=311|pages=1406–9|pmid=16527966|issue=5766|bibcode=2006Sci...311.1406D|last1=Dougherty|first1=M. K.|last2=Khurana|first2=K. K.|s2cid=42050327|display-authors=0}}</ref><ref name="Hansen">{{cite journal|doi=10.1126/science.1121254|title=Enceladus' Water Vapor Plume|date=2006|journal=Science|volume=311|pages=1422–5|pmid=16527971|issue=5766|bibcode=2006Sci...311.1422H|last1=Hansen|first1=Candice J.|last2=Esposito|first2=L.|s2cid=2954801|display-authors=0}}</ref>
| atmosphere_composition = 91% [[Chemistry:Water|water]] vapor<br />4% [[Chemistry:Nitrogen|nitrogen]]<br />3.2% [[Chemistry:Carbon dioxide|carbon dioxide]]<br />1.7% [[Chemistry:Methane|methane]]<ref name="Waite">{{cite journal |doi=10.1126/science.1121290 |title=Cassini Ion and Neutral Mass Spectrometer: Enceladus Plume Composition and Structure |date=2006 |journal=Science |volume=311 |pages=1419–22 |pmid=16527970 |issue=5766 |bibcode=2006Sci...311.1419W | last1=Waite | first1=Jack Hunter Jr. |last2=Combi|first2=M. R. |s2cid=3032849 |display-authors=0 }}</ref>
}}

'''Enceladus''' is the sixth-largest moon of Saturn (19th largest in the [[Astronomy:Solar System|Solar System]]). It is about {{convert|500|km|lk=on|abbr=off|sp=us}} in diameter,<ref name="Facts">{{cite web|url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sat_Enceladus&Display=Facts|work=Solar System Exploration|title=Enceladus: Facts & Figures|publisher=NASA|date=August 12, 2013|access-date=April 26, 2014|url-status=dead|archive-url=https://web.archive.org/web/20131016093801/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sat_Enceladus&Display=Facts|archive-date=October 16, 2013}}</ref> about a tenth of that of [[Astronomy:Saturn|Saturn]]'s largest moon, [[Astronomy:Titan (moon)|Titan]]. It is mostly covered by fresh, clean ice, making it one of the most reflective bodies of the Solar System. Consequently, its surface temperature at noon reaches only {{convert|-198|°C|K F|lk=on|abbr=on}}, far colder than a light-absorbing body would be. Despite its small size, Enceladus has a wide range of surface features, ranging from old, heavily [[Astronomy:Impact crater|cratered]] regions to young, [[Earth:Tectonics|tectonically]] deformed [[Terrain|terrain]].

Enceladus was discovered on August 28, 1789 by William Herschel,<ref name="Discovery">{{cite web|url=http://planetarynames.wr.usgs.gov/append7.html|title=Planetary Body Names and Discoverers|work=Gazetteer of Planetary Nomenclature|publisher=[[Earth:Astrogeology Research Program|USGS Astrogeology Science Center]]|access-date=January 12, 2015|archive-date=August 25, 2009|archive-url=https://web.archive.org/web/20090825153846/http://planetarynames.wr.usgs.gov/append7.html|url-status=live}}</ref><ref name="Herschel_1790">{{cite journal|last=Herschel|first=W.|title=Account of the Discovery of a Sixth and Seventh Satellite of the Planet Saturn; With Remarks on the Construction of Its Ring, Its Atmosphere, Its Rotation on an Axis, and Its Spheroidal Figure|url=http://rstl.royalsocietypublishing.org/content/80.toc|journal=Philosophical Transactions of the Royal Society of London|volume=80|date=January 1, 1790|doi=10.1098/rstl.1790.0004|pages=1–20|access-date=April 27, 2014|archive-date=April 27, 2014|archive-url=https://archive.today/20140427153244/http://rstl.royalsocietypublishing.org/content/80.toc|url-status=live}}</ref><ref name="Herschel_1795"/> but little was known about it until the two [[Astronomy:Voyager program|Voyager]] spacecraft, ''Voyager&nbsp;1'' and ''Voyager&nbsp;2'', flew by Saturn in 1980 and 1981.<ref name="Lovett_cosmos"/> In 2005, the spacecraft ''[[Astronomy:Cassini–Huygens|Cassini]]'' started multiple close flybys of Enceladus, revealing its surface and environment in greater detail. In particular, ''Cassini'' discovered water-rich [[Physics:Plume (fluid dynamics)|plumes]] venting from the south polar region.<ref name="NYT-20150315">{{cite news|last=Chang|first=Kenneth|title=Suddenly, It Seems, Water Is Everywhere in Solar System|url=https://www.nytimes.com/2015/03/13/science/space/suddenly-it-seems-water-is-everywhere-in-solar-system.html|date=March 12, 2015|work={{wipe|The New York Times}}|access-date=March 13, 2015|archive-date=May 9, 2020|archive-url=https://web.archive.org/web/20200509080640/https://www.nytimes.com/2015/03/13/science/space/suddenly-it-seems-water-is-everywhere-in-solar-system.html|url-status=live}}</ref> [[Astronomy:Cryovolcano|Cryovolcano]]es near the south pole shoot [[Earth:Geyser|geyser]]-like jets of [[Physics:Water vapor|water vapor]], molecular hydrogen, other volatiles, and solid material, including [[Chemistry:Sodium chloride|sodium chloride]] crystals and ice particles, into space, totaling about {{convert|200|kg|lb|lk=on|abbr=off|sp=us}} per second.<ref name="Hansen"/><ref name="Lovett_cosmos">{{cite web|url=http://www.cosmosmagazine.com/features/secret-life-saturns-moon-enceladus/|title=Secret life of Saturn's moon: Enceladus|work=Cosmos Magazine|last=Lovett|first=Richard A.|date=September 4, 2012|access-date=August 29, 2013|archive-date=August 15, 2014|archive-url=https://web.archive.org/web/20140815095512/http://cosmosmagazine.com/features/secret-life-saturns-moon-enceladus/|url-status=live}}</ref><ref name="Spencer2013a">{{Cite journal|last1=Spencer |first1=John R. |last2=Nimmo |first2=F. |s2cid=140646028 |doi=10.1146/annurev-earth-050212-124025 |title=Enceladus: An Active Ice World in the Saturn System |journal=[[Earth:Annual Review of Earth and Planetary Sciences|Annual Review of Earth and Planetary Sciences]] |volume=41 |pages=693–717 |date=May 2013 |bibcode=2013AREPS..41..693S}}</ref> More than 100 geysers have been identified.<ref name="NASA-20140728">{{cite web |display-authors=0 |last1=Dyches |first1=Preston |last2=Brown |first2=Dwayne |title=Cassini Spacecraft Reveals 101 Geysers and More on Icy Saturn Moon |url=http://www.jpl.nasa.gov/news/news.php?release=2014-246&2 |date=July 28, 2014 |work=NASA/JPL |access-date=July 29, 2014 |archive-date=July 14, 2017 |archive-url=https://web.archive.org/web/20170714031155/https://www.jpl.nasa.gov/news/news.php?release=2014-246&2 |url-status=live }}</ref> Some of the water vapor falls back as "snow"; the rest escapes and supplies most of the material making up Saturn's [[Astronomy:Rings of Saturn#E Ring|E&nbsp;ring]].<ref name="E ring">{{cite news|url=http://www.nasa.gov/jpl/cassini/icy-tendrils-reaching-into-saturn-ring-traced-to-their-source/|title=Icy Tendrils Reaching into Saturn Ring Traced to Their Source|work=NASA News|date=April 14, 2015|access-date=April 15, 2015|archive-date=April 16, 2015|archive-url=https://web.archive.org/web/20150416034254/http://www.nasa.gov/jpl/cassini/icy-tendrils-reaching-into-saturn-ring-traced-to-their-source/|url-status=live}}</ref><ref name="E ring 2006">{{cite web|url= https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA08321|title=Ghostly Fingers of Enceladus |work=NASA/JPL/Space Science Institute |date=September 19, 2006 |access-date=April 26, 2014|archive-url=https://web.archive.org/web/20140427010559/http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=2276|archive-date=April 27, 2014}}</ref> According to [[Organization:NASA|NASA]] scientists, the plumes are similar in composition to [[Astronomy:Comet|comet]]s.<ref name="NS-20080326">{{cite web |last=Battersby|first=Stephen |title=Saturn's moon Enceladus surprisingly comet-like |url=https://www.newscientist.com/article/dn13541-saturns-moon-enceladus-surprisingly-cometlike.html|date=March 26, 2008 |work=New Scientist|access-date=April 16, 2015|archive-date=June 30, 2015|archive-url=https://web.archive.org/web/20150630011601/http://www.newscientist.com/article/dn13541-saturns-moon-enceladus-surprisingly-cometlike.html|url-status=live}}</ref> In 2014, NASA reported that ''Cassini'' had found evidence for a large south polar subsurface ocean of liquid water with a thickness of around {{convert|10|km|0|abbr=on}}.<ref name="NASA-20140403"/><ref name="Witze2014"/><ref name="SCI-20140404"/> The existence of Enceladus' subsurface ocean has since been mathematically modelled and replicated.<ref>{{Cite journal|last1=Tjoa|first1=J. N. K. Y.|last2=Mueller|first2=M.|last3=Tak|first3=F. F. S. van der|date=2020-04-01|title=The subsurface habitability of small, icy exomoons |url=https://www.aanda.org/articles/aa/abs/2020/04/aa37035-19/aa37035-19.html |journal=Astronomy & Astrophysics |volume=636 |pages=A50 |doi=10.1051/0004-6361/201937035 |arxiv=2003.09231 |bibcode=2020A&A...636A..50T |s2cid=214605690 |issn=0004-6361}}</ref>

These geyser observations, along with the finding of escaping internal heat and very few (if any) impact craters in the south polar region, show that Enceladus is currently geologically active. Like many other satellites in the extensive systems of the [[Astronomy:Giant planet|giant planet]]s, Enceladus is trapped in an orbital resonance. Its resonance with Dione excites its [[Astronomy:Orbital eccentricity|orbital eccentricity]], which is [[Damping ratio|damped]] by [[Physics:Tidal force|tidal force]]s, [[Astronomy:Tidal heating|tidally heating]] its interior and driving the geological activity.<ref name="Efroimsky_2018a">{{cite journal |last=Efroimsky |first=M. |title=Tidal viscosity of Enceladus |journal=Icarus |volume=300 |date=January 15, 2018 |doi=10.1016/j.icarus.2017.09.013 |pages=223–226 |arxiv=1706.09000 |bibcode=2018Icar..300..223E |s2cid=119462312}}</ref>

''Cassini'' performed chemical analysis of Enceladus's plumes, finding evidence for hydrothermal activity,<ref name="Waite et al. 2018">{{cite journal |pmid=28408597 |year=2017 | last1=Waite | first1=Jack Hunter Jr. |title=Cassini finds molecular hydrogen in the Enceladus plume: Evidence for hydrothermal processes |journal=Science |volume=356 |issue=6334 |pages=155–159 |last2=Glein |first2=C. R. |last3=Perryman |first3=R. S. |last4=Teolis |first4=Ben D. |last5=Magee |first5=B. A. |last6=Miller |first6=G. |last7=Grimes |first7=J. |last8=Perry |first8=M. E. |last9=Miller |first9=K. E. |last10=Bouquet |first10=A. |last11=Lunine |first11=Jonathan I. |last12=Brockwell |first12=T. |last13=Bolton |first13=S. J. |doi=10.1126/science.aai8703 |bibcode=2017Sci...356..155W |doi-access=free }}</ref><ref name="Hsu et al. 2015">{{cite journal|last1=Hsu|first1=Hsiang-Wen|last2=Postberg|first2=Frank|display-authors=0|title=Ongoing hydrothermal activities within Enceladus|journal=Nature|date=March 11, 2015|volume=519|issue=7542|pages=207–10|doi=10.1038/nature14262|bibcode=2015Natur.519..207H|pmid=25762281|s2cid=4466621}}</ref> possibly driving complex chemistry.<ref name="Postberg et al. 2018">{{cite journal |author=Postberg, Frank |display-authors=0 |title=Macromolecular organic compounds from the depths of Enceladus |date=June 27, 2018 |journal=Nature |volume=558 |issue=7711 |pages=564–568 |doi=10.1038/s41586-018-0246-4 |pmid=29950623 |pmc=6027964 |bibcode=2018Natur.558..564P }}</ref> Ongoing research on ''Cassini'' data suggests that Enceladus's hydrothermal environment could be habitable to some of Earth's [[Earth:Hydrothermal vent|hydrothermal vent]]'s microorganisms, and that plume-found methane could be produced by such organisms.<ref name="Taubner et al. 2018">{{Cite journal| doi = 10.1038/s41467-018-02876-y| issn = 2041-1723| volume = 9| issue = 1| pages = 748| last1 = Taubner| first1 = Ruth-Sophie| last2 = Pappenreiter| first2 = Patricia| last3 = Zwicker| first3 = Jennifer| last4 = Smrzka| first4 = Daniel| last5 = Pruckner| first5 = Christian| last6 = Kolar| first6 = Philipp| last7 = Bernacchi| first7 = Sébastien| last8 = Seifert| first8 = Arne H.| last9 = Krajete| first9 = Alexander| last10 = Bach| first10 = Wolfgang| last11 = Peckmann| first11 = Jörn| last12 = Paulik| first12 = Christian| last13 = Firneis| first13 = Maria G.| last14 = Schleper| first14 = Christa| last15 = Rittmann| first15 = Simon K.-M. R.| title = Biological methane production under putative Enceladus-like conditions| journal = Nature Communications| date = 2018-02-27| pmid = 29487311| pmc = 5829080| bibcode = 2018NatCo...9..748T}}</ref><ref name="Affholder et al. 2021">{{cite journal |author=Affholder, Antonin |display-authors=et al. |title=Bayesian analysis of Enceladus's plume data to assess methanogenesis |url=https://hal.sorbonne-universite.fr/hal-03384808/file/Affholder_main_text.pdf |date=7 June 2021 |journal=[[Astronomy:Nature Astronomy|Nature Astronomy]] |volume=5 |issue=8 |pages=805–814 |doi=10.1038/s41550-021-01372-6 |bibcode=2021NatAs...5..805A |s2cid=236220377 |access-date=7 July 2021 |archive-date= |archive-url= |url-status= |url-access= }}</ref>

==History==

===Discovery===
[[File:William_Herschel01.jpg|left|thumb|upright|William Herschel, discoverer of Enceladus]]
Enceladus was discovered by William Herschel on August 28, 1789, during the first use of his new {{convert|1.2|m|in|abbr=on}} [[Astronomy:40-foot telescope|40-foot telescope]], then the largest in the world, at Observatory House in Slough, England.<ref name="Herschel_1795">{{cite journal|last=Herschel|first=W.|date=1795|title=Description of a Forty-feet Reflecting Telescope|journal=Philosophical Transactions of the Royal Society of London|volume=85|pages=347–409|bibcode=1795RSPT...85..347H|doi=10.1098/rstl.1795.0021|s2cid=186212450}} (reported by {{cite web|first=M.|last=Arago|date=1871|url=http://laplaza.org/~tom/People/Herschel.htm|title=Herschel|work=Annual Report of the Board of Regents of the Smithsonian Institution|pages=198–223|archive-url=https://web.archive.org/web/20160113070818/http://laplaza.org/~tom/People/Herschel.htm|archive-date=January 13, 2016}})</ref><ref>{{cite web|last1=Frommert|first1=H.|last2=Kronberg|first2=C.|url=http://messier.seds.org/xtra/Bios/wherschel.html|title=William Herschel (1738–1822)|work=The Messier Catalog|access-date=March 11, 2015|archive-date=May 19, 2013|archive-url=https://web.archive.org/web/20130519083311/http://messier.seds.org/xtra/Bios/wherschel.html|url-status=live}}</ref> Its faint [[Astronomy:Apparent magnitude|apparent magnitude]] (''H<sub>V</sub>'' = +11.7) and its proximity to the much brighter Saturn and Saturn's rings make Enceladus difficult to observe from Earth with smaller telescopes. Like many satellites of Saturn discovered prior to the [[Engineering:Space Age|Space Age]], Enceladus was first observed during a Saturnian equinox, when Earth is within the ring plane. At such times, the reduction in glare from the rings makes the moons easier to observe.<ref name="SP-20130405">{{cite web|last=Redd|first=Nola Taylor|title=Enceladus: Saturn's Tiny, Shiny Moon|url=http://www.space.com/20543-enceladus-saturn-s-tiny-shiny-moon.html|date=April 5, 2013|work=Space.com|access-date=April 27, 2014|archive-date=December 24, 2018|archive-url=https://web.archive.org/web/20181224232040/https://www.space.com/20543-enceladus-saturn-s-tiny-shiny-moon.html|url-status=live}}</ref> Prior to the [[Astronomy:Voyager program|''Voyager'' missions]] the view of Enceladus improved little from the dot first observed by Herschel. Only its orbital characteristics were known, with estimations of its [[Physics:Mass|mass]], [[Physics:Density|density]] and [[Earth:Albedo|albedo]].

===Naming===
[[File:John_F._Hershel.png|left|thumb|upright|John Herschel, the astronomer who suggested that the moons of Saturn be named after the Titans and Giants]]
[[File:Athena Enkelados Louvre CA3662.jpg|thumb|Athena (left) fighting Enceladus (right)]]
Enceladus is named after the giant Enceladus of [[Religion:Greek mythology|Greek mythology]].<ref name="Discovery"/> The name, like the names of each of the first seven satellites of Saturn to be discovered, was suggested by William Herschel's son [[Biography:John Herschel|John Herschel]] in his 1847 publication ''Results of Astronomical Observations made at the Cape of Good Hope''.<ref name="Lassell">As reported by {{cite journal |last=Lassell |first=William |url=http://adsabs.harvard.edu//full/seri/MNRAS/0008//0000042.000.html |title=Names |journal=Monthly Notices of the Royal Astronomical Society |volume=8 |issue=3|pages=42–3|date=January 14, 1848 |bibcode=1848MNRAS...8...42L |doi=10.1093/mnras/8.3.42 |doi-access=free |access-date=July 15, 2004 |archive-date=July 25, 2008 |archive-url=https://web.archive.org/web/20080725063430/http://adsabs.harvard.edu//full/seri/MNRAS/0008//0000042.000.html|url-status=live}}</ref> He chose these names because [[Religion:Saturn (mythology)|Saturn]], known in Greek mythology as [[Religion:Cronus|Cronus]], was the leader of the Titans.

Geological features on Enceladus are named by the [[Organization:International Astronomical Union|International Astronomical Union]] (IAU) after characters and places from Richard Francis Burton's 1885 translation of ''The Book of One Thousand and One Nights''.<ref name="NameCategories">{{cite web|url=http://planetarynames.wr.usgs.gov/append6.html|title=Categories for Naming Features on Planets and Satellites|work=Gazetteer of Planetary Nomenclature|publisher=[[Earth:Astrogeology Research Program|USGS Astrogeology Science Center]]|access-date=January 12, 2015|archive-date=August 25, 2011|archive-url=https://www.webcitation.org/61CvxYaVi?url=http://planetarynames.wr.usgs.gov/Page/Categories|url-status=live}}</ref> [[Astronomy:Impact crater|Impact crater]]s are named after characters, whereas other feature types, such as [[Earth:Fossa (geology)|fossae]] (long, narrow depressions), dorsa (ridges), planitiae ([[Earth:Plain|plain]]s), [[Earth:Sulcus (geology)|sulci]] (long parallel grooves), and [[Astronomy:Rupes|rupes]] (cliffs) are named after places. The IAU has officially named 85 features on Enceladus, most recently Samaria Rupes, formerly called Samaria Fossa.<ref>{{cite web|url=http://planetarynames.wr.usgs.gov/SearchResults?target=ENCELADUS|title=Nomenclature Search Results: Enceladus|work=Gazetteer of Planetary Nomenclature|publisher=USGS Astrogeology Science Center|access-date=January 13, 2015|archive-date=June 17, 2013|archive-url=https://web.archive.org/web/20130617012144/http://planetarynames.wr.usgs.gov/SearchResults?target=ENCELADUS|url-status=live}}</ref><ref>{{GPN|15329|Samaria Rupes}}</ref>

==Shape and size==
[[File:Enceladus_size_corrected.png|left|thumb|Size comparison between Enceladus (lower left), the Moon (upper left) and Earth]]
Enceladus is a relatively small satellite composed of ice and rock.<ref>{{cite web|url=http://www.jpl.nasa.gov/news/news.php?release=2007-025|title=A Hot Start Might Explain Geysers on Enceladus|work=NASA/JPL|date=March 12, 2007|access-date=January 12, 2015|archive-date=November 13, 2014|archive-url=https://web.archive.org/web/20141113071550/http://www.jpl.nasa.gov/news/news.php?release=2007-025|url-status=live}}</ref> It is a scalene [[Ellipsoid|ellipsoid]] in shape; its diameters, calculated from images taken by ''Cassini's'' ISS (Imaging Science Subsystem) instrument, are {{nowrap|513 km}} between the sub- and anti-Saturnian poles, {{nowrap|503 km}} between the leading and trailing hemispheres, and {{nowrap|497 km}} between the north and south poles.<ref name="Porco Helfenstein et al. 2006" />

Enceladus is only one-seventh the diameter of Earth's [[Astronomy:Moon|Moon]]. It ranks sixth in both mass and size among the satellites of Saturn, after [[Astronomy:Titan (moon)|Titan]] ({{nowrap|5,150 km}}), [[Astronomy:Rhea (moon)|Rhea]] ({{nowrap|1,530 km}}), Iapetus ({{nowrap|1,440 km}}), Dione ({{nowrap|1,120 km}}) and Tethys ({{nowrap|1,050 km}}).<ref>{{cite web |url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturniansatfact.html|title=Saturnian Satellite Fact Sheet|work=Planetary Factsheets |publisher=NASA|access-date=July 15, 2016|date=October 13, 2015|archive-date=April 30, 2010|archive-url=https://web.archive.org/web/20100430122034/http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturniansatfact.html|url-status=live}}</ref><ref name="Thomas2007">{{cite journal|display-authors=0|last1=Thomas|first1=P. C.|last2=Burns|first2=J. A.|title=Shapes of the saturnian icy satellites and their significance|journal=Icarus|volume=190|issue=2|pages=573–584|date=2007|doi=10.1016/j.icarus.2007.03.012 |bibcode=2007Icar..190..573T}}</ref>

==Orbit and rotation==
Enceladus is one of the major inner satellites of Saturn along with Dione, Tethys, and Mimas. It orbits at {{Convert|238,000|km|mi|abbr=on}} from Saturn's center and {{Convert|180,000|km|mi|abbr=on}} from its cloud tops, between the orbits of Mimas and Tethys. It orbits Saturn every 32.9 hours, fast enough for its motion to be observed over a single night of observation. Enceladus is currently in a 2:1 mean-motion orbital resonance with Dione, completing two orbits around Saturn for every one orbit completed by Dione.<ref name="Porco Helfenstein et al. 2006"/>

This resonance maintains Enceladus's orbital eccentricity (0.0047), which is known as a forced eccentricity. This non-zero eccentricity results in tidal deformation of Enceladus. The dissipated heat resulting from this deformation is the main heating source for Enceladus's geologic activity.<ref name="Porco Helfenstein et al. 2006"/> Enceladus orbits within the densest part of [[Astronomy:Rings of Saturn#E Ring|Saturn's E ring]], the outermost of [[Astronomy:Rings of Saturn|its major rings]], and is the main source of the ring's material composition.<ref name="ras.377">{{cite journal|last1=Hillier|first1=J. K.|last2=Green|first2=S. F.|title=The composition of Saturn's E ring|journal=[[Organization:Monthly Notices of the Royal Astronomical Society|Monthly Notices of the Royal Astronomical Society]]|volume=377|issue=4|pages=1588–96|date=June 2007|bibcode=2007MNRAS.377.1588H|doi=10.1111/j.1365-2966.2007.11710.x|display-authors=0|doi-access=}}</ref>

Like most of Saturn's larger satellites, Enceladus rotates synchronously with its orbital period, keeping one face pointed toward Saturn. Unlike Earth's [[Astronomy:Moon|Moon]], Enceladus does not appear to [[Astronomy:Libration|librate]] more than 1.5° about its spin axis. However, analysis of the shape of Enceladus suggests that at some point it was in a 1:4 forced secondary spin–orbit libration.<ref name="Porco Helfenstein et al. 2006"/> This libration could have provided Enceladus with an additional heat source.<ref name="Efroimsky_2018a"/><ref name="Efroimsky_2018b">{{cite journal |last=Efroimsky |first=M. |title=Dissipation in a tidally perturbed body librating in longitude |journal=Icarus |volume=306 |date=May 15, 2018 |doi=10.1016/j.icarus.2017.10.020 |pages=328–354 |arxiv=1706.08999 |bibcode=2018Icar..306..328E |s2cid=119093658}}</ref><ref name="Harvard 2008">{{cite journal |title=Implications of Spin-orbit Librations on Enceladus |last1=Hurford |first1=Terry |last2=Bruce |first2=B. |year=2008 |journal=American Astronomical Society, DPS Meeting #40, #8.06 |page=399 |bibcode=2008DPS....40.0806H |volume=40}}</ref>

===Source of the E ring===
[[File:E ring with Enceladus.jpg|thumb|Enceladus orbiting within Saturn's E&nbsp;ring]]
Plumes from Enceladus, which are similar in composition to comets,<ref name="NS-20080326"/> have been shown to be the source of the material in Saturn's [[Astronomy:Rings of Saturn#E Ring|E ring]].<ref name="E ring"/> The E ring is the widest and outermost ring of Saturn (except for the tenuous Phoebe ring). It is an extremely wide but diffuse disk of microscopic icy or dusty material distributed between the orbits of Mimas and [[Astronomy:Titan (moon)|Titan]].<ref name="Hedman_2012">{{cite journal|display-authors=0|last1=Hedman|first1=M. M. |last2=Burns|first2=J. A.|date=2012|title=The three-dimensional structure of Saturn's E ring|journal=Icarus|volume=217|issue=1|pages=322–338 |doi=10.1016/j.icarus.2011.11.006|arxiv=1111.2568|bibcode=2012Icar..217..322H|s2cid=1432112}}</ref>

Mathematical models show that the E ring is unstable, with a lifespan between 10,000 and 1,000,000 years; therefore, particles composing it must be constantly replenished.<ref name="csa.cassini">{{cite web|url=https://www.researchgate.net/publication/251317042|title=Cassini visits Enceladus: New light on a bright world|last=Vittorio|first=Salvatore A.|work=Cambridge Scientific Abstracts|date=July 2006|access-date=April 27, 2014|archive-date=October 1, 2021|archive-url=https://web.archive.org/web/20211001063532/https://www.researchgate.net/publication/251317042_Cassini_visits_Enceladus_-_New_light_on_a_bright_world|url-status=live}}</ref> Enceladus is orbiting inside the ring, at its narrowest but highest density point. In the 1980s some suspected that Enceladus is the main source of particles for the ring.<ref name="Baum 1981">{{cite journal|title=Saturn's E ring: I. CCD observations of March 1980|journal=Icarus|date=July 1981|last1=Baum|first1=W. A.|last2=Kreidl|first2=T.|volume=47|issue=1|pages=84–96|doi=10.1016/0019-1035(81)90093-2|bibcode=1981Icar...47...84B}}</ref><ref name="Haff 1983">{{cite journal|first1=P. K.|last1=Haff|first2=A.|last2=Eviatar|display-authors=0|title=Ring and plasma: Enigmae of Enceladus|journal=Icarus|date=1983|volume=56|issue=3|pages=426–438|bibcode=1983Icar...56..426H|doi=10.1016/0019-1035(83)90164-1}}</ref><ref name="ref.09459">{{cite journal|first1=Kevin D.|last1=Pang|first2=Charles C.|last2=Voge|display-authors=0|title=The E ring of Saturn and satellite Enceladus|journal=Journal of Geophysical Research|date=1984|volume=89|page=9459|doi=10.1029/JB089iB11p09459|bibcode=1984JGR....89.9459P}}</ref><ref name="ssu.241">{{cite book|title=Solar System Update|publisher=Springer Science|location=Berlin Heidelberg|first1=Philippe|last1=Blondel|first2=John|last2=Mason|date=August 23, 2006|pages=241–3|url=https://www.springer.com/us/book/9783540260561|isbn=978-3-540-37683-5|doi=10.1007/3-540-37683-6|bibcode=2006ssu..book.....B|access-date=August 28, 2017|archive-date=December 1, 2018|archive-url=https://web.archive.org/web/20181201005244/https://www.springer.com/us/book/9783540260561|url-status=live}}</ref> This hypothesis was confirmed by ''Cassini's'' first two close flybys in 2005.<ref name="Spahn">{{cite journal|title=Cassini Dust Measurements at Enceladus and Implications for the Origin of the E ring|journal=Science|volume=311|issue=5766|pages=1416–18 |date=2006|doi=10.1126/science.1121375|pmid=16527969|bibcode=2006Sci...311.1416S|last1=Spahn|first1=F.|last2=Schmidt|first2=J.|display-authors=0|citeseerx=10.1.1.466.6748|s2cid=33554377}}</ref><ref name="ut.12710">{{cite news|last=Cain|first=Fraser|url=http://www.universetoday.com/12710/enceladus-is-supplying-ice-to-saturns-a-ring/|title=Enceladus is Supplying Ice to Saturn's A-Ring|work=NASA|publisher=Universe Today|date=February 5, 2008|access-date=April 26, 2014|archive-date=April 26, 2014|archive-url=https://web.archive.org/web/20140426215407/http://www.universetoday.com/12710/enceladus-is-supplying-ice-to-saturns-a-ring/|url-status=live}}</ref>

The [[Astronomy:Cosmic Dust Analyzer|Cosmic Dust Analyzer]] (CDA) "detected a large increase in the number of particles near Enceladus", confirming it as the primary source for the E&nbsp;ring.<ref name="Spahn"/> Analysis of the CDA and INMS data suggest that the gas cloud ''Cassini'' flew through during the July encounter, and observed from a distance with its magnetometer and UVIS, was actually a water-rich cryovolcanic plume, originating from vents near the south pole.<ref name="JPL619">{{cite web| url=http://www.jpl.nasa.gov/news/news.php?release=2005-171| title=NASA's Cassini Images Reveal Spectacular Evidence of an Active Moon| work=NASA/JPL| date=December 5, 2005| access-date=May 4, 2016| archive-date=March 12, 2016| archive-url=https://web.archive.org/web/20160312131720/http://www.jpl.nasa.gov/news/news.php?release=2005-171| url-status=live}}</ref>

Visual confirmation of venting came in November 2005, when ''[[Astronomy:Cassini–Huygens|Cassini]]'' imaged [[Earth:Geyser|geyser]]-like jets of icy particles rising from Enceladus's south polar region.<ref name="Porco Helfenstein et al. 2006" /><ref name="E ring 2006" /> (Although the plume was imaged before, in January and February 2005, additional studies of the camera's response at high phase angles, when the Sun is almost behind Enceladus, and comparison with equivalent high-phase-angle images taken of other Saturnian satellites, were required before this could be confirmed.<ref name="Ciclops1652">{{cite web|url=http://ciclops.org/view.php?id=1652|title=Spray Above Enceladus|work=Cassini Imaging|access-date=March 22, 2005|archive-date=February 25, 2006|archive-url=https://web.archive.org/web/20060225074236/http://ciclops.org/view.php?id=1652|url-status=live}}</ref>)

==Geology==

===Surface features===
[[File:Enceladusstripes cassini.jpg|thumb|A false-color photo of Enceladus, that highlights its ridges, impact craters and plains.]]
''[[Astronomy:Voyager 2|Voyager 2]]'' was the first spacecraft to observe Enceladus's surface in detail, in August 1981. Examination of the resulting highest-resolution imagery revealed at least five different types of terrain, including several regions of cratered terrain, regions of smooth (young) terrain, and lanes of ridged terrain often bordering the smooth areas.<ref name="Rothery"/> Extensive linear cracks<ref name="cracks">{{cite news|work=NASA|title=Cracks on Enceladus Open and Close under Saturn's Pull|url=http://www.nasa.gov/mission_pages/cassini/media/enceladus_cracks.html|date=May 16, 2007|first=Bill|last=Steigerwald|access-date=May 17, 2007|archive-date=January 19, 2009|archive-url=https://web.archive.org/web/20090119120627/http://www.nasa.gov/mission_pages/cassini/media/enceladus_cracks.html|url-status=live}}</ref> and [[Earth:Escarpment|scarps]] were observed. Given the relative lack of craters on the smooth plains, these regions are probably less than a few hundred million years old.<ref name="Solstice"/>

Accordingly, Enceladus must have been recently active with "water volcanism" or other processes that renew the surface.<ref name="Solstice">{{cite web|url=http://saturn.jpl.nasa.gov/science/moons/enceladus/|title=Satun Moons – Enceladus|work=Cassini Solstice Mission Team|publisher=JPL/NASA|access-date=April 26, 2014|archive-date=April 20, 2016|archive-url=https://web.archive.org/web/20160420174722/http://saturn.jpl.nasa.gov/science/moons/enceladus/|url-status=live}}</ref> The fresh, clean ice that dominates its surface makes Enceladus the most reflective body in the Solar System, with a visual [[Physics:Geometric albedo|geometric albedo]] of 1.38<ref name="Verbiscer et al. 2007"/> and bolometric [[Physics:Bond albedo|Bond albedo]] of {{val|0.81|0.04}}.<ref name="Howett_2010" /> Because it reflects so much sunlight, its surface only reaches a mean noon temperature of {{convert|-198|°C|0|abbr=on}}, somewhat colder than other Saturnian satellites.<ref name="Spencer" />

Observations during three flybys on February 17, March 9, and July 14, 2005, revealed Enceladus's surface features in much greater detail than the ''Voyager 2'' observations. The smooth plains, which ''Voyager 2'' had observed, resolved into relatively crater-free regions filled with numerous small [[Earth:Ridge|ridge]]s and scarps. Numerous fractures were found within the older, cratered terrain, suggesting that the surface has been subjected to extensive deformation since the craters were formed.<ref name="Rathbun">{{cite journal|last1=Rathbun|first1=J. A.|last2=Turtle|first2=E. P.|display-authors=0|date=2005|title=Enceladus' global geology as seen by Cassini ISS|journal=Eos Trans. AGU|volume=82|issue=52 (Fall Meeting Supplement), abstract P32A–03|pages=P32A–03|bibcode=2005AGUFM.P32A..03R }}</ref>

Some areas contain no craters, indicating major resurfacing events in the geologically recent past. There are fissures, plains, corrugated terrain and other crustal deformations. Several additional regions of young terrain were discovered in areas not well-imaged by either ''Voyager'' spacecraft, such as the bizarre terrain near the south pole.<ref name="Porco Helfenstein et al. 2006"/> All of this indicates that Enceladus's interior is liquid today, even though it should have been frozen long ago.<ref name="Solstice"/>

====Impact craters====
[[File:PIA20011-SaturnMoon-Enceladus-NPole-Closeup-20151014.jpg|thumb|A close up picture of Al-Haddar (top), Shahrazad (middle) and Dunyazad (bottom) craters]]
[[Astronomy:Impact crater|Impact crater]]ing is a common occurrence on many Solar System bodies. Much of Enceladus's surface is covered with craters at various densities and levels of degradation.<ref name="Smith">{{cite journal|doi=10.1126/science.215.4532.504|title=A New Look at the Saturn System: The Voyager 2 Images|date=1982|journal=Science|volume=215|pages=504–37|pmid=17771273|issue=4532|bibcode=1982Sci...215..504S|last1=Smith|first1=B. A.|last2=Soderblom |first2=L.|s2cid=23835071|display-authors=0}}</ref> This subdivision of cratered terrains on the basis of crater density (and thus surface age) suggests that Enceladus has been resurfaced in multiple stages.<ref name="Solstice" />

''Cassini'' observations provided a much closer look at the crater distribution and size, showing that many of Enceladus's craters are heavily degraded through viscous relaxation and [[Physics:Fracture|fracturing]].<ref name="Turtle">{{cite web|last=Turtle |first=E. P. |display-authors=0 |url=http://saturn.jpl.nasa.gov/multimedia/products/pdfs/CHARM_Turtle_050426.pdf |archive-url=https://web.archive.org/web/20090927115324/http://saturn.jpl.nasa.gov/multimedia/products/pdfs/CHARM_Turtle_050426.pdf |archive-date=September 27, 2009 |title=Enceladus, Curiouser and Curiouser: Observations by ''Cassini's'' Imaging Science Subsystem |work=CHARM Teleconference |publisher=JPL/NASA |url-status=dead |date=April 28, 2005}}</ref> Viscous relaxation allows gravity, over geologic time scales, to deform craters and other topographic features formed in water ice, reducing the amount of topography over time. The rate at which this occurs is dependent on the temperature of the ice: warmer ice is easier to deform than colder, stiffer ice. Viscously relaxed craters tend to have [[Engineering:Dome|dome]]d floors, or are recognized as craters only by a raised, circular rim. Dunyazad crater is a prime example of a viscously relaxed crater on Enceladus, with a prominent domed floor.<ref name="Se-4">{{cite web|title=Shahrazad (Se-4)|work=PIA12783: The Enceladus Atlas|publisher=NASA/Cassini Imaging Team|url=http://photojournal.jpl.nasa.gov/figures/PIA12783_full_5.jpg|access-date=February 4, 2012|archive-date=March 17, 2017|archive-url=https://web.archive.org/web/20170317123123/http://photojournal.jpl.nasa.gov/figures/PIA12783_full_5.jpg|url-status=live}}</ref>

====Tectonic features====
[[File:Enceladus - November 21 2009 (37416820511).jpg|thumb|A close-up view of Enceladus's ridges]]
''Voyager 2'' found several types of tectonic features on Enceladus, including [[Earth:Trough (geology)|troughs]], scarps, and [[Earth:Geology|belts]] of grooves and [[Earth:Ridge|ridge]]s.<ref name="Rothery" /> Results from ''Cassini'' suggest that [[Earth:Tectonics|tectonics]] is the dominant mode of deformation on Enceladus, including rifts, one of the more dramatic types of tectonic features that were noted. These canyons can be up to 200&nbsp;km long, 5–10&nbsp;km wide, and 1&nbsp;km deep. Such features are geologically young, because they cut across other tectonic features and have sharp topographic relief with prominent outcrops along the cliff faces.<ref name="LPS 2006">{{cite conference|display-authors=0 |last1=Helfenstein |first1=P.|last2=Thomas|first2=P. C. |title=Patterns of fracture and tectonic convergence near the south pole of Enceladus |url=http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2182.pdf |conference=Lunar and Planetary Science XXXVII (2006)|access-date=April 27, 2014|archive-date=March 4, 2016|archive-url=https://web.archive.org/web/20160304001737/http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2182.pdf|url-status=live}}</ref>

Evidence of tectonics on Enceladus is also derived from grooved terrain, consisting of lanes of curvilinear grooves and ridges. These bands, first discovered by ''Voyager 2'', often separate smooth plains from cratered regions.<ref name="Rothery"/> Grooved terrains such as the Samarkand Sulci are reminiscent of grooved terrain on [[Astronomy:Ganymede (moon)|Ganymede]]. Unlike those seen on Ganymede, grooved topography on Enceladus is generally more complex. Rather than parallel sets of grooves, these lanes often appear as bands of crudely aligned, chevron-shaped features.<ref name="Turtle"/>

In other areas, these bands bow upwards with fractures and ridges running the length of the feature. ''Cassini'' observations of the Samarkand Sulci have revealed dark spots (125 and 750 m wide) located parallel to the narrow fractures. Currently, these spots are interpreted as collapse pits within these ridged plain belts.<ref name="Turtle"/>

In addition to deep fractures and grooved lanes, Enceladus has several other types of tectonic terrain. Many of these fractures are found in bands cutting across cratered terrain. These fractures probably propagate down only a few hundred meters into the crust. Many have probably been influenced during their formation by the weakened regolith produced by impact craters, often changing the strike of the propagating fracture.<ref name="Turtle"/><ref>{{cite journal|last=Barnash|first=A. N.|display-authors=0|date=2006|bibcode=2006DPS....38.2406B|title=Interactions Between Impact Craters and Tectonic Fractures on Enceladus|journal=Bulletin of the American Astronomical Society|volume=38|issue=3|id=Presentation #24.06|page=522}}</ref>

Another example of tectonic features on Enceladus are the linear grooves first found by ''Voyager 2'' and seen at a much higher resolution by ''Cassini''. These linear grooves can be seen cutting across other terrain types, like the groove and ridge belts. Like the deep rifts, they are among the youngest features on Enceladus. However, some linear grooves have been softened like the craters nearby, suggesting that they are older. Ridges have also been observed on Enceladus, though not nearly to the extent as those seen on [[Astronomy:Europa (moon)|Europa]]. These ridges are relatively limited in extent and are up to one kilometer tall. One-kilometer high domes have also been observed.<ref name="Turtle"/> Given the level of resurfacing found on Enceladus, it is clear that tectonic movement has been an important driver of geology for much of its history.<ref name="LPS 2006"/>

====Smooth plains====
Two regions of smooth plains were observed by ''Voyager 2''. They generally have low relief and have far fewer craters than in the cratered terrains, indicating a relatively young surface age.<ref name="Smith"/> In one of the smooth plain regions, Sarandib Planitia, no impact craters were visible down to the limit of resolution. Another region of smooth plains to the southwest of Sarandib is criss-crossed by several troughs and scarps. ''Cassini'' has since viewed these smooth plains regions, like Sarandib Planitia and Diyar Planitia at much higher resolution. ''Cassini'' images show these regions filled with low-relief ridges and fractures, probably caused by [[Physics:Shear stress|shear deformation]].<ref name="Turtle"/> The high-resolution images of Sarandib Planitia revealed a number of small impact craters, which allow for an estimate of the surface age, either 170 million years or 3.7 billion years, depending on assumed impactor population.<ref name="Porco Helfenstein et al. 2006"/><ref name="crater flux" group="lower-alpha"/>

The expanded surface coverage provided by ''Cassini'' has allowed for the identification of additional regions of smooth plains, particularly on Enceladus's leading hemisphere (the side of Enceladus that faces the direction of motion as it orbits Saturn). Rather than being covered in low-relief ridges, this region is covered in numerous criss-crossing sets of troughs and ridges, similar to the deformation seen in the south polar region. This area is on the opposite side of Enceladus from Sarandib and Diyar Planitiae, suggesting that the placement of these regions is influenced by Saturn's tides on Enceladus.<ref name="Pappalardo">{{cite journal|last1=Nimmo|first1=F.|last2=Pappalardo|first2=R. T.|date=2006|title=Diapir-induced reorientation of Saturn's moon Enceladus|journal=Nature|volume=441|issue=7093|pages=614–16|doi=10.1038/nature04821|pmid=16738654|bibcode=2006Natur.441..614N|s2cid=4339342}}</ref>

====South polar region====
[[File:Enceladus quadrangle Se15 PIA08419 full 30.jpg|thumb|An atlas of Enceladus's south pole quadrangle, which is dominated by the tiger stripes.]]
Images taken by ''Cassini'' during the flyby on July 14, 2005, revealed a distinctive, tectonically deformed region surrounding Enceladus's south pole. This area, reaching as far north as 60° south latitude, is covered in tectonic fractures and ridges.<ref name="Porco Helfenstein et al. 2006"/><ref name="Ciclops1223">{{cite web|url=http://ciclops.org/view.php?id=1223|title=Enceladus in False Color|work=Cassini Imaging|date=July 26, 2005|access-date=March 22, 2006|archive-date=March 9, 2006|archive-url=https://web.archive.org/web/20060309110455/http://ciclops.org/view.php?id=1223|url-status=live}}</ref> The area has few sizable impact craters, suggesting that it is the youngest surface on Enceladus and on any of the mid-sized icy satellites. Modeling of the cratering rate suggests that some regions of the south polar terrain are possibly as young as 500,000 years or less.<ref name="Porco Helfenstein et al. 2006"/>

Near the center of this terrain are four fractures bounded by ridges, unofficially called "tiger stripes".<ref name="NYT-20191209">{{cite news |last=Drake |first=Nadia |title=How an Icy Moon of Saturn Got Its Stripes – Scientists have developed an explanation for one of the most striking features of Enceladus, an ocean world that has the right ingredients for life. |url=https://www.nytimes.com/2019/12/09/science/enceladus-stripes-moon.html |date=9 December 2019 |work={{wipe|The New York Times}} |access-date=11 December 2019 |archive-date=December 11, 2019 |archive-url=https://web.archive.org/web/20191211141151/https://www.nytimes.com/2019/12/09/science/enceladus-stripes-moon.html |url-status=live }}</ref> They appear to be the youngest features in this region and are surrounded by mint-green-colored (in false color, UV–green–near IR images), coarse-grained water ice, seen elsewhere on the surface within outcrops and fracture walls.<ref name="Ciclops1223"/> Here the "blue" ice is on a flat surface, indicating that the region is young enough not to have been coated by fine-grained water ice from the [[Astronomy:Rings of Saturn#E Ring|E ring]].<ref name="VIMS_PR"/>

Results from the visual and infrared spectrometer (VIMS) instrument suggest that the green-colored material surrounding the tiger stripes is chemically distinct from the rest of the surface of Enceladus. VIMS detected crystalline water ice in the stripes, suggesting that they are quite young (likely less than 1,000 years old) or the surface ice has been thermally altered in the recent past.<ref name="VIMS_PR">{{cite web|url=http://www.nasa.gov/mission_pages/cassini/media/cassini-083005.html|title=Cassini Finds Enceladus Tiger Stripes Are Really Cubs|work=NASA|date=August 30, 2005|access-date=April 3, 2014|archive-date=April 7, 2014|archive-url=https://web.archive.org/web/20140407100640/http://www.nasa.gov/mission_pages/cassini/media/cassini-083005.html|url-status=live}}</ref> VIMS also detected simple organic (carbon-containing) compounds in the tiger stripes, chemistry not found anywhere else on Enceladus thus far.<ref name="Brown">{{cite journal|date=2006|title=Composition and Physical Properties of Enceladus' Surface|journal=Science|volume=311|issue=5766|pages=1425–28|doi=10.1126/science.1121031|pmid=16527972|bibcode=2006Sci...311.1425B|last1=Brown|first1=R. H.|last2=Clark |first2=R. N.|s2cid=21624331|display-authors=0}}</ref>

One of these areas of "blue" ice in the south polar region was observed at high resolution during the July 14, 2005, flyby, revealing an area of extreme tectonic deformation and blocky terrain, with some areas covered in boulders 10–100 m across.<ref name="Ciclops1250">{{cite web|url=http://ciclops.org/view.php?id=1250|title=Boulder-Strewn Surface|work=Cassini Imaging|date=July 26, 2005|access-date=March 26, 2006|archive-date=May 11, 2013|archive-url=https://web.archive.org/web/20130511074838/http://www.ciclops.org/view.php?id=1250|url-status=live}}</ref>

The boundary of the south polar region is marked by a pattern of parallel, Y- and V-shaped ridges and valleys. The shape, orientation, and location of these features suggest they are caused by changes in the overall shape of Enceladus. As of 2006 there were two theories for what could cause such a shift in shape: the orbit of Enceladus may have migrated inward, leading to an increase in Enceladus's rotation rate. Such a shift would lead to a more oblate shape;<ref name="Porco Helfenstein et al. 2006"/> or a rising mass of warm, low-density material in Enceladus's interior may have led to a shift in the position of the current south polar terrain from Enceladus's southern mid-latitudes to its south pole.<ref name="Pappalardo"/>

Consequently, the moon's ellipsoid shape would have adjusted to match the new orientation. One problem of the polar flattening hypothesis is that both polar regions should have similar tectonic deformation histories.<ref name="Porco Helfenstein et al. 2006"/> However, the north polar region is densely cratered, and has a much older surface age than the south pole.<ref name="Smith"/> Thickness variations in Enceladus's [[Earth:Lithosphere|lithosphere]] is one explanation for this discrepancy. Variations in lithospheric thickness are supported by the correlation between the Y-shaped discontinuities and the V-shaped cusps along the south polar terrain margin and the relative surface age of the adjacent non-south polar terrain regions. The Y-shaped discontinuities, and the north–south trending tension fractures into which they lead, are correlated with younger terrain with presumably thinner lithospheres. The V-shaped cusps are adjacent to older, more heavily cratered terrains.<ref name="Porco Helfenstein et al. 2006"/>

====South polar plumes====
[[File:Enceladus - November 21 2009 (49728085428).png|center|thumb|upright=1.5|A panorama of Enceladus's plumes taken by the ''Cassini'' spacecraft]]
Following ''Voyager's'' encounters with Enceladus in the early 1980s, scientists postulated it to be geologically active based on its young, reflective surface and location near the core of the E&nbsp;ring.<ref name="Rothery"/> Based on the connection between Enceladus and the E&nbsp;ring, scientists suspected that Enceladus was the source of material in the E&nbsp;ring, perhaps through venting of water vapor.<ref name="Baum 1981"/><ref name="Haff 1983"/> The first ''Cassini'' sighting of a plume of icy particles above Enceladus's south pole came from the Imaging Science Subsystem (ISS) images taken in January and February 2005,<ref name="Porco Helfenstein et al. 2006"/> though the possibility of a camera artifact delayed an official announcement.

Data from the [[Physics:Magnetometer|magnetometer]] instrument during the February 17, 2005, encounter provided evidence for a planetary atmosphere. The magnetometer observed a deflection or "draping" of the magnetic field, consistent with local ionization of neutral gas.<ref name="Dougherty"/> During the two following encounters, the [[Physics:Magnetometer|magnetometer]] team determined that gases in Enceladus's atmosphere are concentrated over the south polar region, with atmospheric density away from the pole being much lower.<ref name="Dougherty"/> Unlike the magnetometer, the [[Physics:Ultraviolet|Ultraviolet]] Imaging Spectrograph failed to detect an atmosphere above Enceladus during the February encounter when it looked over the equatorial region, but did detect water vapor during an occultation over the south polar region during the July encounter.<ref name="Hansen"/>[[File:Enceladus Cold Geyser Model.svg|thumb|One possible scheme for Enceladus's cryovolcanism]]''Cassini'' flew through this gas cloud on a few encounters, allowing instruments such as the ion and neutral [[Physics:Mass spectrometry|mass spectrometer]] (INMS) and the cosmic dust analyzer (CDA) to directly sample the plume. (See 'Composition' section.) The November 2005 images showed the plume's fine structure, revealing numerous jets (perhaps issuing from numerous distinct vents) within a larger, faint component extending out nearly {{Convert|500|km|mi|abbr=on}} from the surface.<ref name="JPL619"/> The particles have a bulk velocity of {{Convert|1.25|+/-|0.1|km/s|mph|lk=on|abbr=off|sp=us}},<ref name="perry2846">{{cite conference |last1=Perry |first1=M. E. |last2=Teolis |first2=Ben D. |last3=Grimes |first3=J. |display-authors=0 |title=Direct Measurement of the Velocity of the Enceladus Vapor Plumes |url=http://www.hou.usra.edu/meetings/lpsc2016/pdf/2846.pdf |conference=47th Lunar and Planetary Science Conference |place=The Woodlands, Texas |date=March 21, 2016 |page=2846 |access-date=May 4, 2016 |archive-date=May 30, 2016 |archive-url=https://web.archive.org/web/20160530112411/http://www.hou.usra.edu/meetings/lpsc2016/pdf/2846.pdf |url-status=live }}</ref> and a maximum velocity of {{Convert|3.40|km/s|mph|abbr=on}}.<ref>{{cite journal |title=Enceladus Plume Structure and Time Variability: Comparison of Cassini Observations |journal=Astrobiology |date=September 5, 2017 |last1=Teolis |first1=Ben D. |last2=Perry |first2=Mark E. |last3=Hansen |first3=Candice J. |last4=Waite, Jr. |first4=Jack Hunter |last5=Porco |first5=Carolyn C. |last6=Spencer |first6=John R. |last7=Howett |first7=Carly J. A. |volume=17 |issue=9 |pages=926–940 |doi=10.1089/ast.2017.1647 |pmid=28872900 |pmc=5610430 |bibcode=2017AsBio..17..926T }}</ref> ''Cassini's'' UVIS later observed gas jets coinciding with the dust jets seen by ISS during a non-targeted encounter with Enceladus in October 2007.

The combined analysis of imaging, mass spectrometry, and magnetospheric data suggests that the observed south polar plume emanates from pressurized subsurface chambers, similar to Earth's [[Earth:Geyser|geyser]]s or fumaroles.<ref name="Porco Helfenstein et al. 2006"/> Fumaroles are probably the closer analogy, since periodic or episodic emission is an inherent property of geysers. The plumes of Enceladus were observed to be continuous to within a factor of a few. The mechanism that drives and sustains the eruptions is thought to be tidal heating.<ref name="Kite 3972–3975">{{cite journal |title=Sustained eruptions on Enceladus explained by turbulent dissipation in tiger stripes |journal=Proceedings of the National Academy of Sciences of the United States of America |date=January 29, 2016 |last1=Kite |first1=Edwin S. |last2=Rubin |first2=Allan M. |volume=113 |issue=15 |doi=10.1073/pnas.1520507113 |pages=3972–3975 |pmid=27035954 |pmc=4839467|arxiv = 1606.00026 |bibcode = 2016PNAS..113.3972K |doi-access=free }}</ref>

The intensity of the eruption of the south polar jets varies significantly as a function of the position of Enceladus in its orbit. The plumes are about four times brighter when Enceladus is at apoapsis (the point in its orbit most distant from Saturn) than when it is at periapsis.<ref name="Spotts2013">{{cite web| last=Spotts| first=P.| title=What's going on inside Saturn moon? Geysers offer intriguing new clue| work=The Christian Science Monitor| url=http://www.csmonitor.com/Science/2013/0731/What-s-going-on-inside-Saturn-moon-Geysers-offer-intriguing-new-clue| date=July 31, 2013| access-date=August 3, 2013| archive-date=August 3, 2013| archive-url=https://web.archive.org/web/20130803072949/http://www.csmonitor.com/Science/2013/0731/What-s-going-on-inside-Saturn-moon-Geysers-offer-intriguing-new-clue| url-status=live}}</ref><ref name="Lakdawalla2013">{{cite web| last=Lakdawalla| first=E.| title=Enceladus huffs and puffs: plumes vary with orbital longitude| work=Planetary Society blogs| publisher=[[Organization:The Planetary Society|The Planetary Society]]| url=http://www.planetary.org/blogs/emily-lakdawalla/2013/12111647-enceladus-huffs-and-puffs.html| date=March 11, 2013| access-date=January 26, 2014| archive-date=February 2, 2014| archive-url=https://web.archive.org/web/20140202102944/http://www.planetary.org/blogs/emily-lakdawalla/2013/12111647-enceladus-huffs-and-puffs.html| url-status=live}}</ref><ref name="Spencer2013b">{{cite journal |last=Spencer |first=John R. |title=Solar system: Saturn's tides control Enceladus' plume |journal=Nature |date=July 31, 2013 |issn=0028-0836 |doi=10.1038/nature12462 |bibcode=2013Natur.500..155S |volume=500 |issue=7461 |pages=155–6 |pmid=23903653 |s2cid=205235182 }}</ref> This is consistent with geophysical calculations which predict the south polar fissures are under compression near periapsis, pushing them shut, and under tension near apoapsis, pulling them open.<ref name="HedmanGosmeyer2013">{{cite journal| last1=Hedman|first1=M. M.| last2=Gosmeyer| first2=C. M.| display-authors=0| title=An observed correlation between plume activity and tidal stresses on Enceladus| journal=Nature| issn=0028-0836|doi=10.1038/nature12371| bibcode=2013Natur.500..182H| volume=500|issue=7461| pages=182–4| pmid=23903658|date=July 31, 2013|s2cid=205234732}}</ref>

Much of the plume activity consists of broad curtain-like eruptions. Optical illusions from a combination of viewing direction and local fracture geometry previously made the plumes look like discrete jets.<ref>{{Cite journal| title=Curtain eruptions from Enceladus' south-polar terrain| journal=Nature| date=May 7, 2015| issn=0028-0836| pages=57–60| volume=521| issue=7550| doi=10.1038/nature14368| first1=Joseph N.| last1=Spitale| first2=Terry A.| last2=Hurford|display-authors=0|bibcode=2015Natur.521...57S| pmid=25951283| s2cid=4394888}}</ref><ref>{{cite web|title='Jets' on Saturn Moon Enceladus May Actually Be Giant Walls of Vapor and Ice|url=http://www.space.com/29330-saturn-moon-enceladus-geysers-curtains.html|access-date=May 8, 2015|work=Space.com|date=May 6, 2015|first=Charles Q.|last=Choi|archive-date=May 9, 2015|archive-url=https://web.archive.org/web/20150509005013/http://www.space.com/29330-saturn-moon-enceladus-geysers-curtains.html|url-status=live}}</ref><ref>{{Cite news| title=Long 'curtains' of material may be shooting off Saturn's moon Enceladus| url=http://www.latimes.com/science/sciencenow/la-sci-sn-enceladus-jets-curtains-20150506-story.html| newspaper=Los Angeles Times| access-date=May 8, 2015| issn=0458-3035| archive-date=May 12, 2015| archive-url=https://web.archive.org/web/20150512023727/http://www.latimes.com/science/sciencenow/la-sci-sn-enceladus-jets-curtains-20150506-story.html| url-status=live}}</ref>

The extent to which cryovolcanism really occurs is a subject of some debate. At Enceladus, it appears that cryovolcanism occurs because water-filled cracks are periodically exposed to vacuum, the cracks being opened and closed by tidal stresses.<ref name="HedmanGosmeyer2013"/><ref name="Nimmo 2016">{{cite journal |title=Ocean worlds in the outer solar system |journal=Journal of Geophysical Research |date=August 8, 2016 |volume=121 |issue=8 |pages=1378–1399 |last1=Nimmo |first1=F. |last2=Pappalardo |first2=R. T. |doi=10.1002/2016JE005081 |url=https://websites.pmc.ucsc.edu/~fnimmo/website/ocean_worlds_new.pdf |access-date=October 1, 2017 |bibcode=2016JGRE..121.1378N |doi-access=free |archive-date=October 1, 2017 |archive-url=https://web.archive.org/web/20171001214043/https://websites.pmc.ucsc.edu/~fnimmo/website/ocean_worlds_new.pdf |url-status=live }}</ref><ref>{{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}}</ref>

===Internal structure===

Before the [[Astronomy:Cassini–Huygens|''Cassini'']] mission, little was known about the interior of Enceladus. However, flybys by ''Cassini'' provided information for models of Enceladus's interior, including a better determination of the mass and shape, high-resolution observations of the surface, and new insights on the interior.<ref name="esa.usea">{{cite web |url=http://www.esa.int/Our_Activities/Space_Science/Cassini-Huygens/Icy_moon_Enceladus_has_underground_sea |title=Icy moon Enceladus has underground sea |work=ESA |date=April 3, 2014 |access-date=April 30, 2014|archive-date=April 21, 2014|archive-url=https://web.archive.org/web/20140421130347/http://www.esa.int/Our_Activities/Space_Science/Cassini-Huygens/Icy_moon_Enceladus_has_underground_sea|url-status=live}}</ref><ref name="aas.44">{{cite conference|display-authors=0|last1=Tajeddine|first1=R.|last2=Lainey|first2=V.|date=October 2012|title=Mimas and Enceladus: Formation and interior structure from astrometric reduction of Cassini images|bibcode=2012DPS....4411203T|conference=American Astronomical Society, DPS meeting #44, #112.03}}</ref>

Initial mass estimates from the [[Astronomy:Voyager program|Voyager program]] missions suggested that Enceladus was composed almost entirely of water ice.<ref name="Rothery"/> However, based on the effects of Enceladus's [[Physics:Gravity|gravity]] on ''Cassini'', its mass was determined to be much higher than previously thought, yielding a density of 1.61 [[Gram|g]]/cm<sup>3</sup>.<ref name="Porco Helfenstein et al. 2006"/> This density is higher than those of Saturn's other mid-sized icy satellites, indicating that Enceladus contains a greater percentage of [[Chemistry:Silicate|silicate]]s and [[Chemistry:Iron|iron]].

{{harvp|Castillo, Matson et al. |2005}} suggested that Iapetus and the other icy satellites of Saturn formed relatively quickly after the formation of the Saturnian subnebula, and thus were rich in short-lived radionuclides.<ref name="Castillo1">{{Cite journal |last1=Castillo |first1=J. C. |last2=Matson |first2=D. L.|display-authors=0|date=2005 |title=<sup>26</sup>Al in the Saturnian System&nbsp;– New Interior Models for the Saturnian satellites|journal=Eos Trans. AGU|volume=82|issue=52 (Fall Meeting Supplement), abstract P32A–01 |pages=P32A–01 |bibcode=2005AGUFM.P32A..01C |ref={{sfnref|Castillo, Matson et al. |2005}}}}</ref><ref name="Bhatia2017">{{cite journal |last1=Bhatia|first1= G.K. |last2=Sahijpal|first2= S. |title=Thermal evolution of trans-Neptunian objects, icy satellites, and minor icy planets in the early solar system |journal=Meteoritics & Planetary Science |doi=10.1111/maps.12952|volume=52|issue=12|year=2017 |pages=2470–2490 |bibcode=2017M&PS...52.2470B|s2cid= 133957919|doi-access=free}}</ref> These radionuclides, like [[Chemistry:Aluminium#Isotopes|aluminium-26]] and [[Chemistry:Iron#Isotopes|iron-60]], have short half-lives and would produce interior heating relatively quickly. Without the short-lived variety, Enceladus's complement of long-lived radionuclides would not have been enough to prevent rapid freezing of the interior, even with Enceladus's comparatively high rock–mass fraction, given its small size.<ref name="Castillo2">{{cite conference |last1=Castillo |first1=J. C. |last2=Matson |first2=D. L. |display-authors=0 |date=2006 |url=http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2200.pdf |title=A New Understanding of the Internal Evolution of Saturnian Icy Satellites from Cassini Observations |conference=37th Annual Lunar and Planetary Science Conference, Abstract 2200 |access-date=February 2, 2006 |archive-date=September 7, 2009|archive-url=https://web.archive.org/web/20090907191749/http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2200.pdf|url-status=live}}</ref>

Given Enceladus's relatively high rock–mass fraction, the proposed enhancement in <sup>26</sup>Al and <sup>60</sup>Fe would result in a [[Astronomy:Planetary differentiation|differentiated]] body, with an icy [[Earth:Mantle (geology)|mantle]] and a rocky [[Astronomy:Planetary core|core]].<ref name="Bhatia2017"/><ref name="Schubert2007">{{cite journal |doi=10.1016/j.icarus.2006.12.012 |title=Enceladus: Present internal structure and differentiation by early and long-term radiogenic heating |date=2007 |last1=Schubert |first1=G. |last2=Anderson |first2=J. |display-authors=0 |journal=Icarus |volume=188 |issue=2 |pages=345–55 |bibcode=2007Icar..188..345S}}</ref> Subsequent radioactive and [[Astronomy:Tide|tidal]] heating would raise the temperature of the core to 1,000 K, enough to melt the inner mantle. For Enceladus to still be active, part of the core must have also melted, forming [[Biology:Magma|magma]] chambers that would flex under the strain of Saturn's tides. Tidal heating, such as from the resonance with Dione or from [[Astronomy:Libration|libration]], would then have sustained these hot spots in the core and would power the current geological activity.<ref name="Harvard 2008"/><ref name="Matson">{{cite web |last=Matson |first=D. L. |date=2006 |url=http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2219.pdf |display-authors=0 |title=Enceladus' Interior and Geysers&nbsp;– Possibility for Hydrothermal Geometry and N<sub>2</sub> Production|work=37th Annual Lunar and Planetary Science Conference, abstract |page=2219 |access-date=February 2, 2006 |archive-date=March 26, 2009 |archive-url=https://web.archive.org/web/20090326070627/http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2219.pdf |url-status=live}}</ref>

In addition to its mass and modeled [[Chemistry:Geochemistry|geochemistry]], researchers have also examined Enceladus's shape to determine if it is differentiated. {{harvp|Porco, Helfenstein et al. |2006}} used limb measurements to determine that its shape, assuming [[Astronomy:Hydrostatic equilibrium|hydrostatic equilibrium]], is consistent with an undifferentiated interior, in contradiction to the geological and geochemical evidence.<ref name="Porco Helfenstein et al. 2006"/> However, the current shape also supports the possibility that Enceladus is not in hydrostatic equilibrium, and may have rotated faster at some point in the recent past (with a differentiated interior).<ref name="Schubert2007"/> Gravity measurements by ''Cassini'' show that the density of the core is low, indicating that the core contains water in addition to silicates.<ref name="Taubner et al. 2014"/>

====Subsurface water ocean====

[[File:PIA19656-SaturnMoon-Enceladus-Ocean-ArtConcept-20150915.jpg|thumb|An artist's impression of a global subsurface ocean of liquid water<ref name="NASA-20140403" /><ref name="SCI-20140404"/> (updated and better scaled version)]]
Evidence of liquid water on Enceladus began to accumulate in 2005, when scientists observed plumes containing water vapor spewing from its south polar surface,<ref name="Porco Helfenstein et al. 2006"/><ref name="rain">{{cite web|url=http://www.esa.int/Our_Activities/Space_Science/Herschel/Enceladus_rains_water_onto_Saturn|title=Enceladus rains water onto Saturn|work=ESA|date=2011|access-date=January 14, 2015|archive-date=November 23, 2017|archive-url=https://web.archive.org/web/20171123034704/http://www.esa.int/Our_Activities/Space_Science/Herschel/Enceladus_rains_water_onto_Saturn|url-status=live}}</ref> with jets moving 250&nbsp;kg of water vapor every second<ref name="rain"/> at up to {{convert|2189|km/h|mph|abbr=on}} into space.<ref name="news9.9422981">{{cite news|agency=The Associated Press|title=Astronomers find hints of water on Saturn moon|url=http://www.news9.com/story/9422981/astronomers-find-hints-of-water-on-saturn-moon?redirected=true|work=News9.com|date=November 27, 2008|access-date=September 15, 2011|archive-date=March 26, 2012|archive-url=https://web.archive.org/web/20120326175020/http://www.news9.com/story/9422981/astronomers-find-hints-of-water-on-saturn-moon?redirected=true|url-status=live}}</ref> Soon after, in 2006 it was determined that Enceladus's plumes are the source of Saturn's [[Astronomy:Rings of Saturn#E Ring|E Ring]].<ref name="Porco Helfenstein et al. 2006"/><ref name="Spahn"/> The sources of salty particles are uniformly distributed along the tiger stripes, whereas sources of "fresh" particles are closely related to the high-speed gas jets. The "salty" particles are heavier and mostly fall back to the surface, whereas the fast "fresh" particles escape to the E ring, explaining its salt-poor composition of 0.5–2% of sodium salts by mass.<ref name="Postberg2011">{{Cite journal|last1=Postberg|first1=F.|last2=Schmidt|first2=J.|display-authors=0|title=A salt-water reservoir as the source of a compositionally stratified plume on Enceladus|doi=10.1038/nature10175|journal=Nature|volume=474|issue=7353|pages=620–2|year=2011|pmid=21697830|bibcode=2011Natur.474..620P|s2cid=4400807}}</ref>

Gravimetric data from ''Cassini'''s December 2010 flybys showed that Enceladus likely has a liquid water ocean beneath its frozen surface, but at the time it was thought the subsurface ocean was limited to the south pole.<ref name="NASA-20140403">{{cite web |last1=Platt |first1=Jane |last2=Bell |first2=Brian |title=NASA Space Assets Detect Ocean inside Saturn Moon |url=http://www.jpl.nasa.gov/news/news.php?release=2014-103 |work=NASA/JPL |date=April 3, 2014 |access-date=April 3, 2014 |archive-date=April 3, 2014 |archive-url=https://web.archive.org/web/20140403235224/http://www.jpl.nasa.gov/news/news.php?release=2014-103|url-status=live}}</ref><ref name="Witze2014">{{Cite journal |doi=10.1038/nature.2014.14985 |title=Icy Enceladus hides a watery ocean |journal=Nature |date=April 3, 2014 |last1=Witze |first1=A. |s2cid=131145017 |url=http://www.nature.com/news/icy-enceladus-hides-a-watery-ocean-1.14985 |access-date=September 4, 2015 |archive-date=September 1, 2015|archive-url=https://web.archive.org/web/20150901000636/http://www.nature.com/news/icy-enceladus-hides-a-watery-ocean-1.14985|url-status=live}}</ref><ref name="SCI-20140404">{{cite journal |last1=Iess |first1=L. |last2=Stevenson |first2=D. J. |last3=Parisi |first3=M. |last4=Hemingway |first4=D. |last5=Jacobson |first5=R.A. |last6=Lunine |first6=Jonathan I. |last7=Nimmo |first7=F. |last8=Armstrong |first8=J. W. |last9=Asmar |first9=S. W. |last10=Ducci |first10=M. |last11=Tortora |first11=P. |title=The Gravity Field and Interior Structure of Enceladus |date=April 4, 2014 |journal=Science |volume=344 |number=6179 |pages=78–80 |doi=10.1126/science.1250551 |bibcode=2014Sci...344...78I |pmid=24700854 |s2cid=28990283 |url=https://authors.library.caltech.edu/45462/7/Iess-SM.pdf |access-date=July 13, 2019|archive-date=December 2, 2017|archive-url=https://web.archive.org/web/20171202120709/https://authors.library.caltech.edu/45462/7/Iess-SM.pdf |url-status=live }}</ref><ref name="Ocean Apr 2014">{{cite news |last=Amos |first=Jonathan |url=https://www.bbc.com/news/science-environment-26872184 |title=Saturn's Enceladus moon hides 'great lake' of water |work=BBC News |date=April 3, 2014 |access-date=April 7, 2014 |archive-date=February 11, 2021 |archive-url=https://web.archive.org/web/20210211065656/https://www.bbc.com/news/science-environment-26872184 |url-status=live}}</ref> The top of the ocean probably lies beneath a {{convert|30|to|40|km|mi|sp=us}} thick ice shelf. The ocean may be {{convert|10|km|mi|sp=us}} deep at the south pole.<ref name="NASA-20140403"/><ref name="Guardian-20140403">{{cite web |last=Sample|first=Ian|date=April 3, 2014|title=Ocean discovered on Enceladus may be best place to look for alien life |url=https://www.theguardian.com/science/2014/apr/03/ocean-enceladus-alien-life-water-saturn-moon|work=The Guardian|access-date=April 3, 2014|archive-date=April 4, 2014|archive-url=https://web.archive.org/web/20140404042504/http://www.theguardian.com/science/2014/apr/03/ocean-enceladus-alien-life-water-saturn-moon |url-status=live}}</ref>

Measurements of Enceladus's "wobble" as it orbits Saturn—called [[Astronomy:Libration|libration]]—suggests that the entire icy crust is detached from the rocky core and therefore that a global ocean is present beneath the surface.<ref name=global>{{cite web|title=Cassini finds global ocean in Saturn's moon Enceladus|url=http://www.astronomy.com/news/2015/09/cassini-finds-global-ocean-in-saturns-moon-enceladus |author=NASA |website=astronomy.com|date=September 15, 2015|access-date=September 15, 2015|archive-date=September 16, 2015|archive-url=https://web.archive.org/web/20150916232834/http://www.astronomy.com/news/2015/09/cassini-finds-global-ocean-in-saturns-moon-enceladus |url-status=live}}</ref> The amount of libration (0.120° ± 0.014°) implies that this global ocean is about {{convert|26|to|31|km|mi|abbr=off|sp=us}} deep.<ref>{{Cite journal|title=Enceladus's measured physical libration requires a global subsurface ocean|journal=Icarus|doi=10.1016/j.icarus.2015.08.037|first1=P. C.|last1=Thomas|first2=R.|last2=Tajeddine|display-authors=0 |arxiv=1509.07555|bibcode=2016Icar..264...37T|volume=264|pages=37–47|year=2016|s2cid=118429372}}</ref><ref name="NASA-Global">{{cite web |title=Cassini Finds Global Ocean in Saturn's Moon Enceladus|url=http://www.nasa.gov/press-release/cassini-finds-global-ocean-in-saturns-moon-enceladus|access-date=September 17, 2015|work=NASA|date=September 15, 2015|archive-date=September 16, 2015|archive-url=https://web.archive.org/web/20150916190919/http://www.nasa.gov/press-release/cassini-finds-global-ocean-in-saturns-moon-enceladus/|url-status=live}}</ref><ref name=sciam>{{cite web |first=Lee|last=Billings|date=September 16, 2015 |title=Cassini Confirms a Global Ocean on Saturn's Moon Enceladus |url=http://blogs.scientificamerican.com/observations/cassini-confirms-a-global-ocean-on-saturn-s-moon-enceladus/ |access-date=September 17, 2015|work=Scientific American|archive-date=September 16, 2015|archive-url=https://web.archive.org/web/20150916221445/http://blogs.scientificamerican.com/observations/cassini-confirms-a-global-ocean-on-saturn-s-moon-enceladus/|url-status=live}}</ref><ref>{{cite web|title=Under Saturnian moon's icy crust lies a 'global' ocean |url=http://www.news.cornell.edu/stories/2015/09/under-saturnian-moons-icy-crust-lies-global-ocean|work=Cornell Chronicle |publisher=Cornell University|access-date=September 17, 2015 |archive-date=September 19, 2015|archive-url=https://web.archive.org/web/20150919031608/http://www.news.cornell.edu/stories/2015/09/under-saturnian-moons-icy-crust-lies-global-ocean|url-status=live}}</ref> For comparison, Earth's ocean has an average depth of 3.7 kilometers.<ref name=sciam/>

====Composition====
[[File:Enceladus plume molecules.jpg|thumb|The chemical composition of Enceladus's plumes]]
The ''Cassini'' spacecraft flew through the southern plumes on several occasions to sample and analyze its composition. As of 2019, the data gathered is still being analyzed and interpreted. The plumes' salty composition (-Na, -Cl, -CO<sub>3</sub>) indicates that the source is a salty subsurface ocean.<ref name="space.090624">{{cite web|url=http://www.space.com/scienceastronomy/090624-enceladus-ocean.html |title=Ocean Hidden Inside Saturn's Moon|work=Space.com|date=June 24, 2009|access-date=January 14, 2015|archive-date=September 16, 2009 |archive-url=https://web.archive.org/web/20090916074015/http://www.space.com/scienceastronomy/090624-enceladus-ocean.html|url-status=live}}</ref>

The INMS instrument detected mostly [[Physics:Water vapor|water vapor]], as well as traces of molecular [[Chemistry:Nitrogen|nitrogen]], [[Chemistry:Carbon dioxide|carbon dioxide]],<ref name="Waite" /> and trace amounts of simple hydrocarbons such as [[Chemistry:Methane|methane]], [[Chemistry:Propane|propane]], [[Chemistry:Acetylene|acetylene]] and [[Physics:Formaldehyde|formaldehyde]].<ref name="space.5179">{{cite news|last=Mosher|first=Dave |url=http://www.space.com/5179-seeds-life-saturn.html |title=Seeds of Life Found Near Saturn |work=Space.com |date=March 26, 2014|access-date=April 9, 2014|archive-date=April 5, 2014|archive-url=https://web.archive.org/web/20140405071813/http://www.space.com/5179-seeds-life-saturn.html|url-status=live}}</ref><ref name="PR03262008">{{cite web|url=http://www.nasa.gov/mission_pages/cassini/media/cassini-20080326.html|title=Cassini Tastes Organic Material at Saturn's Geyser Moon|work=NASA|date=March 26, 2008|access-date=March 26, 2008|archive-date=July 20, 2021|archive-url=https://web.archive.org/web/20210720225341/https://www.nasa.gov/mission_pages/cassini/media/cassini-20080326.html|url-status=live}}</ref> The plumes' composition, as measured by the INMS, is similar to that seen at most comets.<ref name="PR03262008"/> ''Cassini'' also found traces of simple [[Chemistry:Organic compound|organic compound]]s in some dust grains,<ref name="Postberg2011"/><ref name="esa.037PG">{{cite web |url=http://www.esa.int/esaSC/SEMSZ2037PG_index_0.html|title=''Cassini'' samples the icy spray of Enceladus' water plumes |work=ESA |date=2011|access-date=July 24, 2012|archive-date=August 2, 2012|archive-url=https://web.archive.org/web/20120802134920/http://www.esa.int/esaSC/SEMSZ2037PG_index_0.html|url-status=live}}</ref> as well as larger organics such as benzene ({{chem|C|6|H|6}}),<ref>{{cite journal |title=Neutral Gas Composition of Enceladus' Plume – Model Parameter Insights from Cassini-INMS |journal=Lunar and Planetary Science XLVIII |date=March 24, 2017 |last1=Magee |first1=B. A. | last2=Waite | first2=Jack Hunter Jr. |issue=1964 |page=2974 |bibcode=2017LPI....48.2974M |url=https://www.hou.usra.edu/meetings/lpsc2017/pdf/2974.pdf |access-date=September 16, 2017 |archive-date=August 30, 2021 |archive-url=https://web.archive.org/web/20210830234209/https://www.hou.usra.edu/meetings/lpsc2017/pdf/2974.pdf |url-status=live }}</ref> and [[Chemistry:Organic compound|complex macromolecular organics]] as large as 200 [[Physics:Atomic mass|atomic mass units]],<ref name="Postberg et al. 2018"/><ref name="NASA-20180627">{{cite web |last1=McCartney |first1=Gretchen |last2=Brown |first2=Dwayne |last3=Wendel |first3=JoAnna |last4=Bauer |first4=Markus |title=Complex Organics Bubble up from Enceladus |url=https://www.jpl.nasa.gov/news/news.php?feature=7174 |date=June 27, 2018 |work=NASA/JPL |access-date=June 27, 2018 |archive-date=January 5, 2019 |archive-url=https://web.archive.org/web/20190105232840/https://www.jpl.nasa.gov/news/news.php?feature=7174 |url-status=live }}</ref> and at least 15 carbon atoms in size.<ref>{{cite web |url=https://www.space.com/41005-saturn-moon-enceladus-complex-organic-molecules.html?|title=Saturn Moon Enceladus Is First Alien 'Water World' with Complex Organics|first=Charles Q. |last=Choi|work=Space.com|date=June 27, 2018|access-date=September 6, 2019|archive-date=July 15, 2019|archive-url=https://web.archive.org/web/20190715071821/https://www.space.com/41005-saturn-moon-enceladus-complex-organic-molecules.html |url-status=live}}</ref>

The [[Physics:Mass spectrometer|mass spectrometer]] detected [[Chemistry:Hydrogen|molecular hydrogen]] (H<sub>2</sub>) which was in "thermodynamic disequilibrium" with the other components,<ref>{{Cite news |url=http://www.ndtv.com/world-news/nasa-finds-ingredients-for-life-spewing-out-of-saturns-icy-moon-enceladus-1681326 |title=NASA Finds Ingredients For Life Spewing Out Of Saturn's Icy Moon Enceladus|work=NDTV.com|access-date=April 14, 2017 |archive-date=April 14, 2017|archive-url=https://web.archive.org/web/20170414110958/http://www.ndtv.com/world-news/nasa-finds-ingredients-for-life-spewing-out-of-saturns-icy-moon-enceladus-1681326|url-status=live}}</ref> and found traces of [[Chemistry:Ammonia|ammonia]] ({{chem|NH|3}}).<ref name="jpl.2238"/>

A model suggests that Enceladus's salty ocean (-Na, -Cl, -CO<sub>3</sub>) has an alkaline [[Chemistry:PH|pH]] of 11 to 12.<ref name="pH 2015">{{cite journal|title=The pH of Enceladus' ocean |journal=Geochimica et Cosmochimica Acta |date=April 16, 2015 |last1=Glein |first1=Christopher R. |last2=Baross |first2=John A. |display-authors=0 |doi=10.1016/j.gca.2015.04.017 |arxiv=1502.01946 |bibcode=2015GeCoA.162..202G |volume=162 |pages=202–19 |s2cid=119262254}}</ref><ref name="Glein 2015">{{cite conference|last1=Glein|first1=C. R. |last2=Baross |first2=J. A. |display-authors=0 |title=The chemistry of Enceladus' ocean from a convergence of Cassini data and theoretical geochemistry |url=http://www.hou.usra.edu/meetings/lpsc2015/pdf/1685.pdf |conference=46th Lunar and Planetary Science Conference 2015 |date=March 26, 2015 |access-date=September 27, 2015|archive-date=March 4, 2016|archive-url=https://web.archive.org/web/20160304050237/http://www.hou.usra.edu/meetings/lpsc2015/pdf/1685.pdf|url-status=live}}</ref> The high pH is interpreted to be a consequence of [[Earth:Serpentinite|serpentinization]] of chondritic rock that leads to the generation of H<sub>2</sub>, a geochemical source of energy that could support both abiotic and biological synthesis of organic molecules such as those that have been detected in Enceladus's plumes.<ref name="pH 2015"/><ref name="Wall">{{cite news|last=Wall|first=Mike|url=http://www.space.com/29334-enceladus-ocean-energy-source-life.html|title=Ocean on Saturn Moon Enceladus May Have Potential Energy Source to Support Life |work=Space.com|date=May 7, 2015|access-date=May 8, 2015|archive-date=August 20, 2021|archive-url=https://web.archive.org/web/20210820112512/https://www.space.com/29334-enceladus-ocean-energy-source-life.html|url-status=live}}</ref>

Further analysis in 2019 was done of the spectral characteristics of ice grains in Enceladus's erupting plumes. The study found that nitrogen-bearing and oxygen-bearing [[Chemistry:Amine|amine]]s were likely present, with significant implications for the availability of [[Chemistry:Amino acid|amino acid]]s in the internal ocean. The researchers suggested that the compounds on Enceladus could be precursors for "biologically relevant organic compounds".<ref>{{Cite journal |last1=Khawaja |first1=N. |last2=Postberg |first2=F. |last3=Hillier |first3=J. |last4=Klenner |first4=F. |last5=Kempf |first5=S. |last6=Nölle |first6=L. |last7=Reviol |first7=R. |last8=Zou |first8=Z. |last9=Srama |first9=R. |date=2019-11-11 |title=Low-mass nitrogen-, oxygen-bearing, and aromatic compounds in Enceladean ice grains |journal=Monthly Notices of the Royal Astronomical Society |volume=489 |issue=4 |pages=5231–5243 |doi=10.1093/mnras/stz2280 |issn=0035-8711 |doi-access=free}}</ref><ref>{{Cite web |url=https://www.space.com/saturn-moon-enceladus-organic-compounds-in-plumes.html |title=Organic Compounds Found in Plumes of Saturn's Icy Moon Enceladus |last1=Gohd |first1=Chelsea |website=Space.com |date=October 3, 2019 |access-date=2019-10-03 |archive-date=October 3, 2019 |archive-url=https://web.archive.org/web/20191003205820/https://www.space.com/saturn-moon-enceladus-organic-compounds-in-plumes.html |url-status=live}}</ref>

===Possible heat sources===

[[File:Jet Spots in Tiger Stripes PIA10361.jpg|thumb|Heat map of the south polar fractures, dubbed 'tiger stripes']]During the flyby of July 14, 2005, the Composite Infrared Spectrometer (CIRS) found a warm region near the south pole. Temperatures in this region ranged from 85 to 90 K, with small areas showing as high as {{convert|157|K|°C|0|abbr=on}}, much too warm to be explained by solar heating, indicating that parts of the south polar region are heated from the interior of Enceladus.<ref name="Spencer" /> The presence of a subsurface ocean under the south polar region is now accepted,<ref name="Showman 2013">{{cite journal|title=The effect of an asymmetric core on convection in Enceladus' ice shell: Implications for south polar tectonics and heat flux <!-- original "in press" title: The hemispheric dichotomy of surface tectonics and heat flux on Enceladus -->|journal=Geophysical Research Letters| display-authors=0| last1=Showman| first1=Adam P.| last2=Han| first2=Lijie| bibcode=2013GeoRL..40.5610S<!-- original "in press" url: http://www.lpl.arizona.edu/~showman/publications/enceltopo.pdf -->| date=November 2013| volume=40|issue=21 |pages=5610–14 |doi= 10.1002/2013GL057149| citeseerx=10.1.1.693.2896|s2cid=52406337 }}</ref> but it cannot explain the source of the heat, with an estimated heat flux of 200&nbsp;mW/m<sup>2</sup>, which is about 10 times higher than that from radiogenic heating alone.<ref name="heat flux 2016">{{cite conference |last1=Kamata |first1=S. |last2=Nimmo |first2=F. |title=INTERIOR THERMAL STATE OF ENCELADUS INFERRED FROM THE VISCOELASTIC STATE OF ITS ICY SHELL |url=http://www.hou.usra.edu/meetings/lpsc2016/pdf/1097.pdf |conference=47th Lunar and Planetary Science Conference |publisher=Lunar and Planetary Institute |date=March 21, 2016 |access-date=May 4, 2016 |archive-date=March 25, 2017 |archive-url=https://web.archive.org/web/20170325214337/http://www.hou.usra.edu/meetings/lpsc2016/pdf/1097.pdf |url-status=live}}</ref>

Several explanations for the observed elevated temperatures and the resulting plumes have been proposed, including venting from a subsurface reservoir of liquid water, [[Sublimation (phase transition)|sublimation]] of ice,<ref name="DPS meeting #45">{{cite journal|title=Enceladus Near-Fissure Surface Temperatures|journal=American Astronomical Society|volume=45|pages=416.01|date=2013|last1=Howell|first1=Robert R. |last2=Goguen |first2=J. D.|display-authors=0|bibcode=2013DPS....4541601H}}</ref> decompression and dissociation of clathrates, and shear heating,<ref name="spencer 2014">{{cite conference |last1=Abramov |first1=O. |last2=Spencer |first2=John R. |title=New Models of Endogenic Heat from Enceladus' South Polar Fractures |url=http://www.hou.usra.edu/meetings/lpsc2014/eposter/2878.pdf |conference=45th Lunar and Planetary Science Conference 2014 |publisher=LPSC |date=March 17–21, 2014 |access-date=April 10, 2014 |archive-date=April 13, 2014 |archive-url=https://web.archive.org/web/20140413125607/http://www.hou.usra.edu/meetings/lpsc2014/eposter/2878.pdf |url-status=live }}</ref> but a complete explanation of all the heat sources causing the observed thermal power output of Enceladus has not yet been settled.

Heating in Enceladus has occurred through various mechanisms ever since its formation. [[Physics:Radioactive decay|Radioactive decay]] in its core may have initially heated it,<ref name="A Hot Start on Enceladus">{{cite web |url=http://www.astrobio.net/pressrelease/2269/a-hot-start-on-enceladus |title=A Hot Start on Enceladus |work=Astrobio.net |access-date=March 21, 2010 |date=March 14, 2007 |archive-date=May 28, 2020 |archive-url=https://web.archive.org/web/20200528215014/https://www.astrobio.net/enceladus/a-hot-start-on-enceladus/ |url-status=dead}}</ref> giving it a warm core and a subsurface ocean, which is now kept above freezing through unidentified mechanisms. Geophysical models indicate that [[Astronomy:Tidal heating|tidal heating]] is a main heat source, perhaps aided by radioactive decay and some [[Physics:Exothermic reaction|heat-producing chemical reactions]].<ref name="15 gigawatts">{{cite news |url=http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20110307.html |title=Cassini Finds Enceladus is a Powerhouse |work=NASA |date=March 7, 2011 |access-date=April 7, 2014 |archive-date=April 6, 2013 |archive-url=https://web.archive.org/web/20130406151941/http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20110307.html |url-status=live }}</ref><ref name="sd.001237">{{cite journal |title=Non-steady state tidal heating of Enceladus |journal=Icarus |volume=235 |pages=75–85 |date=March 14, 2014 |last1=Shoji |first1=D. |last2=Hussmann |first2=H. |display-authors=0 |doi=10.1016/j.icarus.2014.03.006 |bibcode=2014Icar..235...75S }}</ref><ref name="areps.41">{{cite journal |title=Enceladus: An Active Ice World in the Saturn System |journal=Annual Review of Earth and Planetary Sciences |date=May 2013 |last1=Spencer |first1=John R. |last2=Nimmo |first2=Francis |s2cid=140646028 |volume=41 |pages=693–717 |doi=10.1146/annurev-earth-050212-124025 |bibcode=2013AREPS..41..693S }}</ref><ref name="icarus.226.1">{{cite journal |title=Impact of tidal heating on the onset of convection in Enceladus's ice shell |journal=Icarus |date=September–October 2013 |last1=Běhounková |first1=Marie |last2=Tobie |first2=Gabriel |display-authors=0 |volume=226 |issue=1 |pages=898–904 |doi=10.1016/j.icarus.2013.06.033 |bibcode=2013Icar..226..898B }}</ref> A 2007 study predicted the internal heat of Enceladus, if generated by tidal forces, could be no greater than 1.1 gigawatts,<ref name="EPSC 2013" /> but data from ''Cassini's'' infrared spectrometer of the south polar terrain over 16 months, indicate that the internal heat generated power is about 4.7 gigawatts,<ref name="EPSC 2013">{{cite conference |last=Spencer |first=John R. |title=Enceladus Heat Flow from High Spatial Resolution Thermal Emission Observations |url=http://meetingorganizer.copernicus.org/EPSC2013/EPSC2013-840-1.pdf |conference=European Planetary Science Congress 2013 |publisher=EPSC Abstracts |date=2013 |access-date=April 7, 2014 |archive-date=April 8, 2014 |archive-url=https://web.archive.org/web/20140408230424/http://meetingorganizer.copernicus.org/EPSC2013/EPSC2013-840-1.pdf |url-status=live }}</ref> and suggest that it is in thermal equilibrium.<ref name="Spencer" /><ref name="VIMS_PR" /><ref name="nature.7163">{{cite journal |last1=Spitale |first1=J. N. |last2=Porco |first2=Carolyn C. |title=Association of the jets of Enceladus with the warmest regions on its south-polar fractures |journal=Nature |date=2007 |volume=449 |issue=7163 |pages=695–7 |doi=10.1038/nature06217 |pmid=17928854 |bibcode=2007Natur.449..695S |s2cid=4401321 }}</ref>

The observed power output of 4.7 gigawatts is challenging to explain from tidal heating alone, so the main source of heat remains a mystery.<ref name="Porco Helfenstein et al. 2006"/><ref name="15 gigawatts"/> Most scientists think the observed heat flux of Enceladus is not enough to maintain the subsurface ocean, and therefore any subsurface ocean must be a remnant of a period of higher eccentricity and tidal heating, or the heat is produced through another mechanism.<ref name="icarus.188.2">{{cite journal |last1=Meyer |first1=J. |last2=Wisdom |first2=Jack |title=Tidal heating in Enceladus |journal=Icarus |date=2007 |volume=188 |issue=2 |pages=535–9 |doi=10.1016/j.icarus.2007.03.001 |bibcode=2007Icar..188..535M |citeseerx=10.1.1.142.9123}}</ref><ref name="Roberts2008"/>

====Tidal heating====
[[Astronomy:Tidal heating|Tidal heating]] occurs through the tidal friction processes: orbital and rotational energy are dissipated as heat in the crust of an object. In addition, to the extent that tides produce heat along fractures, [[Astronomy:Libration|libration]] may affect the magnitude and distribution of such tidal shear heating.<ref name="Harvard 2008"/> Tidal dissipation of Enceladus's ice crust is significant because Enceladus has a subsurface ocean. A computer simulation that used data from ''Cassini'' was published in November 2017, and it indicates that friction heat from the sliding rock fragments within the permeable and fragmented core of Enceladus could keep its underground ocean warm for up to billions of years.<ref>{{cite web |url=https://www.space.com/38679-saturn-moon-enceladus-warm-churning-insides.html|title=Saturn Moon Enceladus' Churning Insides May Keep Its Ocean Warm|first=Charles Q.|last=Choi |work=Space.com|date=November 6, 2017|access-date=September 6, 2019 |archive-date=July 31, 2020 |archive-url=https://web.archive.org/web/20200731010505/https://www.space.com/38679-saturn-moon-enceladus-warm-churning-insides.html|url-status=live}}</ref><ref>{{cite web |author=European Space Agency |url=https://phys.org/news/2017-11-ocean-moon-enceladus-billions-years.html |title=Heating ocean moon Enceladus for billions of years |work=Phys.Org |date=November 6, 2017 |archive-url=https://web.archive.org/web/20171108035537/https://phys.org/news/2017-11-ocean-moon-enceladus-billions-years.html |archive-date=November 8, 2017 |url-status=live}}</ref><ref>{{cite journal |doi=10.1038/s41550-017-0289-8 |volume=1 |title=Powering prolonged hydrothermal activity inside Enceladus |year=2017 |journal=Nature Astronomy |pages=841–847 |last1=Choblet |first1=Gaël |last2=Tobie |first2=Gabriel |last3=Sotin |first3=Christophe |last4=Běhounková |first4=Marie |last5=Čadek |first5=Ondřej |last6=Postberg |first6=Frank |last7=Souček |first7=Ondřej| issue=12 |bibcode=2017NatAs...1..841C |s2cid=134008380}}</ref> It is thought that if Enceladus had a more eccentric orbit in the past, the enhanced tidal forces could be sufficient to maintain a subsurface ocean, such that a periodic enhancement in [[Astronomy:Orbital eccentricity|eccentricity]] could maintain a subsurface ocean that periodically changes in size.<ref name="Roberts2008">{{cite journal |last1=Roberts |first1=J. H. |last2=Nimmo |first2=Francis |title=Tidal heating and the long-term stability of a subsurface ocean on Enceladus |journal=Icarus |date=2008 |volume=194 |issue=2 |pages=675–689 |doi=10.1016/j.icarus.2007.11.010 |bibcode=2008Icar..194..675R}}</ref>

A 2016 analysis claimed that "a model of the tiger stripes as tidally flexed slots that puncture the ice shell can simultaneously explain the persistence of the eruptions through the tidal cycle, the phase lag, and the total power output of the tiger stripe terrain, while suggesting that eruptions are maintained over geological timescales."<ref name="Kite 3972–3975"/> Previous models suggest that resonant perturbations of Dione could provide the necessary periodic eccentricity changes to maintain the subsurface ocean of Enceladus, if the ocean contains a substantial amount of [[Chemistry:Ammonia|ammonia]].<ref name="Porco Helfenstein et al. 2006"/> The surface of Enceladus indicates that the entire moon has experienced periods of enhanced heat flux in the past.<ref name="grl.052736">{{cite journal |last1=Bland |first1=M. T. |last2=Singer |first2=Kelsi N. |s2cid=54889900 |display-authors=0 |title=Enceladus' extreme heat flux as revealed by its relaxed craters |journal=Geophysical Research Letters |date=2012 |volume=39 |issue=17 |pages=n/a |doi=10.1029/2012GL052736 |bibcode=2012GeoRL..3917204B |doi-access=free}}</ref>

====Radioactive heating====
The "hot start" model of heating suggests Enceladus began as ice and rock that contained rapidly decaying short-lived radioactive isotopes of [[Physics:Aluminium-26|aluminium]], [[Physics:Iron-60|iron]] and [[Physics:Manganese-60|manganese]]. Enormous amounts of heat were then produced as these isotopes decayed for about 7 million years, resulting in the consolidation of rocky material at the core surrounded by a shell of ice. Although the heat from radioactivity would decrease over time, the combination of radioactivity and tidal forces from Saturn's gravitational tug could prevent the subsurface ocean from freezing.<ref name="A Hot Start on Enceladus"/>

The present-day radiogenic heating rate is 3.2{{E-sp|15}} ergs/s (or 0.32 gigawatts), assuming Enceladus has a composition of ice, iron and silicate materials.<ref name="Porco Helfenstein et al. 2006"/> Heating from long-lived radioactive isotopes [[Physics:Uranium-238|uranium]]-238, [[Physics:Uranium-235|uranium-235]], [[Physics:Thorium-232|thorium]]-232 and [[Physics:Potassium-40|potassium]]-40 inside Enceladus would add 0.3 gigawatts to the observed heat flux.<ref name="15 gigawatts"/> The presence of Enceladus's regionally thick subsurface ocean suggests a heat flux ~10 times higher than that from radiogenic heating in the silicate core.<ref name="perry2846"/>

==== Chemical factors ====
Because no [[Chemistry:Ammonia|ammonia]] was initially found in the vented material by [[Astronomy:Cassini–Huygens#Instruments|INMS]] or UVIS, which could act as an antifreeze, it was thought such a heated, pressurized chamber would consist of nearly pure liquid water with a temperature of at least {{convert|270|K|°C|0|abbr=on}}, because pure water requires more [[Physics:Energy|energy]] to melt.

In July 2009 it was announced that traces of ammonia had been found in the plumes during [[Engineering:Planetary flyby|flyby]]s in July and October 2008.<ref name="jpl.2238">{{cite web|url=http://www.jpl.nasa.gov/news/features.cfm?feature=2238|title=Saturnian Moon Shows Evidence of Ammonia|work=NASA/JPL|date=July 22, 2009|access-date=March 21, 2010|archive-date=June 22, 2010|archive-url=https://web.archive.org/web/20100622101501/http://www.jpl.nasa.gov/news/features.cfm?feature=2238|url-status=live}}</ref><ref name="NAT-20090723-JHW">{{cite journal | last1=Waite | first1=Jack Hunter Jr. |last2=Lewis |first2=W. S. |title=Liquid water on Enceladus from observations of ammonia and 40 Ar in the plume |date=July 23, 2009 |journal=Nature |volume=460 |issue=7254 |pages=487–490 |doi=10.1038/nature08153 |display-authors=0 |bibcode=2009Natur.460..487W |s2cid=128628128 }}</ref> Reducing the freezing point of water with [[Chemistry:Ammonia|ammonia]] would also allow for outgassing and higher gas pressure,<ref name="ucl.84452">{{cite journal|last=Fortes|first=A. D.|date=2007|url=http://discovery.ucl.ac.uk/84452/|title=Metasomatic clathrate xenoliths as a possible source for the south polar plumes of Enceladus|journal=Icarus|volume=191|issue=2|pages=743–8|doi=10.1016/j.icarus.2007.06.013|bibcode=2007Icar..191..743F|access-date=April 8, 2014|archive-url=https://web.archive.org/web/20170323185017/http://discovery.ucl.ac.uk/84452/|archive-date=March 23, 2017|url-status=dead}}</ref> and less heat required to power the water plumes.<ref name="ammonia">{{cite journal|title=Ammonia clathrate hydrates as new solid phases for Titan, Enceladus, and other planetary systems|journal=Proceedings of the National Academy of Sciences of the USA|date=September 11, 2012|last1=Shin|first1=Kyuchul|last2=Kumar|first2=Rajnish|display-authors=0|volume=109|issue=37|pages=14785–90|doi=10.1073/pnas.1205820109|pmid=22908239|bibcode=2012PNAS..10914785S|pmc=3443173|doi-access=free}}</ref> The subsurface layer heating the surface water ice could be an ammonia–water slurry at temperatures as low as {{convert|170|K|°C|0|abbr=on}}, and thus less energy is required to produce the plume activity. However, the observed 4.7 gigawatts heat flux is enough to power the cryovolcanism without the presence of ammonia.<ref name="EPSC 2013"/><ref name="ammonia" />

==Origin==

===Mimas–Enceladus paradox===
Mimas, the innermost of the round moons of Saturn and directly interior to Enceladus, is a geologically dead body, even though it should experience stronger tidal forces than Enceladus. This apparent paradox can be explained in part by temperature-dependent properties of water ice (the main constituent of the interiors of Mimas and Enceladus). The tidal heating per unit mass is given by the formula

:<math>q_{tid}=\frac{63\rho n^{5} r^{4} e^{2}}{38\mu Q},</math>

where ''ρ'' is the (mass) density of the satellite, ''n'' is its mean orbital motion, ''r'' is the satellite's radius, ''e'' is the [[Astronomy:Orbital eccentricity|orbital eccentricity]] of the satellite, ''μ'' is the [[Physics:Shear modulus|shear modulus]] and ''Q'' is the dimensionless [[Physics:Dissipation factor|dissipation factor]]. For a same-temperature approximation, the expected value of ''q<sub>tid</sub>'' for Mimas is about 40 times that of Enceladus. However, the material parameters ''μ'' and ''Q'' are temperature dependent. At high temperatures (close to the melting point), ''μ'' and ''Q'' are low, so tidal heating is high. Modeling suggests that for Enceladus, both a 'basic' low-energy thermal state with little internal temperature gradient, and an 'excited' high-energy thermal state with a significant temperature gradient, and consequent convection (endogenic geologic activity), once established, would be stable.<ref name="asr.013"/>

For Mimas, only a low-energy state is expected to be stable, despite its being closer to Saturn. So the model predicts a low-internal-temperature state for Mimas (values of ''μ'' and ''Q'' are high) but a possible higher-temperature state for Enceladus (values of ''μ'' and ''Q'' are low).<ref name="asr.013">{{cite journal|last=Czechowski|first=Leszek|date=2006|title=Parameterized model of convection driven by tidal and radiogenic heating|journal=Advances in Space Research|volume=38|pages=788–93|doi=10.1016/j.asr.2005.12.013|bibcode=2006AdSpR..38..788C|issue=4}}</ref> Additional historical information is needed to explain how Enceladus first entered the high-energy state (e.g. more radiogenic heating or a more eccentric orbit in the past).<ref name="aj.752">{{cite journal|title=Strong tidal dissipation in Saturn and constraints on Enceladus' thermal state from astrometry|journal=The Astrophysical Journal|date=May 22, 2012|last1=Lainey|first1=Valery|last2=Karatekin|first2=Ozgur|display-authors=0|volume=752|issue=1|page=14|doi=10.1088/0004-637X/752/1/14|arxiv=1204.0895|bibcode=2012ApJ...752...14L|s2cid=119282486}}</ref>

The significantly higher density of Enceladus relative to Mimas (1.61 vs. 1.15 g/cm<sup>3</sup>), implying a larger content of rock and more radiogenic heating in its early history, has also been cited as an important factor in resolving the Mimas paradox.<ref name="Cowen2006">{{cite journal|last=Cowen|first=Ron|title=The Whole Enceladus: A new place to search for life in the outer solar system|journal=Science News|volume=169|issue=15|pages=282–284|date=April 15, 2006|url=http://www.phschool.com/science/science_news/articles/whole_enceladus.html|access-date=April 8, 2014|doi=10.2307/4019332|jstor=4019332|archive-date=April 8, 2014|archive-url=https://web.archive.org/web/20140408215644/http://www.phschool.com/science/science_news/articles/whole_enceladus.html|url-status=live}}</ref>

It has been suggested that for an icy satellite the size of Mimas or Enceladus to enter an 'excited state' of tidal heating and convection, it would need to enter an orbital resonance before it lost too much of its primordial internal heat. Because Mimas, being smaller, would cool more rapidly than Enceladus, its window of opportunity for initiating orbital resonance-driven convection would have been considerably shorter.<ref name="Czechowski2014"/>

===Proto-Enceladus hypothesis===
Enceladus is losing mass at a rate of 200&nbsp;kg/second. If mass loss at this rate continued for 4.5 Gyr, the satellite would have lost approximately 30% of its initial mass. A similar value is obtained by assuming that the initial densities of Enceladus and Mimas were equal.<ref name="Czechowski2014">{{Cite journal|doi=10.1016/j.pss.2014.09.010|title=Some remarks on the early evolution of Enceladus|journal=Planetary and Space Science|volume=104|pages=185–99|date=December 2014|last=Czechowski|first=L.|bibcode=2014P&SS..104..185C}}</ref> It suggests that tectonics in the south polar region is probably mainly related to subsidence and associated subduction caused by the process of mass loss.<ref>Czechowski L. (2015) Mass loss as a driving mechanism of tectonics of Enceladus. 46th Lunar and Planetary Science Conference 2030.pdf.</ref>

===Date of formation===
In 2016, a study of how the orbits of Saturn's moons should have changed due to tidal effects suggested that all of Saturn's satellites inward of Titan, including Enceladus (whose geologic activity was used to derive the strength of tidal effects on Saturn's satellites), may have formed as little as 100 million years ago.<ref>{{cite web|url=http://www.astronomy.com/news/2016/03/moons-of-saturn-may-be-younger-than-the-dinosaurs|title=Moons of Saturn may be younger than the dinosaurs|date=March 25, 2016|author=SETI Institute |website=astronomy.com |access-date=March 30, 2016|archive-date=December 6, 2019|archive-url=https://web.archive.org/web/20191206163235/https://astronomy.com/news/2016/03/moons-of-saturn-may-be-younger-than-the-dinosaurs|url-status=live}}</ref>
A later study from 2019 estimated that the ocean is around one billion years old.<ref name="Anderson 2019">{{cite web |last=Anderson |first=Paul Scott |title=Enceladus' ocean right age to support life |website=EarthSky |date=2019-07-17 |url=https://earthsky.org/space/enceladus-ocean-moon-habitability-right-age-to-support-life |access-date=2020-12-27 |archive-date=January 18, 2021 |archive-url=https://web.archive.org/web/20210118210418/https://earthsky.org/space/enceladus-ocean-moon-habitability-right-age-to-support-life |url-status=live }}</ref>

=== Potential habitability ===
Enceladus ejects plumes of salted water laced with grains of silica-rich sand,<ref name="Tobie 2015">{{cite journal |title=Planetary science: Enceladus' hot springs |journal=Nature |date=March 12, 2015 |last=Tobie |first=Gabriel |volume=519 |issue=7542 |doi=10.1038/519162a|bibcode = 2015Natur.519..162T |pmid=25762276 |pages=162–3|s2cid=205084413 |doi-access=free }}</ref> nitrogen (in ammonia),<ref name="McKay 2014" /> and organic molecules, including trace amounts of simple hydrocarbons such as [[Chemistry:Methane|methane]] ({{chem|C|H|4}}), [[Chemistry:Propane|propane]] ({{chem|C|3|H|8}}), [[Chemistry:Acetylene|acetylene]] ({{chem|C|2|H|2}}) and [[Physics:Formaldehyde|formaldehyde]] ({{chem|C|H|2|O}}), which are carbon-bearing molecules.<ref name="space.5179"/><ref name="PR03262008"/><ref name="Space.com">{{cite web |url=http://www.space.com/29334-enceladus-ocean-energy-source-life.html |title=Ocean on Saturn Moon Enceladus May Have Potential Energy Source to Support Life |work=Space.com |date=May 7, 2015 |access-date=August 15, 2015 |author=Wall, Mike |archive-date=August 20, 2021 |archive-url=https://web.archive.org/web/20210820112512/https://www.space.com/29334-enceladus-ocean-energy-source-life.html |url-status=live}}</ref> This indicates that hydrothermal activity —an energy source— may be at work in Enceladus's subsurface ocean.<ref name="Tobie 2015"/><ref name="hydrothermal vents">{{cite web |url=http://news.discovery.com/space/alien-life-exoplanets/potentially-life-giving-hydrothermal-activity-revealed-on-enceladus-150312.htm |title=Enceladus Has Potentially Life-Giving Hydrothermal Activity |work=Discovery News |date=March 12, 2015 |access-date=August 15, 2015 |author=O' Neill, Ian |archive-date=September 1, 2015 |archive-url=https://web.archive.org/web/20150901010629/http://news.discovery.com/space/alien-life-exoplanets/potentially-life-giving-hydrothermal-activity-revealed-on-enceladus-150312.htm |url-status=live}}</ref> Models indicate<ref>{{cite journal |doi=10.1016/j.pss.2014.09.010 |title=Some remarks on the early evolution of Enceladus |year=2014 |last1=Czechowski |first1=Leszek |journal=Planetary and Space Science |volume=104 |pages=185–199 |bibcode=2014P&SS..104..185C}}</ref> that the large rocky [[Astronomy:Planetary core|core]] is porous, allowing water to flow through it, transferring heat and chemicals. It was confirmed by observations and other research.<ref name="Spotts">{{cite news |last=Spotts |first=Peter |url=http://www.csmonitor.com/Science/2015/0916/Proposed-NASA-mission-to-Saturn-moon-If-there-s-life-we-ll-find-it |title=Proposed NASA mission to Saturn moon: If there's life, we'll find it |work=The Christian Science Monitor |date=September 16, 2015 |access-date=September 27, 2015 |archive-date=September 26, 2015 |archive-url=https://web.archive.org/web/20150926033459/http://www.csmonitor.com/Science/2015/0916/Proposed-NASA-mission-to-Saturn-moon-If-there-s-life-we-ll-find-it |url-status=live }}</ref><ref name="Taubner EPSC 2015">{{cite conference |last1=Taubner |first1=R.-S. |last2=Leitner |first2=J. J. |last3=Firneis |first3=M. G. |last4=Hitzenberger |first4=R. |title=Including Cassini's Gravity Measurements from the Flybys E9, E12, E19 into Interior Structure Models of Enceladus |url=http://meetingorganizer.copernicus.org/EPSC2014/EPSC2014-676.pdf |conference=European Planetary Science Congress 2014 |publisher=EPSC Abstracts |date=September 7, 2014 |access-date=September 27, 2015 |archive-date=September 28, 2015 |archive-url=https://web.archive.org/web/20150928140602/http://meetingorganizer.copernicus.org/EPSC2014/EPSC2014-676.pdf |url-status=live }}</ref><ref>{{Cite web |url=http://meetingorganizer.copernicus.org/EGU2014/EGU2014-9492-1.pdf |title=Czechowski (2014). Enceladus: a cradle of life of the Solar System? Geophysical Research Abstracts Vol. 16, EGU2014-9492-1 |access-date=August 2, 2017 |archive-date=February 13, 2015 |archive-url=https://web.archive.org/web/20150213091102/http://meetingorganizer.copernicus.org/EGU2014/EGU2014-9492-1.pdf |url-status=live }}</ref> Molecular hydrogen ({{chem|H|2}}), a geochemical source of energy that can be [[Biology:Metabolism|metabolized]] by [[Biology:Methanogen|methanogen]] microbes to provide energy for life, could be present if, as models suggest, Enceladus's salty ocean has an alkaline pH from [[Earth:Serpentinite|serpentinization]] of chondritic rock.<ref name="pH 2015"/><ref name="Glein 2015"/><ref name="Wall"/>

The presence of an internal global salty ocean with an aquatic environment supported by global ocean circulation patterns,<ref name="Spotts"/> with an energy source and complex organic compounds<ref name="Postberg et al. 2018"/> in contact with Enceladus's rocky core,<ref name="Witze2014"/><ref name="SCI-20140404" /><ref name="MckayPR">{{cite web |url=http://www.nasa.gov/mission_pages/cassini/media/enceladus-f20080326.html |title=A Perspective on Life on Enceladus: A World of Possibilities |work=NASA |date=March 26, 2008 |access-date=September 15, 2011 |archive-date=September 15, 2011 |archive-url=https://web.archive.org/web/20110915162558/http://www.nasa.gov/mission_pages/cassini/media/enceladus-f20080326.html |url-status=live }}</ref> may advance the study of [[Astronomy:Astrobiology|astrobiology]] and the study of potentially [[Astronomy:Planetary habitability|habitable environments]] for microbial [[Astronomy:Extraterrestrial life|extraterrestrial life]].<ref name="NASA-20140403"/><ref name="Ocean Apr 2014"/><ref name="Guardian-20140403"/><ref name=guardian>{{cite web|last1=McKie|first1=Robin|title=Enceladus: home of alien lifeforms?|url=https://www.theguardian.com/science/2012/jul/29/alien-life-enceladus-saturn-moon|work=The Guardian|access-date=August 16, 2015 |date=July 29, 2012|archive-date=September 2, 2015|archive-url=https://web.archive.org/web/20150902011725/http://www.theguardian.com/science/2012/jul/29/alien-life-enceladus-saturn-moon |url-status=live}}</ref><ref name="Discover Magazine">{{cite web |url=http://blogs.discovermagazine.com/crux/2015/03/12/enceladus-oceans-life/ |title=Warm Oceans on Saturn's Moon Enceladus Could Harbor Life |work=Discover Magazine |date=March 12, 2015 |access-date=August 15, 2015 |author=Coates, Andrew |archive-date=March 13, 2015 |archive-url=https://web.archive.org/web/20150313104945/http://blogs.discovermagazine.com/crux/2015/03/12/enceladus-oceans-life/ |url-status=live}}</ref><ref>{{cite journal |url=http://web.gps.caltech.edu/~jkirschvink/pdfs/Enceladus_OLEB_online_published_paper.pdf |archive-url=https://web.archive.org/web/20180614085929/http://web.gps.caltech.edu/~jkirschvink/pdfs/Enceladus_OLEB_online_published_paper.pdf |doi=10.1007/s11084-008-9135-4|archive-date=2018-06-14 |title=Habitability of Enceladus: Planetary Conditions for Life |year=2008 |last1=Parkinson |first1=Christopher D. |last2=Liang |first2=Mao-Chang |last3=Yung |first3=Yuk L. |last4=Kirschivnk |first4=Joseph L. |journal=Origins of Life and Evolution of Biospheres |volume=38 |issue=4 |pages=355–369 |pmid=18566911 |bibcode=2008OLEB...38..355P |s2cid=15416810 }}</ref> Geochemical modeling results concerning not-yet-detected [[Chemistry:Phosphorus|phosphorus]] indicate the moon meets potential [[Biology:Abiogenesis|abiogenesis]]-requirements.<ref>{{cite news |title=Ocean on Saturn's moon Enceladus could be rich in a key ingredient for life |url=https://physicsworld.com/a/ocean-on-saturns-moon-enceladus-could-be-rich-in-a-key-ingredient-for-life/ |access-date=20 October 2022 |work=Physics World |date=11 October 2022}}</ref><ref>{{cite journal |last1=Hao |first1=Jihua |last2=Glein |first2=Christopher R. |last3=Huang |first3=Fang |last4=Yee |first4=Nathan |last5=Catling |first5=David C. |last6=Postberg |first6=Frank |last7=Hillier |first7=Jon K. |last8=Hazen |first8=Robert M. |title=Abundant phosphorus expected for possible life in Enceladus's ocean |journal=Proceedings of the National Academy of Sciences |date=27 September 2022 |volume=119 |issue=39 |pages=e2201388119 |doi=10.1073/pnas.2201388119 |pmid=36122219 |pmc=9522369 |bibcode=2022PNAS..11901388H |issn=0027-8424}}</ref> However, phosphates have been detected from a cryovolcanic plume detected by Cassini and is discussed in a paper in the June 14, 2023 issue of Nature entitled "Detection of Phosphates Originating From Enceladus's Ocean".<ref name="NAT-20230614fp">{{Cite journal |last1=Postberg |first1=Frank |last2=Sekine |first2=Yasuhito |last3=Klenner |first3=Fabian |last4=Glein |first4=Christopher |last5=Zou |first5=Zenghui |last6=Abel |first6=Bernd |last7=Furuya |first7=Kento |last8=Hillier |first8=Jon |last9=Khawaja |first9=Nozair |last10=Kempf |first10=Sascha |last11=Noelle |first11=Lenz |last12=Saito |first12=Takuya |last13=Schmidt |first13=Juergen |last14=Shibuya |first14=Takazo |last15=Srama |first15=Ralf |date=June 14, 2023 |title=Detection of phosphates originating from Enceladus's ocean |journal=Nature |volume=618 |issue=7965 |pages=489–493 |doi=10.1038/s41586-023-05987-9 |pmid=37316718 |pmc=10266972 |bibcode=2023Natur.618..489P |s2cid=259157087}}</ref>

The presence of a wide range of organic compounds and ammonia indicates their source may be similar to the water/rock reactions known to occur on Earth and that are known to support life.<ref name="NASA strategy 2015">{{cite web| url=https://nai.nasa.gov/media/medialibrary/2015/10/NASA_Astrobiology_Strategy_2015_151008.pdf| access-date=September 26, 2017| title=NASA Astrobiology Strategy| year=2015| work=NASA| archive-url=https://web.archive.org/web/20161222190306/https://nai.nasa.gov/media/medialibrary/2015/10/NASA_Astrobiology_Strategy_2015_151008.pdf| archive-date=December 22, 2016}}</ref> Therefore, several robotic missions have been proposed to further explore Enceladus and assess its habitability. Some of the proposed missions are: [[Astronomy:Journey to Enceladus and Titan|Journey to Enceladus and Titan]] (JET), [[Astronomy:Enceladus Explorer|Enceladus Explorer]] (En-Ex), [[Astronomy:Enceladus Life Finder|Enceladus Life Finder]] (ELF), [[Astronomy:Life Investigation For Enceladus|Life Investigation For Enceladus]] (LIFE), and [[Astronomy:Enceladus Life Signatures and Habitability|Enceladus Life Signatures and Habitability]] (ELSAH).

In June 2023, astronomers reported that the presence of phosphates on Enceladus has been detected, completing the discovery of all the basic chemical ingredients for life on the moon.<ref name="NAT-20230614fp"/><ref name="NYT-20230614km">{{cite news |last=Miller |first=Katrina |title=A 'Soda Ocean' on a Moon of Saturn Has All the Ingredients for Life |quote=Using data from the Cassini spacecraft, scientists discovered the presence of phosphates on icy Enceladus. |date=14 June 2023 |work={{wipe|The New York Times}} |url=https://www.nytimes.com/2023/06/14/science/enceladus-phosphorus-life.html |access-date=15 June 2023 |url-status=live |archive-date=14 June 2023 |archive-url=https://archive.today/20230614152021/https://www.nytimes.com/2023/06/14/science/enceladus-phosphorus-life.html}}</ref>

On 14 December 2023, astronomers reported the first time discovery, in the [[Physics:Plume (fluid dynamics)|plume]]s of Enceladus, of [[Chemistry:Hydrogen cyanide|hydrogen cyanide]], a possible chemical essential for life as we know it, as well as other organic molecules, some of which are yet to be better identified and understood. According to the researchers, "these [newly discovered] compounds could potentially support extant [[Biology:Microorganism|microbial communities]] or drive complex [[Chemistry:Organic synthesis|organic synthesis]] leading to the origin of life."<ref name="NYT-20231214kc">{{cite news |last=Chang |first=Kenneth |title=Poison Gas Hints at Potential for Life on an Ocean Moon of Saturn |quote=A researcher who has studied the icy world said “the prospects for the development of life are getting better and better on Enceladus.” |url=https://www.nytimes.com/2023/12/14/science/enceladus-moon-cyanide-life-saturn.html |date=14 December 2023 |work={{wipe|The New York Times}} |url-status=live |url-access=subscription |archive-url=https://archive.today/20231214210144/https://www.nytimes.com/2023/12/14/science/enceladus-moon-cyanide-life-saturn.html |archive-date=14 December 2023 |access-date=15 December 2023}}</ref><ref name="NA-20231214">{{cite journal |author=Peter, Jonah S. |author2=Nordheim, Tom A. |author3=Hand, Kevin P. |date=14 December 2023 |title=Detection of HCN and diverse redox chemistry in the plume of Enceladus |journal=[[Astronomy:Nature Astronomy|Nature Astronomy]] |doi=10.1038/s41550-023-02160-0 |arxiv=2301.05259}}</ref>

=== Hydrothermal vents ===

[[File:PIA19058-SaturnMoon-Enceladus-PossibleHydrothermalActivity-ArtistConcept-20150311.jpg|thumb|An artist's impression of possible hydrothermal activity on Enceladus's ocean floor<ref name="Hsu et al. 2015"/>]]
On April 13, 2017, NASA announced the discovery of possible hydrothermal activity on Enceladus's sub-surface ocean floor. In 2015, the Cassini probe made a close fly-by of Enceladus's south pole, flying within {{convert|48.3|km|mi|abbr=on}} of the surface, as well as through a plume in the process. A mass spectrometer on the craft detected molecular hydrogen (H<sub>2</sub>) from the plume, and after months of analysis, the conclusion was made that the hydrogen was most likely the result of hydrothermal activity beneath the surface.<ref name="Waite et al. 2018" /> It has been speculated that such activity could be a potential oasis of habitability.<ref name="NYT-20170413">{{cite news |last=Chang |first=Kenneth |title=Conditions for Life Detected on Saturn Moon Enceladus |url=https://www.nytimes.com/2017/04/13/science/saturn-cassini-moon-enceladus.html |date=April 13, 2017 |work={{wipe|The New York Times}} |access-date=April 13, 2017 |archive-date=April 13, 2017 |archive-url=https://web.archive.org/web/20170413185535/https://www.nytimes.com/2017/04/13/science/saturn-cassini-moon-enceladus.html |url-status=live}}</ref><ref>{{Cite news|url=https://www.pbs.org/newshour/rundown/nasa-ocean-saturn-moon-may-possess-life-sustaining-hydrothermal-vents/|title=NASA: Ocean on Saturn moon may possess life-sustaining hydrothermal vents|work=PBS NewsHour|access-date=April 13, 2017|archive-date=April 13, 2017|archive-url=https://web.archive.org/web/20170413183731/http://www.pbs.org/newshour/rundown/nasa-ocean-saturn-moon-may-possess-life-sustaining-hydrothermal-vents/|url-status=live}}</ref><ref>{{Cite web |url=https://www.theverge.com/2017/4/13/15270854/nasa-enceladus-ocean-hydrothermal-vents-alien-life-conditions-cassini-saturn|title=NASA finds more evidence that the ocean on Enceladus could support alien life|date=April 13, 2017|website=The Verge|access-date=April 13, 2017 |archive-date=April 13, 2017|archive-url=https://web.archive.org/web/20170413200049/http://www.theverge.com/2017/4/13/15270854/nasa-enceladus-ocean-hydrothermal-vents-alien-life-conditions-cassini-saturn|url-status=live}}</ref>

The presence of ample hydrogen in Enceladus's ocean means that microbes – if any exist there – could use it to obtain energy by combining the [[Chemistry:Hydrogen|hydrogen]] with [[Chemistry:Carbon dioxide|carbon dioxide]] dissolved in the [[Chemistry:Water|water]]. The chemical reaction is known as "[[Engineering:Methanogenesis|methanogenesis]]" because it produces [[Chemistry:Methane|methane]] as a byproduct, and is at the root of the tree of life on [[Astronomy:Earth|Earth]], the [[Biology:Earliest known life forms|birthplace of all life that is known to exist]].<ref>{{Cite news|url=https://www.nasa.gov/press-release/nasa-missions-provide-new-insights-into-ocean-worlds-in-our-solar-system|title=NASA Missions Provide New Insights into 'Ocean Worlds'|last=Northon|first=Karen|date=April 13, 2017 |work=NASA |access-date=April 13, 2017|archive-date=April 20, 2017|archive-url=https://web.archive.org/web/20170420143202/https://www.nasa.gov/press-release/nasa-missions-provide-new-insights-into-ocean-worlds-in-our-solar-system/|url-status=live}}</ref><ref>{{cite news |last1=Kaplan |first1=Sarah |title=NASA finds ingredients for life spewing out of Saturn's icy moon Enceladus |url=https://www.washingtonpost.com/news/speaking-of-science/wp/2017/04/13/nasa-finds-ingredients-for-life-spewing-out-of-saturns-moon/ |access-date=May 3, 2017|agency=NASA|newspaper=Washington Post|date=April 13, 2017|archive-date=April 30, 2017|archive-url=https://web.archive.org/web/20170430201527/https://www.washingtonpost.com/news/speaking-of-science/wp/2017/04/13/nasa-finds-ingredients-for-life-spewing-out-of-saturns-moon/|url-status=live}}</ref>

== Exploration ==

=== ''Voyager'' missions ===
{{Main|Astronomy:Voyager program}}
[[File:Enceladus from Voyager.jpg|thumb|''Voyager 2''<nowiki/>'s image mosaic of Enceladus]]
The two [[Astronomy:Voyager program|Voyager]] spacecraft made the first close-up images of Enceladus. ''[[Astronomy:Voyager 1|Voyager 1]]'' was the first to fly past Enceladus, at a distance of 202,000&nbsp;km on November 12, 1980.<ref name="Voyager">{{cite web |url=http://pds-rings.seti.org/voyager/mission/ |title=Voyager Mission Description |work=Ring-Moon Systems Node |publisher=SETI |date=February 19, 1997 |access-date=May 29, 2006 |archive-date=April 28, 2014 |archive-url=https://web.archive.org/web/20140428121443/http://pds-rings.seti.org/voyager/mission/ |url-status=live }}</ref> Images acquired from this distance had very poor spatial resolution, but revealed a highly reflective surface devoid of impact craters, indicating a youthful surface.<ref name="Terrile">{{cite web |last1=Terrile |first1=R. J. |last2=Cook |first2=A. F. |date=1981 |url=http://articles.adsabs.harvard.edu//full/seri/LPICo/0428//0000010.000.html |title=Enceladus: Evolution and Possible Relationship to Saturn's E-ring |work=12th Annual Lunar and Planetary Science Conference, Abstract |page=428 |access-date=May 30, 2006 |archive-date=May 28, 2020 |archive-url=https://web.archive.org/web/20200528214941/http://articles.adsabs.harvard.edu//full/seri/LPICo/0428//0000010.000.html |url-status=live }}</ref> ''Voyager 1'' also confirmed that Enceladus was embedded in the densest part of Saturn's diffuse [[Astronomy:Rings of Saturn#E Ring|E ring]]. Combined with the apparent youthful appearance of the surface, ''Voyager'' scientists suggested that the E ring consisted of particles vented from Enceladus's surface.<ref name="Terrile" />

''[[Astronomy:Voyager 2|Voyager 2]]'' passed closer to Enceladus (87,010&nbsp;km) on August 26, 1981, allowing higher-resolution images to be obtained.<ref name="Voyager" /> These images showed a young surface.<ref name="Rothery">{{cite book |last=Rothery |first=David A. |title=Satellites of the Outer Planets: Worlds in their own right |publisher=Oxford University Press |date=1999 |isbn=978-0-19-512555-9 }}</ref> They also revealed a surface with different regions with vastly different surface ages, with a heavily cratered mid- to high-northern latitude region, and a lightly cratered region closer to the equator. This geologic diversity contrasts with the ancient, heavily cratered surface of Mimas, another moon of Saturn slightly smaller than Enceladus. The geologically youthful terrains came as a great surprise to the scientific community, because no theory was then able to predict that such a small (and cold, compared to [[Astronomy:Jupiter|Jupiter]]'s highly active moon [[Astronomy:Io (moon)|Io]]) celestial body could bear signs of such activity.

=== ''Cassini'' ===
{{Main|Astronomy:Cassini–Huygens}}
[[File:Enceladus - January 17 2006 (24199218078).png|thumb|A picture of Enceladus in parallel with Saturn's ring, taken by ''Cassini'' in January 2006]]
The answers to many remaining mysteries of Enceladus had to wait until the arrival of the [[Astronomy:Cassini–Huygens|''Cassini'' spacecraft]] on July 1, 2004, when it entered orbit around Saturn. Given the results from the ''Voyager 2'' images, Enceladus was considered a priority target by the ''Cassini'' mission planners, and several targeted [[Engineering:Planetary flyby|flybys]] within 1,500&nbsp;km of the surface were planned as well as numerous, "non-targeted" opportunities within 100,000&nbsp;km of Enceladus. The flybys have yielded significant information concerning Enceladus's surface, as well as the discovery of [[Physics:Water vapor|water vapor]] with traces of simple [[Chemistry:Hydrocarbon|hydrocarbon]]s venting from the geologically active south polar region.<ref name="Planetary Society">{{cite web |url=http://www.planetary.org/explore/space-topics/space-missions/cassinis-tour.html |title=Cassini's Tour of the Saturn System |work=Planetary Society |access-date=March 11, 2015 |archive-date=April 2, 2015 |archive-url=https://web.archive.org/web/20150402123749/http://www.planetary.org/explore/space-topics/space-missions/cassinis-tour.html |url-status=live }}</ref>

These discoveries prompted the adjustment of ''Cassini's'' flight plan to allow closer flybys of Enceladus, including an encounter in March 2008 that took it to within 48&nbsp;km of the surface.<ref name="Planetary Society"/> ''Cassini's'' extended mission included seven close flybys of Enceladus between July 2008 and July 2010, including two passes at only 50&nbsp;km in the later half of 2008.<ref>{{cite web |last=Moomaw |first=B. |url=http://www.spacedaily.com/reports/Tour_de_Saturn_Set_For_Extended_Play_999.html |title=Tour de Saturn Set For Extended Play |work=Spacedaily |date=February 5, 2007 |access-date=February 5, 2007 |archive-date=May 28, 2020 |archive-url=https://web.archive.org/web/20200528214941/https://www.spacedaily.com/reports/Tour_de_Saturn_Set_For_Extended_Play_999.html |url-status=live }}</ref> ''Cassini'' performed a flyby on October 28, 2015, passing as close as {{convert|49|km|abbr=on}} and through a plume.<ref name="nasa-press-release">{{cite web| url=http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20151028/| title=Deepest-Ever Dive Through Enceladus Plume Completed| work=NASA/JPL| access-date=October 29, 2015| date=October 28, 2015| archive-date=November 2, 2015| archive-url=https://web.archive.org/web/20151102025851/http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20151028/| url-status=live}}</ref> Confirmation of molecular hydrogen ({{chem|H|2}}) would be an independent line of evidence that hydrothermal activity is taking place in the Enceladus seafloor, increasing its habitability.<ref name="Wall"/>

''Cassini'' has provided strong evidence that Enceladus has an ocean with an energy source, nutrients and organic molecules, making Enceladus one of the best places for the study of potentially [[Astronomy:Planetary habitability|habitable environments]] for extraterrestrial life.<ref name="LCPM Enceladus" /><ref name="Ciclops1881">{{cite web |url=http://ciclops.org/view.php?id=1881 |title=Cassini Images of Enceladus Suggest Geysers Erupt Liquid Water at the Moon's South Pole |website=Cassini Imaging |access-date=March 22, 2006 |archive-date=July 25, 2011 |archive-url=https://web.archive.org/web/20110725171423/http://ciclops.org/view.php?id=1881 |url-status=live }}</ref><ref>{{Cite web |last=McKie|first=Robin|date=2020-09-20|title=The search for life – from Venus to the outer solar system |website=the Guardian |url=http://www.theguardian.com/science/2020/sep/20/the-search-for-life-from-venus-to-the-outer-solar-system|access-date=2020-09-21 |archive-date=September 21, 2020|archive-url=https://web.archive.org/web/20200921031839/https://www.theguardian.com/science/2020/sep/20/the-search-for-life-from-venus-to-the-outer-solar-system|url-status=live}}</ref>

On 14 December 2023, astronomers reported the first time discovery, in the [[Physics:Plume (fluid dynamics)|plume]]s of Enceladus, of [[Chemistry:Hydrogen cyanide|hydrogen cyanide]], a possible chemical essential for life as we know it, as well as other organic molecules, some of which are yet to be better identified and understood. According to the researchers, "these [newly discovered] compounds could potentially support extant [[Biology:Microorganism|microbial communities]] or drive complex [[Chemistry:Organic synthesis|organic synthesis]] leading to the origin of life."<ref name="NYT-20231214kc"/><ref name="NA-20231214"/>

=== Proposed mission concepts ===
The discoveries ''Cassini'' made at Enceladus have prompted studies into follow-up mission concepts, including a probe flyby ([[Astronomy:Journey to Enceladus and Titan|Journey to Enceladus and Titan]] or JET) to analyze plume contents ''in situ'',<ref name="Sotin2011">{{cite conference |last1=Sotin |first1=C. |last2=Altwegg |first2=K.|display-authors=0 |title=JET: Journey to Enceladus and Titan |url=http://www.lpi.usra.edu/meetings/lpsc2011/pdf/1326.pdf |conference=42nd Lunar and Planetary Science Conference |publisher=Lunar and Planetary Institute |date=2011 |access-date=August 17, 2015 |archive-date=April 15, 2015 |archive-url=https://web.archive.org/web/20150415113634/http://www.lpi.usra.edu/meetings/lpsc2011/pdf/1326.pdf |url-status=live }}</ref><ref name="JET flyby mission">{{cite web |date=March 21, 2011 |author=Van Kane |url=http://futureplanets.blogspot.com/search?q=jet+enceladus+titan |title=Cost Capped Titan-Enceladus Proposal |work=Future Planetary Exploration |access-date=April 9, 2014 |archive-date=October 1, 2019 |archive-url=https://web.archive.org/web/20191001184553/https://futureplanets.blogspot.com/search?q=jet+enceladus+titan |url-status=live}}</ref> a lander by the [[Organization:German Aerospace Center|German Aerospace Center]] to study the habitability potential of its subsurface ocean ([[Astronomy:Enceladus Explorer|Enceladus Explorer]]),<ref name="Konstantinidis 2015">{{cite journal |title=A lander mission to probe subglacial water on Saturn's moon Enceladus for life |journal=Acta Astronautica |date=February 2015 |last1=Konstantinidis |first1=Konstantinos |last2=Flores Martinez |first2=Claudio L. |last3=Dachwald |first3=Bernd |last4=Ohndorf |first4=Andreas |last5=Dykta |first5=Paul |volume=106 |pages=63–89 |doi=10.1016/j.actaastro.2014.09.012 |url=https://www.researchgate.net/publication/268748795 |access-date=April 11, 2015 |bibcode=2015AcAau.106...63K |archive-date=October 1, 2021 |archive-url=https://web.archive.org/web/20211001063533/https://www.researchgate.net/publication/268748795_A_lander_mission_to_probe_subglacial_water_on_Saturns_moon_Enceladus_for_life |url-status=live }}</ref><ref name="ut.93879">{{cite news |last=Anderson |first=Paul Scott |date=February 29, 2012 |title=Exciting New 'Enceladus Explorer' Mission Proposed to Search for Life |url=http://www.universetoday.com/93879/exciting-new-enceladus-explorer-mission-proposed-to-search-for-life/ |work=Universe Today |access-date=April 9, 2014 |archive-date=April 13, 2014 |archive-url=https://web.archive.org/web/20140413123757/http://www.universetoday.com/93879/exciting-new-enceladus-explorer-mission-proposed-to-search-for-life/ |url-status=live }}</ref><ref name="dlr.4874">{{cite web |url=http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-2751/#gallery/4874 |title=Searching for life in the depths of Enceladus |work=News |publisher=German Aerospace Center (DLR) |date=February 22, 2012 |access-date=April 9, 2014 |archive-date=April 10, 2014 |archive-url=https://web.archive.org/web/20140410101827/http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-2751/#gallery/4874 |url-status=live }}</ref> and two astrobiology-oriented mission concepts (the [[Astronomy:Enceladus Life Finder|Enceladus Life Finder]]<ref name="2015 LPSC">{{cite conference |last1=Lunine |first1=Jonathan I. |last2=Waite |first2=Jack Hunter Jr. |last3=Postberg |first3=Frank |last4=Spilker |first4=Linda J. |title=Enceladus Life Finder: The search for life in a habitable moon |url=http://www.hou.usra.edu/meetings/lpsc2015/pdf/1525.pdf |conference=46th Lunar and Planetary Science Conference (2015) |publisher=Lunar and Planetary Institute |place=Houston (TX) |year=2015 |access-date=February 21, 2015 |archive-date=May 28, 2019 |archive-url=https://web.archive.org/web/20190528200113/https://www.hou.usra.edu/meetings/lpsc2015/pdf/1525.pdf |url-status=live }}</ref><ref name="contestants">{{cite news |last=Clark |first=Stephen |url=http://spaceflightnow.com/2015/04/06/diverse-destinations-considered-for-new-interplanetary-probe/ |title=Diverse destinations considered for new interplanetary probe |work=Space Flight Now |date=April 6, 2015 |access-date=April 7, 2015 |archive-date=January 5, 2017 |archive-url=https://web.archive.org/web/20170105033544/http://spaceflightnow.com/2015/04/06/diverse-destinations-considered-for-new-interplanetary-probe/ |url-status=live }}</ref> and [[Astronomy:Life Investigation For Enceladus|Life Investigation For Enceladus]] (LIFE)).<ref name="McKay 2014">{{cite journal |title=Follow the Plume: The Habitability of Enceladus |journal=Astrobiology |date=April 15, 2014 |display-authors=2 |last1=McKay |first1=Christopher P. |last2=Anbar |first2=Ariel D. |last3=Porco |first3=Carolyn |last4=Tsou |first4=Peter |volume=14 |issue=4 |pages=352–355 |bibcode=2014AsBio..14..352M |doi=10.1089/ast.2014.1158 |pmid=24684187 |url=https://zenodo.org/record/1235251 |access-date=July 13, 2019 |archive-date=July 31, 2020 |archive-url=https://web.archive.org/web/20200731003830/https://zenodo.org/record/1235251 |url-status=live }}</ref><ref name="LCPM Enceladus">{{cite conference |last1=Tsou |first1=P. |last2=Brownlee |first2=D. E. |last3=McKay |first3=C. P. |last4=Anbar |first4=A. |display-authors=2 |date=June 18–20, 2013 |title=Low Cost Enceladus Sample Return Mission Concept |url=http://lcpm10.caltech.edu/pdf/session-5/10_LIFE_LCPM_FINAL.pdf |conference=Low Cost Planetary Missions Conference (LCPM) # 10 |access-date=April 9, 2014 |archive-url=https://web.archive.org/web/20140408060245/http://lcpm10.caltech.edu/pdf/session-5/10_LIFE_LCPM_FINAL.pdf |archive-date=April 8, 2014}}</ref><ref name="Interview2012">{{cite news |last=Wall |first=Mike |url=http://www.space.com/18792-enceladus-sample-return-mission.html |title=Saturn Moon Enceladus Eyed for Sample-Return Mission |work=Space.com |date=December 6, 2012 |access-date=April 10, 2015 |archive-date=September 5, 2017 |archive-url=https://web.archive.org/web/20170905040839/https://www.space.com/18792-enceladus-sample-return-mission.html |url-status=live }}</ref><ref name="Astro2012">{{cite journal |title=LIFE: Life Investigation For Enceladus – A Sample Return Mission Concept in Search for Evidence of Life |journal=Astrobiology |date=August 2012 |last1=Tsou |first1=Peter |last2=Brownlee |first2=D. E. |first3=Christopher |last3=McKay |last4=Anbar |first4=A. D. |last5=Yano |first5=H. |s2cid=34375065 |volume=12 |issue=8 |pages=730–742 |doi=10.1089/ast.2011.0813 |pmid=22970863 |bibcode=2012AsBio..12..730T |format=doc |url=http://discoveringenceladus.com/downloads/LIFE%20-%20Life%20Investigation%20For%20Enceladus%20-%20A%20Sample%20Return%20Mission%20Concept%20in%20Search%20for%20Evidence%20of%20Life.doc |access-date=April 10, 2015 |archive-url=https://web.archive.org/web/20150901121008/http://discoveringenceladus.com/downloads/LIFE%20-%20Life%20Investigation%20For%20Enceladus%20-%20A%20Sample%20Return%20Mission%20Concept%20in%20Search%20for%20Evidence%20of%20Life.doc |archive-date=September 1, 2015|url-access=subscription }}</ref>

The [[Organization:European Space Agency|European Space Agency]] (ESA) was assessing concepts in 2008 to send a probe to Enceladus in a mission to be combined with studies of Titan: [[Astronomy:Titan Saturn System Mission|Titan Saturn System Mission]] (TSSM).<ref name="TandEM">{{cite web |url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=42337 |title=TandEM (Titan and Enceladus Mission) Workshop |website=ESA |date=February 7, 2008 |access-date=March 2, 2008 |archive-date=June 4, 2011 |archive-url=https://web.archive.org/web/20110604043307/http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=42337 |url-status=live }}</ref> TSSM was a joint [[Organization:NASA|NASA]]/ESA flagship-class proposal for exploration of [[Astronomy:Moons of Saturn|Saturn's moons]], with a focus on Enceladus, and it was competing against the Europa Jupiter System Mission (EJSM) proposal for funding. In February 2009, it was announced that NASA/ESA had given the EJSM mission priority ahead of TSSM,<ref name="bbc.7897585">{{cite news |last=Rincon |first=Paul |url=http://news.bbc.co.uk/1/hi/sci/tech/7897585.stm |title=Jupiter in space agencies' sights |work=BBC News |date=February 18, 2009 |access-date=March 13, 2009 |archive-date=February 21, 2009 |archive-url=https://web.archive.org/web/20090221185643/http://news.bbc.co.uk/1/hi/sci/tech/7897585.stm |url-status=live }}</ref> although ''TSSM'' will continue to be studied and evaluated.

In November 2017, Russian billionaire [[Biography:Yuri Milner|Yuri Milner]] expressed interest in funding a "low-cost, privately funded mission to Enceladus which can be launched relatively soon."<ref name="Milner flyby 2017">{{Cite news |url=https://www.newscientist.com/article/mg23631533-900-private-mission-may-get-us-back-to-enceladus-sooner-than-nasa/ |title=Private mission may get us back to Enceladus sooner than NASA|work=New Scientist|access-date=December 31, 2017 |archive-date=December 31, 2017|archive-url=https://web.archive.org/web/20171231103856/https://www.newscientist.com/article/mg23631533-900-private-mission-may-get-us-back-to-enceladus-sooner-than-nasa/|url-status=live}}</ref><ref>{{Cite news |url=https://firenewsfeed.com/lifestyle/720408| title='Looking for a smoking gun': Russian billionaire to fund alien-hunting mission to Saturn moon|access-date=December 31, 2017| language=ru| work=RT| archive-url=https://web.archive.org/web/20171231103746/https://firenewsfeed.com/lifestyle/720408|archive-date=December 31, 2017}}</ref> In September 2018, [[Organization:NASA|NASA]] and the [[Astronomy:Breakthrough Initiatives|Breakthrough Initiatives]], founded by Milner, signed a cooperation agreement for the mission's initial concept phase.<ref>{{Cite news|url=https://spacenews.com/nasa-to-support-initial-studies-of-privately-funded-enceladus-mission/|title=NASA to support initial studies of privately funded Enceladus mission |date=November 9, 2018|work=SpaceNews.com|access-date=November 10, 2018 |archive-date=November 11, 2018|archive-url=https://wayback.archive-it.org/all/20181111072343/https://spacenews.com/nasa%2Dto%2Dsupport%2Dinitial%2Dstudies%2Dof%2Dprivately%2Dfunded%2Denceladus%2Dmission/ |url-status=live}}</ref> The spacecraft would be low-cost, low mass, and would be launched at high speed on an affordable rocket. The spacecraft would be directed to perform a single flyby through Enceladus' plumes in order to sample and analyze its content for [[Astronomy:Biosignature|biosignature]]s.<ref name="SN Nov 2018">{{cite web |url=https://spacenews.com/nasa-to-support-initial-studies-of-privately-funded-enceladus-mission/ |title=NASA to support initial studies of privately funded Enceladus mission |archive-url=https://wayback.archive-it.org/all/20181111072343/https://spacenews.com/nasa%2Dto%2Dsupport%2Dinitial%2Dstudies%2Dof%2Dprivately%2Dfunded%2Denceladus%2Dmission/ |archive-date=November 11, 2018 |url-status=live |author=Jeff Foust |work=SpaceNews.com |date=November 9, 2018}}</ref><ref name="Powell NBC 2018">{{cite web |url=https://www.nbcnews.com/mach/science/billionaire-aims-jump-start-search-alien-life-rewrite-rules-space-ncna949311 |title=Billionaire aims to jump-start search for alien life and rewrite rules of space exploration |url-status=live |archive-url=https://web.archive.org/web/20181220215001/https://www.nbcnews.com/mach/science/billionaire-aims-jump-start-search-alien-life-rewrite-rules-space-ncna949311 |archive-date=December 20, 2018 |author=Corey S. Powell |work=NBC News |date=December 19, 2018}}</ref> NASA provided scientific and technical expertise through various reviews, from March 2019 to December 2019.<ref name="SR Foust 2018">{{cite web |url=http://thespacereview.com/article/3602/1 |title=A different trajectory for funding space science missions |archive-url=https://web.archive.org/web/20181216093053/http://www.thespacereview.com/article/3602/1 |archive-date=December 16, 2018 |url-status=live |author=Jeff Foust |work=Space Review |date=November 12, 2018}}</ref>

In 2022, the [[Astronomy:Planetary Science Decadal Survey|Planetary Science Decadal Survey]] by the National Academy of Sciences recommended that NASA prioritize its newest probe concept, the [[Engineering:Enceladus Orbilander|Enceladus Orbilander]], as a Flagship-class mission, alongside its newest concepts for a [[Astronomy:Mars sample-return mission|Mars sample-return mission]] and the [[Astronomy:Uranus Orbiter and Probe|Uranus Orbiter and Probe]]. The Enceladus Orbilander would be launched on a similarly affordable rocket, but would cost about $5 billion, and be designed to endure eighteen months in orbit inspecting Enceladus' plumes before landing and spending two Earth years conducting surface astrobiology research.<ref name="2022Decadal">{{cite news |last=Foust |first=Jeff |url=https://spacenews.com/planetary-science-decadal-endorses-mars-sample-return-outer-planets-missions/ |title=Planetary science decadal endorses Mars sample return, outer planets missions |work=SpaceNews.com |date=19 April 2022 |access-date=20 July 2022}}</ref>
{| class="wikitable"
|-
! Year proposed !! Proponent !! Project name!! Status!! References
|-
| 2006 || GSFC NASA Academy || EAGLE study || Cancelled ||<ref>{{cite web |title=EAGLE: Mission Overview |date=November 2006 |url=https://www.lpi.usra.edu/opag/meetings/nov2006/presentations/eagle.pdf|url-status=live |archive-date=October 1, 2021|archive-url=https://web.archive.org/web/20211001063531/https://www.lpi.usra.edu/opag/meetings/nov2006/presentations/eagle.pdf}}</ref> 
|-
| 2006 || NASA || 'Titan and Enceladus $1B Mission Feasibility' Study || Cancelled ||<ref>{{cite report |id=JPL D-37401 B  |url=https://www.lpi.usra.edu/opag/TitanEnceladusBillionDollarBox.pdf |author=Kim Reh |date=January 30, 2007 |title=Titan and Enceladus $1B Mission Feasibility Study |publisher=NASA |archive-url=https://web.archive.org/web/20170705145936/http://www.lpi.usra.edu/opag/TitanEnceladusBillionDollarBox.pdf |archive-date=July 5, 2017 |url-status=live}}</ref><ref name="FPE 2011">{{cite web |url=http://futureplanets.blogspot.com/2011/06/enceladus-mission-options.html |author=Van Kane|date=June 20, 2011|title=Enceladus Mission Options |work=Future Planetary Exploration |archive-url=https://web.archive.org/web/20181221134534/http://futureplanets.blogspot.com/2011/06/enceladus-mission-options.html |archive-date=December 21, 2018 |url-status=live}}</ref>
|-
| 2007 || NASA || 'Enceladus Flagship' study || Cancelled ||<ref name="FPE 2011"/>
|-
| 2007 || [[Organization:European Space Agency|ESA]] || Titan and Enceladus Mission (TandEM) || Cancelled ||<ref name="TandEM"/> 
|-
| 2007 || NASA JPL || Enceladus RMA Study || Cancelled ||<ref name="ieee.5747289">{{cite book |last1=Adler |first1=M. |last2=Moeller |first2=R. C. |last3=Borden |first3=C. S. |last4=Smythe |first4=W. D. |last5=Shotwell |first5=R. F. |last6=Cole |first6=B. F. |last7=Spilker |first7=T. R. |last8=Strange |first8=N. J. |last9=Petropoulos |first9=A. E. |last10=Chattopadhyay |first10=D. |last11=Ervin |first11=J. |last12=Deems |first12=E. |last13=Tsou |first13=P. |last14=Spencer |first14=John R. |title=2011 Aerospace Conference |chapter=Rapid Mission Architecture trade study of Enceladus mission concepts |display-authors=2 |publisher=IEEE |date=March 5–12, 2011 |pages=1–13 |doi=10.1109/AERO.2011.5747289 |issn=1095-323X |isbn=978-1-4244-7350-2 |s2cid=32352068 }}</ref>
|-
| 2008 || NASA/ESA || TandEM became [[Astronomy:Titan Saturn System Mission|Titan Saturn System Mission]] (TSSM) || Cancelled ||<ref name="TandEM"/>
|-
| 2010 || PSDS Decadal Survey || Enceladus Orbiter || Cancelled ||<ref>{{cite web |last1=Spencer |first1=John R. |title=Planetary Science Decadal Survey Enceladus Orbiter |work=Mission Concept Study |url=http://sites.nationalacademies.org/cs/groups/ssbsite/documents/webpage/ssb_059320.pdf |publisher=NASA |access-date=June 23, 2016 |date=May 2010 |archive-date=September 29, 2015 |archive-url=https://web.archive.org/web/20150929161156/http://sites.nationalacademies.org/cs/groups/ssbsite/documents/webpage/ssb_059320.pdf |url-status=live }}</ref>
|-
| 2011 || NASA JPL || [[Astronomy:Journey to Enceladus and Titan|Journey to Enceladus and Titan]] (JET) || Under study ||<ref name="JET 204">{{cite news |last=Kane |first=Van |title=Discovery Missions for an Icy Moon with Active Plumes |url=http://www.planetary.org/blogs/guest-blogs/van-kane/20140402-discovery-missions-for-an-icy-moon-with-plumes.html |work=The Planetary Society |date=April 3, 2014 |access-date=April 9, 2015 |archive-date=April 16, 2015 |archive-url=https://web.archive.org/web/20150416094147/http://www.planetary.org/blogs/guest-blogs/van-kane/20140402-discovery-missions-for-an-icy-moon-with-plumes.html |url-status=live }}</ref>
|-
| 2012 || [[Organization:German Aerospace Center|DLR]] || [[Astronomy:Enceladus Explorer|Enceladus Explorer]] (EnEx) lander, employing the [[Engineering:IceMole|IceMole]] || Under study ||<ref>{{cite news |last=Brabaw |first=Kasandra |url=http://www.space.com/28930-icemole-drills-glacier-saturn-moon-enceladus.html?cid=514630_20150408_43377416&adbid=10152745517721466&adbpl=fb&adbpr=17610706465 |title=IceMole Drill Built to Explore Saturn's Icy Moon Enceladus Passes Glacier Test |work=Space.com |date=April 7, 2015 |access-date=April 9, 2015 |archive-date=August 23, 2018 |archive-url=https://web.archive.org/web/20180823041934/https://www.space.com/28930-icemole-drills-glacier-saturn-moon-enceladus.html?cid=514630_20150408_43377416&adbid=10152745517721466&adbpl=fb&adbpr=17610706465 |url-status=live }}</ref>
|-
| 2012 || NASA JPL || [[Astronomy:Life Investigation For Enceladus|Life Investigation For Enceladus]] (LIFE) || Cancelled ||<ref name="Interview2012"/><ref name="Astro2012"/><ref name="Porco 2014">{{cite journal |title=LIFE – Enceladus Plume Sample Return via Discovery |journal=45th Lunar and Planetary Science Conference |year=2014 |last1=Tsou |first1=Peter |last2=Anbar |first2=Ariel |last3=Atwegg |first3=Kathrin |last4=Porco |first4=Carolyn |last5=Baross |first5=John |last6=McKay |first6=Christopher |url=http://www.hou.usra.edu/meetings/lpsc2014/pdf/2192.pdf |access-date=April 10, 2015 |archive-date=March 4, 2016 |archive-url=https://web.archive.org/web/20160304001149/http://www.hou.usra.edu/meetings/lpsc2014/pdf/2192.pdf |url-status=live }}</ref>
|-
| 2015 || NASA JPL ||[[Astronomy:Enceladus Life Finder|Enceladus Life Finder]] (ELF) || Under study ||<ref name="2015 LPSC"/>
|-
| 2017 || ESA/NASA || [[Astronomy:Explorer of Enceladus and Titan|Explorer of Enceladus and Titan]] (E<sup>2</sup>T) || Under study ||<ref name="E2T 2017">{{cite journal |title=Explorer of Enceladus and Titan (E2T): Investigating the habitability and evolution of ocean worlds in the Saturn system |journal=American Astronomical Society |year=2017 |author1=Mitri, Giuseppe |author2=Postberg, Frank |author3=Soderblom, Jason M. |author4=Tobie, Gabriel |author5=Tortora, Paolo |author6=Wurz, Peter |author7=Barnes, Jason W. |author8=Coustenis, Athena |author9=Ferri, Francesca |author10=Hayes, Alexander |author11=Hayne, Paul O. |author12=Hillier, Jon |author13=Kempf, Sascha |author14=Lebreton, Jean-Pierre |author15=Lorenz, Ralph |author16=Orosei, Roberto |author17=Petropoulos, Anastassios |author18=Yen, Chen-wan |author19=Reh, Kim R. |author20=Schmidt, Jürgen |author21=Sims, Jon |author22=Sotin, Christophe |author23=Srama, Ralf |volume=48 |pages=225.01 |bibcode=2016DPS....4822501M}}</ref> 
|-
| 2017 || NASA || [[Astronomy:Enceladus Life Signatures and Habitability|Enceladus Life Signatures and Habitability]] (ELSAH) || Under study ||<ref name="2017 Proposals">{{cite news |url=http://futureplanets.blogspot.com/2017/08/proposed-new-frontiers-missions.html |title=Proposed New Frontiers Missions |work=Future Planetary Exploration |author=Van Kane |date=August 4, 2017 |access-date=September 20, 2017 |archive-url=https://web.archive.org/web/20170920093515/http://futureplanets.blogspot.com/2017/08/proposed-new-frontiers-missions.html |archive-date=September 20, 2017 |url-status=live}}</ref><ref>{{cite news |last=McIntyre |first=Ocean |url=http://www.spaceflightinsider.com/missions/solar-system/cassini-legend-legacy-one-nasa-prolific-missions/ |title=Cassini: The legend and legacy of one of NASA's most prolific missions |work=Spaceflight Insider |date=September 17, 2017 |access-date=September 20, 2017 |archive-url=https://web.archive.org/web/20170920063514/http://www.spaceflightinsider.com/missions/solar-system/cassini-legend-legacy-one-nasa-prolific-missions/ |archive-date=September 20, 2017 |url-status=dead }}</ref>
|-
| 2017 || [[Astronomy:Breakthrough Initiatives|Breakthrough Initiatives]] || [[Biology:Breakthrough Enceladus mission|Breakthrough Enceladus mission]] || Under study ||<ref name="Milner flyby 2017"/>
|-
| 2022 || PSDS Decadal Survey || [[Engineering:Enceladus Orbilander|Enceladus Orbilander]] || Under study ||<ref name="2022Decadal"/>
|}

== See also ==
* Enceladus in fiction
* [[Astronomy:List of extraterrestrial volcanoes|List of extraterrestrial volcanoes]]
* [[Astronomy:List of geological features on Enceladus|List of geological features on Enceladus]]
* [[Astronomy:List of natural satellites|List of natural satellites]]

==References==
=== Informational notes ===
{{Reflist
| group=lower-alpha
| refs=

<ref name="caption" group="lower-alpha">
Photograph of Enceladus, taken by the narrow-angle camera of the [[Astronomy:Cassini–Huygens#Instruments|Imaging Science Subsystem (ISS)]] aboard ''[[Astronomy:Cassini–Huygens|Cassini]]'', during the spacecraft’s October 28, 2015 flyby. It shows the younger terrain of Sarandib and Diyar Planitia, populated with many grooves ([[Earth:Sulcus (geology)|sulci]]) and depressions ([[Earth:Fossa (geology)|fossae]]). Older, cratered terrain can be seen towards Enceladus's north pole. The prominent feature visible near the south pole is Cashmere Sulci.
</ref>
<ref name="crater flux" group="lower-alpha">
Without samples to provide absolute age determinations, crater counting is currently the only method for determining surface age on most planetary surfaces. Unfortunately, there is currently disagreement in the scientific community regarding the flux of impactors in the outer Solar System. These competing models can significantly alter the age estimate even with the same crater counts. For the sake of completeness, both age estimates from {{harvp|Porco, Helfenstein et al. |2006}} are provided.
</ref>
}}

=== Citations ===
{{Reflist
| refs=

<ref name="Howett_2010">{{cite journal
 |title  =Thermal inertia and bolometric Bond albedo values for Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus as derived from Cassini/CIRS measurements
 |last1=Howett |first1=Carly J. A. |last2=Spencer |first2=John R. |last3=Pearl |first3=J. C. |last4=Segura |first4=M.
 |date   =2010
 |journal=Icarus
 |volume =206
 |issue  =2
 |pages  =573–593
 |doi    =10.1016/j.icarus.2009.07.016
 |bibcode=2010Icar..206..573H
}}</ref>

<ref name="Porco Helfenstein et al. 2006">{{cite journal| doi=10.1126/science.1123013| last1=Porco| first1=C. C.|  last2=Helfenstein| first2=P.| last3=Thomas| first3=P. C.| last4=Ingersoll| first4=A. P.| last5=Wisdom| first5=J.| last6=West| first6=R.| last7=Neukum| first7=G.| last8=Denk| first8=T.| last9=Wagner| first9=R.| s2cid=6976648| date=March 10, 2006| title=Cassini Observes the Active South Pole of Enceladus| journal=Science| volume=311| issue=5766| pages=1393–1401| pmid=16527964| bibcode=2006Sci...311.1393P| url=https://authors.library.caltech.edu/36593/7/PorcoCC.SOM.pdf| ref={{sfnRef|Porco, Helfenstein et al. |2006}}| display-authors=2| access-date=August 29, 2020| archive-date=August 6, 2020| archive-url= https://web.archive.org/web/20200806170909/https://authors.library.caltech.edu/36593/7/PorcoCC.SOM.pdf| url-status=live}}</ref>

<ref name="Roatsch et al. 2009">
{{cite book| doi = 10.1007/978-1-4020-9217-6_24| last1 = Roatsch| first1 = T.| last2 = Jaumann| first2 = R.| last3 = Stephan| first3 = K.| last4 = Thomas| first4 = P. C.| year = 2009| chapter = Cartographic Mapping of the Icy Satellites Using ISS and VIMS Data| title = Saturn from Cassini-Huygens| pages = 763–781| isbn = 978-1-4020-9216-9| ref = {{sfnRef|Roatsch Jaumann et al.|2009}}}}
</ref>

<ref name="Verbiscer et al. 2007">
{{cite journal| last1 = Verbiscer| first1 = A.| last2 = French| first2 = R.| last3 = Showalter| first3 = M.| last4 = Helfenstein| first4 = P.| title = Enceladus: Cosmic Graffiti Artist Caught in the Act| journal = Science| volume = 315| issue = 5813| page = 815| date = February 9, 2007| pmid = 17289992| bibcode = 2007Sci...315..815V| doi = 10.1126/science.1134681| s2cid = 21932253|ref = {{sfnRef|Verbiscer French et al.|2007}}}} (supporting online material, table S1)
</ref>

<ref name="Observatorio ARVAL">{{cite web |title=Classic Satellites of the Solar System |website=Observatorio ARVAL |date=April 15, 2007 |url=http://www.oarval.org/ClasSaten.htm |ref={{sfnRef|Observatorio ARVAL}} |access-date=December 17, 2011 |archive-url=https://web.archive.org/web/20110920010842/http://www.oarval.org/ClasSaten.htm |url-status=live |archive-date=September 20, 2011}}</ref>

<ref name="Taubner et al. 2014">
{{cite journal | author=Taubner R. S. | author2=Leitner J. J. | author3=Firneis M. G. | author4=Hitzenberg, R. |title=Including Cassini gravity measurements from the flyby E9, E12, E19 into interior structure models of Enceladus. Presented at EPSC 2014-676 | journal=European Planetary Science Congress 2014 | volume=9 | pages=EPSC2014–676 |date=April 2014  |bibcode=2014EPSC....9..676T }}</ref>

}}

== Further reading ==
* {{cite book |last=Lorenz |first=Ralph |title=NASA/ESA/ASI Cassini-Huygens: 1997–2017: (Cassini orbiter, Huygens probe and future exploration concepts : owners' workshop manual |publisher=Haynes Publishing |publication-place=Yeovil England |year=2017 |isbn=9781785211119 |oclc=982381337 |ref=none}}

== External links ==
* [https://web.archive.org/web/20070801204600/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sat_Enceladus Enceladus Profile] at [http://solarsystem.nasa.gov/ NASA's Solar System Exploration site]
* [http://www.solarviews.com/eng/enceladus.htm Calvin Hamilton's Enceladus page]
* [http://www.planetary.org/blogs/blog-archive.html?keywords=enceladus The Planetary Society: Enceladus blogs]
* [http://saturn.jpl.nasa.gov/video/products/MultimediaProductsCharm/ CHARM: ''Cassini–Huygens'' Analysis and Results from the Mission page, contains presentations on Enceladus results]
* [http://stereomoons.blogspot.com/2009/09/turn-of-enceladus.html Paul Schenk's 3D images and flyover videos of Enceladus and other outer solar system satellites]
* [http://web.gps.caltech.edu/~jkirschvink/pdfs/Enceladus_OLEB_online_published_paper.pdf Habitability of Enceladus: Planetary Conditions for Life]

; Images
* [http://ciclops.org/search.php?x=20&y=7&search=Enceladus ''Cassini'' images of Enceladus] {{Webarchive|url=https://web.archive.org/web/20110813151140/http://ciclops.org/search.php?x=20&y=7&search=Enceladus |date=August 13, 2011 }}
* [http://photojournal.jpl.nasa.gov/target/Enceladus Images of Enceladus at JPL's Planetary Photojournal]
* Movie of [https://web.archive.org/web/20100601171634/http://sos.noaa.gov/videos/Enceladus.mov Enceladus's rotation] from the National Oceanic and Atmospheric Administration
* Enceladus [http://www.ciclops.org/view.php?id=7590 global] {{Webarchive|url=https://web.archive.org/web/20180308200714/http://ciclops.org/view.php?id=7590 |date=March 8, 2018 }} and [http://www.ciclops.org/view.php?id=7591 polar] {{Webarchive|url=https://web.archive.org/web/20180308200714/http://ciclops.org/view.php?id=7591 |date=March 8, 2018 }} basemaps (December 2011) from ''Cassini'' images
* [http://www.ciclops.org/view.php?id=6318 Enceladus atlas (May 2010) from ''Cassini'' images]
* [http://planetarynames.wr.usgs.gov/Page/ENCELADUS/target Enceladus nomenclature] and [http://planetarynames.wr.usgs.gov/images/enceladus_comp.pdf Enceladus map with feature names] from the [http://planetarynames.wr.usgs.gov/ USGS planetary nomenclature page]
* [https://www.google.com/maps/space/enceladus/@-57.2040629,172.8918819,8043544m/data=!3m1!1e3 Google Enceladus 3D], interactive map of the moon
* [https://www.flickr.com/photos/kevinmgill/albums/72157676485084546 Image album] by Kevin M. Gill

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