# Europa (moon) <!-- SOLSIM:BEGIN g31 — Solar System explorer state (hand-built on wt-core, specs/solar/); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Europa in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=Europa&embed=1" data-title="Europa in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=Europa`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.* <!-- SOLSIM:END --> *Try: set the speed to 1 day/s and watch Europa, on the highlighted orbit, circle Jupiter in about three and a half seconds while Io goes round twice and Ganymede only half way; press space when Io and Europa line up on the same side of the planet and press space again to watch them drift apart and meet once more about one Europa orbit later; drag to tip the frame edge-on and see all four large moons riding close to Jupiter's equator.* **Europa** is the second of the four [[Galilean_moons|Galilean moons]] of [[Jupiter]] counting outward and the smallest of them, an ice-covered body with a mean radius of 1,560.8 km, a little smaller than Earth's [[Moon]].[^nasa-jsat] Its density of about 3,010 kg/m³ points to a rocky interior, probably with a metallic core, wrapped in a shell of [[Water|water]] around 100 km deep whose upper part is frozen and whose lower part is widely thought to be a salty liquid ocean.[^kargel2000][^phillips2014][^kivelson2000] The case for that ocean rests mainly on the magnetic field Europa generates in response to Jupiter's, and on a surface so young and so crossed by fractures that it can hardly be resting on solid rock.[^kivelson2000][^greenberg2005] Europa is kept warm inside by [[Tide|tides]]: its slightly elongated orbit is forced by an orbital [[Resonance|resonance]] with [[Io_(moon)|Io]] and [[Ganymede_(moon)|Ganymede]], so Jupiter's pull stretches and relaxes it every few days.[^showman1997] That heat, liquid water in contact with rock, and oxidants made at the surface by radiation have made the moon one of the most studied places in the search for life beyond [[Earth]], although no evidence of life has been found.[^hand2007] It was discovered by Galileo Galilei in 1610, mapped at close range by the Galileo orbiter from 1995 to 2003, and is the target of NASA's Europa Clipper, launched in October 2024.[^usgs-names][^nasa-gem][^nasa-clipper] The explorer at the top of this page opens Jupiter's own frame, with Io, Europa, Ganymede and [[Callisto_(moon)|Callisto]] drawn at their true distances and periods; Europa's orbit is the highlighted ring at 670,900 km.[^jpl-europa] ## Discovery and naming Galileo first saw Europa, a moon of [[Jupiter]], through a refracting telescope of about 20× magnification at Padua. On 7 January 1610 his instrument could not separate it from [[Io_(moon)|Io]], and on the following night the two appeared as distinct points of light; the discovery is therefore dated 8 January 1610.[^usgs-names] Simon Marius reported the moons independently and proposed the mythological names in his *Mundus Iovialis* of 1614, crediting the idea to [[Johannes_Kepler|Johannes Kepler]].[^marius1614] In myth Europa was a Phoenician princess carried to Crete by Zeus, the Greek counterpart of Jupiter, so the name fits a scheme in which each large moon is named after one of the god's lovers.[^arnett-europa] Marius's names were not widely used for three centuries. Astronomers wrote instead "Jupiter II", Galileo's numeral for the second moon from the planet, and only in the mid-twentieth century did the names return to general use.[^marazzini2005] The numeral survived even after the count changed: Amalthea, found in 1892 inside the Galilean orbits, made Europa the third moon outward, and three further inner satellites found by the Voyager probes in 1979 made it the sixth, yet the designation Jupiter II was never altered.[^marazzini2005] The accepted adjective is "Europan".[^greenberg2005] ## Orbit and rotation Europa takes 3.551 days to go once around [[Jupiter]] at a mean distance of about 670,900 km, on an [[Orbit|orbit]] with an eccentricity of 0.009 and an inclination of 0.47° to Jupiter's equator.[^jpl-europa][^nasa-jsat] Like the other Galilean moons it is tidally locked, keeping one face toward the planet, so a fixed sub-Jovian point on its surface defines its prime meridian. [[Io_(moon)|Io]] completes two orbits and [[Ganymede_(moon)|Ganymede]] half an orbit in the time Europa completes one; this 1:2:4 chain, the Laplace [[Resonance|resonance]], keeps the three moons from settling into perfectly circular orbits.[^showman1997] The small eccentricity matters more than its size suggests. Across each orbit Europa's distance from Jupiter changes by about 2 × 0.009 × 670,900 ≈ 12,000 km (derived), so the [[Tide|tidal]] bulge the planet raises grows and shrinks, and it also rocks back and forth around the sub-Jovian point. That repeated flexing turns orbital energy into heat. In the resonance picture, the energy is ultimately drawn from Jupiter's rotation: Io raises tides on Jupiter, gains orbital energy from them and passes it outward to Europa and Ganymede through the resonance.[^showman1997] Europa may not be perfectly locked. The orientations of its older fractures fit a shell that has turned slightly faster than synchronous, which would be possible only if a liquid layer separated the ice from the rock beneath.[^geissler1998] Comparisons of Voyager and Galileo images limit any such drift to less than one full turn of the shell every 12,000 years.[^kattenhorn2002] NASA researchers have also argued from the crack patterns that the spin axis was once tilted, which would raise tidal heating and would mean some fractures are younger than assumed.[^jpl-tilt] In the explorer, each moon moves on a circle at its real period, so the 1:2:4 rhythm of Io, Europa and Ganymede is visible at any speed; the starting positions on those circles are ILLUSTRATIVE, and the moon discs are enlarged so that they stay visible beside Jupiter. ## Bulk properties Europa has a mass of 4.80 × 10²² kg, the least of the four Galilean moons, and a mean [[Density|density]] of 3,010 kg/m³.[^nasa-jsat] Its diameter of just over 3,100 km makes it the sixth-largest moon in the Solar System, and its density is close to that of rocky [[Terrestrial_planet|terrestrial planets]], so most of its mass must be silicate rock.[^kargel2000] Its surface [[Gravity|gravity]] is about G × 4.80 × 10²² kg / (1.5608 × 10⁶ m)² ≈ 1.3 m/s² (derived), roughly 13% of Earth's. ### Internal structure Models place a layer of [[Water|water]] about 100 km thick over the rock, part of it frozen into the [[Crust_(geology)|crust]] and part liquid.[^phillips2014] A metallic [[Iron|iron]] core probably sits at the centre.[^kivelson2000] One sign that the ice floats freely is evidence that parts of the crust have turned by nearly 80°, almost tipping over, which would be hard to arrange if the ice were welded to the mantle.[^cowen2008] ### Subsurface ocean Surface temperatures average about 110 K at the equator and about 50 K near the poles, cold enough that the ice behaves as a hard, brittle solid.[^mcfadden2007] The earliest argument for water beneath it was theoretical: tidal heating from the forced eccentricity could keep a deep layer melted. Galileo imaging then added geological evidence, and the magnetometer supplied the strongest test.[^greenberg2005] The spacecraft found that Europa carries a magnetic moment induced by the changing part of Jupiter's field, with a strength of about 120 nT at its magnetic equator, about one-sixth of [[Ganymede_(moon)|Ganymede]]'s field and six times [[Callisto_(moon)|Callisto]]'s.[^zimmer2000] An induced field of that kind needs an [[Electrical_resistivity_and_conductivity|electrically conducting]] layer near the surface, and a global ocean of salty water is the most plausible one.[^kivelson2000] How thick the ice is remains disputed. In the thick-ice view favoured by most geologists who have mapped the moon, the ocean seldom or never reaches the present surface.[^greeley2004] Estimates from the structure of the largest impact basins and from tidal heating give a solid shell of about 10–30 km, over an ocean perhaps 100 km deep.[^park2015][^levin2025] A thin-ice view allows a shell of only a few kilometres; flexure studies show that the rigid, elastic part of the crust may be as thin as 200 m, but most researchers read that as the top of a thicker shell.[^billings2005] Impacts large enough to punch through the ice could still expose the ocean locally.[^cox2008] At the bottom of the ocean, a 2026 modelling study finds the rocky seafloor probably too strong to be broken by present tides, so any seafloor energy for life would have to come from processes other than active faulting.[^byrne2026] The ocean's chemistry is read indirectly from the surface. Infrared spectra of the reddish material along fractures were first matched with hydrated salts such as [[Magnesium|magnesium]] sulfate, left behind as water from below evaporated, and later with sulfuric acid hydrate.[^mccord1998][^carlson2005] An absorption at 450 nm in Hubble spectra of chaos regions is the signature of irradiated [[Sodium|sodium]] chloride, suggesting ocean salt close to the surface.[^trumbo2019] Carbon dioxide concentrated in Tara Regio, a recently resurfaced area, points to [[Carbon|carbon]] from inside the moon.[^trumbo2023] JWST spectra show that crystalline ice dominates at the surface in Tara and Powys Regiones, while elsewhere radiation keeps the topmost ice amorphous.[^cartwright2025] A weak band at 2.2 μm in reprocessed Galileo spectra has been identified with ammonia along linear features; ammonia dissolved in the ocean would lower the melting point of the ice.[^emran2025] Hubble images from 2012 were interpreted as a plume of water vapour rising up to about 200 km over the south pole, and a 2018 reanalysis of Galileo magnetic and plasma-wave data found a signature consistent with the spacecraft passing through a plume in 1997.[^esa-heic1322][^nasa-plume2013][^jia2018] Plumes, if real and frequent, would allow the ocean to be sampled from orbit. ### Sources of heat Most of Europa's internal heat is thought to come from [[Tide|tides]] rather than from [[Radioactive_decay|radioactive decay]] in the rock; radiogenic heating alone falls about a hundredfold short of the heat flow the observations call for.[^lowell2005][^ruiz2005] Tidal energy is dissipated in three places. In the ocean, Robert Tyler calculated that Europa's small axial tilt could drive a resonant tidal flow, a slow planetary wave carrying about 7.3 × 10¹⁸ J of kinetic energy for a tilt of 0.1°, roughly two thousand times the energy of the flow set up by the main tidal forcing; its dissipation could be the ocean's chief heat source.[^tyler2008] In the ice, laboratory experiments published in 2016 show that deformation of the crystal lattice itself releases heat, about ten times more than had been assumed from friction between grains.[^mccarthy2016] In the rock, flexing of the mantle could warm the seafloor enough for hydrothermal activity.[^nasa-faq] ## Surface environment ### Ice shell and surface No other known solid body in the [[PORTAL_Solar_System|Solar System]] is as smooth as Europa: it has no mountains and very few craters, and the lines that cross it are mostly differences in brightness with little relief.[^galileo2001] Its geometric albedo of 0.64 is among the highest of any moon.[^jpl-europa] Crater counts calibrated against the rate at which comets strike the Jovian system give the surface an age of only about 20–180 million years, so it is being renewed.[^schenk2004] The dominant features are the *lineae*, dark bands that run across the whole globe. The widest exceed 20 km, often with diffuse dark margins, internal striations and a brighter central stripe, and the crust on the two sides has visibly shifted.[^geissler1998b] A common reading is that the crust pulls apart along them and warmer ice wells up to fill the gap, much as new crust forms at mid-ocean ridges on [[Earth]].[^figueredo2004] Tidal stress from [[Jupiter]] should crack a locked shell in a predictable pattern, but only the youngest fractures follow it; older ones are rotated progressively further, as expected if the shell has slipped slowly around the interior.[^hurford2007] Galileo images also show places where one plate of ice appears to have slid beneath another, evidence for subduction and possibly for a form of plate tectonics in ice.[^kattenhorn2014] Modelling suggests, however, that the forces resisting plate motion in such a shell exceed those that could drive it, so the mechanism is unlikely to copy Earth's.[^howell2019] Other terrains are rounded: domes, pits and dark spots called *lenticulae* (Latin for "freckles"), and wider jumbled areas of *chaos* such as Conamara Chaos, where rafts of older crust sit in a hummocky matrix like icebergs frozen into sea ice. One explanation is warm ice rising as diapirs through colder ice and sometimes melting through.[^sotin2002][^goodman2004] In 2011 a team led from the University of Texas argued that some chaos regions lie above lens-shaped lakes of liquid water held within the shell, well above the main ocean.[^schmidt2011] Another hypothesis links chaos to comets that penetrated the crust.[^cox2008] Near the equator, sunlight striking the ice from almost overhead may sculpt it into blades up to about 15 m tall, a possible hazard for any lander.[^hobley2018] ### Radiation environment Europa orbits deep within Jupiter's magnetosphere, and charged particles trapped there strike its surface continuously. The resulting dose at the surface is estimated at about 5.4 Sv per day, enough to cause severe illness or death in a person exposed for a single day.[^ringwald2000][^glasstone1962] The same bombardment breaks up [[Water|water]] molecules in the ice, which drives both the surface chemistry and the thin atmosphere. ### Atmosphere Europa has an extremely tenuous atmosphere of molecular [[Oxygen|oxygen]], first detected in 1995 with Hubble's Goddard High Resolution Spectrograph.[^hall1995] Galileo radio occultations in 1997 then showed an ionosphere above the surface.[^kliore1997] The oxygen is not biological. Ultraviolet [[Photon|light]] and charged particles split water ice into hydrogen and oxygen; the [[Hydrogen|hydrogen]] is light enough to escape, while oxygen molecules, which do not freeze onto the surface on contact, bounce repeatedly and accumulate.[^hubble1995] Gas lost from Europa spreads into a neutral cloud along its orbit that holds more material than the corresponding cloud of [[Io_(moon)|Io]].[^smyth2006] Energetic neutral atom observations, interpreted with modelling, later confirmed this torus.[^smith2019] Measurements by the Juno spacecraft reported in 2024 put the oxygen production rate toward the low end of earlier estimates.[^szalay2024] Laboratory work on oxygen hydrate, stable in ice to at least 2.6 GPa, suggests some radiolytic oxygen may be stored in the shell rather than lost.[^frost2025] ## Exploration Pioneer 10 and 11 passed [[Jupiter]] in 1973 and 1974 and returned only coarse images of Europa; the two Voyagers ([[Voyager_1]] and its twin), in 1979, showed its bright, fractured face in enough detail to raise the idea of water underneath.[^nasa-whyeuropa] The Galileo orbiter, in orbit from 1995 to 2003, made repeated close passes during its primary mission and its Galileo Europa and Millennium extensions, and supplied most of what is known today.[^nasa-gem] New Horizons imaged the moon in 2007 on its way to [[Pluto]], and Juno flew 352 km above the surface in September 2022.[^nasa-pia09246][^nasa-juno2022] The European Space Agency's Jupiter Icy Moons Explorer (Juice), launched on 14 April 2023, is aimed chiefly at [[Ganymede_(moon)|Ganymede]] but includes two flybys of Europa after it reaches Jupiter in July 2031.[^esa-juice][^esa-juicetour] [[NASA]] selected the instruments for a dedicated Europa mission in 2015; Europa Clipper will orbit Jupiter rather than Europa, making repeated low passes with an ice-penetrating radar, an infrared spectrometer, cameras and a mass spectrometer. It was launched on a Falcon Heavy on 14 October 2024.[^nasa-instruments2015][^nasa-launch2021][^nasa-clipper] ### Future missions The possibility of life has kept Europa near the top of proposed destinations, but any spacecraft there must survive Jupiter's radiation belts.[^friedman2005] A Europa Lander concept has been studied by NASA,[^nasa-lander] and JPL is developing BRUIE, a buoyant rover designed to crawl along the underside of floating ice, as a prototype for exploring an ice-covered ocean.[^jpl-bruie] ### Old proposals Earlier concepts were cancelled or merged. NASA's Europa Orbiter, approved in 1999 with an ice-penetrating radar, was cancelled in 2002.[^galileo2001] The nuclear-electric Jupiter Icy Moons Orbiter, part of Project Prometheus, was dropped from NASA's 2006 budget proposal.[^berger2005] In 2009 NASA and ESA gave priority to the joint Europa Jupiter System Mission, a pair of orbiters for Europa and Ganymede, over a mission to [[Titan_(moon)|Titan]].[^nasa-ejsm2009][^rincon2009] Other studies included Ice Clipper, which would have sampled debris thrown up by an impactor, and melt probes that would descend through the ice and release an underwater vehicle, an approach that would require strict sterilization to avoid contaminating the ocean.[^mckay2002][^friedman2005][^nrc2000] ## Habitability No life has been detected on Europa, but the moon combines the ingredients usually listed for habitability: liquid [[Water|water]], probably in contact with rock, a source of energy, and chemistry supplied from both the seafloor and the surface.[^hand2007] Life could in principle live near seafloor vents, within rock below the seafloor, or attached to the underside of the ice, though an ocean that is too cold or too salty would restrict it to specialised organisms.[^marion2003] Oxidants are a key question. Radiation makes [[Oxygen|oxygen]] and [[Hydrogen|hydrogen]] peroxide in the surface ice, and if the shell overturns they could be carried down; Richard Greenberg estimated in 2010 that such transport could make the ocean as oxygen-rich as [[Earth]]'s within about 12 million years.[^greenberg2010] Hand and co-authors estimated that subduction of these oxidants over the surface's lifetime could bring the ocean's free oxygen close to levels in Earth's deep ocean.[^hand2007] On the reducing side, a 2016 NASA study found that reactions between water and rock, such as serpentinization, could supply hydrogen in Earth-like proportions to the oxidants even without volcanism.[^jpl-balance2016] Salt that may come from the ocean has been identified on the surface, clay-like minerals possibly delivered by an impact have been reported, and the lakes proposed within the ice would be further possible habitats.[^jpl-salt2015][^jpl-clay2013][^schmidt2011] Against this, the 2026 finding of a mechanically quiet seafloor would limit hot vents of the "black smoker" type, and the 2024 Juno result lowers the supply of surface oxygen.[^byrne2026][^szalay2024] ### Far future When the [[Sun]] leaves the main sequence in about five billion years and swells into a red giant,[^spiegel2012] the [[Habitable_zone|habitable zone]] will sweep outward past [[Jupiter]]. A 2025 study finds that Europa's ice would then melt or sublimate, giving the moon a temporary water-vapour atmosphere for about 200 million years before the zone moves on.[^mullens2025] ## See also - [[Galilean_moons]] - [[Jupiter]] · [[Io_(moon)|Io]] · [[Ganymede_(moon)|Ganymede]] · [[Callisto_(moon)|Callisto]] - [[Enceladus]] · [[Titan_(moon)|Titan]] - [[Habitable_zone]] - [[Tide]] - Ocean world · Extraterrestrial water - [[PORTAL_Solar_System|Solar System portal]] ## Notes Derived numbers are computed from the cited values: the change in distance across an orbit is 2ae = 2 × 670,900 km × 0.009 ≈ 12,000 km; the surface gravity is GM/r² with G = 6.674 × 10⁻¹¹ m³ kg⁻¹ s⁻², M = 4.80 × 10²² kg and r = 1.5608 × 10⁶ m, giving ≈ 1.3 m/s², or 0.13 of Earth's 9.8 m/s². ## References [^nasa-jsat]: Williams, D. R. "Jovian Satellite Fact Sheet". NASA NSSDCA. https://nssdc.gsfc.nasa.gov/planetary/factsheet/joviansatfact.html (fetched 2026-09-18). [^jpl-europa]: NASA/JPL Project Galileo. "Europa, a Continuing Story of Discovery". http://www2.jpl.nasa.gov/galileo/europa/#overview [^kargel2000]: Kargel, J. S.; Kaye, J. Z.; Head, J. W.; Marion, G. M.; Sassen, R.; Crowley, J. K.; et al. (2000). "Europa's crust and ocean: origin, composition, and the prospects for life". *Icarus* 148: 226–265. https://doi.org/10.1006/icar.2000.6471 [^kivelson2000]: Kivelson, M. G.; Khurana, K. K.; Russell, C. T.; Volwerk, M.; Walker, R. J.; Zimmer, C. (2000). "Galileo magnetometer measurements: a stronger case for a subsurface ocean at Europa". *Science* 289: 1340–1343. https://doi.org/10.1126/science.289.5483.1340 [^greenberg2005]: Greenberg, R. (2005). *Europa: The Ocean Moon: Search for an Alien Biosphere*. Springer–Praxis. ISBN 978-3-540-27053-9. https://doi.org/10.1007/b138547 [^showman1997]: Showman, A. P.; Malhotra, R. (1997). 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ISBN 978-0-387-47936-1. ## External links - NASA Science: Europa — https://science.nasa.gov/jupiter/moons/europa/ - NASA Europa Clipper mission — https://europa.nasa.gov/ - ESA Juice mission — https://sci.esa.int/web/juice - USGS Gazetteer of Planetary Nomenclature: Jupiter system — https://planetarynames.wr.usgs.gov/Page/JUPITER/system - NASA/JPL Photojournal: Europa images — https://photojournal.jpl.nasa.gov/ ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Europa_(moon)) : [Wikitube](https://en.wikitube.io/wiki/Europa_(moon)) · pinned revision [1374725737](https://en.wikipedia.org/w/index.php?oldid=1374725737) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-058 · explorer state `?obj=Europa`.* <!-- hub_tags: Life_Physics · PORTAL_Solar_System -->