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Europa (moon)
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{{Other uses|Europa (disambiguation){{!}}Europa}}{{redirect|Jupiter II|the spaceship in the 1960s television series Lost in Space|Jupiter 2|the airplane engine|Bristol Jupiter|other uses|Jupiter (disambiguation)}}{{short description|The smallest of the four Galilean moons of Jupiter}}{{Use dmy dates|date=July 2019}}







factoids
|epoch = 8 January 2004670900URL=HTTP://SOLARSYSTEM.NASA.GOV/PLANETS/PROFILE.CFM?OBJECT=JUP_EUROPADEADURL=YESARCHIVEDATE=26 MARCH 2014, |eccentricity = 0.009664862|u=km}}Periapsis is derived from the semimajor axis (a) and eccentricity (e): a(1 âˆ’ e).676938|u=km}}Apoapsis is derived from the semimajor axis (a) and eccentricity (e): a(1 + e).3.551181|u=d}}13.740|u=km/s}}|inclination = 0.470° (to Jupiter's equator)1.791° (to the ecliptic)|satellite_of = Jupiter1560.8u=km}} (0.245 Earths)3.09u=km2}} (0.061 Earths)Surface area derived from the radius (r): 4{{pi}}r 2.1.593u=km3}} (0.015 Earths)Volume derived from the radius (r): 4/3{{pi}}r 3.4.799844e=22|u=kg}} (0.008 Earths)3.013u=g/cm3}}1.314|ul=m/s2}} (0.134 g)Surface gravity derived from the mass (m), the gravitational constant (G) and the radius (r): Gm/r2.0.346LAST2= MALHOTRATITLE=THE GALILEAN SATELLITESVOLUME= 286DATE= 1 OCTOBER 1999DOI= 10.1126/SCIENCE.286.5437.77, 10506564, (estimate)2.025|u=km/s}}Escape velocity derived from the mass (m), the gravitational constant (G) and the radius (r): textstylesqrt{2Gm/r}.synchronous rotation>SynchronousTITLE=FREE AND FORCED OBLIQUITIES OF THE GALILEAN SATELLITES OF JUPITERVOLUME=175PAGES=233–247BIBCODE=2005ICAR..175..233BURL=HTTPS://ZENODO.ORG/RECORD/1259023, DATE=13 JULY 2006PUBLISHER=JPL SOLAR SYSTEM DYNAMICSACCESSDATE=5 NOVEMBER 2007, Opposition (astronomy and astrology)>opposition)|temp_name1 = Surfacekelvin>K|mean_temp_1 = 102 K (−171.15 Â°C)|max_temp_1 = 125 K|atmosphere = yesPascal (unit)>µPa (10−12 bar (unit))MCGRATHEDITOR2=MCKINNON, WILLIAM B.TITLE=EUROPAPUBLISHER=UNIVERSITY OF ARIZONA PRESSCHAPTER=ATMOSPHERE OF EUROPA, }}Europa ({{IPAc-en|audio=en-Europa.ogg|j|ʊəˈr|oÊŠ|p|É™}} {{respell|yoor|OH|pÉ™}},or as Jupiter II) is the smallest of the four Galilean moons orbiting Jupiter, and the sixth-closest to the planet of all the 79 known moons of Jupiter. It is also the sixth-largest moon in the Solar System. Europa was discovered in 1610 by Galileo Galilei and was named after Europa, the Phoenician mother of King Minos of Crete and lover of Zeus (the Greek equivalent of the Roman god Jupiter).Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and has a water-ice crustNEWS, Chang, Kenneth, Suddenly, It Seems, Water Is Everywhere in Solar System,weblink 12 March 2015, The New York Times, 13 March 2015, and probably an iron–nickel core. It has a very thin atmosphere, composed primarily of oxygen. Its surface is striated by cracks and streaks, but craters are relatively few. In addition to Earth-bound telescope observations, Europa has been examined by a succession of space-probe flybys, the first occurring in the early 1970s.Europa has the smoothest surface of any known solid object in the Solar System. The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it, which could conceivably harbor extraterrestrial life.WEB,weblink Possibility of Life on Europa, Tritt, Charles S., 10 August 2007, Milwaukee School of Engineering, 2002, yes,weblink" title="web.archive.org/web/20070609150109weblink">weblink 9 June 2007, The predominant model suggests that heat from tidal flexing causes the ocean to remain liquid and drives ice movement similar to plate tectonics, absorbing chemicals from the surface into the ocean below.WEB,weblink Tidal Heating, geology.asu.edu,weblink" title="web.archive.org/web/20060329000051weblink">weblink 29 March 2006, yes, WEB, Dyches, Preston, Brown, Dwayne, Buckley, Michael, Scientists Find Evidence of 'Diving' Tectonic Plates on Europa,weblink 8 September 2014, NASA, 8 September 2014, Sea salt from a subsurface ocean may be coating some geological features on Europa, suggesting that the ocean is interacting with the sea floor. This may be important in determining whether Europa could be habitable.WEB, Dyches, Preston, Brown, Dwayne, NASA Research Reveals Europa's Mystery Dark Material Could Be Sea Salt,weblink 12 May 2015, NASA, 12 May 2015, In addition, the Hubble Space Telescope detected water vapor plumes similar to those observed on Saturn's moon Enceladus, which are thought to be caused by erupting cryogeysers.WEB, Cook, Jia-Rui C., Gutro, Rob, Brown, Dwayne, Harrington, J. D., Fohn, Joe, Hubble Sees Evidence of Water Vapor at Jupiter Moon,weblink 12 December 2013, NASA, In May 2018, astronomers provided supporting evidence of water plume activity on Europa, based on an updated critical analysis of data obtained from the Galileo space probe, which orbited Jupiter from 1995 to 2003. Such plume activity could help researchers in a search for life from the subsurface Europan ocean without having to land on the moon.The Galileo mission, launched in 1989, provides the bulk of current data on Europa. No spacecraft has yet landed on Europa, although there have been several proposed exploration missions. The European Space Agency's Jupiter Icy Moon Explorer (JUICE) is a mission to Ganymede that is due to launch in 2022 and will include two flybys of Europa.NEWS, Jonathan, Amos,weblink Esa selects 1bn-euro Juice probe to Jupiter, 2 May 2012, 2 May 2012, BBC News Online, NASA's planned Europa Clipper should be launched in the mid-2020s.NEWS, Borenstein, Seth, NASA plots daring flight to Jupiter's watery moon,weblink 4 March 2014, Associated Press, 5 March 2014,weblink" title="web.archive.org/web/20140305180141weblink">weblink 5 March 2014, yes, {{TOC limit|4}}

Discovery and naming

Europa, along with Jupiter's three other large moons, Io, Ganymede, and Callisto, was discovered by Galileo Galilei on 8 January 1610, and possibly independently by Simon Marius. The first reported observation of Io and Europa was made by Galileo on 7 January 1610 using a 20×-magnification refracting telescope at the University of Padua. However, in that observation, Galileo could not separate Io and Europa due to the low magnification of his telescope, so that the two were recorded as a single point of light. The following day, 8 January 1610 (used as the discovery date for Europa by the IAU), Io and Europa were seen for the first time as separate bodies during Galileo's observations of the Jupiter system.WEB, Blue, Jennifer, 9 November 2009,weblink Planet and Satellite Names and Discoverers, USGS, Europa is named after Europa, daughter of the king of Tyre, a Phoenician noblewoman in Greek mythology. Like all the Galilean satellites, Europa is named after a lover of Zeus, the Greek counterpart of Jupiter. Europa was courted by Zeus and became the queen of Crete.WEB,weblink Europa, Arnett, Bill, Nine Planets, October 2005, 27 April 2014, The naming scheme was suggested by Simon Marius, who attributed the proposal to Johannes Kepler.Marius, S.; (1614) Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici weblink, where he attributes the suggestion to Johannes KeplerWEB,weblink Simon Marius (January 20, 1573 – December 26, 1624), 9 August 2007, University of Arizona, Students for the Exploration and Development of Space, yes,weblink" title="web.archive.org/web/20070713221843weblink">weblink 13 July 2007, The names fell out of favor for a considerable time and were not revived in general use until the mid-20th century.JOURNAL, Marazzini, Claudio, 2005, I nomi dei satelliti di Giove: da Galileo a Simon Marius (The names of the satellites of Jupiter: from Galileo to Simon Marius), Lettere Italiane, 57, 3, 391–407, In much of the earlier astronomical literature, Europa is simply referred to by its Roman numeral designation as {{Nowrap|Jupiter II}} (a system also introduced by Galileo) or as the "second satellite of Jupiter". In 1892, the discovery of Amalthea, whose orbit lay closer to Jupiter than those of the Galilean moons, pushed Europa to the third position. The Voyager probes discovered three more inner satellites in 1979, so Europa is now considered Jupiter's sixth satellite, though it is still sometimes referred to as {{Nowrap|Jupiter II}}.

Orbit and rotation

(File:Galilean moons around Jupiter.gif|thumb|right|Galilean moons around Jupiter {{legend2|Lime|Jupiter}}{{·}}{{legend2|OrangeRed|Io}}{{·}}{{legend2|RoyalBlue|Europa}}{{·}}{{legend2|Gold|Ganymede}}{{·}}{{legend2|Cyan|Callisto}})File:Galilean moon Laplace resonance animation.gif|thumb|right|Animation showing Io's Laplace resonanceLaplace resonanceEuropa orbits Jupiter in just over three and a half days, with an orbital radius of about 670,900 km. With an orbital eccentricity of only 0.009, the orbit itself is nearly circular, and the orbital inclination relative to Jupiter's equatorial plane is small, at 0.470°.WEB,weblinkweblink" title="web.archive.org/web/19970105180851weblink">weblink yes, 5 January 1997, Europa, a Continuing Story of Discovery, 9 August 2007, Project Galileo, NASA, Jet Propulsion Laboratory, Like its fellow Galilean satellites, Europa is tidally locked to Jupiter, with one hemisphere of Europa constantly facing Jupiter. Because of this, there is a sub-Jovian point on Europa's surface, from which Jupiter would appear to hang directly overhead. Europa's prime meridian is a line passing through this point.WEB, Planetographic Coordinates, Wolfram Research,weblink 2010, 29 March 2010, yes,weblink" title="web.archive.org/web/20090301191456weblink">weblink 1 March 2009, Research suggests that the tidal locking may not be full, as a non-synchronous rotation has been proposed: Europa spins faster than it orbits, or at least did so in the past. This suggests an asymmetry in internal mass distribution and that a layer of subsurface liquid separates the icy crust from the rocky interior.JOURNAL, Geissler, P. E., Greenberg, R., Hoppa, G., Helfenstein, P., McEwen, A., Pappalardo, R., Tufts, R., Ockert-Bell, M., Sullivan, R., Greeley, R., Belton, M. J. S., Denk, T., Clark, B. E., Burns, J., Veverka, J., 1998, Evidence for non-synchronous rotation of Europa, Nature (journal), Nature, 391, 368–70, 1998Natur.391..368G, 10.1038/34869, 9450751, 6665, The slight eccentricity of Europa's orbit, maintained by the gravitational disturbances from the other Galileans, causes Europa's sub-Jovian point to oscillate around a mean position. As Europa comes slightly nearer to Jupiter, Jupiter's gravitational attraction increases, causing Europa to elongate towards and away from it. As Europa moves slightly away from Jupiter, Jupiter's gravitational force decreases, causing Europa to relax back into a more spherical shape, and creating tides in its ocean. The orbital eccentricity of Europa is continuously pumped by its mean-motion resonance with Io.JOURNAL, 10.1006/icar.1996.5669, Showman, Adam P., Malhotra, Renu, Tidal Evolution into the Laplace Resonance and the Resurfacing of Ganymede, Icarus, 127, 1997, 1, 93–111,weblink 1997Icar..127...93S, Thus, the tidal flexing kneads Europa's interior and gives it a source of heat, possibly allowing its ocean to stay liquid while driving subsurface geological processes. The ultimate source of this energy is Jupiter's rotation, which is tapped by Io through the tides it raises on Jupiter and is transferred to Europa and Ganymede by the orbital resonance.JOURNAL, Moore, W. B., Tidal heating and convection in Io, Journal of Geophysical Research, 108, E8, 5096, 2003, 0148-0227, 10.1029/2002JE001943,weblink 2 January 2008, 2003JGRE..108.5096M, 10.1.1.558.6924, Analysis of the unique cracks lining Europa yielded evidence that it likely spun around a tilted axis at some point in time. If correct, this would explain many of Europa's features. Europa's immense network of crisscrossing cracks serves as a record of the stresses caused by massive tides in its global ocean. Europa's tilt could influence calculations of how much of its history is recorded in its frozen shell, how much heat is generated by tides in its ocean, and even how long the ocean has been liquid. Its ice layer must stretch to accommodate these changes. When there is too much stress, it cracks. A tilt in Europa's axis could suggest that its cracks may be much more recent than previously thought. The reason for this is that the direction of the spin pole may change by as much as a few degrees per day, completing one precession period over several months. A tilt could also affect the estimates of the age of Europa's ocean. Tidal forces are thought to generate the heat that keeps Europa's ocean liquid, and a tilt in the spin axis would cause more heat to be generated by tidal forces. Such additional heat would have allowed the ocean to remain liquid for a longer time. However, it has not yet been determined when this hypothesized shift in the spin axis might have occurred.Cook, Jia-Rui C. (18 September 2013) Long-stressed Europa Likely Off-kilter at One Time. jpl.nasa.gov

Physical characteristics

(File:Europa, Earth & Moon size comparison.jpg|thumb|Size comparison of Europa (lower left) with the Moon (top left) and Earth (right))Europa is slightly smaller than the Moon. At just over {{Convert|3100|km|mi}} in diameter, it is the sixth-largest moon and fifteenth-largest object in the Solar System. Though by a wide margin the least massive of the Galilean satellites, it is nonetheless more massive than all known moons in the Solar System smaller than itself combined.Mass of Europa: 48{{e|21}} kg. Mass of Triton plus all smaller moons: 39.5{{e|21}} kg (see note g here) Its bulk density suggests that it is similar in composition to the terrestrial planets, being primarily composed of silicate rock.JOURNAL, Kargel, Jeffrey S., Kaye, Jonathan Z., Head, James W., III, Marion, Giles M., Sassen, Roger, Crowley, James K., Ballesteros, Olga Prieto, Grant, Steven A., Hogenboom, David L., 2000, Europa's Crust and Ocean: Origin, Composition, and the Prospects for Life, Planetary Sciences Group, Brown University, Icarus, 148, 1, 226–265, 2000Icar..148..226K, 10.1006/icar.2000.6471,weblink

Internal structure

It is estimated that Europa has an outer layer of water around {{Convert|100|km|mi|-0|abbr=on}} thick; a part frozen as its crust, and a part as a liquid ocean underneath the ice. Recent magnetic-field data from the Galileo orbiter showed that Europa has an induced magnetic field through interaction with Jupiter's, which suggests the presence of a subsurface conductive layer.JOURNAL, Europa Clipper Mission Concept, Eos, Transactions American Geophysical Union, 20 May 2014, Phillips, Cynthia B., Pappalardo, Robert T., 95, 20, 165–167, 10.1002/2014EO200002, 2014EOSTr..95..165P, This layer is likely a salty liquid-water ocean. Portions of the crust are estimated to have undergone a rotation of nearly 80°, nearly flipping over (see true polar wander), which would be unlikely if the ice were solidly attached to the mantle.NEWS, Ron, Cowen, A Shifty Moon,weblink Science News, 7 June 2008, Europa probably contains a metallic iron core.JOURNAL, Kivelson, Margaret G., Khurana, Krishan K., Russell, Christopher T., Volwerk, Martin, Walker, Raymond J., Zimmer, Christophe, 2000, Galileo Magnetometer Measurements: A Stronger Case for a Subsurface Ocean at Europa, Science (journal), Science, 289, 5483, 1340–1343, 10.1126/science.289.5483.1340, 10958778, 2000Sci...289.1340K, JOURNAL, Bhatia, G.K., Sahijpal, S., Thermal evolution of trans-Neptunian objects, icy satellites, and minor icy planets in the early solar system, Meteoritics & Planetary Science, 10.1111/maps.12952, 52, 12, 2017, 2470–2490, 2017M&PS...52.2470B,

Surface features

(File:PIA01295 Europa Global Views in Natural and Enhanced Colors.jpg|thumb|Approximate natural color (left) and enhanced color (right) Galileo view of leading hemisphere)Europa is the smoothest known object in the Solar System, lacking large-scale features such as mountains and craters.WEB,weblinkweblink" title="web.archive.org/web/20110721210346weblink">weblink 21 July 2011, Europa: Another Water World?, 2001, 9 August 2007, NASA, Jet Propulsion Laboratory, Project Galileo: Moons and Rings of Jupiter, However, according to one study, Europa's equator may be covered in icy spikes called penitentes, which may be up to 15 meters high, due to direct overhead sunlight on the equator, causing the ice to sublime, forming vertical cracks.WEB,weblink Ice blades threaten Europa landing, BBC News, Europa may have towering ice spikes on its surface. Paul Scott Anderson, Earth and Sky. 20 October 2018.Formation of metre-scale bladed roughness on Europa’s surface by ablation of ice. Daniel E. J. Hobley, Jeffrey M. Moore, Alan D. Howard, and Orkan M. Umurhan. Nature Geoscience. 8 October 2018. {{doi|10.1038/s41561-018-0235-0}}. Although the imaging available from the Galileo orbiter does not have the resolution needed to confirm this, radar and thermal data are consistent with this interpretation. The prominent markings crisscrossing Europa appear to be mainly albedo features that emphasize low topography. There are few craters on Europa, because its surface is tectonically too active and therefore young.Arnett, Bill (7 November 1996) Europa. astro.auth.grWEB,weblink Hamilton, Calvin J., Jupiter's Moon Europa, solarviews.com, Europa's icy crust has an albedo (light reflectivity) of 0.64, one of the highest of all moons. This indicates a young and active surface: based on estimates of the frequency of cometary bombardment that Europa experiences, the surface is about 20 to 180 million years old.Schenk, Paul M.; Chapman, Clark R.; Zahnle, Kevin; and Moore, Jeffrey M. (2004) "Chapter 18: Ages and Interiors: the Cratering Record of the Galilean Satellites", pp. 427 ff. in Jupiter: The Planet, Satellites and Magnetosphere, Cambridge University Press, {{ISBN|0-521-81808-7}}. There is currently no full scientific consensus among the sometimes contradictory explanations for the surface features of Europa.WEB,weblink High Tide on Europa, 2007, 20 October 2007, astrobio.net, Astrobiology Magazine, The radiation level at the surface of Europa is equivalent to a dose of about 5400 mSv (540 rem) per day,WEB, 29 February 2000, SPS 1020 (Introduction to Space Sciences), California State University, Fresno, Frederick A. Ringwald,weblink 4 July 2009, yes,weblink" title="web.archive.org/web/20080725050708weblink">weblink 25 July 2008, an amount of radiation that would cause severe illness or death in human beings exposed for a single day.The Effects of Nuclear Weapons, Revised ed., US DOD 1962, pp. 592–593

Lineae

{{See also|List of lineae on Europa}}File:PIA19048 realistic color Europa mosaic.jpg|alt=|thumb|Realistic-color Galileo mosaic of Europa's anti-Jovian hemisphere showing numerous lineaelineae
missing image!
- PIA20028 - Europa's varied surface features (rotated).jpg -
Enhanced-color view showing the intricate pattern of linear fractures on Europa's surface
Europa's most striking surface features are a series of dark streaks crisscrossing the entire globe, called (). Close examination shows that the edges of Europa's crust on either side of the cracks have moved relative to each other. The larger bands are more than {{Convert|20|km|mi|0|abbr=on}} across, often with dark, diffuse outer edges, regular striations, and a central band of lighter material.JOURNAL, Geissler, P., 10.1006/icar.1998.5980, Evolution of Lineaments on Europa: Clues from Galileo Multispectral Imaging Observations, Icarus, 135, 1, 107–337, 1998, 1998Icar..135..107G, The most likely hypothesis is that the lineae on Europa were produced by a series of eruptions of warm ice as the Europan crust spread open to expose warmer layers beneath.JOURNAL, Figueredo, P. H., Greeley, R., 10.1016/j.icarus.2003.09.016, Resurfacing history of Europa from pole-to-pole geological mapping, Icarus, 167, 2, 287, 2004, 2004Icar..167..287F, The effect would have been similar to that seen in Earth's oceanic ridges. These various fractures are thought to have been caused in large part by the tidal flexing exerted by Jupiter. Because Europa is tidally locked to Jupiter, and therefore always maintains approximately the same orientation towards Jupiter, the stress patterns should form a distinctive and predictable pattern. However, only the youngest of Europa's fractures conform to the predicted pattern; other fractures appear to occur at increasingly different orientations the older they are. This could be explained if Europa's surface rotates slightly faster than its interior, an effect that is possible due to the subsurface ocean mechanically decoupling Europa's surface from its rocky mantle and the effects of Jupiter's gravity tugging on Europa's outer ice crust.JOURNAL, Hurford, T. A., Sarid, A. R., Greenberg, R., Cycloidal cracks on Europa: Improved modeling and non-synchronous rotation implications, 10.1016/j.icarus.2006.08.026, Icarus, 186, 1, 218, 2007, 2007Icar..186..218H, Comparisons of Voyager and Galileo spacecraft photos serve to put an upper limit on this hypothetical slippage. A full revolution of the outer rigid shell relative to the interior of Europa takes at least 12,000 years.JOURNAL, Kattenhorn, Simon A., Nonsynchronous Rotation Evidence and Fracture History in the Bright Plains Region, Europa, Icarus, 157, 2, 490–506, 2002, 10.1006/icar.2002.6825, 2002Icar..157..490K, Studies of Voyager and Galileo images have revealed evidence of subduction on Europa's surface, suggesting that, just as the cracks are analogous to ocean ridges,JOURNAL, Fault Offsets and Lateral plate motions on Europa: Evidence for a mobile ice shell, Schenk, Paul, McKinnon, William B., Icarus, 79, 1, 75–100, 1989, 10.1016/0019-1035(89)90109-7, 1989Icar...79...75S, JOURNAL, Evidence for subduction in the ice shell of Europa, Kattenhorn, Simon, Prockter, Louise, Nature Geoscience, 7, 9, 762, 2014, 10.1038/ngeo2245, 2014NatGe...7..762K, so plates of icy crust analogous to tectonic plates on Earth are recycled into the molten interior. Together, the evidence for crustal spreading at bands and convergence at other sites marks the first evidence for plate tectonics on any world other than Earth.{{contradict-inline|article=Plate tectonics|section=Icy satellites|date=May 2017}}

Other geological features

{{See also|List of geological features on Europa}} {{multiple image
|direction = horizontal
|align = right
|width1 = 130
|width2 = 236
|image1 = PIA01092 - Evidence of Internal Activity on Europa.jpg
|image2 = Europa chaotic terrain.jpg
|footer = Left: surface features indicative of tidal flexing: lineae, lenticulae and the Conamara Chaos region (close-up, right) where craggy, 250 m high peaks and smooth plates are jumbled together
}}Other features present on Europa are circular and elliptical lenticulae (Latin for "freckles"). Many are domes, some are pits and some are smooth, dark spots. Others have a jumbled or rough texture. The dome tops look like pieces of the older plains around them, suggesting that the domes formed when the plains were pushed up from below.WEB, Europa: Tidal heating of upwelling thermal plumes and the origin of lenticulae and chaos melting, Sotin, Christophe, Head III, James W., Tobie, Gabriel, 2001,weblink 20 December 2007, One hypothesis states that these lenticulae were formed by diapirs of warm ice rising up through the colder ice of the outer crust, much like magma chambers in Earth's crust. The smooth, dark spots could be formed by meltwater released when the warm ice breaks through the surface. The rough, jumbled lenticulae (called regions of "chaos"; for example, Conamara Chaos) would then be formed from many small fragments of crust, embedded in hummocky, dark material, appearing like icebergs in a frozen sea.JOURNAL, 10.1029/2003JE002073, Hydrothermal plume dynamics on Europa: Implications for chaos formation, Journal of Geophysical Research, 109, E3, E03008, 2004, Goodman, Jason C.,weblink 2004JGRE..109.3008G, yes,weblink" title="web.archive.org/web/20120308061644weblink">weblink 8 March 2012, An alternative hypothesis suggest that lenticulae are actually small areas of chaos and that the claimed pits, spots and domes are artefacts resulting from over-interpretation of early, low-resolution Galileo images. The implication is that the ice is too thin to support the convective diapir model of feature formation.JOURNAL, Tidal Heat in Europa: Ice Thickness and the Plausibility of Melt-Through, O'Brien, David P., Geissler, Paul, Greenberg, Richard, Bulletin of the American Astronomical Society, October 2000, 30, 1066, 2000DPS....32.3802O, BOOK, Unmasking Europa, Greenberg, Richard, 2008,weblink 978-0-387-09676-6, Springer + Praxis Publishing, In November 2011, a team of researchers from the University of Texas at Austin and elsewhere presented evidence in the journal Nature suggesting that many "chaos terrain" features on Europa sit atop vast lakes of liquid water.JOURNAL, Active formation of 'chaos terrain' over shallow subsurface water on Europa, Schmidt, Britney, Blankenship, Don, Patterson, Wes, Schenk, Paul, Nature, 24 November 2011, 479, 502–505, 10.1038/nature10608, 2011Natur.479..502S, 7374, 22089135, WEB, Scientists Find Evidence for "Great Lake" on Europa and Potential New Habitat for Life, Airhart, Marc, 2011, Jackson School of Geosciences,weblink 16 November 2011, These lakes would be entirely encased in Europa's icy outer shell and distinct from a liquid ocean thought to exist farther down beneath the ice shell. Full confirmation of the lakes' existence will require a space mission designed to probe the ice shell either physically or indirectly, for example, using radar.

Subsurface ocean

missing image!
- EuropaInterior1.jpg -
Two possible models of Europa
Scientists' consensus is that a layer of liquid water exists beneath Europa's surface, and that heat from tidal flexing allows the subsurface ocean to remain liquid. Europa's surface temperature averages about {{Convert|110|K|C F|-1|lk=on}} at the equator and only {{Convert|50|K|C F|-1|}} at the poles, keeping Europa's icy crust as hard as granite.BOOK, The Encyclopedia of the Solar System, McFadden, Lucy-Ann, Weissman, Paul, Johnson, Torrence, Elsevier, 2007, 432, 978-0-12-226805-2, The first hints of a subsurface ocean came from theoretical considerations of tidal heating (a consequence of Europa's slightly eccentric orbit and orbital resonance with the other Galilean moons). Galileo imaging team members argue for the existence of a subsurface ocean from analysis of Voyager and Galileo images.Greenberg, Richard (2005) Europa: The Ocean Moon: Search for an Alien Biosphere, Springer + Praxis Books, {{ISBN|978-3-540-27053-9}}. The most dramatic example is "chaos terrain", a common feature on Europa's surface that some interpret as a region where the subsurface ocean has melted through the icy crust. This interpretation is controversial. Most geologists who have studied Europa favor what is commonly called the "thick ice" model, in which the ocean has rarely, if ever, directly interacted with the present surface.Greeley, Ronald; et al. (2004) "Chapter 15: Geology of Europa", pp. 329 ff. in Jupiter: The Planet, Satellites and Magnetosphere, Cambridge University Press, {{ISBN|0-521-81808-7}}. The best evidence for the thick-ice model is a study of Europa's large craters. The largest impact structures are surrounded by concentric rings and appear to be filled with relatively flat, fresh ice; based on this and on the calculated amount of heat generated by Europan tides, it is estimated that the outer crust of solid ice is approximately 10–30 km (6–19 mi) thick,JOURNAL, Improved detection of tides at Europa with radiometric and optical tracking during flybys, Planetary and Space Science, July 2015, Park, Ryan S., Bills, Bruce, Buffington, Brent B., 112, 10–14, 10.1016/j.pss.2015.04.005, 2015P&SS..112...10P, including a ductile "warm ice" layer, which could mean that the liquid ocean underneath may be about {{Convert|100|km|mi|-1|abbr=on}} deep.NEWS, Zaina, Adamu, Water near surface of a Jupiter moon only temporary, 1 October 2012,weblink CNN News, 2 October 2012, This leads to a volume of Europa's oceans of 3 Ã— 1018 m3, between two or three times the volume of Earth's oceans.APOD, All the Water on Europa, 24 May 2012, 8 March 2016, WEB, Williams, Matt,weblink Jupiter's Moon Europa, Universe Today, 15 September 2015, 9 March 2016, The thin-ice model suggests that Europa's ice shell may be only a few kilometers thick. However, most planetary scientists conclude that this model considers only those topmost layers of Europa's crust that behave elastically when affected by Jupiter's tides. One example is flexure analysis, in which Europa's crust is modeled as a plane or sphere weighted and flexed by a heavy load. Models such as this suggest the outer elastic portion of the ice crust could be as thin as {{Convert|200|m|ft}}. If the ice shell of Europa is really only a few kilometers thick, this "thin ice" model would mean that regular contact of the liquid interior with the surface could occur through open ridges, causing the formation of areas of chaotic terrain.JOURNAL, The great thickness debate: Ice shell thickness models for Europa and comparisons with estimates based on flexure at ridges, Billings, Sandra E., Kattenhorn, Simon A., Icarus, 177, 2, 397–412, 2005, 10.1016/j.icarus.2005.03.013, 2005Icar..177..397B,

Composition

The Galileo orbiter found that Europa has a weak magnetic moment, which is induced by the varying part of the Jovian magnetic field. The field strength at the magnetic equator (about 120 nT) created by this magnetic moment is about one-sixth the strength of Ganymede's field and six times the value of Callisto's.JOURNAL, Zimmer, Christophe, Khurana, Krishan K., Subsurface Oceans on Europa and Callisto: Constraints from Galileo Magnetometer Observations, Icarus, 2000, 147, 2, 329–347, 10.1006/icar.2000.6456,weblink 2000Icar..147..329Z, Kivelson, Margaret G., 10.1.1.366.7700, The existence of the induced moment requires a layer of a highly electrically conductive material in Europa's interior. The most plausible candidate for this role is a large subsurface ocean of liquid saltwater.Since the Voyager spacecraft flew past Europa in 1979, scientists have worked to understand the composition of the reddish-brown material that coats fractures and other geologically youthful features on Europa's surface.NEWS,weblink Europa Mission to Probe Magnetic Field and Chemistry, Jet Propulsion Laboratory, 27 May 2015, 29 May 2015, Spectrographic evidence suggests that the dark, reddish streaks and features on Europa's surface may be rich in salts such as magnesium sulfate, deposited by evaporating water that emerged from within.JOURNAL, Salts on Europa's Surface Detected by Galileo's Near Infrared Mapping Spectrometer, McCord, Thomas B., Hansen, Gary B., etal, 1998, 10.1126/science.280.5367.1242, 280, 5367, Science, 1242–1245, 1998Sci...280.1242M, Sulfuric acid hydrate is another possible explanation for the contaminant observed spectroscopically.JOURNAL, Carlson, R. W., Anderson, M. S., Mehlman, R., Johnson, R. E., Distribution of hydrate on Europa: Further evidence for sulfuric acid hydrate, 10.1016/j.icarus.2005.03.026, Icarus, 177, 2, 461, 2005, 2005Icar..177..461C, In either case, because these materials are colorless or white when pure, some other material must also be present to account for the reddish color, and sulfur compounds are suspected.JOURNAL, Calvin, Wendy M., Clark, Roger N., Brown, Robert H., Spencer, John R., Spectra of the ice Galilean satellites from 0.2 to 5 µm: A compilation, new observations, and a recent summary, Journal of Geophysical Research, 1995, 100, E9, 19,041–19,048, 1995JGR...10019041C, 10.1029/94JE03349, Another hypothesis for the colored regions is that they are composed of abiotic organic compounds collectively called tholins.JOURNAL, 10.1029/2002JE001841, 2002JGRE..107.5114B, 107, E11, A new energy source for organic synthesis in Europa's surface ice, 2002, Journal of Geophysical Research: Planets, 24–1–24–5, Borucki, Jerome G., Khare, Bishun, Cruikshank, Dale P., MISE: A Search for Organics on Europa. Whalen, Kelly; Lunine, Jonathan I.; Blaney, Diana L. American Astronomical Society, AAS Meeting #229, id.138.04. January 2017.NEWS,weblink Europa Mission to Probe Magnetic Field and Chemistry, Jet Propulsion Laboratory, 27 May 2015, 23 October 2017, The morphology of Europa's impact craters and ridges is suggestive of fluidized material welling up from the fractures where pyrolysis and radiolysis take place. In order to generate colored tholins on Europa there must be a source of materials (carbon, nitrogen, and water) and a source of energy to make the reactions occur. Impurities in the water ice crust of Europa are presumed both to emerge from the interior as cryovolcanic events that resurface the body, and to accumulate from space as interplanetary dust. Tholins bring important astrobiological implications, as they may play a role in prebiotic chemistry and abiogenesis.JOURNAL, 3796891, 24143126, 10.2174/13852728113179990078, 17, 16, Atmospheric Prebiotic Chemistry and Organic Hazes, 2013, Curr Org Chem, 1710–1723, Trainer, MG, >Prebiotic chemistry on Titan ? The nature of Titan's aerosols and their potential evolution at the satellite surface. Coll, P. J.; Poch, O.; Ramirez, S. I.; Buch, A.; Brassé, C.; Raulin, F. American Geophysical Union, Fall Meeting 2010, abstract #P31C-1551JOURNAL, 21161385, 10.1007/s11084-010-9232-z, 41, 4, Prebiotic synthesis of protobiopolymers under alkaline ocean conditions, 2011, Orig Life Evol Biosph, 331–45, Ruiz-Bermejo, M, Rivas, LA, Palacín, A, Menor-Salván, C, Osuna-Esteban, S, 2011OLEB...41..331R, The presence of sodium chloride in the internal ocean has been suggested by a 450 nm absorption feature, characteristic of irradiated NaCl crystals, that has been spotted in HST observations of the chaos regions, presumed to be areas of recent subsurface upwelling.JOURNAL, Trumbo, S. K., Brown, M. E., Hand, K. P., Sodium chloride on the surface of Europa, Science Advances, 5, 6, 2019, eaaw7123, 10.1126/sciadv.aaw7123,

Sources of heat

Tidal heating occurs through the tidal friction and tidal flexing processes caused by tidal acceleration: orbital and rotational energy are dissipated as heat in the core of the moon, the internal ocean, and the ice crust.NEWS,weblink Frequently Asked Questions about Europa, NASA, 2012, 18 April 2016,weblink" title="web.archive.org/web/20160428110229weblink">weblink 28 April 2016, yes,

Tidal friction

Ocean tides are converted to heat by frictional losses in the oceans and their interaction with the solid bottom and with the top ice crust. In late 2008, it was suggested Jupiter may keep Europa's oceans warm by generating large planetary tidal waves on Europa because of its small but non-zero obliquity. This generates so-called Rossby waves that travel quite slowly, at just a few kilometers per day, but can generate significant kinetic energy. For the current axial tilt estimate of 0.1 degree, the resonance from Rossby waves would contain 7.3{{E|18}} J of kinetic energy, which is two thousand times larger than that of the flow excited by the dominant tidal forces.WEB, Scientist Explains Why Jupiter's Moon Europa Could Have Energetic Liquid Oceans,weblink Lisa, Zyga, PhysOrg.com, 12 December 2008, 28 July 2009, JOURNAL, Tyler, Robert H., Strong ocean tidal flow and heating on moons of the outer planets, Nature, 11 December 2008, 456, 770–772, 10.1038/nature07571, 19079055, 7223, 2008Natur.456..770T, Dissipation of this energy could be the principal heat source of Europa's ocean.

Tidal flexing

Tidal flexing kneads Europa's interior and ice shell, which becomes a source of heat.NEWS,weblink Europa: Energy, NASA, 2012, 18 April 2016, Tidal flexing of the ice shell could create slightly warmer pockets of ice that rise slowly upward to the surface, carrying material from the ocean below.,weblink 28 April 2016, yes, Depending on the amount of tilt, the heat generated by the ocean flow could be 100 to thousands of times greater than the heat generated by the flexing of Europa's rocky core in response to gravitational pull from Jupiter and the other moons circling that planet.NEWS, Tyler, Robert,weblink Jupiter's Moon Europa Does The Wave To Generate Heat, University of Washington, Science Daily, 15 December 2008, 18 April 2016, Europa's seafloor could be heated by the moon's constant flexing, driving hydrothermal activity similar to undersea volcanoes in Earth's oceans.Experiments and ice modeling published in 2016, indicate that tidal flexing dissipation can generate one order of magnitude more heat in Europa's ice than scientists had previously assumed.NEWS, Stacey, Kevin,weblink Europa’s heaving ice might make more heat than scientists thought, Brown University, 14 April 2016, 18 April 2016, JOURNAL, Tidal dissipation in creeping ice and the thermal evolution of Europa, Earth and Planetary Science Letters, 1 June 2016, McCarthy, Christine, Cooper, Reid F., 443, 185–194, 10.1016/j.epsl.2016.03.006, 2016E&PSL.443..185M, Their results indicate that most of the heat generated by the ice actually comes from the ice's crystalline structure (lattice) as a result of deformation, and not friction between the ice grains. The greater the deformation of the ice sheet, the more heat is generated.

Radioactive decay

In addition to tidal heating, the interior of Europa could also be heated by the decay of radioactive material (radiogenic heating) within the rocky mantle.BOOK, Pappalardo, Robert T., McKinnon, William B., Khurana, K., Europa, Heat Transfer in Europa’s Icy Shell, Pappalardo, Robert T., McKinnon, William B., Khurana, K., The University of Arizona Press, 2009, 405, 9780816528448, But the models and values observed are one hundred times higher than those that could be produced by radiogenic heating alone,JOURNAL, Hydrothermal systems on Europa, Geophysical Research Letters, 9 March 2005, Lowell, Robert P., DuBosse, Myesha, 32, 5, L05202, 10.1029/2005GL022375, 2005GeoRL..32.5202L, thus implying that tidal heating has a leading role in Europa.WEB,weblink The heat flow of Europa, Ruiz, Javier, Universidad Complutense de Madrid, Universidad Complutense de Madrid, 25 May 2005, 18 April 2016,

Plumes

File:PIA21922-EuropaPlumesDetectedByGalileoSpacecraft-ArtistConcept-20180514.jpg|thumb|right|150px|Water plumes on Europa detected by the Galileo space probeNEWS, Hubble discovers water vapour venting from Jupiter's moon Europa,weblink 16 December 2013, ESA/Hubble Press Release, ]](File:Photo composite of suspected water plumes on Europa.jpg|thumb|left|150px|Photo composite of suspected water plumes on EuropaWEB, Photo composite of suspected water plumes on Europa,weblink www.spacetelescope.org, 6 October 2016, )The Hubble Space Telescope acquired an image of Europa in 2012 that was interpreted to be a plume of water vapour erupting from near its south pole.WEB,weblink Hubble discovers water vapour venting from Jupiter's moon Europa, 12 December 2013, www.spacetelescope.org, Hubble Space Telescope/European Space Agency, en, 2019-04-16, The image suggests the plume may be {{convert|200|km|mi|abbr=on}} high, or more than 20 times the height of Mt. Everest.NEWS, Leigh, Fletcher, The Plumes of Europa, 12 December 2013,weblink The Planetary Society, 17 December 2013, NEWS, Charles Q., Choi, Jupiter Moon Europa May Have Water Geysers Taller Than Everest, 12 December 2013,weblink Space.com, 17 December 2013, It has been suggested that if they exist, they are episodicNEWS, Dyches, Preston,weblink Signs of Europa Plumes Remain Elusive in Search of Cassini Data, NASA, 30 July 2015, 18 April 2016, and likely to appear when Europa is at its farthest point from Jupiter, in agreement with tidal force modeling predictions.JOURNAL, Roth, L., Saur, J., Retherford, K. D., Strobel, D. F., Feldman, P. D., McGrath, M. A., Nimmo, F., 10.1126/science.1247051,weblink Transient Water Vapor at Europa's South Pole, Science, 343, 6167, 171–174, 26 November 2013, 24336567, 26 January 2014, 2014Sci...343..171R, Additional imaging evidence from the Hubble Space Telescope was presented in September 2016.NEWS, Berger, Eric,weblink Hubble finds additional evidence of water vapor plumes on Europa, NASA, ARS Technica, 26 September 2016, 26 September 2016, NEWS, Amos, Jonathan,weblink Europa moon 'spewing water jets', BBC News, 26 September 2016, 26 September 2016, In May 2018, astronomers provided supporting evidence of water plume activity on Europa, based on an updated critical analysis of data obtained from the Galileo space probe, which orbited Jupiter between 1995 and 2003. Galileo flew by Europa in 1997 within {{convert|206|km|mi|abbr=on}} of the moon's surface and the researchers suggest it may have flown through a water plume.JOURNAL, Jia, Xianzhe, Kivelson, Margaret G., Khurana, Krishan K., Kurth, William S., Evidence of a plume on Europa from Galileo magnetic and plasma wave signatures, 14 May 2018, Nature Astronomy, 2, 6, 459–464, 10.1038/s41550-018-0450-z, 2018NatAs...2..459J, WEB, McCartney, Gretchen, Brown, Dwayne, Wendel, JoAnna, Old Data Reveal New Evidence of Europa Plumes,weblink 14 May 2018, 14 May 2018, NEWS, Chang, Kenneth, NASA Finds Signs of Plumes From Europa, Jupiter’s Ocean Moon,weblink 14 May 2018, The New York Times, 14 May 2018, WEB, Wall, Mike, This May Be the Best Evidence Yet of a Water Plume on Jupiter's Moon Europa,weblink 14 May 2018, Space.com, 14 May 2018, Such plume activity could help researchers in a search for life from the subsurface European ocean without having to land on the moon.The tidal forces are about 1,000 times stronger than the Moon's effect on Earth. The only other moon in the Solar System exhibiting water vapor plumes is Enceladus. The estimated eruption rate at Europa is about 7000 kg/s compared to about 200 kg/s for the plumes of Enceladus.JOURNAL, Hansen, C. J., Esposito, L., Stewart, A. I., Colwell, J., Hendrix, A., Pryor, W., Shemansky, D., West, R., 10.1126/science.1121254, Enceladus' Water Vapor Plume, Science, 311, 5766, 1422–1425, 10 March 2006, 16527971, 2006Sci...311.1422H, JOURNAL, Spencer, J. R., Nimmo, F., 10.1146/annurev-earth-050212-124025, Enceladus: An Active Ice World in the Saturn System, Annual Review of Earth and Planetary Sciences, 41, 693, May 2013, 2013AREPS..41..693S, If confirmed, it would open the possibility of a flyby through the plume and obtain a sample to analyze in situ without having to use a lander and drill through miles of ice.NEWS, O'Neill, Ian,weblink NASA: Activity Spied on Europa, But It's 'NOT Aliens', Discovery News, Space, 22 September 2016, 23 September 2016, JOURNAL, On the in-situ detectability of Europa's water vapour plumes from a flyby mission, Huybrighs, Hans, Futaana, Yoshifumi, Barabash, Stas, Wieser, Martin, Wurz, Peter, Krupp, Norbert, Glassmeier, Karl-Heinz, Vermeersen, Bert, Icarus, June 2017, 289, 270–280, 10.1016/j.icarus.2016.10.026, 1704.00912, 2017Icar..289..270H,

Atmosphere

Observations with the Goddard High Resolution Spectrograph of the Hubble Space Telescope, first described in 1995, revealed that Europa has a thin atmosphere composed mostly of molecular oxygen (O2).JOURNAL, Hall, D. T., Strobel, D. F., Feldman, P. D., McGrath, M. A., Weaver, H. A., Detection of an oxygen atmosphere on Jupiter's moon Europa, 10.1038/373677a0, Nature, 373, 6516, 677–681, 1995, 7854447, 1995Natur.373..677H, WEB, Donald, Savage, Jones, Tammy, Villard, Ray,weblink Hubble Finds Oxygen Atmosphere on Europa, 17 August 2007, NASA, Jet Propulsion Laboratory, 23 February 1995, Project Galileo, The surface pressure of Europa's atmosphere is 0.1 Î¼Pa, or 10−12 times that of the Earth. In 1997, the Galileo spacecraft confirmed the presence of a tenuous ionosphere (an upper-atmospheric layer of charged particles) around Europa created by solar radiation and energetic particles from Jupiter's magnetosphere,JOURNAL, Kliore, Arvydas J., Hinson, D. P., Flasar, F. Michael, Nagy, Andrew F., Cravens, Thomas E., July 1997, The Ionosphere of Europa from Galileo Radio Occultations, Science (magazine), Science, 277, 5324, 355–358, 10.1126/science.277.5324.355, 9219689, 1997Sci...277..355K, WEB, 1997, Galileo Spacecraft Finds Europa has Atmosphere, NASA, Jet Propulsion Laboratory,weblink 10 August 2007, Project Galileo, providing evidence of an atmosphere.
missing image!
- Europa field.png -
upright|Magnetic field around Europa. The red line shows a trajectory of the Galileo spacecraft during a typical flyby (E4 or E14).
Unlike the oxygen in Earth's atmosphere, Europa's is not of biological origin. The surface-bounded atmosphere forms through radiolysis, the dissociation of molecules through radiation.JOURNAL, Planetary applications of ion induced erosion of condensed-gas frosts, Johnson, Robert E., Lanzerotti, Louis J., Brown, Walter L., 1982, 1982NucIM.198..147J, 198, 147, Nuclear Instruments and Methods in Physics Research, 10.1016/0167-5087(82)90066-7, Solar ultraviolet radiation and charged particles (ions and electrons) from the Jovian magnetospheric environment collide with Europa's icy surface, splitting water into oxygen and hydrogen constituents. These chemical components are then adsorbed and "sputtered" into the atmosphere. The same radiation also creates collisional ejections of these products from the surface, and the balance of these two processes forms an atmosphere.JOURNAL, Shematovich, Valery I., Cooper, John F., Johnson, Robert E., April 2003, 13094, Surface-bounded oxygen atmosphere of Europa, EGS – AGU – EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 2003EAEJA....13094S, Molecular oxygen is the densest component of the atmosphere because it has a long lifetime; after returning to the surface, it does not stick (freeze) like a water or hydrogen peroxide molecule but rather desorbs from the surface and starts another ballistic arc. Molecular hydrogen never reaches the surface, as it is light enough to escape Europa's surface gravity.JOURNAL, Liang, Mao-Chang, Lane, Benjamin F., Pappalardo, Robert T., Allen, Mark, Yung, Yuk L., Atmosphere of Callisto, Journal of Geophysical Research, 2005, 110, E2, E02003, 10.1029/2004JE002322,weblink 2005JGRE..11002003L, yes,weblink" title="web.archive.org/web/20090225131107weblink">weblink 25 February 2009, WEB, Smyth, William H., Marconi, Max L.,weblink Processes Shaping Galilean Satellite Atmospheres from the Surface to the Magnetosphere – Workshop on Ices, Oceans, and Fire: Satellites of the Outer Solar System, Boulder, Colorado – Abstracts, 15 August 2007, 131–132, Observations of the surface have revealed that some of the molecular oxygen produced by radiolysis is not ejected from the surface. Because the surface may interact with the subsurface ocean (considering the geological discussion above), this molecular oxygen may make its way to the ocean, where it could aid in biological processes.JOURNAL, Chyba, C. F., Hand, K. P., PLANETARY SCIENCE: Enhanced: Life Without Photosynthesis, 10.1126/science.1060081, Science, 292, 5524, 2026–2027, 2001, 11408649, One estimate suggests that, given the turnover rate inferred from the apparent ~0.5 Gyr maximum age of Europa's surface ice, subduction of radiolytically generated oxidizing species might well lead to oceanic free oxygen concentrations that are comparable to those in terrestrial deep oceans.JOURNAL, Energy, Chemical Disequilibrium, and Geological Constraints on Europa, Hand, Kevin P., Carlson, Robert W., Chyba, Christopher F., Astrobiology, December 2007, 7, 6, 1006–1022, 10.1089/ast.2007.0156, 18163875, 2007AsBio...7.1006H, 10.1.1.606.9956, The molecular hydrogen that escapes Europa's gravity, along with atomic and molecular oxygen, forms a gas torus in the vicinity of Europa's orbit around Jupiter. This "neutral cloud" has been detected by both the Cassini and Galileo spacecraft, and has a greater content (number of atoms and molecules) than the neutral cloud surrounding Jupiter's inner moon Io. Models predict that almost every atom or molecule in Europa's torus is eventually ionized, thus providing a source to Jupiter's magnetospheric plasma.JOURNAL, Smyth, William H., Marconi, Max L., 2006, Europa's atmosphere, gas tori, and magnetospheric implications, Icarus (journal), Icarus, 2006Icar..181..510S, 10.1016/j.icarus.2005.10.019, 181, 2, 510,

Exploration

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|caption1 = In 1973 Pioneer 10 made the first closeup images of Europa – however the probe was too far away to obtain more detailed images
|caption2 = Europa seen in detail in 1979 by Voyager 2
}}Exploration of Europa began with the Jupiter flybys of Pioneer 10 and 11 in 1973 and 1974 respectively. The first closeup photos were of low resolution compared to later missions. The two Voyager probes traveled through the Jovian system in 1979, providing more-detailed images of Europa's icy surface. The images caused many scientists to speculate about the possibility of a liquid ocean underneath. Starting in 1995, the Galileo space probe orbited Jupiter for eight years, until 2003, and provided the most detailed examination of the Galilean moons to date. It included the "Galileo Europa Mission" and "Galileo Millennium Mission", with numerous close flybys of Europa.The Journey to Jupiter: Extended Tours – GEM and the Millennium Mission. Solarsystem.nasa.gov. Retrieved on 23 July 2013. In 2007, New Horizons imaged Europa, as it flew by the Jovian system while on its way to Pluto.WEB,weblink PIA09246: Europa, 2 April 2007, NASA photojournal, 9 March 2016,

Future missions

Conjectures regarding extraterrestrial life have ensured a high profile for Europa and have led to steady lobbying for future missions.WEB, David, Leonard,weblink Europa Mission: Lost In NASA Budget, 7 February 2006, Space.com, WEB, Projects: Europa Mission Campaign; Campaign Update: 2007 Budget Proposal, Friedman, Louis, 14 December 2005,weblink 11 August 2011, yes, The Planetary Society,weblink" title="web.archive.org/web/20110811002508weblink">weblink The aims of these missions have ranged from examining Europa's chemical composition to searching for extraterrestrial life in its hypothesized subsurface oceans.WEB, Thin ice opens lead for life on Europa, David L., Chandler, 20 October 2002,weblink New Scientist, Muir, Hazel (22 May 2002) Europa has raw materials for life, New Scientist. Robotic missions to Europa need to endure the high-radiation environment around itself and Jupiter. Europa receives about 5.40 Sv of radiation per day.Ringwald, Frederick A. (29 February 2000) SPS 1020 (Introduction to Space Sciences) Course Notes {{webarchive|url=https://web.archive.org/web/20080725050708weblink |date=25 July 2008 }}, California State University, csufresno.edu.In 2011, a Europa mission was recommended by the U.S. Planetary Science Decadal Survey.NEWS, Lean U.S. missions to Mars, Jupiter moon recommended,weblink Reuters, 7 March 2011, Zabarenko, Deborah, In response, NASA commissioned Europa lander concept studies in 2011, along with concepts for a Europa flyby (Europa Clipper), and a Europa orbiter.WEB,weblink Europa Lander, 15 January 2014, NASA, yes,weblink" title="web.archive.org/web/20140116103022weblink">weblink 16 January 2014, dmy-all, March 2012 OPAG Meeting. Lunar and Planetary Institute, NASA. Retrieved on 23 July 2013. The orbiter element option concentrates on the "ocean" science, while the multiple-flyby element (Clipper) concentrates on the chemistry and energy science. On 13 January 2014, the House Appropriations Committee announced a new bipartisan bill that includes $80 million funding to continue the Europa mission concept studies.NEWS, Amina, Khan, NASA gets some funding for Mars 2020 rover in federal spending bill, 15 January 2014,weblink Los Angeles Times, NEWS, Frank C., Girardot, JPL's Mars 2020 rover benefits from spending bill, 14 January 2014,weblink Pasadena Star-News,
  • In 2012, Jupiter Icy Moon Explorer (JUICE) was selected by the European Space Agency (ESA) as a planned mission.Selection of the L1 mission. ESA, 17 April 2012. (PDF). Retrieved on 23 July 2013. That mission includes 2 flybys of Europa, but is more focused on Ganymede.WEB,weblink JUICE—Science objectives, 20 April 2012, 16 March 2012, European Space Agency,
  • Europa Clipper — In July 2013 an updated concept for a flyby Europa mission called Europa Clipper was presented by the Jet Propulsion Laboratory (JPL) and the Applied Physics Laboratory (APL).WEB,weblink The Europa Clipper – OPAG Update, 2013, Robert, Pappalardo, Brian, Cooke, Barry, Goldstein, Louise, Prockter, Dave, Senske, Tom, Magner, Jet Propulsion Laboratory, JPL/Applied Physics Laboratory, APL, In May 2015, NASA announced that it had accepted development of the Europa Clipper mission, and revealed the instruments it will use.WEB,weblink NASA's Europa Mission Begins with Selection of Science Instruments, NASA, The aim of Europa Clipper is to explore Europa in order to investigate its habitability, and to aid selecting sites for a future lander. The Europa Clipper would not orbit Europa, but instead orbit Jupiter and conduct 45 low-altitude flybys of Europa during its envisioned mission. The probe would carry an ice-penetrating radar, short-wave infrared spectrometer, topographical imager, and an ion- and neutral-mass spectrometer.
  • Europa Lander (NASA) is a recent concept mission under study. 2018 research suggests Europa may be covered in tall, jagged ice spikes, presenting a problem for any potential landing on its surface.WEB,weblink Future spacecraft landing on Jupiter's moon Europa may have to navigate jagged blades of ice, Grush, Loren, 2018-10-08, The Verge, 2019-04-16, NEWS,weblink Jagged ice spikes cover Jupiter’s moon Europa, study suggests, Guarino, Ben, 8 October 2018, The Washington Post, 2019-04-15,

Old proposals

{{multiple image
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|image2 = JIMO Europa Lander Mission.jpg
|footer = Left: artist's concept of the cryobot and its deployed "hydrobot" submersible. Right: Europa Lander Mission concept, NASA 2005.WEB,weblink Small RPS-Enabled Europa Lander Mission, NASA–JPL, 13 February 2005, yes,weblink" title="web.archive.org/web/20111008230224weblink">weblink 8 October 2011,
}}In the early 2000s, Jupiter Europa Orbiter led by NASA and the Jupiter Ganymede Orbiter led by the ESA were proposed together as an Outer Planet Flagship Mission to Jupiter's icy moons called Europa Jupiter System Mission, with a planned launch in 2020.WEB,weblink NASA and ESA Prioritize Outer Planet Missions, 26 July 2009, NASA, 2009, In 2009 it was given priority over Titan Saturn System Mission.NEWS,weblink Jupiter in space agencies' sights, Paul, Rincon, BBC News, 20 February 2009, 20 February 2009, At that time, there was competition from other proposals.WEB,weblink Cosmic Vision 2015–2025 Proposals, 21 July 2007, ESA, 20 February 2009, Japan proposed Jupiter Magnetospheric Orbiter.Jovian Europa Orbiter was an ESA Cosmic Vision concept study from 2007. Another concept was Ice Clipper,JOURNAL, Planetary protection for a Europa surface sample return: The Ice Clipper mission, Advances in Space Research, 2002, C. P., 30, 6, 1601–1605, 10.1016/S0273-1177(02)00480-5, 2002AdSpR..30.1601M, McKay,weblink which would have used an impactor similar to the Deep Impact mission—it would make a controlled crash into the surface of Europa, generating a plume of debris that would then be collected by a small spacecraft flying through the plume.Goodman, Jason C. (9 September 1998) Re: Galileo at Europa, MadSci Network forums.Jupiter Icy Moons Orbiter (JIMO) was a partially developed fission-powered spacecraft with ion thrusters that was cancelled in 2006. It was part of Project Prometheus.Berger, Brian; NASA 2006 Budget Presented: Hubble, Nuclear Initiative Suffer Space.com (7 February 2005) The Europa Lander Mission proposed a small nuclear-powered Europa lander for JIMO.Abelson & Shirley – Small RPS-Enabled Europa Lander Mission (2005). {{webarchive|url=https://web.archive.org/web/20111008230224weblink |date=8 October 2011 }}. (PDF). Retrieved on 23 July 2013. It would travel with the orbiter, which would also function as a communication relay to Earth.Europa Orbiter — Its objective would be to characterize the extent of the ocean and its relation to the deeper interior. Instrument payload could include a radio subsystem, laser altimeter, magnetometer, Langmuir probe, and a mapping camera.2012 Europa Mission Studies. OPAG 29 March 2012 (PDF). Lunar and Planetary Institute, NASA. Retrieved on 23 July 2013.{{citation | contribution=EUROPA STUDY 2012 REPORT | title=EUROPA ORBITER MISSION | author=Europa Study Team | publisher=JPL – NASA | date=1 May 2012 | contribution-url=http://solarsystem.nasa.gov/europa/docs/ES%202012%20Report%20B%20Orbiter%20-%20Final%20-%2020120501.pdf | url=http://solarsystem.nasa.gov/europa/docs/ES%202012%20Report%20B%20Orbiter%20-%20Final%20-%2020120501.pdf | access-date=17 January 2014 | archive-url=https://web.archive.org/web/20140202105451weblink | archive-date=2 February 2014 | dead-url=yes | df=dmy-all }} The Europa Orbiter received a go-ahead in 1999 but was canceled in 2002. This orbiter featured a special ice-penetrating radar that would allow it to scan below the surface.More ambitious ideas have been put forward including an impactor in combination with a thermal drill to search for biosignatures that might be frozen in the shallow subsurface.JOURNAL, Weiss, P., Yung, K. L., Kömle, N., Ko, S. M., Kaufmann, E., Kargl, G., 10.1016/j.asr.2010.01.015, Thermal drill sampling system onboard high-velocity impactors for exploring the subsurface of Europa, Advances in Space Research, 48, 4, 743, 2011, 2011AdSpR..48..743W, WEB
, Dual Drill Designed for Europa's Ice
, Hsu
, J.
,weblink
, Astrobiology Magazine
, 15 April 2010
,weblink" title="web.archive.org/web/20100418204646weblink">weblink
, 18 April 2010
, yes
,
, Another proposal put forward in 2001 calls for a large nuclear-powered "melt probe" (cryobot) that would melt through the ice until it reached an ocean below.WEB, Knight, Will, 14 January 2002,weblink Ice-melting robot passes Arctic test, New Scientist, Once it reached the water, it would deploy an autonomous underwater vehicle (hydrobot) that would gather information and send it back to Earth.WEB, Bridges, Andrew, 10 January 2000,weblink" title="web.archive.org/web/20090208150400weblink">weblink 8 February 2009, no,weblink Latest Galileo Data Further Suggest Europa Has Liquid Ocean, Space.com, Both the cryobot and the hydrobot would have to undergo some form of extreme sterilization to prevent detection of Earth organisms instead of native life and to prevent contamination of the subsurface ocean.BOOK, National Academy of Sciences Space Studies Board,weblink" title="web.archive.org/web/20080213194509weblink">weblink 13 February 2008, no,weblink Preventing the Forward Contamination of Europa, National Academy Press, Washington (DC), 2000, 978-0-309-57554-6, This suggested approach has not yet reached a formal conceptual planning stage.JOURNAL, NEMO: A mission to search for and return to Earth possible life forms on Europa, Powell, Jesse, Powell, James, Maise, George, Paniagua, John, Acta Astronautica, 57, 2–8, 579–593, 2005, 10.1016/j.actaastro.2005.04.003, 2005AcAau..57..579P,

Habitability potential

File:Blacksmoker in Atlantic Ocean.jpg|thumb|upright|A black smoker in the Atlantic Ocean. Driven by geothermal energy, this and other types of hydrothermal vents create chemical disequilibria that can provide energy sources for life.]]So far, there is no evidence that life exists on Europa, but Europa has emerged as one of the most likely locations in the Solar System for potential habitability.WEB, Alternative Energy Sources Could Support Life on Europa,weblink Schulze-Makuch, Dirk, Irwin, Louis N., Departments of Geological and Biological Sciences, University of Texas at El Paso, 2001, 21 December 2007,weblink" title="web.archive.org/web/20060703033956weblink">weblink 3 July 2006, Life could exist in its under-ice ocean, perhaps in an environment similar to Earth's deep-ocean hydrothermal vents.WEB, Nicola, Jones, 11 December 2001,weblink Bacterial explanation for Europa's rosy glow, New Scientist, 26 September 2016, Even if Europa lacks volcanic hydrothermal activity, a 2016 NASA study found that Earth-like levels of hydrogen and oxygen could be produced through processes related to serpentinization and ice-derived oxidants, which do not directly involve volcanism.{{Citation| url=http://www.jpl.nasa.gov/news/news.php?feature=6514 |title=Europa's Ocean May Have An Earthlike Chemical Balance| newspaper=Jpl.nasa.gov| accessdate=18 May 2016}} In 2015, scientists announced that salt from a subsurface ocean may likely be coating some geological features on Europa, suggesting that the ocean is interacting with the seafloor. This may be important in determining if Europa could be habitable.NEWS, Wall, Mike,weblink NASA Aiming for Multiple Missions to Jupiter Moon Europa, Space.com, 9 June 2015, 10 June 2015, The likely presence of liquid water in contact with Europa's rocky mantle has spurred calls to send a probe there.Phillips, Cynthia (28 September 2006) Time for Europa, Space.com.(File:PIA22479-Europa-JupiterMoon-ArtistConcept-20180723.jpg|thumb|400px|Europa – possible effect of radiation on biosignature chemicals)The energy provided by tidal forces drives active geological processes within Europa's interior, just as they do to a far more obvious degree on its sister moon Io. Although Europa, like the Earth, may possess an internal energy source from radioactive decay, the energy generated by tidal flexing would be several orders of magnitude greater than any radiological source.WEB, Tidal Heating on Io and Europa and its Implications for Planetary Geophysics, Wilson, Colin P., Geology and Geography Dept., Vassar College,weblink 2007, 21 December 2007, Life on Europa could exist clustered around hydrothermal vents on the ocean floor, or below the ocean floor, where endoliths are known to inhabit on Earth. Alternatively, it could exist clinging to the lower surface of Europa's ice layer, much like algae and bacteria in Earth's polar regions, or float freely in Europa's ocean.JOURNAL, The Search for Life on Europa: Limiting Environmental Factors, Potential Habitats, and Earth Analogues, Marion, Giles M., Fritsen, Christian H., Eicken, Hajo, Payne, Meredith C., Astrobiology, 2003, 10.1089/153110703322736105, 14987483, 3, 4, 785–811, 2003AsBio...3..785M, If Europa's ocean is too cold, biological processes similar to those known on Earth could not take place. If it is too salty, only extreme halophiles could survive in that environment. In 2010, a model proposed by Richard Greenberg of the University of Arizona proposed that irradiation of ice on Europa's surface could saturate its crust with oxygen and peroxide, which could then be transported by tectonic processes into the interior ocean. Such a process could render Europa's ocean as oxygenated as our own within just 12 million years, allowing the existence of complex, multicellular lifeforms.JOURNAL, Transport Rates of Radiolytic Substances into Europa's Ocean: Implications for the Potential Origin and Maintenance of Life, Richard Greenberg, Astrobiology, May 2010, 10, 3, 275–283, 10.1089/ast.2009.0386, 2010AsBio..10..275G, Evidence suggests the existence of lakes of liquid water entirely encased in Europa's icy outer shell and distinct from a liquid ocean thought to exist farther down beneath the ice shell. If confirmed, the lakes could be yet another potential habitat for life. Evidence suggests that hydrogen peroxide is abundant across much of the surface of Europa.NASA – Mapping the Chemistry Needed for Life at Europa. {{webarchive|url=https://web.archive.org/web/20130408082530weblink |date=8 April 2013 }}. Nasa.gov (4 April 2013). Retrieved on 23 July 2013. Because hydrogen peroxide decays into oxygen and water when combined with liquid water, the authors argue that it could be an important energy supply for simple life forms.Clay-like minerals (specifically, phyllosilicates), often associated with organic matter on Earth, have been detected on the icy crust of Europa.WEB, Cook, Jia-Rui C., Clay-Like Minerals Found on Icy Crust of Europa,weblink 11 December 2013, NASA, The presence of the minerals may have been the result of a collision with an asteroid or comet. Some scientists have speculated that life on Earth could have been blasted into space by asteroid collisions and arrived on the moons of Jupiter in a process called lithopanspermia.NEWS, Charles Q., Choi,weblink Life Could Have Hitched a Ride to Outer Planet Moons, Astrobiology Magazine, Astrobiology Web, 8 December 2013,

See also

{hide}cmn|colwidth=30em| {edih}

Notes

{{reflist|group=lower-alpha}}

References

{{Reflist}}

Further reading

  • BOOK


, David A., Rothery, 1999
, Satellites of the Outer Planets: Worlds in Their Own Right
, Oxford University Press US
, 978-0-19-512555-9,
  • BOOK


, David M., Harland, 2000
, Jupiter Odyssey: The Story of NASA's Galileo Mission
, Springer, 978-1-85233-301-0,
  • BOOK


, Richard, Greenberg, 2005
, EUROPA The Ocean Moon
, Springer, 978-3-540-22450-1,

External links

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