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Line 1: − + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + − '''Europa''' is the fourth largest moon of [[Jupiter]]. Discovered by [[Galileo]], it has a diameter of 3,318 km (about 2,068 miles, which would make it slightly smaller than the Earth's [[Moon]]). It is surrounded by a layer of [[ice]], which makes it part of the search for [[extraterrestrial life]]. There are almost no craters on Europa.+ + − + − John Gribbin, ''Companion to the Cosmos'' (Little, Brown & Company, 1996)+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + − http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jup_Europa − [[Category: Astronomy]]
Brand-new article on Europa, with richer details, references, and illustrations
[[Image:Europamoon.jpg|right|thumb|Europa, a moon of Jupiter.]]
{{Planet
| name=Europa
| image=Europa_true_color.jpg
| caption=True-color image of Europa from the second orbit around it by [[Galileo Project|Galileo]].
| date=January 7, 1610<ref name=disc>"[http://www.solarviews.com/eng/galdisc.htm The Discovery of the Galilean Satellites]," [[JPL]], [[NASA]], n.d. Quoted by Hamilton, Calvin J., ''[http://www.solarviews.com/ SolarViews].'' Accessed February 18, 2008.</ref>
| discname=[[Galileo Galilei]], [[Simon Marius]]<ref name=disc/>
| origname=[[Phoenicia]]n princess from whom [[Europe]] gets its name; mistress of [[Zeus]], mother of [[Rhadamanthus]] and [[Minos]].<ref name=Arnett>Arnett, Bill. "[http://www.nineplanets.org/europa.html Entry for Europa]." ''The <s>Nine</s>8 Planets'', October 1, 2005. Accessed February 29, 2008.</ref>
| primary=Jupiter
| order=6
| periapsis=664,792 km<ref name=calc>Calculated</ref>
| apoapsis=677,408 km<ref name=calc/>
| semimajor=671,100 km<ref name=gal1>"[http://www2.jpl.nasa.gov/galileo/europa/ Europa, a Continuing Story of Discovery]." ''Galileo Project/NASA''. Accessed February 29, 2008.</ref><ref name=ssdelem>"[Planetary Satellite Mean Orbital Parameters]." ''Solar System Dynamics'', [[JPL]], [[NASA]]. Accessed February 29, 2008.</ref>
| eccentricity=0.0094<ref name=gal1/><ref name=ssdelem/>
| sidereal=3.551181 da<ref name=gal1/><ref name=ssdelem/>
| orbitspeed=13.74 km/s<ref name=gal1/><ref name=solarviews>Hamilton, Calvin J. "[http://www.solarviews.com/eng/europa.htm Jupiter's Moon Europa]." ''Solarviews'', 1997. Accessed February 29, 2008.</ref>
| inclination=0.470°<ref name=gal1/>
| siderealday=3.551181 da<ref name=gal1/>
| rotatespeed=31.96 m/s<ref name=calc/>
| mass=4.8 * 10<sup>22</sup> kg (0.83021% earth)<ref name=gal1/><ref name=solarviews/>
| density=3,013 kg/m³<ref name=gal1/><ref name=ssdphys>"[http://ssd.jpl.nasa.gov/?sat_phys_par Planetary Body Physical Parameters]." ''Solar System Dynamics'', [[JPL]], [[NASA]]. Accessed February 29, 2008.</ref>
| meanradius=1,560.8 km<ref name=ssdphys/>
| surfacegrav= 1.3239 m/s² (0.135 ''g'')<ref name=gal1/>
| escapespeed=2.02 km/s<ref name=gal1/><ref name=solarviews/>
| surfacearea=30,612,893 km² (6.00175% earth)<ref name=calc/>
| mintemp=50 K<ref name=planetsoc>"[http://www.planetary.org/explore/topics/our_solar_system/jupiter/europa.html Jupiter's Moon Europa]." ''The Planetary Society''. Accessed February 29, 2008.</ref>
| maxtemp=110 K<ref name=planetsoc/>
| composition=Water ice and rock<ref name=gal1/>
| color=Marble white with reddish-brown streaks
| albedo=0.64<ref name=gal1/><ref name=solarviews/>
}}'''Europa''' is the second innermost of the [[Galilean moons]] of [[Jupiter]], and also the smallest and lightest. Europa is named for the Phoenician princess who, according to legend, gave Europe its name, and was carried off to [[Crete]] by [[Zeus]], who had assumed the shape of a bull. Today Europa, more than any other object in the [[solar system]], continues to be the subject of speculation as to whether it harbors [[extraterrestrial life]].
== Discovery and naming ==
[[Galileo Galilei]] observed Jupiter beginning on January 7, 1610, with his famous telescope. He at first thought he had discovered three stars near Jupiter, but on the next night those "stars" seemed to have moved. (Those "stars" were actually [[Io]], Europa, and [[Callisto]].) He continued to observe Jupiter and its companion "stars" for seven days, during which time a fourth "star" ([[Ganymede]]) appeared and all four of these objects appeared to move with Jupiter. Finally he concluded that these objects were not stars at all, but satellites of Jupiter. This was the first direct observation that provided evidence for [[Nicolaus Copernicus]]'s heliocentric model of the solar system.<ref name=disc/>
==References==
[[Simon Marius]] claimed to observe Jupiter and these satellites independently of Galileo and beginning five weeks earlier. However, he did not ''publish'' his findings, while Galileo published his. Furthermore, Galileo's notes were more reliable and extensive than those of Marius, which is why Ganymede and the three other satellites he observed (Io, Europa, and Callisto) are called the Galilean satellites and not the Galilean-Marian satellites.<ref name=disc/>
Marius does, however, receive credit for providing the names that the satellites have today. He named Ganymede, the largest, for the mythical son of King [[Tros]] of [[Troy]], carried aloft to [[Mount Olympus]] by [[Zeus]] to be the cupbearer to the gods. (The names ''Zeus'' and ''Jupiter'' refer to the same classical deity from whom Jupiter gets its name.) The other three satellites are named for three of Zeus' most famous mistresses.<ref name=disc/> Marius propounded these names after [[Johannes Kepler]] suggested them to him.
Galileo, for his part, called them the "Medicean planets" after the [[Medici]] family and simply numbered the moons I, II, III, and IV. The satellites carried these names for two centuries until the discovery of other moons of Jupiter made that naming system untenably confusing.
== Orbital and rotational characteristics ==
[[Image:Galilean moon Laplace resonance animation.gif|thumb|150 px|left|Orbital resonance of Io, Europa, and Ganymede]]Europa is sixth in order of all of Jupiter's moons, and orbits Jupiter at a semi-major axis of 671,100 km. Its orbit is more eccentric than that of volcanic [[Io]] and is slightly inclined to Jupiter's equator. Europa is in [[tidal lock]] (thus keeping the same face toward Jupiter as it revolves).
Europa maintains a three-part [[orbital resonance]] or Laplace resonance with Io and Ganymede. These three satellites complete their orbits around Jupiter in the ratio 1:2:4. They also conjoin with one another at their apsides, in a mutually self-correcting manner that allows the resonance to persist. (Most such resonances are unstable and end with one or all bodies changing their orbits.)
The origin of the resonance itself is controversial. Peale and Lee, in 2002, presented a model suggesting that this resonance is primordial and thus was part of the Jovian system since its formation.<ref name=Peale>Peale, S. J., and Lee, Man Hoi. "[http://adsabs.harvard.edu/abs/2002Sci...298..593P A Primordial Origin of the Laplace Relation Among the Galilean Satellites]." ''Science'', 298(5593):593-597, October 2002. Accessed February 18, 2008.</ref> Their model, however, derives from the [[nebular hypothesis]] of the formation of the solar system and thus depends on [[uniformitarianism|uniformitarian]] assumptions. Showman and Malhotra suggest a model by which the resonance developed after the solar system had formed.<ref name=Showman>Showman, Adam P., and Malhotra, Renu. "[http://www.lpl.arizona.edu/~showman/publications/showman-malhotra-1997.pdf Tidal Evolution into the Laplace Resonance and the Resurfacing of Ganymede]." ''Icarus'', 127:93-111, 1997. Accessed February 18, 2008.</ref>
Most scientists hold that this resonance make Europa warmer than it otherwise would be. This, they say, would explain the persistence of a liquid water ocean on Europa, though it should be far too cold for this.<ref name=Arnett/> More to the point, the same tidal flexion that might be heating Europa is cited as the cause of the irregular striations observed on Europa's surface.<ref name=planetsoc/>
== Physical characteristics ==
[[Image:Europa interior.jpg|thumb|150px|left|Europa cutaway]]At 1,560.8 km in radius, Europa is the fifteenth largest body in the [[solar system]].<ref name=planetsoc/> It is very lightly cratered and thus "geologically young" by [[uniformitarianism|uniformitarian]] standards;<ref name=Arnett/> some authorities suggest an apparent age of 30 million Julian years.<ref name=planetsoc/> Indeed only three craters larger than 5 km in diameter are known to exist on Europa's surface.<ref name=solarviews/><ref name=Arnett/>
Europa is likely surfaced with [[water]] ice and rock, with a 50-km-deep subterranean ocean deep to this, resting on a silicate floor that surrounds a metallic core. The surface has multiple dark streaks, some measuring 20 km broad. Their origin is uncertain but suspected to be volcanic or geothermal.<ref name=Arnett/>
The [[Hubble Space Telescope]] has detected evidence of an "atmosphere" at pressure 10<sup>-11</sup> bar, containing [[oxygen]].<ref name=Arnett/><ref name=planetsoc/> Most authorities ''do not'' suggest that this oxygen is biotic. Rather, they suggest that sunlight and the charged particles of Jupiter's radiation belt cause the water ice on Europa to vaporize and then to split into [[hydrogen]] and oxygen--and the hydrogen, being much lighter, tends to escape Europa's light gravity (0.135 ''g'') and leave the oxygen.<ref name=Arnett/><ref name=planetsoc/>
[[Image:Europa sulfuric.jpg|150 px|right]]More recently, [[sulfuric acid]] has been found on Europa.<ref name=acid>"[http://science.nasa.gov/newhome/headlines/ast30sep99_1.htm Sulfuric Acid Found on Europa]." ''Science@NASA'', September 30, 1999. Accessed February 29, 2008.</ref> This might explain the red striations; they might be composed largely of elemental sulfur.
=== Controversy over Europa's surface ===
[[Image:Europa surface models.jpg|200 px|left]]At least two models of the surface of Europa are in current circulation: the "thick ice" model and the "liquid ocean" model. The "thick ice," if present, would be convective, and this would explain the slight magnetic moment that Europa possesses. Whether Europa has "thick ice" or a liquid ocean under ''thin'' ice remains an open question.
Faced with the risk of terrestrial contamination of Europa's possible thinly-covered ocean, an event that would severely complicate any evaluation of the finding of [[extremophile]]s or other microbes on Europa, led the Galileo mission planners to make a hard decision. They deliberately crash-dived the Galileo orbiter onto Jupiter so that it would never risk crashing onto the surface of Europa.<ref name=rense>"[http://www.rense.com/general41/galui.htm Galileo Crashes into Jupiter]." ''ABC News Online'', Australian Broadcasting Company, September 21, 2003. Accessed February 20, 2008.</ref><ref name=Macey>Macey, Richard. "[http://www.smh.com.au/articles/2003/09/14/1063478069136.html Galileo's death leap onto Jupiter]." ''Sydney Morning Herald Online'', September 15, 2003. Accessed February 20, 2008.</ref>
== Speculation about Life ==
More than any other extraterrestrial body (except perhaps for [[Mars]]), Europa has long been the subject of intense speculation as to whether it harbors [[extraterrestrial life]]. The Voyager data fueled this speculation, which began with Richard C. Hoagland's startling proposal that Europa might have a liquid ocean that harbored life.<ref name=hoagland>Hoagland, Richard C. "The Europa Enigma." ''Star and Sky'', January, 1980, pp. 16-31. Reproduced in full at ''[http://www.enterprisemission.com/europa.html The Enterprise Mission]'', 1996. Accessed February 29, 2008.</ref> NASA authorities, except for [[Robert Jastrow]], were unsympathetic and highly skeptical of Hoagland's theories--but at least one writer of [[science fiction]] would utilize them in one of his [[novel]]s that later became a major [[motion picture]].<ref name=enterprise>Review and history of the Richard C. Hoagland paper at ''[http://www.enterprisemission.com/europa.html The Enterprise Mission]'', 1996. Accessed February 29, 2008.</ref>
With the arrival of Galileo and the data from Galileo's early exploration of Europa, the speculation became even more feverish.<ref name=resa>"[http://www.resa.net/nasa/europa_life.htm Life on Europa]." ''[http://www.resa.net/nasa/xlife_intro.htm Life on Other Planets in the Solar System]'', October 22, 1998. Accessed February 29, 2008.</ref><ref name=solarviews/><ref name=Arnett/> Even the finding of sulfuric acid, a harsh chemical, on Europa has not dampened that speculation.<ref name=acid/>
== Exploration ==
=== History ===
[[Image:Hydrobot.jpg|150 px|right]]The first spacecraft to visit the Jovian system were [[Pioneer 10]] and [[Pioneer 11]]. Though these two probes demonstrated the feasibility of deep-space exploration, they took only a few dim and blurred images of Europa and the other Galilean moons.
[[Voyager 1]] and [[Voyager 2]] provided richly detailed images and other findings that made possible the first models of Europa's interior and surface composition.
In 1995, the [[Galileo Project|Galileo]] spacecraft arrived in the Jovian system. After releasing a probe to dive into Jupiter and report on Jupiter's atmosphere, Galileo began a detailed reconnaissance of the Galilean moons. Europa was one of its first targets, on account of the relative safety of a close approach to Europa and the urgency of the speculation about Europa's composition and possible harboring of [[extraterrestrial life]].
=== Plans and proposals ===
The proposed [[Jupiter Icy Moons Orbiter]], which would almost certainly have had Europa as a prime target, has recently been cancelled.<ref name=JIMO>"[http://www.planetsurveyor.com/latest-space-exploration-news/jupiter-icy-moons-orbiter-victim-of-budget-cut.html Jupiter Icy Moons Orbiter Victim of Budget Cut]." ''Planet Surveyor'', May 23, 2005. Accessed February 18, 2008.</ref> But other proposals for further exploration remain under consideration. At least one such proposal calls for a probe to crash through the ice and release a robotic submersible to explore the water beneath the ice.<ref>[http://sbir.gsfc.nasa.gov/SBIR/abstracts/98/sbir/phase1/SBIR-98-1-25.01-9300.html A Submersible Remotely Operated Vehicle System for Exploration of Large Subglacial Bodies of Water]. Proposal 98-1 25.01-9300, SBIR Phase I, [[NASA]]. Accessed February 29, 2008.</ref>
== References ==
{{reflist|2}}
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