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| − | [[Image:Jupitermoon.jpg|thumb|250px|[[Jupiter (planet)|Jupiter]]'s four Galilean moons, in a composite image comparing their sizes and the size of Jupiter ([[Great Red Spot]] visible). From the top, they are [[Io (moon)|Io]], [[Europa (moon)|Europa]], [[Ganymede (moon)|Ganymede]], [[Callisto (moon)|Callisto]].]]
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| − | The '''Galilean moons''' are the four moons of [[Jupiter]] discovered by [[Galileo Galilei]]. They are the largest of the many [[moons of Jupiter]] and have been named ''[[Io (moon)|Io]]'', ''[[Europa (moon)|Europa]]'', ''[[Ganymede (moon)|Ganymede]]'', and ''[[Callisto (moon)|Callisto]]''. Ganymede, Europa and Io participate in a 1:2:4 [[orbital resonance]]. They are among [[List of moons by diameter|the most massive objects]] in the [[Solar System]] outside the [[Sun]] and the eight planets, with radii larger than those of the [[dwarf planet]]s.
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| − | The four moons were discovered sometime between 1609 and 1610, when Galileo made improvements to his [[telescope]], enabling him to observe celestial bodies more distinctly than had ever been possible before. Galileo’s discovery showed the importance of the telescope as a tool for astronomers by proving that there were objects in space that cannot be seen by the naked eye. More importantly, the discovery of celestial bodies orbiting something other than the Earth dealt a serious blow to the then-accepted [[geocentric model]] (or [[Ptolemy|Ptolemaic]] world system), according to which every celestial body was thought to orbit around the [[Earth]].
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| − | Galileo initially named his discovery the '''Cosmica Sidera''' ("Cosimo's stars"), but names that eventually prevailed were chosen by [[Simon Marius]]. Marius claimed to have discovered the moons at the same time as Galileo, and gave them their present names in his ''Mundus Jovialis'', published in 1614.
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| − | ==Historical highlights==
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| − | ===Discovery===
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| − | [[Image:Galileo.arp.300pix.jpg|thumb|right|150px|[[Galileo Galilei]], discoverer of the four Galilean moons.]]
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| − | As a result of improvements [[Galileo Galilei]] made to the [[telescope]], with a magnifying capability of 30×,<ref>Van Helden, Albert. 1974. The Telescope in the Seventeenth Century. ''Isis''. 65(1):38–58.</ref> he was able to see celestial bodies more distinctly than was ever possible before. This allowed Galilei to discover sometime between December 1609 and January 1610 what came to be known as the Galilean moons.<ref name=Galileo89>Galilei, and Van Helden. 1989. pages 14–16.</ref> Nevertheless, a Chinese historian of astronomy, [[Xi Zezong]], claimed that the Chinese astronomer [[Gan De]] observed one of Jupiter's moons in 362 B.C.E., nearly 2 millennia earlier than Galileo.<ref>Zezong, Xi. 1982. The Discovery of Jupiter's Satellite Made by Gan De 2000 years Before Galileo. ''Chinese Physics'' 2(3):664–67.</ref>
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| − | On January 7, 1610, Galileo wrote a letter containing the first mention of Jupiter’s moons. At the time, he saw only three of them, and he believed them to be fixed stars near Jupiter. He continued to observe these [[Celestial spheres|celestial orb]]s from January 8 to March 2, 1610. In these observations, he discovered a fourth body, and also observed that the four were not fixed stars, but rather were orbiting Jupiter.<ref name=Galileo89/>
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| − | Galileo’s discovery proved the importance of the telescope as a tool for astronomers by showing that there were objects in space to be discovered that until then had remained unseen by the naked eye. More importantly, the incontrovertible discovery of celestial bodies orbiting something other than the Earth dealt a serious blow to the then-accepted [[Geocentric model|Ptolemaic world system]], which held that the Earth was at the center of the universe and all other celestial bodies revolved around it. That Jupiter has four moons while Earth has only one further undercut the near-universal belief that the Earth was the center of the universe both in position and in importance. Galileo's ''Sidereus Nuncius'' ''(Starry Messenger)'', which announced celestial observations through his telescope, does not explicitly mention [[Copernican heliocentrism]], a theory that placed the [[Sun]] at the center of the universe. Nevertheless, Galileo believed in the Copernican theory.<ref name=Galileo89/> As a result of these discoveries, Galileo was able to develop a method of determining [[longitude]] based on the timing of the orbits of the Galilean moons.
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| − | === Dedication to the Medicis===
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| − | [[Image:Jupiter.moons1.jpg|thumb|Surface features of the four members.]]
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| − | In 1605, Galileo had been employed as a mathematics tutor for [[Cosimo II de' Medici, Grand Duke of Tuscany|Cosimo II de’ Medici]] (1590–1621). In 1609, Cosimo became Grand Duke Cosimo II of [[Tuscany]]. Galileo, seeking patronage from his now-wealthy former student and his powerful family, used the discovery of Jupiter’s moons to gain it.<ref name=Galileo89/> On February 13, 1610, Galileo wrote to the Grand Duke’s secretary:
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| − | <blockquote>God graced me with being able, through such a singular sign, to reveal to my Lord my devotion and the desire I have that his glorious name live as equal among the stars, and since it is up to me, the first discoverer, to name these new planets, I wish, in imitation of the great sages who placed the most excellent heroes of that age among the stars, to inscribe these with the name of the Most Serene Grand Duke.<ref name=Galileo89/></blockquote>
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| − | Galileo asked whether he should name the moons the ''Cosmica Sidera'' ("Cosimo's stars") after Cosimo alone, or ''Medicea Sidera'' ("the Medician Stars"), which would honor all four brothers (Cosimo, Francesco, [[Carlo de' Medici (1595-1666)|Carlo]], and Lorenzo) in the Medici clan. The secretary replied that the latter name would be best.<ref name=Galileo89/>
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| − | On March 12, 1610, Galileo wrote his dedicatory letter to the Duke of Tuscany, and sent a copy to him the next day, hoping to obtain his support as quickly as possible. On March 19, he sent the telescope he had used to first view Jupiter’s moons to the Grand Duke, along with an official copy of ''Sidereus Nuncius'' (''The Starry Messenger'') that, following the secretary's advice, named the four moons ''Medicea Sidera''.<ref name=Galileo89/> In his dedicatory introduction, Galileo wrote:
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| − | <blockquote>
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| − | [S]carcely have the immortal graces of your soul begun to shine forth on earth than bright stars offer themselves in the heavens which, like tongues, will speak of and celebrate your most excellent virtues for all time. Behold, therefore, four stars reserved for your illustrious name … which … make their journeys and orbits with a marvelous speed around the star of Jupiter … like children of the same family … Indeed, it appears the Maker of the Stars himself, by clear arguments, admonished me to call these new planets by the illustrious name of Your Highness before all others.<ref name=Galileo89/>
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| − | </blockquote>
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| − | ===Naming the moons===
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| − | Galileo received several suggestions for names for the moons. They included:
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| − | * ''Principharus'', ''Victipharus'', ''Cosmipharus'', and ''Ferdinandipharus'', for each of the four Medici brothers - by [[Giovanni Batista Hodierna]], a disciple of Galileo and author of the first [[Ephemeris|ephemerides]] (''Medicaeorum Ephemerides'', 1656);
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| − | * ''Circulatores Jovis'', or ''Jovis Committees'' - by [[Johannes Hevelius]];
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| − | * ''Gardes'', or ''Satellites'' (from the [[Latin]] ''satelles, satellitis'', meaning "escorts") - by [[Jacques Ozanam]].
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| − | The names that eventually prevailed were chosen by [[Simon Marius]], who claimed to have discovered the moons at the same time as Galileo. He named them after lovers of the god Zeus (the Greek equivalent of Jupiter): ''[[Io (moon)|Io]]'', ''[[Europa (moon)|Europa]]'', ''[[Ganymede (moon)|Ganymede]]'' and ''[[Callisto (moon)|Callisto]]'', in his ''Mundus Jovialis'', published in 1614.<ref name="marazzini"/>
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| − | Galileo steadfastly refused to use Marius' names and invented as a result the numbering scheme that is still used nowadays, in parallel with proper moon names. The numbers run from Jupiter outward: I, II, III, and IV corresponding to Io, Europa, Ganymede, and Callisto, respectively.<ref name="marazzini"/> Although Galileo used this system in his notebooks, he never published it. The numbered names (Jupiter ''x'') were used until the mid-twentieth century, when other inner moons were discovered and Marius' names became widely used.<ref name="marazzini">Marazzini, C. 2005. The names of the satellites of Jupiter: from Galileo to Simon Marius. ''Lettere Italiana''. 57(3):391–407.</ref>
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| − | == Some details about the Galilean moons ==
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| − | The Galilean moons are, in increasing order of distance from Jupiter:
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| − | {| class="sortable wikitable"
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| − | |-
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| − | ! Name<br/>
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| − | ! class="unsortable" |Image
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| − | ! Diameter<br/>([[Kilometer|km]])
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| − | ! Mass<br/>([[Kilogram|kg]])
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| − | ! Density<br/>(g/cm³)
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| − | ! [[Semi-major axis]]<br/>([[Kilometer|km]])<ref>[http://cfa-www.harvard.edu/iau/NatSats/NaturalSatellites.html µ value.] IAU-MPC Satellites Ephemeris Service. Retrieved January 10, 2009.</ref>
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| − | ! [[Orbital period]]([[Day|d]])<br/><ref>[http://sse.jpl.nasa.gov/planets/profile.cfm?Object=Jupiter&Display=Facts Jupiter: Facts & Figures.] JPL/NASA. Retrieved January 10, 2009.</ref>(relative)
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| − | ! [[Inclination]]<br/>([[Degree (angle)|°]])<ref>[http://www.hnsky.org/iau-iag.htm Report of the IAU/IAG working group on cartographic coordinates and rotational elements of the planets and satellites: 2000.] IAG Travaux 2001. Retrieved January 10, 2009.</ref>
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| − | ! [[Orbital eccentricity|Eccentricity]]<br/>
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| − | |-
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| − | |align="center"| '''[[Io (moon)|Io]]''' <br/>''(Jupiter I)''
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| − | |bgcolor=black align="center"|[[Image:Io, moon of Jupiter, NASA.jpg|121px]]
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| − | |align="center"| 3660.0×3637.4×3630.6
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| − | |align="center"| {{sort|0893|8.93×10<sup>22</sup>}}
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| − | |align="center"| 3.528
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| − | |align="center"| 421,800
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| − | |align="center"| 1.769 <br/><br/>'''(1)'''
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| − | |align="center"| 0.050
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| − | |align="center"| 0.0041
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| − | |-
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| − | |align="center"| '''[[Europa (moon)|Europa]]''' <br/>''(Jupiter II)''
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| − | |bgcolor=black align="center"|[[Image:Europa-moon.jpg|104px]]
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| − | |align="center"| 3121.6
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| − | |align="center"| {{sort|0480|4.8×10<sup>22</sup>}}
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| − | |align="center"| 3.014
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| − | |align="center"| 671,100
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| − | |align="center"| 3.551 <br/><br/>'''(2)'''
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| − | |align="center"| 0.471
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| − | |align="center"| 0.0094
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| − | |-
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| − | |align="center"| '''[[Ganymede (moon)|Ganymede]]''' <br/>''(Jupiter III)''
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| − | |bgcolor=black align="center"|[[Image:Ganymede, moon of Jupiter, NASA.jpg|175px]]
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| − | |align="center"| 5262.4
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| − | |align="center"| {{sort|1480|1.48×10<sup>23</sup>}}
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| − | |align="center"| 1.942
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| − | |align="center"| 1,070,400
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| − | |align="center"| 7.155 <br/><br/>'''(4)'''
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| − | |align="center"| 0.204
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| − | |align="center"| 0.0011
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| − | |-
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| − | |align="center"| '''[[Callisto (moon)|Callisto]]''' <br/>''(Jupiter IV)''
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| − | |bgcolor=black align="center"|[[Image:Callisto, moon of Jupiter, NASA.jpg|160px]]
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| − | |align="center"| 4820.6
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| − | |align="center"| {{sort|1080|1.08×10<sup>23</sup>}}
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| − | |align="center"| 1.834
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| − | |align="center"| 1,882,700
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| − | |align="center"| 16.69 <br/><br/>(9.4)
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| − | |align="center"| 0.205
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| − | |align="center"| 0.0074
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| − | |}
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| − |
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| − | ===Io===
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| − | [[Image:Galilean moon Laplace resonance animation.gif|250px|thumb|The three inner Galilean moons revolve in a 4:2:1 resonance.]]
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| − | {{main article|Io (moon)}}
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| − | Io is the innermost of the four Galilean moons [[natural satellite|moons]] of [[Jupiter]] and, with a [[diameter]] of 3,642 [[Kilometre|kilometers]], the [[List of moons by diameter|fourth-largest moon]] in the [[Solar System]]. It was named after [[Io (mythology)|Io]], a priestess of [[Hera]] who became one of the lovers of [[Zeus]]. Nevertheless, it was simply referred to as “Jupiter I,” or “The first satellite of Jupiter,” until mid-20th century.<ref name="marazzini"/>
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| − | With over 400 active volcanoes, Io is the most geologically active object in the Solar System.<ref name=Lopes2004>Lopes, R.M.C., Lucas W. Kamp, William D. Smythe, Peter Mouginis-Mark, Jeff Kargel, Jani Radebaugh, Elizabeth P. Turtle, Jason Perry, David A. Williams, R.W. Carlson, and S. Douté. 2004. [http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004Icar..169..140L&db_key=AST&link_type=ABSTRACT&high=45f9f8ee3003884 Lava Lakes on Io: Observations of Io's Volcanic Activity from Galileo NIMS During the 2001 Fly-bys.] ''Icarus''. 169(1):140–174. Retrieved January 10, 2009.</ref> Its surface is dotted with more than 100 mountains some which are taller than Earth's [[Mount Everest]].<ref name=Schenk2001>Schenk, P., Henrik Hargitai, Ronda Wilson, Alfred McEwen, and Peter Thomas. 2001. [http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001JGR...10633201S&db_key=AST&link_type=ABSTRACT&high=45f9f8ee3004779 The Mountains of Io: Global and Geological Perspectives from ''Voyager'' and ''Galileo'']. ''Journal of Geophysical Research.'' 106(E12):33201–33222. Retrieved January 10, 2009.</ref> Unlike most satellites in the outer Solar System (which have a thick coating of ice), Io is primarily composed of silicate rock surrounding a molten iron or iron sulfide core.
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| − | Although not proven, recent data from the Galileo orbiter indicates that Io might have its own magnetic field.<ref>Porco, C.C., et al. 2003. Cassini imaging of Jupiter's atmosphere, satellites, and rings. ''Science''. 299:1541–1547.</ref> Io has an extremely thin atmosphere made up mostly of [[sulfur dioxide]] (SO<sub>2</sub>).<ref>McEwen, A.S., et al. 1998. High-temperature silicate volcanism on Jupiter's moon Io. ''Science''. 281:87–90.</ref> If a surface data or collection vessel were to land on Io in the future, it would have to be extremely tough (similar to the [[tank]]-like bodies of the Soviet [[Venera]] landers) to survive the radiation and magnetic fields that originate from Jupiter.<ref>Fanale, F.P., T.V. Johnson, and D.L. Matson. 1974. [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1974Sci...186..922F&db_key=AST&data_type=HTML&format=&high=45f9f8ee3017014 Io: A Surface Evaporite Deposit?] ''Science''. 186(4167):922–925. Retrieved January 10, 2009.</ref>
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| − | ===Europa===
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| − | [[Image:Masses of Jovian moons.png|thumb|250 px|The relative masses of the Galilean moons. Io and Callisto together are 50%, as are Europa and Ganymede. The Galileans so dominate the system that all the other Jovian moons put together are not visible at this scale.]]
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| − | {{main article|Europa (moon)}}
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| − | Europa, the second of the four Galilean moons, is the second closest to Jupiter and the smallest at 3121.6 kilometers in diameter, which is slightly smaller than [[Earth]]'s [[Moon]]. The name, Europa was after a mythical [[Phoenicia]]n noblewoman, [[Europa (mythology)|Europa]], who was courted by [[Zeus]] and became the queen of [[Crete]], but did not become widely used until the mid-20th century.<ref name="marazzini"/>
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| − | It is one of the smoothest objects in the solar system,<ref>[http://www2.jpl.nasa.gov/galileo/moons/europa.html Europa: Another Water World?] [[NASA]], Jet Propulsion Laboratory. Retrieved January 10, 2009.</ref> with a layer of water surrounding the mantle of the planet, thought to be 100 kilometers thick.<ref>Schenk, Chapman, Zahnle, and Moore. 2004.</ref> The smooth surface includes a layer of ice, while the bottom of the ice is theorized to be liquid water.<ref name="EuropaAlbedo">Hamilton, C.J. [http://www.solarviews.com/eng/europa.htm Jupiter's Moon Europa.] Solar Views. Retrieved January 10, 2009.</ref> The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it, which could conceivably serve as an abode for [[extraterrestrial life]].<ref>Tritt, Charles S. 2002. [http://people.msoe.edu/~tritt/sf/europa.life.html Possibility of Life on Europa.] Milwaukee School of Engineering. Retrieved January 10, 2009.</ref> Heat energy from [[tidal flexing]] ensures that the ocean remains liquid and drives geological activity.<ref name=geology>[http://web.archive.org/web/20060329000051/http://geology.asu.edu/~glg_intro/planetary/p8.htm Tidal Heating]. geology.asu.edu. Retrieved January 10, 2009.</ref> Life may exist in Europa's under-ice ocean, perhaps subsisting in an environment similar to Earth's deep-ocean [[hydrothermal vent]]s or the Antarctic [[Lake Vostok]].<ref>[http://science.nasa.gov/newhome/headlines/ast10dec99_2.htm Exotic Microbes Discovered near Lake Vostok.] Science@NASA. Retrieved January 10, 2009.</ref> Life in such an ocean could possibly be similar to [[Microorganism|microbial]] life on Earth in the [[deep ocean]].<ref>Jones, N. 2001. [http://www.newscientist.com/article.ns?id=dn1647 Bacterial explanation for Europa's rosy glow.] NewScientist.com. Retrieved January 10, 2009.</ref> So far, there is no evidence that life exists on Europa, but the likely presence of liquid water has spurred calls to send a probe there.<ref>Phillips, C. 2006. [http://www.space.com/searchforlife/seti_europa_060928.html Time for Europa.] Space.com. Retrieved January 10, 2009.</ref>
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| − | The prominent markings that criss-cross the moon seem to be mainly [[albedo feature]]s, which emphasize low topography. There are few [[Impact crater|crater]]s on Europa because its surface is tectonically active and young.<ref>Arnett, B. 1996. [http://www.astro.auth.gr/ANTIKATOPTRISMOI/nineplanets/nineplanets/europa.html Europa.] Retrieved January 10, 2009.</ref> Some theories suggest that Jupiter’s gravity is causing these markings, as one side of Europa is constantly facing Jupiter. Also, volcanic water eruptions splitting the surface of Europa, and even geysers have been considered as a cause. The color of the markings, reddish-brown, is theorized to be caused by sulfur, but scientists cannot confirm that, because no data collection devices have been sent to Europa. Europa is primarily made of [[silicate]] rock and likely has an [[iron]] core. It has a tenuous atmosphere composed primarily of [[oxygen]].
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| − | ===Ganymede===
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| − | {{main article|Ganymede (moon)}}
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| − | Ganymede, the third Galilean is named the mythological [[Ganymede (mythology)|Ganymede]], cupbearer of the [[Greek mythology|Greek gods]] and [[Zeus|Zeus's]] beloved.<ref>[http://galileo.rice.edu/sci/observations/jupiter_satellites.html Satellites of Jupiter.] The Galileo Project. Retrieved January 10, 2009.</ref> Ganymede is [[List of natural satellites by diameter|largest natural satellite]] in the [[Solar System]] at 5262.4 kilometers in diameter, which makes it larger than the planet [[Mercury (planet)|Mercury]] - although only at about half of its mass.<ref name="nineplanets.org-Ganymede">[http://www.nineplanets.org/ganymede.html Ganymede.] nineplanets.org. Retrieved January 10, 2009.</ref> It is the only satellite in the Solar System known to possess a [[magnetosphere]], likely created through [[convection]] within the liquid iron core.<ref>Kivelson, M.G., K.K. Khurana, and M. Volwerk. 2002. [http://www.igpp.ucla.edu/people/mkivelson/Publications/ICRUS1572507.pdf The Permanent and Inductive Magnetic Moments of Ganymede.] ''Icarus''. 157:507–522. Retrieved January 10, 2009.</ref>
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| − | Ganymede is composed primarily of [[silicate|silicate rock]] and water ice, and a salt-water ocean is believed to exist nearly 200 [[km]] below Ganymede's surface, sandwiched between layers of ice. The metallic core of Ganymede suggests a greater heat at some time in its past than had previously been proposed. The surface is a mix of two types of terrain – highly cratered dark regions and younger, but still ancient, regions with a large array of grooves and ridges. Ganymede has a high number of craters, but many are gone or barely visible due to its icy crust forming over them. The satellite has a thin [[oxygen]] [[atmosphere]] that includes O, O<sub>2</sub>, and possibly O<sub>3</sub> ([[ozone]]), and some [[atomic hydrogen]].<ref>Hall, D.T., P.D. Feldman, M.A. McGrath, and D.F. Strobel. 1998. [http://adsabs.harvard.edu/abs/1998ApJ...499..475H The Far-Ultraviolet Oxygen Airglow of Europa and Ganymede]. ''The Astrophysical Journal''. 499:475–481. Retrieved January 10, 2009.</ref><ref>Eviatar, Aharon, Vytenis M. Vasyliunas, Donald A. Gurnett, et al. 2001. The ionosphere of Ganymede. ''Plan. Space Sci. 49:327–336.</ref>
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| − | ===Callisto===
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| − | {{main article|Callisto (moon)}}
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| − | Callisto is the fourth and last Galilean moon, and is the second largest of the four, and at 4820.6 kilometers in diameter, it is [[List of moons by diameter|the third largest moon]] in the [[Solar System]]. It does not form part of the [[orbital resonance]] that affects three inner Galilean satellites and thus does not experience appreciable [[Tidal acceleration#Tidal heating|tidal heating]].<ref>Musotto, Susanna, Ferenc Varadi, William Moore, and Gerald Schubert. 2002. [http://adsabs.harvard.edu/abs/2002Icar..159..500M Numerical Simulations of the Orbits of the Galilean Satellites.] ''Icarus''. 159:500–504. Retrieved January 10, 2009.</ref> Callisto is composed of approximately equal amounts of [[rock (geology)|rock]] and [[Volatiles|ices]], which makes it the least dense of the Galilean moons. It is one of the most heavily cratered satellites in the solar system, and one major feature is a basin around 3000 km wide called [[Valhalla (crater)|Valhalla]].
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| − | Callisto is surrounded by an extremely thin [[atmosphere]] composed of [[carbon dioxide]]<ref>Carlson, R.W. 1999. [http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/16785/1/99-0186.pdf A Tenuous Carbon Dioxide Atmosphere on Jupiter's Moon Callisto.] ''Science''. 283:820–821. Retrieved January 10, 2009.</ref>and probably [[molecular oxygen]].<ref>Liang, M.C., B.F. Lane, R.T. Pappalardo, Mark Allen, and Yuk L. Yung. 2005. [http://yly-mac.gps.caltech.edu/ReprintsYLY/N164Liang_Callisto%2005/Liang_callisto_05.pdf Atmosphere of Callisto.] ''Journal of Geophysics''. 110:E02003. Retrieved January 10, 2009.</ref> Investigation revealed that Callisto may have possibly a subsurface ocean of liquid water at depths greater than 100 kilometers.<ref>Showman, Adam P., and Renu Malhotra. 1999. [http://www.lpl.arizona.edu/~showman/publications/showman-malhotra-1999.pdf The Galilean Satellites.] ''Science''. 286:77–84. Retrieved January 10, 2009.</ref> The likely presence of an ocean within Callisto indicates that it can or could harbor [[extraterrestrial life|life]]. However, this is less likely than on nearby [[Europa (moon)|Europa]].<ref>Lipps, Jere H., Gregory Delory, Joe Pitman, and Sarah Rieboldta. 2004. [http://learning.berkeley.edu/astrobiology/2004ppt/jupiter.pdf Astrobiology of Jupiter’s Icy Moons]. ''Proc. SPIE.'' 5555:10. Retrieved January 10, 2009.</ref> Callisto has long been considered the most suitable place for a human base for future exploration of the system of Jupiter.<ref>Trautman, Pat, and Kristen Bethke. 2003. [http://www.nasa-academy.org/soffen/travelgrant/bethke.pdf Revolutionary Concepts for Human Outer Planet Exploration(HOPE)]. [[NASA]]. Retrieved January 10, 2009.</ref>
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| − | == Visibility ==
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| − | [[Image:Thomas Bresson - Jupiter(2) (by).jpg|thumb|300px|The Galilean moons seen with an amateur telescope.]]
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| − | All four Galilean moons are bright enough that they could, if they were farther away from Jupiter, be sighted from Earth without a telescope. They have [[apparent magnitude]]s between 4.6 and 5.6 when Jupiter is in [[Opposition (astronomy)|opposition]] with the Sun,<ref name=jplfact>Yeomans, Donald K. 2006. [http://ssd.jpl.nasa.gov/?sat_phys_par Planetary Satellite Physical Parameters.] JPL Solar System Dynamics. Retrieved January 10, 2009.</ref> and are about one unit of magnitude dimmer when Jupiter is in [[Conjunction (astronomy and astrology)|conjunction]]. The main difficulty in observing the moons from Earth is their proximity to Jupiter since they are obscured by its brightness.<ref>Jupiter is about 750 times brighter than Ganymede and about 2000 times brighter than Callisto.<br/>Ganymede: (5th root of 100)^(4.4 Ganymede [[Apparent magnitude|APmag]] - (-2.8 Jup APmag)) = 758<br/>Callisto: (5th root of 100)^(5.5 Callisto APmag - (-2.8 Jup APmag)) = 2089</ref> The maximum angular separations of the moons are between 2 and 10 [[minute of arc|minutes of arc]] from Jupiter,<ref>Jupiter near [[apsis|perihelion]] 2010-Sep-19: 656.7 (Callisto angular separation arcsec) - 24.9 (jup angular [[radius]] arcsec) = 631 arcsec = 10 arcmin</ref> close to the limit of human [[visual acuity]]. Ganymede and Callisto, at their maximum separation, are the likeliest targets for potential naked-eye observation. The easiest way to observe them is to cover Jupiter with an object, e.g. atree limb or a power line that is perpendicular to the plane of moons' orbits.
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| − | {{Moons of Jupiter}}
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| − | {{Solar System moons (compact)}}
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| − | {{Jupiter}}
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| − | {{Solar System}}
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| − | ==See also==
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| − | * [[Copernicus]]
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| − | * [[Galileo Galilei]]
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| − | * [[Jupiter]]
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| − | * [[Natural satellite]]
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| − | * [[Ptolemy]]
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| − | * [[Solar System]]
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| − | * [[Telescope]]
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| − | == Notes ==
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| − | <references/>
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| − | ==References==
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| − | * Galilei, Galileo, Albert Van Helden trans. 1989. ''Sidereus Nuncius''. Chicago, IL; London, UK: University of Chicago Press. ISBN 9780226279039.
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| − | * Leutwyler, Kristin, and John R. Casani. 2003. ''The Moons of Jupiter''. New York, NY: W.W. Norton. ISBN 0393050602.
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| − | * Schenk, P.M., C.R. Chapman, K. Zahnle, and J.M. Moore. "Chapter 18: Ages and Interiors: the Cratering Record of the Galilean Satellites," in Bagenal, Fran, Timothy E. Dowling, William B. McKinnon eds. 2004. ''Jupiter: The Planet, Satellites and Magnetosphere''. Cambridge, UK; New York, NY: Cambridge University Press. ISBN 9780521818087.
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| − | == External links ==
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| − | * [http://strangepaths.com/observation-of-jupiter-moons-march-1613/2007/04/22/en/ Animation of Galileo's observation, march 1613]. Retrieved January 10, 2009.
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| − | [[Category:Physical sciences]]
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| − | [[Category:Astronomy]]
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| − | {{credit|261948827}}
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