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[[Image:JupiterGalileans.jpg|thumb|250 px|right|The four Galilean satellites [[Io]], [[Europa]], [[Ganymede]], and [[Callisto]], shown next to [[Jupiter]]'s Great Red Spot to give perspective.]]The '''Galilean moons''' are the four satellites of [[Jupiter]] discovered in 1610 by [[Galileo Galilei]]. They were the first natural satellites of any planet other than [[Earth]] to be discovered and are in fact the only four known [[dwarf planet]] sized satellites that Jupiter has.
== 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. He continued to observe Jupiter and its companion "stars" for seven days, during which time a fourth "star" (actually [[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>"[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>
[[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 whose patronage he was actively seeking at the time. He did so on the advice of the Grand Duke of [[Tuscany]], whom he had tutored in mathematics as a boy. Rather than assign the moons individual names, Galileo simply numbered the moons I, II, III, and IV and prefaced each Roman numeral by the name of Jupiter.
The satellites carried these names for two centuries. At that time, the technology of telescopes had improved tremendously, so that telescopes could now detect objects much smaller than these four moons. The discovery of other moons of Jupiter made the simple naming system untenably confusing. Only then did the Keplerian/Marian names come into common usage.
== Resonating orbits ==
[[Image:Galilean moon Laplace resonance animation.gif|thumb|150 px|left|Orbital resonance of Io, Europa, and Ganymede]]The inner three of the Galilean moons ([[Io]], [[Europa]], and [[Ganymede]]) participate in a complex [[orbital resonance]] or Laplace resonance. 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.)
According to the most currently popular models of this resonance, [[Ganymede]]'s orbit should not be as eccentric as it now is. Others have suggested that an [[asteroid]]al or [[comet]]ary impact has produced this eccentricity, but Showman and Malhotra, in 1997, showed that such an impact cannot account for the observed eccentricity.<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>
The resonance ''is'' held responsible for the relatively pronounced eccentricity of the innermost satellite, Io--and also for the tremendous tidal reshaping that Io undergoes during its less-than-two-day month. Most scientist hold that the latter phenomenon heats Io and causes its remarkable volcanism.
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/>
== Io ==
[[Image:Io galileo 19951217 sc 0-002-005.jpg|100px|right]]: ''Main article: [[Io]]''
'''Io''' is the innermost, the most dense, and in many other ways the most remarkable of the Galilean moons. Since the [[Voyager 1]] and [[Voyager 2]] flybys in 1979 and 1980, Io has been known to be [[volcano|volcanically]] active--the most active body in all the solar system. [[Tidal heating]] is the most popular theory to explain this activity, but even that cannot account completely for the total amount of heat that flows out from Io at all times.
== Europa ==
[[Image:Europamoon.jpg|left]]: ''Main article: [[Europa]]''
'''Europa''', the second innermost, is also the smallest of the Galileans. It has very few craters, and thus by [[uniformitarianism|uniformitarian]] criteria it is considered "young." The thick ice sheet that overlays it has fueled speculation, in the scientific community and in [[science fiction]], that it harbors life. The [[Galileo project|Galileo]] spacecraft discovered evidence that Europa has a sub-crust saltwater ocean under a very thin rock-and-ice layer--and in order to prevent the spacecraft from crashing into Europa, rupturing the crust, and contaminating Europa's oceans with earthly [[microbe]]s, Galileo's mission controllers steered Galileo into a crash dive into Jupiter in September of 2003.<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>
== Ganymede ==
[[Image:Ganymede2 galileo big.jpg|100px|right]]: ''Main article: [[Ganymede]]''
'''Ganymede''' is the largest moon not only of Jupiter but in all the solar system--larger than [[Mercury]] so that in theory it could be a true [[planet]] if it orbited the [[Sun]]. In sharp contrast to Io and Europa, Ganymede is very heavily cratered and is therefore considered "old." Ganymede has its own independent magnetic field, and a quite strong one. Like Europa, it has a saltwater ocean, but under a much thicker crust than that of Europa.
== Callisto ==
: ''Main article: [[Callisto]]''
'''Callisto''', the outermost of the Galilean moons, does not participate in the Laplace resonance with the other three and also lies beyond the Van Allen-like radiation belt that surrounds Jupiter. Like Ganymede, Callisto is very heavily cratered.
== Problems for Uniformitarianism Posed by the Galileans ==
The three-part orbital resonance of [[Io]], [[Europa]], and [[Ganymede]] becomes very difficult to explain in light of Ganymede appearing to be so much "older" than Io or Europa. At least some astronomers believe that this resonance is primordial.<ref name=Peale/> Not all astronomers agree.<ref name=Showman/> But if the resonance ''is'' primordial, then the presence within it of bodies of differing "geological ages" begs explanation. Most observers consider that Io is older than it looks, because the volcanoes and their associated lava flows have erased the craters. But that cannot apply to Europa, which looks at least as "young" as does Io.
Io's volcanism testifies to its tremendous internal heat. In fact, Io is radiating far more heat than tidal heating alone could generate. And yet no astronomer accepts [[radioactive decay]] as a significant source of Io's heat. Either Io has another, still-unknown source of heat, or else the present rate of heat outflow is unsustainable and temporary.
== References ==
{{reflist|2}}
{{SolarSystem}}
== 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. He continued to observe Jupiter and its companion "stars" for seven days, during which time a fourth "star" (actually [[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>"[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>
[[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 whose patronage he was actively seeking at the time. He did so on the advice of the Grand Duke of [[Tuscany]], whom he had tutored in mathematics as a boy. Rather than assign the moons individual names, Galileo simply numbered the moons I, II, III, and IV and prefaced each Roman numeral by the name of Jupiter.
The satellites carried these names for two centuries. At that time, the technology of telescopes had improved tremendously, so that telescopes could now detect objects much smaller than these four moons. The discovery of other moons of Jupiter made the simple naming system untenably confusing. Only then did the Keplerian/Marian names come into common usage.
== Resonating orbits ==
[[Image:Galilean moon Laplace resonance animation.gif|thumb|150 px|left|Orbital resonance of Io, Europa, and Ganymede]]The inner three of the Galilean moons ([[Io]], [[Europa]], and [[Ganymede]]) participate in a complex [[orbital resonance]] or Laplace resonance. 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.)
According to the most currently popular models of this resonance, [[Ganymede]]'s orbit should not be as eccentric as it now is. Others have suggested that an [[asteroid]]al or [[comet]]ary impact has produced this eccentricity, but Showman and Malhotra, in 1997, showed that such an impact cannot account for the observed eccentricity.<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>
The resonance ''is'' held responsible for the relatively pronounced eccentricity of the innermost satellite, Io--and also for the tremendous tidal reshaping that Io undergoes during its less-than-two-day month. Most scientist hold that the latter phenomenon heats Io and causes its remarkable volcanism.
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/>
== Io ==
[[Image:Io galileo 19951217 sc 0-002-005.jpg|100px|right]]: ''Main article: [[Io]]''
'''Io''' is the innermost, the most dense, and in many other ways the most remarkable of the Galilean moons. Since the [[Voyager 1]] and [[Voyager 2]] flybys in 1979 and 1980, Io has been known to be [[volcano|volcanically]] active--the most active body in all the solar system. [[Tidal heating]] is the most popular theory to explain this activity, but even that cannot account completely for the total amount of heat that flows out from Io at all times.
== Europa ==
[[Image:Europamoon.jpg|left]]: ''Main article: [[Europa]]''
'''Europa''', the second innermost, is also the smallest of the Galileans. It has very few craters, and thus by [[uniformitarianism|uniformitarian]] criteria it is considered "young." The thick ice sheet that overlays it has fueled speculation, in the scientific community and in [[science fiction]], that it harbors life. The [[Galileo project|Galileo]] spacecraft discovered evidence that Europa has a sub-crust saltwater ocean under a very thin rock-and-ice layer--and in order to prevent the spacecraft from crashing into Europa, rupturing the crust, and contaminating Europa's oceans with earthly [[microbe]]s, Galileo's mission controllers steered Galileo into a crash dive into Jupiter in September of 2003.<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>
== Ganymede ==
[[Image:Ganymede2 galileo big.jpg|100px|right]]: ''Main article: [[Ganymede]]''
'''Ganymede''' is the largest moon not only of Jupiter but in all the solar system--larger than [[Mercury]] so that in theory it could be a true [[planet]] if it orbited the [[Sun]]. In sharp contrast to Io and Europa, Ganymede is very heavily cratered and is therefore considered "old." Ganymede has its own independent magnetic field, and a quite strong one. Like Europa, it has a saltwater ocean, but under a much thicker crust than that of Europa.
== Callisto ==
: ''Main article: [[Callisto]]''
'''Callisto''', the outermost of the Galilean moons, does not participate in the Laplace resonance with the other three and also lies beyond the Van Allen-like radiation belt that surrounds Jupiter. Like Ganymede, Callisto is very heavily cratered.
== Problems for Uniformitarianism Posed by the Galileans ==
The three-part orbital resonance of [[Io]], [[Europa]], and [[Ganymede]] becomes very difficult to explain in light of Ganymede appearing to be so much "older" than Io or Europa. At least some astronomers believe that this resonance is primordial.<ref name=Peale/> Not all astronomers agree.<ref name=Showman/> But if the resonance ''is'' primordial, then the presence within it of bodies of differing "geological ages" begs explanation. Most observers consider that Io is older than it looks, because the volcanoes and their associated lava flows have erased the craters. But that cannot apply to Europa, which looks at least as "young" as does Io.
Io's volcanism testifies to its tremendous internal heat. In fact, Io is radiating far more heat than tidal heating alone could generate. And yet no astronomer accepts [[radioactive decay]] as a significant source of Io's heat. Either Io has another, still-unknown source of heat, or else the present rate of heat outflow is unsustainable and temporary.
== References ==
{{reflist|2}}
{{SolarSystem}}