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'''Callisto''' is [[Jupiter]]'s eighth and second largest [[moon]]. It is approximately 780,213,000 km from the [[sun]]. Callisto’s rotation and revolution periods are both approximately 17 days. It is approximately 1,883,000 km from Jupiter, and its diameter is about 4800 km. Callisto is made up of about 40% ice and 60% rock and iron, and its surface is covered entirely with craters.
{{Planet
| name=Callisto
| image=Callisto true color.jpg
| caption=True-color full-face image of Callisto by spacecraft [[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=[[Nymph]] or daughter of [[Lycaeon]], servant of [[Artemis]], mistress of [[Zeus]]
| primary=Jupiter
| order=8
| periapsis=1,868,768 km<ref name=calc>Calculated</ref>
| apoapsis=1,896,632 km<ref name=calc/>
| semimajor=1,882,700 km<ref name=ssdelem>"[http://ssd.jpl.nasa.gov/?sat_elem Planetary Satellite Mean Orbital Parameters]," Solar System Dynamics, [[JPL]], [[NASA]]. Accessed March 1, 2008.</ref>
| eccentricity=0.0074<ref name=ssdelem/>
| sidereal=16.68902 da<ref name=ssdelem/><ref name=solarviews>Hamilton, Calvin J. "[http://www.solarviews.com/eng/callisto.htm Entry for Callisto]." ''SolarViews'', 2001. Accessed March 1, 2008.</ref>
| orbitspeed=8.21 km/s<ref name=solarviews/>
| inclination=0.281°<ref name=solarviews/><ref name=resa>"[http://www.resa.net/nasa/callisto.htm Callisto: The Ancient Moon]." ''Galileo at Jupiter''. Accessed March 1, 2008.</ref>
| siderealday=16.68902 da<ref name=solarviews/>
| rotatespeed=10.50 m/s<ref name=calc/>
| mass=1.076 * 10<sup>23</sup> kg (1.80% earth)<ref name=solarviews/>
| density=1,834 kg/m³<ref name=ssdphys>"[http://ssd.jpl.nasa.gov/?sat_phys_par Planetary Satellite Physical Parameters]." Solar System Dynamics, [[JPL]], [[NASA]]. Accessed March 1, 2008.</ref>
| meanradius=2,410.3 km<ref name=ssdphys/>
| surfacegrav=1.23577 m/s² (1.26014 ''g'')<ref name=calc/>
| escapespeed=2.45 km/s<ref name=solarviews/>
| surfacearea=73,004,909 km² (14.312% earth)<ref name=calc/>
| meantemp=126 K<ref name=gal1>"[http://www2.jpl.nasa.gov/galileo/callisto/ Callisto, a Continuing Story of Discovery]." ''Galileo Project''. Accessed March 1, 2008.</ref>
| maxtemp=168 K<ref name=gal1/>
| composition=60% rock and 40% ice<ref name=solarviews/><ref name=arnett>Arnett, Bill. "[http://www.nineplanets.org/callisto.html Entry for Callisto]." ''The <s>Nine</s> 8 Planets'', November 20, 2001. Accessed March 1, 2008.</ref>
| color=Gray-maroon-brown
| albedo=0.20<ref name=solarviews/>
}}'''Callisto''' is the outermost of the [[Galilean moons]] of Jupiter, and the second-largest moon of that primary and the third-largest natural [[satellite]] in all the [[solar system]] after [[Ganymede]] and [[Titan (moon)|Titan]]. Once considered a boring, undifferentiated lump of rock, Callisto has surprised planetary scientists at least as much as has other moons like Titan or Ganymede or [[Europa]] after the kind of close-up observation that deep-space orbiters can provide.
== 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/>
[[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 moons]] and not the Galilean-Marian moons.<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 characteristics ==
Callisto orbits [[Jupiter]] at a mean distance of about 1.9 million kilometers and takes 17 days to make a complete orbit around Jupiter (and a complete rotation on its own axis). Like other moons that orbit their primaries as closely as does Callisto, Callisto is [[tidal lock|tidally locked]] and keeps the same hemisphere toward Jupiter at all times. Callisto's orbit is more eccentric than those of the other Galilean moons (except for [[Europa]]), and is slightly inclined to Jupiter's equator.
Callisto does not resonate in its orbit with [[Ganymede]] and the other two Galilean moons; in fact its sidereal month is slightly more than twice as long as that of Ganymede. Callisto is, however, subject to tidal stresses on account of the eccentricity of its orbit.
== Physical characteristics ==
=== Surface ===
[[Image:Callisto equatorial.jpg|thumb|150 px|left|Callisto's equatorial surface]]Callisto is one of the most heavily cratered celestial bodies, if not the most heavily cratered body, in the solar system. Because of this cratering, [[uniformitarianism|uniformitarian]] [[astronomy|astronomers]] commonly assume Callisto to be 4.5 billion years old, or not much "younger" than they consider the solar system itself to be.<ref name=darling>"[http://www.daviddarling.info/encyclopedia/C/Callisto.html Callisto (moon of Jupiter)]." ''The Internet Encyclopedia of Science''. Accessed March 1, 2008.</ref><ref name=spires>"[http://science.nasa.gov/headlines/y2001/ast22aug_1.htm The Strange Spires of Callisto]." ''Science@NASA''. Accessed March 1, 2008.</ref>
[[Image:Callisto scarp.jpg|thumb|200 px|right|Escarpment in Valhalla region]]Callisto's craters are remarkably flat, and lack the high ridges, radial rays, and central depressions observed on impact craters on other bodies like the [[Moon]] and [[Mercury]].<ref name=arnett/> Moreover, many of the smaller craters have been obscured, as if by subsequent impacts.<ref name=arnett/> Some authorities hold that this relative flattening and lack of large mountains is due primarily to Callisto having an icy surface.<ref name=solarviews/>
Callisto has two very broad basins that most consider the result of repeated impacts. They are named for the two fabled homes of the [[Viking]] gods, [[Valhalla]] and [[Asgard]].<ref name=darling/> Valhalla measures 3,000 km across; Asgard 1,600 km.<ref name=solarviews/>
Spectroscopy reveals a layer of solid [[carbon dioxide]] ("dry ice") on Callisto's surface and a thin, constantly escaping carbon-dioxide "atmosphere."<ref name=darling/><ref name=britannica>"[http://www.britannica.com/eb/article-9018714 Entry for Callisto]." ''Encyclopædia Britannica'' 2008, Encyclopædia Britannica Online. Accessed March 1, 2008.</ref>
=== Interior ===
[[Image:Callisto interior.jpg|thumb|250 px|left|Callisto cutaway]]Callisto is remarkably lightweight (1.8% of earth's mass), and its density, at 1,834 kg/m³, is consistent with an interior that has almost as much water as it has ice. Until the [[Galileo Project]], Callisto was considered to have an undifferentiated interior. Detailed examination by the Galileo orbiter suggests an interior composed of 40% ice and 60% rock, with the percentage of rock gradually increasing with increasing depth. Unique among the Galilean moons, Callisto does ''not'' have a metallic core.
Callisto has a very weak magnetic field that causes definite perturbations in the magnetic field of Jupiter as it passes in its orbit. Magnetic fields require currents, and current in turn require either metal or an electrolytic solution.<ref name=solarviews/> The most likely explanation is that Callisto has a saltwater ocean, which according to current models lies 200 km deep to the surface and is 10 km deep.<ref name=interior>"[http://photojournal.jpl.nasa.gov/catalog/PIA01478 Catalog page for Image PIA01478]." [[Jet Propulsion Laboratory]], [[NASA]], October 21, 1998. Accessed March 1, 2008.</ref> Further evidence in favor of this saltwater ocean lies in the relatively flat nature of the features of the Valhalla basin. Dr. David A. Williams of [[Arizona State University]] has long held that such an ocean must exist and help absorb the shock of meteoric impacts; the recent photographs of the region support this hypothesis.<ref name=Williams>"[http://www.jpl.nasa.gov/releases/2001/release_2001_230.html Ocean Inside Jupiter's Moon Callisto May Have Cushioned Big Impact]." [[NASA]] news release, November 29, 2001. Accessed March 1, 2008.</ref>
== Exploration ==
The first close observation of Callisto was by [[Voyager 1]] and [[Voyager 2]]. The Galileo probe visited Callisto twice, in 1998 and again in 2001, in an effort to steer it toward [[Io]], the last of its targets.<ref name=Britt>Britt, Robert Roy. "[http://www.space.com/missionlaunches/missions/callisto_flyby_010523.html Callisto Close-up: Galileo's Final Swing Past 'Ugly Duckling' Moon]." ''<Space.com>'', May 23, 2001. Accessed March 1, 2008.</ref>
No further intensive observation of Callisto is currently planned.
== References ==
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