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| {{other uses|Callisto (disambiguation)}}
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| {{Infobox Planet
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| | name = Callisto
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| | alt_names = Jupiter IV
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| | adjectives = Callistoan, Callistonian
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| | image = [[File:Callisto.jpg|250px|Callisto]]
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| | caption =
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| | bgcolour = #a0ffa0
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| | discovery = yes
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| | discoverer = [[Galileo Galilei]]
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| | discovered = January 7, 1610<ref name=Galilei/>
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| | semimajor = 1 882 700 km<ref name=orbit/>
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| | eccentricity = {{val|0.0074}}<ref name=orbit/>
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| | periapsis = {{val|1869000|u=km}}<ref group=lower-alpha>Periapsis is derived from the semimajor axis (''a'') and eccentricity (''e''): <math>a(1-e)</math>.</ref>
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| | apoapsis = {{val|1897000|u=km}}<ref group=lower-alpha>Apoapsis is derived from the semimajor axis (''a'') and eccentricity (''e''): <math>a(1+e)</math>.</ref>
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| | period = {{val|16.6890184|ul=d}}<ref name=orbit/>
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| | avg_speed = {{val|8.204|u=km/s}}
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| | inclination = 0.192° (to local [[Laplace plane]]s)<ref name=orbit/>
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| | satellite_of = [[Jupiter]]
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| | physical_characteristics = yes
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| | mean_radius = {{val|2410.3|1.5|u=km}} (0.378 Earths)<ref name="Anderson 2001"/>
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| | surface_area = {{val|7.30|e=7|u=km2}} (0.143 Earths)<ref group=lower-alpha>Surface area derived from the radius (''r''): <math>4\pi r^2</math>.</ref>
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| | volume = {{val|5.9|e=10|u=km3}} (0.0541 Earths)<ref group=lower-alpha>Volume derived from the radius (''r''): <math>\frac{4}{3}\pi r^3</math>.</ref>
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| | mass = {{val|1.075938|0.000137|e=23|u=kg}} (0.018 Earths)<ref name="Anderson 2001"/>
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| | density = {{val|1.8344|0.0034|u=g/cm3}}<ref name="Anderson 2001"/>
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| | surface_grav = {{val|1.235|ul=m/s2}} (0.126 ''[[g-force|g]]'')<ref group=lower-alpha>Surface gravity derived from the mass (''m''), the [[gravitational constant]] (''G'') and the radius (''r''): <math>\frac{Gm}{r^2}</math>.</ref>
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| | escape_velocity = 2.440 km/s<ref group=lower-alpha>Escape velocity derived from the mass (''m''), the [[gravitational constant]] (''G'') and the radius (''r''): <math>\textstyle\sqrt{\frac{2Gm}{r}}</math>.</ref>
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| | rotation = [[synchronous rotation|synchronous]]<ref name="Anderson 2001"/>
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| | axial_tilt = zero<ref name="Anderson 2001"/>
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| | albedo = 0.22 (geometric)<ref name=Moore2004/>
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| | magnitude = 5.65 ([[Opposition (astronomy)|opposition]])<ref name=magnitude>{{cite web|title=Classic Satellites of the Solar System|url=http://www.oarval.org/ClasSaten.htm|publisher=Observatorio ARVAL|accessdate=2007-07-13}}</ref>
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| | temperatures=yes
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| | temp_name1 = K<ref name=Moore2004/>
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| | max_temp_1 = {{val|165|5}}
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| | mean_temp_1 = {{val|134|11}}
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| | min_temp_1 = {{val|80|5}}
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| | atmosphere = yes
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| | surface_pressure = {{val|7.5|u=[[bar (unit)|pbar]]}}<ref name="Carlson 1999"/>
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| | atmosphere_composition = ≈ {{val|4|e=8|u=molecules/cm<sup>3</sup>}} [[carbon dioxide]];<ref name="Carlson 1999"/><br>up to {{val|2|e=10|u=molecules/cm<sup>3</sup>}} [[molecular oxygen]](O<sub>2</sub>)<ref name="Liang 2005"/>
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| }}
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| '''Callisto''' {{IPAc-en|k|ə|ˈ|l|ɪ|s|t|oʊ}}<ref>{{respell|kə|LIS|toh}}, or as {{lang-el|''Καλλιστώ''}}</ref> ('''Jupiter IV''') is a [[Moons of Jupiter|moon of the planet Jupiter]]. It was discovered in 1610 by [[Galileo Galilei]]. It is the [[List of moons#Moons of planets and dwarf planets|third-largest moon]] in the [[Solar System]] and the second largest in the Jovian system, after [[Ganymede (moon)|Ganymede]]. Callisto has about 99% the diameter of the planet [[Mercury (planet)|Mercury]] but only about a third of its mass. It is the fourth [[Galilean moon]] of [[Jupiter]] by distance, with an orbital radius of about 1,880,000 km.<ref name=orbit/> It does not form part of the [[orbital resonance]] that affects three inner Galilean satellites—[[Io (moon)|Io]], [[Europa (moon)|Europa]] and [[Ganymede (moon)|Ganymede]]—and thus does not experience appreciable [[Tidal acceleration#Tidal heating|tidal heating]].<ref name=Musotto2002/> Callisto's rotation is [[tidal locking|tidally locked]] to its revolution around Jupiter, so that the same hemisphere always faces inward; Jupiter appears to stand still in Callisto's sky. It is less affected by Jupiter's [[magnetosphere]] than the other [[inner satellite]]s because it orbits farther away.<ref name=Cooper2001/>
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| Callisto is composed of approximately equal amounts of [[rock (geology)|rock]] and [[Volatiles|ice]]s, with a mean [[density]] of about 1.83 g/cm<sup>3</sup>. Compounds detected [[Spectroscopy|spectroscopically]] on the surface include [[ice|water ice]], [[carbon dioxide]], [[silicate]]s, and [[organic compound]]s. Investigation by the ''[[Galileo (spacecraft)|Galileo]]'' spacecraft revealed that Callisto may have a small [[silicate]] [[Planetary core|core]] and possibly a subsurface [[ocean]] of liquid [[water]] at depths greater than 100 km.<ref name=Kuskov2005/><ref name=Showman1999/>
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| The surface of Callisto is heavily [[impact crater|cratered]] and extremely old. It does not show any signatures of [[endogenic|subsurface]] processes such as [[plate tectonics]] or [[volcano|volcanism]], and is thought to have evolved predominantly under the influence of [[impact crater|impacts]].<ref name="Greeley 2000"/> Prominent surface features include multi-ring structures, variously shaped [[impact crater]]s, and chains of craters (''catenae'') and associated [[escarpment|scarps]], ridges and deposits.<ref name="Greeley 2000"/> At a small scale, the surface is varied and made up of small, sparkly frost [[deposit (geology)|deposits]] at the tips of high spots, surrounded by a low-lying, smooth blanket of dark material.<ref name=Moore2004/> This is thought to result from the [[sublimation (chemistry)|sublimation]]-driven degradation of small [[landform]]s, which is supported by the general deficit of small impact craters and the presence of numerous small knobs, considered to be their remnants.<ref name=Moore1999/> The absolute ages of the landforms are not known.
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| Callisto is surrounded by an extremely thin [[atmosphere]] composed of [[carbon dioxide]]<ref name="Carlson 1999"/> and probably [[molecular oxygen]],<ref name="Liang 2005"/> as well as by a rather intense [[ionosphere]].<ref name="Kliore 2002"/> Callisto is thought to have formed by slow [[accretion (astrophysics)|accretion]] from the disk of the gas and dust that surrounded Jupiter after its formation.<ref name=Canup2002/> Callisto's gradual accretion and the lack of [[Tidal acceleration#Tidal heating|tidal heating]] meant that not enough heat was available for rapid [[planetary differentiation|differentiation]]. The slow [[convection]] in the interior of Callisto, which commenced soon after formation, led to partial differentiation and possibly to the formation of a subsurface ocean at a depth of 100–150 km and a small, rocky [[planetary core|core]].<ref name="Spohn 2003"/>
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| The likely presence of an ocean within Callisto leaves open the possibility that it could harbor [[extraterrestrial life|life]]. However, conditions are thought to be less favorable than on nearby [[Europa (moon)|Europa]].<ref name=Lipps2004/> Various space probes from ''[[Pioneer 10|Pioneers 10]]'' and ''[[Pioneer 11|11]]'' to ''[[Galileo (spacecraft)|Galileo]]'' and ''[[Cassini–Huygens|Cassini]]'' have studied Callisto. Because of its low [[radiation]] levels, Callisto has long been considered the most suitable place for a human base for future exploration of the Jovian system.<ref name=HOPE/>
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| ==Discovery and naming==
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| Callisto was discovered by Galileo in January 1610 along with three other large Jovian moons—[[Ganymede (moon)|Ganymede]], [[Io (moon)|Io]], and [[Europa (moon)|Europa]].<ref name=Galilei>Galilei, G.; [http://www.physics.emich.edu/jwooley/chapter9/Chapter9.html ''Sidereus Nuncius''] (March 13, 1610)</ref> Callisto is named after one of [[Zeus]]'s many lovers in [[Greek mythology]]. [[Callisto (mythology)|Callisto]] was a nymph (or, according to some sources, the daughter of [[Lycaon (mythology)|Lycaon]]) who was associated with the goddess of the hunt, [[Artemis]].<ref name=Galileo/> The name was suggested by [[Simon Marius]] soon after Callisto's discovery.<ref name=Marius>{{cite book|author=[[Marius, S.]]|title=Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici|url=http://galileo.rice.edu/sci/marius.html|year=1614}}</ref> Marius attributed the suggestion to [[Johannes Kepler]].<ref name=Galileo>{{cite web|title=Satellites of Jupiter|publisher=The Galileo Project| url=http://galileo.rice.edu/sci/observations/jupiter_satellites.html|accessdate=2007-07-31}}</ref> However, the names of the [[Galilean moons|Galilean satellites]] fell into disfavor for a considerable time, and were not revived in common use until the mid-20th century. In much of the earlier astronomical literature, Callisto is referred to by its Roman numeral designation, a system introduced by Galileo, as '''{{nowrap|Jupiter IV}}''' or as "the fourth satellite of Jupiter".<ref name=Barnard1892>{{cite journal|last=Barnard|first=E. E.|url= http://adsabs.harvard.edu//full/seri/AJ.../0012//0000081.000.html|title=Discovery and Observation of a Fifth Satellite to Jupiter|journal=Astronomical Journal|volume=12|year=1892|pages=81–85|doi=10.1086/101715|bibcode=1892AJ.....12...81B}}</ref> In scientific writing, the adjectival form of the name is ''Callistoan'',<ref name=Klemaszewski2001>{{cite web|last= Klemaszewski|first= J.A.|coauthors= Greeley, R.|title= Geological Evidence for an Ocean on Callisto |year=2001|publisher=Lunar and Planetary Science XXXI|page=1818|url=http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1818.pdf|format=PDF}}</ref>
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| pronounced {{IPAc-en|ˌ|k|æ|l|ɨ|ˈ|s|t|oʊ|.|ə|n}}, or ''Callistan''.<ref name=Moore1999/>
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| ==Orbit and rotation==
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| [[File:001221 Cassini Jupiter & Europa & Callisto.jpg|left|thumb|Callisto (bottom left), Jupiter (top right) and Europa (below and left of Jupiter's [[Great Red Spot]]) as viewed by ''[[Cassini–Huygens|Cassini]]'']]
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| Callisto is the outermost of the four Galilean moons of Jupiter. It orbits at a distance of approximately 1 880 000 km (26.3 times the 71 492 km radius of Jupiter itself).<ref name=orbit>{{cite web|title=Planetary Satellite Mean Orbital Parameters|publisher=Jet Propulsion Laboratory, California Institute of Technology|url=http://ssd.jpl.nasa.gov/?sat_elem}}</ref> This is significantly larger than the orbital radius—1 070 000 km—of the next-closest Galilean satellite, Ganymede. As a result of this relatively distant orbit, Callisto does not participate in the [[orbital resonance|mean-motion resonance]]—in which the three inner Galilean satellites are locked—and probably never has.<ref name=Musotto2002>{{cite journal|last=Musotto|first=Susanna|coauthors=Varadi, Ferenc; Moore, William; Schubert, Gerald|title=Numerical Simulations of the Orbits of the Galilean Satellites|year=2002|volume=159|issue=2|pages=500–504|doi=10.1006/icar.2002.6939| bibcode=2002Icar..159..500M | journal = Icarus}}</ref>
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| Like most other regular planetary moons, Callisto's rotation is locked to be [[synchronous rotation|synchronous]] with its orbit.<ref name="Anderson 2001">{{cite journal|last=Anderson|first=J. D. |coauthors=Jacobson, R. A.; McElrath, T. P.; ''et al.'' |title=Shape, mean radius, gravity field and interior structure of Callisto |journal=Icarus|year=2001|volume=153|issue=1|pages=157–161|doi=10.1006/icar.2001.6664| bibcode=2001Icar..153..157A}}</ref> The length of the Callistoan day, simultaneously its [[orbital period]], is about 16.7 days. Its orbit is very slightly eccentric and inclined to the Jovian [[equator]], with the [[orbital eccentricity|eccentricity]] and [[inclination]] changing [[almost periodic function|quasi-periodic]]ally due to solar and planetary gravitational perturbations on a timescale of centuries. The ranges of change are 0.0072–0.0076 and 0.20–0.60°, respectively.<ref name=Musotto2002/> These orbital variations cause the [[axial tilt]] (the angle between rotational and orbital axes) to vary between 0.4 and 1.6°.<ref name=Bills2005>{{cite journal|last=Bills|first=Bruce G.|title=Free and forced obliquities of the Galilean satellites of Jupiter|year=2005|volume=175|issue=1|pages=233–247|doi=10.1016/j.icarus.2004.10.028| bibcode=2005Icar..175..233B | journal = Icarus}}</ref>
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| The dynamical isolation of Callisto means that it has never been appreciably [[Tidal acceleration#Tidal heating|tidally heated]], which has had important consequences for its internal structure and [[evolution]].<ref name=Freeman2006/> Its distance from Jupiter also means that the [[charged particle|charged-particle]] [[flux]] from Jupiter's [[magnetosphere]] at its surface is relatively low—about 300 times lower than, for example, that at [[Europa (moon)|Europa]]. Hence, unlike the other Galilean moons, charged-particle [[irradiation]] has had a relatively minor effect on the Callistoan surface.<ref name=Cooper2001>{{cite journal|last=Cooper|first=John F.|coauthors=Johnson, Robert E.; Mauk, Barry H.; et al.|title=Energetic Ion and Electron Irradiation of the Icy Galilean Satellites|year=2001|volume=139|issue=1|pages=133–159|doi=10.1006/icar.2000.6498| url=http://people.virginia.edu/~rej/Icarus_Jan2001_Cooper_et_al.pdf|format=PDF | journal = Icarus|bibcode=2001Icar..149..133C}}</ref> The radiation level at the surface of Callisto is equivalent to a dose of about 0.01 [[Röntgen equivalent man|rem]] (0.1 [[millisievert|mSv]]) per day.<ref name="ringwald">{{cite web |date=2000-02-29 |title=SPS 1020 (Introduction to Space Sciences) |publisher=California State University, Fresno |author=Frederick A. Ringwald |url=http://zimmer.csufresno.edu/~fringwal/w08a.jup.txt |accessdate=2009-07-04}} [http://www.webcitation.org/5jwBSgPuV (Webcite from 2009-09-20)]</ref>
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| ==Physical characteristics==
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| ===Composition===
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| [[File:PIA00844 NIMS spectra.gif|thumb|right|220px|[[Galileo (spacecraft)#Near-Infrared Mapping Spectrometer (NIMS)|Near-IR spectra]] of dark cratered plains (red) and the [[Asgard (crater)|Asgard impact structure]] (blue), showing the presence of more water ice ([[Water absorption|absorption bands]] from 1 to 2 [[micrometer (unit)|µm]])<ref name = "Clark">{{Cite journal | last = Clark | first = R. N. | title = Water frost and ice: the near-infrared spectral reflectance 0.65–2.5 μm | journal = [[Journal of Geophysical Research]] | volume = 86 | issue = B4 | pages = 3087–3096 | date = 1981-04-10 | url = http://www.agu.org/pubs/crossref/1981/JB086iB04p03087.shtml | doi = 10.1029/JB086iB04p03087| accessdate = 2010-03-03 | bibcode=1981JGR....86.3087C}}</ref> and less rocky material within Asgard.]]
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| The average [[density]] of Callisto, 1.83 g/cm<sup>3</sup>,<ref name="Anderson 2001"/> suggests a composition of approximately equal parts of rocky material and [[ice|water ice]], with some additional volatile ices such as [[ammonia]].<ref name=Kuskov2005>{{cite journal|last=Kuskov|first=O.L.|coauthors=Kronrod, V.A.|title=Internal structure of Europa and Callisto|year=2005|volume=177|issue=2|pages=550–369|doi=10.1016/j.icarus.2005.04.014| bibcode=2005Icar..177..550K | journal = Icarus}}</ref> The mass fraction of ices is between 49–55%.<ref name=Kuskov2005/><ref name="Spohn 2003"/> The exact composition of Callisto's [[Rock (geology)|rock]] component is not known, but is probably close to the composition of L/LL type [[ordinary chondrite]]s, which are characterized by less total [[iron]], less metallic iron and more [[iron oxide]] than [[H chondrite]]s. The weight ratio of iron to [[silicon]] is 0.9—1.3 in Callisto, whereas the [[Sun|solar ratio]] is around 1:8.<ref name=Kuskov2005/>
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| Callisto's surface has an [[albedo]] of about 20%.<ref name=Moore2004/> Its surface composition is thought to be broadly similar to its composition as a whole. Near-infrared [[spectroscopy]] has revealed the presence of water ice [[absorption band]]s at wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0 micrometers.<ref name=Moore2004/> Water ice seems to be ubiquitous on the surface of Callisto, with a mass fraction of 25–50%.<ref name=Showman1999>{{cite journal|last=Showman |first=Adam P.|coauthors=Malhotra, Renu|title=The Galilean Satellites|year=1999|journal=Science|volume=286|pages=77–84|doi=10.1126/science.286.5437.77| url=http://www.lpl.arizona.edu/~showman/publications/showman-malhotra-1999.pdf|format=PDF|pmid=10506564|issue=5437}}</ref> The analysis of high-resolution, [[near-infrared]] and [[Ultraviolet|UV]] [[spectrum|spectra]] obtained by the ''[[Galileo (spacecraft)|Galileo]]'' spacecraft and from the ground has revealed various non-ice materials: [[magnesium]]- and [[iron]]-bearing hydrated [[silicates]],<ref name=Moore2004/> [[carbon dioxide]],<ref name=Brown2003/> [[sulfur dioxide]],<ref name=Noll1996>{{cite web|last=Noll|first=K.S.|title=Detection of SO<sub>2</sub> on Callisto with the Hubble Space Telescope|year=1996|publisher=Lunar and Planetary Science XXXI| url=http://www.lpi.usra.edu/meetings/lpsc97/pdf/1852.PDF|page=1852|format=PDF}}</ref> and possibly [[ammonia]] and various [[organic compounds]].<ref name=Moore2004/><ref name=Showman1999/> Spectral data indicate that Callisto's surface is extremely heterogeneous at the small scale. Small, bright patches of pure water ice are intermixed with patches of a rock–ice mixture and extended dark areas made of a non-ice material.<ref name=Moore2004/><ref name="Greeley 2000"/>
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| The Callistoan surface is asymmetric: the leading hemisphere<ref group=lower-alpha name=footnote2>The leading hemisphere is the hemisphere facing the direction of the orbital motion; the trailing hemisphere faces the reverse direction.</ref> is darker than the trailing one. This is different from other [[Galilean satellites]], where the reverse is true.<ref name=Moore2004/> The trailing hemisphere<ref group=lower-alpha name=footnote2 /> of Callisto appears to be enriched in [[carbon dioxide]], whereas the leading hemisphere has more [[sulfur dioxide]].<ref name=Hibbitts1998>{{cite web|last=Hibbitts |first=C.A.|coauthors=McCord, T. B.; Hansen, G.B.|title=Distributions of CO<sub>2</sub> and SO<sub>2</sub> on the Surface of Callisto|year=1998|publisher=Lunar and Planetary Science XXXI|url=http://www.lpi.usra.edu/meetings/lpsc2000/pdf/1908.pdf|page=1908|format=PDF}}</ref> Many fresh [[impact crater]]s like [[Adlinda (crater)|Lofn]] also show enrichment in carbon dioxide.<ref name=Hibbitts1998/> Overall, the chemical composition of the surface, especially in the dark areas, may be close to that seen on [[D-type asteroid]]s,<ref name="Greeley 2000"/> whose surfaces are made of [[carbon]]aceous material.
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| ===Internal structure===
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| [[File:PIA01478 Interior of Callisto.jpg|thumb|left|150px|Model of Callisto's internal structure showing a surface ice layer, a possible liquid water layer, and an ice-rock interior]]
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| Callisto's battered surface lies on top of a cold, stiff, and icy [[lithosphere]] that is between 80 and 150 km thick.<ref name=Kuskov2005/><ref name="Spohn 2003"/> A salty ocean 50–200 km deep may lie beneath the [[crust (geology)|crust]],<ref name=Kuskov2005/><ref name="Spohn 2003"/> indicated by studies of the [[magnetic field]]s around Jupiter and its moons.<ref name="Khurana 2000">{{cite journal |last=Khurana|first=K. K.|coauthors=''et al.''|title=Induced magnetic fields as evidence for subsurface oceans in Europa and Callisto|journal=Nature|year=1998|volume=395|pages=777–780|doi=10.1038/27394| url=http://www.igpp.ucla.edu/people/mkivelson/Publications/N395777.pdf|format=PDF |pmid=9796812 |issue=6704|bibcode = 1998Natur.395..777K }}</ref><ref name="Zimmer 2000">{{cite journal|last=Zimmer|first=C.|coauthors=Khurana, K. K.|title=Subsurface Oceans on Europa and Callisto: Constraints from Galileo Magnetometer Observations|journal=Icarus|year=2000|volume=147|issue=2|pages=329–347|doi=10.1006/icar.2000.6456| url=http://www.igpp.ucla.edu/people/mkivelson/Publications/ICRUS147329.pdf|format=PDF|bibcode=2000Icar..147..329Z}}</ref> It was found that Callisto responds to Jupiter's varying background magnetic field like a perfectly [[electrical conductivity|conducting]] sphere; that is, the field cannot penetrate inside Callisto, suggesting a layer of highly conductive fluid within it with a thickness of at least 10 km.<ref name="Zimmer 2000"/> The existence of an ocean is more likely if water contains a small amount of [[ammonia]] or other [[antifreeze]], up to 5% by weight.<ref name="Spohn 2003">{{cite journal |last=Spohn|first=T.|coauthors=Schubert, G.|title=Oceans in the icy Galilean satellites of Jupiter?|journal=Icarus|year=2003|volume=161 |issue=2|pages=456–467|doi=10.1016/S0019-1035(02)00048-9| url=http://lasp.colorado.edu/icymoons/europaclass/Spohn_Schubert_oceans.pdf|format=PDF|bibcode=2003Icar..161..456S}}</ref> In this case the ocean can be as thick as 250–300 km.<ref name=Kuskov2005/> Failing an ocean, the icy lithosphere may be somewhat thicker, up to about 300 km.
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| Beneath the lithosphere and putative ocean, Callisto's interior appears to be neither entirely uniform nor particularly variable. ''[[Galileo (spacecraft)|Galileo]]'' orbiter data<ref name="Anderson 2001"/> (especially the dimensionless [[moment of inertia]]<ref group=lower-alpha>The dimensionless moment of inertia referred to is ''<tt>I</tt>''/(''mr''<sup>2</sup>), where ''<tt>I</tt>'' is the moment of inertia, ''m'' the mass, and ''r'' the maximal radius. It is 0.4 for a homogenous spherical body, but less than 0.4 if density increases with depth.</ref>—0.3549 ± 0.0042—determined during close flybys) suggest that its interior is composed of compressed [[rocks]] and [[ice]]s, with the amount of rock increasing with depth due to partial settling of its constituents.<ref name=Kuskov2005/><ref name="Anderson 1998">{{cite journal|last=Anderson|first=J. D. |coauthors=Schubert, G.; Jacobson, R. A.; ''et al.''|title=Distribution of Rock, Metals and Ices in Callisto|journal=Science|year=1998|volume=280|pages=1573–1576|doi=10.1126/science.280.5369.1573| url=http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/19178/1/98-0442.pdf|format=PDF|pmid=9616114|issue=5369|bibcode = 1998Sci...280.1573A }}</ref> In other words, Callisto is only partially [[planetary differentiation|differentiated]]. The density and moment of inertia are compatible with the existence of a small [[silicate]] core in the center of Callisto. The radius of any such core cannot exceed 600 km, and the density may lie between 3.1 and 3.6 g/cm<sup>3</sup>.<ref name="Anderson 2001"/><ref name=Kuskov2005/> Callisto's interior is in stark contrast to [[Ganymede (moon)#Internal structure|that of Ganymede]], which appears to be fully differentiated.<ref name=Showman1999/><ref name=Sohl2002>{{cite journal|last=Sohl|first=F.|coauthors=Spohn, T; Breuer, D.; Nagel, K.|title=Implications from Galileo Observations on the Interior Structure and Chemistry of the Galilean Satellites|journal=Icarus|year=2002|volume=157|issue=1|pages=104–119| doi=10.1006/icar.2002.6828|bibcode=2002Icar..157..104S}}</ref>
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| ===Surface features===
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| {{See also|List of geological features on Callisto}}
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| [[File:Cratered plains PIA00745.jpg|thumb|left|200px|''Galileo'' image of cratered plains, illustrating the pervasive local smoothing of Callisto's surface]]
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| The ancient surface of Callisto is one of the most heavily cratered in the Solar System.<ref name="Zahnle 1998">{{cite journal|last=Zahnle|first=K.|coauthors=Dones, L. |title=Cratering Rates on the Galilean Satellites|journal=Icarus|year=1998|volume=136|issue=2|pages=202–222|doi=10.1006/icar.1998.6015| url=http://lasp.colorado.edu/icymoons/europaclass/Zahnle_etal_1998.pdf|format=PDF|pmid=11878353|bibcode=1998Icar..136..202Z}}</ref> In fact, the [[impact crater|crater]] density is close to [[wikt:saturation|saturation]]: any new crater will tend to erase an older one. The large-scale [[geology]] is relatively simple; there are no large Callistoan mountains, volcanoes or other [[endogeny|endogenic]] [[tectonic]] features.<ref name="Bender 1997">{{Cite journal|author=Bender, K. C.; Rice, J. W.; Wilhelms, D. E.; Greeley, R. |title=Geological map of Callisto |publisher=U.S. Geological Survey |year=1997 |url=http://astrogeology.usgs.gov/Projects/PlanetaryMapping/DIGGEOL/galsats/callisto/jcglobal.htm }}</ref> The impact craters and multi-ring structures—together with associated [[fracture (geology)|fractures]], [[escarpment|scarps]] and [[deposit (geology)|deposits]]—are the only large features to be found on the surface.<ref name="Greeley 2000"/><ref name="Bender 1997"/>
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| Callisto's surface can be divided into several geologically different parts: cratered plains, light plains, bright and dark smooth plains, and various units associated with particular multi-ring structures and impact craters.<ref name="Greeley 2000">{{cite journal|last=Greeley|first=R.|coauthors=Klemaszewski, J. E.; Wagner, L.; ''et al.''|title=Galileo views of the geology of Callisto|journal=Planetary and Space Science|year=2000|volume=48|issue=9|pages=829–853| bibcode=2000P&SS...48..829G|doi=10.1016/S0032-0633(00)00050-7}}</ref><ref name="Bender 1997"/> The cratered plains constitute most of the surface area and represent the ancient lithosphere, a mixture of ice and rocky material. The light plains include bright impact craters like [[Asgard (crater)|Burr]] and [[Adlinda (crater)|Lofn]], as well as the effaced remnants of old large craters called [[Palimpsest (planetary astronomy)|palimpsest]]s,{{#tag:ref|In the case of icy satellites, palimpsests are defined as bright circular surface features, probably old impact craters; see Greeley et al. 2000.<ref name="Greeley 2000"/>|group=lower-alpha}} the central parts of multi-ring structures, and isolated patches in the cratered plains.<ref name="Greeley 2000"/> These light plains are thought to be icy impact deposits. The bright, smooth plains constitute a small fraction of the Callistoan surface and are found in the ridge and [[trough (geology)|trough]] zones of the [[Valhalla (crater)|Valhalla]] and [[Asgard (crater)|Asgard]] formations and as isolated spots in the cratered plains. They were believed to be connected with [[endogenic]] activity, but the high-resolution ''Galileo'' images showed that the bright, smooth plains correlate with heavily fractured and knobby terrain and do not show any signs of resurfacing.<ref name="Greeley 2000"/> The ''Galileo'' images also revealed small, dark, smooth areas with overall coverage less than 10,000 km<sup>2</sup>, which appear to embay<ref group=lower-alpha>To ''embay'' means to shut in, or shelter, as in a bay.</ref> the surrounding terrain. They are possible [[cryovolcano|cryovolcanic]] deposits.<ref name="Greeley 2000"/> Both the light and the various smooth plains are somewhat younger and less cratered than the background cratered plains.<ref name="Greeley 2000"/><ref name="Wagner 2001">{{cite conference |last=Wagner |first=R. |coauthors=Neukum, G.; Greeley, R; ''et al.'' |title=Fractures, Scarps, and Lineaments on Callisto and their Correlation with Surface Degradation |booktitle=32nd Annual Lunar and Planetary Science Conference |date=March 12–16, 2001 |url=http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1838.pdf|format=PDF}}</ref>
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| [[File:Callisto Har PIA01054.jpg|thumb|right|250px|Impact crater [[Hár (crater)|Hár]] with a central dome. [[Crater chain|Chains]] of [[secondary crater]]s from formation of the more recent crater [[Tindr (crater)|Tindr]] at upper right crosscut the terrain.]]
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| Impact crater diameters seen range from 0.1 km—a limit defined by the [[image resolution|imaging resolution]]—to over 100 km, not counting the multi-ring structures.<ref name="Greeley 2000"/> Small craters, with diameters less than 5 km, have simple bowl or flat-floored shapes. Those 5–40 km across usually have a central peak. Larger impact features, with diameters in the range 25–100 km, have central pits instead of peaks, such as [[Tindr (crater)|Tindr]] crater.<ref name="Greeley 2000"/> The largest craters with diameters over 60 km can have central domes, which are thought to result from central [[tectonic uplift]] after an impact;<ref name="Greeley 2000"/> examples include [[Asgard (crater)|Doh]] and [[Hár (crater)|Hár]] craters. A small number of very large—more 100 km in diameter—and bright impact craters show anomalous dome geometry. These are unusually shallow and may be a transitional [[landform]] to the multi-ring structures, as with the [[Adlinda (crater)|Lofn]] impact feature.<ref name="Greeley 2000"/> Callistoan craters are generally shallower than those on the [[Moon]].
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| [[File:Valhalla crater on Callisto.jpg|thumb|250px|''[[Voyager 1]]'' image of [[Valhalla (crater)|Valhalla]], a [[Complex crater|multi-ring impact structure]] 3800 km in diameter]]
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| The largest impact features on the Callistoan surface are multi-ring basins.<ref name="Greeley 2000"/><ref name="Bender 1997"/> Two are enormous. [[Valhalla (crater)|Valhalla]] is the largest, with a bright central region 600 kilometers in diameter, and rings extending as far as 1,800 kilometers from the center (see figure).<ref name="Map 2002">{{cite map|title=Controlled Photomosaic Map of Callisto JC 15M CMN |publisher=U.S. Geological Survey |edition=2002 |url=http://geopubs.wr.usgs.gov/i-map/i2770/}}</ref> The second largest is [[Asgard (crater)|Asgard]], measuring about 1,600 kilometers in diameter.<ref name="Map 2002"/> Multi-ring structures probably originated as a result of a post-impact [[concentric]] fracturing of the lithosphere lying on a layer of soft or liquid material, possibly an ocean.<ref name=Klemaszewski2001/> The catenae—for example [[Gomul Catena]]—are long chains of impact craters lined up in straight lines across the surface. They were probably created by objects that were tidally disrupted as they passed close to Jupiter prior to the impact on Callisto, or by very [[wikt:oblique|oblique]] impacts.<ref name="Greeley 2000"/> A historical example of a disruption was [[Comet Shoemaker-Levy 9]].
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| As mentioned above, small patches of pure water ice with an [[albedo]] as high as 80% are found on the surface of Callisto, surrounded by much darker material.<ref name=Moore2004/> High-resolution ''[[Galileo (spacecraft)|Galileo]]'' images showed the bright patches to be predominately located on elevated surface features: [[rim (craters)|crater rims]], [[Escarpment|scarps]], ridges and knobs.<ref name=Moore2004/> They are likely to be thin [[frost|water frost]] [[deposit (geology)|deposits]]. Dark material usually lies in the lowlands surrounding and mantling bright features and appears to be smooth. It often forms patches up to 5 km across within the crater floors and in the intercrater depressions.<ref name=Moore2004/>
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| [[File:Landslides and knobs PIA01095.jpg|thumb|left|200px|Two [[landslide]]s 3–3.5 km long are visible on the right sides of the floors of the two large craters on the right.]]
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| On a sub-kilometer scale the surface of Callisto is more degraded than the surfaces of other icy [[Galilean moons]].<ref name=Moore2004/> Typically there is a deficit of small impact craters with diameters less than 1 km as compared with, for instance, the dark plains on [[ganymede (moon)|Ganymede]].<ref name="Greeley 2000"/> Instead of small craters, the almost ubiquitous surface features are small knobs and pits.<ref name=Moore2004/> The knobs are thought to represent remnants of crater rims degraded by an as-yet uncertain process.<ref name=Moore1999>{{cite journal|last=Moore|first=Jeffrey M.|coauthors=Asphaug, Erik; Morrison, David; et al.|title=Mass Movement and Landform Degradation on the Icy Galilean Satellites: Results of the Galileo Nominal Mission|year=1999|volume=140|issue=2 |pages=294–312|doi=10.1006/icar.1999.6132|bibcode=1999Icar..140..294M | journal = Icarus}}</ref> The most likely candidate process is the slow [[sublimation (chemistry)|sublimation]] of ice, which is enabled by a temperature of up to 165 [[kelvin|K]], reached at a subsolar point.<ref name=Moore2004/> Such sublimation of water or other [[volatiles]] from the dirty ice that is the [[bedrock]] causes its decomposition. The non-ice remnants form [[debris]] avalanches descending from the slopes of the crater walls.<ref name=Moore1999/> Such avalanches are often observed near and inside impact craters and termed "debris aprons".<ref name=Moore2004/><ref name="Greeley 2000"/><ref name=Moore1999/> Sometimes crater walls are cut by sinuous valley-like incisions called "gullies", which resemble certain [[Mars|Martian]] surface features.<ref name=Moore2004/> In the ice sublimation hypothesis, the low-lying dark material is interpreted as a blanket of primarily non-ice debris, which originated from the degraded rims of craters and has covered a predominantly icy bedrock.
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| The relative ages of the different surface units on Callisto can be determined from the density of impact craters on them. The older the surface, the denser the crater population.<ref name=Chapman1997>{{cite web|last=Chapman|first= C.R.|coauthors= Merline, W.J.; Bierhaus, B.; et al.|title= Populations of Small Craters on Europa, Ganymede, and Callisto: Initial Galileo Imaging Results |year=1997|publisher=Lunar and Planetary Science XXXI|url=http://www.lpi.usra.edu/meetings/lpsc97/pdf/1221.pdf|format=PDF|page=1221}}</ref> Absolute dating has not been carried out, but based on theoretical considerations, the cratered plains are thought to be ~4.5 [[1000000000 (number)|billion]] years old, dating back almost to the formation of the [[Solar System]]. The ages of multi-ring structures and impact craters depend on chosen background cratering rates and are estimated by different authors to vary between 1 and 4 billion years.<ref name="Greeley 2000"/><ref name="Zahnle 1998"/>
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| ===Atmosphere and ionosphere===
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| [[File:Callisto field.svg|thumb|300px|right|Induced magnetic field around Callisto]]
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| Callisto has a very tenuous atmosphere composed of [[carbon dioxide]].<ref name="Carlson 1999">{{cite journal |last=Carlson |first=R. W.|coauthors=''et al.''|title=A Tenuous Carbon Dioxide Atmosphere on Jupiter's Moon Callisto|journal=Science|year=1999 |volume=283|pages=820–821|doi=10.1126/science.283.5403.820| url=http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/16785/1/99-0186.pdf|format=PDF|pmid=9933159 |issue=5403|bibcode = 1999Sci...283..820C }}</ref> It was detected by the ''Galileo'' Near Infrared Mapping Spectrometer (NIMS) from its absorption feature near the wavelength 4.2 [[micrometers]]. The surface pressure is estimated to be 7.5 {{Esp|−12}} [[bar (unit)|bar]] (0.75 [[micropascal|µPa]]) and particle density 4{{Esp|8}} cm<sup>−3</sup>. Because such a thin atmosphere would be lost in only about 4 days ''(see [[atmospheric escape]])'', it must be constantly replenished, possibly by slow sublimation of carbon dioxide ice from Callisto's icy crust,<ref name="Carlson 1999"/> which would be compatible with the sublimation–degradation hypothesis for the formation of the surface knobs.
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| Callisto's ionosphere was first detected during ''Galileo'' flybys;<ref name="Kliore 2002">{{cite journal |last=Kliore|first=A. J. |coauthors=Anabtawi, A; Herrera, R. G.; ''et al.''|title=Ionosphere of Callisto from Galileo radio occultation observations |journal=Journal of Geophysics Research|year=2002|volume=107 |issue=A11|page=1407|doi=10.1029/2002JA009365| bibcode=2002JGRA.107kSIA19K}}</ref> its high electron density of 7–17{{Esp|4}} cm<sup>−3</sup> cannot be explained by the photoionization of the atmospheric [[carbon dioxide]] alone. Hence, it is suspected that the atmosphere of Callisto is actually dominated by [[molecular oxygen]] (in amounts 10–100 times greater than {{chem|CO|2}}).<ref name="Liang 2005">{{cite journal|last=Liang|first=M. C.|coauthors=Lane, B. F.; Pappalardo, R. T.; ''et al.''|title=Atmosphere of Callisto|journal=Journal of Geophysics Research|year=2005|volume=110|issue=E2|pages=E02003|doi=10.1029/2004JE002322| url=http://yly-mac.gps.caltech.edu/ReprintsYLY/N164Liang_Callisto%2005/Liang_callisto_05.pdf|format=PDF|bibcode=2005JGRE..11002003L}}</ref>
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| However, [[oxygen]] has not yet been directly detected in the atmosphere of Callisto. Observations with the [[Hubble Space Telescope]] (HST) placed an upper limit on its possible concentration in the atmosphere, based on lack of detection, which is still compatible with the ionospheric measurements.<ref name=Strobel2002>{{cite journal|last=Strobel|first=Darrell F.|coauthors=Saur, Joachim; Feldman, Paul D.; et al.|title=Hubble Space Telescope Space Telescope Imaging Spectrograph Search for an Atmosphere on Callisto: a Jovian Unipolar Inductor|year=2002|volume=581|issue=1|pages=L51–L54|doi=10.1086/345803|bibcode=2002ApJ...581L..51S | journal = The Astrophysical Journal}}</ref> At the same time HST was able to detect [[condensation|condensed]] oxygen trapped on the surface of Callisto.<ref name=Spencer2002>{{cite journal|last= Spencer|first=John R.|coauthors=Calvin, Wendy M.|title=Condensed O2 on Europa and Callisto|year=2002|volume=124|issue= 6|pages=3400–3403| doi=10.1086/344307|url=http://www.boulder.swri.edu/~spencer/o2europa.pdf|format=PDF | journal = The Astronomical Journal|bibcode=2002AJ....124.3400S}}</ref>
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| ==Origin and evolution==
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| The partial [[planetary differentiation|differentiation]] of Callisto (inferred e.g. from moment of inertia measurements) means that it has never been heated enough to melt its ice component.<ref name="Spohn 2003"/> Therefore, the most favorable model of its formation is a slow [[accretion (astrophysics)|accretion]] in the low-density Jovian [[solar nebula|subnebula]]—a disk of the gas and dust that existed around Jupiter after its formation.<ref name=Canup2002/> Such a prolonged accretion stage would allow cooling to largely keep up with the heat accumulation caused by impacts, radioactive decay and contraction, thereby preventing melting and fast differentiation.<ref name=Canup2002>{{cite journal|last=Canup|first=Robin M.|coauthors=Ward, William R.|title=Formation of the Galilean Satellites: Conditions of Accretion|year=2002|volume=124|issue=6|pages=3404–3423|doi=10.1086/344684| url=http://www.boulder.swri.edu/~robin/cw02final.pdf|format=PDF | journal = The Astronomical Journal|bibcode=2002AJ....124.3404C}}</ref> The allowable timescale of formation of Callisto lies then in the range 0.1 million–10 million years.<ref name=Canup2002/>
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| [[File:Jagged Hills PIA03455.jpg|thumb|left|200px|Views of eroding (top) and mostly eroded (bottom) ice knobs (~100 m high), possibly formed from the [[Ejecta blanket|ejecta]] of an ancient [[impact crater|impact]]]]
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| The further evolution of Callisto after [[accretion (astrophysics)|accretion]] was determined by the balance of the [[radioactive]] heating, cooling through [[thermal conduction]] near the surface, and solid state or subsolidus [[convection]] in the interior.<ref name=Freeman2006>{{cite journal|last=Freeman|first=J.|title=Non-Newtonian stagnant lid convection and the thermal evolution of Ganymede and Callisto|year=2006|volume=54|issue=1|pages=2–14|doi=10.1016/j.pss.2005.10.003| url=http://bowfell.geol.ucl.ac.uk/~lidunka/EPSS-papers/pete2.pdf|format=PDF | journal = Planetary and Space Science|bibcode=2006P&SS...54....2F}}</ref> Details of the subsolidus convection in the ice is the main source of uncertainty in the models of all [[icy moon]]s. It is known to develop when the temperature is sufficiently close to the [[melting point]], due to the temperature dependence of ice [[viscosity]].<ref name=McKinnon2006/> Subsolidus convection in icy bodies is a slow process with ice motions of the order of 1 centimeter per year, but is, in fact, a very effective cooling mechanism on long timescales.<ref name=McKinnon2006>{{cite journal|last=McKinnon|first=William B.|title=On convection in ice I shells of outer Solar System bodies, with detailed application to Callisto|year=2006|volume=183|issue=2|pages=435–450|doi=10.1016/j.icarus.2006.03.004| bibcode=2006Icar..183..435M | journal = Icarus}}</ref> It is thought to proceed in the so-called stagnant lid regime, where a stiff, cold outer layer of Callisto conducts heat without convection, whereas the ice beneath it convects in the subsolidus regime.<ref name="Spohn 2003"/><ref name=McKinnon2006/> For Callisto, the outer conductive layer corresponds to the cold and rigid [[lithosphere]] with a thickness of about 100 km. Its presence would explain the lack of any signs of the [[endogenic]] activity on the Callistoan surface.<ref name=McKinnon2006/><ref name=Nagel2004/> The convection in the interior parts of Callisto may be layered, because under the high pressures found there, [[ice|water ice]] exists in different crystalline phases beginning from the [[ice I]] on the surface to [[ice VII]] in the center.<ref name=Freeman2006/> The early onset of subsolidus convection in the Callistoan interior could have prevented large-scale ice melting and any resulting [[planetary differentiation|differentiation]] that would have otherwise formed a large rocky [[core (geology)|core]] and icy [[mantle (geology)|mantle]]. Due to the convection process, however, very slow and partial separation and differentiation of rocks and ices inside Callisto has been proceeding on timescales of billions of years and may be continuing to this day.<ref name=Nagel2004>{{cite journal|last=Nagel|first=K.a|coauthors=Breuer, D.; Spohn, T.|title=A model for the interior structure, evolution, and differentiation of Callisto|year=2004|volume=169|issue=2|pages=402–412|doi=10.1016/j.icarus.2003.12.019| bibcode=2004Icar..169..402N | journal = Icarus}}</ref>
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| The current understanding of the evolution of Callisto allows for the existence of a layer or "ocean" of liquid water in its interior. This is connected with the anomalous behavior of ice I phase's melting temperature, which decreases with [[pressure]], achieving temperatures as low as 251 K at 2,070 bar (207 [[megapascal|MPa]]).<ref name="Spohn 2003"/> In all realistic models of Callisto the temperature in the layer between 100 and 200 km in depth is very close to, or exceeds slightly, this anomalous melting temperature.<ref name=Freeman2006/><ref name=McKinnon2006/><ref name=Nagel2004/> The presence of even small amounts of [[ammonia]]—about 1–2% by weight—almost guarantees the liquid's existence because ammonia would lower the melting temperature even further.<ref name="Spohn 2003"/>
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| Although Callisto is very similar in bulk properties to [[Ganymede (moon)|Ganymede]], it apparently had a much simpler [[geological history]]. The surface appears to have been shaped mainly by impacts and other [[exogenic]] forces.<ref name="Greeley 2000"/> Unlike neighboring Ganymede with its grooved terrain, there is little evidence of [[plate tectonics|tectonic]] activity.<ref name=Showman1999/> Explanations that have been proposed for the contrasts in internal heating and consequent differentiation and geologic activity between Callisto and Ganymede include differences in formation conditions,<ref name = "Barr2">{{Cite journal
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| | last = Barr
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| | first = A. C.
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| | coauthors = Canup, R. M.
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| | title = Constraints on gas giant satellite formation from the interior states of partially differentiated satellites
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| | journal = [[Icarus (journal)|Icarus]]
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| | volume = 198
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| | issue = 1
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| | pages = 163–177
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| | date = 2008-08-03
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| | doi = 10.1016/j.icarus.2008.07.004
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| | bibcode=2008Icar..198..163B}}</ref> the greater tidal heating experienced by Ganymede,<ref name = "Showman2">{{Cite journal
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| | last = Showman
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| | first = A. P.
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| | coauthors = Malhotra, R.
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| | title = Tidal evolution into the Laplace resonance and the resurfacing of Ganymede
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| | journal = [[Icarus (journal)|Icarus]]
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| | volume = 127
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| | issue = 1
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| | pages = 93–111
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| | date = March 1997
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| | doi = 10.1006/icar.1996.5669
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| | bibcode=1997Icar..127...93S}}</ref> and the more numerous and energetic impacts that would have been suffered by Ganymede during the [[Late Heavy Bombardment]].<ref name = "Baldwin">{{cite web
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| | last = Baldwin
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| | first = E.
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| | title = Comet impacts explain Ganymede-Callisto dichotomy
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| | work = [[Astronomy Now]]
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| | date = 2010-01-25
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| | url = http://www.astronomynow.com/news/n1001/25galilean/
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| | accessdate = 2010-03-01}}</ref><ref name = "LPI1158">{{Cite conference
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| | first = A. C.
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| | last = Barr
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| | coauthors = Canup, R. M.
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| | title = Origin of the Ganymede/Callisto dichotomy by impacts during an outer solar system late heavy bombardment
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| | booktitle = 41st Lunar and Planetary Science Conference (2010)
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| | pages =
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| | publisher =
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| | date = March 2010
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| | location = Houston
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| | url = http://www.lpi.usra.edu/meetings/lpsc2010/pdf/1158.pdf
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| | doi =
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| | accessdate = 2010-03-01}}</ref><ref name = "Barr">{{Cite journal
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| | last = Barr
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| | first = A. C.
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| | coauthors = Canup, R. M.
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| | title = Origin of the Ganymede–Callisto dichotomy by impacts during the late heavy bombardment
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| | journal = [[Nature Geoscience]]
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| | volume = 3
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| | issue = March 2010
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| | pages = 164–167
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| | date = 2010-01-24
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| | doi = 10.1038/NGEO746
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| | bibcode = 2010NatGe...3..164B
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| }}</ref> The relatively simple geological history of Callisto provides planetary scientists with a reference point for comparison with other more active and complex worlds.<ref name=Showman1999/>
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| [[File:Callisto Earth Moon Comparison.png|thumb|right|250px|Size comparison of [[Earth]], [[Moon]] and Callisto]]
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| ==Possibility of life in the ocean ==
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| As with [[Europa (moon)|Europa]] and [[Ganymede (moon)|Ganymede]], the idea has been raised that [[extraterrestrial life|extraterrestrial microbial life]] may exist in a salty ocean under the Callistoan surface.<ref name=Lipps2004>{{cite journal|last=Lipps|first=Jere H.|coauthors=Delory, Gregory; Pitman, Joe; et al.|title=Astrobiology of Jupiter's Icy Moons|journal=Proc. SPIE|year=2004|volume=5555|page=10|doi=10.1117/12.560356| url=http://learning.berkeley.edu/astrobiology/2004ppt/jupiter.pdf|format=PDF}}</ref> However, the conditions for life appear to be less favorable on Callisto than on Europa. The principal reasons are the lack of contact with rocky material and the lower heat flux from the interior of Callisto.<ref name=Lipps2004/> Scientist Torrence Johnson said the following about comparing the odds of life on Callisto with the odds on other [[Galilean moons]]:<ref name=Phillips>{{cite web|last=Phillips|first=T.| url=http://science.nasa.gov/newhome/headlines/ast22oct98_2.htm|title=Callisto makes a big splash|publisher=Science@NASA|date=1998-10-23}}</ref>
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| {{cquote|The basic ingredients for life—what we call 'pre-biotic chemistry'—are abundant in many solar system objects, such as comets, asteroids and icy moons. Biologists believe liquid water and energy are then needed to actually support life, so it's exciting to find another place where we might have liquid water. But, energy is another matter, and currently, Callisto's ocean is only being heated by radioactive elements, whereas Europa has tidal energy as well, from its greater proximity to Jupiter.}}
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| Based on the considerations mentioned above and on other scientific observations, it is thought that of all of Jupiter's Galilean moons, Europa has the greatest chance of supporting [[bacteria|microbial life]].<ref name=Lipps2004/><ref name="François2005">{{cite journal|last=François|first=Raulin|title=Exo-Astrobiological Aspects of Europa and Titan: from Observations to speculations|year=2005|volume=116|issue=1–2|pages=471–487| url=http://www.springerlink.com/content/u8112784gx7j6266/fulltext.pdf| format=PDF|doi=10.1007/s11214-005-1967-x | journal = Space Science Reviews|bibcode = 2005SSRv..116..471R }}</ref>
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| ==Exploration==
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| The ''[[Pioneer 10]]'' and ''[[Pioneer 11]]'' Jupiter encounters in the early 1970s contributed little new information about Callisto in comparison with what was already known from Earth-based observations.<ref name=Moore2004>{{cite encyclopedia|last=Moore|first=Jeffrey M.|coauthors=Chapman, Clark R.; Bierhaus, Edward B. et al. |title=Callisto|encyclopedia=Jupiter: The planet, Satellites and Magnetosphere|year=2004|publisher=Cambridge University Press|editor=Bagenal, F.; Dowling, T.E.; McKinnon, W.B.| url=http://lasp.colorado.edu/~espoclass/homework/5830_2008_homework/Ch17.pdf|format=PDF}}</ref> The real breakthrough happened later with the ''[[Voyager 1]]'' and ''[[Voyager 2|2]]'' flybys in 1979–1980. They imaged more than half of the Callistoan surface with a resolution of 1–2 km, and precisely measured its temperature, mass and shape.<ref name=Moore2004/> A second round of exploration lasted from 1994 to 2003, when the ''[[Galileo (spacecraft)|Galileo]]'' spacecraft had eight close encounters with Callisto, the last flyby during the C30 orbit in 2001 came as close as 138 km to the surface. The ''Galileo'' orbiter completed the global imaging of the surface and delivered a number of pictures with a resolution as high as 15 meters of selected areas of Callisto.<ref name="Greeley 2000"/> In 2000, the ''[[Cassini–Huygens|Cassini]]'' spacecraft en route to [[Saturn]] acquired high-quality infrared spectra of the Galilean satellites including Callisto.<ref name=Brown2003>{{cite journal |last=Brown |first=R. H.|coauthors=Baines, K. H.; Bellucci, G.; ''et al.''|title=Observations with the Visual and Infrared Mapping Spectrometer (VIMS) during Cassini's Flyby of Jupiter |year=2003 |journal=Icarus |volume=164 |issue=2 |pages=461–470 |doi=10.1016/S0019-1035(03)00134-9 |bibcode=2003Icar..164..461B}}</ref> In February–March 2007, the ''[[New Horizons]]'' probe on its way to Pluto obtained new images and spectra of Callisto.<ref name=Morring2007>{{cite journal|last=Morring |first=F.|title=Ring Leader |journal=Aviation Week & Space Technology|date=2007-05-07|pages=80–83}}</ref>
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| The next planned mission to the Jovian system is the [[European Space Agency]]'s [[Jupiter Icy Moon Explorer]] (JUICE), due to launch in 2022.<ref name='selection'>{{cite news|first = Jonathan Amos|url = http://www.bbc.co.uk/news/science-environment-17917102 |title = Esa selects 1bn-euro Juice probe to Jupiter |accessdate = 2012-05-02|date = 2 May 2012|work = [[BBC News Online]]}}</ref> Several close flybys of Callisto are planned during the mission.<ref name='selection'/>
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| ===Old proposals===
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| Formerly proposed for a launch in 2020, the [[Europa Jupiter System Mission]] (EJSM) was a joint [[NASA]]/[[ESA]] proposal for exploration of [[Jupiter]]'s moons. In February 2009 it was announced that ESA/NASA had given this mission priority ahead of the [[Titan Saturn System Mission]].<ref>{{cite news|url=http://news.bbc.co.uk/1/hi/sci/tech/7897585.stm|title=Jupiter in space agencies' sights|first=Paul|last=Rincon|publisher=BBC News|accessdate=2009-02-20|date=2009-02-20}}</ref> ESA's contribution still faced funding competition from other ESA projects.<ref>{{cite web|url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=41177|title=Cosmic Vision 2015–2025 Proposals|date=2007-07-21|publisher=ESA|accessdate=2009-02-20}}</ref> EJSM consisted of the NASA-led [[Jupiter Europa Orbiter]], the ESA-led [[Jupiter Ganymede Orbiter]], and possibly a [[JAXA]]-led [[Jupiter Magnetospheric Orbiter]].
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| ==Potential colonization==
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| [[File:Callisto base.PNG|thumb|right|180px|Artist's impression of a base on Callisto<ref name="CallistoBase"/>]]
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| In 2003 [[NASA]] conducted a conceptual study called [[Human Outer Planets Exploration]] (HOPE) regarding the future human exploration of the [[outer Solar System]]. The target chosen to consider in detail was Callisto.<ref name=HOPE>{{cite web|title=Revolutionary Concepts for Human Outer Planet Exploration (HOPE)|last=Trautman|first=Pat|coauthors=Bethke, Kristen|publisher=NASA|year=2003|url=http://www.nasa-academy.org/soffen/travelgrant/bethke.pdf|format=PDF}}</ref><ref>{{cite journal|last=Troutman|first=Patrick A.|coauthors=Bethke, Kristen; Stillwagen, Fred; Caldwell, Darrell L. Jr.; Manvi, Ram; Strickland, Chris; Krizan, Shawn A.|title=Revolutionary Concepts for Human Outer Planet Exploration (HOPE)|journal=American Institute of Physics Conference Proceedings|date=28 January 2003|volume=654|pages=821–828|doi=10.1063/1.1541373}}</ref>
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| The study proposed a possible surface base on Callisto that would produce fuel for further exploration of the Solar System.<ref name="CallistoBase">{{cite web|title=Vision for Space Exploration|url=http://www.nasa.gov/pdf/55583main_vision_space_exploration2.pdf|publisher=[[NASA]]|year=2004|format=PDF}}</ref> Advantages of a base on this moon include low radiation (due to Callisto's distance from Jupiter) and geological stability. Such a base could facilitate remote exploration of [[Europa (moon)|Europa]], or be an ideal location for a Jovian system waystation servicing spacecraft heading farther into the outer Solar System, using a [[gravity assist]] from a close flyby of Jupiter after departing Callisto.<ref name=HOPE/>
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| A December 2003 NASA report expressed the belief that a manned mission to Callisto may be possible in the 2040s.<ref>{{cite web|title=High Power MPD Nuclear Electric Propulsion (NEP) for Artificial Gravity HOPE Missions to Callisto|url=http://trajectory.grc.nasa.gov/aboutus/papers/STAIF-2003-177.pdf|publisher=[[NASA]]|year=2003|format=PDF}}</ref>
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| == See also ==
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| {{Portal|Solar System}}
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| * [[Jupiter's moons in fiction]]
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| * [[List of craters on Callisto]]
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| * [[List of geological features on Callisto]]
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| * [[Moons of Jupiter]]
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| * [[Valhalla (crater)]]
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| ==Notes==
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| {{Reflist|group=lower-alpha}}
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| ==References==
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| {{Reflist|colwidth=30em}}
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| ==External links==
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| {{Commons|Callisto}}
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| * [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jup_Callisto Callisto Profile] at [http://solarsystem.nasa.gov NASA's Solar System Exploration site]
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| * [http://www.nineplanets.org/callisto.html Callisto page] at ''The Nine Planets''
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| * [http://www.solarviews.com/eng/callisto.htm Callisto page] at ''Views of the Solar System''
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| * [http://www.lpi.usra.edu/resources/cc/cchome.html Callisto Crater Database] from the Lunar and Planetary Institute
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| * [http://photojournal.jpl.nasa.gov/target/Callisto Images of Callisto at JPL's Planetary Photojournal]
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| * Movie of [http://sos.noaa.gov/videos/Callisto.mov Callisto's rotation] from the National Oceanic and Atmospheric Administration
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| * [http://photojournal.jpl.nasa.gov/catalog/PIA03876 Callisto map with feature names] from [http://photojournal.jpl.nasa.gov/ Planetary Photojournal]
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| *[http://planetarynames.wr.usgs.gov/Page/CALLISTO/target Callisto nomenclature] and [http://planetarynames.wr.usgs.gov/images/callisto_comp.pdf Callisto map with feature names] from the [http://planetarynames.wr.usgs.gov USGS planetary nomenclature page]
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| * [http://stereomoons.blogspot.com/2009/10/galileo-4-moons-at-400-years.html Paul Schenk's 3D images and flyover videos of Callisto and other outer solar system satellites]
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| *[http://www.nasa-academy.org/soffen/travelgrant/bethke.pdf '''Human Outer Planet Exploration''' (2003) - NASA] (with Callisto probes and manned mission concepts)
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