View source for Astronomy:Callisto (moon)

{{Short description|Second-largest moon of Jupiter}}
{{Infobox planet
| name = Callisto
| alt_names = Jupiter {{rn|IV}}
| pronounced = {{IPAc-en|k|ə|ˈ|l|ɪ|s|t|oʊ}} ({{respell|kə|LIST|oh}})<ref>{{Cite dictionary |url=http://www.lexico.com/definition/Callisto |archive-url=https://web.archive.org/web/20200322182103/https://www.lexico.com/definition/callisto |url-status=dead |archive-date=22 March 2020 |title=Callisto |dictionary=Lexico UK English Dictionary |publisher=[[Organization:Oxford University Press|Oxford University Press]]}}</ref>
| adjectives = Callistoan {{IPAc-en|ˌ|k|æ|l|ɪ|ˈ|s|t|oʊ|.|ə|n}} ({{respell|KAL|iss|TOH|ən}})<br>''etc.'' (see text)
| named_after = [[Astronomy:Callisto (mythology)|Καλλιστώ]] ''Kallistō''
| image = Callisto - July 8 1979 (38926064465).jpg
| caption = Callisto as imaged in true color by the ''[[Astronomy:Voyager 2|Voyager 2]]'' spacecraft on 8 July 1979. The image is composed from orange, green and blue color channels.
| background = Silver
| discoverer = [[Biography:Galileo Galilei|Galileo Galilei]]<br/>Simon Marius
| discovered = 7 January 1610<ref name=Galilei/>
| semimajor = 1,882,700&nbsp;km<ref name=orbit/>
| eccentricity = {{val|0.0074}}<ref name=orbit/>
| periapsis = {{val|1869000|fmt=commas|u=km}}<ref group=lower-alpha>Periapsis is derived from the semimajor axis (''a'') and eccentricity (''e''): <math>a(1-e)</math>.</ref>
| apoapsis = {{val|1897000|fmt=commas|u=km}}<ref group=lower-alpha>Apoapsis is derived from the semimajor axis (''a'') and eccentricity (''e''): <math>a(1+e)</math>.</ref>
| period = {{val|16.6890184|fmt=none|ul=d}}<ref name=orbit/>
| avg_speed = {{val|8.204|u=km/s}}
| inclination = 2.017° (to the [[Astronomy:Ecliptic|ecliptic]])<br>0.192° (to local [[Astronomy:Laplace plane|Laplace plane]]s)<ref name=orbit/>
| satellite_of = [[Astronomy:Jupiter|Jupiter]]
| group = Galilean moon
| mean_radius = {{val|2410.3|1.5|fmt=commas|u=km}} (0.378 Earths)<ref name="Anderson 2001"/>
| 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>
| 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>
| mass = {{val|1.075938|0.000137|e=23|fmt=commas|u=kg}} (0.018 Earths)<ref name="Anderson 2001"/>
| density = {{val|1.8344|0.0034|u=g/cm3}} (0.333 Earths)<ref name="Anderson 2001"/>
| surface_grav = {{val|1.235|ul=m/s2}} (0.126 ''[[Physics:G-force|g]]'')<ref group=lower-alpha>Surface gravity derived from the mass (''m''), the [[Physics:Gravitational constant|gravitational constant]] (''G'') and the radius (''r''): <math>\frac{Gm}{r^2}</math>.</ref>
| moment_of_inertia_factor = {{val|0.3549|0.0042}}<ref name="Schubert2004">{{cite book|last1= Schubert|first1= G.|last2= Anderson|first2= J. D.|last3= Spohn|first3= T.|last4= McKinnon|first4= W. B.|editor1-last= Bagenal|editor1-first= F.|editor2-last= Dowling|editor2-first= T. E.|editor3-last= McKinnon|editor3-first= W. B.|title= Jupiter : the planet, satellites, and magnetosphere|chapter= Interior composition, structure and dynamics of the Galilean satellites|date= 2004|publisher= Cambridge University Press|location= New York|url= https://books.google.com/books?id=aMERHqj9ivcC|chapter-url= https://books.google.com/books?id=aMERHqj9ivcC&pg=PA281|isbn= 978-0521035453|pages= 281–306|oclc= 54081598|access-date= 23 July 2019|archive-date= 16 April 2023|archive-url= https://web.archive.org/web/20230416151559/https://books.google.com/books?id=aMERHqj9ivcC|url-status= live}}</ref>
| escape_velocity = 2.441&nbsp;km/s<ref group=lower-alpha>Escape velocity derived from the mass (''m''), the [[Physics:Gravitational constant|gravitational constant]] (''G'') and the radius (''r''): <math>\textstyle\sqrt{\frac{2Gm}{r}}</math>.</ref>
| rotation = synchronous<ref name="Anderson 2001"/>
| axial_tilt = zero<ref name="Anderson 2001"/>
| albedo = 0.22 (geometric)<ref name=Moore2004/>
| magnitude = 5.65 ([[Astronomy:Opposition|opposition]])<ref name=magnitude>{{cite web|title=Classic Satellites of the Solar System|url=http://www.oarval.org/ClasSaten.htm|publisher=Observatorio ARVAL|access-date=13 July 2007|archive-url=https://web.archive.org/web/20110709203915/http://www.oarval.org/ClasSaten.htm|archive-date=9 July 2011|url-status=dead}}</ref>
| temp_name1 = K<ref name=Moore2004/>
| max_temp_1 = {{val|165|5}}
| mean_temp_1 = {{val|134|11}}
| min_temp_1 = {{val|80|5}}
| atmosphere = yes
| surface_pressure = {{cvt|0.75|μPa|atm|sigfig=3}}<ref name="Carlson 1999"/>
| atmosphere_composition = ≈ {{val|4|e=8|u=molecules/cm<sup>3</sup>}} [[Chemistry:Carbon dioxide|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"/>
}}

'''Callisto''' ({{IPAc-en|k|ə|ˈ|l|ɪ|s|t|oʊ}}, {{respell|kə|LIST|oh}}), or '''Jupiter IV''', is the second-largest moon of Jupiter, after [[Astronomy:Ganymede (moon)|Ganymede]]. In the [[Astronomy:Solar System|Solar System]] it is the third-largest moon after Ganymede and [[Astronomy:Saturn|Saturn]]'s largest moon [[Astronomy:Titan (moon)|Titan]], and as large as the smallest planet [[Astronomy:Mercury (planet)|Mercury]], though only about a third as massive. Callisto is, with a diameter of {{val|4821|fmt=commas|u=km}}, roughly a third larger than Earth's [[Astronomy:Moon|Moon]] and orbits Jupiter on average at a distance of {{val|1883000|fmt=commas|u=km}}, which is about six times further out than the Moon orbiting Earth. It is the outermost of the four large [[Astronomy:Galilean moons|Galilean moons]] of Jupiter,<ref name=orbit/> which were discovered in 1610 with one of the first [[Astronomy:Telescope|telescope]]s, being visible from Earth with common binoculars.

The surface of Callisto is the oldest and most heavily [[Astronomy:Impact crater|cratered]] object in the Solar System.<ref>{{cite web|url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jup_Callisto|title=Callisto – Overview – Planets – NASA Solar System Exploration|website=NASA Solar System Exploration|url-status=dead|archive-url=https://web.archive.org/web/20140328070532/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jup_Callisto|archive-date=28 March 2014}}</ref> Its surface is completely covered with impact craters.<ref>{{Cite book|title=Guinness Book of World Records 2014|last=Glenday|first=Craig|publisher=Guinness World Records Limited|year=2013|isbn=978-1-908843-15-9|page=[https://archive.org/details/guinnessworldrec0000unse_r3e7/page/187 187]|url=https://archive.org/details/guinnessworldrec0000unse_r3e7/page/187}}</ref> It does not show any signatures of subsurface processes such as [[Earth:Plate tectonics|plate tectonics]] or [[Earth:Volcano|volcanism]], with no signs that geological activity in general has ever occurred, and is thought to have evolved predominantly under the influence of [[Astronomy:Impact crater|impacts]].<ref name="Greeley 2000"/> Prominent surface features include multi-ring structures, variously shaped [[Astronomy:Impact crater|impact crater]]s, and chains of craters (''catenae'') and associated [[Earth:Escarpment|scarps]], ridges and deposits.<ref name="Greeley 2000"/> At a small scale, the surface is varied and made up of small, sparkly frost 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-driven degradation of small [[Earth:Landform|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.
Callisto is composed of approximately equal amounts of [[Earth:Rock (geology)|rock]] and [[Earth:Volatile (astrogeology)|ice]], with a [[Physics:Density|density]] of about {{val|1.83|u=g/cm3}}, the lowest density and surface gravity of Jupiter's major moons. Compounds detected [[Physics:Spectroscopy|spectroscopically]] on the surface include [[Earth:Ice|water ice]],<ref name="NYT-20150315">{{cite news |last=Chang |first=Kenneth |title=Suddenly, It Seems, Water Is Everywhere in Solar System |url=https://www.nytimes.com/2015/03/13/science/space/suddenly-it-seems-water-is-everywhere-in-solar-system.html |date=12 March 2015 |work={{wipe|The New York Times}} |access-date=12 March 2015 |archive-date=9 May 2020 |archive-url=https://web.archive.org/web/20200509080640/https://www.nytimes.com/2015/03/13/science/space/suddenly-it-seems-water-is-everywhere-in-solar-system.html |url-status=live }}</ref> [[Chemistry:Carbon dioxide|carbon dioxide]], [[Chemistry:Silicate|silicate]]s and [[Chemistry:Organic compound|organic compound]]s. Investigation by the ''[[Engineering:Galileo (spacecraft)|Galileo]]'' spacecraft revealed that Callisto may have a small [[Chemistry:Silicate|silicate]] [[Astronomy:Planetary core|core]] and possibly a subsurface ocean of liquid [[Chemistry:Water|water]]<ref name="NYT-20150315"/> at depths greater than {{val|100|u=km}}.<ref name=Kuskov2005/><ref name="Showman1999">{{cite journal|last1= Showman|first1=A. P.|last2= Malhotra|first2= R.|s2cid=9492520|title=The Galilean Satellites|journal= Science|volume= 286|issue= 5437|date= 1 October 1999|pages =77–84|doi= 10.1126/science.286.5437.77|pmid=10506564}}</ref>

It is not in an orbital resonance like the three other Galilean satellites—[[Astronomy:Io (moon)|Io]], [[Astronomy:Europa (moon)|Europa]] and [[Astronomy:Ganymede (moon)|Ganymede]]—and is thus not appreciably [[Astronomy:Tidal heating|tidally heated]].<ref name=Musotto2002/> Callisto's rotation is tidally locked to its orbit around Jupiter, so that it always faces the same direction, making Jupiter appear to hang directly overhead over its near-side. It is less affected by Jupiter's [[Astronomy:Magnetosphere|magnetosphere]] than the other inner satellites because of its more remote orbit, located just outside Jupiter's main radiation belt.<ref name=Cooper2001/><ref>{{cite web|url=http://www.spacetoday.org/SolSys/Jupiter/CallistoInfo.html|title=Exploring Jupiter – JIMO – Jupiter Icy Moons Orbiter – the moon Callisto|publisher=Space Today Online|access-date=11 October 2014|archive-date=26 June 2018|archive-url=https://web.archive.org/web/20180626102045/http://www.spacetoday.org/SolSys/Jupiter/CallistoInfo.html|url-status=live}}</ref> Callisto is surrounded by an extremely thin [[Astronomy:Atmosphere|atmosphere]] composed of [[Chemistry:Carbon dioxide|carbon dioxide]]<ref name="Carlson 1999"/> and probably molecular oxygen,<ref name="Liang 2005"/> as well as by a rather intense [[Astronomy:Ionosphere|ionosphere]].<ref name="Kliore 2002"/> Callisto is thought to have formed by slow [[Physics: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 heating meant that not enough heat was available for rapid [[Astronomy:Planetary differentiation|differentiation]]. The slow [[Astronomy:Convection|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&nbsp;km and a small, rocky [[Astronomy:Planetary core|core]].<ref name="Spohn 2003"/>

The likely presence of an ocean within Callisto leaves open the possibility that it could harbor [[Astronomy:Extraterrestrial life|life]]. However, conditions are thought to be less favorable than on nearby [[Astronomy:Europa (moon)|Europa]].<ref name=Lipps2004/> Various space probes from ''[[Astronomy:Pioneer 10|Pioneers 10]]'' and ''[[Astronomy:Pioneer 11|11]]'' to ''[[Engineering:Galileo (spacecraft)|Galileo]]'' and ''[[Astronomy:Cassini–Huygens|Cassini]]'' have studied Callisto. Because of its low [[Physics:Radiation|radiation]] levels, Callisto has long been considered the most suitable to base possible future crewed missions on to study the Jovian system.<ref name=HOPE/>

==History==

===Discovery===
Callisto was discovered independently by Simon Marius and [[Biography:Galileo Galilei|Galileo Galilei]] in 1610, along with the three other large Jovian moons—[[Astronomy:Ganymede (moon)|Ganymede]], [[Astronomy:Io (moon)|Io]] and [[Astronomy:Europa (moon)|Europa]].<ref name=Galilei>{{cite book |last=Galilei |first=G. |title=Sidereus Nuncius |date=13 March 1610|title-link=Sidereus Nuncius }}</ref>

===Name===
Callisto, like all of Jupiter's moons, is named after one of [[Religion:Zeus|Zeus]]'s many lovers or other sexual partners in [[Religion:Greek mythology|Greek mythology]]. [[Astronomy:Callisto (mythology)|Callisto]] was a nymph (or, according to some sources, the daughter of Lycaon) who was associated with the goddess of the hunt, [[Religion:Artemis|Artemis]].<ref name=Galileo/> The name was suggested by Simon Marius soon after Callisto's discovery.<ref name="Marius">{{cite book |last=Marius |first=S. |url=http://galileo.rice.edu/sci/marius.html |title=Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici |year=1614 |access-date=15 April 2007 |archive-url=https://web.archive.org/web/20190929023619/http://galileo.rice.edu/sci/marius.html |archive-date=29 September 2019 |url-status=live}}</ref> Marius attributed the suggestion to [[Biography:Johannes Kepler|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|access-date=31 July 2007|archive-date=11 February 2012|archive-url=https://web.archive.org/web/20120211140650/http://galileo.rice.edu/sci/observations/jupiter_satellites.html|url-status=live}}</ref> 

{{quote|Jupiter is much blamed by the poets on account of his irregular loves. Three maidens are especially mentioned as having been clandestinely courted by Jupiter with success. Io, daughter of the River Inachus, Callisto of Lycaon, Europa of Agenor. Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell... I think, therefore, that I shall not have done amiss if the First is called by me Io, the Second Europa, the Third, on account of its majesty of light, Ganymede, the Fourth Callisto...<ref name=Helden>{{cite journal |last1=Van Helden |first1=Albert |title=Naming the Satellites of Jupiter and Saturn |journal=The Newsletter of the Historical Astronomy Division of the American Astronomical Society |date=August 1994 |issue=32 |url=https://had.aas.org/sites/had.aas.org/files/HADN32.pdf |access-date=10 March 2023 |archive-date=7 December 2022 |archive-url=https://web.archive.org/web/20221207151902/https://had.aas.org/sites/had.aas.org/files/HADN32.pdf |url-status=live }}</ref><ref>{{cite book |last1=Marius |first1=Simon |title=Mundus Iovialis: anno MDCIX detectus ope perspicilli Belgici, hoc est, quatuor Jovialium planetarum, cum theoria, tum tabulæ |date=1614 |publisher=Sumptibus & Typis Iohannis Lauri |location=Nuremberg |page=B2, recto and verso (images 35 and 36), with erratum on last page (image 78) |url=https://repository.ou.edu/uuid/748b6fe7-62da-5877-ae84-885372b3030c |access-date=30 June 2020 |archive-date=2 July 2020 |archive-url=https://web.archive.org/web/20200702154450/https://repository.ou.edu/uuid/748b6fe7-62da-5877-ae84-885372b3030c |url-status=live }}</ref>}}

However, the names of the [[Astronomy: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.|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>  

There's no established English adjectival form of the name. The adjectival form of Greek Καλλιστῴ ''Kallistōi'' is Καλλιστῴος ''Kallistōi-os'', from which one might expect Latin ''Callistōius'' and English *Callistóian (with 5 syllables), parallel to Sapphóian (4 syllables) for ''[[Biology:Sappho|Sapphō<sub>i</sub>]]''<ref>''The Thistle'', January 1903, vol. I, no. 2, p. 4</ref> and Letóian for ''[[Biology:Leto|Lētō<sub>i</sub>]]''.<ref>E. Alan Roberts (2013) ''The Courage of Innocence: (The Virgin of Phileros)'', p. 191</ref> However, the iota subscript is often omitted from such Greek names (cf. ''Inóan''<ref>George Stuart (1882) ''The Eclogues, Georgics, and Moretum of Virgil'', p. 271</ref> from ''Īnō<sub>i</sub>''<ref>{{L&S|Ino|ref}}</ref> and ''Argóan''<ref>Noah Webster (1832) ''A Dictionary of the English Language''</ref> from ''Argō<sub>i</sub>''<ref>{{L&S|Argo|ref}}</ref>), and indeed the analogous form '''Callistoan''' is found.<ref name=Klemaszewski2001>{{cite web|last= Klemaszewski|first= J.A.|last2=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 |archive-url=https://ghostarchive.org/archive/20221009/http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1818.pdf |archive-date=2022-10-09 |url-status=live}}</ref><ref>Steven Croft (1985) "Ripple Ring Basins on Ganymede and Callisto", [ibid] p. 206</ref><ref>David M. Harland (2000) ''Jupiter Odyssey: The Story of NASA's Galileo Mission'', p. 165</ref>
In Virgil, a second oblique stem appears in Latin: ''Callistōn-,''<ref>Genitive ''Callistūs'' or ''Callistōnis''. {{L&S|Callisto|ref}}</ref> but the corresponding '''Callistonian''' has rarely appeared in English.<ref>''Monthly notices of the Royal Astronomical Society,'' v.71, 1911</ref> One also sees ''ad hoc'' forms, such as '''Callistan''',<ref name=Moore1999/> '''Callistian'''<ref>P. Leonardi (1982), Geological results of twenty years of space enterprises: Satellites of Jupiter and Saturn, in ''Geologica romana,'' p. 468.</ref> and '''Callistean'''.<ref>Pierre Thomas & Philippe Mason (1985) "Tectonics of the Vahalla Structure on Callisto", ''Reports of Planetary Geology and Geophysics Program – 1984'', NASA Technical Memorandum 87563, p. 535</ref><ref>Jean-Pierre Burg & Mary Ford (1997) ''Orogeny Through Time'', p. 55</ref>

==Orbit and rotation==
[[File:Galilean moons around Jupiter.gif|thumb|right|Galilean moons around Jupiter {{legend2|Lime|Jupiter}}{{·}}{{legend2|OrangeRed|Io}}{{·}}{{legend2|RoyalBlue|Europa}}{{·}}{{legend2|Gold|Ganymede}}{{·}}{{legend2|Cyan|Callisto}}]]
[[File:001221 Cassini Jupiter & Europa & Callisto.jpg|thumb|upright|Callisto (bottom left), Jupiter (top right) and Europa (below and left of Jupiter's [[Astronomy:Great Red Spot|Great Red Spot]]) as viewed by ''Cassini–Huygens'']]
Callisto is the outermost of the four Galilean moons of Jupiter. It orbits at a distance of approximately 1,880,000&nbsp;km (26.3 times the 71,492&nbsp;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| access-date=6 July 2007| archive-date=3 November 2013| archive-url=https://web.archive.org/web/20131103134221/http://ssd.jpl.nasa.gov/?sat_elem| url-status=live}}</ref> This is significantly larger than the orbital radius—1,070,000&nbsp;km—of the next-closest Galilean satellite, Ganymede. As a result of this relatively distant orbit, Callisto does not participate in 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|last2=Varadi, Ferenc |last3=Moore, William |last4= 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> Callisto is expected to be captured into the resonance in about 1.5 billion years, completing the 1:2:4:8 chain.<ref>{{cite journal |last1=Lari |first1=Giacomo |last2=Saillenfest |first2=Melaine |first3=Marco |last3=Fenucci |date=2020 |title=Long-term evolution of the Galilean satellites: the capture of Callisto into resonance |url=https://www.aanda.org/articles/aa/full_html/2020/07/aa37445-20/aa37445-20.html |journal=Astronomy & Astrophysics |volume=639 |pages=A40 |doi=10.1051/0004-6361/202037445 |arxiv=2001.01106 |bibcode=2020A&A...639A..40L |s2cid=209862163 |access-date=1 August 2022 |archive-date=11 June 2022 |archive-url=https://web.archive.org/web/20220611193930/https://www.aanda.org/articles/aa/full_html/2020/07/aa37445-20/aa37445-20.html |url-status=live }}</ref>

Like most other regular planetary moons, Callisto's rotation is locked to be synchronous with its orbit.<ref name="Anderson 2001">{{cite journal|last=Anderson|first=J. D. |last2=Jacobson, R. A. |last3=McElrath, T. P. |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| last4=Moore|first4=W. B.|last5=Schubert |first5=G. |last6=Thomas |first6=P. C.|s2cid=120591546 }}</ref> The length of Callisto's day, simultaneously its [[Astronomy:Orbital period|orbital period]], is about 16.7 Earth days. Its orbit is very slightly eccentric and inclined to the Jovian [[Astronomy:Equator|equator]], with the [[Astronomy: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 [[Astronomy:Axial tilt|axial tilt]] (the angle between the 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|url=https://zenodo.org/record/1259023|access-date=26 August 2018|archive-date=27 July 2020|archive-url=https://web.archive.org/web/20200727063125/https://zenodo.org/record/1259023|url-status=live}}</ref>

The dynamical isolation of Callisto means that it has never been appreciably [[Astronomy:Tidal heating|tidally heated]], which has important consequences for its internal structure and [[Biology:Evolution|evolution]].<ref name=Freeman2006/> Its distance from Jupiter also means that the [[Physics:Charged particle|charged-particle]] [[Physics:Flux|flux]] from Jupiter's [[Astronomy:Magnetosphere|magnetosphere]] at its surface is relatively low—about 300 times lower than, for example, that at [[Astronomy:Europa (moon)|Europa]]. Hence, unlike the other Galilean moons, charged-particle [[Biology:Irradiation|irradiation]] has had a relatively minor effect on Callisto's surface.<ref name=Cooper2001>{{cite journal|last=Cooper|first=John F.|last2=Johnson, Robert E.|last3=Mauk, Barry H.|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|journal=Icarus|bibcode=2001Icar..149..133C|last4=Garrett|first4=Garry H.|last5=Gehrels|first5=Neil|access-date=25 October 2011|archive-url=https://web.archive.org/web/20120116225011/http://people.virginia.edu/~rej/Icarus_Jan2001_Cooper_et_al.pdf|archive-date=16 January 2012|url-status=dead}}</ref> The radiation level at Callisto's surface is equivalent to a dose of about 0.01 rem (0.1 mSv) per day, which is just over ten times higher than Earth's average background radiation,<ref>{{Cite book|url=http://www.unscear.org/unscear/en/publications/2008_1.html|title=United Nations Scientific Committee on the Effects of Atomic Radiation|publisher=United Nations|year=2008|isbn=978-92-1-142274-0|location=New York|pages=4|access-date=5 January 2017|archive-date=16 July 2019|archive-url=https://web.archive.org/web/20190716210835/http://www.unscear.org/unscear/en/publications/2008_1.html|url-status=live}}</ref><ref name="ringwald">{{cite web |last=Ringwald |first=Frederick A. |date=29 February 2000 |title=SPS 1020 (Introduction to Space Sciences) |url=http://zimmer.csufresno.edu/~fringwal/w08a.jup.txt |url-status=dead |archive-url=https://web.archive.org/web/20080725050708/http://zimmer.csufresno.edu/~fringwal/w08a.jup.txt |archive-date=25 July 2008 |access-date=4 July 2009 |publisher=California State University, Fresno}}</ref> but less than in [[Engineering:Low Earth Orbit|Low Earth Orbit]] or on [[Astronomy:Mars|Mars]].

==Physical characteristics==

===Composition===
[[File:Callisto, Earth & Moon size comparison.jpg|thumb|left|Size comparison of [[Astronomy:Earth|Earth]], Moon and Callisto]]
[[File:PIA00844 NIMS spectra.gif|thumb|right|Near-IR spectra of dark cratered plains (red) and the Asgard impact structure (blue), showing the presence of more water ice (absorption bands from 1 to 2 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 = 10 April 1981 | url = http://www.agu.org/pubs/crossref/1981/JB086iB04p03087.shtml | doi = 10.1029/JB086iB04p03087 | access-date = 3 March 2010 | bibcode = 1981JGR....86.3087C | archive-date = 6 June 2011 | archive-url = https://web.archive.org/web/20110606002239/http://www.agu.org/pubs/crossref/1981/JB086iB04p03087.shtml | url-status = dead }}</ref> and less rocky material within Asgard.]]
The average [[Physics:Density|density]] of Callisto, 1.83&nbsp;g/cm<sup>3</sup>,<ref name="Anderson 2001"/> suggests a composition of approximately equal parts of rocky material and [[Earth:Ice|water ice]], with some additional volatile ices such as [[Chemistry:Ammonia|ammonia]].<ref name=Kuskov2005>{{cite journal| last=Kuskov|first=O.L.|last2=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 49–55%.<ref name=Kuskov2005/><ref name="Spohn 2003"/> The exact composition of Callisto's [[Earth:Rock (geology)|rock]] component is not known, but is probably close to the composition of L/LL type ordinary chondrites,<ref name=Kuskov2005/> which are characterized by less total [[Chemistry:Iron|iron]], less metallic iron and more [[Chemistry:Iron oxide|iron oxide]] than H chondrites. The weight ratio of iron to [[Chemistry:Silicon|silicon]] is 0.9–1.3 in Callisto, whereas the [[Astronomy:Sun|solar ratio]] is around 1:8.<ref name=Kuskov2005/>

Callisto's surface has an [[Earth:Albedo|albedo]] of about 20%.<ref name=Moore2004/> Its surface composition is thought to be broadly similar to its composition as a whole. Near-infrared [[Physics:Spectroscopy|spectroscopy]] has revealed the presence of water ice [[Physics:Absorption band|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/> The analysis of high-resolution, near-infrared and [[Physics:Ultraviolet|UV]] [[Spectrum|spectra]] obtained by the ''[[Engineering:Galileo (spacecraft)|Galileo]]'' spacecraft and from the ground has revealed various non-ice materials: [[Chemistry:Magnesium|magnesium]]- and [[Chemistry:Iron|iron]]-bearing hydrated silicates,<ref name=Moore2004/> [[Chemistry:Carbon dioxide|carbon dioxide]],<ref name=Brown2003/> [[Chemistry:Sulfur dioxide|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|access-date=25 July 2007|archive-url=https://web.archive.org/web/20160604011832/http://www.lpi.usra.edu/meetings/lpsc97/pdf/1852.PDF|archive-date=4 June 2016|url-status=dead}}</ref> and possibly [[Chemistry:Ammonia|ammonia]] and various organic compounds.<ref name=Showman1999/><ref name=Moore2004/> 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"/>

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 [[Chemistry:Carbon dioxide|carbon dioxide]], whereas the leading hemisphere has more [[Chemistry:Sulfur dioxide|sulfur dioxide]].<ref name=Hibbitts1998>{{cite web|last=Hibbitts|first=C.A.|last2=McCord, T. B.|last3=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|access-date=10 July 2007|archive-url=https://web.archive.org/web/20160604011832/http://www.lpi.usra.edu/meetings/lpsc2000/pdf/1908.pdf|archive-date=4 June 2016|url-status=dead}}</ref> Many fresh [[Astronomy:Impact crater|impact crater]]s like 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 [[Astronomy:D-type asteroid|D-type asteroid]]s,<ref name="Greeley 2000"/> whose surfaces are made of [[Chemistry:Carbon|carbon]]aceous material.

===Internal structure===
[[File:Callisto diagram.svg|thumb|left|Model of Callisto's internal structure showing a surface ice layer, a possible liquid water layer, and an ice–rock interior]]
Callisto's battered surface lies on top of a cold, stiff and icy [[Earth:Lithosphere|lithosphere]] that is between 80 and 150&nbsp;km thick.<ref name=Kuskov2005/><ref name="Spohn 2003"/> A salty ocean 150–200&nbsp;km deep may lie beneath the [[Earth:Crust (geology)|crust]],<ref name=Kuskov2005/><ref name="Spohn 2003"/> indicated by studies of the [[Physics:Magnetic field|magnetic field]]s around Jupiter and its moons.<ref name="Khurana 2000">{{cite journal |last1=Khurana|first1=K. K.|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 |archive-url=https://ghostarchive.org/archive/20221009/http://www.igpp.ucla.edu/people/mkivelson/Publications/N395777.pdf |archive-date=2022-10-09 |url-status=live|pmid=9796812 | issue=6704| bibcode = 1998Natur.395..777K |last2=Kivelson|first2=M. G.| last3=Stevenson| first3=D. J.| last4=Schubert|first4=G.|last5=Russell|first5=C. T.|last6=Walker|first6=R. J.| last7=Polanskey| first7=C.|s2cid=4424606}}</ref><ref name="Zimmer 2000">{{cite journal| last=Zimmer| first=C.|last2=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 |archive-url=https://ghostarchive.org/archive/20221009/http://www.igpp.ucla.edu/people/mkivelson/Publications/ICRUS147329.pdf |archive-date=2022-10-09 |url-status=live| bibcode=2000Icar..147..329Z| last3=Kivelson| first3=Margaret G.| citeseerx=10.1.1.366.7700}}</ref> It was found that Callisto responds to Jupiter's varying background magnetic field like a perfectly [[Physics: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&nbsp;km.<ref name="Zimmer 2000"/> The existence of an ocean is more likely if water contains a small amount of [[Chemistry:Ammonia|ammonia]] or other [[Chemistry:Antifreeze|antifreeze]], up to 5% by weight.<ref name="Spohn 2003">{{cite journal |last=Spohn|first=T.|last2=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://www.igpp.ucla.edu/public/mkivelso/refs/PUBLICATIONS/SpohnSchubrt03GLLsats.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.igpp.ucla.edu/public/mkivelso/refs/PUBLICATIONS/SpohnSchubrt03GLLsats.pdf |archive-date=2022-10-09 |url-status=live|bibcode=2003Icar..161..456S}}</ref> In this case the water+ice layer can be as thick as 250–300&nbsp;km.<ref name=Kuskov2005/> Failing an ocean, the icy lithosphere may be somewhat thicker, up to about 300&nbsp;km.

Beneath the lithosphere and putative ocean, Callisto's interior appears to be neither entirely uniform nor particularly variable. ''[[Engineering:Galileo (spacecraft)|Galileo]]'' orbiter data<ref name="Anderson 2001"/> (especially the dimensionless [[Physics:Moment of inertia|moment of inertia]]<ref group="lower-alpha">The dimensionless moment of inertia referred to is <math>I / (mr^2)</math>, where {{var|I}} is the moment of inertia, {{var|m}} the mass, and {{var|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&nbsp;±&nbsp;0.0042—determined during close flybys) suggest that, if Callisto is in [[Astronomy:Hydrostatic equilibrium|hydrostatic equilibrium]], its interior is composed of compressed [[Earth:Rock (geology)|rocks]] and [[Earth:Ice|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.|last2=Schubert, G.|last3=Jacobson, R. A.|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|pmid=9616114|issue=5369|bibcode=1998Sci...280.1573A|last4=Lau|first4=E. L.|last5=Moore|first5=W. B.|last6=Sjo Gren|first6=W. L.|url-status=dead|archive-url=https://web.archive.org/web/20070926195310/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/19178/1/98-0442.pdf|archive-date=26 September 2007}}</ref> In other words, Callisto may be only partially [[Astronomy:Planetary differentiation|differentiated]]. The density and moment of inertia for an equilibrium Callisto are compatible with the existence of a small [[Chemistry:Silicate|silicate]] core in the center of Callisto. The radius of any such core cannot exceed 600&nbsp;km, and the density may lie between 3.1 and 3.6&nbsp;g/cm<sup>3</sup>.<ref name="Anderson 2001"/><ref name=Kuskov2005/> In this case, Callisto's interior would be in stark contrast to [[Astronomy: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. |last2=Spohn |first2=T. |last3=Breuer |first3=D. |last4=Nagel |first4=K. |year=2002 |title=Implications from Galileo Observations on the Interior Structure and Chemistry of the Galilean Satellites |journal=Icarus |volume=157 |issue=1 |pages=104–119 |bibcode=2002Icar..157..104S |doi=10.1006/icar.2002.6828}}</ref>

However, a 2011 reanalysis of Galileo data suggests that Callisto is not in hydrostatic equilibrium.<ref name=Monteux2014>{{cite journal |last1=Monteux |first1=J. |last2=Tobie |first2=G. |last3=Choblet |first3=G. |last4=Le Feuvre |first4=M. |title=Can large icy moons accrete undifferentiated? |journal=Icarus |year= 2014 |volume=237 |pages=377–387 |doi=10.1016/j.icarus.2014.04.041|bibcode=2014Icar..237..377M |s2cid=46172826 |url=https://hal.uca.fr/hal-01636068/file/Monteux-Icarus-V3-1-Final-2014.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://hal.uca.fr/hal-01636068/file/Monteux-Icarus-V3-1-Final-2014.pdf |archive-date=2022-10-09 |url-status=live }}</ref> In that case, the gravity data may be more consistent with a more thoroughly differentiated Callisto with a hydrated silicate core.<ref name="Castillo-Rogez2011">{{cite journal |last1=Castillo-Rogez |first1=J. C.|display-authors=0 |title=How differentiated is Callisto |journal=42nd Lunar and Planetary Science Conference |date=2011 |pages=2580 |url=https://www.lpi.usra.edu/meetings/lpsc2011/pdf/2580.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.lpi.usra.edu/meetings/lpsc2011/pdf/2580.pdf |archive-date=2022-10-09 |url-status=live |access-date=2 January 2020}}</ref>

===Surface features===
[[File:Cratered plains PIA00745.jpg|thumb|upright|left|''Galileo'' image of cratered plains, illustrating the pervasive local smoothing of Callisto's surface]]
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. |last2=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 |pmid=11878353 |bibcode=1998Icar..136..202Z |last3=Levison |first3=Harold F. |url-status=dead |archive-url=https://web.archive.org/web/20080227015923/http://lasp.colorado.edu/icymoons/europaclass/Zahnle_etal_1998.pdf |archive-date=27 February 2008}}</ref> In fact, the [[Astronomy:Impact crater|crater]] density is close to saturation: any new crater will tend to erase an older one. The large-scale [[Earth:Geology|geology]] is relatively simple; on Callisto there are no large mountains, volcanoes or other endogenic tectonic features.<ref name="Bender 1997">{{Cite journal |last1=Bender |first1=K. C. |last2=Rice |first2=J. W. |last3=Wilhelms |first3=D. E. |last4=Greeley |first4=R. |title=Geological map of Callisto |journal=Abstracts of the 25th Lunar and Planetary Science Conference |volume=25 |pages=91 |year=1997 |url=https://astrogeology.usgs.gov/Projects/PlanetaryMapping/DIGGEOL/galsats/callisto/jcglobal.htm |bibcode=1994LPI....25...91B |access-date=28 August 2017 |archive-url=https://web.archive.org/web/20150124085702/http://astrogeology.usgs.gov/Projects/PlanetaryMapping/DIGGEOL/galsats/callisto/jcglobal.htm |archive-date=24 January 2015 |url-status=dead }}</ref> The impact craters and multi-ring structures—together with associated [[Earth:Fracture (geology)|fractures]], [[Earth:Escarpment|scarps]] and deposits—are the only large features to be found on the surface.<ref name="Greeley 2000"/><ref name="Bender 1997"/>

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.|last2=Klemaszewski, J. E. |last3=Wagner, L. |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| display-authors=0}}</ref><ref name="Bender 1997"/> The cratered plains make up 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 Burr and Lofn, as well as the effaced remnants of old large craters called [[Astronomy:Palimpsest (planetary astronomy)|palimpsest]]s,{{refn|In the case of icy satellites, palimpsests are defined as bright circular surface features, probably old impact craters<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 make up a small fraction of Callisto's surface and are found in the ridge and [[Earth:Trough (geology)|trough]] zones of the Valhalla and Asgard formations and as isolated spots in the cratered plains. They were thought 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&nbsp;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 [[Astronomy: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. |last2=Neukum, G. |last3=Greeley, R |title=Fractures, Scarps, and Lineaments on Callisto and their Correlation with Surface Degradation |work=32nd Annual Lunar and Planetary Science Conference |date= 12–16 March 2001 |url=http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1838.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1838.pdf |archive-date=2022-10-09 |url-status=live|display-authors=0}}</ref>

[[File:Callisto Har PIA01054.jpg|thumb|right|Impact crater Hár with a central dome. Chains of secondary craters from formation of the more recent crater Tindr at upper right crosscut the terrain.]]
Impact crater diameters seen range from 0.1&nbsp;km—a limit defined by the [[Image resolution|imaging resolution]]—to over 100&nbsp;km, not counting the multi-ring structures.<ref name="Greeley 2000"/> Small craters, with diameters less than 5&nbsp;km, have simple bowl or flat-floored shapes. Those 5–40&nbsp;km across usually have a central peak. Larger impact features, with diameters in the range 25–100&nbsp;km, have central pits instead of peaks, such as Tindr crater.<ref name="Greeley 2000"/> The largest craters with diameters over 60&nbsp;km can have central domes, which are thought to result from central [[Earth:Tectonic uplift|tectonic uplift]] after an impact;<ref name="Greeley 2000"/> examples include Doh and Hár craters. A small number of very large—more than 100&nbsp;km in diameter—and bright impact craters show anomalous dome geometry. These are unusually shallow and may be a transitional [[Earth:Landform|landform]] to the multi-ring structures, as with the Lofn impact feature.<ref name="Greeley 2000"/> Callisto's craters are generally shallower than those on the [[Astronomy:Moon|Moon]].

[[File:Valhalla crater on Callisto.jpg|thumb|''Voyager 1'' image of Valhalla, a multi-ring impact structure 3,800&nbsp;km in diameter]]
The largest impact features on Callisto's surface are multi-ring basins.<ref name="Greeley 2000"/><ref name="Bender 1997"/> Two are enormous. Valhalla is the largest, with a bright central region 600&nbsp;km in diameter, and rings extending as far as 1,800&nbsp;km 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/ |access-date=17 April 2007 |archive-date=9 May 2013 |archive-url=https://web.archive.org/web/20130509055309/http://geopubs.wr.usgs.gov/i-map/i2770/ |url-status=live }}</ref> The second largest is Asgard, measuring about 1,600&nbsp;km 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 oblique impacts.<ref name="Greeley 2000"/> A historical example of a disruption was [[Astronomy:Comet Shoemaker–Levy 9|Comet Shoemaker–Levy 9]].

As mentioned above, small patches of pure water ice with an [[Earth:Albedo|albedo]] as high as 80% are found on the surface of Callisto, surrounded by much darker material.<ref name=Moore2004/> High-resolution ''[[Engineering:Galileo (spacecraft)|Galileo]]'' images showed the bright patches to be predominately located on elevated surface features: crater rims, [[Earth:Escarpment|scarps]], ridges and knobs.<ref name=Moore2004/> They are likely to be thin water [[Earth:Frost|frost]] 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&nbsp;km across within the crater floors and in the intercrater depressions.<ref name=Moore2004/>

[[File:Landslides and knobs PIA01095.jpg|thumb|upright|left|Two [[Earth:Landslide|landslide]]s 3–3.5&nbsp;km long are visible on the right sides of the floors of the two large craters on the right.]]
On a sub-kilometer scale the surface of Callisto is more degraded than the surfaces of other icy [[Astronomy:Galilean moons|Galilean moons]].<ref name=Moore2004/> Typically there is a deficit of small impact craters with diameters less than 1&nbsp;km as compared with, for instance, the dark plains on [[Astronomy: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. |last2=Asphaug |first2=Erik |last3=Morrison, David |last4=Spencer |first4=John R. |last5=Chapman |first5=Clark R. |last6=Bierhaus |first6=Beau |last7=Sullivan |first7=Robert J. |last8=Chuang |first8=Frank C. |last9=Klemaszewski |first9=James E. |last10=Greeley |first10=Ronald |last11=Bender |first11=Kelly C. |last12=Geissler |first12=Paul E. |last13=Helfenstein |first13=Paul |last14=Pilcher |first14=Carl B. |year=1999 |title=Mass Movement and Landform Degradation on the Icy Galilean Satellites: Results of the Galileo Nominal Mission |url=https://zenodo.org/record/1229836 |url-status=live |journal=Icarus |volume=140 |issue=2 |pages=294–312 |bibcode=1999Icar..140..294M |doi=10.1006/icar.1999.6132 |archive-url=https://web.archive.org/web/20190129011552/https://zenodo.org/record/1229836 |archive-date=29 January 2019 |access-date=26 August 2018}}</ref> The most likely candidate process is the slow sublimation of ice, which is enabled by a temperature of up to 165&nbsp;[[Kelvin|K]], reached at a subsolar point.<ref name=Moore2004/> Such sublimation of water or other [[Earth:Volatile (astrogeology)|volatiles]] from the dirty ice that is the [[Earth:Bedrock|bedrock]] causes its decomposition. The non-ice remnants form [[Physics:Debris|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 [[Astronomy: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.

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|last1=Chapman |first1=C.R. |last2=Merline |first2=W.J. |last3=Bierhaus |first3=B.|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 |archive-url=https://ghostarchive.org/archive/20221009/http://www.lpi.usra.edu/meetings/lpsc97/pdf/1221.pdf |archive-date=2022-10-09 |url-status=live| page=1221|display-authors=0}}</ref> Absolute dating has not been carried out, but based on theoretical considerations, the cratered plains are thought to be ~4.5&nbsp;billion years old, dating back almost to the formation of the [[Astronomy:Solar System|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&nbsp;billion years.<ref name="Greeley 2000"/><ref name="Zahnle 1998"/>

===Atmosphere and ionosphere===
[[File:Callisto field.svg|thumb|right|Induced magnetic field around Callisto]]
Callisto has a very tenuous atmosphere composed of [[Chemistry:Carbon dioxide|carbon dioxide]].<ref name="Carlson 1999">{{cite journal|last=Carlson|first=R. W.|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|pmid=9933159|issue=5403|bibcode=1999Sci...283..820C|display-authors=0|citeseerx=10.1.1.620.9273|access-date=10 July 2007|archive-date=3 October 2008|archive-url=https://web.archive.org/web/20081003231710/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/16785/1/99-0186.pdf|url-status=dead}}</ref> It was detected by the ''Galileo'' Near Infrared Mapping Spectrometer (NIMS) from its absorption feature near the wavelength 4.2&nbsp;micrometers. The surface pressure is estimated to be 7.5 pico[[Bar (unit)|bar]] (0.75 μPa) and particle density 4{{E-sp|8}}&nbsp;cm<sup>−3</sup>. Because such a thin atmosphere would be lost in only about four years ''(see [[Earth:Atmospheric escape|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.

Callisto's ionosphere was first detected during ''Galileo'' flybys;<ref name="Kliore 2002">{{cite journal |last=Kliore|first=A. J. |last2=Anabtawi, A. |last3=Herrera, R. G. |title=Ionosphere of Callisto from Galileo radio occultation observations |journal=Journal of Geophysical Research|year=2002|volume=107 |issue=A11|page=1407|doi=10.1029/2002JA009365| bibcode=2002JGRA..107.1407K|display-authors=0|url=https://deepblue.lib.umich.edu/bitstream/2027.42/95670/1/jgra16576.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://deepblue.lib.umich.edu/bitstream/2027.42/95670/1/jgra16576.pdf |archive-date=2022-10-09 |url-status=live|hdl=2027.42/95670 |doi-access=free}}</ref> its high electron density of 7–17{{E-sp|4}}&nbsp;cm<sup>−3</sup> cannot be explained by the photoionization of the atmospheric [[Chemistry:Carbon dioxide|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.|last2=Lane, B. F.|last3=Pappalardo, R. T.|title=Atmosphere of Callisto|journal=Journal of Geophysical Research|year=2005|volume=110|issue=E2|pages=E02003|doi=10.1029/2004JE002322|bibcode=2005JGRE..11002003L|display-authors=0|doi-access=free}}</ref> However, [[Chemistry:Oxygen|oxygen]] has not yet been directly detected in the atmosphere of Callisto. Observations with the [[Astronomy:Hubble Space Telescope|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.|last2=Saur, Joachim |last3=Feldman, Paul D. |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|display-authors=0|doi-access=free}}</ref> At the same time, HST was able to detect [[Physics:Condensation|condensed]] oxygen trapped on the surface of Callisto.<ref name="Spencer2002">{{cite journal|last= Spencer|first=John R.|last2=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 |archive-url=https://ghostarchive.org/archive/20221009/http://www.boulder.swri.edu/~spencer/o2europa.pdf |archive-date=2022-10-09 |url-status=live| journal = The Astronomical Journal|bibcode=2002AJ....124.3400S|s2cid=51792560 }}</ref>

Atomic hydrogen has also been detected in Callisto's atmosphere via recent analysis of 2001 Hubble Space Telescope data.<ref name=":0">{{Cite journal|last=Roth|first=Lorenz|display-authors=et al|date=27 May 2017|title=Detection of a hydrogen corona at Callisto|journal=Journal of Geophysical Research: Planets|volume=122|issue=5|pages=1046–1055|doi=10.1002/2017JE005294|bibcode=2017JGRE..122.1046R|s2cid=125830948 }}</ref> Spectral images taken on 15 and 24 December 2001 were re-examined, revealing a faint signal of scattered light that indicates a hydrogen corona. The observed brightness from the scattered sunlight in Callisto's hydrogen corona is approximately two times larger when the leading hemisphere is observed. This asymmetry may originate from a different hydrogen abundance in both the leading and trailing hemispheres. However, this hemispheric difference in Callisto's hydrogen corona brightness is likely to originate from the extinction of the signal in Earth's [[Earth:Geocorona|geocorona]], which is greater when the trailing hemisphere is observed.<ref>{{Cite journal|date=15 November 2017|title=New constraints on Ganymede's hydrogen corona: Analysis of Lyman-α emissions observed by HST/STIS between 1998 and 2014|journal=Planetary and Space Science|volume=148|pages=35–44|doi=10.1016/j.pss.2017.10.006|issn=0032-0633|last1=Alday|first1=Juan|last2=Roth|first2=Lorenz|last3=Ivchenko|first3=Nickolay|last4=Retherford|first4=Kurt D|last5=Becker|first5=Tracy M|last6=Molyneux|first6=Philippa|last7=Saur|first7=Joachim|bibcode=2017P&SS..148...35A}}</ref>

Callisto's atmosphere have been modelled to gain better understanding of impact of collisional molecular interactions.<ref name="auto">{{Cite journal |last1=Carberry Mogan |first1=Shane R. |last2=Tucker |first2=Orenthal J. |last3=Johnson |first3=Robert E. |last4=Sreenivasan |first4=Katepalli R. |last5=Kumar |first5=Sunil |date=2020-12-01 |title=The influence of collisions and thermal escape in Callisto's atmosphere |journal=Icarus |language=en |volume=352 |pages=113932 |doi=10.1016/j.icarus.2020.113932 |bibcode=2020Icar..35213932C |s2cid=225656570 |issn=0019-1035|doi-access=free }}</ref> Researchers used a kinetic method to model collisions between the constituent elements of Callisto's atmosphere (carbon dioxide, molecular oxygen and molecular hydrogen). The modeling took into account the thermal desorption of these compounds due to solar exposure and the resulting variations in temperature on the surface. The simulation showed that the density of Callisto's atmosphere could be explained by the trapping of hydrogen by the heavier gases, carbon dioxide and oxygen. The model shows how kinetic interactions between molecules affect the atmosphere, although it has limitations in terms of variables considered. The simulated densities correlate with expected thresholds for experimental detection.<ref name="auto1">{{Cite journal |last1=Carberry Mogan |first1=Shane R. |last2=Tucker |first2=Orenthal J. |last3=Johnson |first3=Robert E. |last4=Vorburger |first4=Audrey |last5=Galli |first5=Andre |last6=Marchand |first6=Benoit |last7=Tafuni |first7=Angelo |last8=Kumar |first8=Sunil |last9=Sahin |first9=Iskender |last10=Sreenivasan |first10=Katepalli R. |date=2021-11-01 |title=A tenuous, collisional atmosphere on Callisto |url=https://www.sciencedirect.com/science/article/pii/S0019103521002621 |journal=Icarus |language=en |volume=368 |pages=114597 |doi=10.1016/j.icarus.2021.114597 |arxiv=2107.12341 |bibcode=2021Icar..36814597C |s2cid=236428141 |issn=0019-1035}}</ref><ref>{{Cite journal |last1=Alexander |first1=Francis J. |last2=Garcia |first2=Alejandro L. |date=1997 |title=The Direct Simulation Monte Carlo Method |journal=Computers in Physics |language=en |volume=11 |issue=6 |pages=588 |doi=10.1063/1.168619 |bibcode=1997ComPh..11..588A |doi-access=free }}</ref>

==Origin and evolution==
The partial [[Astronomy: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 [[Physics:Accretion (astrophysics)|accretion]] in the low-density Jovian 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. |last2=Ward |first2=William R. |year=2002 |title=Formation of the Galilean Satellites: Conditions of Accretion |url=http://www.boulder.swri.edu/~robin/cw02final.pdf |url-status=live |journal=The Astronomical Journal |volume=124 |issue=6 |pages=3404–3423 |bibcode=2002AJ....124.3404C |doi=10.1086/344684 |s2cid=47631608 |archive-url=https://ghostarchive.org/archive/20221009/http://www.boulder.swri.edu/~robin/cw02final.pdf |archive-date=2022-10-09}}</ref> The allowable timescale for the formation of Callisto lies then in the range 0.1&nbsp;million–10&nbsp;million years.<ref name=Canup2002/>

[[File:Jagged Hills PIA03455.jpg|thumb|left|Views of eroding (top) and mostly eroded (bottom) ice knobs (~100&nbsp;m high), possibly formed from the ejecta of an ancient impact]]
The further evolution of Callisto after [[Physics:Accretion (astrophysics)|accretion]] was determined by the balance of the [[Physics:Radioactive|radioactive]] heating, cooling through [[Physics:Thermal conduction|thermal conduction]] near the surface, and solid state or subsolidus [[Astronomy:Convection|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 |journal=Planetary and Space Science |bibcode=2006P&SS...54....2F |url-status=dead |archive-url=https://web.archive.org/web/20070824155106/http://bowfell.geol.ucl.ac.uk/~lidunka/EPSS-papers/pete2.pdf |archive-date=24 August 2007 }}</ref> Details of the subsolidus convection in the ice is the main source of uncertainty in the models of all [[Astronomy:Icy moon|icy moon]]s. It is known to develop when the temperature is sufficiently close to the [[Physics:Melting point|melting point]], due to the temperature dependence of ice [[Physics:Viscosity|viscosity]].<ref name=McKinnon2006/> Subsolidus convection in icy bodies is a slow process with ice motions of the order of 1&nbsp;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 [[Earth:Lithosphere|lithosphere]] with a thickness of about 100&nbsp;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, water [[Earth:Ice|ice]] exists in different crystalline phases beginning from the ice I on the surface to [[Earth:Ice VII|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 [[Astronomy:Planetary differentiation|differentiation]] that would have otherwise formed a large rocky core and icy [[Earth: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|last2=Breuer, D. |last3=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>

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 [[Physics:Pressure|pressure]], achieving temperatures as low as 251&nbsp;K at 2,070&nbsp;bar (207&nbsp;MPa).<ref name="Spohn 2003"/> In all realistic models of Callisto the temperature in the layer between 100 and 200&nbsp;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 [[Chemistry:Ammonia|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"/>

Although Callisto is very similar in bulk properties to [[Astronomy:Ganymede (moon)|Ganymede]], it apparently had a much simpler [[Earth:Historical geology|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 [[Earth: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
| last = Barr
| first = A. C.
|last2=Canup, R. M.
| title = Constraints on gas giant satellite formation from the interior states of partially differentiated satellites
| journal = Icarus
| volume = 198
| issue = 1
| pages = 163–177
| date = 3 August 2008
| doi = 10.1016/j.icarus.2008.07.004
| bibcode=2008Icar..198..163B}}</ref> the greater tidal heating experienced by Ganymede,<ref name = "Showman2">{{Cite journal
| last = Showman
| first = A. P.
|last2=Malhotra, R.
| s2cid = 55790129
| title = Tidal evolution into the Laplace resonance and the resurfacing of Ganymede
| journal = Icarus
| volume = 127
| issue = 1
| pages = 93–111
| date = March 1997
| doi = 10.1006/icar.1996.5669
| bibcode=1997Icar..127...93S}}</ref> and the more numerous and energetic impacts that would have been suffered by Ganymede during the [[Astronomy:Late Heavy Bombardment|Late Heavy Bombardment]].<ref name = "Baldwin">{{cite web
 | last = Baldwin
 | first = E.
 | title = Comet impacts explain Ganymede-Callisto dichotomy
 | website = [[Astronomy:Astronomy Now|Astronomy Now]]
 | date = 25 January 2010
 | url = http://www.astronomynow.com/news/n1001/25galilean/
 | access-date = 1 March 2010
 | archive-date = 30 January 2010
 | archive-url = https://web.archive.org/web/20100130231918/http://www.astronomynow.com/news/n1001/25galilean/
 | url-status = dead
 }}</ref><ref name="LPI1158">{{Cite conference |last=Barr |first=A. C. |last2=Canup |first2=R. M. |date=March 2010 |title=Origin of the Ganymede/Callisto dichotomy by impacts during an outer solar system late heavy bombardment |url=http://www.lpi.usra.edu/meetings/lpsc2010/pdf/1158.pdf |conference= |location=Houston |archive-url=https://web.archive.org/web/20110605044843/http://www.lpi.usra.edu/meetings/lpsc2010/pdf/1158.pdf |archive-date=5 June 2011 |access-date=1 March 2010 |work=41st Lunar and Planetary Science Conference (2010) |url-status=live}}</ref><ref name="Barr">{{Cite journal |last=Barr |first=A. C. |last2=Canup |first2=R. M. |date=24 January 2010 |title=Origin of the Ganymede–Callisto dichotomy by impacts during the late heavy bombardment |url=http://www.planetary.brown.edu/pdfs/ab19.pdf |url-status=dead |journal=[[Earth:Nature Geoscience|Nature Geoscience]] |volume=3 |issue=March 2010 |pages=164–167 |bibcode=2010NatGe...3..164B |doi=10.1038/NGEO746 |archive-url=https://web.archive.org/web/20210301065853/http://www.planetary.brown.edu/pdfs/ab19.pdf |archive-date=1 March 2021 |access-date=12 April 2020}}</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/>

==Habitability==
It is speculated that there could be life in Callisto's subsurface ocean. Like [[Astronomy:Europa (moon)|Europa]] and [[Astronomy:Ganymede (moon)|Ganymede]], as well as [[Astronomy:Saturn|Saturn]]'s [[Astronomy:Moons of Saturn|moons]] [[Astronomy:Enceladus|Enceladus]], Dione and [[Astronomy:Titan (moon)|Titan]] and [[Astronomy:Neptune|Neptune]]'s moon [[Astronomy:Triton (moon)|Triton]],<ref name="Triton subsurface ocean">{{cite journal |last1=Nimmo |first1=Francis |title=Powering Triton's recent geological activity by obliquity tides: Implications for Pluto geology |journal=Icarus |date=15 January 2015 |volume=246 |pages=2–10 |doi=10.1016/j.icarus.2014.01.044 |bibcode=2015Icar..246....2N |s2cid=40342189 |url=https://escholarship.org/content/qt99s8t6zm/qt99s8t6zm.pdf?t=nnm476 |access-date=12 April 2020 |archive-date=27 July 2020 |archive-url=https://web.archive.org/web/20200727083959/https://escholarship.org/content/qt99s8t6zm/qt99s8t6zm.pdf?t=nnm476 |url-status=live }}</ref> a possible subsurface ocean might be composed of [[Chemistry:Saline water|salt water]].

It is possible that [[Biology:Halophile|halophile]]s could thrive in the ocean.<ref name="NASA finds ocean">{{cite web | url=https://science.nasa.gov/science-news/science-at-nasa/1998/ast22oct98_2/ | title=Callisto makes a big splash | publisher=NASA | date=23 October 1998 | access-date=15 August 2015 | last=Phillips | first=Tony | archive-date=28 May 2019 | archive-url=https://web.archive.org/web/20190528024102/https://science.nasa.gov/science-news/science-at-nasa/1998/ast22oct98_2 | url-status=live }}</ref>
As with [[Astronomy:Europa (moon)|Europa]] and [[Astronomy:Ganymede (moon)|Ganymede]], the idea has been raised that [[Astronomy:Planetary habitability|habitable]] conditions and even [[Astronomy:Extraterrestrial life|extraterrestrial microbial life]] may exist in the salty ocean under the Callistoan surface.<ref name=Lipps2004>{{cite journal|last=Lipps |first=Jere H. |last2=Delory, Gregory |last3=Pitman, Joe |editor3-first=Alexei Y |editor3-last=Rozanov |editor2-first=Gilbert V |editor2-last=Levin |editor1-first=Richard B |editor1-last=Hoover |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 |series=Instruments, Methods, and Missions for Astrobiology VIII |display-authors=0 |url-status=dead |archive-url=https://web.archive.org/web/20080820014713/http://learning.berkeley.edu/astrobiology/2004ppt/jupiter.pdf |archive-date=20 August 2008|bibcode=2004SPIE.5555...78L |s2cid=140590649 }}</ref> However, the environmental conditions necessary 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/> Callisto's ocean is heated only by radioactive decay, while Europa's is also heated by tidal energy, as it is much closer to Jupiter.<ref name="NASA finds ocean"/> It is thought that of all of Jupiter's moons, Europa has the greatest chance of supporting [[Biology:Microorganism|microbial life]].<ref name=Lipps2004/><ref name="François2005">{{cite journal|last=François|first=Raulin|s2cid=121543884|title=Exo-Astrobiological Aspects of Europa and Titan: from Observations to speculations|year=2005|volume=116|issue=1–2|pages=471–487|doi=10.1007/s11214-005-1967-x | journal = Space Science Reviews|bibcode = 2005SSRv..116..471R }}</ref>

==Exploration==
===Past===
The ''[[Astronomy:Pioneer 10|Pioneer 10]]'' and ''[[Astronomy:Pioneer 11|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 |last1=Moore |first1=Jeffrey M. |last2=Chapman |first2=Clark R. |last3=Bierhaus |first3=Edward B. |chapter=Callisto |title=Jupiter: The planet, Satellites and Magnetosphere |year=2004 |publisher=Cambridge University Press |editor=Bagenal, Fran |editor2=Dowling, Timothy E. |editor3=McKinnon, William B. |chapter-url=http://lasp.colorado.edu/~espoclass/homework/5830_2008_homework/Ch17.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://lasp.colorado.edu/~espoclass/homework/5830_2008_homework/Ch17.pdf |archive-date=2022-10-09 |url-status=live |display-authors=0 }}</ref> The real breakthrough happened later with the ''[[Astronomy:Voyager 1|Voyager 1]]'' and ''[[Astronomy:Voyager 2|Voyager 2]]'' flybys in 1979. They imaged more than half of the Callistoan surface with a resolution of 1–2&nbsp;km, and precisely measured its temperature, mass and shape.<ref name=Moore2004/> A second round of exploration lasted from 1994 to 2003, when the ''[[Engineering:Galileo (spacecraft)|Galileo]]'' spacecraft had eight close encounters with Callisto, the last flyby during the C30 orbit in 2001 came as close as 138&nbsp;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&nbsp;meters of selected areas of Callisto.<ref name="Greeley 2000"/> In 2000, the ''[[Astronomy:Cassini–Huygens|Cassini]]'' spacecraft en route to [[Astronomy:Saturn|Saturn]] acquired high-quality infrared spectra of the Galilean satellites including Callisto.<ref name=Brown2003>{{cite journal |last1=Brown |first1=R. H.|last2=Baines |first2=K. H. |last3=Bellucci |first3=G. |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 |last4=Bibring|first4=J-P. |last5=Buratti|first5=B. J. |last6=Capaccioni|first6=F. |last7=Cerroni|first7=P. |last8=Clark|first8=R. N. |last9=Coradini|first9=A. |last10=Cruikshank|first10=D. P. |last11=Drossart|first11=P.|last12=Formisano|first12=V.|last13=Jaumann|first13=R.|last14=Langevin|first14=Y.|last15=Matson|first15=D. L. |last16=McCord|first16=T. B.|last17=Mennella|first17=V.|last18=Nelson|first18=R. M.|last19=Nicholson|first19=P. D. |last20=Sicardy|first20=B.|last21=Sotin|first21=C.|last22=Amici|first22=S.|last23=Chamberlain|first23=M. A. |last24=Filacchione|first24=G.|last25=Hansen|first25=G.|last26=Hibbitts|first26=K.|last27=Showalter|first27=M.}}</ref> In February–March 2007, the ''[[Astronomy:New Horizons|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=7 May 2007|pages=80–83|url=https://archive.aviationweek.com/issue/20070507|access-date=30 September 2020|archive-date=8 July 2022|archive-url=https://web.archive.org/web/20220708084420/https://archive.aviationweek.com/issue/20070507|url-status=live}}<!-- paywalled, cannot link to exact article --></ref>

===Future exploration===
Callisto will be visited by three spacecraft in the near future. 

The [[Organization:European Space Agency|European Space Agency]]'s [[Astronomy:Jupiter Icy Moons Explorer|Jupiter Icy Moons Explorer]] (JUICE), which launched on April 14, 2023, will perform 21 close flybys of Callisto between 2031 and 2034.<ref>{{cite web |url=https://sci.esa.int/web/juice |title=ESA Science & Technology – JUICE |publisher=ESA |date=8 November 2021 |access-date=10 November 2021 |archive-date=21 September 2019 |archive-url=https://web.archive.org/web/20190921101151/https://sci.esa.int/web/juice |url-status=live }}</ref><ref name='selection'>{{cite news|first = Jonathan|last = Amos|url = https://www.bbc.co.uk/news/science-environment-17917102|title = Esa selects 1bn-euro Juice probe to Jupiter|access-date = 2 May 2012|date = 2 May 2012|work = BBC News Online|archive-date = 11 May 2020|archive-url = https://web.archive.org/web/20200511181342/https://www.bbc.co.uk/news/science-environment-17917102|url-status = live}}</ref>

NASA's [[Astronomy:Europa Clipper|Europa Clipper]], which is scheduled to launch in October 2024, will conduct nine close flybys of Callisto beginning in 2030.<ref>{{Cite journal|title=Tour Design Techniques for the Europa Clipper Mission|first1=Stefano|last1=Campagnola|first2=Brent B.|last2=Buffington|first3=Try|last3=Lam|first4=Anastassios E.|last4=Petropoulos|first5=Etienne|last5=Pellegrini|date=17 December 2019|journal=Journal of Guidance, Control, and Dynamics|volume=42|issue=12|pages=2615–2626|doi=10.2514/1.G004309|bibcode=2019JGCD...42.2615C |s2cid=203030313 |doi-access=free}}</ref>

China's CNSA [[Astronomy:Tianwen-4|Tianwen-4]] is planned to launch to Jupiter around 2030 before entering orbit around Callisto.<ref>{{Cite web |last=Theresa |first=Deena |date=2022-09-23 |title=China's Tianwen 4 to target Jupiter and Uranus with two spacecraft on one rocket |url=https://interestingengineering.com/innovation/tianwen-4-jupiter-and-uranus-two-spacecraft-one-rocket |access-date=2023-04-17 |website=interestingengineering.com |language=en-US}}</ref><ref>{{Cite magazine |date=2022-09-23 |title=China Unveils Plans to Send Spacecraft to Jupiter and Uranus |url=https://time.com/6216133/china-aims-for-jupiter-and-uranus/ |access-date=2023-04-17 |magazine=Time |language=en}}</ref><ref>{{Cite web |last=Andrew Jones published |date=2022-09-22 |title=China wants to probe Uranus and Jupiter with 2 spacecraft on one rocket |url=https://www.space.com/china-probes-jupiter-uranus-same-launch |access-date=2023-04-17 |website=Space.com |language=en}}</ref>

===Old proposals===
Formerly proposed for a launch in 2020, the Europa Jupiter System Mission (EJSM) was a joint [[Organization:NASA|NASA]]/ESA proposal for exploration of [[Astronomy:Jupiter|Jupiter]]'s moons. In February 2009 it was announced that ESA/NASA had given this mission priority ahead of the [[Astronomy:Titan Saturn System Mission|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|work=BBC News|access-date=20 February 2009|date=20 February 2009|archive-date=21 February 2009|archive-url=https://web.archive.org/web/20090221185643/http://news.bbc.co.uk/1/hi/sci/tech/7897585.stm|url-status=live}}</ref> At the time 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=21 July 2007|publisher=ESA|access-date=20 February 2009|archive-date=2 September 2011|archive-url=https://web.archive.org/web/20110902033453/http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=41177|url-status=live}}</ref> EJSM consisted of the NASA-led [[Astronomy:Jupiter Europa Orbiter|Jupiter Europa Orbiter]], the ESA-led [[Astronomy:Jupiter Ganymede Orbiter|Jupiter Ganymede Orbiter]] and possibly a [[Organization:JAXA|JAXA]]-led [[Astronomy:Jupiter Magnetospheric Orbiter|Jupiter Magnetospheric Orbiter]].

=== Potential crewed exploration and habitation ===
[[File:Callisto base.PNG|thumb|right|Artist's impression of a base on Callisto<ref name="CallistoBase"/>]]
In 2003 [[Organization:NASA|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|last2=Bethke, Kristen|publisher=NASA|year=2003|url=http://www.nasa-academy.org/soffen/travelgrant/bethke.pdf|url-status=dead|archive-url=https://web.archive.org/web/20120119170143/http://www.nasa-academy.org/soffen/travelgrant/bethke.pdf|archive-date=19 January 2012}}</ref><ref>{{cite journal |last=Troutman |first=Patrick A. |last2=Bethke |first2=Kristen |last3=Stillwagen |first3=Fred |last4=Caldwell |first4=Darrell L. Jr. |last5=Manvi |first5=Ram |last6=Strickland |first6=Chris |last7=Krizan |first7=Shawn A. |date=28 January 2003 |title=Revolutionary Concepts for Human Outer Planet Exploration (HOPE) |journal=AIP Conference Proceedings |volume=654 |pages=821–828 |bibcode=2003AIPC..654..821T |doi=10.1063/1.1541373 |s2cid=109235313 |hdl-access=free |hdl=2060/20030063128}}</ref>

The study proposed a possible surface base on Callisto that would produce [[Chemistry:Rocket propellant|rocket propellant]] 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 |archive-url=https://ghostarchive.org/archive/20221009/http://www.nasa.gov/pdf/55583main_vision_space_exploration2.pdf |archive-date=2022-10-09 |url-status=live|publisher=[[Organization:NASA|NASA]]|year=2004}}</ref> Advantages of a base on Callisto include low radiation (due to its distance from Jupiter) and geological stability. Such a base could facilitate remote exploration of [[Astronomy:Europa (moon)|Europa]], or be an ideal location for a Jovian system waystation servicing spacecraft heading farther into the outer Solar System, using a [[Astronomy:Gravity assist|gravity assist]] from a close flyby of Jupiter after departing Callisto.<ref name=HOPE/>

In December 2003, NASA reported that a crewed mission to Callisto might 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=[[Organization:NASA|NASA]]|year=2003|access-date=25 June 2009|archive-url=https://web.archive.org/web/20120305055810/http://trajectory.grc.nasa.gov/aboutus/papers/STAIF-2003-177.pdf|archive-date=5 March 2012|url-status=dead}}</ref>

== See also ==
{{div col|colwidth=20em}}
* [[Astronomy:List of former planets|List of former planets]]
* Jupiter's moons in fiction
* [[Astronomy:List of craters on Callisto|List of craters on Callisto]]
* List of geological features on Callisto
* [[Astronomy:List of natural satellites|List of natural satellites]]
{{div col end}}

==Notes==
{{Reflist|group=lower-alpha}}

==References==
{{Reflist}}

==External links==
{{Commons}}
* [https://web.archive.org/web/20140328070532/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jup_Callisto Callisto Profile] at [http://solarsystem.nasa.gov NASA's Solar System Exploration site]
* [http://www.nineplanets.org/callisto.html Callisto page] at ''The Nine Planets''
* [https://web.archive.org/web/20110604053256/http://www.solarviews.com/eng/callisto.htm Callisto page] at ''Views of the Solar System''
* [http://www.lpi.usra.edu/resources/cc/cchome.html Callisto Crater Database] from the Lunar and Planetary Institute
* [http://photojournal.jpl.nasa.gov/target/Callisto Images of Callisto at JPL's Planetary Photojournal]
* Movie of [https://web.archive.org/web/20100601171427/http://sos.noaa.gov/videos/Callisto.mov Callisto's rotation] from the National Oceanic and Atmospheric Administration
* [http://photojournal.jpl.nasa.gov/catalog/PIA03876 Callisto map with feature names] from [http://photojournal.jpl.nasa.gov/ Planetary Photojournal]
*[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]
* [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]
* [https://www.google.com/maps/space/callisto/@13.1830935,-94.1384306,6879202m/data=!3m1!1e3 Google Callisto 3D], interactive map of the moon

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