View source for Ganymede (moon)

{{otheruses4|the natural satellite of Jupiter|the asteroid|1036 Ganymed}}
<!--
 Scroll down to edit the contents of this page.
 Additional parameters for this template are available at [[Template:Infobox Planet]].
-->
{{Infobox Planet
| name               = Ganymede
| alt_names = Jupiter III
| adjectives = Ganymedian, Ganymedean
| image              = [[Image:Ganymede g1 true.jpg|250px|True-color image taken by the ''Galileo'' probe]]
| caption            = True-color image taken by the ''[[Galileo probe|Galileo]]'' probe
| bgcolour           = #a0ffa0
| discovery          = yes
| discoverer         = [[Galileo Galilei|G. Galilei]]<br/>[[Simon Marius|S. Marius]]
| discovered         = January 11, 1610
| mean_orbit_radius  = 1&nbsp;070&nbsp;400&nbsp;km<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>
| eccentricity       = 0.001&nbsp;3<ref name=orbit/>
| periapsis          = 1&nbsp;069&nbsp;200&nbsp;km{{Ref_label|B|b|none}}
| apoapsis           = 1&nbsp;071&nbsp;600&nbsp;km{{Ref_label|A|a|none}}
| period             = 7.154&nbsp;552&nbsp;96&nbsp;[[day|d]]<ref name=orbit/>
| avg_speed          = 10.880&nbsp;km/s
| inclination        = 0.20° (to Jupiter's equator)<ref name=orbit/>
| satellite_of       = [[Jupiter]]
| physical_characteristics = yes
| mean_radius        = {{nowrap|2634.1 ± 0.3 km}} (0.413 Earths)<ref name=Showman1999/>
| surface_area       = 87.0 million&nbsp;[[square kilometre|km<sup>2</sup>]] (0.171 Earths){{Ref_label|C|c|none}}
| volume             = 7.6{{Esp|10}}&nbsp;[[cubic kilometre|km<sup>3</sup>]] (0.0704 Earths){{Ref_label|D|d|none}}
| mass               = {{nowrap|1.4819{{Esp|23}} [[kilogram|kg]]}} (0.025 Earths)<ref name=Showman1999/>
| density            = 1.936&nbsp;[[Gram|g]]/[[cubic centimetre|cm<sup>3</sup>]]<ref name=Showman1999/>
| surface_grav       = 1.428&nbsp;[[acceleration|m/s<sup>2</sup>]] (0.146 ''[[g-force|g]]''){{Ref_label|E|e|none}}
| escape_velocity    = 2.741&nbsp;km/s{{Ref_label|F|f|none}}
| rotation           = [[synchronous rotation|synchronous]]
| axial_tilt         = 0&ndash;0.33°<ref name=Bills2005/>
| albedo             = {{nowrap|0.43 ± 0.02}}<ref name=jplfact>{{cite web|last=Yeomans|first=Donald K.|date=2006-07-13|title=Planetary Satellite Physical Parameters|publisher=JPL Solar System Dynamics|url=http://ssd.jpl.nasa.gov/?sat_phys_par|accessdate=2007-11-05}}</ref>
| magnitude          = 4.61 ([[Opposition (astronomy)|opposition]]) <ref name=jplfact/>
| temperatures=yes
| temp_name1 = K
| max_temp_1 = 152<ref name=Orton1996>{{cite journal|last=Orton|first=G.S.|coauthors=Spencer, G.R.; Travis, L.D. et al.|title=Galileo Photopolarimeter-radiometer observations of Jupiter and the Galilean Satellites|journal=Science|year=1996|volume=274|pages=389&ndash;391|url=http://adsabs.harvard.edu/abs/1996Sci...274..389O|doi=10.1126/science.274.5286.389}}</ref>
| mean_temp_1 = 110<ref name=Delitsky1998>{{cite journal|last=Delitsky|first=Mona L.|coauthors=Lane, Arthur L. |title=Ice chemistry of Galilean satellites|journal=J.of Geophys. Res.|year=1998|volume=103|issue=E13| pages=31,391&ndash;31,403|doi=10.1029/1998JE900020| url=http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/20675/1/98-1725.pdf|format=pdf}}</ref>
| min_temp_1 = 70<ref name=Delitsky1998/>
| atmosphere         = yes
| surface_pressure   = trace
| atmosphere_composition = [[oxygen]]<ref name=Hall1998/>
}}
'''Ganymede''' ({{pron-en|ˈɡænɨmiːd}},<ref>In US dictionary transcription, {{USdict|găn′·<s>ı</s>·mēd}}</ref> or as Greek ''Γανυμήδης)'' is a [[Moons of Jupiter|moon of Jupiter]] and the [[List of natural satellites by diameter|largest moon]] in the [[Solar System]]. Completing an orbit in roughly seven days, it is the seventh moon and third [[Galilean moon]] from  Jupiter.<ref name="Planetary Society"/> Ganymede participates in a 1:2:4 [[orbital resonance]] with the moons [[Europa (moon)|Europa]] and [[Io (moon)|Io]], respectively. It is larger in diameter than the planet [[Mercury (planet)|Mercury]] but has only about half its mass. It has the highest mass of all planetary satellites with 2.01 times the mass of the Earth's moon.<ref name="nineplanets.org-Ganymede">{{cite web|publisher=nineplanets.org |title=Ganymede|date=October 31, 1997|url=http://www.nineplanets.org/ganymede.html|accessdate=2008-02-27}}</ref>

Ganymede is composed primarily of [[silicate|silicate rock]] and water ice. It is a fully [[Planetary differentiation|differentiated]] body with an iron-rich, liquid core. A saltwater ocean is believed to exist nearly 200&nbsp;[[km]] below Ganymede's surface, sandwiched between layers of ice.<ref name=JPLDec>{{cite web|url=http://www.jpl.nasa.gov/releases/2000/aguganymederoundup.html|title=Solar System's largest moon likely has a hidden ocean|accessdate=2008-01-11|date=2000-12-16|work=Jet Propulsion Laboratory |publisher=NASA}}</ref> Its surface comprises two main types of terrain. Dark regions, saturated with [[impact crater]]s and dated to four billion years ago, cover about a third of the satellite. Lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient, cover the remainder. The cause of the light terrain's disrupted geology is not fully known, but was likely the result of [[tectonics|tectonic]] activity brought about by [[tidal heating]].<ref name=Showman1999/> 

Ganymede is the only satellite in the Solar System known to possess a [[magnetosphere]], likely created through [[convection]] within the liquid iron core.<ref name=Kivelson2002/> The meager magnetosphere is buried within Jupiter's much larger [[magnetic field]] and connected to it through open [[field line]]s. The satellite has a thin [[oxygen]] [[atmosphere]] that includes O, O<sub>2</sub>, and possibly O<sub>3</sub> ([[ozone]]).<ref name=Hall1998/> [[Atomic hydrogen]] is a minor atmospheric constituent. Whether the satellite has an [[ionosphere]] to correspond to its atmosphere is unresolved.<ref name=Eviatar2001/>

Ganymede's discovery is credited to [[Galileo Galilei]], who observed it in 1610.<ref name="Sidereus Nuncius">{{cite web|url=http://www.physics.emich.edu/jwooley/chapter9/Chapter9.html|title=Sidereus Nuncius|work=Eastern Michigan University|accessdate=2008-01-11}}</ref> The satellite's name was soon suggested by astronomer [[Simon Marius]], for the mythological [[Ganymede (mythology)|Ganymede]], cupbearer of the [[Greek mythology|Greek gods]] and [[Zeus|Zeus's]] beloved.<ref name="Naming"/> Beginning with ''[[Pioneer 10]]'', spacecraft have been able to examine Ganymede closely.<ref name="Pioneer 11"/> The ''[[Voyager program|Voyager]]'' probes refined measurements of its size, while the ''[[Galileo (spacecraft)|Galileo]]'' craft discovered its underground ocean and magnetic field. A new mission to Jupiter's icy moons, the [[Europa Jupiter System Mission]] (EJSM) is proposed for a launch in 2020.

Ganymede receives about 8 [[Röntgen equivalent man|rem]] of radiation per day.<ref>http://zimmer.csufresno.edu/~fringwal/w08a.jup.txt</ref>

==Discovery and naming==
On January 11, 1610, Galileo Galilei observed what he believed were three stars near Jupiter; the next night he noticed that they had moved. He found a fourth supposed star, which would turn out to be Ganymede, on January 13. By January 15, Galileo came to the conclusion that the stars were actually bodies orbiting Jupiter.<ref name="Discovery">{{cite web | url = http://www.iki.rssi.ru/solar/eng/galdisc.htm | title = The Discovery of the Galilean Satellites |publisher=Space Research Institute, Russian Academy of Sciences| work = Views of the Solar System | accessdate = 2007-11-24}}</ref> 	
He claimed the right to name the moons; he considered "Cosmian Stars" and settled on "[[Galilean_Satellites#Dedication_to_the_Medicis|Medicean Stars]]".<ref name="Naming">{{cite web | url = http://galileo.rice.edu/sci/observations/jupiter_satellites.html | title = Satellites of Jupiter | work = The Galileo Project | accessdate = 2007-11-24}}</ref> 

The French astronomer [[Nicolas-Claude Fabri de Peiresc]] suggested individual names from the [[Medici]] family for the moons, but his proposal was not taken up.<ref name="Naming"/> [[Simon Marius]], who had originally claimed to have found the Galilean satellites,<ref name="College">{{cite web | url = http://www.cascadia.ctc.edu/facultyweb/instructors/jvanleer/astro%20sum01/astro101/discovery.htm | title = DISCOVERY | work = Cascadia Community College | accessdate = 2007-11-24}}{{Dead link|date=March 2009}}</ref> tried to name the moons the "Saturn of Jupiter", the "Jupiter of Jupiter" (this was Ganymede), the "Venus of Jupiter", and the "Mercury of Jupiter", another nomenclature that never caught on. From a suggestion by [[Johannes Kepler]], Marius once again tried to name the moons:<ref name="Naming"/>

{{cquote|…Then there was [[Ganymede (Greek mythology)|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&nbsp;… the Third, on account of its majesty of light, Ganymede …<ref name="Discovery"/>}}

This name and those of the other Galilean satellites fell into disfavor for a considerable time, and were not in common use until the mid-20th century. In much of the earlier astronomical literature, Ganymede is referred to instead by its Roman numeral designation (a system introduced by Galileo) as {{nowrap|Jupiter III}} or as the "third satellite of Jupiter". Following the discovery of moons of Saturn, a naming system based on that of Kepler and Marius was used for Jupiter’s moons.<ref name="Naming"/> Ganymede is the only Galilean moon of Jupiter named after a male figure.

==Orbit and rotation==
Ganymede [[orbit]]s Jupiter at a distance of 1&nbsp;070&nbsp;400&nbsp;km, third among the Galilean satellites,<ref name="Planetary Society">{{cite web| url=http://www.planetary.org/explore/topics/our_solar_system/jupiter/moons.html|title=Jupiter's Moons| work=The Planetary Society|accessdate=2007-12-07}}</ref> and completes a revolution every seven days and three hours. Like most known moons, Ganymede is [[tidal locking|tidally locked]], with one face always pointing toward the planet.<ref name="The Grand Tour">{{cite book |title=The Grand Tour: A Traveler's Guide to the Solar System|last= Miller|first= Ron|authorlink= Ron Miller (artist and author)|coauthors=William K. Hartmann|year=2005|month=May|pages=108&ndash;114|publisher= Workman Publishing|location=Thailand|edition=3rd|isbn=0-7611-3547-2}}</ref> 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 [[Perturbation (astronomy)|perturbation]]s on a timescale of centuries. The ranges of change are 0.0009&ndash;0.0022 and 0.05&ndash;0.32°, respectively.<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|pages=500&ndash;504|doi=10.1006/icar.2002.6939| url=http://adsabs.harvard.edu/abs/2002Icar..159..500M | journal = Icarus}}</ref> These orbital variations cause the [[axial tilt]] (the angle between rotational and orbital axes) to vary between 0 and 0.33°.<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|pages=233&ndash;247| doi=10.1016/j.icarus.2004.10.028|url=http://adsabs.harvard.edu/abs/2005Icar..175..233B | journal = Icarus}}</ref>   
[[Image:Galilean moon Laplace resonance animation.gif|The [[Laplace resonance]]s of Ganymede, [[Europa (moon)|Europa]], and [[Io (moon)|Io]]|left|thumb]] 
Ganymede participates in [[orbital resonance]]s with Europa and Io: for every orbit of Ganymede, Europa orbits twice and Io orbits four times.<ref name=Musotto2002/><ref name="SPACE.com">{{cite web|url=http://www.space.com/searchforlife/seti_tidal_europa_021003.html | title=High Tide on Europa|work=SPACE.com|accessdate=2007-12-07}}</ref> The superior [[Astronomical conjunction|conjunction]] between Io and Europa always occurs when Io is at [[periapsis]] and Europa at [[apoapsis]]. The superior conjunction between Europa and Ganymede occurs when Europa is at periapsis.<ref name=Musotto2002/> The longitudes of the Io&ndash;Europa and Europa&ndash;Ganymede conjunctions change with the same rate, making the triple conjunctions possible. Such a complicated resonance is called the [[Laplace resonance]].<ref name=Showman1997a>{{cite journal|last=Showman|first=Adam P.|coauthors=Malhotra, Renu|title=Tidal Evolution into the Laplace Resonance and the Resurfacing of Ganymede|journal=Icarus|year=1997|volume=127|pages=93&ndash;111|doi=10.1006/icar.1996.5669| url=http://www.lpl.arizona.edu/~showman/publications/showman-malhotra-1997.pdf|format=pdf}}</ref>

The current Laplace resonance is unable to pump the orbital eccentricity of Ganymede to a higher value.<ref name=Showman1997a/> The value of about 0.0013 is probably a remnant from a previous epoch, when such pumping was possible.<ref name="SPACE.com"/> The ganymedian orbital eccentricity is somewhat puzzling; if it is not pumped now it should have decayed long ago due to the tidal [[dissipation]] in the interior of Ganymede.<ref name=Showman1997a/> This means that the last episode of the eccentricity excitation happened only several hundred million years ago.<ref name=Showman1997a/> Because the orbital eccentricity of Ganymede is relatively low&mdash;0.0015 on average<ref name="SPACE.com"/>&mdash;the [[tidal heating]] of this moon is negligible now.<ref name=Showman1997a/> However, in the past Ganymede may have passed through one or more Laplace-like resonances{{Ref_label|J|j|none}} which were able to pump the orbital eccentricity to a value as high as 0.01&ndash;0.02.<ref name=Showman1999/><ref name=Showman1997a/> This probably caused a significant tidal heating of the interior of Ganymede; the  formation of the grooved terrain may be a result of one or more heating episodes.<ref name=Showman1999/><ref name=Showman1997a/>

The origin of the Laplace resonance among Io, Europa, and Ganymede is not known. Two hypotheses exist: that it is [[Primordial elements|primordial]] and has existed from the beginning of the Solar System;<ref name=Peale2002>{{cite journal|last=Peale|first=S.J.|coauthors=Lee, Man Hoi|title=A Primordial Origin of the Laplace Relation Among the Galilean Satellites|journal=Science|year=2002|volume=298|pages=593&ndash;597|doi=10.1126/science.1076557| url=http://adsabs.harvard.edu/abs/2002Sci...298..593P|pmid=12386333}}</ref> or that it developed after the [[Formation and evolution of the Solar System|formation of the Solar System]]. A possible sequence of the events is as follows: Io raised tides on Jupiter, causing its orbit to expand until it encountered 2:1 resonance with Europa; after that the expansion continued, but some of the angular [[moment (physics)|moment]] was transferred to Europa as the resonance caused its orbit to expand as well; the process continued until Europa encountered 2:1 resonance with Ganymede.<ref name=Showman1997a/> Eventually the drift rates of conjunctions between all three moons were synchronized and locked in the Laplace resonance.<ref name=Showman1997a/>

==Physical characteristics==
===Composition===
[[Image:PIA00519 Interior of Ganymede.jpg|right|thumb|Interior of Ganymede]]
The average [[density]] of Ganymede, 1.936&nbsp;[[gram|g]]/[[cubic centimetre|cm<sup>3</sup>]], suggests a composition of approximately equal parts rocky material and water, which is mainly in the form of ice.<ref name=Showman1999/> The [[mass fraction]] of ices is between 46–50%, slightly lower than that in [[Callisto (moon)|Callisto]].<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|pages=550&ndash;369|doi=10.1016/j.icarus.2005.04.014| url=http://adsabs.harvard.edu/abs/2005Icar..177..550K | journal = Icarus}}</ref> Some additional volatile ices such as [[ammonia]] may also be present.<ref name=Kuskov2005/><ref name=Spohn2003>{{cite journal |last=Spohn|first=T.|coauthors=Schubert, G.|title=Oceans in the icy Galilean satellites of Jupiter?|journal=Icarus|year=2003|volume=161|pages=456&ndash;467|doi=10.1016/S0019-1035(02)00048-9| url=http://lasp.colorado.edu/icymoons/europaclass/Spohn_Schubert_oceans.pdf|format=pdf}}</ref> The exact composition of Ganymede's [[Rock (geology)|rock]] is not known, but is probably close to the composition of [[L chondrite|L]]/[[LL chondrite|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 1.05&ndash;1.27 in Ganymede, whereas the [[Sun|solar ratio]] is around 1.8.<ref name=Kuskov2005/> 

Ganymede's surface has an [[albedo]] of about 43%.<ref name=Calvin1995/> Water ice seems to be ubiquitous on the surface, with a mass fraction of 50–90%,<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&ndash;84|doi=10.1126/science.286.5437.77| url=http://www.lpl.arizona.edu/~showman/publications/showman-malhotra-1999.pdf|format=pdf|pmid=10506564}}</ref> significantly more than in Ganymede as a whole. [[Near-infrared]] [[spectroscopy]] has revealed the presence of strong water ice [[absorption band]]s at wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0&nbsp;[[micrometre|μm]].<ref name=Calvin1995>{{cite journal|last=Calvin|first=Wendy M.|coauthors=Clark, Roger N.;Brown, Robert H.; and Spencer John R.|title=Spectra of the ice Galilean satellites from 0.2 to 5 µm: A compilation, new observations, and a recent summary|journal=J.of Geophys. Res.|year=1995|volume=100|pages=19,041&ndash;19,048| url=http://adsabs.harvard.edu/abs/1995JGR...10019041C|doi=10.1029/94JE03349}}</ref> The grooved terrain is brighter and has more icy composition than the dark terrain.<ref name="RESA">{{cite web|url=http://www.resa.net/nasa/ganymede.htm|title=Ganymede: the Giant Moon|work=Wayne RESA|accessdate=2007-12-31}}</ref> The analysis of high-resolution, near-infrared and [[UV]] [[spectrum|spectra]] obtained by the [[Galileo (spacecraft)|''Galileo'']] spacecraft and from the ground has revealed various non-water materials: [[carbon dioxide]], [[sulfur dioxide]] and, possibly, [[cyanogen]], [[hydrogen sulfate]] and various [[organic compound]]s.<ref name=Showman1999/><ref name=McCord1998>{{cite journal|last=McCord|first=T.B.|coauthors=Hansen, G.V.; Clark, R.N. et al.|title=Non-water-ice constituents in the surface material of the icy Galilelean satellites from Galileo near-infrared mapping spectrometer investigation|journal=J. Of Geophys. Res.|year=1998|volume=103|issue=E4| pages=8,603&ndash;8,626|url=http://adsabs.harvard.edu/abs/1998JGR...103.8603M|doi=10.1029/98JE00788}}</ref> ''Galileo'' results have also shown [[magnesium sulfate]] (MgSO<sub>4</sub>) and, possibly, [[sodium sulfate]] (Na<sub>2</sub>SO<sub>4</sub>) on Ganymede's surface.<ref name="The Grand Tour"/><ref name=McCord2001>{{cite journal|last=McCord|first=Thomas B.|coauthors=Hansen, Gary B.; Hibbitts, Charles A.|title=Hydrated Salt Minerals on Ganymede’s Surface: Evidence of an Ocean Below|journal=Science|year=2001|volume=292|pages=1523&ndash;1525| doi=10.1126/science.1059916|url=http://adsabs.harvard.edu/abs/2001Sci...292.1523M|pmid=11375486}}</ref> These salts may originate from the subsurface ocean.<ref name=McCord2001/>

The ganymedian surface is asymmetric; the leading hemisphere&mdash;that facing the direction of the orbital motion{{Ref_label|G|g|none}}&mdash;is brighter than the trailing one.<ref name=Calvin1995/> This is similar to Europa, but the reverse is true on Callisto.<ref name=Calvin1995/> The trailing hemisphere of Ganymede appears to be enriched in sulfur dioxide.<ref name=Domingue1996>{{cite journal|last=Domingue |first=Deborah|coauthors=Lane, Arthur; Moth, Pimol|title= Evidence from IUE for Spatial and Temporal Variations in the Surface Composition of the Icy Galilean Satellites |journal=Bulletin of the American Astronomical Society|year=1996|volume=28|pages=1070|url=http://adsabs.harvard.edu/abs/1996DPS....28.0404D }}</ref><ref name=Domingue1998>{{cite journal|last=Domingue|first=Deborah L.|coauthors= Lane, Arthur L.; Beyer, Ross A.|title=IEU’s detection of tenuous SO2 frost on Ganymede and its rapid time variability|journal=Geophys. Res. Lett.|year=1998|volume=25|issue=16|pages=3,117&ndash;3,120|doi=10.1029/98GL02386| url=http://adsabs.harvard.edu/abs/1998GeoRL..25.3117D}}</ref> The distribution of carbon dioxide does not demonstrate any hemispheric asymmetry, although it is not observed near the poles.<ref name=McCord1998/><ref name=Hibbitts2003>{{cite journal|last=Hibbitts|first=C.A.|coauthors= Pappalardo, R.; Hansen, G.V.; McCord, T.B.|title=Carbon dioxide on Ganymede|journal=J.of Geophys. Res.|year=2003|volume=108|issue=E5|pages=5,036|doi=10.1029/2002JE001956| url=http://adsabs.harvard.edu/abs/2003JGRE..108.5036H}}</ref> [[Impact crater]]s on Ganymede (except one) do not show any enrichment in carbon dioxide, which also distinguishes it from Callisto. Ganymede's carbon dioxide levels were probably depleted in the past.<ref name=Hibbitts2003/>

===Internal structure===
[[Image:Ganymede terrain.jpg|thumb|250px|A sharp boundary divides the dark Nicholson Regio from the bright Harpagia Sulcus]]
Ganymede appears to be fully differentiated, consisting of an [[iron sulfide]]&ndash;[[iron]] [[core (geology)|core]], [[silicate]] [[mantle (geology)|mantle]] and an outer ice mantle.<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|pages=104&ndash;119| doi=10.1006/icar.2002.6828|url=http://adsabs.harvard.edu/abs/2002Icar..157..104S}}</ref> This model is supported by the low value of its dimensionless{{Ref_label|H|h|none}} [[moment of inertia]]&mdash;{{nowrap|0.3105 ± 0.0028}}&mdash;which was measured during ''Galileo'' flybys.<ref name=Showman1999/><ref name=Sohl2002/> In fact, Ganymede has the lowest moment of inertia among the solid solar system bodies. The existence of a liquid, iron-rich core provides a natural explanation for the intrinsic [[magnetosphere|magnetic field]] of Ganymede detected by ''Galileo''.<ref name=Hauk2006/> The [[convection]] in the liquid iron, which has high [[electrical conductivity]], is the most reasonable model of magnetic field generation.<ref name=Kivelson2002/>

The precise thicknesses of the different layers in the interior of Ganymede depend on the assumed composition of silicates (fraction of [[olivine]] and [[pyroxene]]) and amount of [[sulfur]] in the core.<ref name=Kuskov2005/><ref name=Sohl2002/> The most probable values are 700&ndash;900&nbsp;km for the core radius and 800&ndash;1000&nbsp;km for the thickness of the outer ice mantle, with the remainder being made by the silicate mantle.<ref name=Sohl2002/><ref name=Hauk2006/><ref name=Kuskov2005b>{{cite journal|last=Kuskov|first=O.L.|coauthors=Kronrod, V.A.; Zhidicova, A.P.|title=Internal Structure of Icy Satellites of Jupiter|journal=Geophysical Research Abstracts|publisher=European Geosciences Union|year=2005|volume=7|pages=01892|url=http://www.cosis.net/abstracts/EGU05/01892/EGU05-J-01892.pdf |format=pdf}}</ref><ref name=Freeman2006>{{cite journal|last=Freeman|first=J.|title=Non-Newtonian stagnant lid convection and the thermal evolution of Ganymede and Callisto|journal=Planetary and Space Science|year=2006|volume=54|pages=2&ndash;14|doi=10.1016/j.pss.2005.10.003| url=http://bowfell.geol.ucl.ac.uk/~lidunka/EPSS-papers/pete2.pdf|format=pdf}}</ref> The density of the core is 5.5&ndash;6&nbsp;g/cm<sup>3</sup> and the silicate mantle is 3.4&ndash;3.6&nbsp;g/cm<sup>3</sup>.<ref name=Kuskov2005/><ref name=Sohl2002/><ref name=Hauk2006/><ref name=Kuskov2005b/> Some models of the magnetic field generation require the existence of a solid core made of pure iron inside the liquid Fe&ndash;FeS core&mdash;similar to the structure of the Earth's core. The radius of this core may be up to 500&nbsp;km.<ref name=Hauk2006/> The temperature in the core of Ganymede is probably 1500&ndash;1700&nbsp;K and pressure up to 100&nbsp;[[bar (unit)|kBar]] (10 [[Pascal (unit)|Gpa]]).<ref name=Sohl2002/><ref name=Hauk2006/>

===Surface features===
{{seealso|List of geological features on Ganymede}}
[[Image:PIA00081 Ganymede Voyager 2 mosaic.jpg|thumb|250px|''[[Voyager 2]]'' image mosaic of Ganymede's anti-Jovian hemisphere. The ancient dark area of [[Galileo Regio]] lies at the upper right. It is separated from the smaller dark region of Marius Regio to its left by the brighter and younger band of Uruk Sulcus. Fresh ice ejected from the relatively recent Osiris Crater created the bright rays at the bottom.]]
The Ganymedian surface is a mix of two types of terrain: very old, highly [[impact crater|cratered]], dark regions and somewhat younger (but still ancient), lighter regions marked with an extensive array of grooves and ridges. The dark terrain, which comprises about one-third of the surface,<ref name=Petterson2007>{{cite journal|last=Petterson|first=Wesley|coauthors=Head, James W.; Collins, Geoffrey C. et al.|title=A Global Geologic Map of Ganymede|journal=Lunar and Planetary Science|year=2007|volume=XXXVIII|pages=1098| url=http://www.lpi.usra.edu/meetings/lpsc2007/pdf/1098.pdf|format=pdf}}</ref> contains clays and organic materials that could indicate the composition of the impactors from which Jovian satellites accreted.<ref name=Pappalardo2001/>

The heating mechanism required for the formation of the grooved terrain on Ganymede is an unsolved problem in the [[planetary sciences]]. The modern view is that the grooved terrain is mainly [[tectonic]] in nature.<ref name=Showman1999/> [[Cryovulcanism]] is thought to have played only a minor role, if any.<ref name=Showman1999/> The forces that caused the strong stresses in the ganymedian ice [[lithosphere]] necessary to initiate the tectonic activity may be connected to the [[tidal heating]] events in the past, possibly caused when the satellite passed through unstable [[orbital resonance]]s.<ref name=Showman1999/><ref name=Showman1997b>{{cite journal|last=Showman|first=Adam P.|coauthors=Stevenson, David J.; Malhotra, Renu|title=Coupled Orbital and Thermal Evolution of Ganymede|journal=Icarus|year=1997|volume=129|pages=367&ndash;383|doi=10.1006/icar.1997.5778| url=http://www.lpl.arizona.edu/~showman/publications/showman-etal-1997.pdf|format=pdf}}</ref> The tidal flexing of the ice may have heated the interior and strained the lithosphere, leading to the development of cracks and [[horst and graben]] faulting, which erased the old, dark terrain on 70% of the surface.<ref name=Showman1999/><ref name=Bland2007/> The formation of the grooved terrain may also be connected with the early core formation and subsequent tidal heating of the moon's interior, which may have caused a slight expansion of Ganymede by 1&ndash;6% due to [[phase transition]]s in ice and [[thermal expansion]].<ref name=Showman1999/> During subsequent evolution deep, hot water [[Plume (hydrodynamics)|plume]]s may have risen from the core to the surface, leading to the tectonic deformation of the lithosphere.<ref name=Barr>{{cite journal |last=Barr|first=A.C.|coauthors=Pappalardo, R. T. et al.|year=2001|title=Rise of Deep Melt into Ganymede's Ocean and Implications for Astrobiology|journal=Lunar and Planetary Science Conference|volume=32 |url=http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1781.pdf|pages=1781|format=pdf}}</ref> [[Radioactive decay|Radiogenic heating]] within the satellite is the most relevant current heat source, contributing, for instance, to thickness of the ocean. Research models have found that if the orbital eccentricity were an order of magnitude greater than current (as it may have been in the past), tidal heating would be a more substantial heat source than radiogenic heating.<ref>{{cite journal|last=Huffmann|first=H. |coauthors=Sohl, F. et al.|year=2004|title=Internal Structure and Tidal Heating of Ganymede |journal=European Geosciences Union, Geophysical Research Abstracts|volume=6 |url=http://www.cosis.net/abstracts/EGU04/05114/EGU04-J-05114.pdf|format=PDF}}</ref>

[[Image:Craters on Ganymede.jpg|thumb|Fresh impact craters on the grooved terrain of Ganymede]]
Cratering is seen on both types of terrain, but is especially extensive on the dark terrain: it appears to be saturated with impact craters and has evolved largely through impact events.<ref name=Showman1999/> The brighter, grooved terrain contains many fewer impact features, which have been only of a minor importance to its tectonic evolution.<ref name=Showman1999/> The density of cratering indicates an age of 4 billion years for the dark terrain, similar to the highlands of the [[Moon]], and a somewhat younger age for the grooved terrain (but how much younger is uncertain).<ref name=Zahnle1998>{{cite journal|last=Zahnle|first=K.|coauthors=Dones, L. |title=Cratering Rates on the Galilean Satellites|journal=Icarus|year=1998|volume=136|pages=202&ndash;222|doi=10.1006/icar.1998.6015| url=http://lasp.colorado.edu/icymoons/europaclass/Zahnle_etal_1998.pdf|format=pdf}}</ref> Ganymede may have experienced a period of heavy cratering 3.5 to 4 billion years ago similar to that of the Moon.<ref name=Zahnle1998/> If true, the vast majority of impacts happened in that epoch, while the cratering rate has been much smaller since.<ref name="nineplanets.org-Ganymede"/> Craters both overlay and are crosscut by the groove systems, indicating that some of the grooves are quite ancient. Relatively young craters with rays of ejecta are also visible.<ref name="nineplanets.org-Ganymede"/><ref name="Ganymede">{{cite web|work=Lunar and Planetary Institute|title=Ganymede |year=1997|url=http://www.lpi.usra.edu/resources/outerp/gany.html}}</ref> Ganymedian craters are flatter than those on the Moon and Mercury. This is probably due to the relatively weak nature of Ganymede's icy crust, which can (or could) flow and thereby soften the relief. Ancient craters whose relief has disappeared leave only a "ghost" of a crater known as a  [[Palimpsest#Extended usages|palimpsest]].<ref name="nineplanets.org-Ganymede"/>

One significant feature on Ganymede is a dark plain named Galileo Regio, which contains a series of concentric grooves, or furrows, likely created during a period of geologic activity.<ref name="Casacchia">{{cite journal|last=Casacchia|first=R.|coauthors=Strom, R.G.|year=1984|title=Geologic evolution of Galileo Regio|journal=Journal of Geophysical Research|volume=89|pages=B419&ndash;B428|bibcode= 1984LPSC...14..419C|url=http://adsabs.harvard.edu/abs/1984JGRS...89..419C|doi=10.1029/JB089iS02p0B419}}</ref> Another prominent feature on Ganymede are polar caps, likely composed of water frost. The frost extends to 40° latitude.<ref name="The Grand Tour"/> These polar caps were first seen by the [[Voyager program|''Voyager'']] spacecraft. Theories on the caps' formation include the migration of water to higher latitudes and bombardment by plasma of the ice. Data from ''Galileo'' suggests the latter is correct.<ref name="Polar caps">{{cite journal|last=Khurana|first=Krishan K.|coauthors=Pappalardo, Robert T.; Murphy, Nate; Denk, Tilmann|year=2007|title=The origin of Ganymede's polar caps|journal= Icarus|volume=191|issue=1|pages=193&ndash;202|doi=10.1016/j.icarus.2007.04.022|url= http://adsabs.harvard.edu/abs/2007Icar..191..193K}}</ref>

===Atmosphere and ionosphere===
In 1972, a team of Indian, British and American astronomers working at [[Indonesia]]'s [[Bosscha Observatory]] claimed that they had detected a thin atmosphere around the satellite during an [[occultation]], when it and Jupiter passed in front of a star.<ref name=Carlson1973>{{cite journal|last=Carlson|first=R.W.|coauthors=Bhattacharyya, J.C.; Smith, B.A. et al.|title=Atmosphere of Ganymede from its occultation of SAO 186800 on 7 June 1972|journal=Science|year=1973|volume=53|pages=182|url=http://adsabs.harvard.edu/abs/1973Sci...182...53C}}</ref> They estimated that the surface pressure was around 1&nbsp;[[Bar (unit)|μBar]] (0.1&nbsp;[[Pascal (unit)|Pa]]).<ref name=Carlson1973/> However, in 1979 ''[[Voyager 1]]'' observed an occultation of a star ([[Kappa Centauri|&kappa; Centauri]]) during its flyby of the planet, with differing results.<ref name=Broadfoot1981>{{cite journal|last=Broadfoot|first=A.L.|coauthors=Sandel, B.R.; Shemansky, D.E. et al.|title=Overview of the Voyager Ultraviolet Spectrometry Results through Jupiter Encounter|journal=Science|year=1981|volume=86|pages=8259&ndash;8284| url=http://www-personal.umich.edu/~atreya/Articles/1981_Overview_Voyager.pdf|format=pdf}}</ref> The occultation measurements were conducted in the [[far-ultraviolet]] spectrum with [[wavelength]] shorter than 200&nbsp;[[nanometre|nm]]; they were much more sensitive to the presence of gases than measurements in the [[visible spectrum]] in 1972. No atmosphere was revealed by the ''Voyager'' data. The upper limit on the surface particle [[number density]] was found to be {{nowrap|1.5{{Esp|9}} cm<sup>−3</sup>}}, which corresponds to a surface pressure of less than {{nowrap|2.5{{Esp|&minus;5}} μBar}}.<ref name=Broadfoot1981/> The latter value is almost five orders of magnitude less than that measured in 1972, indicating that the earlier interpretation was too optimistic.<ref name=Broadfoot1981/> 

Despite the ''Voyager'' data, evidence for a tenuous [[oxygen]] atmosphere on Ganymede, very similar to the one found on [[Europa (moon)|Europa]], was found by the [[Hubble Space Telescope]] (HST) in 1995.<ref name=Hall1998/><ref name=JPLAtmosphere>{{cite web |url=http://www2.jpl.nasa.gov/galileo/hst7.html|title=Hubble Finds Thin Oxygen Atmosphere on Ganymede |accessdate=2008-01-15 |work=Jet Propulsion Laboratory|publisher=NASA|month=October|year=1996}}</ref> HST actually observed [[airglow]] of [[atomic oxygen]] in the far-ultraviolet at the wavelengths 130.4&nbsp;nm and 135.6&nbsp;nm. Such an airglow is excited when [[molecular oxygen]] is [[dissociation (chemistry)|dissociated]] by electron impacts,<ref name=Hall1998/> evidence of a significant neutral atmosphere composed predominantly of O<sub>2</sub> molecules. The surface number density probably lies in the {{nowrap|1.2&ndash;7{{Esp|8}} cm<sup>&minus;3</sup>}} range, corresponding to the surface pressure of {{nowrap|0.2&ndash;1.2{{Esp|&minus;5}} μBar}}.<ref name=Hall1998>{{cite journal|last=Hall|first=D.T.|coauthors=Feldman, P.D.; McGrath, M.A. et al.|title=The Far-Ultraviolet Oxygen Airglow of Europa and Ganymede|journal=The Astrophysical Journal|year=1998|volume=499|pages=475&ndash;481| doi=10.1086/305604| url=http://adsabs.harvard.edu/abs/1998ApJ...499..475H}}</ref>{{Ref_label|I|i|none}} These values are in agreement with the ''Voyager''<nowiki>'</nowiki>s upper limit set in 1981. The oxygen is not evidence of life; it is thought to be produced when water ice on Ganymede's surface is split into [[hydrogen]] and oxygen by radiation, with the hydrogen then being more rapidly lost due to its low atomic mass.<ref name=JPLAtmosphere/> The airglow observed over Ganymede is not spatially homogeneous like that over Europa. HST observed two bright spots located in the northern and southern hemispheres, near ±&nbsp;50° latitude, which is exactly the boundary between the open and closed field lines of the ganymedian magnetosphere (see below).<ref name=Feldman2000/> The bright spots are probably polar [[aurora (astronomy)|auroras]], caused by plasma precipitation along the open field lines.<ref name=Johnson1997>{{cite journal |last=Johnson |first=R.E.|year=1997|title=Polar “Caps” on Ganymede and Io Revisited|journal=Icarus| volume=128|issue=2|pages=469&ndash;471|url=http://adsabs.harvard.edu/abs/1997Icar..128..469J| doi=10.1006/icar.1997.5746}}</ref>

[[Image:Map of temparatureof ganymede.jpg|thumb|False color temperature map of Ganymede]]
The existence of a neutral atmosphere implies that an [[ionosphere]] should exist, because oxygen molecules are ionized by the impacts of the energetic [[electron]]s coming from the magnetosphere<ref name=Paranicas1999/> and by solar [[EUV]] radiation.<ref name=Eviatar2001>{{cite journal|last=Eviatar|first=Aharon|coauthors=Vasyliunas, Vytenis M.; Gurnett, Donald A. et al.|title=The ionosphere of Ganymede|journal=Plan.Space Sci.|year=2001|volume=49|pages=327&ndash;336| doi=10.1016/S0032-0633(00)00154-9|url=http://www.tau.ac.il/~arkee/ganymop.ps|format=ps}}</ref> However, the nature of the ganymedian ionosphere is as controversial as the nature of the atmosphere. Some ''Galileo'' measurements found an elevated electron density near the moon, suggesting an ionosphere, while others failed to detect anything.<ref name=Eviatar2001/> The electron density near the surface is estimated by different sources to lie in the range 400&ndash;2,500&nbsp;cm<sup>−3</sup>.<ref name=Eviatar2001/> As of 2008, the parameters of the ionosphere of Ganymede are not well constrained.

Additional evidence of the oxygen atmosphere comes from spectral detection of gases trapped in the ice at the surface of Ganymede.  The detection of [[ozone]] (O<sub>3</sub>) bands was announced in 1996.<ref name=Noll1996>{{cite journal |last= Noll|first= Keith S. |coauthors= Johnson, Robert E. et al.|year=1996|month=July|title=Detection of Ozone on Ganymede |journal= Science|volume=273|issue=5273|pages=341&ndash;343|url=http://www.sciencemag.org/cgi/content/abstract/273/5273/341|accessdate= 2008-01-13|doi=10.1126/science.273.5273.341|pmid=8662517}}</ref> In 1997 spectroscopic analysis revealed the [[dimer]] (or [[diatomic]]) absorption features of the [[molecular oxygen]]. Such an absorption can arise only if the oxygen is in a dense phase. The best candidate is the molecular oxygen trapped in ice. The depth of the dimer absorption bands depends on [[latitude]] and [[longitude]], rather than on surface [[albedo]]&mdash;they tend to decrease with increasing latitude on Ganymede, while the O<sub>3</sub> shows an opposite effect.<ref name=Oxygen97>{{cite journal|last=Calvin|first=Wendy M.|coauthors=Spencer, John R.|month=December|year=1997 |title=Latitudinal Distribution of O2on Ganymede: Observations with the Hubble Space Telescope|journal=Icarus|volume=130 |issue=2|pages=505–516|doi=10.1006/icar.1997.5842|url=http://adsabs.harvard.edu/abs/1997Icar..130..505C}}</ref> Laboratory work has found that O<sub>2</sub> would not cluster and bubble but dissolve in ice at Ganymede's relatively warm surface temperature of 100 K.<ref>{{cite journal|last=Vidal|first=R. A.|coauthors=Bahr, D. et al.|year=1997|title=Oxygen on Ganymede: Laboratory Studies |journal=Science|volume=276| issue=5320|pages=1839&ndash;1842|url=http://adsabs.harvard.edu/abs/1997Sci...276.1839V| doi=10.1126/science.276.5320.1839|pmid=9188525}}</ref>

A search for [[sodium]] in the atmosphere, just after such a finding on Europa, turned up nothing in 1997. Sodium is at least 13 times less abundant around Ganymede than around Europa, possibly because of a relative deficiency at the surface or because the magnetosphere fends off energetic particles.<ref>{{cite journal |last=Brown|first=Michael E.|year=1997|title=A Search for a Sodium Atmosphere around Ganymede|journal=Icarus|volume=126|issue=1|pages=236&ndash;238  |url=http://adsabs.harvard.edu/abs/1997Icar..126..236B|doi=10.1006/icar.1996.5675}}</ref> Another minor constituent of the ganymedian atmosphere is [[atomic hydrogen]]. Hydrogen atoms were observed as far as 3,000&nbsp;km from the surface of the moon. Their density on the surface is about {{nowrap|1.5{{Esp|4}}&nbsp;cm<sup>−3</sup>}}.<ref name=Barth1997>{{cite journal|last=Barth|first=C.A.|coauthors=Hord, C.W.; Stewart, A.I. et al.|title=Galileo ultraviolet spectrometer observations of atomic hydrogen in the atmosphere of Ganymede|journal=Geophys. Res. Lett.|year=1997|volume=24|issue=17|pages=2147&ndash;2150| url=http://adsabs.harvard.edu/abs/1997GeoRL..24.2147B|doi=10.1029/97GL01927}}</ref>

{{Atmospheres}}

===Magnetosphere===
[[Image:Ganymede-moon.jpg|thumb|Enhanced-color ''Galileo'' spacecraft image of Ganymede's trailing hemisphere<ref name="Spaceflight Now">{{cite web|url=http://spaceflightnow.com/news/n0012/29ganyflyby/|title=Galileo has successful flyby of Ganymede during eclipse|work=Spaceflight Now|accessdate=2008-01-19}}</ref>]]
The ''Galileo'' craft made six close flybys of Ganymede from 1995–2000 (G1, G2, G7, G8, G28 and G29)<ref name=Kivelson2002>{{cite journal|last=Kivelson|first=M.G.|coauthors=Khurana, K.K.; Coroniti, F.V. et al.|title=The Permanent and Inductive Magnetic Moments of Ganymede |journal=Icarus|year=2002|volume=157|pages=507&ndash;522|doi=10.1006/icar.2002.6834| url=http://www.igpp.ucla.edu/people/mkivelson/Publications/ICRUS1572507.pdf|format=pdf}}</ref> and discovered that Ganymede has a permanent (intrinsic) [[magnetic moment]] independent of the Jovian magnetic field.<ref name=Kivelson1997>{{cite journal|last=Kivelson|first=M.G.|coauthors=Khurana, K.K.; Coroniti, F.V. et al.|title=The magnetic field and magnetosphere of Ganymede|journal=Geophys. Res. Lett.|year=1997|volume=24|issue=17|pages=2155&ndash;2158|doi=10.1029/97GL02201| url=http://www.igpp.ucla.edu/people/mkivelson/Publications/97GL02201.pdf|format=pdf}}</ref> The value of the moment is about {{nowrap|1.3{{Esp|13}} T·m<sup>3</sup>}},<ref name=Kivelson2002/> which is three times larger than the magnetic moment of [[Mercury (planet)|Mercury]]. The magnetic dipole is tilted with respect to the rotational axis of Ganymede by 176°, which means that it is directed against the Jovian magnetic moment.<ref name=Kivelson2002/> Its north pole lies below the [[Orbital plane (astronomy)|orbital plane]]. The [[dipole|dipole magnetic field]] created by this permanent moment has a strength of 719&nbsp;±&nbsp;2&nbsp;[[Tesla (unit)|nT]] at the equator of the moon,<ref name=Kivelson2002/> which should be compared with the Jovian magnetic field at the distance of Ganymede&mdash;about 120&nbsp;nT.<ref name=Kivelson1997/> The equatorial field of Ganymede is directed against the Jovian field, meaning [[magnetosphere|reconnection]] is possible. The intrinsic field strength at the poles is two times that at the equator&mdash;1440&nbsp;nT.<ref name=Kivelson2002/>

The permanent magnetic moment carves a part of space around Ganymede, creating a tiny [[magnetosphere]] embedded inside [[Jupiter's magnetosphere|that of Jupiter]]; it is the only moon in the Solar System known to possess the feature.<ref name=Kivelson1997/> Its diameter is 4&ndash;5&nbsp;R<sub>G</sub> (R<sub>G</sub>&nbsp;=&nbsp;2,631.2&nbsp;km).<ref name=Kivelson1998>{{cite journal|last=Kivelson|first=M.G.|coauthors=Warnecke, J.; Bennett, L. et al.|title=Ganymede’s magnetosphere: magnetometer overview|journal=J.of Geophys. Res.|year=1998|volume=103|issue=E9|pages=19,963&ndash;19,972|doi=10.1029/98JE00227| url=http://www.igpp.ucla.edu/people/mkivelson/Publications/98JE00227.pdf|format=pdf}}</ref> The ganymedian magnetosphere has a region of closed [[field line]]s located below 30° [[latitude]], where [[charged particle]]s ([[electron]]s and [[ion]]s) are trapped, creating a kind of [[radiation belt]].<ref name=Kivelson1998/> The main ion species in the magnetosphere is single ionized [[oxygen]]&mdash;O<sup>+</sup><ref name=Eviatar2001>{{cite journal|last=Eviatar|first=Aharon|coauthors=Vasyliunas, Vytenis M.; Gurnett, Donald A. et al.|title=The ionosphere of Ganymede|journal=Plan. Space Sci.|year=2001|volume=49|pages=327&ndash;336| doi=10.1016/S0032-0633(00)00154-9| url=http://adsabs.harvard.edu/abs/2001P%26SS...49..327E}}</ref>&mdash;which fits well with the tenuous oxygen [[atmosphere]] of the moon. In the polar cap regions, at latitudes higher than 30°, magnetic field lines are open, connecting Ganymede with Jupiter's [[ionosphere]].<ref name=Kivelson1998/> In these areas, the energetic (tens and hundreds of [[kiloelectronvolt|keV]]) electrons and ions have been detected,<ref name=Paranicas1999/> which may be responsible for the [[aurora (astronomy)|aurora]]s observed around the ganymedian poles.<ref name=Feldman2000>{{cite journal|last=Feldman|first=Paul D.|title=HST/STIS Ultraviolet Imaging of Polar Aurora on Ganymede|journal=The Astrophysical Journal|year=2000|volume=535|pages=1085&ndash;1090| doi=10.1086/308889|url=http://adsabs.harvard.edu/abs/2000ApJ...535.1085F|coauthors=McGrath, Melissa A.; Strobell, Darrell F. et al.}}</ref> In addition, heavy ions continuously precipitate on the polar surface of the moon, [[sputtering]] and darkening the ice.<ref name=Paranicas1999>{{cite journal|last=Paranicas|first=C.|coauthors=Paterson, W.R.; Cheng, A.F. et al.|title=Energetic particles observations near Ganymede|journal=J.of Geophys.Res.|year=1999|volume=104|issue=A8|pages=17,459&ndash;17,469| doi=10.1029/1999JA900199|url=http://adsabs.harvard.edu/abs/1999JGR...10417459P}}</ref>

[[Image:Ganymede_magnetic_field.jpg#Summary|thumb|Magnetic field of the Jovian satellite Ganymede, which is embedded into the magnetosphere of Jupiter. Closed field lines are marked with green color.]]
The interaction between the ganymedian magnetosphere and Jovian [[Plasma (physics)|plasma]] is in many respects similar to that of the [[solar wind]] and Earth's magnetosphere.<ref name=Kivelson1998/><ref name=Volwerk1999/> The plasma co-rotating with Jupiter impinges on the trailing side of the ganymedian magnetosphere much like the solar wind impinges on the Earth's magnetosphere. The main difference is the speed of plasma flow&mdash;[[supersonic]] in the case of Earth and [[Speed of sound|subsonic]] in the case of Ganymede. Because of the subsonic flow, there is no [[bow shock]] off the trailing hemisphere of Ganymede.<ref name=Volwerk1999>{{cite journal|last=Volwerk|first=M.|coauthors=Kivelson, M.G.; Khurana, K.K.; McPherron, R.L.|title= Probing Ganymede’s magnetosphere with field line resonances|journal=J.of Geophys. Res.|year=1999|volume=104|issue=A7| pages=14,729&ndash;14,738|doi=10.1029/1999JA900161| url=http://www.igpp.ucla.edu/people/mkivelson/Publications/1999JA900161.pdf|format=pdf}}</ref>

In addition to the intrinsic magnetic moment, Ganymede has an induced dipole magnetic field.<ref name=Kivelson2002/> Its existence is connected with the variation of the Jovian magnetic field near the moon. The induced moment is directed radially to or from Jupiter following the direction of the varying part of the planetary magnetic field. The induced magnetic moment is an order of magnitude weaker than the intrinsic one. The [[field strength]] of the induced field at the magnetic equator is about 60&nbsp;nT&mdash;half of that of the ambient Jovian field.<ref name=Kivelson2002/> The induced magnetic field of Ganymede is similar to those of [[Callisto (moon)|Callisto]] and [[Europa (moon)|Europa]], indicating that this moon also has a subsurface water ocean with a high [[electrical conductivity]].<ref name=Kivelson2002/>

Given that Ganymede is completely differentiated and has a metallic core,<ref name=Showman1999/><ref name=Hauk2006>{{cite journal|last=Hauk|first=Steven A.|coauthors=Aurnou, Jonathan M.; Dombard, Andrew J.|title=Sulfur’s impact on core evolution and magnetic field generation on Ganymede|journal=J. Of Geophys. Res.|year=2006|volume=111|pages=E09008| doi=10.1029/2005JE002557|url=http://geology.case.edu/~hauck/papers/hauck_jgr_2006.pdf|format=pdf}}</ref> its intrinsic magnetic field is probably generated in a similar fashion to the Earth's: as a result of conducting material moving in the interior.<ref name=Kivelson2002/><ref name=Hauk2006/> The magnetic field detected around Ganymede is likely to be caused by compositional convection in the core,<ref name=Hauk2006/> if the magnetic field is the product of dynamo action, or magnetoconvection.<ref name=Kivelson2002/><ref name=Hauck2002>{{cite journal|last=Hauck|first=Steven A.|title=Internal structure and mechanism of core convection on Ganymede|journal=Lunar and Planetary Science|volume=XXXIII|year=2002|pages=1380|url=http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1380.pdf|format=pdf}}</ref> 

Despite the presence of an iron core, Ganymede's magnetosphere remains enigmatic, particularly given that similar bodies lack the feature.<ref name=Showman1999/> Some research has suggested that, given its relatively small size, the core ought to have sufficiently cooled to the point where fluid motions and a magnetic field would not be sustained. One explanation is that the same orbital resonances proposed to have disrupted the surface also allowed the magnetic field to persist: with Ganymede's eccentricity pumped and tidal heating increased during such resonances, the mantle may have insulated the core, preventing it from cooling.<ref name=Bland2007>{{cite journal|last=Bland|first=|coauthors=Showman, A.P.; Tobie, G.|title=Ganymede's orbital and thermal evolution and its effect on magnetic field generation|journal=Lunar and Planetary Society Conference|month=March|year=2007|volume=38|pages=2020| url=http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2020.pdf|format=pdf}}</ref> Another explanation is a remnant magnetization of silicate rocks in the mantle, which is possible if the satellite had a more significant dynamo-generated field in the past.<ref name=Showman1999/>

==Origin and evolution==
Ganymede likely formed by an [[accretion (astrophysics)|accretion]] in Jupiter’s [[solar nebula|subnebula]], a disk of gas and dust surrounding Jupiter after its formation.<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|pages=3404&ndash;3423|doi=10.1086/344684| url=http://www.boulder.swri.edu/~robin/cw02final.pdf|format=pdf | journal = The Astronomical Journal}}</ref> The accretion of Ganymede probably took about 10&nbsp;000 years,<ref name=Mosqueira2003>{{cite journal|last=Mosqueira|first=Ignacio|coauthors=Estrada, Paul R|title=Formation of the regular satellites of giant planets in an extended gaseous nebula I: subnebula model and accretion of satellites|year=2003|volume=163|pages=198&ndash;231| doi=10.1016/S0019-1035(03)00076-9|url=http://adsabs.harvard.edu/abs/2003Icar..163..198M | journal = Icarus}}</ref> much shorter than the 100&nbsp;000 years estimated for Callisto. The Jovian subnebula may have been relatively "gas-starved" when the Galilean satellites formed; this would have allowed for the lengthy accretion times required for Callisto.<ref name=Canup2002/> In contrast Ganymede formed closer to Jupiter, where the subnebular was denser, which explains its shorter formation timescale.<ref name=Mosqueira2003/> This relatively fast formation prevented the escape of accretional heat, which led to ice melt and [[Planetary differentiation|differentiation]]: the separation of the rocks and ice. The rocks settled to the center, forming the core. In this respect, Ganymede is different from Callisto, which failed to melt and differentiate early due to loss of the accretional heat during its slower formation.<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|pages=435&ndash;450|doi=10.1016/j.icarus.2006.03.004| url=http://adsabs.harvard.edu/abs/2006Icar..183..435M | journal = Icarus}}</ref> This hypothesis explains why the two Jovian moons look so dissimilar, despite their similar mass and composition.<ref name=Freeman2006/><ref name=McKinnon2006/> 

After formation, the ganymedian core largely retained the heat accumulated during accretion and differentiation, only slowly releasing it to the ice mantle like a kind thermal battery.<ref name=McKinnon2006/> The mantle, in turn, transported it to the surface by convection.<ref name=Freeman2006/> Soon the decay of [[radioactivity|radioactive elements]] within rocks further heated the core, causing increased differentiation: an inner, [[iron]]&ndash;[[iron sulfide]] core and a [[silicate]] mantle formed.<ref name=Hauk2006/><ref name=McKinnon2006/> With this, Ganymede became a fully differentiated body. By comparison, the radioactive heating of undifferentiated Callisto caused convection in its icy interior, which effectively cooled it and prevented large-scale melting of ice and rapid differentiation.<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|pages=402&ndash;412|doi=10.1016/j.icarus.2003.12.019| url=http://adsabs.harvard.edu/abs/2004Icar..169..402N | journal = Icarus}}</ref> The convective motions in Callisto have caused only a partial separation of rock and ice.<ref name=Nagel2004/> Today, Ganymede continues to cool slowly.<ref name=Hauk2006/> The heat being released from its core and silicate mantle enables the subsurface ocean to exist,<ref name=Spohn2003/> while the slow cooling of the liquid Fe&ndash;FeS core causes convection and supports magnetic field generation.<ref name=Hauk2006/> The current [[heat flux]] out of Ganymede is probably higher than that out of Callisto.<ref name=McKinnon2006/>

==Exploration==
[[Image:Voyager.jpg|thumb|The [[Voyager spacecraft|''Voyager'' spacecraft]]]]
Several probes flying by or orbiting Jupiter have explored Ganymede in detail. The first probes to explore were ''[[Pioneer 10]]'' and ''[[Pioneer 11]]'',<ref name="Pioneer 11">{{cite web|url=http://sse.jpl.nasa.gov/missions/profile.cfm?Sort=Advanced&MCode=Pioneer_11|title=Pioneer 11|work=Solar System Exploration|accessdate=2008-01-06}}</ref> neither of which returned much information about the satellite.<ref name="Terraformers">{{cite web|url=http://society.terraformers.ca/content/view/63/112/|title=Exploration of Ganymede|work=Terraformers Society of Canada|accessdate=2008-01-06}}</ref> ''[[Voyager 1]]'' and ''[[Voyager 2]]'' were next, passing by Ganymede in 1979. They refined its size, revealing it was larger than [[Saturn]]'s moon [[Titan (moon)|Titan]], which was previously thought to have been bigger.<ref name="Voyager">{{cite web|url=http://library.thinkquest.org/J0112188/voyager_1_and_2.htm|title=Voyager 1 and 2|work=ThinkQuest|accessdate=2008-01-06}}</ref> The grooved terrain was also seen.<ref name="Voyager Mission">{{cite web|url=http://www.solarviews.com/eng/vgrfs.htm|title=The Voyager Planetary Mission|work=Views of the Solar System|accessdate=2008-01-06}}</ref> 

In 1995, the ''[[Galileo (spacecraft)|Galileo]]'' spacecraft entered orbit around Jupiter and between 1996 and 2000 made six close flybys to explore Ganymede.<ref name="The Grand Tour"/> These flybys are G1, G2, G7, G8, G28 and G29.<ref name=Kivelson2002/> During the closest flyby&mdash;G2&mdash;''Galileo'' passed just 264&nbsp;km from the surface of Ganymede.<ref name=Kivelson2002/> During a G1 flyby in 1996, the ganymedian magnetic field was discovered,<ref name="Magnetic Field Discovery">{{cite web|url=http://www2.jpl.nasa.gov/galileo/status961212.html|title=New Discoveries From Galileo|work=Jet Propulsion Laboratory|accessdate=2008-01-06}}</ref> while the discovery of the ocean was announced in 2001.<ref name=Kivelson2002/><ref name="The Grand Tour"/> ''Galileo'' transmitted a large number of spectral images and discovered several non-ice compounds on the surface of Ganymede.<ref name=McCord1998/> The most recent spacecraft to explore Ganymede up close was ''[[New Horizons]]'', which passed by in 2007 on its way to [[Pluto]]. ''New Horizons'' made topography and composition maps of Ganymede as it sped by.<ref name="New Horizons">{{cite web|url=http://www.spacedaily.com/reports/Pluto_Bound_New_Horizons_Spacecraft_Gets_A_Boost_From_Jupiter_999.html|title=Pluto-Bound New Horizons Spacecraft Gets A Boost From Jupiter|work=Space Daily|accessdate=2008-01-06}}</ref><ref name=Grundy2007>{{cite journal|last=Grundy|first=W.M.|coauthors=Buratti, B.J.; Cheng, A.F. et al.|title=New Horizons Mapping of Europa and Ganymede| journal=Science|year=2007|volume=318|pages=234&ndash;237|doi=10.1126/science.1147623| url=http://adsabs.harvard.edu/abs/2007Sci...318..234G|pmid=17932288}}</ref>

Proposed for a launch in [[2020]], the [[Europa Jupiter System Mission]] (EJSM) is 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 web|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 will still face 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&ndash;2025 Proposals|date=2007-07-21|publisher=ESA|accessdate=2009-02-20}}</ref> EJSM consists of the NASA-led [[Jupiter Europa Orbiter]], the ESA-led [[Jupiter Ganymede Orbiter]], and possibly a [[JAXA]]-led [[Jupiter Magnetospheric Orbiter]].

One canceled proposal to orbit Ganymede was the [[Jupiter Icy Moons Orbiter]]. [[Nuclear fission]] would have been used to power the craft, which would have been able to study Ganymede in detail.<ref name="JIMO">{{cite web|url=http://www.daviddarling.info/encyclopedia/J/JIMO.html|title=Jupiter Icy Moons Orbiter (JIMO)|work=The Internet Encyclopedia of Science|accessdate=2008-01-06}}</ref> However, the mission was canceled in 2005 because of budget cuts.<ref name="JIMO cancellation">{{cite web|url=http://www.planetsurveyor.com/latest-space-exploration-news/jupiter-icy-moons-orbiter-victim-of-budget-cut.html|title=Jupiter Icy Moons Orbiter Victim of Budget Cut|work=Planet Surveyor|accessdate=2008-01-06}}</ref> Another old proposal was called The Grandeur of Ganymede.<ref name=Pappalardo2001>{{cite journal|last=Pappalardo|first=R.T.|coauthors=Khurana, K.K.; Moore, W.B.|title= The Grandeur of Ganymede: Suggested Goals for an Orbiter Mission|journal=Lunar and Planetary Science|year=2001|volume=XXXII|pages=4062|url=http://www.lpi.usra.edu/meetings/outerplanets2001/pdf/4065.pdf| format=pdf}}</ref>

== See also ==
{{portal|Solar System|Solar system.jpg}}
* [[Colonization of the outer Solar System#Ganymede|Colonization of Ganymede]]
* [[Jupiter's moons in fiction]]
* [[List of craters on Ganymede]]
* [[List of geological features on Ganymede]]
* [[Lunar and Planetary Institute]]

== Notes ==
<div class="references-small">
<ol type="a">
<li>{{Note_label|A|a|none}} Apoapsis is derived from semimajor axis ''a'' and eccentricity ''e'': <math>a*(1+e)</math>'''.
<li>{{Note_label|B|b|none}} Periapsis is derived from semimajor axis ''a'' and eccentricity ''e'': <math>a*(1-e)</math>'''.
<li>{{Note_label|C|c|none}} Surface area derived from the radius ''r'': <math>4\pi r^2</math>'''.
<li>{{Note_label|D|d|none}} Volume ''v'' derived from the radius ''r'': <math>4\pi r^3/3</math>'''. 
<li>{{Note_label|E|e|none}} Surface gravity derived from the mass ''m'', the [[gravitational constant]] ''G'' and the radius ''r'': <math>Gm/r^2</math>. 
<li>{{Note_label|F|f|none}} Escape velocity derived from the mass ''m'', the [[gravitational constant]] ''G'' and the radius ''r'': <math>\textstyle\sqrt{\frac{2Gm}{r}}</math>.
<li>{{Note_label|G|g|none}} Leading hemisphere is a hemisphere looking in the direction of the orbital motion, the trailing hemisphere looks in the reverse direction.
<li>{{Note_label|H|h|none}} The dimensionless moment of inertia referred to is I/(mr²), where I 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.
<li>{{Note_label|I|i|none}} The surface number density and pressure were calculated from the column densities reported in Hall, et al. 1998, assuming a [[scale height]] of 20&nbsp;km and temperature 120&nbsp;K.
<li>{{Note_label|J|j|none}}Laplace-like resonance is similar to the current Laplace resonance among Galilean moons with the only difference that longitudes of the Io&ndash;Europa and Europa&ndash;Ganymede conjunctions change with rates, whose ratio is a rational number&mdash;not unity as in the case of the Laplace resonance</li>
</ol>
</div>

==References==
{{Reflist|2}}

==External links==
{{commonscat|Ganymede (moon)}}
*[http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jup_Ganymede Ganymede Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
*[http://www.nineplanets.org/ganymede.html Ganymede page] on ''The <strike>Nine</strike>8 Planets''
*[http://www.solarviews.com/eng/ganymede.htm Jupiter's Moon Ganymede] on ''Views of the Solar System''
*[http://www.lpi.usra.edu/resources/gc/gchome.shtml Ganymede Crater Database] Lunar and Planetary Institute

{{Moons of Jupiter}}
{{Solar System moons (compact)}}
{{Jupiter}}
{{Solar System}}

{{Featured article}}

[[Category:Moons of Jupiter]]

{{Link FA|it}}
{{Link FA|zh}}
{{Link FA|vi}}

[[af:Ganymedes]]
[[als:Ganymed (Mond)]]
[[ar:جانيميد]]
[[frp:Ganimède (satèlite)]]
[[bs:Ganimed (mjesec)]]
[[br:Ganymede (loarenn)]]
[[bg:Ганимед (спътник)]]
[[ca:Ganimedes (satèl·lit)]]
[[cs:Ganymed (měsíc)]]
[[co:Ganimede]]
[[da:Ganymedes (måne)]]
[[de:Ganymed (Mond)]]
[[et:Ganymedes (kuu)]]
[[el:Γανυμήδης (δορυφόρος)]]
[[es:Ganímedes (satélite)]]
[[eo:Ganimedo (luno)]]
[[eu:Ganimede]]
[[fa:گانمید (ماه)]]
[[fr:Ganymède (lune)]]
[[ga:Ganymede (gealach)]]
[[gl:Ganímedes (lúa)]]
[[ko:가니메데]]
[[hr:Ganimed (mjesec)]]
[[io:Ganimede]]
[[id:Ganymede]]
[[is:Ganýmedes (tungl)]]
[[it:Ganimede (astronomia)]]
[[he:גנימד]]
[[la:Ganymedes (satelles)]]
[[lv:Ganimēds (pavadonis)]]
[[lt:Ganimedas (palydovas)]]
[[hu:Ganümédész (hold)]]
[[mk:Ганимед]]
[[mr:गॅनिमीड (उपग्रह)]]
[[nl:Ganymedes (maan)]]
[[ja:ガニメデ (衛星)]]
[[no:Ganymedes (måne)]]
[[nn:Jupitermånen Ganymede]]
[[nds:Ganymed (Maand)]]
[[pl:Ganimedes (księżyc)]]
[[pt:Ganímedes (satélite)]]
[[ro:Ganymede (satelit)]]
[[ru:Ганимед (спутник)]]
[[simple:Ganymede (moon)]]
[[sk:Ganymedes (mesiac)]]
[[sl:Ganimed (luna)]]
[[sr:Ганимед (сателит)]]
[[sh:Ganimed (mjesec)]]
[[fi:Ganymedes]]
[[sv:Ganymedes (måne)]]
[[tt:Ганимед (юлдаш)]]
[[th:แกนีมีด]]
[[tg:Ганимед (радиф)]]
[[tr:Ganymede (uydu)]]
[[ur:گانیمید]]
[[vi:Ganymede (vệ tinh)]]
[[zh-classical:木衛三]]
[[wuu:木卫三]]
[[zh-yue:木衛三]]
[[zh:木卫三]]