Editing Ganymede (moon) (section)

==Physical characteristics==
[[Image:Noaa ganymede.jpg|thumb|Depiction of Ganymede centered over 45° W. longitude; dark areas are Perrine (upper) and Nicholson (lower) regions; prominent craters are Tros (upper right) and Cisti (lower left).]]
[[File:Ganymede - Voyager 2 (26670869304).png|thumb|Three high-resolution views of Ganymede taken by [[Voyager 1]] near closest approach on July 9, 1979]]

===Size===
{{See also|List of Solar System objects by size}}
With a diameter of about {{convert|5270|km|mi}} and a mass of {{convert|1.48E20|tonne|kg lbs}}, Ganymede is the largest and most massive [[Satellite system (astronomy)|moon]] in the [[Solar System]].<ref>{{Cite web|title=Ganymede|url=https://solarsystem.nasa.gov/moons/jupiter-moons/ganymede/in-depth|access-date=June 15, 2021|website=NASA Solar System Exploration|archive-date=November 12, 2018|archive-url=https://web.archive.org/web/20181112163337/https://solarsystem.nasa.gov/moons/jupiter-moons/ganymede/in-depth/|url-status=live}}</ref> It is slightly more massive than the second most massive moon, Saturn's satellite [[Titan (moon)|Titan]], and is more than twice as massive as the Earth's Moon. It is larger than the planet [[Mercury (planet)|Mercury]], which has a diameter of {{convert|4880|km|mi}} but is only 45 percent of Mercury's mass. Ganymede is the ninth-largest object in the solar system, but the tenth-most massive.

===Composition===
The average [[density]] of Ganymede, 1.936 g/cm<sup>3</sup> (a bit greater than Callisto's), suggests a composition of about equal parts rocky material and mostly water [[Ice (planetary science)|ices]].<ref name="Showman1999" /> Some of the water is liquid, forming an underground ocean.<ref name="NYT-20150315" /> The [[mass fraction (chemistry)|mass fraction]] of ices is between 46 and 50 percent, which is slightly lower than that in Callisto.<ref name="Kuskov2005">{{cite journal |last1=Kuskov |first1=O.L. |last2=Kronrod |first2=V.A. |title=Internal structure of Europa and Callisto |date=2005 |volume=177 |issue=2 |pages=550–569|doi=10.1016/j.icarus.2005.04.014 |bibcode=2005Icar..177..550K |journal=Icarus }}</ref> Some additional volatile ices such as [[ammonia]] may also be present.<ref name="Kuskov2005" /><ref name="Spohn2003" /> 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,<ref name="Kuskov2005" /> 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]] ranges between 1.05 and 1.27 in Ganymede, whereas the [[Sun|solar ratio]] is around 1.8.<ref name="Kuskov2005" />

===Surface features===
{{See also|List of geological features on Ganymede}}
{{multiple image
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{{Multiple image|align=left|direction=vertical|width=180px
|image1=PIA24681-1041-Ganymede-JupiterMoon-Juno-20210607.jpg
|image2=PIA24682-Ganymede-DarkSide-JupiterMoon-20210607.jpg
|footer=<div align="center">Ganymede (''Juno''; June 7, 2021)</div>}}
[[File:Tros Crater, Ganymede - PJ34-1 - Detail - Map Projected.png|thumb|left|[[Tros (crater)|Tros]] crater (''Juno''; June 7, 2021)]]
[[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 |access-date=January 19, 2008 |archive-date=November 19, 2018 |archive-url=https://web.archive.org/web/20181119064953/https://spaceflightnow.com/news/n0012/29ganyflyby/ |url-status=live }}</ref> The crater Tashmetum's prominent rays are at lower right, and the large ejecta field of Hershef at upper right. Part of dark Nicholson Regio is at lower left, bounded on its upper right by Harpagia Sulcus.]]
[[File:PIA26075-JupiterMoon-GanymedeTerrain-20231030.jpg|thumb|left|Ganymede grooved terrain<br />(''Juno''; June 7, 2021)]]

Ganymede's surface has an [[albedo]] of about 43 percent.<ref name="Calvin1995">{{cite journal |last1=Calvin |first1=Wendy M. |last2=Clark |first2=Roger N. |last3=Brown |first3=Robert H. |last4=Spencer |first4=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. Geophys. Res. |date=1995 |volume=100 |issue=E9 |pages=19,041–19,048 |bibcode=1995JGR...10019041C |doi=10.1029/94JE03349 }}</ref> Water ice seems to be ubiquitous on its surface, with a mass fraction of 50–90 percent,<ref name="Showman1999" /> 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 [[micrometre|μm]].<ref name="Calvin1995" /> The grooved terrain is brighter and has a more icy composition than the dark terrain.<ref name="RESA">{{cite web |url=http://www.resa.net/nasa/ganymede.htm |archive-url=https://web.archive.org/web/20071202132022/http://www.resa.net/nasa/ganymede.htm |archive-date=December 2, 2007 |title=Ganymede: the Giant Moon |work=Wayne RESA |access-date=December 31, 2007 }}</ref> The analysis of high-resolution, [[near-infrared]] and [[Ultraviolet|UV]] [[spectrum|spectra]] obtained by the [[Galileo (spacecraft)|''Galileo'']] spacecraft and from Earth observations 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 |last1=McCord |first1=T.B. |last2=Hansen |first2=G.V. |last3=Clark |first3=R.N. |last4=Martin |first4=P. D. |last5=Hibbitts |first5=C. A. |last6=Fanale |first6=F. P. |last7=Granahan |first7=J. C. |last8=Segura |first8=M. |last9=Matson |first9=D. L. |display-authors=2 |title=Non-water-ice constituents in the surface material of the icy Galilelean satellites from Galileo near-infrared mapping spectrometer investigation |journal=J. Geophys. Res.|date=1998 |volume=103 |issue=E4 |pages=8,603–8,626 |bibcode=1998JGR...103.8603M |doi=10.1029/98JE00788 |doi-access=free }}</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 |last1=McCord |first1=Thomas B. |last2=Hansen |first2=Gary B. |last3=Hibbitts |first3=Charles A. |title=Hydrated Salt Minerals on Ganymede's Surface: Evidence of an Ocean Below |journal=Science |date=2001 |volume=292 |pages=1523–1525 |doi=10.1126/science.1059916 |bibcode=2001Sci...292.1523M |pmid=11375486 |issue=5521 |s2cid=40346198 }}</ref> These salts may originate from the subsurface ocean.<ref name="McCord2001" />

[[Image:Craters on Ganymede.jpg|thumb|The craters [[Gula (crater)|Gula]] and [[Achelous (crater)|Achelous]] (bottom), in the grooved terrain of Ganymede, with [[ejecta blanket|ejecta]] "[[pedestal crater|pedestals]]" and [[rampart crater|ramparts]]]]
The Ganymedian surface albedo is very asymmetric; the leading hemisphere<ref name="hemispherecomment" group=lower-alpha /> is brighter than the trailing one.<ref name="Calvin1995" /> This is similar to Europa, but the reverse for Callisto.<ref name="Calvin1995" /> The trailing hemisphere of Ganymede appears to be enriched in sulfur dioxide.<ref name="Domingue1996">{{cite journal |last1=Domingue |first1=Deborah |last2=Lane |first2=Arthur |last3=Moth |first3=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 |date=1996 |volume=28 |page=1070 |bibcode=1996DPS....28.0404D }}</ref><ref name="Domingue1998">{{cite journal |last1=Domingue |first1=Deborah L. |last2=Lane |first2=Arthur L. |last3=Beyer |first3=Ross A. |title=IEU's detection of tenuous SO2 frost on Ganymede and its rapid time variability |journal=Geophys. Res. Lett. |date=1998 |volume=25 |issue=16 |pages=3,117–3,120 |doi=10.1029/98GL02386 |bibcode=1998GeoRL..25.3117D |s2cid=128823420 |doi-access=free }}</ref> The distribution of carbon dioxide does not demonstrate any hemispheric asymmetry, but little or no carbon dioxide is observed near the poles.<ref name="McCord1998" /><ref name="Hibbitts2003">{{cite journal |last1=Hibbitts |first1=C.A. |last2=Pappalardo |first2=R. |last3=Hansen |first3=G.V. |last4=McCord |first4=T.B. |title=Carbon dioxide on Ganymede |journal=J. Geophys. Res. |date=2003 |volume=108 |issue=E5 |pages=5,036 |doi=10.1029/2002JE001956 |bibcode=2003JGRE..108.5036H |doi-access=free }}</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 gas was probably depleted in the past.<ref name="Hibbitts2003" />
Ganymede's surface is a mix of two types of terrain: very old, highly 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="Patterson2007">{{cite journal |last1=Patterson |first1=Wesley |last2=Head |first2=James W. |last3=Collins |first3=Geoffrey C. |display-authors=2 |title=A Global Geologic Map of Ganymede |journal=Lunar and Planetary Science |date=2007 |volume=XXXVIII |page=1098 |url=http://www.lpi.usra.edu/meetings/lpsc2007/pdf/1098.pdf |access-date=January 30, 2008 |archive-date=March 27, 2009 |archive-url=https://web.archive.org/web/20090327050809/http://www.lpi.usra.edu/meetings/lpsc2007/pdf/1098.pdf |url-status=live }}</ref> contains clays and organic materials that could indicate the composition of the impactors from which Jovian satellites accreted.<ref name="Pappalardo2001">{{cite journal |last1=Pappalardo |first1=R.T. |last2=Khurana |first2=K.K. |last3=Moore |first3=W.B. |title=The Grandeur of Ganymede: Suggested Goals for an Orbiter Mission |journal=Lunar and Planetary Science |date=2001 |volume=XXXII |page=4062 |url=http://www.lpi.usra.edu/meetings/outerplanets2001/pdf/4065.pdf |bibcode=2001iaop.work...62P |access-date=October 21, 2007 |archive-date=March 27, 2009 |archive-url=https://web.archive.org/web/20090327050808/http://www.lpi.usra.edu/meetings/outerplanets2001/pdf/4065.pdf |url-status=live }}</ref>

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" /> [[Cryovolcano|Cryovolcanism]] 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 resonances.<ref name="Showman1999" /><ref name="Showman1997b">{{cite journal |last1=Showman |first1=Adam P. |last2=Stevenson |first2=David J. |last3=Malhotra |first3=Renu |title=Coupled Orbital and Thermal Evolution of Ganymede |journal=Icarus |date=1997 |volume=129 |issue=2 |pages=367–383 |doi=10.1006/icar.1997.5778 |url=http://www.lpl.arizona.edu/~showman/publications/showman-etal-1997.pdf |bibcode=1997Icar..129..367S |access-date=January 30, 2008 |archive-date=June 3, 2019 |archive-url=https://web.archive.org/web/20190603184639/http://www.lpl.arizona.edu/~showman/publications/showman-etal-1997.pdf |url-status=live }}</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 percent of the surface.<ref name="Showman1999" /><ref name="Bland2007">{{cite journal |last1=Bland |last2=Showman |first2=A.P. |last3=Tobie |first3=G. |title=Ganymede's orbital and thermal evolution and its effect on magnetic field generation |journal=Lunar and Planetary Society Conference |date=March 2007 |volume=38 |issue=1338 |page=2020 |url=http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2020.pdf |bibcode=2007LPI....38.2020B |access-date=January 16, 2008 |archive-date=March 27, 2009 |archive-url=https://web.archive.org/web/20090327050813/http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2020.pdf |url-status=live }}</ref>  The formation of the grooved terrain may also be connected with the early core formation and subsequent tidal heating of Ganymede's interior, which may have caused a slight expansion of Ganymede by one to six percent due to [[phase transition]]s in ice and [[thermal expansion]].<ref name="Showman1999" /> During subsequent evolution deep, hot water [[plume (hydrodynamics)|plumes]] may have risen from the core to the surface, leading to the tectonic deformation of the lithosphere.<ref name="Barr">{{cite journal |last1=Barr |first1=A.C. |last2=Pappalardo |first2=R. T. |last3=Pappalardo |first3=Stevenson |date=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 |page=1781 |bibcode=2001LPI....32.1781B |access-date=January 10, 2008 |archive-date=March 27, 2009 |archive-url=https://web.archive.org/web/20090327050810/http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1781.pdf |url-status=live }}</ref> [[Radiogenic heat]]ing within the satellite is the most relevant current heat source, contributing, for instance, to ocean depth. Research models have found that if the orbital eccentricity were an order of magnitude greater than currently (as it may have been in the past), tidal heating would be a more substantial heat source than radiogenic heating.<ref name="gra.6">{{cite journal |last1=Huffmann |first1=H. |last2=Sohl |first2=F. |date=2004 |title=Internal Structure and Tidal Heating of Ganymede |journal=Geophysical Research Abstracts |volume=6 |url=http://www.cosis.net/abstracts/EGU04/05114/EGU04-J-05114.pdf |display-authors=1 |access-date=January 21, 2008 |archive-date=March 27, 2009 |archive-url=https://web.archive.org/web/20090327050805/http://www.cosis.net/abstracts/EGU04/05114/EGU04-J-05114.pdf |url-status=live }}</ref>

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 minor importance to its tectonic evolution.<ref name="Showman1999" /> The density of cratering indicates an age of 4&nbsp;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|last1=Zahnle |first1=K. |last2=Dones |first2=L. |title=Cratering Rates on the Galilean Satellites |journal=Icarus |date=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 |url-status=dead |archive-url=https://web.archive.org/web/20080227015923/http://lasp.colorado.edu/icymoons/europaclass/Zahnle_etal_1998.pdf |archive-date=February 27, 2008 }}</ref> Ganymede may have experienced a period of heavy cratering 3.5 to 4&nbsp;billion years ago similar to that of the Moon.<ref name="Zahnle1998" /> If true, the vast majority of impacts happened in that epoch, whereas the cratering rate has been much smaller since.<ref name="nineplanets.org-Ganymede">{{cite web |publisher=nineplanets.org |title=Ganymede |date=October 31, 1997 |url=http://www.nineplanets.org/ganymede.html |access-date=February 27, 2008 |archive-date=August 27, 2019 |archive-url=https://web.archive.org/web/20190827231253/https://nineplanets.org/ganymede.html |url-status=live }}</ref> 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 |date=1997 |url=http://www.lpi.usra.edu/resources/outerp/gany.html |access-date=February 7, 2007 |archive-date=February 11, 2017 |archive-url=https://web.archive.org/web/20170211103518/http://www.lpi.usra.edu/resources/outerp/gany.html |url-status=live }}</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 (planetary astronomy)|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 |last1=Casacchia |first1=R. |last2=Strom |first2=R.G. |date=1984 |title=Geologic evolution of Galileo Regio |journal=Journal of Geophysical Research |volume=89 |issue=S02 |pages=B419–B428 |bibcode=1984LPSC...14..419C |doi=10.1029/JB089iS02p0B419 }}</ref>

Ganymede also has 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 formation of the caps include the migration of water to higher latitudes and the bombardment of the ice by plasma. Data from ''Galileo'' suggests the latter is correct.<ref name="Polar caps" /> The presence of a magnetic field on Ganymede results in more intense charged particle bombardment of its surface in the unprotected polar regions; sputtering then leads to redistribution of water molecules, with frost migrating to locally colder areas within the polar terrain.<ref name="Polar caps">{{cite journal |last1=Khurana |first1=Krishan K. |last2=Pappalardo |first2=Robert T. |last3=Murphy |first3=Nate |last4=Denk |first4=Tilmann |date=2007 |title=The origin of Ganymede's polar caps |journal=Icarus |volume=191 |issue=1 |pages=193–202 |doi=10.1016/j.icarus.2007.04.022 |bibcode=2007Icar..191..193K }}</ref>

A crater named [[Anat (crater)|Anat]] provides the reference point for measuring longitude on Ganymede. By definition, Anat is at 128° longitude.<ref name="iau.table2">{{cite web |url=https://astrogeology.usgs.gov/Projects/WGCCRE/constants/iau2000_table2.html |title=USGS Astrogeology: Rotation and pole position for planetary satellites (IAU WGCCRE) |access-date=August 28, 2017 |archive-url=https://web.archive.org/web/20111024101848/http://astrogeology.usgs.gov/Projects/WGCCRE/constants/iau2000_table2.html |archive-date=October 24, 2011 |url-status=dead }}</ref> The 0° longitude directly faces Jupiter, and unless stated otherwise longitude increases toward the west.<ref name="targetcoordsys">{{cite web|title=Planetary Names: Target Coordinate Systems|url=http://planetarynames.wr.usgs.gov/TargetCoordinates|website=planetarynames.wr.usgs.gov|publisher=International Astronomical Union|access-date=May 21, 2016|archive-url=https://web.archive.org/web/20160527032231/http://planetarynames.wr.usgs.gov/TargetCoordinates|archive-date=May 27, 2016|url-status=dead}}</ref>

===Internal structure===
Ganymede appears to be fully [[Planetary differentiation|differentiated]], with an internal structure consisting of an [[iron(II) sulfide|iron-sulfide]]–iron [[core (geology)|core]], a [[silicate]] [[mantle (geology)|mantle]], and outer layers of water ice and liquid water.<ref name="Showman1999" /><ref name="Sohl2002">{{cite journal |last1=Sohl |first1=F. |last2=Spohn |first2=T |last3=Breuer |first3=D. |last4=Nagel |first4=K. |title=Implications from Galileo Observations on the Interior Structure and Chemistry of the Galilean Satellites |journal=Icarus |date=2002 |volume=157 |issue=1 |pages=104–119 |doi=10.1006/icar.2002.6828 |bibcode=2002Icar..157..104S }}</ref>
<ref name="Bhatia2017">{{cite journal|last1=Bhatia|first1= G.K.|last2=Sahijpal|first2= S.|title=Thermal evolution of trans-Neptunian objects, icy satellites, and minor icy planets in the early solar system |journal=Meteoritics & Planetary Science |doi=10.1111/maps.12952|volume=52|issue= 12|year=2017|pages=2470–2490|bibcode=2017M&PS...52.2470B|s2cid= 133957919|doi-access=free}}</ref> 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" /><ref name="Kuskov2005b">{{cite journal |last1=Kuskov |first1=O.L. |last2=Kronrod |first2=V.A. |last3=Zhidikova |first3=A.P. |title=Internal Structure of Icy Satellites of Jupiter |journal=Geophysical Research Abstracts |date=2005 |volume=7 |page=01892 |url=http://www.cosis.net/abstracts/EGU05/01892/EGU05-J-01892.pdf |access-date=January 21, 2008 |archive-date=March 27, 2009 |archive-url=https://web.archive.org/web/20090327050809/http://www.cosis.net/abstracts/EGU05/01892/EGU05-J-01892.pdf |url-status=live }}</ref><ref name="Kuskov2005c">{{Cite journal |last1=Kuskov |first1=O. L. |last2=Kronrod |first2=V. A. |last3=Zhidikova |first3=A. P. |date=May 2010 |title=Internal Structure of Icy Satellites of Jupiter |journal=Advances in Geosciences |language=en |publisher=World Scientific |volume=19 |pages=365–376 |bibcode=2010aogs...19..365K |doi=10.1142/9789812838162_0028 |doi-broken-date=November 1, 2024 |isbn=9789812838162 |editor-first1=Anil |editor-last1=Bhardwaj}}</ref> Ganymede has the lowest [[moment of inertia factor]], 0.31,<ref name="Showman1999" /> among the solid Solar System bodies. This is a consequence of its substantial water content and fully differentiated interior.

====Subsurface oceans====
[[File:Ganymede diagram.svg|thumb|upright=2|Artist's cut-away representation of the internal structure of Ganymede. Layers drawn to scale.]]
In the 1970s, NASA scientists first suspected that Ganymede had a thick ocean between two layers of ice, one on the surface and one beneath a liquid ocean and atop the rocky mantle.<ref name="Showman1999" /><ref name="clubsandwich 2014" /><ref name="Sohl2002" /><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 |date=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 |archive-url=https://web.archive.org/web/20070824155106/http://bowfell.geol.ucl.ac.uk/~lidunka/EPSS-papers/pete2.pdf |archive-date=August 24, 2007 |bibcode=2006P&SS...54....2F }}</ref><ref name="amount of water in ocean">{{cite web | url=http://earthsky.org/space/underground-ocean-on-jupiters-largest-moon | title=Underground ocean on Jupiter's largest moon | publisher=EarthSky | date=March 15, 2015 | access-date=August 14, 2015 | archive-date=October 11, 2019 | archive-url=https://web.archive.org/web/20191011045129/https://earthsky.org/space/underground-ocean-on-jupiters-largest-moon | url-status=live }}</ref> In the 1990s, NASA's ''Galileo'' mission flew by Ganymede, and found indications of such a subsurface ocean.<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=March 12, 2015 |work=[[New York Times]] |access-date=March 12, 2015 |archive-date=May 9, 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> An analysis published in 2014, taking into account the realistic thermodynamics for water and effects of salt, suggests that Ganymede might have a stack of several ocean layers separated by different [[phases of ice]], with the lowest liquid layer adjacent to the rocky [[Mantle (geology)|mantle]].<ref name="clubsandwich 2014" /><ref name="Vance" /><ref name="NASA-20140501c" /><ref name="Hubble 2015">{{cite news |url=http://phys.org/news/2015-03-hubble-underground-ocean-jupiter-largest.html |title=Hubble observations suggest underground ocean on Jupiter's largest moon Ganymede |work=NASA |publisher=PhysOrg |date=March 12, 2015 |access-date=March 13, 2015 |archive-date=March 28, 2022 |archive-url=https://web.archive.org/web/20220328173030/https://phys.org/news/2015-03-hubble-underground-ocean-jupiter-largest.html |url-status=live }}</ref> Water–rock contact may be an important factor in the [[Abiogenesis|origin of life]].<ref name="clubsandwich 2014" /> The analysis also notes that the extreme depths involved (~800&nbsp;km to the rocky "seafloor") mean that temperatures at the bottom of a convective (adiabatic) ocean can be up to 40 K higher than those at the ice–water interface.

In March 2015, scientists reported that measurements with the Hubble Space Telescope of how the [[aurora]]e moved confirmed that Ganymede has a subsurface ocean.<ref name="NYT-20150315"/> A large saltwater ocean affects Ganymede's magnetic field, and consequently, its aurorae.<ref name="Ocean Hubble"/><ref name="Hubble 2015"/><ref name="sciencedaily1503">{{Cite web | url=https://www.sciencedaily.com/releases/2015/03/150312112112.htm | title=Underground ocean on Jupiter's largest moon, Ganymede | access-date=March 9, 2018 | archive-date=November 16, 2018 | archive-url=https://web.archive.org/web/20181116100344/https://www.sciencedaily.com/releases/2015/03/150312112112.htm | url-status=live }}</ref><ref name="sswater1">{{cite journal | last1 = Saur | first1 = Joachim | last2 = Duling | first2 = Stefan | last3 = Roth | first3 = Lorenz | last4 = Jia | first4 = Xianzhe | last5 = Strobel | first5 = Darrell F. | last6 = Feldman | first6 = Paul D. | last7 = Christensen | first7 = Ulrich R. | last8 = Retherford | first8 = Kurt D. | last9 = McGrath | first9 = Melissa A. | last10 = Musacchio | first10 = Fabrizio | last11 = Wennmacher | first11 = Alexandre | last12 = Neubauer | first12 = Fritz M. | last13 = Simon | first13 = Sven | last14 = Hartkorn | first14 = Oliver | year = 2015 | title = The Search for a Subsurface Ocean in Ganymede with Hubble Space Telescope Observations of its Auroral Ovals | url = http://kth.diva-portal.org/smash/get/diva2:814598/FULLTEXT01 | journal = Journal of Geophysical Research: Space Physics | volume = 120 | issue = 3 | pages = 1715–1737 | doi = 10.1002/2014JA020778 | bibcode = 2015JGRA..120.1715S | hdl = 2027.42/111157 | doi-access = free | access-date = August 25, 2019 | archive-date = July 20, 2018 | archive-url = https://web.archive.org/web/20180720185410/http://kth.diva-portal.org/smash/get/diva2:814598/FULLTEXT01 | url-status = live | hdl-access = free }}</ref> The evidence suggests that Ganymede's oceans might be the largest in the entire Solar System.<ref name='Sci Am 2017'>{{Cite news |url=https://www.scientificamerican.com/article/overlooked-ocean-worlds-fill-the-outer-solar-system/ |title=Overlooked Ocean Worlds Fill the Outer Solar System |first=John |last=Wenz |work=Scientific American |date=October 4, 2017 |access-date=January 6, 2018 |archive-date=December 26, 2018 |archive-url=https://web.archive.org/web/20181226133924/https://www.scientificamerican.com/article/overlooked-ocean-worlds-fill-the-outer-solar-system/ |url-status=live }}</ref> These observations were later supported by ''[[Juno (spacecraft)|Juno]]'', which detected various salts and other compounds on Ganymede's surface, including [[hydrohalite|hydrated sodium chloride]], [[ammonium chloride]], [[sodium bicarbonate]], and possibly [[aldehyde|aliphatic aldehydes]]. These compounds were potentially deposited from Ganymede's ocean in past resurfacing events and were discovered to be most abundant in Ganymede's lower latitudes, shielded by its small magnetosphere.<ref>{{Cite web |title=Ganymede moon has a huge internal ocean and salty surface |url=https://www.earth.com/news/jupiters-moon-ganymede-has-huge-internal-ocean-and-salty-surface/ |access-date=November 18, 2023 |website=Earth.com |language=en}}</ref> As a result of these findings, there is increasing speculation on the potential [[planetary habitability|habitability]] of Ganymede's ocean.<ref name="amount of water in ocean"/><ref name="subsurface ocean found">{{cite news | url=https://www.independent.co.uk/news/science/ganymede-oceans-on-jupiters-moon-could-have-been-home-to-alien-life-10106286.html | archive-url= https://web.archive.org/web/20150313232038/https://www.independent.co.uk/news/science/ganymede-oceans-on-jupiters-moon-could-have-been-home-to-alien-life-10106286.html| archive-date= March 13, 2015| url-status= dead| title=Ganymede: oceans on Jupiter's moon could have been home to alien life | newspaper=The Independent | date=March 13, 2015 | access-date= February 19, 2018 | author=Griffin, Andrew}}</ref>

====Core====
The existence of a liquid, [[iron–nickel alloy|iron–nickel]]-rich core<ref name="Bhatia2017" /> provides a natural explanation for the intrinsic [[magnetosphere|magnetic field]] of Ganymede detected by ''Galileo'' spacecraft.<ref name="Hauk2006">{{cite journal |last1=Hauck |first1=Steven A. |last2=Aurnou |first2=Jonathan M. |last3=Dombard |first3=Andrew J. |title=Sulfur's impact on core evolution and magnetic field generation on Ganymede |journal=J. Geophys. Res.|date=2006 |volume=111 |issue=E9 |pages=E09008 |doi=10.1029/2005JE002557 |bibcode=2006JGRE..111.9008H |doi-access=free }}</ref> The [[convection]] in the liquid iron, which has high [[electrical conductivity]], is the most reasonable model of magnetic field generation.<ref name="Kivelson2002" /> The density of the core is 5.5–6 g/cm<sup>3</sup> and the silicate mantle is 3.4–3.6 g/cm<sup>3</sup>.<ref name="Kuskov2005" /><ref name="Sohl2002" /><ref name="Kuskov2005b" /><ref name="Hauk2006" /> The radius of this core may be up to 500&nbsp;km.<ref name="Hauk2006" /> The temperature in the core of Ganymede is probably 1500–1700 K and pressure up to {{convert|10|GPa|atm|abbr=on}}.<ref name="Sohl2002" /><ref name="Hauk2006" />

===Atmosphere and ionosphere===
In 1972, a team of Indian, British and American astronomers working in [[Java]], [[Indonesia]] and [[Kavalur]], India claimed that they had detected a thin atmosphere during an [[occultation]], when it and Jupiter passed in front of a [[star]].<ref name="Carlson1973">{{cite journal |last1=Carlson |first1=R.W. |last2=Bhattacharyya |first2=J. C. |author-link2=J. C. Bhattacharyya |last3=Smith |first3=B.A. |last4=Johnson |first4=T. V. |last5=Hidayat |first5=B. |last6=Smith |first6=S. A. |last7=Taylor |first7=G. E. |last8=O'Leary |first8=B. |last9=Brinkmann |first9=R. T. |display-authors=2 |title=Atmosphere of Ganymede from its occultation of SAO 186800 on 7 June 1972 |journal=Science |date=1973 |volume=182 |bibcode=1973Sci...182...53C |doi=10.1126/science.182.4107.53 |issue=4107 |pmid=17829812 |pages=53–5 |s2cid=33370778 |url=http://authors.library.caltech.edu/61963/1/1736235.pdf |access-date=April 20, 2018 |archive-date=December 2, 2017 |archive-url=https://web.archive.org/web/20171202104832/https://authors.library.caltech.edu/61963/1/1736235.pdf |url-status=live }}</ref> They estimated that the surface pressure was around 0.1 [[Pascal (unit)|Pa]] (1 microbar).<ref name="Carlson1973" /> However, in 1979, ''[[Voyager 1]]'' observed an occultation of the star [[Kappa Centauri|κ Centauri]] during its flyby of Jupiter, with differing results.<ref name="Broadfoot1981">{{cite journal |last1=Broadfoot |first1=A.L. |last2=Sandel |first2=B.R. |last3=Shemansky |first3=D.E. |last4=McConnell |first4=J. C. |last5=Smith |first5=G. R. |last6=Holberg |first6=J. B. |last7=Atreya |first7=S. K. |last8=Donahue |first8=T. M. |last9=Strobel |first9=D. F. |display-authors=2 |title=Overview of the Voyager Ultraviolet Spectrometry Results through Jupiter Encounter |journal=Journal of Geophysical Research |date=1981 |volume=86 |issue=A10 |pages=8259–8284 |url=http://www-personal.umich.edu/~atreya/Articles/1981_Overview_Voyager.pdf |bibcode=1981JGR....86.8259B |doi=10.1029/JA086iA10p08259 |access-date=January 16, 2008 |archive-date=March 27, 2009 |archive-url=https://web.archive.org/web/20090327050807/http://www-personal.umich.edu/~atreya/Articles/1981_Overview_Voyager.pdf |url-status=live }}</ref> The occultation measurements were conducted in the [[far-ultraviolet]] spectrum at [[wavelength]]s shorter than 200 [[nanometre|nm]], which were much more sensitive to the presence of gases than the 1972 measurements made in the [[visible spectrum]]. No atmosphere was revealed by the ''Voyager'' data. The upper limit on the surface particle [[number density]] was found to be {{nowrap|1.5{{E|9}} cm<sup>−3</sup>}}, which corresponds to a surface pressure of less than 2.5 μPa (25 picobar).<ref name="Broadfoot1981" /> The latter value is almost five orders of magnitude less than the 1972 estimate.<ref name="Broadfoot1981" />
[[Image:Map of temparatureof ganymede.jpg|thumb|left|False-color temperature map of Ganymede]]
Despite the ''Voyager'' data, evidence for a tenuous oxygen atmosphere ([[exosphere]]) on Ganymede, very similar to the one found on Europa, was found by the [[Hubble Space Telescope]] (HST) in 1995.<ref name="Hall1998" /><ref name="JPLAtmosphere">{{cite web |archive-url= https://web.archive.org/web/20090504072525/http://www2.jpl.nasa.gov/galileo/hst7.html |url= http://www2.jpl.nasa.gov/galileo/hst7.html|title=Hubble Finds Thin Oxygen Atmosphere on Ganymede |work=Jet Propulsion Laboratory |publisher=NASA |date=October 23, 1996 |access-date= February 17, 2017|url-status= dead |archive-date= May 4, 2009}}</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" /> which is evidence of a significant neutral atmosphere composed predominantly of O<sub>2</sub> molecules. The surface number density probably lies in the {{nowrap |(1.2–7){{E|8}} cm<sup>−3</sup>}} range, corresponding to the surface pressure of {{nowrap |0.2–1.2 μPa}}.<ref name="Hall1998" /><ref name="surfacedensitynumber" group=lower-alpha /> These values are in agreement with ''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 observed over Europa. HST observed two bright spots located in the northern and southern hemispheres, near ± 50° latitude, which is exactly the boundary between the open and closed field lines of the Ganymedian magnetosphere (see below).<ref name="Feldman2000">{{cite journal |last1=Feldman |first1=Paul D. |last2=McGrath |first2=Melissa A. |last3=Strobell |first3=Darrell F. |last4=Moos |first4=H. Warren |last5=Retherford |first5=Kurt D. |last6=Wolven |first6=Brian C. |display-authors=2 |title=HST/STIS Ultraviolet Imaging of Polar Aurora on Ganymede |journal=The Astrophysical Journal |date=2000 |volume=535 |issue=2 |pages=1085–1090 |doi=10.1086/308889 |bibcode=2000ApJ...535.1085F |arxiv=astro-ph/0003486 |s2cid=15558538 }}</ref> 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. |date=1997 |title=Polar "Caps" on Ganymede and Io Revisited |journal=Icarus |volume=128 |issue=2 |pages=469–471 |bibcode=1997Icar..128..469J |doi=10.1006/icar.1997.5746 }}</ref>

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">{{cite journal |last1=Paranicas |first1=C. |last2=Paterson |first2=W. R. |last3=Cheng |first3=A. F. |last4=Mauk |first4=B. H. |last5=McEntire |first5=R. W. |last6=Frank |first6=L. A. |last7=Williams |first7=D. J. |display-authors=2 |title=Energetic particles observations near Ganymede |journal=J. Geophys. Res. |date=1999 |volume=104 |issue=A8 |pages=17,459–17,469 |doi=10.1029/1999JA900199 |bibcode=1999JGR...10417459P }}</ref> and by solar [[Extreme ultraviolet|EUV]] radiation.<ref name="Eviatar2001" /> 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 Ganymede, suggesting an ionosphere, whereas others failed to detect anything.<ref name="Eviatar2001" /> The electron density near the surface is estimated by different sources to lie in the range 400–2,500&nbsp;cm<sup>−3</sup>.<ref name="Eviatar2001" /> As of 2008, the parameters of the ionosphere of Ganymede were 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 |last1=Noll |first1=Keith S. |last2=Johnson |first2=Robert E. |last3=Domingue |first3=D. L. |last4=Weaver |first4=H. A. |display-authors=2 |date=July 1996 |title=Detection of Ozone on Ganymede |journal=Science |volume=273 |issue=5273 |pages=341–343 |doi=10.1126/science.273.5273.341 |pmid=8662517 |bibcode=1996Sci...273..341N |s2cid=32074586 }}</ref> In 1997 spectroscopic analysis revealed the [[Dimer (chemistry)|dimer]] (or [[diatomic]]) absorption features of molecular oxygen. Such an absorption can arise only if the oxygen is in a dense phase. The best candidate is molecular oxygen trapped in ice. The depth of the dimer absorption bands depends on [[latitude]] and [[longitude]], rather than on surface albedo—they tend to decrease with increasing latitude on Ganymede, whereas O<sub>3</sub> shows an opposite trend.<ref name="Oxygen97">{{cite journal |last1=Calvin |first1=Wendy M. |last2=Spencer |first2=John R. |date=December 1997 |title=Latitudinal Distribution of O<sub>2</sub> on Ganymede: Observations with the Hubble Space Telescope |journal=Icarus |volume=130 |issue=2 |pages=505–516 |doi=10.1006/icar.1997.5842 |bibcode=1997Icar..130..505C |url=https://zenodo.org/record/1229830 |access-date=July 13, 2019 |archive-date=December 2, 2020 |archive-url=https://web.archive.org/web/20201202001450/https://zenodo.org/record/1229830 |url-status=live }}</ref> Laboratory work has found that O<sub>2</sub> would not cluster or bubble but would dissolve in ice at Ganymede's relatively warm surface temperature of 100 K (−173.15&nbsp;°C).<ref name="sci.5320">{{cite journal |last1=Vidal |first1=R. A. |last2=Bahr |first2=D. |s2cid=27378519 |date=1997 |title=Oxygen on Ganymede: Laboratory Studies |journal=Science |volume=276 |issue=5320 |pages=1839–1842 |bibcode=1997Sci...276.1839V |doi=10.1126/science.276.5320.1839 |pmid=9188525 |display-authors=1 }}</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 name="ic.126.1">{{cite journal |last=Brown |first=Michael E. |date=1997 |title=A Search for a Sodium Atmosphere around Ganymede |journal=Icarus |volume=126 |issue=1 |pages=236–238  |bibcode=1997Icar..126..236B |doi=10.1006/icar.1996.5675 |citeseerx=10.1.1.24.7010 }}</ref> Another minor constituent of the Ganymedian atmosphere is [[atomic hydrogen]]. Hydrogen atoms were observed as far as 3,000&nbsp;km from Ganymede's surface. Their density on the surface is about {{nowrap |1.5{{E|4}} cm<sup>−3</sup>}}.<ref name="Barth1997">{{cite journal |last1=Barth |first1=C.A. |last2=Hord |first2=C.W. |last3=Stewart |first3=A.I. |last4=Pryor |first4=W. R. |last5=Simmons |first5=K. E. |last6=McClintock |first6=W. E. |last7=Ajello |first7=J. M. |last8=Naviaux |first8=K. L. |last9=Aiello |first9=J. J. |s2cid=123038216 |display-authors=2 |title=Galileo ultraviolet spectrometer observations of atomic hydrogen in the atmosphere of Ganymede |journal=Geophys. Res. Lett. |date=1997 |volume=24 |issue=17 |pages=2147–2150 |bibcode=1997GeoRL..24.2147B |doi=10.1029/97GL01927 |doi-access=free }}</ref>

In 2021, water vapour was detected in the atmosphere of Ganymede.<ref>[https://www.space.com/jupiter-moon-ganymede-water-vapor-discovery Water vapor detected on huge Jupiter moon Ganymede for 1st time] {{Webarchive|url=https://web.archive.org/web/20210806162110/https://www.space.com/jupiter-moon-ganymede-water-vapor-discovery |date=August 6, 2021 }}, Space.com</ref>

===Magnetosphere===
[[Image:Ganymede magnetic field.svg#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 ''Galileo'' craft made six close flybys of Ganymede from 1995 to 2000 (G1, G2, G7, G8, G28 and G29)<ref name="Kivelson2002" /> and discovered that Ganymede has a permanent (intrinsic) [[magnetic moment]] independent of the Jovian magnetic field.<ref name="Kivelson1997">{{cite journal |last1=Kivelson |first1=M.G. |last2=Khurana |first2=K.K. |last3=Coroniti |first3=F.V. |last4=Joy |first4=S. |last5=Russell |first5=C. T. |last6=Walker |first6=R. J. |last7=Warnecke |first7=J. |last8=Bennett |first8=L. |last9=Polanskey |first9=C. |display-authors=2 |title=The magnetic field and magnetosphere of Ganymede |journal=Geophys. Res. Lett. |date=1997 |volume=24 |issue=17 |pages=2155–2158 |doi=10.1029/97GL02201 |url=http://www.igpp.ucla.edu/people/mkivelson/Publications/97GL02201.pdf |bibcode=1997GeoRL..24.2155K |doi-access=free |access-date=January 15, 2008 |archive-date=March 27, 2009 |archive-url=https://web.archive.org/web/20090327050805/http://www.igpp.ucla.edu/people/mkivelson/Publications/97GL02201.pdf |url-status=live }}</ref> The value of the moment is about {{nowrap |1.3{{E-sp |13}} T·m<sup>3</sup>}},<ref name="Kivelson2002" /> which is three times larger than the [[Mercury's magnetic field|magnetic moment of 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 ± 2 [[Tesla (unit)|nT]] at Ganymede's equator,<ref name="Kivelson2002" /> which should be compared with the Jovian magnetic field at the distance of Ganymede—about 120 nT.<ref name="Kivelson1997" /> The equatorial field of Ganymede is directed against the Jovian field, meaning [[Magnetic reconnection|reconnection]] is possible. The intrinsic field strength at the poles is two times that at the equator—1440 nT.<ref name="Kivelson2002" />
[[Image:15-33i2-JupiterMoon-Ganymede-Aurora-20150312.png|thumb|right|Aurorae on Ganymede—auroral belt shifting may indicate a subsurface saline ocean.]]
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 4–5 Ganymede radii.<ref name="Kivelson1998">{{cite journal |last1=Kivelson |first1=M.G. |last2=Warnecke |first2=J. |last3=Bennett |first3=L. |last4=Joy |first4=S. |last5=Khurana |first5=K. K. |last6=Linker |first6=J. A. |last7=Russell |first7=C. T. |last8=Walker |first8=R. J. |last9=Polanskey |first9=C. |display-authors=2 |title=Ganymede's magnetosphere: magnetometer overview |journal=J. Geophys. Res. |date=1998 |volume=103 |issue=E9 |pages=19,963–19,972 |doi=10.1029/98JE00227 |url=http://www.igpp.ucla.edu/people/mkivelson/Publications/98JE00227.pdf |bibcode=1998JGR...10319963K |doi-access=free |access-date=January 15, 2008 |archive-date=March 27, 2009 |archive-url=https://web.archive.org/web/20090327050819/http://www.igpp.ucla.edu/people/mkivelson/Publications/98JE00227.pdf |url-status=live }}</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<ref name="Eviatar2001" /> (O<sup>+</sup>) which fits well with Ganymede's tenuous oxygen [[atmosphere]]. 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]]) electrons and ions have been detected,<ref name="Paranicas1999" /> which may cause the auroras observed around the Ganymedian poles.<ref name="Feldman2000" /> In addition, heavy ions precipitate continuously on Ganymede's polar surface, [[sputtering]] and darkening the ice.<ref name="Paranicas1999" />

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">{{cite journal |last1=Volwerk |first1=M. |last2=Kivelson |first2=M.G. |last3=Khurana |first3=K.K. |last4=McPherron |first4=R.L. |title=Probing Ganymede's magnetosphere with field line resonances |journal=J. Geophys. Res. |date=1999 |volume=104 |issue=A7 |pages=14,729–14,738 |doi=10.1029/1999JA900161 |url=http://www.igpp.ucla.edu/people/mkivelson/Publications/1999JA900161.pdf |bibcode=1999JGR...10414729V |doi-access=free |access-date=January 15, 2008 |archive-date=March 27, 2009 |archive-url=https://web.archive.org/web/20090327050807/http://www.igpp.ucla.edu/people/mkivelson/Publications/1999JA900161.pdf |url-status=live }}</ref> 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—[[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" />

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 Ganymede. 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 nT—half of that of the ambient Jovian field.<ref name="Kivelson2002" /> The induced magnetic field of Ganymede is similar to those of Callisto and Europa, indicating that Ganymede 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" /> 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 |last1=Hauck |first1=Steven A. |last2=Dombard |first2=A. J. |last3=Solomon |first3=S. C. |last4=Aurnou |first4=J. M. |title=Internal structure and mechanism of core convection on Ganymede |journal=Lunar and Planetary Science |volume=XXXIII |date=2002 |page=1380 |url=http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1380.pdf |bibcode=2002LPI....33.1380H |access-date=October 21, 2007 |archive-date=March 27, 2009 |archive-url=https://web.archive.org/web/20090327050805/http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1380.pdf |url-status=live }}</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, hence 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 of the mantle increased during such resonances, reducing heat flow from the core, leaving it fluid and convective.<ref name="Bland2007" /> 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" />

===Radiation environment===
The radiation level at the surface of Ganymede is considerably lower than on Europa, being 50–80 mSv (5–8 rem) per day, an amount that would cause severe illness or death in human beings exposed for two months.<ref name= "Podzolko">{{cite conference | last1= Podzolko | first1= M.V. | last2= Getselev | first2= I.V. | title= Radiation Conditions of a Mission to Jupiterʼs Moon Ganymede | book-title= International Colloquium and Workshop "Ganymede Lander: Scientific Goals and Experiments | publisher= Moscow State University | date= March 8, 2013 | location= IKI, Moscow, Russia | url= https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=32688.0;attach=541277 | access-date= January 6, 2020 | archive-date= March 9, 2021 | archive-url= https://web.archive.org/web/20210309003045/https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=32688.0;attach=541277 | url-status= live }}</ref>