# Ganymede (moon) <!-- SOLSIM:BEGIN g31 — Solar System explorer state (hand-built on wt-core, specs/solar/); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Ganymede in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=Ganymede&embed=1" data-title="Ganymede in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=Ganymede`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.* <!-- SOLSIM:END --> *Try: set the speed to 1 day/s and follow Ganymede round the highlighted ring for one seven-second circuit while Europa laps twice and Io four times; press space to freeze the scene and drag the view round to compare Ganymede's ring with Callisto's far wider one outside it; scroll in toward Jupiter and release the pause to watch the inner three moons keep their 1:2:4 beat.* **Ganymede** is the largest [[Galilean_moons|Galilean moon]] of [[Jupiter]] and the largest and most massive moon in the [[PORTAL_Solar_System|Solar System]]: about 5,270 km across, it is wider than the planet [[Mercury_(planet)|Mercury]], though it has less than half of Mercury's mass.[^nasa-ganymede][^nasa-jsat][^nasa-fs] It is built of rock and [[Water|water]] in roughly equal shares, separated into an iron-rich core, a rocky mantle and thick layers of ice, with a salty ocean believed to lie deep inside the ice.[^showman1999][^vance2014] Its low density gives it a weaker surface [[Gravity|gravity]] than Earth's [[Moon]] despite its size.[^jpl-ganymede] Ganymede is the only moon known to generate its own magnetic field, most likely by [[Convection|convection]] in a liquid metal core, and so the only one with a small magnetosphere of its own nested inside Jupiter's.[^kivelson1997][^kivelson2002] It has a very thin [[Oxygen|oxygen]] atmosphere and a surface split between ancient dark, cratered terrain and younger bright terrain cut by grooves.[^hall1998][^showman1999] It orbits every 7.155 days, locked in a 1:2:4 resonance with [[Europa_(moon)|Europa]] and [[Io_(moon)|Io]].[^nasa-ganymede-facts][^musotto2002] Discovered by Galileo Galilei in January 1610, it has been passed at close range by six spacecraft, and ESA's Juice mission is due to become the first to orbit it.[^galileo1610][^chang2021][^esa-juice] The explorer at the top of this page opens Jupiter's own frame with the four large moons at true distance and period; Ganymede's orbit is the highlighted ring at 1,070,400 km.[^nasa-jsat] ## History Records of the Chinese astronomer Gan De from 365 BC mention a small reddish star beside Jupiter, which some historians have read as a naked-eye sighting of one of its moons, possibly Ganymede; the reported colour is hard to reconcile with a body that faint.[^brecher1981][^huang1997] The documented discovery is Galileo's. On 7 January 1610 his telescope showed three "stars" in a line near Jupiter, one of them Ganymede; they had moved by the next night, on 13 January he saw all four moons together for the first time, and by 15 January he had concluded that they circle the planet.[^galileo1610][^nasa-ganymede] ### Name Galileo claimed the naming rights and called the four moons the Medicean Stars after his patron Cosimo II de' Medici. Nicolas-Claude Fabri de Peiresc suggested names of individual Medici, and Simon Marius, who claimed an independent discovery, first tried a planetary scheme in which Ganymede was the "Jupiter of Jupiter". Neither caught on. Marius then adopted a suggestion from [[Johannes_Kepler|Johannes Kepler]] to name the moons after lovers of Zeus, and in this scheme the third moon became Ganymede, the Trojan prince whom Zeus carried off to Olympus to pour wine for the gods.[^rice-satellites] Like the others, the name fell out of use until the mid-twentieth century; older literature calls the moon Jupiter III, Galileo's numbering.[^rice-satellites] It is the only one of the four named after a male figure. The English form drops the final syllable of the Latin *Ganymēdēs*.[^mw1995] ## Orbit and rotation Ganymede circles [[Jupiter]] at 1,070,400 km, third of the four Galilean moons, once every 7.155 days, and like most large moons it is tidally locked, so its day equals its orbit.[^nasa-ganymede-facts][^nasa-jsat] Its [[Orbit|orbit]] is almost circular. The eccentricity and the inclination to Jupiter's equator vary over centuries under the pull of the Sun and the planets, within 0.0009–0.0022 and 0.05–0.32°.[^musotto2002] For each orbit of Ganymede, Europa completes two and Io four; the periods of 1.769, 3.551 and 7.155 days stand almost exactly in the ratio 1:2.007:4.044 (derived).[^nasa-jsat] Io–Europa conjunctions occur with Io at its closest point to Jupiter and Europa at its farthest, Europa–Ganymede conjunctions with Europa at its closest, and because the two sets of conjunctions drift at the same rate the three moons are never all lined up at once. This locked arrangement is the Laplace [[Resonance|resonance]].[^musotto2002][^showman1997] The resonance cannot at present force Ganymede's eccentricity upward, so its value of about 0.0013 is probably left over from an earlier configuration; tidal friction inside the moon should otherwise have erased it, which places the last excitation only several hundred million years ago.[^showman1997] With so small an eccentricity, [[Tide|tidal]] heating is negligible today. Earlier, the moon may have crossed Laplace-like resonances that pumped its eccentricity to 0.01–0.02, heating the interior strongly enough to help form the grooved terrain.[^showman1997][^showman1999] How the resonance arose is debated: it may date from the formation of the moons, or it may have been assembled later as Io's orbit expanded under tides it raised on Jupiter, capturing Europa and then Ganymede in turn.[^peale2002][^showman1997] In the explorer the moons move on circles at their true periods, so the 1:2:4 beat of Io, Europa and Ganymede can be counted directly; the starting positions are ILLUSTRATIVE, and the moons are drawn larger than scale so that they remain visible. ## Bulk properties ### Size Ganymede's diameter of about 5,270 km and mass of about 1.48 × 10²³ kg make it the largest and heaviest moon in the Solar System.[^nasa-ganymede] The NASA fact sheet gives a mean radius of 2,631.2 km and a mass of 1.4819 × 10²³ kg (the explorer's caption uses the radius 2,634.1 km).[^nasa-jsat] That is about 1.1 times the mass of [[Titan_(moon)|Titan]], Saturn's largest moon, and about twice that of Earth's [[Moon]] (both derived). Mercury, at 4,880 km across, is the smaller body but the denser one, and Ganymede carries only about 45% of its mass (derived).[^nasa-ssat][^nasa-fs] Surface gravity follows as G × 1.4819 × 10²³ kg / (2.6312 × 10⁶ m)² ≈ 1.43 m/s² (derived). ### Composition Ganymede's mean [[Density|density]], about 1,940 kg/m³, a little above Callisto's, implies a body of roughly half rock and half water, mostly as ice.[^showman1999][^nasa-jsat] Interior models put the ice fraction at 46–50% by mass, slightly less than in [[Callisto_(moon)|Callisto]], perhaps with some ammonia as well; the rock is probably similar to L or LL ordinary chondrite meteorites, with an iron-to-silicon ratio of 1.05–1.27 against a solar value near 1.8.[^kuskov2005] ### Internal structure Ganymede appears fully differentiated: an iron and iron-sulfide core, a silicate mantle, and outer shells of ice and liquid water.[^showman1999][^sohl2002] Its moment of inertia factor, 0.31, is the lowest measured for any solid body in the Solar System, a sign of how strongly its mass is concentrated toward the centre.[^showman1999] The core may reach a radius of 500 km, with a density of 5,500–6,000 kg/m³, a temperature of perhaps 1,500–1,700 K and pressures up to 10 GPa; the mantle has a density of 3,400–3,600 kg/m³.[^hauck2006][^sohl2002] The ocean is thought to lie between an outer ice shell and a layer of high-pressure ice on top of the mantle. A 2014 model that includes realistic thermodynamics for salty water suggests instead a stack of several oceans separated by different phases of ice, the deepest one touching the rocky seafloor about 800 km down, where temperatures could be up to 40 K higher than at the top of the ocean.[^vance2014][^jpl-sandwich2014] Hubble observations published in 2015 supported the ocean: Ganymede's auroral ovals rock back and forth less than they would over a solid interior, because a conducting salty layer opposes the changing Jovian field.[^nasa-hubble2015] ### Magnetosphere Galileo made six close flybys between 1996 and 2000 and found that Ganymede has a permanent magnetic moment of its own, about 1.3 × 10¹³ T·m³, three times that of Mercury.[^kivelson1997][^kivelson2002] The dipole is tilted 176° to the rotation axis, pointing opposite to Jupiter's, and produces a field of 719 ± 2 nT at the equator and about 1,440 nT at the poles, several times the Jovian field of about 120 nT at Ganymede's distance.[^kivelson2002][^kivelson1997] The result is a miniature magnetosphere 4–5 Ganymede radii across. Below about 30° latitude its field lines are closed and trap charged particles in a small radiation belt; above that latitude they connect to Jupiter, letting energetic particles reach the surface near the poles.[^kivelson1998] Jupiter's co-rotating [[Plasma_(physics)|plasma]] flows past it the way the [[Solar_wind|solar wind]] flows past Earth, but the flow is subsonic rather than supersonic, so no bow shock forms.[^volwerk1999] Galileo also found a weaker induced field of about 60 nT, of the kind that at Europa and Callisto points to a salty ocean.[^kivelson2002] The field is most likely made by a dynamo in the liquid core, driven by compositional convection as the iron–sulfur core slowly cools.[^hauck2006] Why a body this small still has an active dynamo is unresolved; one proposal is that past resonance passages heated the mantle and slowed the core's heat loss, keeping it convecting.[^bland2007] ## Surface environment ### Surface features About two-thirds of Ganymede is bright terrain, crossed by sets of parallel grooves and ridges, and about one-third is dark, older and saturated with craters.[^patterson2007][^showman1999] Crater densities give the dark terrain an age of about 4 billion years, comparable to the lunar highlands, while the grooved terrain is somewhat younger by an uncertain amount.[^zahnle1998] The grooves are now thought to be mostly tectonic: stretching of the icy lithosphere, perhaps during episodes of tidal heating, broke it into blocks of horst and graben faults that erased the old dark crust over much of the surface, with cryovolcanism playing at most a minor part.[^showman1999][^bland2007] A 2020 analysis of the furrows of the dark regions concludes that they form one concentric system produced by a single giant impact about 4 billion years ago.[^hirata2020] Water ice makes up 50–90% of the surface by mass, and near-infrared spectra show strong ice bands at 1.04, 1.25, 1.5, 2.0 and 3.0 μm.[^showman1999][^calvin1995] Galileo's mapping spectrometer found [[Carbon|carbon]] dioxide, sulfur dioxide and possibly other compounds, together with hydrated salts such as magnesium sulfate that may have come from the ocean.[^mccord1998][^mccord2001] Carbon dioxide is spread evenly in longitude but is scarce near the poles.[^hibbitts2003] The hemisphere that faces forward along the orbit is the brighter one.[^calvin1995] Polar caps of frost extend toward 40° latitude; Galileo data favour their formation by plasma striking the unprotected polar ice and redistributing water molecules.[^khurana2007] The oldest craters have relaxed into faint, flat "palimpsests" such as Memphis Facula, while young craters still show bright rays of ejecta.[^nineplanets-ganymede] The dark plain Galileo Regio carries a system of concentric furrows formed during an early period of geological activity.[^casacchia1984] ### Atmosphere and ionosphere A 1972 stellar occultation observed from Indonesia and India was reported as showing a thin atmosphere with a surface pressure of about 0.1 Pa, but Voyager 1's far-ultraviolet occultation in 1979 found none, setting an upper limit of 2.5 μPa.[^carlson1973][^broadfoot1981] In 1996 the Hubble Space Telescope detected ultraviolet airglow of atomic oxygen at 130.4 nm and 135.6 nm, the signature of molecular oxygen being broken apart by electron impact; the implied surface pressure is 0.2–1.2 μPa, consistent with the Voyager limit.[^hall1998][^jpl-hubble1996] As on Europa, the oxygen comes from radiation splitting surface ice, with the hydrogen escaping faster. The airglow is brightest in two bands near ±50° latitude, at the boundary between open and closed field lines, and is interpreted as aurora.[^feldman2000] Oxygen is also locked in the ice: ozone was detected in 1996 and molecular oxygen trapped in the ice in 1997.[^noll1996][^calvin1997] Around Ganymede, [[Sodium|sodium]] is scarcer than around Europa by a factor of at least 13, and atomic [[Hydrogen|hydrogen]] extends some 3,000 km from the surface.[^brown1997][^barth1997] Whether Ganymede has an ionosphere is still unsettled: Galileo measurements of electron density near the moon range from 400 to 2,500 per cm³.[^eviatar2001] ### Radiation environment Radiation at the surface is far lower than at Europa, about 50–80 mSv per day, a dose that, sustained for about two months, would make a person gravely ill or kill them.[^podzolko2013] ## Origin and evolution Ganymede probably formed by accretion within the disc of gas and dust around the young Jupiter.[^canup2002] Models suggest it assembled in about 10,000 years, far faster than the roughly 100,000 years proposed for Callisto, because the disc was denser closer to Jupiter.[^mosqueira2003] Rapid growth traps accretional heat, which could have melted ice and let rock sink to form the core; slow-growing Callisto lost that heat and stayed only partly separated, which may explain why two moons of similar mass and bulk make-up look so different.[^mckinnon2006] Alternative explanations invoke stronger tidal heating of Ganymede, or more energetic bombardment in the [[Late_Heavy_Bombardment|Late Heavy Bombardment]], which in one model makes differentiation run away at Ganymede but not at Callisto.[^showman1997][^barr2010] After formation, the core released its stored heat slowly to the ice above, where convection carried it outward; [[Radioactive_decay|radioactive decay]] in the rock heated the core further and completed its separation into an iron–iron-sulfide centre and a silicate mantle.[^hauck2006][^mckinnon2006] Heat escaping from the rock today keeps the ocean liquid, and the slow cooling of the core sustains the convection that generates the magnetic field.[^spohn2003][^hauck2006] ### Geological periods The USGS global geologic map divides Ganymede's history into three periods. In the Nicholsonian, the original bright icy surface was darkened by non-ice material from space, like the dark Nicholson Regio seen today. In the Harpagian, named after Harpagia Sulcus, tectonic resurfacing replaced about two-thirds of the ancient dark terrain with bright grooved terrain. The Gilgameshan began with the impact that formed the Gilgamesh basin and has been dominated by cratering; its craters grow progressively crisper with time, suggesting that heat flow fell and the ice shell thickened.[^usgs2013] ## Exploration Six spacecraft have made close flybys of Ganymede, two Pioneers, two Voyagers, Galileo and Juno, and two more have observed it from a distance; none has yet orbited it.[^chang2021] Pioneer 10 passed in 1973 and Pioneer 11 in 1974, returning images with a resolution of up to 400 km.[^nasa-pioneer11][^nasa-sp349] The Voyager flybys of 1979 showed that Ganymede is larger than Titan, which had been thought the bigger of the two, and gave the first views of its grooved terrain; [[Voyager_1]] imaged the Jupiter-facing hemisphere and Voyager 2 the opposite one.[^usgs2013] ### Completed flybys Galileo, in orbit around Jupiter from 1995, made six close passes between 1996 and 2000, known as G1, G2, G7, G8, G28 and G29. G1 in 1996 detected the magnetic field, and G2 came within 264 km of the surface, still the closest approach by any spacecraft.[^kivelson2002][^jpl-galileo1996] Galileo data later revealed the induced field that points to an ocean.[^kivelson2002] Cassini, passing Jupiter in December 2000 on the way to [[Saturn]], imaged Ganymede from 10.35 million km, and New Horizons from 3.5 million km in 2007 en route to [[Pluto]].[^nasa-cassini2000][^nasa-nh2007] Juno made a distant flyby on 25 December 2019 that imaged the polar regions, and on 7 June 2021 passed 1,038 km above the surface, a close encounter that also shortened Juno's orbit around Jupiter from 53 to 43 days.[^swri-juno2020][^chang2021] ### Future missions Juice was launched on an Ariane 5 on 14 April 2023 and is due to reach Jupiter in July 2031. After a tour of flybys of Europa, Ganymede and Callisto it is to enter orbit around Ganymede, the first spacecraft to orbit a moon other than Earth's, within a nominal science phase of about four years.[^esa-juice-fs] NASA's Europa Clipper, launched in October 2024, is also scheduled to make close flybys of Ganymede during its tour of the Jupiter system.[^campagnola2019] ### Cancelled proposals The Jupiter Icy Moons Orbiter, which would have studied Ganymede closely, was cancelled in NASA's 2005 budget.[^peplow2005] A Juno-derived Ganymede orbiter studied for the 2011 planetary decadal survey was passed over in favour of a Europa mission.[^nrc2011] The joint NASA–ESA Europa Jupiter System Mission, given priority over a Titan mission in 2009, was to include an ESA-led Jupiter Ganymede Orbiter; when NASA's part was dropped, ESA went ahead alone in 2012 with the mission that became Juice.[^rincon2009][^esa-l1-2012] A Russian lander concept, Laplace-P, was cancelled for lack of funding in 2017.[^tass2017] ## See also - [[Galilean_moons]] - [[Jupiter]] · [[Io_(moon)|Io]] · [[Europa_(moon)|Europa]] · [[Callisto_(moon)|Callisto]] - [[Titan_(moon)|Titan]] - [[Resonance]] - Moons of Jupiter · List of geological features on Ganymede - [[PORTAL_Solar_System|Solar System portal]] ## Notes Derived numbers are computed from the cited values. The period ratios are 3.551181 / 1.769138 ≈ 2.007 and 7.154553 / 1.769138 ≈ 4.044. The mass ratios use 1.4819 × 10²³ kg for Ganymede, 1.3455 × 10²³ kg for Titan (ratio ≈ 1.10), 7.346 × 10²² kg for the Moon (≈ 2.02) and 3.30 × 10²³ kg for Mercury (≈ 0.45). The magnetic moment is consistent with the equatorial field: 719 nT × (2.634 × 10⁶ m)³ ≈ 1.3 × 10¹³ T·m³. Surface gravity is GM/r² with G = 6.674 × 10⁻¹¹ m³ kg⁻¹ s⁻² and r = 2.6312 × 10⁶ m, giving ≈ 1.43 m/s². ## References [^nasa-ganymede]: NASA Solar System Exploration. "Ganymede: in depth". https://solarsystem.nasa.gov/moons/jupiter-moons/ganymede/in-depth/ [^nasa-ganymede-facts]: NASA Science. "Ganymede: facts". https://science.nasa.gov/jupiter/moons/ganymede/facts/ [^nasa-jsat]: Williams, D. R. "Jovian Satellite Fact Sheet". NASA NSSDCA. https://nssdc.gsfc.nasa.gov/planetary/factsheet/joviansatfact.html (fetched 2026-09-18). [^nasa-ssat]: Williams, D. R. "Saturnian Satellite Fact Sheet". 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"The Galilean satellites". *Science* 286: 77–84. https://doi.org/10.1126/science.286.5437.77 ## External links - NASA Science: Ganymede — https://science.nasa.gov/jupiter/moons/ganymede/ - ESA Juice mission — https://sci.esa.int/web/juice - USGS Global Geologic Map of Ganymede (SIM 3237) — https://pubs.usgs.gov/sim/3237/ - USGS Gazetteer of Planetary Nomenclature — https://planetarynames.wr.usgs.gov/ ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Ganymede_(moon)) : [Wikitube](https://en.wikitube.io/wiki/Ganymede_(moon)) · pinned revision [1375329281](https://en.wikipedia.org/w/index.php?oldid=1375329281) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-059 · explorer state `?obj=Ganymede`.* <!-- hub_tags: Life_Physics · PORTAL_Solar_System -->