Ganymede (moon)

Ganymede (moon)

Ganymede, international name Jupiter III Ganymede, is a natural satellite of Jupiter. On the scale of distances to the center of Jupiter, it is the seventh known natural satellite on the planet and the third Galilean satellite. Completing an orbit in approximately seven Earth days, it participates in a so-called Laplace orbital resonance, of the 1:2:4 type, with Europa and Io. With a diameter of 5,268 kilometers, 8% higher than that of the planet Mercury and 2% that of Titan, Saturn’s largest moon, Ganymede is the largest natural satellite of Jupiter and the largest in the entire Solar System. Being made up of roughly equal amounts of silicate rock and water ice, its mass is only 45% that of Mercury (consisting of rocks and metals), but remains the largest of all planetary satellites in the Solar System, reaching 2.02 times that of the Moon.

Type Natural satellite of Jupiter
Ganymede Designations
Name Ganymede
Alternative Name Jupiter III
Name origin Ganymede (Greek mythology)
Named by Simon Marius
Ganymede Discovery
Discoverer Galileo
Discovery method Observation with a telescope
Discovery date January 7, 1610
Discovery place Padua
Characteristics of the atmosphere
Atmosphere composition Traces of oxygen
Ganymede Orbital characteristics
Semi-major axis 1 070 400 km
Periapsis 1 069 008 km
Apoapsis 1 071 792 km
Eccentricity 0.001 5 to 0.001 3
Orbital circumference 6 725 518,71 km
Revolution period 7,154 553 days
Average orbital speed 10,879 km/s
Inclination (relative to Jupiter’s equatorial plane) 0,21°
Inclination (relative to Jupiter’s Laplace plane) 0,117°
Ganymede Physical characteristics
Diameter 5 262,4 km (average)
Mass 1,482 x 1023 kg
Area 86 999 665,93 km2
Volume 76 304 506 998 km3
Average volumic mass 1,940 to 1,942 × 103 kg/m3
Surface gravity 1,428 m/s2
Release speed 2,742 km/s
Rotation period 7,154 553 days (synchronous)
Apparent magnitude 4,61 ± 0,03
Middle albedo 0,44
Minimum surface temperature 70 K (-203,15°C)
Average surface temperature 110 K (-163,15°C)
Maximum surface temperature 152 K (-121,15°C)

Ganymede is a totally differentiated body, consisting of an iron-rich liquid core and an ocean beneath surface ice, which could hold more water than all of Earth’s oceans combined. The two major terrain types on its surface cover about one-third of dark, impact crater-ridden and four-billion-year-old regions, and the remaining two-thirds lighter regions, intersected by wide, barely younger grooves. The cause of this geological disturbance is not well known, but is likely the result of tectonic activity caused by tidal warming.

It is the only known satellite in the Solar System with a magnetosphere, probably created by convection inside the liquid iron core. Its weak magnetosphere is within Jupiter’s much larger magnetic field and connected to it by open field lines. The satellite has a thin atmosphere that contains atomic oxygen (O), oxygen (02) and possibly ozone (O3); atomic hydrogen is also present in small proportions. It is not yet known whether the satellite has an ionosphere associated with its atmosphere or not.

Although Ganymede can be seen with the naked eye in the night sky, it is considered to form, along with Io, the first pair of objects to have been both detected and resolved using an optical instrument. Their discovery is indeed attributed to Galileo, who observed them separately for the first time on January 7, 1610, in Padua with an astronomical telescope of his manufacture.

The name of the Galilean satellite was later suggested by the astronomer Simon Marius, after the mythological Ganymede. Pioneer 10 is the first probe to examine it closely. The Voyager probes have refined measurements of its size, while the Galileo probe has discovered its subterranean ocean and magnetic field. The next mission scheduled to explore the Jovian system is the  European Space Agency’s Jupiter Icy Moon Explorer (JUICE), which was launched on April 14, 2023, for just over 12 years with orbit around Ganymede starting in December 2034.

Ganymede Discovery and naming

On January 7, 1610, in Padua, Galileo Galileo observed with a telescope of his own making what he took for three stars close to Jupiter, which turned out to be Ganymede, Callisto and the combined light of Io and Europa. The next night, he notices that they have moved. On January 13, he saw the four objects in one piece for the first time, although he had seen each of the moons at least once before. On January 15, Galileo concluded that these stars are actually bodies orbiting Jupiter. He claims the right to name moons; he considered Cosmica Sidera for a time before opting for Medicea Sidera (“Medici Stars”).

The French astronomer Nicolas-Claude Fabri de Peiresc suggested names taken from the Medici family for the moons, but his proposal was not accepted. Simon Marius, who claims to have discovered the Galilean satellites as early as November 1609 but did not publish his observations until 1614, tries to name the moons the “Saturn of Jupiter”, the “Jupiter of Jupiter” (for Ganymede), the “Venus of Jupiter” and the “Mercury of Jupiter”, but this nomenclature has never been retained. At the suggestion of Johannes Kepler, Marius again attempted to name the moons:

… Iupiter à poëtis ob illicitos maximè amores arguitur: Inprimis autem celebrantur tres fœminæ Virgines, quarum furtivo amore Iupiter captus & potitus est, videlicet Io Ianachi Amnis filia: Deinde Calisto Lycaonis, & deniq; Europe Agenoris filia: Quin etiam impensius amavit Ganymedem puerum formosum, Trois Regis filium, adeo etiam ut assumptâ àquilæ figurâ, illum humeris impositum, in cœlum transportavit, prout fabulantur poetæ, inprimis autem Ovidius lib. I o.fab.6. Itaque non male fecisse videor, si Primus à me vocatur Io. Secundus Europa: Tertius ob luminis Majestatem Ganymedes Quartus denique Calisto….”

Simon Marius, Mundus Iovialis anno M.DC.IX Detectus ope perspicilli Belgici, 1614

“Jupiter is accused by poets of the most illicit loves; three young virgin women are especially mentioned, for Jupiter was seized and possessed of a hidden love for them, namely Io, daughter of Inachos, then Callisto daughter of Lycaon, and finally Europa daughter of Agenor; moreover he loved Ganymede, a handsome young man son of King Tros, so much so that having taken the form of an eagle he carried him to heaven resting on his shoulders, according to what the poets say, especially Ovid [Metamorphoses] I, 6. Thus, I do not think I have done wrong if the first is called by me Io, the second Europe, the third Ganymede because of the majesty of its light, and finally the fourth Callisto.”

The name Ganymede and the other Galilean satellites fell into disuse until the middle of the twentieth century. In most early astronomical documents, Ganymede is rather evoked by its numerical designation in Roman numerals, a system introduced by Galileo: Jupiter III or the “third satellite of Jupiter”. Following the discovery of Saturn’s moons, a naming system based on that of Kepler and Marius was used for Jupiter’s moons. Ganymede is Jupiter’s only Galilean moon named after a male character.

The discovery of Ganymede is generally credited to Galileo. However, according to the archives of Chinese astronomy, Gan De discovered it in 362 BC. a moon of Jupiter with the naked eye, probably Ganymede, nearly two millennia before the Italian astronomer. The Galilean moons can indeed be distinguished with the naked eye, during their maximum elongation and under exceptional observation conditions.

Orbit and rotation of Ganymede

Ganymede orbits on average 1,070,400 kilometers from Jupiter, the third largest Galilean satellite and the seventh known natural satellite on the planet. He carries out a revolution every seven days and three hours. Like most moons, Ganymede’s rotation is locked by gravitational tidal effects, causing the satellite to have one face permanently facing the planet. Its orbit is very slightly eccentric and inclined at the Jovian equator, whose eccentricity and inclination change almost periodically under the effect of solar and planetary gravitational perturbations on a time scale of several centuries. The ranges of change are 0.0009–0.0022 and 0.05–0.32°, respectively. These orbital variations cause the inclination of the axis (the angle between the axis of rotation and the orbital axis) to oscillate between 0 and 0.33°.

Ganymede is in an orbital resonance with Europa and Io: for every revolution of Ganymede around Jupiter, Europa performs two, and Io four. The superior conjunction between Io and Europa always occurs when Io is at its perizene (closest to Jupiter) and Europa at its apozene (farthest from Jupiter). The superior conjunction between Europa and Ganymede occurs when Europa is at its perizene. The longitudes of the Io–Europa and Europa–Ganymede conjunctions change at the same rate, which prevents any triple conjunction of the moons. A complex resonance of this kind is called “Laplace resonance”.

The current Laplace resonance is unable to oscillate the eccentricity of Ganymede’s orbit to a higher value. Its value of about 0.0013 is probably the remnant of a time when this oscillation was possible. The Ganymedian orbital eccentricity is a bit confusing; without the current oscillation, it should have decreased long ago due to the dissipation of tides inside Ganymede. This means that the last episode of the excitement of eccentricity took place only a few hundred million years ago. Because of this relatively low orbital eccentricity, the tidal warming of the moon is now negligible. But it may be that Ganymede once passed through one or more Laplace resonances capable of strengthening its orbital eccentricity to a higher value of 0.01 to 0.02. This is the probable cause of the significant tidal warming inside Ganymede; The formation of grooved surfaces could be due to one or more warming episodes.

Two hypotheses would explain the origin of the Laplace resonance between Io, Europa and Ganymede. It would have existed since the beginning of the Solar System for the first, while it would have developed after its formation for the second. In the latter scenario, the proposed sequence is as follows: the tides between Io and Jupiter would have increased, causing the satellite’s orbit to widen to cause a 2:1 ratio resonance with Europa; the enlargement would have continued, but part of the angular momentum would have transferred to Europa via resonance, which would also have expanded its orbit; the process would have continued until Europa in turn provoked a similar resonance, this time with Ganymede. Finally, the drift velocities of conjunctions between the three moons would have synchronized and blocked in the form of the Laplace resonance.

Physical characteristics

Mass and dimensions of Ganymede

Size comparison of Earth, Moon and Ganymede
Size comparison of Earth, Moon and Ganymede

At about 5,260 km in diameter, Ganymede is the largest natural satellite in the Solar System, slightly larger than the second, Titan (5,150 km), a satellite of Saturn. It is also larger than the planet Mercury (4,878 km) and the dwarf planet Pluto. In the Jovian system, the second largest satellite is Callisto (4,821 km).

Ganymede, if it remains the most massive of all natural satellites with 1.481 9 × 10 kg, represents 45% of the mass of Mercury (3.302 × 10 kg) because of its lower density (1.942 × 10 kg / m against 5.427 × 10 kg / m), indicative of an internal composition with a high proportion of ice rather than rock. In fact, although it is almost one and a half times larger, the gravity on the surface of Ganymede is weaker than on the Moon (0.146  g  versus 0.165 4 g).


The average density of Ganymede, 1.936 g/cm, suggests a composition comprising rock matter and water in equal parts, the latter being mainly in the form of ice. The mass fraction of ice is 46 to 50 percent lower than on Callisto, but its total mass remains the largest of all planetary satellites in the Solar System, reaching 2.02 times that of the Moon. Additional volatile ice such as ammonia may also be present. The exact composition of Ganymede rock is not known, but it is probably close to that of ordinary L/LL type chondrites, which are characterized by less total iron, less metallic iron, and less iron oxide than H chondrites. The ratio by weight of iron to silicon is 1.05 to 1.27 on Ganymede, while the solar ratio is about 1.8.

The surface of Ganymede has an albedo of about 43%. Water ice seems to be ubiquitous on the surface, with a mass proportion of 50-90%, significantly more than for the entire Moon. Near-infrared spectroscopy revealed the presence of a strong water ice absorption band at wavelengths 1.04, 1.25, 1.5, 2.0 and 3.0 μm. Grooved terrain is brighter and its composition is more icy than dark terrain. High-resolution analysis of the near-infrared and ultraviolet spectra obtained by the Galileo spacecraft and  from Earth revealed non-aqueous materials: carbon dioxide, sulfur dioxide, and possibly cyanogen, sulfuric acid and various organic compounds. Galileo results also showed magnesium sulfate (MgSO 4) and possibly sodium sulfate (Na2SO4) on the surface of Ganymede. These salts could come from the subglacial ocean.

The Ganymedian surface is asymmetrical; The front hemisphere is brighter than the rear hemisphere. This is the same situation as Europe, but the reverse is also true for Callisto. The rear hemisphere of Ganymede appears to be enriched with sulfur dioxide. The distribution of carbon dioxide does not show hemispheric asymmetry, although this element is not observed at the poles. All but one of the satellite’s impact craters show no carbon dioxide enrichment, which also distinguishes it from Callisto. Ganymede’s carbon dioxide levels have likely been depleted in the past.

Ganymede internal structure

Ganymede appears to be completely differentiated into an iron-sulfide core of iron(II) and a mantle of silicates. The exact thicknesses of the inner layers depend on the assumed composition of the silicates (including olivine and pyroxene) and the amount of sulfur in the core. Because of the large presence of water and its differentiated interior, Ganymede is the celestial body with the lowest normalized moment of inertia in the solar system (0.31).

Subglacial oceans

In the 1970s, NASA scientists suspected the presence of a thick ocean between two layers of ice, one at the top and the other at the bottom. In the 1990s, NASA’s Galileo spacecraft flew by Ganymede and confirmed the existence of the lunar ocean. A study published in 2014 taking into account realistic thermodynamics for water and the effects of salt suggests that Ganymede may have several layers of oceans separated by different phases of ice.

The lowest liquid layer would be right next to the rocky mantle. The contact between rock and water could be an important factor in the origin of life. The study also mentions that because of the extreme depths (about 800 km to the rocky “seafloor”), temperatures at the bottom of a convective (adiabatic) ocean can be up to 40 K above those of the ice-water interface. In March 2015, researchers reported that measurements made by the Hubble Space Telescope proved the presence of a subglacial ocean on Ganymede by studying how its auroras move on the surface. A large saltwater ocean containing more water than all the Earth’s oceans combined affects Ganymede’s magnetic field, and therefore its auroras.

The ocean of Ganymede has been the subject of speculation about its potential habitability.


The existence of an iron-rich liquid core is a natural explanation for the presence of an intrinsic magnetic field, as detected by the Galileo spacecraft. The convection movements of liquid iron, whose electrical conductivity is high, is the most likely magnetic field generator. The density of the core is 5.5–6 g/cm and that of the silicate mantle is 3.4–3.6 g/cm. The radius of the core could reach up to 500 km. The temperature within the core is probably 1,500–1,700 K and the pressure should reach 10 GPa (100,000 atm).



Representation of Ganymede centered at 45° West longitude
Representation of Ganymede centered at 45° West longitude. The dark areas to the north and south are the Perrine and Nicholson regions; the striped white craters are Tros (to the northeast) and Cisti (to the southwest)


Mosaic of Voyager 2 images of the anti-Jovian hemisphere of Ganymede
Mosaic of Voyager 2 images of the anti-Jovian hemisphere of Ganymede. The ancient dark region of Galileo Regio lies to the northeast. It is separated from the smaller dark region Marius Regio to the west by the brighter and younger band of Uruk Sulcus. Fresh ice ejected from the relatively recent crater Osiris has created bright stripes to the south

The surface of Ganymede is a mixture of two types of terrain: very ancient dark regions, heavily covered with impact craters, and lighter and younger (but nevertheless ancient) regions marked by numerous furrows and ridges. The dark terrain, which occupies about a third of the surface, contains clays and organic matter that could indicate the composition of the impactors from which the Jovian satellites accrete.

The warming mechanism required for the formation of the Ganymede grooved terrain is an unsolved problem in planetary science. The modern view is that the topography of this terrain is tectonic in nature. It is suggested that cryovolcanism played only a minor role, if any.

The forces that caused the stresses in Ganymede’s icy lithosphere to start tectonic activity could be related to past episodes of tidal warming, perhaps caused when the satellite passed through unstable orbital resonances. It is possible that the tidal bending of the ice warmed the interior and strained the lithosphere, leading to the development of faults forming horsts and grabens, which erased the ancient dark terrain over 70% of the surface. It is also possible that the formation of the grooved terrain is related to early core formation and subsequent tidal warming inside the moon, which may have caused a slight Ganymede expansion of the order of 1–6% due to phase transitions in the ice and thermal expansion.

During the evolution that followed, deep plumes of hot water may have risen from the core to the surface of the satellite, leading to tectonic deformation of the lithosphere. Radioactive heat inside the satellite is the most likely heat source. It is what allows the existence of a subglacial ocean. Research models have revealed that if the orbital eccentricity were an order of magnitude greater than today (as it might have been in the past), tidal warming would have been a larger source of heat than radioactive warming.

Gula and Achelous craters (bottom of image) in the grooved terrain of Ganymede, with ejecta "pedestals" and ramparts
Gula and Achelous craters (bottom of image) in the grooved terrain of Ganymede, with ejecta “pedestals” and ramparts

All terrains have traces of impact craters, but their number is particularly important for the dark parts, which appear to be riddled with them and have greatly evolved according to the impacts received. The lighter-striated terrain contains far fewer traces of impacts, which are of little importance due to its tectonic evolution. The density of these craters gives an age of 4 billion years for dark regions, similar to that of the highlands of the Moon, and younger for light regions, but without being able to determine how much. It is possible that Ganymede was subjected to a period of intense bombardment like the Moon 3.5 to 4 billion years ago.

If so, then the vast majority of impacts occurred at that time, with the bombing rate being much lower since then. The craters cover some furrows and are sheared by others, indicating that these are ancient. Younger craters, with ejecta lines, are also visible. Unlike those on the Moon and Mercury, the craters of Ganymede are fairly flat, lacking the central rings and depressions that are common on these bodies. It is possible that this is due to the nature of the Ganymede ice crust that can flow and soften the reliefs. Ancient craters whose relief has disappeared and which have left only a kind of “ghost” crater are known as palimpsests.

One of the important structures on Ganymede is a dark plain named Galileo Regio, as well as a series of concentric furrows, probably created during a period of geological activity.

Ganymede also has polar ice caps, probably composed of water ice. Frost extends up to 40° latitude. These polar ice caps were first observed by the Voyager probe. Theories about the formation of ice caps include the migration of water at high latitudes and the bombardment of ice by plasma. Galileo data suggest that the second explanation is valid. The presence of a magnetic field on Ganymede results in a more intense bombardment of charged particles on the surface of unprotected polar regions; A spray that leads to the redistribution of water molecules, with frost migrating to locally cooler areas within the polar regions.

Ganymede has an equatorial hump about 600 km in diameter and three kilometers high. Its discovery was announced in March 2015.

Coordinate system

A crater named Anat provides the reference point for measuring longitude on Ganymede. By definition, Anat is at 128 degrees longitude.


As for the other objects of the Solar System, the toponymy of the surface of Ganymede obeys a strict nomenclature on the part of the International Astronomical Union:

  •  catenae are named after deities and heroes of the Fertile Crescent civilizations. Only four are named: Enki, Khnum, Nanshe and Terah;
  •  craters also have such names, such as Gilgamesh, Ilos or Nidaba. Ganymede has 131 named craters;
  • the faculae are named after places associated with Egyptian myths, such as Memphis or Thebes. Ganymede has 17;
  • the fossae are named after the deities of the Fertile Crescent: Lakhamu, Lakhmu and Zu;
  • paterae refer to dry canals in the Fertile Crescent region. Ganymede has 6 named;
  • the five regions pay tribute to the astronomers who discovered the satellites of Jupiter: Barnard, Galileo, Marius, Nicholson and Perrine;
  •  Sulcia bear the names of places associated with the myths of the Fertile Crescent.

Ganymede atmosphere and ionosphere

In 1972, a team of Indian, British and American astronomers working in Java and Kavalur claimed to have detected a thin atmosphere around the satellite during an occultation, during which he and Jupiter passed in front of a star. She estimated a surface pressure of about 0.1 Pa. However, Voyager 1 observed the occultation of the star κ Centauri in 1979 during its flyby of the planet and provides different results. Occultation measurements were conducted in the ultraviolet spectrum (in wavelengths below 200 nm); They were much more sensitive to the presence of gases than the 1972 measurements in the visible spectrum. No atmosphere was revealed by Voyager’s data. The upper limit of the numerical density of surface particles was established at 1.5 × 10 cm, which corresponds to a surface pressure of less than 2.5 μPa. The latter value is almost five orders of magnitude lower than the 1972 estimate.

Despite Voyager’s data, evidence of a tenuous oxygen atmosphere on Ganymede, an exosphere very similar to that found on Europa, was found by the Hubble Space Telescope in 1995. He actually observed the night sky light of atomic oxygen (O) in the far ultraviolet at wavelengths 130.4 nm and 135.6 nm. Such luminescence is excited when molecular oxygen is dissociated by electron impacts, evidence of the existence of a significant atmosphere composed mainly of oxygen molecules (O2). The numerical surface density is probably between 1.2 × 10 cm and 7 × 10 cm, which corresponds to a surface pressure between 0.2 μPa and 1.2 μPa.

These values are in line with the maximum value established by Voyager in 1981. Oxygen is not proof of the existence of life; the researchers speculate that it is produced when water ice on Ganymede’s surface is separated into hydrogen and oxygen by radiation, with hydrogen lost to space much faster due to its low atomic mass. The luminescence observed on Ganymede is not as spatially homogeneous as that of Europa. The Hubble Space Telescope observed two bright points in the northern and southern hemispheres at about 50° latitude, exactly where the boundary between the open and closed lines of the Ganymedian magnetic field lies (cf. infra). The bright spots are probably polar auroras created by plasma precipitation along open field lines.

The existence of a neutral atmosphere implies the existence of an ionosphere, because oxygen molecules are ionized by the impacts of energetic electrons from the magnetosphere and extreme ultraviolet radiation. However, the nature of the Ganymedian ionosphere is as controversial as the nature of its atmosphere. Indeed, some Galileo measurements found a high electron density near Ganymede, suggesting an ionosphere, while others failed to detect anything. The density of electrons near the surface is estimated by different sources between 400 cm and 2,500 cm. In 2008, the parameters of the ionosphere of Ganymede are not yet very limited.

Further evidence for the existence of an oxygen atmosphere comes from spectral detections of gases trapped in ice on Ganymede’s surface. The detection of ozone spectral bands (O3) was announced in 1996. In 1997, a spectroscopic analysis revealed the absorption characteristics of molecular oxygen dimers (or diatomic molecules). This kind of absorption can only occur 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 and not on surface albedo; they tend to decrease with increasing latitude on Ganymede, while those of O3 show an opposite trend. A laboratory experiment found that O2 does not produce groups or bubbles, but rather dissolution in ice at the relatively warm temperature of Ganymede’s surface at 100 K (−173 °C).

A search for sodium in the atmosphere in 1997 following its discovery on Europa yielded nothing. Sodium is at least 13 times less abundant around Ganymede than in Europe, perhaps because of a relative lack of surface or because its magnetosphere repels energetic particles. The hydrogen atom is another minor component of the Ganymedian atmosphere. Hydrogen atoms were observed up to 3,000 km from Ganymede’s surface. Their density on the surface is about 1.5 × 10 cm.


The first flyby of Ganymede by the Galileo probe discovered that Ganymede has its own magnetic field, contained in Jupiter’s magnetosphere. Ganymede is the only natural satellite with a magnetosphere. Ganymede’s intrinsic magnetic field is probably generated similarly to Earth’s, by moving conductive material through its inner layers, probably into its metallic core. Ganymede also has an induced magnetic field, indicating that it has a layer that acts as a conductor. The hypothesis is that this conductive material is a layer of liquid water containing salt, located 150 km below the surface and sandwiched between two layers of ice of different densities.

Ganymede is the most concentrated solid body known in the Solar System, suggesting that it is totally differentiated and has a metallic core. Ganymede’s magnetic field would be produced by thermal convection in the core. Convection movements inside the mantle may have occurred in the past.

The Galileo spacecraft made six close flybys of Ganymede during the period 1995-2000 (G1, G2, G7, G8, G28 and G29) and discovered that Ganymede has a permanent (intrinsic) magnetic moment independent of Jupiter’s magnetic field. The value of the moment is about 1.3 × 10 T m, which is three times more than the magnetic moment of Mercury. The magnetic dipole is tilted 176° relative to the axis of rotation, which means that it is directed against Jupiter’s magnetic moment.

Its north pole lies below the orbital plane. The magnetic field of the dipole created by this permanent moment has an intensity of 719 ± 2 nT at the equator of Ganymede, which is stronger than the Jovian magnetic field at the distance of Ganymede (about 120 nT). The equatorial field of Ganymede is directed against that of Jupiter, which makes reconnection possible. The strength of the intrinsic field is twice as great at the poles as at the equator (1,440 nT).

Auroras over Ganymede - variations in the aurora belt could indicate the presence of a subglacial salty ocean
Auroras over Ganymede – variations in the aurora belt could indicate the presence of a subglacial salty ocean

The permanent magnetic moment cuts through some of the space around Ganymede, creating a tiny magnetosphere integrated with Jupiter’s, making it the only moon in the solar system with this feature. Its diameter is 4–5 R G  (RG = 2,631.2 km). The Ganymedian magnetosphere has a region of closed field lines below 30° latitude, where charged particles (electrons and ions) are trapped, creating a kind of Van Allen belt. The main ion species in the magnetosphere is ionized oxygen (O), which fits well with Ganymede’s tenuous oxygen atmosphere.

In polar sea ice regions, at latitudes above 30°), the magnetic field lines are open, connecting Ganymede with Jupiter’s ionosphere. In these areas, energetic electrons and ions (tens of thousands of electron volts) were detected, which could be the cause of the polar auroras observed around the poles of the satellite. Also, heavy ions continuously rush onto Ganymede’s polar surface, pulverizing and darkening the ice.

The interaction between plasma in the Ganymedian and Jovian atmospheres closely resembles that between the solar wind and the Earth’s magnetosphere. The plasma in co-rotation with Jupiter influences the drag side of the satellite’s magnetosphere much like the solar wind does on Earth’s magnetosphere. The main difference is that the speed of the plasma flow is supersonic in the case of Earth and subsonic in the case of Ganymede. Because of this subsonic flow, there is no arc of shock in the hemisphere of Ganymede’s magnetic tail.

In addition to an induced magnetic moment, Ganymede has an induced dipole magnetic field. Its existence is connected with the variation of the Jovian magnetic field near the satellite. The induced moment is directed radially to or from Jupiter following the direction of the variable part of the planetary magnetic field. The induced magnetic moment is an order of magnitude lower than the intrinsic. The intensity of the magnetic field induced at the magnetic equator is about 60 nT, half of the ambient Jovian field. The induced magnetic field of Ganymede is similar to those of Callisto and Europa, indicating that this moon also has a subglacial ocean with high resistivity.

Since Ganymede’s internal structure is completely differentiated and has a metallic core, its own magnetic field is probably generated in a manner similar to that of Earth, i.e. as a result of the movement of conductive materials inside. The magnetic field detected around Ganymede is probably caused by compositional convection in its core if produced by a dynamo effect, or by magnetoconvection.

Despite the presence of an iron heart, the magnetosphere of Ganymede remains an enigma, especially because of the absence of this element for similar stars. Some research has suggested that given its relatively small size, the core should have cooled enough to reach the point where fluid movement and magnetic field should have stopped. One explanation would be that the same orbital resonances proposed about disturbances on its surface would have allowed the magnetic field to persist. With Ganymede’s eccentricity producing a pumping effect and tidal heating increasing during these resonances, the mantle may have warmed the core and thus avoided its cooling. Another explanation proposes the magnetization remains of the silicate rocks of the mantle, which would be possible if the satellite had a much larger dynamo-generated field in the past.


In 1999, a ring-shaped debris disk was highlighted by the Heidelberg dust detector on board Galileo, as well as for Europa and Callisto.

Its discovery was announced on June 2, 1999, by a press release from the Max Planck Institute for Nuclear Physics and NASA.

Ganymede origin and evolution

Ganymede was probably formed by accretion in Jupiter’s subnebula, a disk of gas and dust surrounding Jupiter after its formation. The accretion of Ganymede probably took about 10,000 years, much less than the estimated 100,000 years for Callisto. The Jovian subnebula may have been relatively “gas-starving” when the Galilean satellites formed; this would explain the longest accretion of Callisto. In contrast, Ganymede formed closer to Jupiter, where the subnebula was denser, which explains shorter formation times. This relatively rapid formation prevented accretional heat leakage, which may have led to ice melting and differentiation, namely the separation between rocks and ice.

The rocks gathered in the center, forming the core. In this respect, Ganymede is different from Callisto, which failed to melt and differentiate early due to accretional heat loss during its slower formation. This hypothesis explains why the two Jovian moons look so different despite their similar mass and composition. Alternative theories explain Ganymede’s greater internal heat from tidal bends or more intense pounding during the Late Great Bombardment.

After its formation, the Ganymedian core largely retained the heat accumulated during accretion and differentiation, releasing it only slowly into the ice mantle like a thermal battery. The mantle then transferred this heat by convection to the surface. Quickly, the decay of radioactive elements inside the rocks warmed the core even more, causing increased differentiation: an inner core of iron and ferrous sulfide and a mantle of silicates was formed.

Thanks to this, Ganymede became a fully differentiated body. By comparison, the radioactive heat of Callisto, which is not differentiated, caused convection inside the ice, which had the effect of cooling it and preventing its large-scale melting and thus rapid differentiation. Callisto’s convection movements led to only partial differentiation of rock and ice. Today, Ganymede continues to cool slowly. The heat released by its silicate core and mantle allows for the existence of a subglacial ocean, while the slow cooling of the liquid core of Fe–FeS creates convection and generates a magnetic field. The current outflow from Ganymede is probably greater than that from Callisto.


Triple eclipse of Jupiter by the Galilean moons Ganymede (blue disk at right; leftmost shadow), Io (white disk; shadow slightly to left) and Callisto (not visible; shadow at right)
Triple eclipse of Jupiter by the Galilean moons Ganymede (blue disk at right; leftmost shadow), Io (white disk; shadow slightly to left) and Callisto (not visible; shadow at right). False color image taken at different infrared wavelengths

At opposition, the apparent magnitude of Ganymede reaches 4.61 ± 0.03; at its maximum elongation, it may be possible to distinguish it from Jupiter with the naked eye from Earth under favorable viewing conditions, preferably by hiding Jupiter’s brightness with an object. During the opposition of Jupiter in 2022, two French amateur astronomers managed to observe and identify several structures on the surface of Ganymede, in particular the great dark region of Galileo Regio.

Ganymede, like all other Galilean satellites, produces eclipses on Jupiter’s surface, sometimes at the same time as others. It is also occulted by the planet for the terrestrial observer.


Past missions

A few probes flying over or orbiting Jupiter have explored Ganymede more closely, including four flybys in the 1970s and several passes in the 1990s and 2000s.

Pioneer 10 came first in 1973 and Pioneer 11 in 1974. These probes sent back information back to the satellite, with a more precise determination of physical characteristics and a resolution of 400 km of the surface elements. The closest distance between Pioneer 10 and Ganymede has been 446,250 km.

Voyager 1 and Voyager 2 were next, missing Ganymede in 1979. They clarified its size and revealed that it is larger than that of Titan, a moon of Saturn that was previously thought to be larger. The grooved terrain was also seen.

In 1995, the Galileo spacecraft orbited Jupiter and then made six close exploration flybys of Ganymede between 1996 and 2000. These overviews are G1, G2, G7, G8, G28 and G29. On its closest flyby, G2, Galileo passed only 264 km from the surface of Ganymede. The Ganymedian magnetic field was discovered during the 1996 G1 flyby, while the discovery of the ocean was announced in 2001. Galileo transmitted a large number of spectral images and discovered some of the non-icy compounds on Ganymede’s surface. The probe that most recently explored Ganymede up close was New Horizons, which passed nearby in 2007 on its way to Pluto. New Horizons made maps of Ganymede’s topography and composition as it passed at high speed.

Current mission projects

The Europa Jupiter System Mission (EJSM) was a joint mission project between NASA and ESA to explore many of Jupiter’s moons, including Ganymede. A launch date had been proposed for 2020. In February 2009, the agencies announced that priority had been given to this mission over the Titan Saturn System Mission. The EJSM consisted of NASA’s  Jupiter Europa Orbiter, ESA’s Jupiter Ganymede Orbiter, and possibly the Japan Aerospace Exploration Agency’s Jupiter Magnetospheric Orbiter. ESA’s contribution faced financial competition from other ESA projects, but on 2 May 2012, the European part of the mission, renamed Jupiter Icy Moon Explorer (JUICE), received an L1 launch slot with an Ariane 5 for 2022 in ESA’s Cosmic Vision science program. The probe will orbit Ganymede and conduct several studies of Callisto and Europa by flybys.

An orbiter around Ganymede based on the Juno probe was proposed in 2010 for the Planetary Science Decadal Survey. Likely instruments included medium resolution camera, flux grid magnetometer, visible and near-infrared imaging spectrometer, laser altimeter, low and high energy plasma packets, ion and neutral mass spectrometer, ultraviolet imaging spectrometer, radio and plasma wave sensor, narrow-angle camera,  and subsurface radar.

The Space Research Institute of the Russian Academy of Sciences discussed the Laplace-P mission, with a focus on astrobiology. This lander for Ganymede would be a partner mission of JUICE. If selected, the launch would take place in 2024, although its schedule could be revised and aligned with that of JUICE. At the end of 2013, the Russian government allocated 50 million rubles to the Laplace-P mission, the project’s former name, for a technical proposal in 2015. A Roscosmos promotional video posted in 2016 suggests a launch for the next decade, if Russia manages to overcome the technical and financial difficulties weighing on the mission.

Canceled mission projects

Another proposal to orbit Ganymede was the Jupiter Icy Moons Orbiter. It was designed to use nuclear fission to provide electricity to the probe, an ion engine for propulsion, and would have studied Ganymede in more detail than before. However, the mission was canceled in 2005 due to budget cuts. Another ancient proposition was “The Grandeur of Ganymede”.

Human settlement about Ganymede

In fiction

Ganymede has been colonized (or even terraformed) in several notable science fiction (SF) works. This is the case in:

  • Robert A. Heinlein’s Farmer in the Sky, a novel published in 1953, and other of his works such as  The Rolling Stones (1952), Double Star (1956), The Ravine of Darkness  (  1970) and Variable Star;
  • the rest of the SF literature of the same half-century, with the short novel The Snows of Ganymede by  Poul Anderson from 1954, the novel 2061: Odyssey Three by  Arthur C. Clarke from 1987 and the two short stories by Isaac Asimov Not Final! (1941) and Victory ‘Unintentional’ from 1942;
  • The novel series and television series The Expanse (2017);
  • the animated series Cowboy Bebop, where Ganymede is also the moon where sea rats are caught.

Settlement projects

The possibility of settling on Ganymede, or even terraforming it, has been studied several times. Thus, the Lifeboat Foundation published in 2012 a study on the colonization of Jupiter’s moons as potential alternative colonies to that of Mars. In September 2016, Space X revealed the Interplanetary Transport System project, whose role has been extended to transport humans to distant destinations in the Solar System including Europa and other Jovian moons.

The interest of this natural satellite is explained by several potential advantages. Indeed, with its gravity close to that of the Moon, the effects of musculoskeletal degeneration would be limited and the rockets would spend little fuel taking off. Its small magnetosphere would protect the colonists better than on other stars. As for the water ice present in large quantities in the subsoil, it would generate oxygen for the colonists to breathe, drinking water and rocket fuel. The sub-frigid ocean would make it possible to make important scientific advances by mounting numerous exploration missions.

But the colonization of Ganymede should also overcome many difficulties. Thus, regarding the health of the colonists, the small magnetosphere of the satellite is dominated by that of Jupiter, exposing them to high levels of radiation, while low gravity causes degeneration of muscles and bone density. Finally, the length of the journey and the financial costs associated with the lack of infrastructure and remoteness from Earth are additional risks to consider.

References (sources)