Ganymede'smagnetic field is probably created byconvection within its core, and influenced by tidal forces from Jupiter's far greater magnetic field.[23] Ganymede has a thinoxygenatmosphere that includes O, O2, and possibly O3 (ozone).[17]Atomic hydrogen is a minor atmospheric constituent. Whether Ganymede has anionosphere associated with its atmosphere is unresolved.[24]
Ganymede's surface is composed of two main types of terrain, the first of which are lighter regions, generally crosscut by extensive grooves and ridges, dating from slightly less than 4 billion years ago, covering two-thirds of Ganymede. The cause of the light terrain's disrupted geology is not fully known, but may be the result oftectonic activity due totidal heating. The second terrain type are darker regions saturated withimpact craters, which are dated to four billion years ago.[9]
Ganymede's discovery is credited toSimon Marius andGalileo Galilei, who both observed it in 1610,[2][g] as the third of theGalilean moons, the first group of objects discovered orbiting another planet.[26] Its name was soon suggested by astronomer Simon Marius, after themythologicalGanymede, aTrojan prince desired byZeus (the Greek counterpart ofJupiter), who carried him off to be the cupbearer of the gods.[27]
Chinese astronomical records report that in 365 BC,Gan De detected what might have been a moon of Jupiter, probably Ganymede, with the naked eye.[31] However, Gan De reported the color of the companion as reddish, which is puzzling since moons are too faint for their color to be perceived with the naked eye.[32]Shi Shen and Gan De together made fairly accurate observations of the five major planets.[33][34]
On January 7, 1610,Galileo Galilei used a telescope to observe what he thought were threestars near Jupiter, including what turned out to be Ganymede,Callisto, and one body that turned out to be the combined light fromIo andEuropa; the next night he noticed that they had moved. On January 13, he saw all four at once for the first time, but had seen each of the moons before this date at least once. By January 15, Galileo concluded that the stars were actually bodies orbitingJupiter.[2][3][g]
Galileo claimed the right to name the moons he had discovered. He considered "Cosmian Stars" and settled on "Medicean Stars", in honor ofCosimo II de' Medici.[27]
The French astronomerNicolas-Claude Fabri de Peiresc suggested individual names from theMedici family for the moons, but his proposal was not taken up.[27]Simon Marius, who had originally claimed to have found the Galilean satellites,[35] tried to name the moons the "Saturn of Jupiter", the "Jupiter of Jupiter" (this was Ganymede), the "Venus of Jupiter", and the "Mercury of Jupiter", another nomenclature that never caught on. Later on, after finding out about a suggestion fromJohannes Kepler, Marius agreed with Kepler's proposal and so he then proposed a naming system based onGreek mythology instead. This final Kepler/Marius proposal was ultimately successful.[27]
Jupiter is much blamed by the poets on account of his irregular loves. Three maidens are especially mentioned as having been clandestinely courted by Jupiter with success. Io, daughter of the River Inachus, Callisto of Lycaon, Europa of Agenor. Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell... I think, therefore, that I shall not have done amiss if the First is called by me Io, the Second Europa, the Third, on account of its majesty of light, Ganymede, the Fourth Callisto...[36][37]
This name and those of the other Galilean satellites fell into disfavor for a considerable time, and were not in common use until the mid-20th century. In much of the earlier astronomical literature, Ganymede is referred to instead by its Roman numeral designation,Jupiter III (a system introduced by Galileo), in other words "the third satellite of Jupiter". Following the discovery of moons of Saturn, a naming system based on that of Kepler and Marius was used for Jupiter's moons.[27] Ganymede is the only Galilean moon of Jupiter named after a male figure—like Io, Europa, and Callisto, he was a lover of Zeus.
In English, the Galilean satellites Io, Europa and Callisto have the Latin spellings of their names, but the Latin form of Ganymede isGanymēdēs, which would be pronounced/ˌɡænɪˈmiːdiːz/.[38] However, the final syllable is dropped in English, perhaps under the influence of FrenchGanymède ([ɡanimɛd]).
Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of thedwarf planet symbols, proposed a Greekgamma (the initial of Ganymede) combined with the cross-bar of the Jupiter symbol as the symbol of Ganymede (). This symbol is not widely used.[39]
Ganymedeorbits Jupiter at a distance of 1,070,400 kilometres (665,100 mi), third among the Galilean satellites,[26] and completes a revolution every seven days and three hours (7.155 days[40]). Like most known moons, Ganymede istidally locked, with one side always facing toward the planet, hence its day is also seven days and three hours.[41] Its orbit is very slightly eccentric and inclined to the Jovianequator, with theeccentricity andinclination changingquasi-periodically due to solar and planetary gravitationalperturbations on a timescale of centuries. The ranges of change are 0.0009–0.0022 and 0.05–0.32°, respectively.[42] These orbital variations cause theaxial tilt (the angle between the rotational and orbital axes) to vary between 0 and 0.33°.[11]
Ganymede participates inorbital resonances with Europa and Io: for every orbit of Ganymede, Europa orbits twice and Io orbits four times.[42][43]Conjunctions (alignment on the same side of Jupiter) between Io and Europa occur when Io is atperiapsis and Europa atapoapsis. Conjunctions between Europa and Ganymede occur when Europa is at periapsis.[42] The longitudes of the Io–Europa and Europa–Ganymede conjunctions change at the same rate, making triple conjunctions impossible. Such a complicated resonance is called theLaplace resonance.[44]The current Laplace resonance is unable to pump the orbital eccentricity of Ganymede to a higher value.[44] The value of about 0.0013 is probably a remnant from a previous epoch, when such pumping was possible.[43] The Ganymedian orbital eccentricity is somewhat puzzling; if it is not pumped now it should have decayed long ago due to the tidaldissipation in the interior of Ganymede.[44] This means that the last episode of the eccentricity excitation happened only several hundred million years ago.[44] Because Ganymede's orbital eccentricity is relatively low—on average 0.0015[43]—tidal heating is negligible now.[44] However, in the past Ganymede may have passed through one or more Laplace-like resonances[h] that were able to pump the orbital eccentricity to a value as high as 0.01–0.02.[9][44] This probably caused a significant tidal heating of the interior of Ganymede; the formation of the grooved terrain may be a result of one or more heating episodes.[9][44]
There are two hypotheses for the origin of the Laplace resonance among Io, Europa, and Ganymede: that it is primordial and has existed from the beginning of the Solar System;[45] or that it developed after theformation of the Solar System. A possible sequence of events for the latter scenario is as follows: Io raised tides on Jupiter, causing Io's orbit to expand (due to conservation of momentum) until it encountered the 2:1 resonance with Europa; after that, the expansion continued, but some of the angularmoment was transferred to Europa as the resonance caused its orbit to expand as well; the process continued until Europa encountered the 2:1 resonance with Ganymede.[44] Eventually the drift rates of conjunctions between all three moons were synchronized and locked in the Laplace resonance.[44]
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).Three high-resolution views of Ganymede taken byVoyager 1 near closest approach on July 9, 1979
With a diameter of about 5,270 kilometres (3,270 mi) and a mass of 1.48×1020 tonnes (1.48×1023 kg; 3.26×1023 lb), Ganymede is the largest and most massivemoon in theSolar System.[46] It is slightly more massive than the second most massive moon, Saturn's satelliteTitan, and is more than twice as massive as the Earth's Moon. It is larger than the planetMercury, which has a diameter of 4,880 kilometres (3,030 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.
The averagedensity of Ganymede, 1.936 g/cm3 (a bit greater than Callisto's), suggests a composition of about equal parts rocky material and mostly waterices.[9] Some of the water is liquid, forming an underground ocean.[47] Themass fraction of ices is between 46 and 50 percent, which is slightly lower than that in Callisto.[48] Some additional volatile ices such asammonia may also be present.[48][49] The exact composition of Ganymede'srock is not known, but is probably close to the composition ofL/LL typeordinary chondrites,[48] which are characterized by less total iron, less metallic iron and moreiron oxide thanH chondrites. The weight ratio of iron tosilicon ranges between 1.05 and 1.27 in Ganymede, whereas thesolar ratio is around 1.8.[48]
Tros crater (Juno; June 7, 2021)Enhanced-colorGalileo spacecraft image of Ganymede's trailing hemisphere.[50] 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.Ganymede grooved terrain (Juno; June 7, 2021)
Ganymede's surface has analbedo of about 43 percent.[51] Water ice seems to be ubiquitous on its surface, with a mass fraction of 50–90 percent,[9] significantly more than in Ganymede as a whole. Near-infraredspectroscopy has revealed the presence of strong water iceabsorption bands at wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0μm.[51] The grooved terrain is brighter and has a more icy composition than the dark terrain.[52] The analysis of high-resolution,near-infrared andUVspectra obtained by theGalileo spacecraft and from Earth observations has revealed various non-water materials:carbon dioxide,sulfur dioxide and, possibly,cyanogen,hydrogen sulfate and variousorganic compounds.[9][53]Galileo results have also shownmagnesium sulfate (MgSO4) and, possibly,sodium sulfate (Na2SO4) on Ganymede's surface.[41][54] These salts may originate from the subsurface ocean.[54]
The Ganymedian surface albedo is very asymmetric; the leading hemisphere[i] is brighter than the trailing one.[51] This is similar to Europa, but the reverse for Callisto.[51] The trailing hemisphere of Ganymede appears to be enriched in sulfur dioxide.[55][56] The distribution of carbon dioxide does not demonstrate any hemispheric asymmetry, but little or no carbon dioxide is observed near the poles.[53][57]Impact craters 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.[57]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,[58] contains clays and organic materials that could indicate the composition of the impactors from which Jovian satellites accreted.[59]
The heating mechanism required for the formation of the grooved terrain on Ganymede is an unsolved problem in theplanetary sciences. The modern view is that the grooved terrain is mainlytectonic in nature.[9]Cryovolcanism is thought to have played only a minor role, if any.[9] The forces that caused the strong stresses in the Ganymedian icelithosphere necessary to initiate the tectonic activity may be connected to thetidal heating events in the past, possibly caused when the satellite passed through unstable orbital resonances.[9][60] The tidal flexing of the ice may have heated the interior and strained the lithosphere, leading to the development of cracks andhorst and graben faulting, which erased the old, dark terrain on 70 percent of the surface.[9][61] 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 tophase transitions in ice andthermal expansion.[9] During subsequent evolution deep, hot waterplumes may have risen from the core to the surface, leading to the tectonic deformation of the lithosphere.[62]Radiogenic heating 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.[63]
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.[9] The brighter, grooved terrain contains many fewer impact features, which have been only of minor importance to its tectonic evolution.[9] The density of cratering indicates an age of 4 billion years for the dark terrain, similar to the highlands of the Moon, and a somewhat younger age for the grooved terrain (but how much younger is uncertain).[64] Ganymede may have experienced a period of heavy cratering 3.5 to 4 billion years ago similar to that of the Moon.[64] If true, the vast majority of impacts happened in that epoch, whereas the cratering rate has been much smaller since.[65] 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.[65][66] 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 apalimpsest.[65]
One significant feature on Ganymede is a dark plain namedGalileo Regio, which contains a series of concentric grooves, or furrows, likely created during a period of geologic activity.[67]
Ganymede also has polar caps, likely composed of water frost. The frost extends to 40° latitude.[41] These polar caps were first seen by theVoyager 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 fromGalileo suggests the latter is correct.[68] 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.[68]
A crater namedAnat provides the reference point for measuring longitude on Ganymede. By definition, Anat is at 128° longitude.[69] The 0° longitude directly faces Jupiter, and unless stated otherwise longitude increases toward the west.[70]
Ganymede appears to be fullydifferentiated, with an internal structure consisting of aniron-sulfide–ironcore, asilicatemantle, and outer layers of water ice and liquid water.[9][71][72] The precise thicknesses of the different layers in the interior of Ganymede depend on the assumed composition of silicates (fraction ofolivine andpyroxene) and amount ofsulfur in the core.[48][71][73][74] Ganymede has the lowestmoment of inertia factor, 0.31,[9] among the solid Solar System bodies. This is a consequence of its substantial water content and fully differentiated interior.
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.[9][20][71][75][76] In the 1990s, NASA'sGalileo mission flew by Ganymede, and found indications of such a subsurface ocean.[47] 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 differentphases of ice, with the lowest liquid layer adjacent to the rockymantle.[20][21][22][77] Water–rock contact may be an important factor in theorigin of life.[20] The analysis also notes that the extreme depths involved (~800 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 theaurorae moved confirmed that Ganymede has a subsurface ocean.[47] A large saltwater ocean affects Ganymede's magnetic field, and consequently, its aurorae.[19][77][78][79] The evidence suggests that Ganymede's oceans might be the largest in the entire Solar System.[80] These observations were later supported byJuno, which detected various salts and other compounds on Ganymede's surface, includinghydrated sodium chloride,ammonium chloride,sodium bicarbonate, and possiblyaliphatic 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.[81] As a result of these findings, there is increasing speculation on the potentialhabitability of Ganymede's ocean.[76][82]
The existence of a liquid,iron–nickel-rich core[72] provides a natural explanation for the intrinsicmagnetic field of Ganymede detected byGalileo spacecraft.[83] Theconvection in the liquid iron, which has highelectrical conductivity, is the most reasonable model of magnetic field generation.[23] The density of the core is 5.5–6 g/cm3 and the silicate mantle is 3.4–3.6 g/cm3.[48][71][73][83] The radius of this core may be up to 500 km.[83] The temperature in the core of Ganymede is probably 1500–1700 K and pressure up to 10 GPa (99,000 atm).[71][83]
In 1972, a team of Indian, British and American astronomers working inJava,Indonesia andKavalur, India claimed that they had detected a thin atmosphere during anoccultation, when it and Jupiter passed in front of astar.[84] They estimated that the surface pressure was around 0.1Pa (1 microbar).[84] However, in 1979,Voyager 1 observed an occultation of the starκ Centauri during its flyby of Jupiter, with differing results.[85] The occultation measurements were conducted in thefar-ultraviolet spectrum atwavelengths shorter than 200nm, which were much more sensitive to the presence of gases than the 1972 measurements made in thevisible spectrum. No atmosphere was revealed by theVoyager data. The upper limit on the surface particlenumber density was found to be1.5×109 cm−3, which corresponds to a surface pressure of less than 2.5 μPa (25 picobar).[85] The latter value is almost five orders of magnitude less than the 1972 estimate.[85]
False-color temperature map of Ganymede
Despite theVoyager data, evidence for a tenuous oxygen atmosphere (exosphere) on Ganymede, very similar to the one found on Europa, was found by theHubble Space Telescope (HST) in 1995.[17][86] HST actually observedairglow of atomic oxygen in the far-ultraviolet at the wavelengths 130.4 nm and 135.6 nm. Such an airglow is excited whenmolecular oxygen isdissociated by electron impacts,[17] which is evidence of a significant neutral atmosphere composed predominantly of O2 molecules. The surface number density probably lies in the(1.2–7)×108 cm−3 range, corresponding to the surface pressure of0.2–1.2 μPa.[17][j] These values are in agreement withVoyager'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 intohydrogen and oxygen by radiation, with the hydrogen then being more rapidly lost due to its low atomic mass.[86] 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).[87] The bright spots are probably polarauroras, caused by plasma precipitation along the open field lines.[88]
The existence of a neutral atmosphere implies that anionosphere should exist, because oxygen molecules are ionized by the impacts of the energeticelectrons coming from the magnetosphere[89] and by solarEUV radiation.[24] However, the nature of the Ganymedian ionosphere is as controversial as the nature of the atmosphere. SomeGalileo measurements found an elevated electron density near Ganymede, suggesting an ionosphere, whereas others failed to detect anything.[24] The electron density near the surface is estimated by different sources to lie in the range 400–2,500 cm−3.[24] 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 ofozone (O3) bands was announced in 1996.[90] In 1997 spectroscopic analysis revealed thedimer (ordiatomic) 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 onlatitude andlongitude, rather than on surface albedo—they tend to decrease with increasing latitude on Ganymede, whereas O3 shows an opposite trend.[91] Laboratory work has found that O2 would not cluster or bubble but would dissolve in ice at Ganymede's relatively warm surface temperature of 100 K (−173.15 °C).[92]
A search forsodium 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.[93] Another minor constituent of the Ganymedian atmosphere isatomic hydrogen. Hydrogen atoms were observed as far as 3,000 km from Ganymede's surface. Their density on the surface is about1.5×104 cm−3.[94]
In 2021, water vapour was detected in the atmosphere of Ganymede.[95]
Magnetic field of the Jovian satellite Ganymede, which is embedded into the magnetosphere of Jupiter. Closed field lines are marked with green color.
TheGalileo craft made six close flybys of Ganymede from 1995 to 2000 (G1, G2, G7, G8, G28 and G29)[23] and discovered that Ganymede has a permanent (intrinsic)magnetic moment independent of the Jovian magnetic field.[96] The value of the moment is about1.3 × 1013 T·m3,[23] which is three times larger than themagnetic 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.[23] Its north pole lies below theorbital plane. Thedipole magnetic field created by this permanent moment has a strength of 719 ± 2nT at Ganymede's equator,[23] which should be compared with the Jovian magnetic field at the distance of Ganymede—about 120 nT.[96] The equatorial field of Ganymede is directed against the Jovian field, meaningreconnection is possible. The intrinsic field strength at the poles is two times that at the equator—1440 nT.[23]
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 tinymagnetosphere embedded insidethat of Jupiter; it is the only moon in the Solar System known to possess the feature.[96] Its diameter is 4–5 Ganymede radii.[97] The Ganymedian magnetosphere has a region of closedfield lines located below 30° latitude, wherecharged particles (electrons andions) are trapped, creating a kind ofradiation belt.[97] The main ion species in the magnetosphere is single ionized oxygen[24] (O+) which fits well with Ganymede's tenuous oxygenatmosphere. In the polar cap regions, at latitudes higher than 30°, magnetic field lines are open, connecting Ganymede with Jupiter's ionosphere.[97] In these areas, the energetic (tens and hundreds ofkiloelectronvolt) electrons and ions have been detected,[89] which may cause the auroras observed around the Ganymedian poles.[87] In addition, heavy ions precipitate continuously on Ganymede's polar surface,sputtering and darkening the ice.[89]
The interaction between the Ganymedian magnetosphere and Jovianplasma is in many respects similar to that of thesolar wind and Earth's magnetosphere.[97][98] 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 andsubsonic in the case of Ganymede. Because of the subsonic flow, there is nobow shock off the trailing hemisphere of Ganymede.[98]
In addition to the intrinsic magnetic moment, Ganymede has an induced dipole magnetic field.[23] 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. Thefield strength of the induced field at the magnetic equator is about 60 nT—half of that of the ambient Jovian field.[23] 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.[23]
Given that Ganymede is completely differentiated and has a metallic core,[9][83] 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.[23][83] The magnetic field detected around Ganymede is likely to be caused by compositional convection in the core,[83] if the magnetic field is the product of dynamo action, or magnetoconvection.[23][99]
Despite the presence of an iron core, Ganymede's magnetosphere remains enigmatic, particularly given that similar bodies lack the feature.[9] 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.[61] 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.[9]
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.[100]
A sharp boundary divides the ancient dark terrain of Nicholson Regio from the younger, finely striated bright terrain of Harpagia Sulcus.
Ganymede probably formed by anaccretion in Jupiter'ssubnebula, a disk of gas and dust surrounding Jupiter after its formation.[101] The accretion of Ganymede probably took about 10,000 years,[102] much shorter than the 100,000 years estimated for Callisto. The Jovian subnebula may have been relatively "gas-starved" when the Galilean satellites formed; this would have allowed for the lengthy accretion times required for Callisto.[101] In contrast, Ganymede formed closer to Jupiter, where the subnebula was denser, which explains its shorter formation timescale.[102] This relatively fast formation prevented the escape of accretional heat, which may have led to ice melt anddifferentiation: the separation of the rocks and ice. The rocks settled to the center, forming the core.[72] In this respect, Ganymede is different from Callisto, which apparently failed to melt and differentiate early due to loss of the accretional heat during its slower formation.[103] This hypothesis explains why the two Jovian moons look so dissimilar, despite their similar mass and composition.[75][103] Alternative theories explain Ganymede's greater internal heating on the basis of tidal flexing[104] or more intense pummeling by impactors during theLate Heavy Bombardment.[105][106][107][108] In the latter case, modeling suggests that differentiation would become arunaway process at Ganymede but not Callisto.[107][108]
After formation, Ganymede's core largely retained the heat accumulated during accretion and differentiation, only slowly releasing it to the ice mantle.[103] The mantle, in turn, transported it to the surface by convection.[75] The decay ofradioactive elements within rocks further heated the core, causing increased differentiation: an inner, iron–iron-sulfide core and a silicate mantle formed.[83][103] With this, Ganymede became a fully differentiated body.[72] By comparison, the radioactive heating of undifferentiated Callisto caused convection in its icy interior, which effectively cooled it and prevented large-scale melting of ice and rapid differentiation.[109] The convective motions in Callisto have caused only a partial separation of rock and ice.[109] Today, Ganymede continues to cool slowly.[83] The heat being released from its core and silicate mantle enables the subsurface ocean to exist,[49] whereas the slow cooling of the liquid Fe–FeS core causes convection and supports magnetic field generation.[83] The currentheat flux out of Ganymede is probably higher than that out of Callisto.[103]
A study from 2020 by Hirata, Suetsugu and Ohtsuki suggests that Ganymede probably was hit by a massive asteroid 4 billion years ago; an impact so violent that may have shifted the moon's axis. The study came to this conclusion analyzing images of the furrows system in the satellite's surface.[110]
Several spacecraft have performed closeflybys of Ganymede: twoPioneer and twoVoyager spacecraft made a single flyby each between 1973 and 1979; theGalileo spacecraft made six passes between 1996 and 2000; and theJuno spacecraft performed two flybys in 2019 and 2021.[111] No spacecraft has yet orbited Ganymede, but theJUICE mission, which launched in April 2023, intends to do so.
The first spacecraft to approach close to Ganymede wasPioneer 10, which performed a flyby in 1973 as it passed through the Jupiter system at high speed.Pioneer 11 made a similar flyby in 1974.[28] Data sent back by the two spacecraft was used to determine the moon's physical characteristics[112] and provided images of the surface with up to 400 km (250 mi) resolution.[113] Pioneer 10's closest approach was 446,250 km, about 85 times Ganymede's diameter.[114]
Voyager 1 andVoyager 2 both studied Ganymede when passing through the Jupiter system in 1979. Data from those flybys were used to refine the size of Ganymede, revealing it was larger thanSaturn's moon Titan, which was previously thought to have been bigger.[115] Images from theVoyagers provided the first views of the moon's grooved surface terrain.[116]
ThePioneer andVoyager flybys were all at large distances and high speeds, as they flew onunbound trajectories through the Jupiter system. Better data can be obtained from a spacecraft which is orbiting Jupiter, as it can encounter Ganymede at a lower speed and adjust the orbit for a closer approach. In 1995, theGalileo spacecraft entered orbit around Jupiter and between 1996 and 2000 made six close flybys of Ganymede.[41] These flybys were denoted G1, G2, G7, G8, G28 and G29.[23] During the closest flyby (G2),Galileo passed just 264 km from the surface of Ganymede (five percent of the moon's diameter),[23] which remains the closest approach by any spacecraft. During the G1 flyby in 1996,Galileo instruments detected Ganymede's magnetic field.[117] Data from theGalileo flybys was used to discover the sub-surface ocean, which was announced in 2001.[23][41] High spatial resolution spectra of Ganymede taken byGalileo were used to identify several non-ice compounds on the surface.[53]
TheNew Horizons spacecraft also observed Ganymede, but from a much larger distance as it passed through the Jupiter system in 2007 (en route toPluto). The data were used to perform topographic and compositional mapping of Ganymede.[118][119]
LikeGalileo, theJuno spacecraft orbited Jupiter. On 2019 December 25,Juno performed a distant flyby of Ganymede during its 24th orbit of Jupiter, at a range of 97,680 to 109,439 kilometers (60,696 to 68,002 mi). This flyby provided images of the moon's polar regions.[120][121] In June 2021,Juno performed a second flyby, at a closer distance of 1,038 kilometers (645 mi).[111][122] This encounter was designed to provide agravity assist to reduceJuno's orbital period from 53 days to 43 days. Additional images of the surface were collected.[111]
TheJupiter Icy Moons Explorer (JUICE) will be the first to enter orbit around Ganymede itself. JUICE was launched on April 14, 2023.[123] It is intended to perform its first flyby of Ganymede in 2031, then enter orbit of the moon in 2032. When the spacecraft consumes its propellant, JUICE is planned to be deorbited and impact Ganymede in February 2034.[124]
In addition to JUICE, NASA'sEuropa Clipper, which was launched in October 2024, will conduct 4 close flybys of Ganymede beginning in 2030.[125] It may also crash into Ganymede at the end of its mission to aid JUICE in studying the surface's geochemistry.[126][127]
Several other missions have been proposed to flyby or orbit Ganymede, but were either not selected for funding or cancelled before launch.
TheJupiter Icy Moons Orbiter would have studied Ganymede in greater detail.[128] However, the mission was canceled in 2005.[129] Another old proposal was called The Grandeur of Ganymede.[59]
TheEuropa Jupiter System Mission had a proposed launch date of 2020, and was a joint NASA and ESA proposal for exploration of many of Jupiter's moons including Ganymede. In February 2009 it was announced that ESA and NASA had given this mission priority ahead of theTitan Saturn System Mission.[132] The mission was to consist of the NASA-ledJupiter Europa Orbiter, the ESA-ledJupiter Ganymede Orbiter, and possibly aJAXA-ledJupiter Magnetospheric Orbiter. The NASA and JAXA components were later cancelled, and ESA's appeared likely to be cancelled too,[133] but in 2012 ESA announced it would go ahead alone. The European part of the mission became the Jupiter Icy Moon Explorer (JUICE).[134]
^Surface gravity derived from the mass (m), thegravitational constant (G) and the radius (r):.
^Escape velocity derived from the mass (m), thegravitational constant (G) and the radius (r):.
^abIt is probable that the German astronomerSimon Marius discovered it independently the same year.[25]
^A Laplace-like resonance is similar to the current Laplace resonance among the Galilean moons with the only difference being that longitudes of the Io–Europa and Europa–Ganymede conjunctions change with rates whose ratio is a non-unity rational number. If the ratio is unity, then the resonance is the Laplace resonance.
^The leading hemisphere is the hemisphere facing the direction of orbital motion; the trailing hemisphere faces the reverse direction.
^The surface number density and pressure were calculated from the column densities reported in Hall, et al. 1998, assuming ascale height of 20 km and temperature 120 K.
^abcGalilei, Galileo; translated by Edward Carlos (March 1610). Barker, Peter (ed.)."Sidereus Nuncius"(PDF). University of Oklahoma History of Science. Archived fromthe original(PDF) on December 20, 2005. RetrievedJanuary 13, 2010.
^Quinn Passey & E. M. Shoemaker (1982) "Craters on Ganymede and Callisto", in David Morrison, ed.,Satellites of Jupiter, vol. 3, International Astronomical Union, pp. 385–386, 411.
^E. M. Shoemaker et al. (1982) "Geology of Ganymede", in David Morrison, ed.,Satellites of Jupiter, vol. 3, International Astronomical Union, pp. 464, 482, 496.
^abVance, Steve; Bouffard, Mathieu; Choukroun, Mathieu; Sotina, Christophe (April 12, 2014). "Ganymede's internal structure including thermodynamics of magnesium sulfate oceans in contact with ice".Planetary and Space Science.96:62–70.Bibcode:2014P&SS...96...62V.doi:10.1016/j.pss.2014.03.011.
^Brecher, K. (1981). "Ancient Astronomy in Modern China".Bulletin of the Astronomical Society.13: 793.Bibcode:1981BAAS...13..793B.
^Yi-Long, Huang (1997)."Gan De". InHelaine Selin (ed.).Encyclopaedia of the history of science, technology, and medicine in non-western cultures. Springer. p. 342.ISBN978-0-7923-4066-9.
^"Discovery".Cascadia Community College. Archived fromthe original on September 20, 2006. RetrievedNovember 24, 2007.
^Van Helden, Albert (August 1994)."Naming the Satellites of Jupiter and Saturn"(PDF).The Newsletter of the Historical Astronomy Division of the American Astronomical Society (32).Archived(PDF) from the original on December 7, 2022. RetrievedMarch 10, 2023.
^abcdeMiller, Ron; Hartmann, William K. (May 2005).The Grand Tour: A Traveler's Guide to the Solar System (3rd ed.). Thailand: Workman Publishing. pp. 108–114.ISBN978-0-7611-3547-0.
^abcMusotto, Susanna; Varadi, Ferenc; Moore, William; Schubert, Gerald (2002). "Numerical Simulations of the Orbits of the Galilean Satellites".Icarus.159 (2):500–504.Bibcode:2002Icar..159..500M.doi:10.1006/icar.2002.6939.
^abcdCalvin, Wendy M.; Clark, Roger N.; Brown, Robert H.; Spencer, John R. (1995). "Spectra of the ice Galilean satellites from 0.2 to 5 μm: A compilation, new observations, and a recent summary".J. Geophys. Res.100 (E9): 19,041–19, 048.Bibcode:1995JGR...10019041C.doi:10.1029/94JE03349.
^Domingue, Deborah; Lane, Arthur; Moth, Pimol (1996). "Evidence from IUE for Spatial and Temporal Variations in the Surface Composition of the Icy Galilean Satellites".Bulletin of the American Astronomical Society.28: 1070.Bibcode:1996DPS....28.0404D.
^Patterson, Wesley; Head, James W.; et al. (2007)."A Global Geologic Map of Ganymede"(PDF).Lunar and Planetary Science.XXXVIII: 1098.Archived(PDF) from the original on March 27, 2009. RetrievedJanuary 30, 2008.
^abcdeSohl, F.; Spohn, T; Breuer, D.; Nagel, K. (2002). "Implications from Galileo Observations on the Interior Structure and Chemistry of the Galilean Satellites".Icarus.157 (1):104–119.Bibcode:2002Icar..157..104S.doi:10.1006/icar.2002.6828.
^Kuskov, O. L.; Kronrod, V. A.; Zhidikova, A. P. (May 2010). Bhardwaj, Anil (ed.). "Internal Structure of Icy Satellites of Jupiter".Advances in Geosciences.19. World Scientific:365–376.Bibcode:2010aogs...19..365K.doi:10.1142/9789812838162_0028 (inactive November 1, 2024).ISBN9789812838162.{{cite journal}}: CS1 maint: DOI inactive as of November 2024 (link)
^Podzolko, M.V.; Getselev, I.V. (March 8, 2013)."Radiation Conditions of a Mission to Jupiterʼs Moon Ganymede".International Colloquium and Workshop "Ganymede Lander: Scientific Goals and Experiments. IKI, Moscow, Russia: Moscow State University.Archived from the original on March 9, 2021. RetrievedJanuary 6, 2020.
^National Research Council (March 7, 2011).Vision and Voyages for Planetary Science in the Decade 2013–2022. Washington DC, US: The National Academies Press.doi:10.17226/13117.ISBN978-0-309-22464-2.Archived from the original on February 11, 2021. RetrievedJune 18, 2021.The committee identified a number of additional large missions that are of high scientific value but are not recommended for the decade 2013-2022 for a variety of reasons. In alphabetical order, these missions are as follows: Ganymede Orbiter [...]
.svg)
.png)
