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'''Ganymede''' [[pronounced]] ''"GAN uh meed"'', or as [[Greek language|Greek]] ''Γανυμήδης'' is a the largest [[natural satellite|satellite]] of [[Jupiter]], and the largest satellite in the [[Solar System]]. It orbits third in distance of the [[Galilean satellites]] around Jupiter and seventh among all the planet's moons.<ref name="Planetary Society"/> Ganymede is larger in diameter than [[Mercury (planet)|Mercury]] but has only about half its mass.<ref name="nineplanets.org-Ganymede"/> Ganymede's discovery is credited to [[Galileo Galilei]], who observed it in [[1610]].<ref name="Sidereus Nuncius">{{cite web|url=http://www.physics.emich.edu/jwooley/chapter9/Chapter9.html|title=Sidereus Nuncius|work=Eastern Michigan University|accessdate=2008-01-11}}</ref> The name Ganymede was suggested soon after by [[Simon Marius]], for the cup-bearer of the [[Greek mythology|Greek gods]], beloved of [[Zeus]] (see [[Ganymede]]).<ref name="Marius, S">Marius, S.; ([[1614]]); [http://galileo.rice.edu/sci/marius.html ''Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici''], where he [http://galileo.rice.edu/sci/observations/jupiter_satellites.html attributes the suggestion] to [[Johannes Kepler]]</ref>
'''Ganymede''' ({{pronEng|ˈgænɨmiːd}} ''<small>GAN</small>-ə-meed,'' or as Greek ''Γανυμήδης)'' is a [[natural satellite]] of [[Jupiter]] and the [[List of natural satellites by diameter|largest natural satellite]] in the [[Solar System]]. It orbits third in distance of the [[Galilean satellites]] around Jupiter and seventh among all the planet's moons.<ref name="Planetary Society"/> Ganymede is larger in diameter than [[Mercury (planet)|Mercury]] but has only about half its mass.<ref name="nineplanets.org-Ganymede"/> Ganymede's discovery is credited to [[Galileo Galilei]], who observed it in [[1610]].<ref name="Sidereus Nuncius">{{cite web|url=http://www.physics.emich.edu/jwooley/chapter9/Chapter9.html|title=Sidereus Nuncius|work=Eastern Michigan University|accessdate=2008-01-11}}</ref> The name Ganymede was suggested soon after by [[Simon Marius]], for the cup-bearer of the [[Greek mythology|Greek gods]], beloved of [[Zeus]] (see [[Ganymede]]).<ref name="Marius, S">Marius, S.; ([[1614]]); [http://galileo.rice.edu/sci/marius.html ''Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici''], where he [http://galileo.rice.edu/sci/observations/jupiter_satellites.html attributes the suggestion] to [[Johannes Kepler]]</ref>


Ganymede is primarily composed of [[silicate|silicate rock]] and [[water ice]]. Its generally ancient surface is pockmarked by craters and also features extensive regions of grooves and ridges,<ref name="The Grand Tour"/> created during a period of internal heating. Ganymede has a thin oxygen [[atmosphere]]<ref name=Broadfoot1981/> and is the only moon known to have a [[magnetosphere]].<ref name="nineplanets.org-Ganymede">{{cite web| work=nineplanets.org |title=Ganymede page |date=[[October 31]], [[1997]] |url=http://www.nineplanets.org/ganymede.html}}</ref> A salty ocean is believed to exist nearly 200 [[km]] below Ganymede's surface,<ref name=JPLDec/> which has led to speculation about the possibility of life.
Ganymede is primarily composed of [[silicate|silicate rock]] and [[water ice]]. Its generally ancient surface is pockmarked by craters and also features extensive regions of grooves and ridges,<ref name="The Grand Tour"/> created during a period of internal heating. Ganymede has a thin oxygen [[atmosphere]]<ref name=Broadfoot1981/> and is the only moon known to have a [[magnetosphere]].<ref name="nineplanets.org-Ganymede">{{cite web| work=nineplanets.org |title=Ganymede page |date=[[October 31]], [[1997]] |url=http://www.nineplanets.org/ganymede.html}}</ref> A salty ocean is believed to exist nearly 200 [[km]] below Ganymede's surface,<ref name=JPLDec/> which has led to speculation about the possibility of life.

Revision as of 23:53, 24 January 2008

This article is about the natural satellite of Jupiter. For the asteroid, see 1036 Ganymed.
Ganymede
True color image taken by the Galileo probe
True color image taken by the Galileo probe
Discovery
Discovered byG. Galilei
S. Marius
Discovery dateJanuary 11, 1610
Designations
AdjectivesGanymedian
Orbital characteristics
Periapsis1,069,200 km (0.007147 AU)
Apoapsis1,071,600 km (0.007163 AU)
Mean orbit radius
1,070,400 km (0.007155 AU)
Eccentricity0.002[1]
7.15455296 d (0.019588 a)
10.880 km/s
Inclination2.21° (to the ecliptic)
0.20° to Jupiter's equator)
Satellite ofJupiter
Physical characteristics
Mean radius
2631.2 km (0.413 Earths)
87.0 million km² (0.171 Earths) [2]
Volume7.6×1010 km³ (0.0704 Earths)
Mass1.4819×1023 kg (0.025 Earths)
Mean density
1.942 g/cm³
1.428 m/s2 (0.146 g)
2.741 km/s (6,130 mph)
synchronous
zero
Albedo0.43 ± 0.02[3]
Temperature~109 K (−172°C)
4.61 (opposition) [3]
Atmosphere
Surface pressure
trace
Composition by volumeoxygen

Ganymede (Template:PronEng GAN-ə-meed, or as Greek Γανυμήδης) is a natural satellite of Jupiter and the largest natural satellite in the Solar System. It orbits third in distance of the Galilean satellites around Jupiter and seventh among all the planet's moons.[4] Ganymede is larger in diameter than Mercury but has only about half its mass.[5] Ganymede's discovery is credited to Galileo Galilei, who observed it in 1610.[6] The name Ganymede was suggested soon after by Simon Marius, for the cup-bearer of the Greek gods, beloved of Zeus (see Ganymede).[7]

Ganymede is primarily composed of silicate rock and water ice. Its generally ancient surface is pockmarked by craters and also features extensive regions of grooves and ridges,[8] created during a period of internal heating. Ganymede has a thin oxygen atmosphere[9] and is the only moon known to have a magnetosphere.[5] A salty ocean is believed to exist nearly 200 km below Ganymede's surface,[10] which has led to speculation about the possibility of life.

Beginning with Pioneer 10, spacecraft have been exploring Ganymede up close.[11] Galileo discovered the ocean[8] and the magnetic field.[12] The Jupiter Icy Moons Orbiter was meant to orbit Ganymede, but was cancelled.[13]

Discovery and naming

On January 7, 1610, Galileo Galilei observed what he believed were three stars near Jupiter; the next night he noticed that they had moved. A fourth supposed star, which would turn out to be Ganymede, was found on January 11. By January 15, Galileo came to the conclusion that the stars were actually bodies orbiting Jupiter.[14] He claimed the right to name the moons; he considered "Cosmian Stars” and settled on “Medicean Stars”.[15]

Nicholas Clade Fabri de Peiresc suggested individual names from the Medici family for the moons, but his system was not used.[15] Simon Marius, who had originally claimed to have found the Galilean Satellites,[16] 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,' a system which never caught on. From a suggestion by Johannes Kepler, Marius once again tried to name the moons.[15]

…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… the Third, on account of its majesty of light, Ganymede…[14]

This name and the names of the other Galilean satellites fell into disfavor for a considerable time, and were not revived in common use until the mid-20th century. In much of the earlier astronomical literature, Ganymede is simply referred to by its Roman numeral designation (a system introduced by Galileo) as Jupiter III or as the "third satellite of Jupiter". Following the discovery of moons of Saturn, a naming system based on the one Kepler and Marius had created was put to use for Jupiter’s moons.[15] Ganymede is the only Galilean moon of Jupiter named after a male figure.

Orbit and rotation

Ganymede orbits at a distance of 1,070,400 kilometers, third among the Galilean satellites,[4] and completes a revolution around Jupiter every seven days and three hours. Like most moons, Ganymede is tidally locked, with one face always pointing toward the planet.[8] Its orbit is very slightly eccentric and inclined to the Jovian equator, with the eccentricity and inclination changing quasi-periodically due to solar and planetary gravitational perturbations on a timescale of centuries. The ranges of change are 0.0009–0.0022 and 0.05–0.32°, respectively.[17] These orbital variations cause the axial tilt (the angle between rotational and orbital axes) to vary between 0 and 0.33°.[18]

Ganymede participates in orbital resonances with Europa and Io, in which for every orbit of Ganymede, Europa orbits twice and Io orbits four times.[17][19] The superior conjunction between Io and Europa always occurs when Io is at periapsis and Europa at the apoapsis. The superior conjunction between Europa and Ganymede occurs when Europa is at the periapsis.[17] The longitudes of the Io–Europa and Europa–Ganymede conjuntions change with the same rate making the triple conjuctions possible. Such a complicated resonance is called the Laplace resonance.[20]

However the Laplace resonance is unable to pump the orbital eccentricity of Ganymede.[20] The current value of about 0.002 is probably a remnant from the previous epoch, when such pumping was possible.[1][19] The Ganymedian orbital eccentricity is to some extent puzzling. If it is not pumped now it should have decayed long ago due to the tidal dissipation in the interior of Ganymede.[20] This means that the last episode of the eccentricity excitation happened only several hundreds million years ago.[20] Because the orbital eccentricity of Ganymede is relatively low—0.0015 on average,[19] the tidal heating of this moon is negligible now.[20] However in the past Ganymede may have passed through one or more Laplace-like resonances,[21] which were able to pump the orbital eccentricity to a value as high as 0.01–0.02.[22][20] This probably caused in a significant tidal heating of the interior of Ganymede; the formation of the grooved terrain may be a result of such a heating episode (or episodes).[22][20]

The origin of the Laplace resonance among Io, Europa and Ganymede is not known. However two hypothesis exist. The first states that it is primordial and has existed from the begining of the Solar System.[23] The second states that it has developed after the Solar System formation. A possible sequence of the events was: Io raised tides on Jupiter and as a result its orbit expanded until it encounterd 2:1 resonance with Europa, after that the expansion continued but some of the angular moment was transfered to Europa by the resonance causing its orbit to expand as well, the process continued until Europa encountared 2:1 resonance with Ganymede.[20] Eventually the drift rates of conjunctions between all three moons were synchronised and locked in the Laplace resonance.[20]

Physical characteristics

Composition

Interior of Ganymede

The average density of Ganymede, 1.936 g/cm³, suggests a composition of approximately equal parts rocky material and water ice.[22] The mass fraction of ices is between 46–50%, slightly lower than that in Callisto.[24] Some additional volatile ices such as ammonia may also be present.[24][25] The exact composition of Ganymede's rock is not known, but is probably close to the composition of L/LL type ordinary chondrites, which are characterized by less total iron, less metallic iron and more iron oxide than H chondrites. The weight ratio of iron to silicon is 1.05-1.27 in Ganymede, whereas the solar ratio is around 1.8.[24]

Ganymede's surface has an albedo of about 43%.[26] Water ice seems to be ubiquitous on the surface, with a mass fraction of 50–90%,[22] significantly more than in Ganymede as a whole. Near-infrared spectroscopy has revealed the presence of strong water ice absorption bands at wavelengths 1.04, 1.25, 1.5, 2.0 and 3.0 micrometers.[26] The grooved terrain is brighter and has more icy composition than the dark terrain.[27] The analysis of high resolution near-infrared and UV spectra obtained by the Galileo spacecraft and from the ground has revealed various non-ice materials: carbon dioxide, sulfur dioxide and, possibly, cyanogen, hydrogen sulfate and various organic compounds.[22][28] Galileo results have also shown magnesium sulfate (MgSO4) and, possibly, sodium sulfate (Na2SO4) on Ganymede's surface.[8][29] These salts may originate from the subsurface ocean.[29]

The Ganymedian surface is asymmetric: the leading hemisphere—that facing the direction of the orbital motion—[30] is brighter than the trailing one.[26] This is similar to Europa, but the reverse is true on Callisto.[26] The trailing hemisphere of Ganymede appears to be enriched in sulfur dioxide.[31][32] The distribution of carbon dioxide does not demonstrate any hemispheric asymmetry, although it is not observed near the poles.[28][33] Impact craters on Ganymede (except one) don't show any enrichment in carbon dioxide, which also distinguishes it from Callisto. The carbon dioxide was probably seriously depleted on Ganymede.[33]

Internal structure

A sharp boundary divides the dark Nicholson Regio from the bright Harpagia Sulcus.

Ganymede appears to be a fully differentiated body consisting of a Fe–FeS core (iron sulfate–iron), silicate mantle and an outer ice mantle.[34][22] This model is supported by the low value of its dimensionless moment of inertia—0.3105 ± 0.0028, which was measured during Galileo flybys.[34][22] In fact, the Ganymede has the lowest moment of inertia among the solid solar system bodies. The existence of the liquid iron rich core provides a natural explanation for the intrinsic magnetic field of Ganymede detected by Galileo.[35] The convection in this iron rich liquid, which has a high electrical conductivity, is the most reasonable model of the magnetic field generation.[36] The precise thicknesses of the different layers in the interior of Ganymede depend on the assumed composition of silicates (fraction of olivine and pyroxene) and amount of sulfur in the core.[24][34] The most probable values are: core radius—700–900 km, the thickness of the outer ice mantle—800–1000 km with the remainder being made by the silicate mantle.[35][34][37][38] The density of the core is 5.5–6 g/cm3 and of the silicate mantle—3.4–3.6 g/cm3.[35][34][37][24] Some models of the magnetic field generation require the existence of a solid core made of pure iron inside the liquid Fe–FeS core—similar to the structure of the Earth’s core. The radius of this core may be up to 500 km.[35] The temperature in the core of Ganymede is probably 1500–1700 K and pressure up to 100 kBar (10 Gpa).[34][35]

Subsurface ocean

The discovery of the induced magnetic moment of Ganymede in 2000 confirmed that all icy Galilean moons—Europa, Ganymede and Callisto have salty water oceans in their interiors (see Magnetosphere).[36][10][39] The existence of the oceans is connected with the anomalous behavior of the ice I melting temperature—it decreases with pressure reaching as low as 251 K at 2070 bar (207 Mpa), which is the water–Ice I–ice III triple point.[24][25] At higher pressures the stable solid phase is ice III and the melting temperature starts to increase.[24] The maximum possible thickness of the solid crust (lid), which overlays the ocean in Ganymede, is 146 km, which corresponds to the pressure of 2070 bar.[40] However the lid is probably somewhat thinner—closer to 100 km.[41] In this case the depth of the ocean is around 230 km.[25] The existence of the ocean can be made more likely if an antifreeze is solved in the water. The best candidates are ammonia and various salts like sulfates and chlorides.[25] Ammonia, for instance, can depress the melting temperature to as low as 175 K.[25]

In many respects Ganymedian ocean is similar to that of Callisto.[25] There are important differences with Europa, however. Europa's ocean is much closer to the surface and is in direct contact with hydrothermal systems on its seafloor. Ganymede's ocean, deeper and sandwiched between layers of ice, has thus been seen as a lesser candidate for extraterrestrial life. Research suggests that interior magmatic activity might still generate pockets of water melt that would supply the ocean with nutrients, possibly sustaining a biota.[40]

The detailed analysis of the trajectory of Galileo spacecraft during the closest G2 flyby (minimum distance 264 km) revealed a few mass anomalies in the interior of Ganymede. The data can be fitted with two or three point-like masses located near the surface or near the ice-rock bounbary. The locations are not correlated with any surface features and their origin is not known.[42]

Surface features

Voyager 2 image mosaic of Ganymede's anti-Jovian hemisphere. The ancient dark area of Galileo Regio lies at the upper right. It is separated from the smaller dark region of Marius Regio to its left by the brighter and younger band of Uruk Sulcus. Fresh ice ejected from the relatively recent Osiris Crater created the bright rays at the bottom.

The Ganymedean 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 contains clays and organic materials that could indicate the composition of the impactors from which Jovian satellites accreted.[43]

The origin of the younger, disrupted terrain is of a tectonic nature, probably formed by the extension, stretching, and faulting of the icy crust.[5] The heating mechanism required to produce this surface geology is not yet understood, however. One possibility is that the satellite passed through unstable orbital resonances that generated tidal heating and, in turn, expansion and surface melt.[44] Radiogenic heating within the satellite is the most relevant current heat source, contributing, for instance, to thickness of the ocean. Research models have found that if the orbital eccentricity were an order of magnitude greater than current (as it may have been in the past) tidal heating would be a more a substantial heat source than radiogenic heating.[45]

Extensive cratering is seen on both types of terrain. 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 bright grooved terrain (but how much younger is uncertain). Craters both overlay and are cross-cut by the groove systems indicating that some of the grooves are quite ancient. Relatively young craters with rays of ejecta are also visible.[46][5] Unlike on the Moon, however, Ganymedean craters are quite flat, lacking the ring mountains and central depressions common to craters on the Moon and Mercury. This is probably due to the relatively weak nature of Ganymede's icy crust which can flow and thereby soften the relief. Ancient craters whose relief has disappeared leaving only a "ghost" of a crater are known as palimpsests.[5]

One feature on Ganymede is a dark plain named Galileo Regio, which contains a series of concentric grooves, or furrows, likely created during a period of geologic activity.[47] Another prominent feature on Ganymede are polar caps, likely composed of water frost. The frost extends to 40° latitude.[8] These polar caps were first seen by the Voyager spacecraft. Theories on the caps' formation include the migration of water to higher latitudes and bombardment by plasma of the ice. Data from Galileo suggests the latter is correct.[48]

Atmosphere and ionosphere

The existence and possible nature of the atmosphere of Ganymede have been controversial. In 1972, a team of Indian, British and American astronomers working at Indonesia's Bosscha Observatory claimed that they had detected a thin atmosphere around the satellite during an occultation, when it and Jupiter passed in front of a star.[49] They estimated that the surface pressure was around 1 μBar (0.1 Pa).[49] However, in 1979 Voyager 1 observed an occultation of a star (κ Centauri) during its flyby of the planet, with differing results.[9] The occultation measurements were conducted in the far-ultraviolet spectrum with wavelength shorter than 200 nm; they were much more sensitive to the presence of gases than measurements in the visible spectrum in 1972. No atmosphere was revealed by the Voyager data. The upper limit on the surface particle number density was found to be 1.5 ×109 cm-3, which corresponds to a surface pressure of less than 2.5 ×10−5 μBar.[9] The latter value is almost five orders of magnitude less than that measured in 1972, indicating that the earlier interpretation was too optimistic.[9]

Despite the Voyager data, evidence for a tenuous oxygen atmosphere on Ganymede, very similar to the one found on Europa, was been found by the Hubble Space Telescope (HST) in 1995.[50][51] HST actually observed airglow of atomic oxygen in the far-ultraviolet at the wavelengths 130.4 nm and 135.6 nm. Such an airglow is excited when molecular oxygen is dissociated by electron impacts,[50] 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 of 0.2–1.2 ×10−5 μBar.[50][52] These values are in agreement with the Voyager's upper limit set in 1981. The oxygen is not evidence of life: it is thought to be produced when water ice on Ganymede's surface is split into hydrogen and oxygen by radiation, with the hydrogen then being more rapidly lost due to its low atomic mass.[51] The airglow observed over Ganymede is not spacially homogeneous like that over Europa. HST observed two bright spots located in the northern and southern hemispheres, respectively, near the ± 50° of latitude, which is exactly the boundary between the open and closed field lines of the Ganymedian magnetosphere (see below).[53] The bright spots are probably polar auroras.[53]

The existence of a neutral atmosphere implies that an ionosphere should exist, because oxygen molecules are ionized by the impacts of the energetic electrons coming from the magnetosphere[54] and by solar EUV radiation.[55] However, the nature of the Ganymedian ionosphere is as controversial as the nature of the atmosphere. Some Galileo measurements found an elevated electron density near the moon, suggesting an ionosphere, while others failed to detect anything.[55] The electron density near the surface is estimated by different sources to lie in the range 400–2500 cm-3.[55] As of 2008, the parameters of the ionosphere of Ganymede are not well constrained.

Additional evidence of the oxygen atmosphere comes from spectral detection of gases trapped in the ice at the surface of Ganymede. The detection of ozone (O3) bands was announced in 1996.[56] In 1997 spectroscopic analysis revealed the dimer (or diatomic) absorption features of the molecular oxygen. Such an absorption can arise only if the oxygen is in a dense phase. The best candidate is the molecular oxygen trapped in ice. The depth of the dimer absorption bands depends on latitude and longitude, rather than on surface albedo—they tend to decrease with increasing latitude on Ganymede, while the O3 shows an opposite effect.[57] Laboratory work has found that O2 would not cluster and bubble but dissolve in ice at Ganymede's relatively warm surface temperature of 100 K.[58]

A search for sodium in the atmosphere, just after such a finding on Europa, turned up nothing in 1997. Sodium is at least 13 times less abundant around Ganymede than around Europa, possibly because of a relative deficiency at the surface or because the magnetosphere fends off energetic particles.[59] Another minor constituent of the Ganymedian atmosphere is atomic hydrogen. Hydrogen atoms were observed as far as 3,000 km from the surface of the moon. Their density on the surface is about 1.5 ×104 cm-3.[60]

Magnetosphere

Enhanced-color Galileo spacecraft image of Ganymede's trailing hemisphere[61]

The Galileo craft made six close flybys of Ganymede in 1995-2000 (G1, G2, G7, G8, G28 and G29)[36] and discovered that Ganymede has a permanent (intrinsic) magnetic moment independent of the Jovian magnetic field.[62] The value of the moment is about 1.3 ×1013 T·m3,[36] which is three times larger than the magnetic moment of Mercury. The magnetic dipole is tilted with respect to the rotational axis of Ganymede by 176°, which means that it is directed against the Jovian magnetic moment.[36] Its north pole lies below the orbital plane. The dipole magnetic field created by this permanent moment has a strength of 719 ± 2 nT at the equator of the moon,[36] which should be compared with the Jovian magnetic field at the distance of Ganymede—about 120 nT.[62] The equatorial field of Ganymede is directed against the Jovian field meaning reconnection is possible. The intrisic field strength at poles is two times that at the equator—1440 nT.[36]

The permament magnetic moment carves a part of space around Ganymede creating a tiny magnetosphere embedded inside that of Jupiter; it is the only moon in the Solar System known to possess the feature.[62] Its diameter is 4–5 RG (RG = 2,631.2 km).[63] The ganymedian magnetosphere has the region of closed field lines located below 30° latitude, where charged particles (electrons and ions) are trapped, creating a kind of radiation belt.[63] The main ion species in the magnetosphere is single ionized oxygen ion—O+,[55] which fits well with the tenuous oxygen atmosphere of the moon. In the polar cap regions, at latitudes higher than 30°, magnetic field lines are open, connecting Ganymede with Jupiter's ionosphere.[63] In these areas, the energetic (tens and hundreds kev) electrons and ions have been detected,[54] which may be responsible for the auroras observed around the ganymedian poles.[53] In addition, heavy ions continuously precipitate on the polar surface of the moon sputtering and darkening the ice.[54]

Magnetic field of the Jovian satellite Ganymede, which is embedded into the magnetosphere of Jupiter. Closed field lines are marked with green color.

The interaction between the Ganymedian magnetosphere and Jovian plasma is in many respect similar to that of the solar wind and Earth's magnetosphere.[63][64] The plasma co-rotating with Jupiter impinges on the trailing side of the Ganymedian magnetosphere much like the solar wind impinges on the Earth's magnetosphere. The main difference is the speed of plasma flow—supersonic in the case of Earth and subsonic in the case of Ganymede. Because of the subsonic flow, there is no bow shock off the trailing hemisphere of Ganymede.[64]

In addition to the intrinsic magnetic moment, Ganymede has an induced dipole magnetic field.[36] Its existence is connected with the variation of the jovian magnetic field near the moon. The induced moment is directed radially to or from Jupiter following the direction of the varying part of the planetary magnetic field. The induced magnetic moment is an order of magnitude weaker than the intrinsic one. The field strength of the induced field at the magnetic equator is about 60 nT—half of that of the ambient jovian field.[36] The induced magnetic field of Ganymede is similar to those of Callisto and Europa indicating that this moon also has a subsurface water ocean with a high electrical conductivity.[36]

Given that Ganymede is completely differentiated and has a metallic core,[22][35] 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, likely originating in the core.[36][35] This theory is supported by the low strength of the higher quadrupole harmonics—they have not been detected so far.[36] The magnetic field detected around Ganymede is likely to be caused by compositional convection in the core,[35] if the magnetic field is the product of dynamo action, or magnetoconvection.[36][65] Some research has suggested that, given its relatively small size, the core ought to have sufficiently cooled to the point where fluid motions and a magnetic field would not be sustained. One explanation: the same orbital resonances proposed to have disrupted the surface also allowed the magnetic field to persist. With Ganymede's eccentricity pumped and tidal heating increased during such resonances, the mantle may have insulated the core, preventing it from cooling.[44]

Origin and evolution

Ganymede likely formed by an accretion in Jupiter’s subnebula, which was a disk of the gas and dust that existed around Jupiter after its formation.[66] The accretion of Ganymede probably took about 10,000 years, which is much shorter than the accretion timescale of Callisto— more than 100,000 years.[66] The relatively fast formation prevented the accretional heat from escaping, which led to the ice melting and differentiation—separation of the rocks and ice. The rocks settled to the center forming a rocky core. In this respect Ganymede is different from Callisto, which was formed much slower and as a result failed to melt and differentiate early due to loss of the accretional heat.[67] The hyposesis described above explaines why these two Jovian moons look so dissimilar, while having similar mass and composition.[38][67]

After the early formation Ganymedian core continued to keep the heat accumulated during the accretion and differentiation only slowly releasing it to the ice mantle functioning like a kind thermal battery.[67] The ice mantle in turn transported it to the surface by convection.[38] Soon the decay of radioactive elementss mixed in rocks further heated the rocky core making it differentiate with formation of an inner iron–iron sulfide core and a silicate mantle.[35][67] After that Ganymede became a fully differentiated body. In comparison the radioactive heating of undifferentiated Callisto caused convection in its icy interior, which effectivelly cooled it and prevented large scale melting of the ice and rapid differentiation.[68] The convective motions in Callisto have caused only a partial separation of rock and ice.[68] Today Ganymede continues to cool slowly.[35] The heat being released from its core and silicate mantle enables the subsurface ocean to exist,[25] while the slow cooling of the liquid Fe–FeS core causes convection in it and supports magnetic field generation.[35] The current heat flux out of Ganymede is probably higher than that out of Callisto.[67]

The formation of the grooved terrain on Ganymede is one of the unsolved problems in the planetary sciences. The modern point of view is that the grooved terrain in mainly tectonic in nature.[22] The cryovulcanism is thought to have played only a minor role if any.[22] The forces that caused the strong stresses in the Ganymedian ice lithosphere necessary to initiate the tectonic activity may be connected to the tidal heating events in the past.[22][69] The tidal flexing of the ice may have heated the interior and strained the lithosphere leading to the development of cracks and horst and graben faulting, which erased the old dark terrain on the 70% of the surface.[22] The formation of the grooved terrain may also be connected with the early core formation. During subsequent evolution deep hot water plumes may have risen from the core to the surface leading to the tectonic deformation of the lithosphere.[40]

The impacts on the surface are another major force that shaped Ganymede.[22] In fact the dark terrain evolved only under the influence of impacts.[22] It appears to be saturated with impact craters. Ganymede may have experienced a period of heavy cratering 3 to 3.5 billion years ago similar to that of the Earth's moon.[5]

Exploration

The Voyager spacecraft.

To study Ganymede in more detail, several probes flying by or orbiting Jupiter have explored Ganymede. The first probes to explore this realm were Pioneer 10 and Pioneer 11,[11] neither of which returned much information about Ganymede.[70] Voyager 1 and Voyager 2 were next, passing by Ganymede in 1979. They refined Ganymede's size, showing it was larger than Saturn's moon Titan, which had previously thought to have been bigger.[71] The grooved terrain was also seen.[72]

In 1995, the Galileo spacecraft entered orbit around Jupiter and between 1996 and 2000 made six close flybys to explore Ganymede.[8] These flybys are G1, G2, G7, G8, G28 and G29.[36] During the closest flyby—G2 Gallileo passed just 264 km from the surface of Ganymede.[36] During a G1 flyby in 1996, the Ganymedian magnetic field was discovered.[12] In 2001, it was announced that an ocean of liquid water likely exists 150 kilometers underneath the frozen surface of this moon.[8][36] Galileo trasmitted a large number of spectral images and discovered several non-ice compounds on the surface of Ganymede.[28] The most recent spacecraft to explore Ganymede up close was New Horizons, which passed by in 2007 on its way to Pluto. New Horizons made topography and composition maps of Ganymede as it sped by.[73][74]

One proposal to orbit Ganymede was the Jupiter Icy Moons Orbiter. It would have used nuclear fission to power it, and would have been able to study Ganymede in detail.[75] However, because of budget cuts, the mission was cancelled in 2005.[13] There is a proposal to send a dedicated mission to orbit Ganymede. The orbiter, which is tentetively called The Grandeur of Ganymede, will stay (if flown) in a low altitude polar orbit around the moon for at least a year. [76]

See also

References

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  2. ^ Using the mean radius
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  21. ^ Laplace-like resonance is similar to the current Laplace resonance among Galilean moons with the only difference that longitudes of the Io–Europa and Europa–Ganymede conjuntions change with rates, whose ratio is a rational number—not unity as in the case of the Laplace resonance
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  30. ^ Leading hemisphere is a hemisphere looking in the direction of the orbital motion, the trailing hemisphere looks in the reverse direction
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External links

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