Ganymede is a moon of Jupiter and the largest moon in the
Solar SystemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
. Completing an orbit in roughly seven days, it is the seventh moon and third Galilean moon from
JupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with a mass slightly less than one-thousandth that of the Sun but is two and a half times the mass of all of the other planets in our Solar System combined. Jupiter is classified as a gas...
. Ganymede participates in a 1:2:4
orbital resonanceIn celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
with the moons
EuropaEuropa is the sixth moon of the planet Jupiter. Europa was discovered in 1610 by Galileo Galilei , and named after a mythical Phoenician noblewoman, Europa, who was courted by Zeus and became the queen of Crete...
and
IoIo is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of 3,642 kilometres, the fourth-largest moon in the Solar System. It was named after Io, a priestess of Hera who became one of the lovers of Zeus.With over 400 active volcanoes, Io is the most...
, respectively. It is larger in diameter than the planet
MercuryFor the liquid metallic element, see Mercury .Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three...
but has only about half its mass. It has the highest mass of all planetary satellites with 2.01 times the mass of the Earth's moon.
Ganymede is composed primarily of
silicate rockA silicate is a compound containing an ion in which one or more central silicon atoms are surrounded by electronegative ligands. This definition is broad enough to include species such as hexafluorosilicate , [SiF6]2−, but the silicate species that are encountered most often...
and water ice. It is a fully
differentiatedIn planetary science, planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, whereby the body evolves into compositionally distinct layers; the denser materials of a planet sink to the center,...
body with an iron-rich, liquid core. A saltwater ocean is believed to exist nearly 200
kmKM, Km, or km may stand for:*Kilometre *KM - the Michaelis constant in Michaelis-Menten kinetics*Kernel methods*Kettle Moraine High School*Khmer language...
below Ganymede's surface, sandwiched between layers of ice. Its surface comprises two main types of terrain. Dark regions, saturated with
impact craterIn the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
s and dated to four billion years ago, cover about a third of the satellite. Lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient, cover the remainder. The cause of the light terrain's disrupted geology is not fully known, but was likely the result of
tectonicTectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...
activity brought about by tidal heating.
Ganymede is the only satellite in the Solar System known to possess a
magnetosphereA magnetosphere is a highly magnetized region around and possessed by an astronomical object. Earth is surrounded by a magnetosphere, as are the magnetized planets Mercury, Jupiter, Saturn, Uranus, and Neptune. Jupiter's moon Ganymede is magnetized, but too weak to trap solar wind plasma. Mars has...
, likely created through
convectionConvection is the movement of molecules within fluids . Convection is one of the major modes of heat transfer and mass transfer...
within the liquid iron core. The meager magnetosphere is buried within Jupiter's much larger
magnetic fieldMagnetic fields surround magnetic materials and electric currents and are detected by the force they exert on other magnetic materials and moving electric charges...
and connected to it through open
field lineA field line is a locus that is defined by a vector field and a starting location within the field. Field lines are useful for visualizing vector fields, which are otherwise hard to depict...
s. The satellite has a thin
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
atmosphereAn atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low...
that includes O, O
2, and possibly O
3 (
ozoneOzone or trioxygen is a simple triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic O
2. Ground-level ozone is an air pollutant with harmful effects on the respiratory systems of animals...
). Atomic hydrogen is a minor atmospheric constituent. Whether the satellite has an
ionosphereThe ionosphere is the uppermost part of the atmosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
to correspond to its atmosphere is unresolved.
Ganymede's discovery is credited to
Galileo GalileiGalileo Galilei was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations, and support for Copernicanism...
, who observed it in 1610. The satellite's name was soon suggested by astronomer
Simon MariusSimon Marius was a German astronomer. He was born in Gunzenhausen near Nuremberg, but he spent most of his life in the city of Ansbach....
, for the mythological
GanymedeIn Greek mythology, Ganymede, or Ganymedes is a divine hero whose homeland was Troy. He was a Trojan prince, son of the eponymous Tros of Dardania, and of Callirrhoe, and brother of Ilus and Assaracus...
, cupbearer of the
Greek godsGreek mythology is the body of myths and legends belonging to the ancient Greeks concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...
and
Zeus'sIn Greek mythology, Zeus is the king of the gods, the ruler of Mount Olympus and the god of the sky and thunder. His symbols are the thunderbolt, eagle, bull, and oak. In addition to his Indo-European inheritance, the classical "cloud-gatherer" also derives certain iconographic traits from the...
beloved. Beginning with
Pioneer 10Pioneer 10 was the first spacecraft to travel through the asteroid belt, which it entered on July 15, 1972, and to make direct observations of Jupiter, which it passed by on December 3, 1973. It was launched from Cape Canaveral Air Force Station's Launch Complex 36A on March 3, 1972 at 01:49:00 UTC...
, spacecraft have been able to examine Ganymede closely. The
VoyagerThe Voyager program is a series of U.S. unmanned space missions that consists of a pair of unmanned scientific probes, Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s...
probes refined measurements of its size, while the
Galileo craft discovered its underground ocean and magnetic field. A new mission to Jupiter's icy moons, the
Europa Jupiter System MissionThe Europa Jupiter System Mission is a proposed joint NASA/ESA unmanned space mission slated to launch around 2020 for the in-depth exploration of Jupiter's moons with a focus on Europa, Ganymede and Jupiter's magnetosphere...
(EJSM) is proposed for a launch in 2020.
Ganymede receives about 8
remThe röntgen equivalent in man or rem is a unit of radiation dose. It is the product of the absorbed dose in röntgens and the biological efficiency of the radiation. More precisely, assuming a radiation weighting factor rW=1, 1 rem equals 1.07185 röntgen...
of radiation per day.
Discovery and naming
On January 11, 1610, Galileo Galilei observed what he believed were three stars near Jupiter; the next night he noticed that they had moved. He found a fourth supposed star, which would turn out to be Ganymede, on January 13. By January 15, Galileo came to the conclusion that the stars were actually bodies orbiting Jupiter.
He claimed the right to name the moons; he considered "Cosmian Stars" and settled on "Medicean Stars".
The French astronomer
Nicolas-Claude Fabri de PeirescNicolas-Claude Fabri de Peiresc was a French astronomer, antiquary and savant who maintained a wide correspondence with scientists and was a successful organizer of scientific inquiry...
suggested individual names from the
MediciThe House of Medici was a political dynasty, banking family and later royal house who first began to gather prominence under Cosimo de' Medici in the Republic of Florence during the late 14th century. The family originated in the Mugello region of the Tuscan countryside, gradually rising until...
family for the moons, but his proposal was not taken up.
Simon MariusSimon Marius was a German astronomer. He was born in Gunzenhausen near Nuremberg, but he spent most of his life in the city of Ansbach....
, who had originally claimed to have found the Galilean satellites, 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. From a suggestion by
Johannes KeplerJohannes Kepler was a German mathematician, astronomer and astrologer, and key figure in the 17th century scientific revolution. He is best known for his eponymous laws of planetary motion, codified by later astronomers based on his works Astronomia nova, Harmonices Mundi, and Epitome of...
, Marius once again tried to name the moons:
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 (a system introduced by Galileo) as or as 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. Ganymede is the only Galilean moon of Jupiter named after a male figure.
Orbit and rotation
Ganymede
orbitIn physics, an orbit is the gravitationally curved path of one object around a point or another body, for example the gravitational orbit of a planet around a star....
s Jupiter at a distance of 1 070 400 km, third among the Galilean satellites, and completes a revolution every seven days and three hours. Like most known moons, Ganymede is
tidally lockedTidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, one side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. This...
, with one face always pointing toward the planet. Its orbit is very slightly eccentric and inclined to the Jovian
equatorThe equator is the intersection of the Earth's surface with the plane perpendicular to the Earth's axis of rotation and containing the Earth's center of mass. In simpler language, it is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole that divides the Earth...
, with the
eccentricityIn astrodynamics, under standard assumptions, any orbit must be of conic section shape. The eccentricity of this conic section, the orbit's eccentricity, is an important parameter of the orbit that defines its absolute shape...
and
inclinationInclination in general is the angle between a reference plane and another plane or axis of direction.- Orbits :The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...
changing
quasi-periodicIn mathematics, an almost periodic function is, loosely speaking, a function of a real number that is periodic to within any desired level of accuracy, given suitably long "almost-periods". The concept was first studied by Harald Bohr and later generalized by Vyacheslav Stepanov, Hermann Weyl and...
ally due to solar and planetary gravitational
perturbationPerturbation is a term used in astronomy in connection with descriptions of the complex motion of a massive body which is subject to appreciable gravitational effects from more than one other massive body....
s on a timescale of centuries. The ranges of change are 0.0009–0.0022 and 0.05–0.32°, respectively. These orbital variations cause the
axial tiltIn astronomy, axial tilt is the angle between an object's rotational axis and a line perpendicular to its orbital plane. The angle is measured between the line perpendicular to object's orbital plane and object's rotational axis passing through north pole at which the planet appears to rotate...
(the angle between rotational and orbital axes) to vary between 0 and 0.33°.
Ganymede participates in
orbital resonanceIn celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
s with Europa and Io: for every orbit of Ganymede, Europa orbits twice and Io orbits four times. The superior conjunction between Io and Europa always occurs when Io is at periapsis and Europa at apoapsis. The superior conjunction between Europa and Ganymede occurs when Europa is at periapsis. The longitudes of the Io–Europa and Europa–Ganymede conjunctions change with the same rate, making the triple conjunctions possible. Such a complicated resonance is called the Laplace resonance.
The current Laplace resonance is unable to pump the orbital eccentricity of Ganymede to a higher value. The value of about 0.0013 is probably a remnant from a previous epoch, when such pumping was possible. The ganymedian orbital eccentricity is somewhat puzzling; if it is not pumped now it should have decayed long ago due to the tidal
dissipationIn physics, dissipation embodies the concept of a dynamical system where important mechanical modes, such as waves or oscillations, lose energy over time, typically due to the action of friction or turbulence. The lost energy is converted into heat, raising the temperature of the system...
in the interior of Ganymede. This means that the last episode of the eccentricity excitation happened only several hundred million years ago. Because the orbital eccentricity of Ganymede is relatively low—0.0015 on average—the tidal heating of this moon is negligible now. However, in the past Ganymede may have passed through one or more Laplace-like resonances which were able to pump the orbital eccentricity to a value as high as 0.01–0.02. 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.
The origin of the Laplace resonance among Io, Europa, and Ganymede is not known. Two hypotheses exist: that it is
primordialIn geochemistry, Primordial elements are chemical elements found on the earth that have existed in their current form since before the earth was formed, according to accepted stellar evolution theory...
and has existed from the beginning of the Solar System; or that it developed after the
formation of the Solar SystemThe formation and evolution of the Solar System is estimated to have begun 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud...
. A possible sequence of the events is as follows: Io raised tides on Jupiter, causing its orbit to expand until it encountered 2:1 resonance with Europa; after that the expansion continued, but some of the angular
momentIn physics, the term "moment" can refer to many different concepts:*Moment of force is a synonym for torque, an important basic concept in physics, civil engineering, and mechanical engineering. In the context of mechanical engineering, the terms are not necessarily interchangeable, but one or the...
was transferred to Europa as the resonance caused its orbit to expand as well; the process continued until Europa encountered 2:1 resonance with Ganymede. Eventually the drift rates of conjunctions between all three moons were synchronized and locked in the Laplace resonance.
Composition
The average
densityThe density of a material is defined as its mass per unit volume. The symbol of density is ρ .- Formula :Mathematically:where: is the density, is the mass, is the volume....
of Ganymede, 1.936
gThe gram , ; symbol g, is a unit of mass.Originally defined as "the absolute weight of a volume of pure water equal to the cube of the hundredth part of a metre, and at the temperature of melting ice" , a gram is now defined as one one-thousandth of the SI base unit, the kilogram, or...
/
cm3A cubic centimetre or cubic centimeter is a commonly used unit of volume extending the derived SI-unit cubic metre and corresponds to the volume of a cube measuring 1×1×1 cm...
, suggests a composition of approximately equal parts rocky material and water, which is mainly in the form of ice. The
mass fractionIn aerospace engineering, the propellant mass fraction is a measure of a vehicle's performance, determined as the portion of the vehicle's mass which does not reach the destination. In a spacecraft, this is an orbit, while for aircraft it is their landing location. A higher mass fraction...
of ices is between 46–50%, slightly lower than that in
CallistoCallisto is a moon of the planet Jupiter, discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the planet Mercury but only about a third of its mass...
. Some additional volatile ices such as
ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula NH
3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers...
may also be present. The exact composition of Ganymede's
rockIn geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
is not known, but is probably close to the composition of
LThe L type ordinary chondrites are the second most common type of meteorite, accounting for approximately 35% of all those catalogued, and 40% of the ordinary chondrites....
/
LL typeThe LL chondrites are the least abundant group of the ordinary chondrites, accounting for about 10-11% of observed ordinary-chondrite falls and 8-9% of all meteorite falls ....
ordinary chondriteThe Ordinary chondrites are a class of stony chondritic meteorites. They are by far the most numerous group and comprise about 87% of all finds...
s, which are characterized by less total
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
, less metallic iron and more
iron oxideIron oxides are chemical compounds composed of iron and oxygen. Altogether, there are sixteen known iron oxides and oxyhydroxides.-Oxides:* FeO, iron oxide, * Fe3O4, iron oxide,...
than
H chondriteThe H type ordinary chondrites are the most common type of meteorite, accounting for approximately 40% of all those catalogued, 46% of the ordinary chondrites, and 44% of the chondrites[1]....
s. The weight ratio of iron to
siliconSilicon is the most common metalloid. It is a chemical element, which has the symbol Si and atomic number 14. A tetravalent metalloid, silicon is less reactive than its chemical analog carbon...
is 1.05–1.27 in Ganymede, whereas the
solar ratioThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
is around 1.8.
Ganymede's surface has an
albedoThe albedo of an object is the extent to which it diffusely reflects light from light sources such as the Sun. It is therefore a more specific form of the term reflectivity. Albedo is defined as the ratio of diffusely reflected to incident electromagnetic radiation. It is a unitless measure...
of about 43%. Water ice seems to be ubiquitous on the surface, with a mass fraction of 50–90%, significantly more than in Ganymede as a whole. Near-infrared
spectroscopySpectroscopy was originally the study of the interaction between radiation and matter as a function of wavelength . In fact, historically, spectroscopy referred to the use of visible light dispersed according to its wavelength, e.g. by a prism. Later the concept was expanded greatly to comprise...
has revealed the presence of strong water ice
absorption bandAn absorption band is a range of wavelengths in the electromagnetic spectrum which are able to excite a particular transition in a substance. See absorption spectrum...
s at wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0
μmA micrometre or micron is one millionth of a metre,or equivalently one thousandth of a millimetre....
. The grooved terrain is brighter and has more icy composition than the dark terrain. The analysis of high-resolution, near-infrared and UV
spectraA spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
obtained by the
Galileo spacecraft and from the ground has revealed various non-water materials:
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
,
sulfur dioxideSulfur dioxide is the chemical compound with the formula SO
2. It is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide...
and, possibly,
cyanogenCyanogen is the chemical compound with the formula 2. It is a colorless, toxic gas with a pungent odor.The molecule is a pseudohalogen. Cyanogen molecules consist of two CN groups — analogous to diatomic halogen molecules, such as Cl2, but far less oxidizing...
, hydrogen sulfate and various
organic compoundAn organic compound is any member of a large class of chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of compounds such as carbonates, simple oxides of carbon and cyanides, as well as the allotropes of carbon, are considered inorganic...
s.
Galileo results have also shown
magnesium sulfateMagnesium sulfate is a chemical compound containing magnesium and sulfur, with the formula MgSO4. In its hydrated form the pH is 6.0 . It is often encountered as the heptahydrate, MgSO4·7H2O, commonly called Epsom salt. Anhydrous magnesium sulfate is used as a...
(MgSO
4) and, possibly,
sodium sulfateSodium sulfate is the sodium salt of sulfuric acid. Anhydrous, it is a white crystalline solid of formula Na
2SO
4 known as the mineral thenardite; the decahydrate Na
2SO
4·10H
2O has been known as Glauber's salt or, historically, sal mirabilis...
(Na
2SO
4) on Ganymede's surface. These salts may originate from the subsurface ocean.
The ganymedian surface is asymmetric; the leading hemisphere—that facing the direction of the orbital motion—is brighter than the trailing one. This is similar to Europa, but the reverse is true on Callisto. The trailing hemisphere of Ganymede appears to be enriched in sulfur dioxide. The distribution of carbon dioxide does not demonstrate any hemispheric asymmetry, although it is not observed near the poles.
Impact craterIn the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
s on Ganymede (except one) do not show any enrichment in carbon dioxide, which also distinguishes it from Callisto. Ganymede's carbon dioxide levels were probably depleted in the past.
Internal structure
Ganymede appears to be fully differentiated, consisting of an
iron sulfideIron sulfide or Iron sulphide may refer to a chemical compound of iron and sulfur with a formula*FeS*FeS2...
–
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
core,
silicateA silicate is a compound containing an ion in which one or more central silicon atoms are surrounded by electronegative ligands. This definition is broad enough to include species such as hexafluorosilicate , [SiF6]2−, but the silicate species that are encountered most often...
mantleThe mantle is a part of an astronomical object. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core. Earth's mantle is about 2,970 km thick rocky shell that...
and an outer ice mantle. This model is supported by the low value of its dimensionless
moment of inertiaMoment of inertia, also called mass moment of inertia or the angular mass, is a measure of an object's resistance to changes in its rotation rate. It is the rotational analog of mass. That is, it is the inertia of a rigid rotating body with respect to its rotation...
——which was measured during
Galileo flybys. In fact, Ganymede has the lowest moment of inertia among the solid solar system bodies. The existence of a liquid, iron-rich core provides a natural explanation for the intrinsic
magnetic fieldA magnetosphere is a highly magnetized region around and possessed by an astronomical object. Earth is surrounded by a magnetosphere, as are the magnetized planets Mercury, Jupiter, Saturn, Uranus, and Neptune. Jupiter's moon Ganymede is magnetized, but too weak to trap solar wind plasma. Mars has...
of Ganymede detected by
Galileo. The
convectionConvection is the movement of molecules within fluids . Convection is one of the major modes of heat transfer and mass transfer...
in the liquid iron, which has high
electrical conductivityElectrical conductivity or specific conductance is a measure of a material's ability to conduct an electric current. When an electrical potential difference is placed across a conductor, its movable charges flow, giving rise to an electric current...
, is the most reasonable model of magnetic field generation.
The precise thicknesses of the different layers in the interior of Ganymede depend on the assumed composition of silicates (fraction of
olivineThe mineral olivine is a magnesium iron silicate with the formula
2SiO
4...
and
pyroxeneThe pyroxenes are a group of important rock-forming silicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
) and amount of
sulfurSulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant, multivalent non-metal. Sulfur, in its native form, is a yellow crystalline solid. In nature, it can be found as the pure element and as sulfide and sulfate minerals...
in the core. The most probable values are 700–900 km for the core radius and 800–1000 km for the thickness of the outer ice mantle, with the remainder being made by the silicate mantle. The density of the core is 5.5–6 g/cm
3 and the silicate mantle is 3.4–3.6 g/cm
3. 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. The temperature in the core of Ganymede is probably 1500–1700 K and pressure up to 100
kBarThe bar is a unit of pressure equal to 100 kilopascals, and rougly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the decibar , centibar , and millibar...
(10
GpaThe pascal is the SI derived unit of pressure, stress, Young's modulus and tensile strength. It is a measure of force per unit area, defined as one newton per square metre...
).
Surface features
The Ganymedian surface is a mix of two types of terrain: very old, highly
crateredIn the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
, 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, contains clays and organic materials that could indicate the composition of the impactors from which Jovian satellites accreted.
The heating mechanism required for the formation of the grooved terrain on Ganymede is an unsolved problem in the planetary sciences. The modern view is that the grooved terrain is mainly tectonic in nature. Cryovulcanism is thought to have played only a minor role, if any. The forces that caused the strong stresses in the ganymedian ice
lithosphereThe lithosphere is the rigid outermost shell of a rocky planet.- Earth's lithosphere :...
necessary to initiate the tectonic activity may be connected to the tidal heating events in the past, possibly caused when the satellite passed through unstable
orbital resonanceIn celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
s. The tidal flexing of the ice may have heated the interior and strained the lithosphere, leading to the development of cracks and
horst and grabenIn geology, horst and graben are terms referring to regions that lie between normal faults and are either above or lower than the area beyond the faults. A horst represents a block pushed upward by the faulting, and a graben is a block that has dropped due to the faulting....
faulting, which erased the old, dark terrain on 70% of the surface. The formation of the grooved terrain may also be connected with the early core formation and subsequent tidal heating of the moon's interior, which may have caused a slight expansion of Ganymede by 1–6% due to
phase transitionA phase transition is a natural physical process. It has the characteristic of taking a given medium with given properties and transforming some or all of that medium, into a new medium with new properties. Phase transitions occur frequently and are found everywhere in the natural world...
s in ice and
thermal expansionThermal expansion is the tendency of a matter to change in volume in response to a change in temperature. When a substance is heated, its particles begin moving and become active thus maintaining a greater average separation. Materials which contract with increasing temperature are rare; this...
. During subsequent evolution deep, hot water
plumeIn hydrodynamics, a plume is a column of one fluid moving through another. Several effects control the motion of the fluid, including momentum, diffusion, and buoyancy...
s may have risen from the core to the surface, leading to the tectonic deformation of the lithosphere.
Radiogenic heatingRadioactive decay is the process in which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, named the daughter...
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 substantial heat source than radiogenic heating.
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. The brighter, grooved terrain contains many fewer impact features, which have been only of a minor importance to its tectonic evolution. The density of cratering indicates an age of 4 billion years for the dark terrain, similar to the highlands of the
MoonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
, and a somewhat younger age for the grooved terrain (but how much younger is uncertain). Ganymede may have experienced a period of heavy cratering 3.5 to 4 billion years ago similar to that of the Moon. If true, the vast majority of impacts happened in that epoch, while the cratering rate has been much smaller since. 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. Ganymedian craters are flatter than those on the Moon and Mercury. This is probably due to the relatively weak nature of Ganymede's icy crust, which can (or could) flow and thereby soften the relief. Ancient craters whose relief has disappeared leave only a "ghost" of a crater known as a palimpsest.
One significant feature on Ganymede is a dark plain named Galileo Regio, which contains a series of concentric grooves, or furrows, likely created during a period of geologic activity. Another prominent feature on Ganymede are polar caps, likely composed of water frost. The frost extends to 40° latitude. These polar caps were first seen by the
VoyagerThe Voyager program is a series of U.S. unmanned space missions that consists of a pair of unmanned scientific probes, Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s...
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.
Atmosphere and ionosphere
In 1972, a team of Indian, British and American astronomers working at
IndonesiaThe Republic of Indonesia is a country in Southeast Asia and Oceania. Indonesia comprises 17,508 islands. With an estimated population of around 237 million people, it is the world's fourth most populous country, with the world's largest population of Muslims.Indonesia is a republic, with an...
's
Bosscha ObservatoryBosscha Observatory is the oldest observatory in Indonesia. The observatory is located in Lembang, West Java, approximately north of Bandung. It is situated on a hilly six hectares of land and is above mean sea level plateau...
claimed that they had detected a thin atmosphere around the satellite during an
occultationAn occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy and can also be used in a general sense to describe when an object in the foreground occults objects in the background...
, when it and Jupiter passed in front of a star. They estimated that the surface pressure was around 1
μBarThe bar is a unit of pressure equal to 100 kilopascals, and rougly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the decibar , centibar , and millibar...
(0.1
PaThe pascal is the SI derived unit of pressure, stress, Young's modulus and tensile strength. It is a measure of force per unit area, defined as one newton per square metre...
). However, in 1979
Voyager 1The Voyager 1 spacecraft is a 722-kilogram robotic space probe of the outer Solar System and beyond, launched September 5, 1977. It remains operational, currently pursuing its extended mission to locate and study the boundaries of the Solar System, including the Kuiper belt and beyond...
observed an occultation of a star (
κ CentauriKappa Centauri is a binary star in the constellation Centaurus. It is approximately 540 light years from the Earth. It was one of 騎官 - 騎官三, meaning "the Third the Cavalry Officer", in traditional Chinese astronomy.The primary component, κ Centauri A, is a blue-white B-type subgiant with an...
) during its flyby of the planet, with differing results. The occultation measurements were conducted in the far-ultraviolet spectrum with
wavelengthIn physics, the wavelength of a sinusoidal wave is the spatial period of the wave – the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
shorter than 200
nmA nanometre is a unit of length in the metric system, equal to one billionth of a meter....
; they were much more sensitive to the presence of gases than measurements in the
visible spectrumThe visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 380 to 750 nm...
in 1972. No atmosphere was revealed by the
Voyager data. The upper limit on the surface particle
number densityIn physics, astronomy, and chemistry, number density is an intensive quantity used to describe the degree of concentration of countable objects in the three-dimensional physical space...
was found to be , which corresponds to a surface pressure of less than . The latter value is almost five orders of magnitude less than that measured in 1972, indicating that the earlier interpretation was too optimistic.
Despite the
Voyager data, evidence for a tenuous
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
atmosphere on Ganymede, very similar to the one found on
EuropaEuropa is the sixth moon of the planet Jupiter. Europa was discovered in 1610 by Galileo Galilei , and named after a mythical Phoenician noblewoman, Europa, who was courted by Zeus and became the queen of Crete...
, was found by the
Hubble Space TelescopeThe Hubble Space Telescope is a space telescope that was carried into orbit by the space shuttle in April 1990. It is named after the American astronomer Edwin Hubble. Although not the first space telescope, the Hubble is one of the largest and most versatile, and is well-known as both a vital...
(HST) in 1995. HST actually observed
airglowAirglow is the very weak emission of light by a planetary atmosphere. In the case of Earth's atmosphere, this phenomenon causes the night sky to never be completely dark .-Development:The airglow phenomenon was first identified in 1868 by Swedish scientist Anders...
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
dissociatedDissociation in chemistry and biochemistry is a general process in which ionic compounds separate or split into smaller particles, ions, or radicals, usually in a reversible manner...
by electron impacts, evidence of a significant neutral atmosphere composed predominantly of O
2 molecules. The surface number density probably lies in the range, corresponding to the surface pressure of . 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
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
and oxygen by radiation, with the hydrogen then being more rapidly lost due to its low atomic mass. The airglow observed over Ganymede is not spatially homogeneous like that 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). The bright spots are probably polar
aurorasAuroras, sometimes called the northern and southern lights or aurorae , are natural light displays in the sky, usually observed at night, particularly in the polar regions. They typically occur in the ionosphere. They are also referred to as polar auroras...
, caused by plasma precipitation along the open field lines.
The existence of a neutral atmosphere implies that an
ionosphereThe ionosphere is the uppermost part of the atmosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
should exist, because oxygen molecules are ionized by the impacts of the energetic
electronAn electron is a subatomic particle that carries a negative electric charge. It has no known substructure and is believed to be a point particle. An electron has a mass that is approximately 1836 times less than that of the proton. The intrinsic angular momentum of the electron is a half integer...
s coming from the magnetosphere and by solar EUV radiation. 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. The electron density near the surface is estimated by different sources to lie in the range 400–2,500 cm
−3. 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
ozoneOzone or trioxygen is a simple triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic O
2. Ground-level ozone is an air pollutant with harmful effects on the respiratory systems of animals...
(O
3) bands was announced in 1996. In 1997 spectroscopic analysis revealed the
dimerA dimer is a chemical or biological entity consisting of two structurally similar subunits called monomers, which are joined by bonds, which can be strong or weak.- Organic chemistry :...
(or
diatomicDiatomic molecules are molecules composed only of two atoms, of either the same or different chemical elements. The prefix di- means two in Greek. Common diatomic molecules are hydrogen, nitrogen, oxygen, and carbon monoxide...
) 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
latitudeLatitude, usually denoted by the Greek letter phi gives the location of a place on Earth north or south of the equator. Lines of Latitude are the imaginary horizontal lines shown running east-to-west on maps that run either north or south of the equator...
and
longitudeLongitude , identified by the Greek letter lambda , is the geographic coordinate most commonly used in cartography and global navigation for east-west measurement...
, rather than on surface
albedoThe albedo of an object is the extent to which it diffusely reflects light from light sources such as the Sun. It is therefore a more specific form of the term reflectivity. Albedo is defined as the ratio of diffusely reflected to incident electromagnetic radiation. It is a unitless measure...
—they tend to decrease with increasing latitude on Ganymede, while the O
3 shows an opposite effect. Laboratory work has found that O
2 would not cluster and bubble but dissolve in ice at Ganymede's relatively warm surface temperature of 100 K.
A search for
sodiumSodium is a metallic element with a symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals within "group 1"...
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. 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 .
Magnetosphere
The
Galileo craft made six close flybys of Ganymede from 1995–2000 (G1, G2, G7, G8, G28 and G29) and discovered that Ganymede has a permanent (intrinsic)
magnetic momentThe magnetic moment of a system is a measure of the strength and the direction of its magnetism. More technically , the term magnetic moment of a system usually refers to its magnetic dipole moment, and quantifies the contribution...
independent of the Jovian magnetic field. The value of the moment is about , which is three times larger than the magnetic moment of
MercuryFor the liquid metallic element, see Mercury .Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three...
. 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. Its north pole lies below the
orbital planeThe orbital plane of an object orbiting another is the geometrical plane in which the orbit is embedded. Three non-colinear points in space suffice to define the orbital plane...
. The
dipole magnetic fieldIn physics, there are two kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some, usually small, distance. A permanent electric dipole is called an...
created by this permanent moment has a strength of 719 ± 2
nTThe tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honor of the Serbian-American inventor, physicist, and electrical engineer Nikola Tesla...
at the equator of the moon, which should be compared with the Jovian magnetic field at the distance of Ganymede—about 120 nT. The equatorial field of Ganymede is directed against the Jovian field, meaning
reconnectionA magnetosphere is a highly magnetized region around and possessed by an astronomical object. Earth is surrounded by a magnetosphere, as are the magnetized planets Mercury, Jupiter, Saturn, Uranus, and Neptune. Jupiter's moon Ganymede is magnetized, but too weak to trap solar wind plasma. Mars has...
is possible. The intrinsic field strength at the poles is two times that at the equator—1440 nT.
The permanent magnetic moment carves a part of space around Ganymede, creating a tiny
magnetosphereA magnetosphere is a highly magnetized region around and possessed by an astronomical object. Earth is surrounded by a magnetosphere, as are the magnetized planets Mercury, Jupiter, Saturn, Uranus, and Neptune. Jupiter's moon Ganymede is magnetized, but too weak to trap solar wind plasma. Mars has...
embedded inside
that of JupiterThe magnetosphere of Jupiter is the cavity created in the solar wind by the planet's immensely strong magnetic field. Extending up to seven million kilometers in the Sun's direction and almost to the orbit of Saturn in the opposite direction, Jupiter's magnetosphere is the largest and most powerful...
; it is the only moon in the Solar System known to possess the feature. Its diameter is 4–5 R
G (R
G = 2,631.2 km). The ganymedian magnetosphere has a region of closed
field lineA field line is a locus that is defined by a vector field and a starting location within the field. Field lines are useful for visualizing vector fields, which are otherwise hard to depict...
s located below 30°
latitudeLatitude, usually denoted by the Greek letter phi gives the location of a place on Earth north or south of the equator. Lines of Latitude are the imaginary horizontal lines shown running east-to-west on maps that run either north or south of the equator...
, where
charged particleIn physics, a charged particle is a particle with an electric charge. It may be either a subatomic particle or an ion. A collection of charged particles, or even a gas containing a proportion of charged particles, is called a plasma, which is called the fourth state of matter because its...
s (
electronAn electron is a subatomic particle that carries a negative electric charge. It has no known substructure and is believed to be a point particle. An electron has a mass that is approximately 1836 times less than that of the proton. The intrinsic angular momentum of the electron is a half integer...
s and
ionAn ion is an atom or molecule where the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge...
s) are trapped, creating a kind of radiation belt. The main ion species in the magnetosphere is single ionized
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
—O
+—which fits well with the tenuous oxygen
atmosphereAn atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low...
of the moon. In the polar cap regions, at latitudes higher than 30°, magnetic field lines are open, connecting Ganymede with Jupiter's
ionosphereThe ionosphere is the uppermost part of the atmosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
. In these areas, the energetic (tens and hundreds of keV) electrons and ions have been detected, which may be responsible for the
auroraAuroras, sometimes called the northern and southern lights or aurorae , are natural light displays in the sky, usually observed at night, particularly in the polar regions. They typically occur in the ionosphere. They are also referred to as polar auroras...
s observed around the ganymedian poles. In addition, heavy ions continuously precipitate on the polar surface of the moon,
sputteringSputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic ions. It is commonly used for thin-film deposition, etching and analytical techniques .- Physics of sputtering :...
and darkening the ice.
The interaction between the ganymedian magnetosphere and Jovian
plasmaIn physics and chemistry, plasma is a partially ionized gas, in which a certain proportion of electrons are free rather than being bound to an atom or molecule. The ability of the positive and negative charges to move somewhat independently makes the plasma electrically conductive so that it...
is in many respects similar to that of the
solar windThe solar wind is a stream of charged particles ejected from the upper atmosphere of the sun. It consists mostly of electrons and protons with energies of about 1 keV. The stream of particles varies in temperature and speed with the passage of time...
and Earth's magnetosphere. 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—
supersonicThe term supersonic is used to define a speed that is over the speed of sound . In dry air at 20 °C , the threshold value required for an object to be traveling at a supersonic speed is approximately 343 m/s, . Speeds greater than 5 times the speed of sound are often referred to as hypersonic...
in the case of Earth and
subsonicSound is a vibration that travels through an elastic medium as a wave. The speed of sound describes how far this wave travels in a given amount of time. In dry air at , the speed of sound is . This equates to , or about one mile in five seconds...
in the case of Ganymede. Because of the subsonic flow, there is no
bow shockA bow shock is a boundary between a magnetosphere and an ambient medium. For stars, this is typically the boundary between their stellar wind and the interstellar medium....
off the trailing hemisphere of Ganymede.
In addition to the intrinsic magnetic moment, Ganymede has an induced dipole magnetic field. 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 strengthIn physics, the field strength of a field is the magnitude of its vector value.In theoretical physics, field strength is another name for the curvature form...
of the induced field at the magnetic equator is about 60 nT—half of that of the ambient Jovian field. The induced magnetic field of Ganymede is similar to those of
CallistoCallisto is a moon of the planet Jupiter, discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the planet Mercury but only about a third of its mass...
and
EuropaEuropa is the sixth moon of the planet Jupiter. Europa was discovered in 1610 by Galileo Galilei , and named after a mythical Phoenician noblewoman, Europa, who was courted by Zeus and became the queen of Crete...
, indicating that this moon also has a subsurface water ocean with a high
electrical conductivityElectrical conductivity or specific conductance is a measure of a material's ability to conduct an electric current. When an electrical potential difference is placed across a conductor, its movable charges flow, giving rise to an electric current...
.
Given that Ganymede is completely differentiated and has a metallic core, 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. The magnetic field detected around Ganymede is likely to be caused by compositional convection in the core, if the magnetic field is the product of dynamo action, or magnetoconvection.
Despite the presence of an iron core, Ganymede's magnetosphere remains enigmatic, particularly given that similar bodies lack the feature. 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 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 increased during such resonances, the mantle may have insulated the core, preventing it from cooling. 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.
Origin and evolution
Ganymede likely formed by an
accretionIn astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...
in Jupiter’s
subnebulaIn cosmogony, the nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. It was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation is now thought to be at...
, a disk of gas and dust surrounding Jupiter after its formation. The accretion of Ganymede probably took about 10 000 years, 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. In contrast Ganymede formed closer to Jupiter, where the subnebular was denser, which explains its shorter formation timescale. This relatively fast formation prevented the escape of accretional heat, which led to ice melt and
differentiationIn planetary science, planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, whereby the body evolves into compositionally distinct layers; the denser materials of a planet sink to the center,...
: the separation of the rocks and ice. The rocks settled to the center, forming the core. In this respect, Ganymede is different from Callisto, which failed to melt and differentiate early due to loss of the accretional heat during its slower formation. This hypothesis explains why the two Jovian moons look so dissimilar, despite their similar mass and composition.
After formation, the Ganymedian core largely retained the heat accumulated during accretion and differentiation, only slowly releasing it to the ice mantle like a kind thermal battery. The mantle, in turn, transported it to the surface by convection. Soon the decay of radioactive elements within rocks further heated the core, causing increased differentiation: an inner,
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
–
iron sulfideIron sulfide or Iron sulphide may refer to a chemical compound of iron and sulfur with a formula*FeS*FeS2...
core and a
silicateA silicate is a compound containing an ion in which one or more central silicon atoms are surrounded by electronegative ligands. This definition is broad enough to include species such as hexafluorosilicate , [SiF6]2−, but the silicate species that are encountered most often...
mantle formed. With this, Ganymede became a fully differentiated body. 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. The convective motions in Callisto have caused only a partial separation of rock and ice. Today, Ganymede continues to cool slowly. The heat being released from its core and silicate mantle enables the subsurface ocean to exist, while the slow cooling of the liquid Fe–FeS core causes convection and supports magnetic field generation. The current
heat fluxHeat flux or thermal flux, sometimes also referred to as heat flux density or heat flow rate intensity is a flow of energy per unit of area per unit of time. In SI units, it is measured in
[W·m
-2]. It has both a direction and a magnitude so it is a...
out of Ganymede is probably higher than that out of Callisto.
Exploration
Several probes flying by or orbiting Jupiter have explored Ganymede in detail. The first probes to explore were
Pioneer 10Pioneer 10 was the first spacecraft to travel through the asteroid belt, which it entered on July 15, 1972, and to make direct observations of Jupiter, which it passed by on December 3, 1973. It was launched from Cape Canaveral Air Force Station's Launch Complex 36A on March 3, 1972 at 01:49:00 UTC...
and
Pioneer 11Pioneer 11 was the second mission of the Pioneer program to investigate Jupiter and the outer solar system and the first to explore Saturn and its main rings. Pioneer 11 used Jupiter's mass in a gravity assist to alter its trajectory toward Saturn...
, neither of which returned much information about the satellite.
Voyager 1The Voyager 1 spacecraft is a 722-kilogram robotic space probe of the outer Solar System and beyond, launched September 5, 1977. It remains operational, currently pursuing its extended mission to locate and study the boundaries of the Solar System, including the Kuiper belt and beyond...
and
Voyager 2The Voyager 2 spacecraft is an unmanned interplanetary space probe launched on August 20, 1977. Identical in form to its sister Voyager program craft Voyager 1, Voyager 2 followed a slower trajectory that allowed it to be kept in the ecliptic so that it could be sent to Uranus and Neptune by means...
were next, passing by Ganymede in 1979. They refined its size, revealing it was larger than
SaturnSaturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant...
's moon
TitanTitan , or Saturn VI, is the largest moon of Saturn, the only moon known to have a dense atmosphere,
and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....
, which was previously thought to have been bigger. The grooved terrain was also seen.
In 1995, the
Galileo spacecraft entered orbit around Jupiter and between 1996 and 2000 made six close flybys to explore Ganymede. These flybys are G1, G2, G7, G8, G28 and G29. During the closest flyby—G2—
Galileo passed just 264 km from the surface of Ganymede. During a G1 flyby in 1996, the ganymedian magnetic field was discovered, while the discovery of the ocean was announced in 2001.
Galileo transmitted a large number of spectral images and discovered several non-ice compounds on the surface of Ganymede. The most recent spacecraft to explore Ganymede up close was
New HorizonsNew Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, and Hydra. NASA may also approve flybys of one or more other Kuiper Belt Objects.New Horizons was launched...
, which passed by in 2007 on its way to
PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
.
New Horizons made topography and composition maps of Ganymede as it sped by.
Proposed for a launch in 2020, the
Europa Jupiter System MissionThe Europa Jupiter System Mission is a proposed joint NASA/ESA unmanned space mission slated to launch around 2020 for the in-depth exploration of Jupiter's moons with a focus on Europa, Ganymede and Jupiter's magnetosphere...
(EJSM) is a joint
NASAThe National Aeronautics and Space Administration is an agency of the United States government, responsible for the nation's public space program. NASA was established by the National Aeronautics and Space Act on July 29, 1958, replacing its predecessor, the National Advisory Committee for...
/ESA proposal for exploration of
JupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with a mass slightly less than one-thousandth that of the Sun but is two and a half times the mass of all of the other planets in our Solar System combined. Jupiter is classified as a gas...
's moons. In February 2009 it was announced that ESA/NASA had given this mission priority ahead of the
Titan Saturn System MissionTitan Saturn System Mission is a joint NASA/ESA proposal for an exploration of Saturn and its moons Titan and Enceladus where many complex phenomena have been revealed by the recent Cassini–Huygens mission. TSSM was competing against the Europa Jupiter System Mission proposal for funding...
. ESA's contribution will still face funding competition from other ESA projects. EJSM consists of the NASA-led
Jupiter Europa OrbiterAs a part of the Europa Jupiter System Mission , the Jupiter Europa Orbiter is a proposed orbiter probe slated for lift-off in 2020 and planned for detailed studies of Jupiter's moons Europa and Io as well as the Jovian magnetosphere...
, the ESA-led
Jupiter Ganymede OrbiterAs a part of the Europa Jupiter System Mission , the Jupiter Ganymede Orbiter is a proposed orbiter probe slated for lift-off in 2020 and planned for detailed studies of Jupiter's moons Ganymede and Callisto as well as the Jovian magnetosphere....
, and possibly a JAXA-led
Jupiter Magnetospheric OrbiterAs a part of the Europa Jupiter System Mission , the Jupiter Magnetospheric Orbiter is an orbiter probe proposed by the Japanese Aerospace Exploration Agency ....
.
One canceled proposal to orbit Ganymede was the
Jupiter Icy Moons OrbiterThe Jupiter Icy Moons Orbiter was a proposed spacecraft designed to explore the icy moons of Jupiter. The main target was Europa, the suspected ocean of which is one of the places where simple alien life is a possibility in our solar system...
.
Nuclear fissionIn nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts, often producing free neutrons and lighter nuclei, which may eventually produce photons...
would have been used to power the craft, which would have been able to study Ganymede in detail. However, the mission was canceled in 2005 because of budget cuts. Another old proposal was called The Grandeur of Ganymede.
See also
External links