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Ganymede, a satellite of Jupiter (Jupiter III), is the largest and most massive of the Solar System's moons. The ninth-largest object (including the Sun) of the Solar System, it is the largest without a substantial atmosphere. It has a diameter of , making it 26% larger than the planet
Mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

Mercury
by volume, although it is only 45% as massive. Possessing a metallic core, it has the lowest
moment of inertia factor Moment or Moments may refer to: * Present The present (or here and now) is the time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparently irreversible process, irreversib ...
of any solid body in the Solar System and is the only
moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

moon
known to have a
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

magnetic field
. Outward from
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
, it is the seventh
satellite In the context of spaceflight Spaceflight (or space flight) is an application of astronautics to fly spacecraft into or through outer space, either human spaceflight, with or uncrewed spaceflight, without humans on board. Most spaceflight ...

satellite
and the third of the
Galilean moon 's four Galilean moons, in a composite image depicting part of Jupiter and their relative sizes (positions are illustrative, not actual). From top to bottom: Io (moon), Io, Europa (moon), Europa, Ganymede (moon), Ganymede, Callisto (moon), Callisto. ...
s, the first group of objects discovered orbiting another planet. Ganymede
orbit In celestial mechanics, an orbit is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an satellite, artificial satellite around an object or po ...

orbit
s Jupiter in roughly seven days and is in a 1:2:4
orbital resonance In celestial mechanics Celestial mechanics is the branch of astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical obje ...
with the moons
Europa Europa may refer to: Places *Europe Europe is a continent A continent is one of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven regions are commonly regard ...
and Io, respectively. Ganymede is composed of approximately equal amounts of silicate rock and
water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known li ...
. It is a fully differentiated body with an iron-rich, liquid core, and an
internal ocean The ocean (also the sea or the world ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
that may contain more water than all of Earth's oceans combined. Its surface is composed of two main types of terrain. Dark regions, saturated with
impact crater An impact crater is an approximately circular depression (geology), depression in the surface of a planet, natural satellite, moon, or other solid body in the Solar System or elsewhere, formed by the hypervelocity collision, impact of a smaller ...

impact crater
s and dated to four billion years ago, cover about a third of it. 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
tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building A mountain is an elevated portion of the Earth's crust, gen ...
activity due to
tidal heating Tidal heating (also known as tidal working or tidal flexing) occurs through the tidal friction processes: orbital and rotational energy is dissipated as heat in either (or both) the surface ocean or interior of a planet or satellite. When an object ...
. Ganymede's magnetic field is probably created by
convection Convection is single or multiphase fluid flow that occurs Spontaneous process, spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When t ...

convection
within its liquid iron core, also created by Jupiter's tidal forces. The meager magnetic field is buried within Jupiter's far larger
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

magnetic field
and would show only as a local perturbation of the
field line A field line is a graphical visual aid Visual communication is the use of visual elements to convey ideas and information"7 Paramount Components of Visual Communication". ''Infographic Design Team - Infographics Design - Data Visualization''. R ...
s. Ganymede has a thin
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...

atmosphere
that includes O, O2, and possibly O3 (
ozone Ozone (), or trioxygen, is an inorganic molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a cha ...

ozone
).
Atomic hydrogen A hydrogen atom is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touch ...
is a minor atmospheric constituent. Whether Ganymede has an
ionosphere The ionosphere () is the ionized part of Earth's upper atmosphere, from about to altitude, a region that includes the thermosphere The thermosphere is the layer in the directly above the and below the . Within this layer of the atmosphere, ...
associated with its atmosphere is unresolved. Ganymede's discovery is credited to
Galileo Galilei Galileo di Vincenzo Bonaiuti de' Galilei ( , ; 15 February 1564 – 8 January 1642), commonly referred to as Galileo, was an astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific q ...

Galileo Galilei
, the first to observe it, on January 7, 1610. Its name was soon suggested by astronomer
Simon Marius Simon Marius (latinized Latinisation or Latinization can refer to: * Latinisation of names, the practice of rendering a non-Latin name in a Latin style * Latinisation in the Soviet Union, the campaign in the USSR during the 1920s and 1930s to re ...

Simon Marius
, after the
mythological Myth is a folklore genre consisting of narratives that play a fundamental role in a society, such as foundational tales or origin myths. The main characters in myths are usually non-humans, such as gods, demigods, and other supernatural figures ...
Ganymede, a
Trojan Trojan or Trojans may refer to: * Of or from the ancient city of Troy Troy (Greek language, Greek: Τροία) or Ilium (Greek language, Greek: Ίλιον) was an ancient city located at Hisarlik in present-day Turkey, south-west of Çanakkal ...

Trojan
prince desired by
Zeus Zeus or , , ; grc, Δῐός, ''Diós'', label=genitive In grammar In linguistics Linguistics is the scientific study of language, meaning that it is a comprehensive, systematic, objective, and precise study of language. Ling ...

Zeus
(the Greek counterpart of
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
), who carried him off to be the cupbearer of the gods. Beginning with ''
Pioneer 10 ''Pioneer 10'' (originally designated Pioneer F) is an American space probe, launched in 1972 and weighing , that completed the first mission to the planet Jupiter. Thereafter, ''Pioneer 10'' became the List of artificial objects leaving the S ...

Pioneer 10
'', several spacecraft have explored Ganymede. The
Voyager Voyager may refer to: Computing and communications * LG Voyager The LG VX10000, also known as the Verizon Voyager or LG VX10K, is an Internet-enabled multimedia phone designed by LG Electronics and carried by Verizon Wireless, Telus, and Bel ...
probes, ''
Voyager 1 ''Voyager 1'' is a space probe A space probe or a spaceprobe is a robotic spacecraft that doesn't Earth orbit, orbit the Earth (planet), Earth, but instead explores farther into outer space. A space probe may approach the Moon; travel ...
'' and ''
Voyager 2 ''Voyager 2'' is a space probe A space probe or a spaceprobe is a robotic spacecraft that doesn't Earth orbit, orbit the Earth (planet), Earth, but instead explores farther into outer space. A space probe may approach the Moon; travel thro ...
'', refined measurements of its size, while ''
Galileo Galileo di Vincenzo Bonaiuti de' Galilei ( , ; 15 February 1564 – 8 January 1642), commonly referred to as Galileo, was an astronomer An astronomer is a in the field of who focuses their studies on a specific question or field o ...
'' discovered its underground ocean and magnetic field. The next planned mission to the Jovian system is the
European Space Agency , owner = , headquarters = Paris Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, most populous city of France, with an estimated population of 2,175,601 residents , in an area ...

European Space Agency
's
Jupiter Icy Moon Explorer The Jupiter Icy Moons Explorer (JUICE) is an interplanetary spacecraft in development by the European Space Agency (ESA) with Airbus Defence and Space as the main contractor. The mission will study three of Jupiter's Galilean moons: Ganymed ...
(JUICE), due to launch in 2023. After flybys of all three icy Galilean moons, it is planned to enter orbit around Ganymede.


History

Chinese astronomical records report that in 365 BC,
Gan De Gan De (; fl. 4th century BC), also known as the Lord Gan (Gan Gong), was an ancient Chinese astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Eart ...
detected what might have been a moon of Jupiter, probably Ganymede, with the naked eye. However, Gan De reported the color of the companion as reddish, which is puzzling since the moons are too faint for their color to be perceived with the naked eye.
Shi ShenShi Shen (, fl. 4th century BC) was a Chinese astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, ...

Shi Shen
and Gan De together made fairly accurate observations of the five major planets. On January 7, 1610,
Galileo Galilei Galileo di Vincenzo Bonaiuti de' Galilei ( , ; 15 February 1564 – 8 January 1642), commonly referred to as Galileo, was an astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific q ...

Galileo Galilei
used a telescope to observe what he thought were three stars near Jupiter, including what turned out to be Ganymede, Callisto, and one body that turned out to be the combined light from Io and
Europa Europa may refer to: Places *Europe Europe is a continent A continent is one of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven regions are commonly regard ...
; the next night he noticed that they had moved. On January 13, he saw all four at once for the first time, but had seen each of the moons before this date at least once. By January 15, Galileo came to the conclusion that the stars were actually bodies orbiting Jupiter.


Name

Galileo claimed the right to name the moons he had discovered. He considered "Cosmian Stars" and settled on " Medicean Stars", in honor of
Cosimo II de' Medici Cosimo II de' Medici (12 May 1590 – 28 February 1621) was Grand Duke of Tuscany from 1609 until his death. He was the elder son of Ferdinando I de' Medici, Grand Duke of Tuscany, and Christina of Lorraine. For the majority of his twelve- ...

Cosimo II de' Medici
. The French astronomer Nicolas-Claude Fabri de Peiresc suggested individual names from the
Medici The House of Medici ( , ) was an Italian banking family and political dynasty Politics (from , ) is the set of activities that are associated with Decision-making, making decisions in Social group, groups, or other forms of Power (social ...

Medici
family for the moons, but his proposal was not taken up.
Simon Marius Simon Marius (latinized Latinisation or Latinization can refer to: * Latinisation of names, the practice of rendering a non-Latin name in a Latin style * Latinisation in the Soviet Union, the campaign in the USSR during the 1920s and 1930s to re ...

Simon Marius
, 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 Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer An astronomer is a in the field of who focuses their studies on a specific question or field outside the scope of . They observe s such as s, s, , s and ...

Johannes Kepler
, Marius suggested a different naming system based on
Greek mythology Greek mythology is the body of myth Myth is a folklore genre Folklore is the expressive body of culture shared by a particular group of people; it encompasses the tradition A tradition is a belief A belief is an Attitude (psyc ...
: 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), in other words "the third satellite of Jupiter". Following the discovery of moons of Saturn, a naming system based on that of Kepler and Marius was used for Jupiter's moons. Ganymede is the only Galilean moon of Jupiter named after a male figure—like Io, Europa, and Callisto, he was a lover of Zeus. The Galilean satellites retain the Italian spellings of their names. In the cases of Io, Europa and Callisto, these are identical to the Latin, but the Latin form of Ganymede is ''Ganymedes''. In English, the final 'e' is silent, perhaps under the influence of French, unlike later names taken from Latin and Greek.


Orbit and rotation

Ganymede
orbit In celestial mechanics, an orbit is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an satellite, artificial satellite around an object or po ...

orbit
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 locked Tidal locking (also called gravitational locking, captured rotation and spin–orbit locking), in the best-known case, occurs when an orbiting astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science tha ...
, with one side always facing toward the planet, hence its day is also seven days and three hours. Its orbit is very slightly eccentric and inclined to the Jovian
equator The Equator is a circle of latitude, about in circumference, that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the N ...

equator
, with the
eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off- center, in geometry * Eccentricity (graph theory) of a ...
and
inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a Plane of reference, reference plane and the orbital plane or Axis of rotation, axis of direction of the orbiting object. ...

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. These orbital variations cause the
axial tilt In , axial tilt, also known as obliquity, is the angle between an object's and its al axis, or, equivalently, the angle between its ial plane and . It differs from . At an obliquity of 0 degrees, the two axes point in the same direction; i.e., ...
(the angle between rotational and orbital axes) to vary between 0 and 0.33°. Ganymede participates in
orbital resonance In celestial mechanics Celestial mechanics is the branch of astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical obje ...
s with Europa and Io: for every orbit of Ganymede, Europa orbits twice and Io orbits four times.
Conjunctions ''Conjunctions'' is a biannual American literature, American literary journal based at Bard College. It was founded in 1981 and is currently edited by Bradford Morrow. Morrow received the PEN/Nora Magid Award for Magazine Editing in 2007. The jou ...
(alignment on the same side of Jupiter) between Io and Europa occur when Io is at
periapsis File:Periapsis_apoapsis.png, upright=1.15, The two-body system of interacting elliptic orbits: The smaller, satellite body (blue) orbits the primary (astronomy), primary body (yellow); both are in elliptic orbits around their center of mass, co ...
and Europa at
apoapsis upright=1.15, The two-body system of interacting primary body (yellow); both are in elliptic orbits around their center of mass">common center of mass (or barycenter), (red +). ∗Periapsis and apoapsis as distances: The smallest and largest ...
. Conjunctions between Europa and Ganymede occur when Europa is at periapsis. The longitudes of the Io–Europa and Europa–Ganymede conjunctions change with the same rate, making triple conjunctions impossible. 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
dissipation In , dissipation is the result of an that takes place in homogeneous . In a dissipative process, (, bulk flow , or system ) from an initial form to a final form, where the capacity of the final form to do is less than that of the initial form. ...
in the interior of Ganymede. This means that the last episode of the eccentricity excitation happened only several hundred million years ago. Because Ganymede's orbital eccentricity is relatively low—on average 0.0015—tidal heating is negligible now. However, in the past Ganymede may have passed through one or more Laplace-like resonances that 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. There are two hypotheses for the origin of the Laplace resonance among Io, Europa, and Ganymede: that it is primordial and has existed from the beginning of the Solar System; or that it developed after the
formation of the Solar System The formation and evolution of the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all indi ...
. A possible sequence of events for the latter scenario is as follows: Io raised tides on Jupiter, causing Io's orbit to expand (due to conservation of momentum) until it encountered the 2:1 resonance with Europa; after that the expansion continued, but some of the angular moment was transferred to Europa as the resonance caused its orbit to expand as well; the process continued until Europa encountered the 2:1 resonance with Ganymede. Eventually the drift rates of conjunctions between all three moons were synchronized and locked in the Laplace resonance.


Physical characteristics


Size

Ganymede is the largest and most massive moon in the Solar System. Its diameter of 5,268 km is 0.41 times that of
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
, 0.77 times that of
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

Mars
, 1.02 times that of Saturn's
Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictional ...
(Solar System's second largest moon), 1.08 times
Mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

Mercury
's, 1.09 times Callisto's, 1.45 times Io's and 1.51 times the Moon's. Its mass is 10% greater than Titan's, 38% greater than Callisto's, 66% greater than Io's and 2.02 times that of the Moon.


Composition

The average
density The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its per unit . The symbol most often used for density is ''ρ'' (the lower case Greek letter ), although the Latin letter ''D'' can also ...

density
of Ganymede, 1.936 g/cm3, suggests a composition of about equal parts rocky material and mostly water ices. Some of the water is liquid, forming an underground ocean. The mass fraction of ices is between 46 and 50%, which is slightly lower than that in Callisto. Some additional volatile ices such as
ammonia Ammonia is a chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula NH3. A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct ch ...

ammonia
may also be present. The exact composition of Ganymede's
rock Rock most often refers to: * Rock (geology) A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition and the way in w ...
is not known, but is probably close to the composition of L/ LL type
ordinary chondrite File:Ordinary chondrite (Ochansk Meteorite).jpg, Ochansk Meteorite, an ordinary chondrite with a fusion crust, found in 1887 in Russia. The ordinary chondrites (sometimes called the O chondrites) are a class of stony chondritic meteorites. They ar ...
s, which are characterized by less total
iron Iron () is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behav ...

iron
, less metallic iron and more
iron oxide Iron oxides are chemical compounds composed of iron and oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, ...

iron oxide
than H chondrites. The weight ratio of iron to
silicon Silicon is a chemical element with the Symbol (chemistry), symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a Tetravalence, tetravalent metalloid and semiconductor. It is a member ...

silicon
ranges between 1.05 and 1.27 in Ganymede, whereas the is around 1.8.


Surface features

Ganymede's surface has an
albedo Albedo (prounounced ; la, albedo, meaning 'whiteness') is the measure of the diffuse reflection Diffuse reflection is the reflectionReflection or reflexion may refer to: Philosophy * Self-reflection Science * Reflection (physics), a comm ...

albedo
of about 43%. Water ice seems to be ubiquitous on its surface, with a mass fraction of 50–90%, significantly more than in Ganymede as a whole.
Near-infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natur ...
spectroscopy Spectroscopy is the study of the interaction Interaction is a kind of action that occurs as two or more objects have an effect upon one another. The idea of a two-way effect is essential in the concept of interaction, as opposed to a one-way ...

spectroscopy
has revealed the presence of strong water ice
absorption band According to quantum mechanics Quantum mechanics is a fundamental theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and ration ...
s at wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0 μm. The grooved terrain is brighter and has a more icy composition than the dark terrain. The analysis of high-resolution, near-infrared and obtained by the ''Galileo'' spacecraft and from Earth observations has revealed various non-water materials:
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
,
sulfur dioxide Sulfur dioxide (IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering OrganizationsNational Adhering Organizations in chemistry are the organizations that work as the autho ...
and, possibly,
cyanogen Cyanogen is the chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volum ...

cyanogen
,
hydrogen sulfate Hydrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the sam ...

hydrogen sulfate
and various
organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
s. ''Galileo'' results have also shown
magnesium sulfate Magnesium sulfate or magnesium sulphate (in British English British English (BrE) is the standard dialect A standard language (also standard variety, standard dialect, and standard) is a language variety that has undergone substantial ...

magnesium sulfate
(MgSO4) and, possibly,
sodium sulfate Sodium sulfate (also known as sodium sulphate or sulfate of soda) is the inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compou ...

sodium sulfate
(Na2SO4) on Ganymede's surface. These salts may originate from the subsurface ocean. The Ganymedian surface albedo is very asymmetric; the leading hemisphere is brighter than the trailing one. This is similar to Europa, but the reverse for 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 crater An impact crater is an approximately circular depression (geology), depression in the surface of a planet, natural satellite, moon, or other solid body in the Solar System or elsewhere, formed by the hypervelocity collision, impact of a smaller ...

Impact crater
s on Ganymede (except one) do not show any enrichment in carbon dioxide, which also distinguishes it from Callisto. Ganymede's carbon dioxide gas was probably depleted in the past. Ganymede's surface is a mix of two types of terrain: very old, highly , 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 Planetary science or, more rarely, planetology, is the scientific study of planets (including Earth), Natural satellite, moons, and planetary systems (in particular those of the Solar System) and the processes that form them. It studies objects ra ...
. The modern view is that the grooved terrain is mainly
tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building A mountain is an elevated portion of the Earth's crust, gen ...
in nature.
Cryovolcanism , one of the most reliably identified cryovolcanoes on Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of ...
is thought to have played only a minor role, if any. The forces that caused the strong stresses in the Ganymedian ice
lithosphere A lithosphere ( grc, λίθος [] for "rocky", and [] for "sphere") is the rigid, outermost shell of a terrestrial planet, terrestrial-type planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion o ...
necessary to initiate the tectonic activity may be connected to the
tidal heating Tidal heating (also known as tidal working or tidal flexing) occurs through the tidal friction processes: orbital and rotational energy is dissipated as heat in either (or both) the surface ocean or interior of a planet or satellite. When an object ...
events in the past, possibly caused when the satellite passed through unstable orbital resonances. 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 In geology Geology (from the γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is a branch of concerned with both the liquid and , the of which it is composed, and the processes by which they change over t ...
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 Ganymede's interior, which may have caused a slight expansion of Ganymede by 1–6% due to
phase transition In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in ...
s in ice and
thermal expansion Thermal expansion is the tendency of matter to change its shape A shape or figure is the form of an object or its external boundary, outline, or external surface File:Water droplet lying on a damask.jpg, Water droplet lying on a damask ...
. During subsequent evolution deep, hot water plumes may have risen from the core to the surface, leading to the tectonic deformation of the lithosphere.
Radiogenic heat A radiogenic nuclide is a nuclide A nuclide (or nucleide, from atomic nucleus, nucleus, also known as nuclear species) is a class of atoms characterized by their number of protons, ''Z'', their number of neutrons, ''N'', and their nuclear energy ...
ing within the satellite is the most relevant current heat source, contributing, for instance, to ocean depth. Research models have found that if the orbital eccentricity were an order of magnitude greater than currently (as it may have been in the past), tidal heating would be a more substantial heat source than radiogenic heating. 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 Moon, 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, whereas 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 In textual studiesTextual scholarship (or textual studies) is an umbrella term In linguistics Linguistics is the science, scientific study of language. It encompasses the analysis of every aspect of language, as well as the methods fo ...
. 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. Ganymede also has polar caps, likely composed of water frost. The frost extends to 40° latitude. These polar caps were first seen by the Voyager program, ''Voyager'' spacecraft. Theories on the formation of the caps include the migration of water to higher latitudes and bombardment of the ice by plasma. Data from ''Galileo'' suggests the latter is correct. The presence of a magnetic field on Ganymede results in more intense charged particle bombardment of its surface in the unprotected polar regions; sputtering then leads to redistribution of water molecules, with frost migrating to locally colder areas within the polar terrain. A crater named Anat (crater), Anat provides the reference point for measuring longitude on Ganymede. By definition, Anat is at 128° longitude. The 0° longitude directly faces Jupiter, and unless stated otherwise longitude increases toward the west.


Internal structure

Ganymede appears to be fully differentiated, with an internal structure consisting of an iron(II) sulfide, iron-sulfide–iron core (geology), core, a silicate mantle (geology), mantle and outer layers of water ice and liquid water. 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. Ganymede has the lowest
moment of inertia factor Moment or Moments may refer to: * Present The present (or here and now) is the time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparently irreversible process, irreversib ...
, 0.31, among the solid Solar System bodies. This is a consequence of its substantial water content and fully differentiated interior.


Subsurface oceans

In the 1970s, NASA scientists first suspected that Ganymede has a thick ocean between two layers of ice, one on the surface and one beneath a liquid ocean and atop the rocky mantle. In the 1990s, NASA's ''Galileo'' mission flew by Ganymede, and found indications of such a subsurface ocean. An analysis published in 2014, taking into account the realistic thermodynamics for water and effects of salt, suggests that Ganymede might have a stack of several ocean layers separated by different phases of ice, with the lowest liquid layer adjacent to the rocky Mantle (geology), mantle. Water–rock contact may be an important factor in the Abiogenesis, origin of life. The analysis also notes that the extreme depths involved (~800 km to the rocky "seafloor") mean that temperatures at the bottom of a convective (adiabatic) ocean can be up to 40 K higher than those at the ice–water interface. In March 2015, scientists reported that measurements with the Hubble Space Telescope of how the aurorae moved confirmed that Ganymede has a subsurface ocean. A large salt-water ocean affects Ganymede's magnetic field, and consequently, its aurora. The evidence suggests that Ganymede's oceans might be the largest in the entire Solar System. There is some speculation on the potential planetary habitability, habitability of Ganymede's ocean.


Core

The existence of a liquid, iron–nickel alloy, iron–nickel-rich core provides a natural explanation for the intrinsic magnetosphere, magnetic field of Ganymede detected by ''Galileo'' spacecraft. The
convection Convection is single or multiphase fluid flow that occurs Spontaneous process, spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When t ...

convection
in the liquid iron, which has high electrical conductivity, is the most reasonable model of magnetic field generation. The density of the core is 5.5–6 g/cm3 and the silicate mantle is 3.4–3.6 g/cm3. 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 .


Atmosphere and ionosphere

In 1972, a team of Indian, British and American astronomers working in Java (Indonesia) and Kavalur (India) claimed that they had detected a thin atmosphere during an occultation, when it and Jupiter passed in front of a star. They estimated that the surface pressure was around 0.1 Pascal (unit), Pa (1 microbar). However, in 1979, ''
Voyager 1 ''Voyager 1'' is a space probe A space probe or a spaceprobe is a robotic spacecraft that doesn't Earth orbit, orbit the Earth (planet), Earth, but instead explores farther into outer space. A space probe may approach the Moon; travel ...
'' observed an occultation of the star Kappa Centauri, κ Centauri during its flyby of Jupiter, with differing results. The occultation measurements were conducted in the far-ultraviolet spectrum at wavelengths shorter than 200 nanometre, nm, which were much more sensitive to the presence of gases than the 1972 measurements made in the visible spectrum. No atmosphere was revealed by the ''Voyager'' data. The upper limit on the surface particle number density was found to be , which corresponds to a surface pressure of less than 2.5 µPa (25 picobar). The latter value is almost five orders of magnitude less than the 1972 estimate. Despite the ''Voyager'' data, evidence for a tenuous oxygen atmosphere (exosphere) on Ganymede, very similar to the one found on Europa, was found by the Hubble Space Telescope (HST) in 1995. 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 dissociation (chemistry), dissociated by electron impacts, which is evidence of a significant neutral atmosphere composed predominantly of O2 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 hydrogen 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 aurora (astronomy), auroras, caused by plasma precipitation along the open field lines. The existence of a neutral atmosphere implies that an
ionosphere The ionosphere () is the ionized part of Earth's upper atmosphere, from about to altitude, a region that includes the thermosphere The thermosphere is the layer in the directly above the and below the . Within this layer of the atmosphere, ...
should exist, because oxygen molecules are ionized by the impacts of the energetic electrons coming from the magnetosphere and by solar Extreme ultraviolet, 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 Ganymede, suggesting an ionosphere, whereas 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
ozone Ozone (), or trioxygen, is an inorganic molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a cha ...

ozone
(O3) bands was announced in 1996. In 1997 spectroscopic analysis revealed the Dimer (chemistry), dimer (or diatomic) absorption features of molecular oxygen. Such an absorption can arise only if the oxygen is in a dense phase. The best candidate is molecular oxygen trapped in ice. The depth of the dimer absorption bands depends on latitude and longitude, rather than on surface albedo—they tend to decrease with increasing latitude on Ganymede, whereas O3 shows an opposite trend. Laboratory work has found that O2 would not cluster or bubble but dissolve in ice at Ganymede's relatively warm surface temperature of 100 K (−173.15 °C). 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. Another minor constituent of the Ganymedian atmosphere is atomic hydrogen. Hydrogen atoms were observed as far as 3,000 km from Ganymede's surface. Their density on the surface is about . In 2021 water vapour was detected in the atmosphere of Ganymede.


Magnetosphere

The ''Galileo'' craft made six close flybys of Ganymede from 1995 to 2000 (G1, G2, G7, G8, G28 and G29) and discovered that Ganymede has a permanent (intrinsic) magnetic moment independent of the Jovian magnetic field. The value of the moment is about , which is three times larger than the Mercury's magnetic field, 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. Its north pole lies below the orbital plane (astronomy), orbital plane. The dipole, dipole magnetic field created by this permanent moment has a strength of 719 ± 2 Tesla (unit), nT at Ganymede's equator, 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 Magnetic reconnection, reconnection 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 magnetosphere embedded inside Jupiter's magnetosphere, that of Jupiter; it is the only moon in the Solar System known to possess the feature. Its diameter is 4–5 Ganymede radii. The Ganymedian magnetosphere has a region of closed
field line A field line is a graphical visual aid Visual communication is the use of visual elements to convey ideas and information"7 Paramount Components of Visual Communication". ''Infographic Design Team - Infographics Design - Data Visualization''. R ...
s located below 30° latitude, where charged particles (electrons and ions) are trapped, creating a kind of radiation belt. The main ion species in the magnetosphere is single ionized oxygen—O+—which fits well with Ganymede's tenuous oxygen
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...

atmosphere
. In the polar cap regions, at latitudes higher than 30°, magnetic field lines are open, connecting Ganymede with Jupiter's ionosphere. In these areas, the energetic (tens and hundreds of kiloelectronvolt) electrons and ions have been detected, which may cause the auroras observed around the Ganymedian poles. In addition, heavy ions precipitate continuously on Ganymede's polar surface, sputtering and darkening the ice. The interaction between the Ganymedian magnetosphere and Jovian plasma (physics), plasma is in many respects similar to that of the solar wind 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—supersonic in the case of Earth and Speed of sound, subsonic in the case of Ganymede. Because of the subsonic flow, there is no bow shock 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 Ganymede. The induced moment is directed radially to or from Jupiter following the direction of the varying part of the planetary magnetic field. The induced magnetic moment is an order of magnitude weaker than the intrinsic one. The field strength 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 Callisto and Europa, indicating that Ganymede also has a subsurface water ocean with a high electrical conductivity. 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, hence a magnetic field would not be sustained. One explanation is that the same orbital resonances proposed to have disrupted the surface also allowed the magnetic field to persist: with Ganymede's eccentricity pumped and tidal heating of the mantle increased during such resonances, reducing heat flow from the core, leaving it fluid and convective. 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.


Radiation environment

The radiation level at the surface of Ganymede is considerably lower than at Europa, being 50–80 mSv (5–8 rem) per day, an amount that would cause severe illness or death in human beings exposed for two months.


Origin and evolution

Ganymede probably formed by an accretion (astrophysics), accretion in Jupiter's solar nebula, subnebula, 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 subnebula was denser, which explains its shorter formation timescale. This relatively fast formation prevented the escape of accretional heat, which may have led to ice melt and Planetary differentiation, differentiation: 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 apparently 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. Alternative theories explain Ganymede's greater internal heating on the basis of tidal flexing or more intense pummeling by impactors during the Late Heavy Bombardment. In the latter case, modeling suggests that differentiation would become a thermal runaway, runaway process at Ganymede but not Callisto. After formation, Ganymede's core largely retained the heat accumulated during accretion and differentiation, only slowly releasing it to the ice mantle. The mantle, in turn, transported it to the surface by convection. The decay of radioactivity, radioactive elements within rocks further heated the core, causing increased differentiation: an inner, iron–iron-sulfide core and a silicate 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, whereas the slow cooling of the liquid Fe–FeS core causes convection and supports magnetic field generation. The current heat flux out of Ganymede is probably higher than that out of Callisto.


Exploration

Several spacecraft have performed close flyby (spaceflight), flybys of Ganymede: two ''Pioneer'' and two ''Voyager'' spacecraft made a single flyby each between 1973 and 1979; the ''Galileo'' spacecraft made six passes between 1996 and 2000; and the Juno (spacecraft), ''Juno'' spacecraft performed two flybys in 2019 and 2021. No spacecraft has yet orbited Ganymede, but there have been multiple proposals to do so, including the JUICE mission which is currently under construction ().


Completed flybys

The first spacecraft to approach close to Ganymede was ''
Pioneer 10 ''Pioneer 10'' (originally designated Pioneer F) is an American space probe, launched in 1972 and weighing , that completed the first mission to the planet Jupiter. Thereafter, ''Pioneer 10'' became the List of artificial objects leaving the S ...

Pioneer 10
'', which performed a flyby in 1973 as it passed through the Jupiter system at high speed. ''Pioneer 11'' made a similar flyby in 1974. Data sent back by the two spacecraft was used to determine the moon's physical characteristics and provided images of the surface with up to resolution. Pioneer 10's closest approach was 446,250 km, about 85 times Ganymede's diameter. ''
Voyager 1 ''Voyager 1'' is a space probe A space probe or a spaceprobe is a robotic spacecraft that doesn't Earth orbit, orbit the Earth (planet), Earth, but instead explores farther into outer space. A space probe may approach the Moon; travel ...
'' and ''
Voyager 2 ''Voyager 2'' is a space probe A space probe or a spaceprobe is a robotic spacecraft that doesn't Earth orbit, orbit the Earth (planet), Earth, but instead explores farther into outer space. A space probe may approach the Moon; travel thro ...
'' both studied Ganymede when passing through the Jupiter system in 1979. Data from those flybys were used to refine the size of Ganymede, revealing it was larger than Saturn's moon Titan, which was previously thought to have been bigger. Images from the ''Voyagers'' provided the first views of the moon's grooved surface terrain. The ''Pioneer'' and ''Voyager'' flybys were all at large distances and high speeds, as they flew on hyperbolic orbit, unbound trajectories through the Jupiter system. Better data can be obtained from a spacecraft which is orbiting Jupiter, as it can encounter Ganymede at a lower speed and adjust the orbit for a closer approach. In 1995, the ''
Galileo Galileo di Vincenzo Bonaiuti de' Galilei ( , ; 15 February 1564 – 8 January 1642), commonly referred to as Galileo, was an astronomer An astronomer is a in the field of who focuses their studies on a specific question or field o ...
'' spacecraft entered orbit around Jupiter and between 1996 and 2000 made six close flybys of Ganymede. These flybys were denoted G1, G2, G7, G8, G28 and G29. During the closest flyby (G2), ''Galileo'' passed just 264 km from the surface of Ganymede (5% of the moon's diameter), which remains the closest approach by any spacecraft. During the G1 flyby in 1996, ''Galileo'' instruments detected Ganymede's magnetic field. Data from the ''Galileo'' flybys was used to discover the sub-surface ocean, which was announced in 2001. High spatial resolution spectra of Ganymede taken by ''Galileo'' were used to identify several non-ice compounds on the surface. The ''New Horizons'' spacecraft also observed Ganymede, but from a much larger distance as it passed through the Jupiter system in 2007 (en route to Pluto). The data were used to perform topographic and compositional mapping of Ganymede. Like ''Galileo'', the ''Juno (spacecraft), Juno'' spacecraft orbited around Jupiter. On 2019 December 25, ''Juno'' performed a distant flyby of Ganymede during its 24th orbit of Jupiter, at a range of . This flyby provided images of the moon's polar regions. In June 2021, ''Juno'' performed a second flyby, at a closer distance of . This encounter was designed to provide a gravity assist to reduce ''Junos orbital period from 53 days to 43 days. Additional images of the surface were collected.


Future missions

The
Jupiter Icy Moon Explorer The Jupiter Icy Moons Explorer (JUICE) is an interplanetary spacecraft in development by the European Space Agency (ESA) with Airbus Defence and Space as the main contractor. The mission will study three of Jupiter's Galilean moons: Ganymed ...
(JUICE) will be the first to enter orbit around Ganymede itself. , JUICE is under construction, with launch planned for August 2023. It is intended to perform its first flyby of Ganymede in 2031, then enter orbit of the moon in 2032. It will de-orbit and crash into Jupiter when the spacecraft fuel begins to run out, probably in 2034, to avoid potential contamination of Ganymede's subsurface ocean (a form of planetary protection).


Proposed missions

Several other missions have been proposed to flyby or orbit Ganymede, but were either not selected for funding or cancelled before launch. The Jupiter Icy Moons Orbiter would have studied Ganymede in greater detail. However, the mission was canceled in 2005. Another old proposal was called The Grandeur of Ganymede. A Ganymede orbiter based on the ''Juno'' probe was proposed in 2010 for the Planetary Science Decadal Survey. The mission was not supported, with the Decadal Survey preferring the Europa Clipper mission instead. The Europa Jupiter System Mission (EJSM) had a proposed launch date in 2020, and was a joint NASA and ESA proposal for exploration of many of
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
's moons including Ganymede. In February 2009 it was announced that ESA and NASA had given this mission priority ahead of the Titan Saturn System Mission. EJSM was to consist of the NASA-led Jupiter Europa Orbiter, the ESA-led Jupiter Ganymede Orbiter, and possibly a JAXA-led Jupiter Magnetospheric Orbiter. The NASA and JAXA components were later cancelled, and ESA's appeared likely to be cancelled too, but in 2012 ESA announced it would go ahead alone. The European part of the mission became the Jupiter Icy Moon Explorer (JUICE) (see above). The Russian Space Research Institute proposed a Ganymede lander (GL) astrobiology mission called Laplace-P, possibly in partnership with JUICE. If selected, it would be launched in 2023.


Gallery

File:Hubble’s View of Ganymede.jpg, Hubble Space Telescope image of Ganymede taken in 1996. File:Ganymede infrared NASA Juno JIRAM.jpg, Infrared image of Ganymede taken during the Juno flyby in July 2021. Image Credits: A. Mura -Juno/JIRAM - ASI/INAF/JPL-CalTech/SwRI


See also

* Cold trap (astronomy) * Jupiter's moons in fiction * List of craters on Ganymede * List of geological features on Ganymede * List of natural satellites * Lunar and Planetary Institute


Notes


References


External links


Ganymede page
a
NASA's Solar System Exploration site


at ''The Nine Planets''

at ''Views of the Solar System''
Ganymede Crater Database
from the Lunar and Planetary Institute
Images of Ganymede at JPL's Planetary Photojournal
* Movie o
Ganymede's rotation
from the National Oceanic and Atmospheric Administration
Ganymede map
fro
''Scientific American'' article

Ganymede map with feature names
fro
Planetary Photojournal

Ganymede nomenclature
an
Ganymede map with feature names
from th
USGS planetary nomenclature page


* [http://www.baen.com/TerraformingGanymede1.asp "Terraforming Ganymede with Robert A. Heinlein" (part 1)], article by Gregory Benford, 2011 *
Part 2

Ganymede Orbiter Concept

Global Geologic Map of Ganymede
(USGS)
Google Ganymede 3D
interactive map of the moon
Video (animation; 4:00): Flyby of Ganymede and Jupiter
(NASA; 15 July 2021). {{Authority control Ganymede (moon),