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Callisto , or Jupiter IV, is the second-largest moon of Jupiter, after
Ganymede
Ganymede
. It is the
third-largest moon
third-largest moon
in 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 individual astronomical objects but uses mixed "Sola ...

Solar System
after Ganymede and
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 Earth. It only has one-eighth the average density of Earth; how ...

Saturn
's largest moon
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 ...
, and the largest object in the Solar System that may not be properly differentiated. Callisto was discovered in 1610 by
Galileo Galilei Galileo di Vincenzo Bonaiuti de' Galilei (; 15 February 1564 – 8 January 1642) was an Italian astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the ...

Galileo Galilei
. At in diameter, Callisto has about 99% the diameter of 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
but only about a third of its mass. It is the fourth
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. ...
of
Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant A gas giant is a giant planet composed mainly of hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and at ...

Jupiter
by distance, with an orbital radius of about . It is not in an
orbital resonance . 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 ...
like the three other Galilean satellites— Io,
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 —and is thus not appreciably tidally heated. Callisto's rotation is
tidally locked and Charon are tidally locked to each other. Charon is massive enough that the barycenter of Pluto's system lies outside of Pluto; thus Pluto and Charon are sometimes considered to be a binary system. Tidal locking (also called gravitational loc ...
to its orbit around Jupiter, so that the same hemisphere always faces inward. Because of this, there is a sub-Jovian point on Callisto's surface, from which Jupiter would appear to hang directly overhead. It is less affected by Jupiter's
magnetosphere In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses m ...

magnetosphere
than the other
inner satellite In astronomy, an inner moon or inner natural satellite is a natural satellite following a prograde orbit, prograde, low-orbital inclination, inclination orbit inwards of the large satellites of the parent planet. They are generally thought to have b ...
s because of its more remote orbit, located just outside Jupiter's main radiation belt. Callisto is composed of approximately equal amounts of
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 ...
and
ice Ice is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known forms of , eve ...
s, with a
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 about , the lowest density and surface gravity of Jupiter's major moons. Compounds detected on the surface include
water iceWater ice could refer to: *Ice Ice is water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constit ...

water ice
,
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
,
silicate In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms, ...
s, and
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. Investigation by 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 scientist in the field of astronomy who focuses their studies on a specific qu ...
'' spacecraft revealed that Callisto may have a small
silicate In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms, ...
core Core or cores may refer to: Science and technology * Core (anatomy) In common parlance, the core of the body is broadly considered to be the torso. Functional movements are highly dependent on this part of the body, and lack of core muscular dev ...
and possibly a
subsurface 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.
of liquid
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 ...

water
at depths greater than . The surface of Callisto is the oldest and most heavily in the Solar System. Its surface is completely covered with impact craters. It does not show any signatures of
subsurface Bedrock in geology is Lithification, lithified Rock (geology), rock that lies under loose softer material called regolith within the surface of the Earth's crust or other terrestrial planets. Components of bedrock Bedrock essentially refers ...
processes such as
plate tectonics Plate tectonics (from the la, label=Late Latin Late Latin ( la, Latinitas serior) is the scholarly name for the written Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. L ...
or
volcanism Volcanism (or volcanicity) is the phenomenon of eruption of molten rock (magma) onto the Earth#Surface, surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics and volcanic gases erupt through a break in the surface called ...

volcanism
, with no signs that geological activity in general has ever occurred, and is thought to have evolved predominantly under the influence of . Prominent surface features include multi-ring structures, variously shaped
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 chains of craters (''catenae'') and associated , ridges and deposits. At a small scale, the surface is varied and made up of small, sparkly frost
deposits A deposit account is a bank account maintained by a financial institution in which a customer can deposit and withdraw money. Deposit accounts can be savings accounts, Transaction account#Current accounts, current accounts or any of several other ...
at the tips of high spots, surrounded by a low-lying, smooth blanket of dark material. This is thought to result from the sublimation-driven degradation of small
landform A landform is a natural or artificial feature of the solid surface of the Earth or other planetary body A planet is an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws ...

landform
s, which is supported by the general deficit of small impact craters and the presence of numerous small knobs, considered to be their remnants. The absolute ages of the landforms are not known. Callisto is surrounded by an extremely thin
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
composed of
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
and probably
molecular oxygenThere are several known allotropes Allotropy or allotropism () is the property of some chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an elemen ...

molecular oxygen
, as well as by a rather intense
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, ...
. Callisto is thought to have formed by slow
accretion Accretion may refer to: Science * Accretion (astrophysics), the formation of planets and other bodies by collection of material through gravity * Accretion (meteorology), the process by which water vapor in clouds forms water droplets around nucle ...
from the disk of the gas and dust that surrounded Jupiter after its formation. Callisto's gradual accretion and the lack of tidal heating meant that not enough heat was available for rapid
differentiation Differentiation may refer to: Business * Differentiation (economics), the process of making a product different from other similar products * Product differentiation, in marketing * Differentiated service, a service that varies with the identity o ...
. The slow
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 interior of Callisto, which commenced soon after formation, led to partial differentiation and possibly to the formation of a subsurface ocean at a depth of 100–150 km and a small, rocky
core Core or cores may refer to: Science and technology * Core (anatomy) In common parlance, the core of the body is broadly considered to be the torso. Functional movements are highly dependent on this part of the body, and lack of core muscular dev ...
. The likely presence of an ocean within Callisto leaves open the possibility that it could harbor
life Life is a characteristic that distinguishes physical entities A bubble of exhaled gas in water In common usage and classical mechanics, a physical object or physical body (or simply an object or body) is a collection of matter within a ...
. However, conditions are thought to be less favorable than on nearby
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 ...
. Various space probes from '''' and '' 11'' to ''
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 scientist in the field of astronomy who focuses their studies on a specific qu ...
'' and '' Cassini'' have studied Callisto. Because of its low
radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and f ...

radiation
levels, Callisto has long been considered the most suitable place for a human base for future exploration of the Jovian system.


History


Discovery

Callisto was discovered by Galileo in January 1610, along with the three other large Jovian moons—, 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 ...
.


Name

Callisto is named after one of
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
's many lovers in
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 ...
. Callisto was a nymph (or, according to some sources, the daughter of Lycaon) who was associated with the goddess of the hunt,
Artemis Artemis (; grc-gre, Ἄρτεμις Artemis, ) is the Greek goddess Greek mythology is the body of myth Myth is a folklore genre consisting of narratives that play a fundamental role in a society, such as foundational tales or ori ...

Artemis
. The name was suggested by
Simon Marius Simon Marius ( latinized form of Simon Mayr; January 10, 1573 – January 5, 1625) was a German 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 Ea ...

Simon Marius
soon after Callisto's discovery. Marius attributed the suggestion to
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
. However, the names of the
Galilean satellites '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. ...
fell into disfavor for a considerable time, and were not revived in common use until the mid-20th century. In much of the earlier astronomical literature, Callisto is referred to by its Roman numeral designation, a system introduced by Galileo, as or as "the fourth satellite of Jupiter". There's no established English adjectival form of the name. The adjectival form of Greek Καλλιστῴ ''Kallistōi'' is Καλλιστῴος ''Kallistōi-os'', from which one might expect Latin ''Callistōius'' and English *Callistóian, parallel to Sapphóian for '''' and Letóian for ''''. However, the
iota subscript The iota subscript is a diacritic A diacritic (also diacritical mark, diacritical point, diacritical sign, or accent) is a glyph added to a letter Letter, letters, or literature may refer to: Characters typeface * Letter (alphabet) A le ...
is often omitted from such Greek names (cf. ''Inóan'' from '' Īnōi'' and ''Argóan'' from ''''), and indeed the analogous form Callistoan is found. In Virgil, a second oblique stem appears in Latin: ''Callistōn-,'' but the corresponding Callistonian has rarely appeared in English. One also sees ''ad hoc'' forms, such as Callistan, Callistian and Callistean.


Orbit and rotation

Callisto is the outermost of the four Galilean moons of Jupiter. It orbits at a distance of approximately 1 880 000 km (26.3 times the 71 492 km radius of Jupiter itself). This is significantly larger than the orbital radius—1 070 000 km—of the next-closest Galilean satellite, Ganymede. As a result of this relatively distant orbit, Callisto does not participate in the
mean-motion resonance . Conjunctions are highlighted by brief color changes. There are two Io-Europa conjunctions (green) and three Io-Ganymede conjunctions (grey) for each Europa-Ganymede conjunction (magenta). This diagram is not to scale. In celestial mechanics, ...
—in which the three inner Galilean satellites are locked—and probably never has. Like most other regular planetary moons, Callisto's rotation is locked to be
synchronous Synchronization is the coordination of events to operate a system in unison. For example, the Conductor (music), conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are sai ...

synchronous
with its orbit. The length of Callisto's day, simultaneously its
orbital period The orbital period is the time a given astronomical object takes to complete one orbit around another object, and applies in astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of th ...
, is about 16.7 Earth days. 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 0.0072–0.0076 and 0.20–0.60°, 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 0.4 and 1.6°. The dynamical isolation of Callisto means that it has never been appreciably tidally heated, which has important consequences for its internal structure and
evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

evolution
. Its distance from Jupiter also means that the charged-particle
flux Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...

flux
from Jupiter's
magnetosphere In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses m ...

magnetosphere
at its surface is relatively low—about 300 times lower than, for example, that at
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 ...
. Hence, unlike the other Galilean moons, charged-particle
irradiation Irradiation is the process by which an object is exposed to radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior th ...

irradiation
has had a relatively minor effect on Callisto's surface. The radiation level at Callisto's surface is equivalent to a dose of about 0.01 rem (0.1
mSv The sievert (symbol: SvNot be confused with the sverdrup In oceanography, the sverdrup (symbol: Sv) is a non-International System of Units, SI Metric_units#Volume_flow_rate, metric unit of Volumetric flow rate, flow, with equal to ; it is eq ...
) per day, which is over ten times higher than Earth's average background radiation.


Physical characteristics


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 Callisto, 1.83 g/cm3, suggests a composition of approximately equal parts of rocky material and
water iceWater ice could refer to: *Ice Ice is water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constit ...

water ice
, with 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
. The mass fraction of ices is 49–55%. The exact composition of Callisto'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 ...
component 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
is 0.9–1.3 in Callisto, whereas the is around 1:8. Callisto'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 20%. Its surface composition is thought to be broadly similar to its composition as a whole. Near-infrared
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 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 micrometers. Water ice seems to be ubiquitous on the surface of Callisto, with a mass fraction of 25–50%. The analysis of high-resolution,
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 ...
and obtained by 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 scientist in the field of astronomy who focuses their studies on a specific qu ...
'' spacecraft and from the ground has revealed various non-ice materials:
magnesium Magnesium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

magnesium
- and
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
-bearing hydrated
silicates In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they unde ...
,
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
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
and various
organic compounds 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 ...
. Spectral data indicate that Callisto's surface is extremely heterogeneous at the small scale. Small, bright patches of pure water ice are intermixed with patches of a rock–ice mixture and extended dark areas made of a non-ice material. The Callistoan surface is asymmetric: the leading hemisphereThe leading hemisphere is the hemisphere facing the direction of the orbital motion; the trailing hemisphere faces the reverse direction. is darker than the trailing one. This is different from other
Galilean satellites '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. ...

Galilean satellites
, where the reverse is true. The trailing hemisphere of Callisto appears to be enriched in
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
, whereas the leading hemisphere has more
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 ...
. Many fresh
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 like
Lofn In Norse mythology Norse or Scandinavian mythology is the body of mythology, myths of the North Germanic peoples, stemming from Norse paganism and continuing after the Christianization of Scandinavia, and into the Scandinavian folklore of the mod ...
also show enrichment in carbon dioxide. Overall, the chemical composition of the surface, especially in the dark areas, may be close to that seen on D-type asteroids, whose surfaces are made of carbonaceous material.


Internal structure

Callisto's battered surface lies on top of a cold, stiff, and icy lithosphere that is between 80 and 150 km thick. A salty ocean 150–200 km deep may lie beneath the crust (geology), crust, indicated by studies of the magnetic fields around Jupiter and its moons. It was found that Callisto responds to Jupiter's varying background magnetic field like a perfectly electrical conductivity, conducting sphere; that is, the field cannot penetrate inside Callisto, suggesting a layer of highly conductive fluid within it with a thickness of at least 10 km. The existence of an ocean is more likely if water contains a small amount of
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
or other antifreeze, up to 5% by weight. In this case the water+ice layer can be as thick as 250–300 km. Failing an ocean, the icy lithosphere may be somewhat thicker, up to about 300 km. Beneath the lithosphere and putative ocean, Callisto's interior appears to be neither entirely uniform nor particularly variable. ''
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 scientist in the field of astronomy who focuses their studies on a specific qu ...
'' orbiter data (especially the dimensionless moment of inertiaThe dimensionless moment of inertia referred to is I / (mr^2), where is the moment of inertia, the mass, and the maximal radius. It is 0.4 for a homogenous spherical body, but less than 0.4 if density increases with depth.—0.3549 ± 0.0042—determined during close flybys) suggest that, if Callisto is in hydrostatic equilibrium, its interior is composed of compressed Rock (geology), rocks and ices, with the amount of rock increasing with depth due to partial settling of its constituents. In other words, Callisto may be only partially differentiated. The density and moment of inertia for an equilibrium Callisto are compatible with the existence of a small
silicate In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms, ...
core in the center of Callisto. The radius of any such core cannot exceed 600 km, and the density may lie between 3.1 and 3.6 g/cm3. In this case, Callisto's interior would be in stark contrast to Ganymede (moon)#Internal structure, that of Ganymede, which appears to be fully differentiated. However, a 2011 reanalysis of Galileo data suggests that Callisto is not in hydrostatic equilibrium; its S22 coefficient from gravity data is an anomalous 10% of its C22 value, which is not consistent with a body in hydrostatic equilibrium and thus significantly increases the error bars on Callisto's moment of inertia. Further, an undifferentiated Callisto is inconsistent with the presence of a substantial internal ocean as inferred by magnetic data, and it would be difficult for an object as large as Callisto to fail to differentiate at any point. In that case, the gravity data may be more consistent with a more thoroughly differentiated Callisto with a hydrated silicate core.


Surface features

The ancient surface of Callisto is one of the most heavily cratered in the Solar System. In fact, the impact crater, crater density is close to wikt:saturation, saturation: any new crater will tend to erase an older one. The large-scale geology is relatively simple; there are no large mountains on Callisto, volcanoes or other endogeny, endogenic tectonic features. The impact craters and multi-ring structures—together with associated fracture (geology), fractures, and
deposits A deposit account is a bank account maintained by a financial institution in which a customer can deposit and withdraw money. Deposit accounts can be savings accounts, Transaction account#Current accounts, current accounts or any of several other ...
—are the only large features to be found on the surface. Callisto's surface can be divided into several geologically different parts: cratered plains, light plains, bright and dark smooth plains, and various units associated with particular multi-ring structures and impact craters. The cratered plains constitute most of the surface area and represent the ancient lithosphere, a mixture of ice and rocky material. The light plains include bright impact craters like Asgard (crater), Burr and
Lofn In Norse mythology Norse or Scandinavian mythology is the body of mythology, myths of the North Germanic peoples, stemming from Norse paganism and continuing after the Christianization of Scandinavia, and into the Scandinavian folklore of the mod ...
, as well as the effaced remnants of old large craters called Palimpsest (planetary astronomy), palimpsests, the central parts of multi-ring structures, and isolated patches in the cratered plains. These light plains are thought to be icy impact deposits. The bright, smooth plains constitute a small fraction of Callisto's surface and are found in the ridge and trough (geology), trough zones of the Valhalla (crater), Valhalla and Asgard (crater), Asgard formations and as isolated spots in the cratered plains. They were thought to be connected with endogenic activity, but the high-resolution ''Galileo'' images showed that the bright, smooth plains correlate with heavily fractured and knobby terrain and do not show any signs of resurfacing. The ''Galileo'' images also revealed small, dark, smooth areas with overall coverage less than 10,000 km2, which appear to embayTo ''embay'' means to shut in, or shelter, as in a bay. the surrounding terrain. They are possible cryovolcano, cryovolcanic deposits. Both the light and the various smooth plains are somewhat younger and less cratered than the background cratered plains. Impact crater diameters seen range from 0.1 km—a limit defined by the image resolution, imaging resolution—to over 100 km, not counting the multi-ring structures. Small craters, with diameters less than 5 km, have simple bowl or flat-floored shapes. Those 5–40 km across usually have a central peak. Larger impact features, with diameters in the range 25–100 km, have central pits instead of peaks, such as Tindr (crater), Tindr crater. The largest craters with diameters over 60 km can have central domes, which are thought to result from central tectonic uplift after an impact; examples include Asgard (crater), Doh and Hár (crater), Hár craters. A small number of very large—more than 100 km in diameter—and bright impact craters show anomalous dome geometry. These are unusually shallow and may be a transitional
landform A landform is a natural or artificial feature of the solid surface of the Earth or other planetary body A planet is an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws ...

landform
to the multi-ring structures, as with the
Lofn In Norse mythology Norse or Scandinavian mythology is the body of mythology, myths of the North Germanic peoples, stemming from Norse paganism and continuing after the Christianization of Scandinavia, and into the Scandinavian folklore of the mod ...
impact feature. Callisto's craters are generally shallower than those on the Moon. The largest impact features on Callisto's surface are multi-ring basins. Two are enormous. Valhalla (crater), Valhalla is the largest, with a bright central region 600 kilometers in diameter, and rings extending as far as 1,800 kilometers from the center (see figure). The second largest is Asgard (crater), Asgard, measuring about 1,600 kilometers in diameter. Multi-ring structures probably originated as a result of a post-impact concentric fracturing of the lithosphere lying on a layer of soft or liquid material, possibly an ocean. The catenae—for example Gomul Catena—are long chains of impact craters lined up in straight lines across the surface. They were probably created by objects that were tidally disrupted as they passed close to Jupiter prior to the impact on Callisto, or by very wikt:oblique, oblique impacts. A historical example of a disruption was Comet Shoemaker-Levy 9. As mentioned above, small patches of pure water ice with 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
as high as 80% are found on the surface of Callisto, surrounded by much darker material. High-resolution ''
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 scientist in the field of astronomy who focuses their studies on a specific qu ...
'' images showed the bright patches to be predominately located on elevated surface features: rim (craters), crater rims, Escarpment, scarps, ridges and knobs. They are likely to be thin frost, water frost
deposits A deposit account is a bank account maintained by a financial institution in which a customer can deposit and withdraw money. Deposit accounts can be savings accounts, Transaction account#Current accounts, current accounts or any of several other ...
. Dark material usually lies in the lowlands surrounding and mantling bright features and appears to be smooth. It often forms patches up to 5 km across within the crater floors and in the intercrater depressions. On a sub-kilometer scale the surface of Callisto is more degraded than the surfaces of other icy Galilean moons. Typically there is a deficit of small impact craters with diameters less than 1 km as compared with, for instance, the dark plains on ganymede (moon), Ganymede. Instead of small craters, the almost ubiquitous surface features are small knobs and pits. The knobs are thought to represent remnants of crater rims degraded by an as-yet uncertain process. The most likely candidate process is the slow sublimation of ice, which is enabled by a temperature of up to 165 kelvin, K, reached at a subsolar point. Such sublimation of water or other volatiles from the dirty ice that is the bedrock causes its decomposition. The non-ice remnants form debris avalanches descending from the slopes of the crater walls. Such avalanches are often observed near and inside impact craters and termed "debris aprons". Sometimes crater walls are cut by sinuous valley-like incisions called "gullies", which resemble certain Mars, Martian surface features. In the ice sublimation hypothesis, the low-lying dark material is interpreted as a blanket of primarily non-ice debris, which originated from the degraded rims of craters and has covered a predominantly icy bedrock. The relative ages of the different surface units on Callisto can be determined from the density of impact craters on them. The older the surface, the denser the crater population. Absolute dating has not been carried out, but based on theoretical considerations, the cratered plains are thought to be ~4.5 1000000000 (number), billion years old, dating back almost to the formation 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 individual astronomical objects but uses mixed "Sola ...

Solar System
. The ages of multi-ring structures and impact craters depend on chosen background cratering rates and are estimated by different authors to vary between 1 and 4 billion years.


Atmosphere and ionosphere

Callisto has a very tenuous atmosphere composed of
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
. It was detected by the ''Galileo'' Near Infrared Mapping Spectrometer (NIMS) from its absorption feature near the wavelength 4.2 micrometers. The surface pressure is estimated to be 7.5 picobar (unit), bar (0.75 micropascal, μPa) and particle density 4 cm−3. Because such a thin atmosphere would be lost in only about 4 days ''(see atmospheric escape)'', it must be constantly replenished, possibly by slow sublimation of carbon dioxide ice from Callisto's icy crust, which would be compatible with the sublimation–degradation hypothesis for the formation of the surface knobs. Callisto's ionosphere was first detected during ''Galileo'' flybys; its high electron density of 7–17 cm−3 cannot be explained by the photoionization of the atmospheric
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
alone. Hence, it is suspected that the atmosphere of Callisto is actually dominated by
molecular oxygenThere are several known allotropes Allotropy or allotropism () is the property of some chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an elemen ...

molecular oxygen
(in amounts 10–100 times greater than ). However, oxygen has not yet been directly detected in the atmosphere of Callisto. Observations with the Hubble Space Telescope (HST) placed an upper limit on its possible concentration in the atmosphere, based on lack of detection, which is still compatible with the ionospheric measurements. At the same time, HST was able to detect condensation, condensed oxygen trapped on the surface of Callisto. Atomic hydrogen has also been detected in Callisto's atmosphere via recent analysis of 2001 Hubble Space Telescope data. Spectral images taken on 15 and 24 December 2001 were re-examined, revealing a faint signal of scattered light that indicates a hydrogen corona. The observed brightness from the scattered sunlight in Callisto's hydrogen corona is approximately two times larger when the leading hemisphere is observed. This asymmetry may originate from a different hydrogen abundance in both leading and trailing hemispheres. However, this hemispheric difference in Callisto's hydrogen corona brightness is likely to originate from the extinction of the signal in the Earth's geocorona, which is greater when the trailing hemisphere is observed.


Origin and evolution

The partial
differentiation Differentiation may refer to: Business * Differentiation (economics), the process of making a product different from other similar products * Product differentiation, in marketing * Differentiated service, a service that varies with the identity o ...
of Callisto (inferred e.g. from moment of inertia measurements) means that it has never been heated enough to melt its ice component. Therefore, the most favorable model of its formation is a slow
accretion Accretion may refer to: Science * Accretion (astrophysics), the formation of planets and other bodies by collection of material through gravity * Accretion (meteorology), the process by which water vapor in clouds forms water droplets around nucle ...
in the low-density Jovian solar nebula, subnebula—a disk of the gas and dust that existed around Jupiter after its formation. Such a prolonged accretion stage would allow cooling to largely keep up with the heat accumulation caused by impacts, radioactive decay and contraction, thereby preventing melting and fast differentiation. The allowable timescale of formation of Callisto lies then in the range 0.1 million–10 million years. The further evolution of Callisto after
accretion Accretion may refer to: Science * Accretion (astrophysics), the formation of planets and other bodies by collection of material through gravity * Accretion (meteorology), the process by which water vapor in clouds forms water droplets around nucle ...
was determined by the balance of the radioactive heating, cooling through thermal conduction near the surface, and solid state or subsolidus
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 interior. Details of the subsolidus convection in the ice is the main source of uncertainty in the models of all icy moons. It is known to develop when the temperature is sufficiently close to the melting point, due to the temperature dependence of ice viscosity. Subsolidus convection in icy bodies is a slow process with ice motions of the order of 1 centimeter per year, but is, in fact, a very effective cooling mechanism on long timescales. It is thought to proceed in the so-called stagnant lid regime, where a stiff, cold outer layer of Callisto conducts heat without convection, whereas the ice beneath it convects in the subsolidus regime. For Callisto, the outer conductive layer corresponds to the cold and rigid lithosphere with a thickness of about 100 km. Its presence would explain the lack of any signs of the endogenic activity on the Callistoan surface. The convection in the interior parts of Callisto may be layered, because under the high pressures found there,
water iceWater ice could refer to: *Ice Ice is water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constit ...

water ice
exists in different crystalline phases beginning from the ice I on the surface to ice VII in the center. The early onset of subsolidus convection in the Callistoan interior could have prevented large-scale ice melting and any resulting
differentiation Differentiation may refer to: Business * Differentiation (economics), the process of making a product different from other similar products * Product differentiation, in marketing * Differentiated service, a service that varies with the identity o ...
that would have otherwise formed a large rocky core (geology), core and icy mantle (geology), mantle. Due to the convection process, however, very slow and partial separation and differentiation of rocks and ices inside Callisto has been proceeding on timescales of billions of years and may be continuing to this day. The current understanding of the evolution of Callisto allows for the existence of a layer or "ocean" of liquid water in its interior. This is connected with the anomalous behavior of ice I phase's melting temperature, which decreases with pressure, achieving temperatures as low as 251 K at 2,070 bar (207 megapascal, MPa). In all realistic models of Callisto the temperature in the layer between 100 and 200 km in depth is very close to, or exceeds slightly, this anomalous melting temperature. The presence of even small amounts of
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
—about 1–2% by weight—almost guarantees the liquid's existence because ammonia would lower the melting temperature even further. Although Callisto is very similar in bulk properties to , it apparently had a much simpler Historical geology, geological history. The surface appears to have been shaped mainly by impacts and other exogenic forces. Unlike neighboring Ganymede with its grooved terrain, there is little evidence of plate tectonics, tectonic activity. Explanations that have been proposed for the contrasts in internal heating and consequent differentiation and geologic activity between Callisto and Ganymede include differences in formation conditions, the greater tidal heating experienced by Ganymede, and the more numerous and energetic impacts that would have been suffered by Ganymede during the Late Heavy Bombardment. The relatively simple geological history of Callisto provides planetary scientists with a reference point for comparison with other more active and complex worlds.


Potential habitability

It is speculated that there could be life in Callisto's subsurface ocean. Like
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 , as well as
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 Earth. It only has one-eighth the average density of Earth; how ...

Saturn
's Moons of Saturn, moons Enceladus, Dione (moon), Dione and
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 ...
and Neptune's moon Triton (moon), Triton, a possible subsurface ocean might be composed of Saline water, salt water. It is possible that halophiles could thrive in the ocean. As with
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 , the idea has been raised that Planetary habitability, habitable conditions and even extraterrestrial life, extraterrestrial microbial life may exist in the salty subsurface ocean, ocean under the Callistoan surface. However, the environmental conditions necessary for life appear to be less favorable on Callisto than on Europa. The principal reasons are the lack of contact with rocky material and the lower heat flux from the interior of Callisto. Scientist Torrence Johnson said the following about comparing the odds of life on Callisto with the odds on other Galilean moons:
The basic ingredients for life—what we call 'pre-biotic chemistry'—are abundant in many solar system objects, such as comets, asteroids and icy moons. Biologists believe liquid water and energy are then needed to actually support life, so it's exciting to find another place where we might have liquid water. But, energy is another matter, and currently, Callisto's ocean is only being heated by radioactive elements, whereas Europa has tidal energy as well, from its greater proximity to Jupiter.
Based on the considerations mentioned above and on other scientific observations, it is thought that of all of Jupiter's moons, Europa has the greatest chance of supporting microorganism, microbial life.


Exploration

The ''Pioneer 10'' and ''Pioneer 11'' Jupiter encounters in the early 1970s contributed little new information about Callisto in comparison with what was already known from Earth-based observations. The real breakthrough happened later with the ''Voyager 1'' and ''Voyager 2'' flybys in 1979. They imaged more than half of the Callistoan surface with a resolution of 1–2 km, and precisely measured its temperature, mass and shape. A second round of exploration lasted from 1994 to 2003, when 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 scientist in the field of astronomy who focuses their studies on a specific qu ...
'' spacecraft had eight close encounters with Callisto, the last flyby during the C30 orbit in 2001 came as close as 138 km to the surface. The ''Galileo'' orbiter completed the global imaging of the surface and delivered a number of pictures with a resolution as high as 15 meters of selected areas of Callisto. In 2000, the '' Cassini'' spacecraft en route to
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 Earth. It only has one-eighth the average density of Earth; how ...

Saturn
acquired high-quality infrared spectra of the Galilean satellites including Callisto. In February–March 2007, the ''New Horizons'' probe on its way to Pluto obtained new images and spectra of Callisto. The next planned mission to the Jovian system is the European Space Agency's Jupiter Icy Moon Explorer (JUICE), due to launch in 2022. Several close flybys of Callisto are planned during the mission.


Old proposals

Formerly proposed for a launch in 2020, the Europa Jupiter System Mission (EJSM) was a joint NASA/ESA proposal for exploration of
Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant A gas giant is a giant planet composed mainly of hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and at ...

Jupiter
's moons. In February 2009 it was announced that ESA/NASA had given this mission priority ahead of the Titan Saturn System Mission. At the time ESA's contribution still faced funding competition from other ESA projects. EJSM consisted of the NASA-led Jupiter Europa Orbiter, the ESA-led Jupiter Ganymede Orbiter, and possibly a JAXA-led Jupiter Magnetospheric Orbiter.


Potential humanization

In 2003 NASA conducted a conceptual study called Human Outer Planets Exploration (HOPE) regarding the future human exploration of the outer Solar System. The target chosen to consider in detail was Callisto. The study proposed a possible surface base on Callisto that would produce rocket propellant for further exploration of the Solar System. Advantages of a base on Callisto include low radiation (due to its distance from Jupiter) and geological stability. Such a base could facilitate remote exploration of
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 ...
, or be an ideal location for a Jovian system waystation servicing spacecraft heading farther into the outer Solar System, using a gravity assist from a close flyby of Jupiter after departing Callisto. In December 2003, NASA reported that a manned mission to Callisto might be possible in the 2040s.


See also

* Planet#Objects formerly considered planets, Former classification of planets * Jupiter's moons in fiction * List of craters on Callisto * List of geological features on Callisto * List of natural satellites


Notes


References


External links


Callisto Profile
a
NASA's Solar System Exploration site


at ''The Nine Planets''

at ''Views of the Solar System''

from the Lunar and Planetary Institute
Images of Callisto at JPL's Planetary Photojournal
* Movie o
Callisto's rotation
from the National Oceanic and Atmospheric Administration
Callisto map with feature names
fro
Planetary PhotojournalCallisto nomenclature
an
Callisto map with feature names
from th
USGS planetary nomenclature page


* [https://www.google.com/maps/space/callisto/@13.1830935,-94.1384306,6879202m/data=!3m1!1e3 Google Callisto 3D], interactive map of the moon {{DEFAULTSORT:Callisto (Moon) Callisto (moon),