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Callisto (moon)From Wikipedia, the free encyclopedia
Callisto
Image of Callisto obtained in 2001 by NASA'sGalileospacecraft
Discovery
Discovered by G. Galilei
S. Marius[1]
Discovery date January 7, 1610[1]
Designations
Alternate name(s) Jupiter IV
Adjective Callistoan, Callistan
Orbital characteristics
Periapsis 1 869 000 km[b]
Apoapsis 1 897 000 km[a]
Mean orbit radius 1 882 700 km[2]
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Eccentricity 0.007 4[2]
Orbital period 16.689 018 4 d[2]
Averageorbital speed
8.204 km/s
Inclination 0.192 (to localLaplace planes)[2]
Satellite of Jupiter
Physical characteristics
Mean radius 2410.3 1.5 km (0.378 Earths) [3]
Surface area 7.30 107km2 (0.143 Earths)[c]
Volume 5.9 1010km3 (0.0541 Earths)[d]
Mass 1.075 938 0.000 137 1023kg(0.018
Earths)[3]
Meandensity 1.834 4 0.003 4 g/cm3[3]
Equatorial
surface gravity1.235m/s2 (0.126g)[e]
Escape velocity 2.440 km/s[f]
Rotation period synchronous[3]
Axial tilt zero[3]Greek
Albedo 0.22 (geometric)[4]
Surfacetemp.
K[4]
min mean max
80 5 134 11 165 5
Apparent magnitude 5.65 (opposition)[5]
Atmosphere
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Surfacepressure 7.5pbar[6]
Composition ~4 108 cm3carbon dioxide[6]
up to 2 1010 cm3molecular oxygen[7]
Callisto (pronounced /k lsto/,[8]named after the Greekmythological figure
ofCallisto,Greek: ) is a moon of the planet Jupiter, discovered in 1610 by Galileo Galilei.[1] It is
the third-largest moonin theSolar System and the second largest in the Jovian system, afterGanymede.
Callisto has about 99% the diameter of the planet Mercurybut only about a third of its mass. It is the
fourthGalilean moon ofJupiterby distance, with an orbital radius of about 1,880,000 km.[2] It does not
form part of theorbital resonancethat affects three inner Galilean satellitesIo, Europa and Ganymede
and thus does not experience appreciabletidal heating.
[9]
Callistorotates synchronously with its orbitalperiod, so the same face is always turned toward Jupiter. Callisto's surface is less affected by
Jupiter's magnetosphere than the otherinner satellitesbecause it orbits farther away.[10]
Callisto is composed of approximately equal amounts ofrock and ices, with a meandensity of about
1.83 g/cm3. Compounds detected spectroscopically on the surface include water ice, carbon
dioxide, silicates, and organic compounds. Investigation by the Galileo spacecraft revealed that Callisto
may have a smallsilicatecoreand possibly a subsurface ocean of liquid water at depths greater than
100 km.[11][12]
The surface of Callisto is heavily cratered and extremely old. It does not show any signatures
ofsubsurface processes such asplate tectonicsorvolcanism, and is thought to have evolved
predominantly under the influence ofimpacts.[13]Prominent surface features include multi-ring structures,
variously shaped impact craters, and chains of craters (catenae) and associated scarps, ridges and
deposits.[13] At a small scale, the surface is varied and consists of small, bright frostdeposits at the tops of
elevations, surrounded by a low-lying, smooth blanket of dark material.[4] This is thought to result from
the sublimation-driven degradation of smalllandforms, which is supported by the general deficit of small
impact craters and the presence of numerous small knobs, considered to be their remnants.[14]The
absolute ages of the landforms are not known.
Callisto is surrounded by an extremely thin atmosphere composed ofcarbon dioxide[6] and
probably molecular oxygen,[7] as well as by a rather intense ionosphere.[15]Callisto is thought to have
formed by slow accretion from the disk of the gas and dust that surrounded Jupiter after its formation.
[16] Its slowness and the lack oftidalheating prevented rapid differentiation. The slow convection in the
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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 100150 km and a small, rocky core.[17]
The likely presence of an ocean within Callisto leaves open the possibility that it could harborlife.
However, conditions are thought to be less favorable than on nearbyEuropa.[18] Various space probes
from Pioneers 10and11 to Galileo and Cassinihave studied the moon. Because of its low radiation
levels, Callisto has long been considered the most suitable place for a human base for future exploration
of the Jovian system.[19]
Contents
[hide]
1 Discovery and naming
2 Orbit and rotation
3 Physical characteristics
3.1 Composition
3.2 Internal structure
3.3 Surface features
3.4 Atmosphere and
ionosphere
4 Origin and evolution
5 Possibility of life in the
ocean
6 Exploration
7 Potential colonization
8 See also
9 Notes
10 References
11 External links
[edit]Discovery and naming
Callisto was discovered by Galileo in January 1610 along with three other large Jovian moons
Ganymede,Io, and Europa.[1]Callisto is named after one ofZeus's many lovers in Greek
mythology. Callisto was a nymph (or, according to some sources, the daughter ofLycaon) who was
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associated with the goddess of the hunt, Artemis.[20]The name was suggested by Simon Mariussoon after
the moon's discovery.[21] Marius attributed the suggestion to Johannes Kepler.[20]However, the names of
theGalilean satellites fell into disfavor for a considerable time, and were not revived in common use until
the mid-20th century. In much of the earlier astronomical literature, Callisto is referred to by its Roman
numeral designation, a system introduced by Galileo, as Jupiter IV or as "the fourth satellite of Jupiter".
[22] In scientific writing, the adjectival form of the name is Callistoan,[23] pronounced/kl sto n/,
orCallistan.[14]
[edit]Orbit and rotation
Callisto (bottom left), Jupiter (top right) and Europa (below and left of Jupiter'sGreat Red Spot) as viewed byCassini
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 71 398 km radius of Jupiter itself).[2] This is significantly larger than the
orbital radius1 070 000 kmof the next-closest Galilean satellite, Ganymede. As a result of this
relatively distant orbit, Callisto does not participate in the mean-motion resonancein which the three
inner Galilean satellites are lockedand probably never has.[9]
Like most other regular planetary moons, Callisto's rotation is locked to be synchronouswith its orbit.
[3]
The length of the Callistoan day, simultaneously its orbital period, is about 16.7 Earth days. Its orbit is
very slightly eccentric and inclined to the Jovianequator, with
the eccentricity and inclinationchanging quasi-periodically due to solar and planetary gravitational
perturbations on a timescale of centuries. The ranges of change are 0.00720.0076 and 0.200.60,
respectively.[9] These orbital variations cause the axial tilt(the angle between rotational and orbital axes)
to vary between 0.4 and 1.6.[24]
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The dynamical isolation of Callisto means that it has never been appreciablytidally heated, which has had
important consequences for its internal structure andevolution.[25]Its distance from Jupiter also means that
the charged-particleflux from the planet's magnetosphereat its surface is relatively lowabout 300 times
lower than, for example, that at Europa. Hence, unlike the other Galilean moons, charged-
particle irradiation has had a relatively minor effect on the Callistoan surface.[10] The radiation level at the
surface of Callisto is equivalent to a dose of about 0.01 rem (0.1 mSv) per day.[26]
[edit]Physical characteristics
[edit]Composition
Near-infrared spectrum of a dark cratered plains area, indicating the presence of relatively less water (between 1 and
2micrometers) and more rocky material than in impact basins
The average density of Callisto, 1.83 g/cm3,[3]suggests a composition of approximately equal parts of
rocky material andwater ice, with some additional volatile ices such as ammonia.[11] The mass fraction of
ices is between 4955%.[11][17] The exact composition of Callisto's rock component is not known, but is
probably close to the composition of L/LL type ordinary chondrites, which are characterized by less
total iron, less metallic iron and more iron oxide than H chondrites. The weight ratio of iron tosiliconis
0.9:1.3 in Callisto, whereas thesolar ratiois around 1:8.[11]
Callisto's surface has an albedo of about 20%.[4] Its surface composition is thought to be broadly similar to
its composition as a whole. Near-infrared spectroscopy has revealed the presence of water ice absorption
bandsat wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0 micrometers.[4] Water ice seems to be ubiquitous on
the surface of Callisto, with a mass fraction of 2550%.[12] The analysis of high-resolution, near-
infrared and UVspectra obtained by the Galileo spacecraft and from the ground has revealed various
non-ice materials: magnesium- and iron-bearing hydrated silicates,[4]carbon dioxide,[27]sulfur dioxide,
[28] and possibly ammonia and various organic compounds.[4][12]Spectral data indicate that the moon's
surface is extremely heterogeneous at the small scale. Small, bright patches of pure water ice are
intermixed with patches of a rockice mixture and extended dark areas made of a non-ice material.[4][13]
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The Callistoan surface is asymmetric: the leading hemispherethe hemisphere facing the direction of the
orbital motion[g]is darker than the trailing one. This is different from otherGalilean satellites, where the
reverse is true.[4] The trailing hemisphere[g]of Callisto appears to be enriched incarbon dioxide, while the
leading hemisphere has moresulfur dioxide.[29] Many fresh impact craters like Lofn also show enrichment
in carbon dioxide.[29] Overall, the chemical composition of the surface, especially in the dark areas, may
be close to that seen on D-type asteroids,[13]whose surfaces are made ofcarbonaceousmaterial.
[edit]Internal structure
Model of Callisto's internal structure showing a surface ice layer, a possible liquid water layer, and an ice-rock interior
Callisto's battered surface lies on top of a cold, stiff, and icy lithosphere that is between 80 and 150 km
thick.[11][17] A salty ocean 50200 km deep may lie beneath thecrust,[11][17]indicated by studies of
the magnetic fieldsaround Jupiter and its moons.[30][31]It was found that Callisto responds to Jupiter's
varying background magnetic field like a perfectlyconducting sphere; that is, the field cannot penetrate
inside the moon, suggesting a layer of highly conductive fluid within it with a thickness of at least 10 km.
[31] The existence of an ocean is more likely if water contains a small amount ofammonia or
otherantifreeze, up to 5% by weight.[17] In this case the ocean can be as thick as 250300 km.[11] 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 orbiter data[3] (especially the dimensionless moment of inertia[h]
0.3549 0.0042determined during close flybys) suggest that its interior is composed of
compressed rocksandices, with the amount of rock increasing with depth due to partial settling of its
constituents.[11][32]In other words, Callisto is only partially differentiated. The density and moment of inertia
are compatible with the existence of a small silicate core in the center of the satellite. The radius of any
such core cannot exceed 600 km, and the density may lie between 3.1 and 3.6 g/cm3.[3][11]Callisto's
interior is in stark contrast to that of Ganymede, which appears to be fully differentiated.[12][33]
[edit]Surface features
See also:List of geological features on Callisto
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Galileo image of cratered plains, illustrating the pervasive local smoothing of Callisto's surface
The ancient surface of Callisto is one of the most heavily cratered in the solar system.[34] In fact,
the craterdensity is close to saturation: any new crater will tend to erase an older one. The large-
scale geologyis relatively simple; there are no large Callistoan mountains, volcanoes or other
endogenic tectonicfeatures.[35] The impact craters and multi-ring structurestogether with
associatedfractures,scarps and depositsare the only large features to be found on the surface.[13][35]
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.[13][35] 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 Burrand Lofn, as well as the effaced remnants of old large craters called palimpsests,[i] the central
parts of multi-ring structures, and isolated patches in the cratered plains.[13] These light plains are thought
to be icy impact deposits. The bright, smooth plains constitute a small fraction of the Callistoan surface
and are found in the ridge and troughzones of the Valhallaand Asgard formations and as isolated spots
in the cratered plains. They were believed 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.[13]The Galileo images also revealed small, dark,
smooth areas with overall coverage less than 10,000 km2, which appear to embay[j]the surrounding
terrain. They are possible cryovolcanic deposits.[13] Both the light and the various smooth plains are
somewhat younger and less cratered than the background cratered plains.[13][36]
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measuring about 1,600 kilometers in diameter.[37]Multi-ring structures probably originated as a result of a
post-impactconcentric fracturing of the lithosphere lying on a layer of soft or liquid material, possibly an
ocean.[23] The catenaefor example Gomul Catenaare 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 oblique impacts.[13] A historical example
of a disruption was Comet Shoemaker-Levy 9.
As mentioned above, small patches of pure water ice with an albedo as high as 80% are found on the
surface of Callisto, surrounded by much darker material.[4] High-resolution Galileo images showed the
bright patches to be predominately located on elevated surface features: crater rims, scarps, ridges and
knobs.[4] They are likely to be thin water frostdeposits. 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.[4]
Twolandslides33.5 km long are visible on the right sides of the floors of the two large craters on the right.
On a sub-kilometer scale the surface of Callisto is more degraded than the surfaces of other icy Galilean
moons.[4]Typically there is a deficit of small impact craters with diameters less than 1 km as compared
with, for instance, the dark plains onGanymede.[13] Instead of small craters, the almost ubiquitous surface
features are small knobs and pits.[4]
The knobs are thought to represent remnants of crater rims degradedby an as-yet uncertain process.[14] The most likely candidate process is the slow sublimation of ice, which
is enabled by a temperature of up to 165 K, reached at a subsolar point.[4]Such sublimation of water or
othervolatiles from the dirty ice that is thebedrock causes its decomposition. The non-ice remnants
form debrisavalanches descending from the slopes of the crater walls.[14]Such avalanches are often
observed near and inside impact craters and termed "debris aprons".[4][13][14] Sometimes crater walls are
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cut by sinuous valley-like incisions called "gullies", which resemble certainMartian surface features.[4]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.[38] Absolute dating has not been
carried out, but based on theoretical considerations, the cratered plains are thought to be
~4.5billionyears old, dating back almost to the formation of the 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.[13][34]
[edit]Atmosphere and ionosphere
Induced magnetic field around Callisto
Callisto has a very tenuous atmosphere composed ofcarbon dioxide.[6] 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 1012bar(0.75Pa) and particle density
4 108 cm3. Because such a thin atmosphere would be lost in only about 4 days (see atmospheric
escape), it must be constantly being replenished, possibly by slow sublimation of carbon dioxide ice from
the satellite's icy crust,[6] which would be compatible with the sublimationdegradation hypothesis for the
formation of the surface knobs.
Callisto's ionosphere was first detected during Galileo flybys;[15] its high electron density of 7
17 104 cm3 cannot be explained by the photoionization of the atmospheric carbon dioxide alone.
Hence, it is suspected that the atmosphere of Callisto is actually dominated bymolecular oxygen (in
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amounts 10100 times greater than CO2).[7]However, oxygenhas 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.[39]At the same time HST was able to detect condensed oxygen trapped on
the surface of Callisto.[40]
[edit]Origin and evolution
The partial differentiation of Callisto (inferred e.g. from moment of inertia measurements) means that it
has never been heated enough to melt its ice component.[17] Therefore, the most favorable model of its
formation is a slowaccretion in the low-density Jovian subnebulaa disk of the gas and dust that existed
around Jupiter after its formation.[16]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.[16] The allowable timescale of formation of Callisto lies then in the range
0.1 million10 million years.[16]
Views of eroding (top) and mostly eroded (bottom) ice knobs (~100 m high), possibly formed from the ejecta of an
ancient impact
The further evolution of Callisto afteraccretionwas determined by the balance of the radioactive heating,
cooling through thermal conduction near the surface, and solid state or subsolidus convection in the
interior.[25] 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.[41] 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.[41]It is thought to proceed in the so-called stagnant lid regime, where a stiff, cold outer
layer of the moon conducts heat without convection, while the ice beneath it convects in the subsolidus
regime.[17][41] 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.[41][42]The convection in the interior parts of Callisto may be layered, because under
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the high pressures found there, water iceexists in different crystalline phases beginning from theice I on
the surface toice VIIin the center.[25]The early onset of subsolidus convection in the Callistoan interior
could have prevented large-scale ice melting and any resultingdifferentiation that would have otherwise
formed a large rocky coreand icy 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.[42]
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 MPa).[17] 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.[25][41][42] The presence of
even small amounts ofammoniaabout 12% by weightalmost guarantees the liquid's existence
because ammonia would lower the melting temperature even further.[17]
While Callisto is very similar in bulk properties to Ganymede, it apparently had a much simplergeological
history. The surface appears to have been shaped mainly by impacts and otherexogenic forces.[13]Unlike
neighboring Ganymede with its grooved terrain, there is little evidence oftectonicactivity.[12] 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,[43]the greater tidal
heating experienced by Ganymede,[44]and the more energetic impacts that would have been suffered by
Ganymede during the Late Heavy Bombardment.[45][46][47] The relatively simple geological history of Callisto
provides planetary scientists with a reference point for comparison with other more active and complex
worlds.[12]
[edit]Possibility of life in the ocean
As with Europa andGanymede, the idea has been raised that extraterrestrial microbial life may exist in a
salty ocean under the Callistoan surface.[18]However, the conditions for life appear to be less favourable
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.[18] Scientist Torrence Johnson said the following about comparing the
odds of life on Callisto with the odds on otherGalilean moons:[48]
Based on the considerations mentioned above and on other scientific observations, it is thought that of all
of Jupiter's Galilean moons, Europa has the greatest chance of supporting microbial life.[18][49]
[edit]Exploration
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The Pioneer 10and 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.[4] The real
breakthrough happened later with theVoyager 1 and 2flybys in 19791980. They imaged more than half
of the Callistoan surface with a resolution of 12 km, and precisely measured its temperature, mass and
shape.[4]A second round of exploration lasted from 1994 to 2003, when theGalileospacecraft 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.[13] In 2000,
the Cassinispacecraft en route to Saturnacquired high-quality infrared spectra of the Galilean satellites
including Callisto.[27]In FebruaryMarch 2007, the New Horizonsprobe on its way to Pluto obtained new
images and spectra of Callisto.[50]
Proposed for a launch in 2020, the Europa Jupiter System Mission (EJSM) is a joint NASA/ESA proposal
for exploration ofJupiter's moons. In February 2009 it was announced that ESA/NASA had given this
mission priority ahead of the Titan Saturn System Mission.[51] ESA's contribution will still face funding
competition from other ESA projects.[52]EJSM consists of the NASA-led Jupiter Europa Orbiter, the ESA-
led Jupiter Ganymede Orbiter, and possibly aJAXA-led Jupiter Magnetospheric Orbiter.
[edit]Potential colonization
Artist's impression of a base on Callisto [53]
In 2003 NASAconducted 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.[19][54]
It was proposed that it could be possible to build a surface base on Callisto that would produce fuel for
further exploration of the Solar System.[53] Advantages of a base on this moon include the low radiation
(due to Callisto's distance from Jupiter) and geological stability. It could facilitate remote exploration
ofEuropa, 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.[19]
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In a December 2003 report, NASA expressed belief that an attempt for a manned mission to Callisto may
be possible in the2040s.[55]
[edit]See also
http://en.wikipedia.org/wiki/2040shttp://en.wikipedia.org/wiki/2040shttp://en.wikipedia.org/w/index.php?title=Callisto_(moon)&action=edit§ion=12http://en.wikipedia.org/wiki/2040shttp://en.wikipedia.org/w/index.php?title=Callisto_(moon)&action=edit§ion=12Top Related