Callisto (moon)

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Callisto
Callisto.jpg
Callisto's anti-Jovian hemisphere imaged in 2001 by NASA's Galileo spacecraft. It shows a heavily cratered terrain. The large impact structure Asgard is on the limb at upper right. The prominent rayed crater below and just right of center is Bran.
Discovery
Discovered by Galileo Galilei
Discovery date7 January 1610 [1]
Designations
Jupiter IV
Adjectives Callistoan, Callistonian
Orbital characteristics
Periapsis 1869000 km [lower-alpha 1]
Apoapsis 1897000 km [lower-alpha 2]
1 882 700 km [2]
Eccentricity 0.0074 [2]
16.6890184  d [2]
Average orbital speed
8.204 km/s
Inclination 2.017° (to the ecliptic)
0.192° (to local Laplace planes) [2]
Satellite of Jupiter
Physical characteristics
Mean radius
2410.3±1.5 km (0.378 Earths) [3]
7.30×107 km2 (0.143 Earths) [lower-alpha 3]
Volume 5.9×1010 km3 (0.0541 Earths) [lower-alpha 4]
Mass (1.075938±0.000137)×1023 kg (0.018 Earths) [3]
Mean density
1.8344±0.0034 g/cm3 [3]
1.235  m/s2 (0.126 g ) [lower-alpha 5]
0.3549±0.0042 [4]
2.440 km/s [lower-alpha 6]
synchronous [3]
zero [3]
Albedo 0.22 (geometric) [5]
Surface temp. minmeanmax
K [5] 80±5134±11165±5
5.65 (opposition) [6]
Atmosphere
Surface pressure
7.5  picobar [7] (7.5×10−10  kPa , 7.4019×10−12  atm )
Composition by volume 4×108 molecules/cm3 carbon dioxide; [7]
up to 2×1010 molecules/cm3 molecular oxygen(O2) [8]

    Callisto /kəˈlɪst/ [9] (Jupiter IV) is the second-largest moon of Jupiter, after Ganymede. It is the third-largest moon in the Solar System after Ganymede and Saturn's largest moon Titan, and the largest object in the Solar System not to be properly differentiated. Callisto was discovered in 1610 by Galileo Galilei. At 4821 km in diameter, Callisto has about 99% the diameter of the planet Mercury but only about a third of its mass. It is the fourth Galilean moon of Jupiter by distance, with an orbital radius of about 1883000 km. [2] It is not in an orbital resonance like the three other Galilean satellites—Io, Europa, and Ganymede—and is thus not appreciably tidally heated. [10] Callisto's rotation is tidally locked to its orbit around Jupiter, so that the same hemisphere always faces inward; Jupiter appears to stand nearly still in Callisto's sky. It is less affected by Jupiter's magnetosphere than the other inner satellites because of its more remote orbit, located just outside Jupiter's main radiation belt. [11] [12]

    Moons of Jupiter The natural satellites of the planet Jupiter

    There are 79 known moons of Jupiter. This gives Jupiter the largest number of known moons with reasonably stable orbits of any planet in the Solar System, if one does not count the moonlets within Saturn's rings. The most massive of the moons are the four Galilean moons, which were independently discovered in 1610 by Galileo Galilei and Simon Marius and were the first objects found to orbit a body that was neither Earth nor the Sun. From the end of the 19th century, dozens of much smaller Jovian moons have been discovered and have received the names of lovers or daughters of the Roman god Jupiter or his Greek equivalent Zeus. The Galilean moons are by far the largest and most massive objects to orbit Jupiter, with the remaining 75 known moons and the rings together comprising just 0.003% of the total orbiting mass.

    Ganymede (moon) The largest moon of Jupiter and in the Solar System

    Ganymede, a satellite of Jupiter, is the largest and most massive of the Solar System's moons. The ninth largest object in the Solar System, it is the largest without a substantial atmosphere. It has a diameter of 5,268 km (3,273 mi) and is 8% larger than the planet Mercury, although only 45% as massive. Possessing a metallic core, it has the lowest moment of inertia factor of any solid body in the Solar System and is the only moon known to have a magnetic field. Outward from Jupiter, it is the seventh satellite and the third of the Galilean moons, the first group of objects discovered orbiting another planet. Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively.

    Solar System Planetary system of the Sun

    The Solar System is the gravitationally bound system of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest are the eight planets, with the remainder being smaller objects, such as the five dwarf planets and small Solar System bodies. Of the objects that orbit the Sun indirectly—the moons—two are larger than the smallest planet, Mercury.

    Contents

    Callisto is composed of approximately equal amounts of rock and ices, with a density of about 1.83 g/cm3, the lowest density and surface gravity of Jupiter's major moons. Compounds detected spectroscopically on the surface include water ice, [13] carbon dioxide, silicates, and organic compounds. Investigation by the Galileo spacecraft revealed that Callisto may have a small silicate core and possibly a subsurface ocean of liquid water [13] at depths greater than 100 km. [14] [15]

    Rock (geology) A naturally occurring solid aggregate of one or more minerals or mineraloids

    A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its chemical composition and the way in which it is formed. Rocks are usually grouped into three main groups: igneous rocks, metamorphic rocks and sedimentary rocks. Rocks form the Earth's outer solid layer, the crust.

    Volatiles In planetary science, chemical elements or compounds with low boiling points associated with a planet’s or moon’s crust or atmosphere; e.g. nitrogen, water, carbon dioxide, ammonia, hydrogen, methane, sulfur dioxide

    In planetary science, volatiles are the group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, methane and sulfur dioxide. In astrogeology, these compounds, in their solid state, often comprise large proportions of the crusts of moons and dwarf planets.

    The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is ρ, although the Latin letter D can also be used. Mathematically, density is defined as mass divided by volume:

    The surface of Callisto is the oldest and most heavily cratered in the Solar System. [16] Its surface is completely covered with impact craters. [17] It does not show any signatures of subsurface processes such as plate tectonics or volcanism, with no signs that geological activity in general has ever occurred, and is thought to have evolved predominantly under the influence of impacts. [18] Prominent surface features include multi-ring structures, variously shaped impact craters, and chains of craters (catenae) and associated scarps, ridges and deposits. [18] At a small scale, the surface is varied and made up of small, sparkly frost deposits at the tips of high spots, surrounded by a low-lying, smooth blanket of dark material. [5] This is thought to result from the sublimation-driven degradation of small landforms, which is supported by the general deficit of small impact craters and the presence of numerous small knobs, considered to be their remnants. [19] The absolute ages of the landforms are not known.

    Impact crater Circular depression on a solid astronomical body formed by a hypervelocity impact of a smaller object

    An impact crater is an approximately circular depression in the surface of a planet, moon, or other solid body in the Solar System or elsewhere, formed by the hypervelocity impact of a smaller body. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters typically have raised rims and floors that are lower in elevation than the surrounding terrain. Impact craters range from small, simple, bowl-shaped depressions to large, complex, multi-ringed impact basins. Meteor Crater is a well-known example of a small impact crater on Earth.

    Plate tectonics The scientific theory that describes the large-scale motions of Earths lithosphere

    Plate tectonics is a scientific theory describing the large-scale motion of seven large plates and the movements of a larger number of smaller plates of the Earth's lithosphere, since tectonic processes began on Earth between 3.3 and 3.5 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century. The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s.

    Volcano A rupture in the crust of a planetary-mass object that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface

    A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.

    Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide [7] and probably molecular oxygen, [8] as well as by a rather intense ionosphere. [20] Callisto is thought to have formed by slow accretion from the disk of the gas and dust that surrounded Jupiter after its formation. [21] Callisto's gradual accretion and the lack of tidal heating meant that not enough heat was available for rapid differentiation. The slow 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. [22]

    Atmosphere The layer of gases surrounding an astronomical body held by gravity

    An atmosphere is a layer or a set of layers of gases surrounding a planet or other material body, that is held in place by the gravity of that body. An atmosphere is more likely to be retained if the gravity it is subject to is high and the temperature of the atmosphere is low.

    Carbon dioxide chemical compound

    Carbon dioxide is a colorless gas with a density about 60% higher than that of dry air. Carbon dioxide consists of a carbon atom covalently double bonded to two oxygen atoms. It occurs naturally in Earth's atmosphere as a trace gas. The current concentration is about 0.04% (410 ppm) by volume, having risen from pre-industrial levels of 280 ppm. Natural sources include volcanoes, hot springs and geysers, and it is freed from carbonate rocks by dissolution in water and acids. Because carbon dioxide is soluble in water, it occurs naturally in groundwater, rivers and lakes, ice caps, glaciers and seawater. It is present in deposits of petroleum and natural gas. Carbon dioxide is odorless at normally encountered concentrations. However, at high concentrations, it has a sharp and acidic odor.

    Ionosphere The ionized part of Earths upper atmosphere

    The ionosphere is the ionized part of Earth's upper atmosphere, from about 60 km (37 mi) to 1,000 km (620 mi) altitude, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an important role in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth. The region below the ionosphere is called neutral atmosphere, or neutrosphere.

    The likely presence of an ocean within Callisto leaves open the possibility that it could harbor life. However, conditions are thought to be less favorable than on nearby Europa. [23] Various space probes from Pioneers 10 and 11 to Galileo and Cassini have studied Callisto. 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. [24]

    Extraterrestrial life Life occurring outside of Earth which did not originate on Earth

    Extraterrestrial life refers to life occurring outside of Earth which did not originate on Earth. Such hypothetical life might range from simple prokaryotes to beings with civilizations far more advanced than humanity. The Drake equation speculates about the existence of intelligent life elsewhere in the universe. The science of extraterrestrial life in all its forms is known as exobiology.

    Europa (moon) The smallest of the four Galilean moons of Jupiter

    Europa is the smallest of the four Galilean moons orbiting Jupiter, and the sixth-closest to the planet of all the 79 known moons of Jupiter. It is also the sixth-largest moon in the Solar System. Europa was discovered in 1610 by Galileo Galilei and was named after Europa, the Phoenician mother of King Minos of Crete and lover of Zeus.

    <i>Pioneer 10</i> space probe launched in March 1972

    Pioneer 10 is an American space probe, launched in 1972 and weighing 258 kilograms, that completed the first mission to the planet Jupiter. Thereafter, Pioneer 10 became the first of five artificial objects to achieve the escape velocity that will allow them to leave the Solar System. This space exploration project was conducted by the NASA Ames Research Center in California, and the space probe was manufactured by TRW Inc.

    History

    Discovery

    Callisto was discovered by Galileo in January 1610, along with the three other large Jovian moons—Ganymede, Io, and Europa. [1]

    Io (moon) Innermost of the four Galilean moons of Jupiter

    Io is the innermost of the four Galilean moons of the planet Jupiter. It is the fourth-largest moon in the solar system, has the highest density of all of them, and has the least amount of water molecules of any known astronomical object in the Solar System. It was discovered in 1610 by Galileo Galilei and was named after the mythological character Io, a priestess of Hera who became one of Zeus' lovers.

    Name

    Callisto is named after one of Zeus's many lovers in Greek mythology. Callisto was a nymph (or, according to some sources, the daughter of Lycaon) who was associated with the goddess of the hunt, Artemis. [25] The name was suggested by Simon Marius soon after Callisto's discovery. [26] Marius attributed the suggestion to Johannes Kepler. [25] However, the names of the Galilean satellites fell into disfavor for a considerable time, and were not revived in common use until the mid-20th century. In much of the earlier astronomical literature, Callisto is referred to by its Roman numeral designation, a system introduced by Galileo, as Jupiter IV or as "the fourth satellite of Jupiter". [27] In scientific writing, the adjectival form of the name is Callistoan, [28] pronounced /ˌkælɪˈst.ən/ , or Callistan. [19]

    Orbit and rotation

    Galilean moons around Jupiter Jupiter *   Io *   Europa *   Ganymede *   Callisto Galilean moons around Jupiter.gif
    Galilean moons around Jupiter   Jupiter ·  Io ·  Europa ·  Ganymede ·  Callisto
    Callisto (bottom left), Jupiter (top right) and Europa (below and left of Jupiter's Great Red Spot) as viewed by Cassini-Huygens 001221 Cassini Jupiter & Europa & Callisto.jpg
    Callisto (bottom left), Jupiter (top right) and Europa (below and left of Jupiter's Great Red Spot) as viewed by Cassini–Huygens

    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). [2] 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—in which the three inner Galilean satellites are locked—and probably never has. [10]

    Like most other regular planetary moons, Callisto's rotation is locked to be synchronous with its orbit. [3] The length of Callisto's day, simultaneously its orbital period, is about 16.7 Earth days. Its orbit is very slightly eccentric and inclined to the Jovian equator, with the eccentricity and inclination changing quasi-periodically due to solar and planetary gravitational perturbations on a timescale of centuries. The ranges of change are 0.0072–0.0076 and 0.20–0.60°, respectively. [10] These orbital variations cause the axial tilt (the angle between rotational and orbital axes) to vary between 0.4 and 1.6°. [29]

    The dynamical isolation of Callisto means that it has never been appreciably tidally heated, which has important consequences for its internal structure and evolution. [30] Its distance from Jupiter also means that the charged-particle flux from Jupiter's magnetosphere at its surface is relatively low—about 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 Callisto's surface. [11] The radiation level at Callisto's surface is equivalent to a dose of about 0.01 rem (0.1 mSv) per day, which is over ten times higher than Earth's average background radiation. [31] [32]

    Physical characteristics

    Composition

    Size comparison of Earth, Moon and Callisto Callisto, Earth & Moon size comparison.jpg
    Size comparison of Earth, Moon and Callisto
    Near-IR spectra of dark cratered plains (red) and the Asgard impact structure (blue), showing the presence of more water ice (absorption bands from 1 to 2 um) and less rocky material within Asgard. PIA00844 NIMS spectra.gif
    Near-IR spectra of dark cratered plains (red) and the Asgard impact structure (blue), showing the presence of more water ice (absorption bands from 1 to 2 µm) and less rocky material within Asgard.

    The average density of Callisto, 1.83 g/cm3, [3] suggests a composition of approximately equal parts of rocky material and water ice, with some additional volatile ices such as ammonia. [14] The mass fraction of ices is 49–55%. [14] [22] The exact composition of Callisto's rock component is not known, but is probably close to the composition of L/LL type ordinary chondrites, [14] which are characterized by less total iron, less metallic iron and more iron oxide than H chondrites. The weight ratio of iron to silicon is 0.9–1.3 in Callisto, whereas the solar ratio is around 1:8. [14]

    Callisto's surface has an albedo of about 20%. [5] 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 bands at wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0 micrometers. [5] Water ice seems to be ubiquitous on the surface of Callisto, with a mass fraction of 25–50%. [15] The analysis of high-resolution, near-infrared and UV spectra obtained by the Galileo spacecraft and from the ground has revealed various non-ice materials: magnesium- and iron-bearing hydrated silicates, [5] carbon dioxide, [34] sulfur dioxide, [35] and possibly ammonia and various organic compounds. [15] [5] 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. [5] [18]

    The Callistoan surface is asymmetric: the leading hemisphere [lower-alpha 7] is darker than the trailing one. This is different from other Galilean satellites, where the reverse is true. [5] The trailing hemisphere [lower-alpha 7] of Callisto appears to be enriched in carbon dioxide, whereas the leading hemisphere has more sulfur dioxide. [36] Many fresh impact craters like Lofn also show enrichment in carbon dioxide. [36] Overall, the chemical composition of the surface, especially in the dark areas, may be close to that seen on D-type asteroids, [18] whose surfaces are made of carbonaceous material.

    Internal structure

    Model of Callisto's internal structure showing a surface ice layer, a possible liquid water layer, and an ice-rock interior Callisto diagram.svg
    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. [14] [22] A salty ocean 150–200 km deep may lie beneath the crust, [14] [22] indicated by studies of the magnetic fields around Jupiter and its moons. [37] [38] It was found that Callisto responds to Jupiter's varying background magnetic field like a perfectly 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. [38] The existence of an ocean is more likely if water contains a small amount of ammonia or other antifreeze, up to 5% by weight. [22] In this case the water+ice layer can be as thick as 250–300 km. [14] 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 [lower-alpha 8] —0.3549 ± 0.0042—determined during close flybys) suggest that its interior is composed of compressed rocks and ices, with the amount of rock increasing with depth due to partial settling of its constituents. [14] [39] 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 Callisto. The radius of any such core cannot exceed 600 km, and the density may lie between 3.1 and 3.6 g/cm3. [3] [14] Callisto's interior is in stark contrast to that of Ganymede, which appears to be fully differentiated. [15] [40]

    Surface features

    Galileo image of cratered plains, illustrating the pervasive local smoothing of Callisto's surface Cratered plains PIA00745.jpg
    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. [41] In fact, the crater density is close to 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 endogenic tectonic features. [42] The impact craters and multi-ring structures—together with associated fractures, scarps and deposits—are the only large features to be found on the surface. [18] [42]

    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. [18] [42] 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 Burr and Lofn, as well as the effaced remnants of old large craters called palimpsests, [lower-alpha 9] the central parts of multi-ring structures, and isolated patches in the cratered plains. [18] 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 zones of the Valhalla and 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. [18] The Galileo images also revealed small, dark, smooth areas with overall coverage less than 10,000 km2, which appear to embay [lower-alpha 10] the surrounding terrain. They are possible cryovolcanic deposits. [18] Both the light and the various smooth plains are somewhat younger and less cratered than the background cratered plains. [18] [43]

    Impact crater Har with a central dome. Chains of secondary craters from formation of the more recent crater Tindr at upper right crosscut the terrain. Callisto Har PIA01054.jpg
    Impact crater Hár with a central dome. Chains of secondary craters from formation of the more recent crater Tindr at upper right crosscut the terrain.

    Impact crater diameters seen range from 0.1 km—a limit defined by the imaging resolution—to over 100 km, not counting the multi-ring structures. [18] 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. [18] The largest craters with diameters over 60 km can have central domes, which are thought to result from central tectonic uplift after an impact; [18] examples include Doh and Hár craters. A small number of very large—more 100 km in diameter—and bright impact craters show anomalous dome geometry. These are unusually shallow and may be a transitional landform to the multi-ring structures, as with the Lofn impact feature. [18] Callisto's craters are generally shallower than those on the Moon.

    Voyager 1 image of Valhalla, a multi-ring impact structure 3800 km in diameter Valhalla crater on Callisto.jpg
    Voyager 1 image of Valhalla, a multi-ring impact structure 3800 km in diameter

    The largest impact features on Callisto's surface are multi-ring basins. [18] [42] Two are enormous. 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). [44] The second largest is Asgard, measuring about 1,600 kilometers in diameter. [44] 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. [28] 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 oblique impacts. [18] 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. [5] High-resolution Galileo images showed the bright patches to be predominately located on elevated surface features: crater rims, scarps, ridges and knobs. [5] They are likely to be thin water frost deposits. 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. [5]

    Two landslides 3-3.5 km long are visible on the right sides of the floors of the two large craters on the right. Landslides and knobs PIA01095.jpg
    Two landslides 3–3.5 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. [5] 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. [18] Instead of small craters, the almost ubiquitous surface features are small knobs and pits. [5] The knobs are thought to represent remnants of crater rims degraded by an as-yet uncertain process. [19] 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. [5] 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. [19] Such avalanches are often observed near and inside impact craters and termed "debris aprons". [5] [18] [19] Sometimes crater walls are cut by sinuous valley-like incisions called "gullies", which resemble certain Martian surface features. [5] 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. [45] Absolute dating has not been carried out, but based on theoretical considerations, the cratered plains are thought to be ~4.5  billion years 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. [18] [41]

    Atmosphere and ionosphere

    Induced magnetic field around Callisto Callisto field.svg
    Induced magnetic field around Callisto

    Callisto has a very tenuous atmosphere composed of carbon dioxide. [7] 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 (0.75 µPa) and particle density 4 × 108 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, [7] 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; [20] its high electron density of 7–17 × 104 cm−3 cannot be explained by the photoionization of the atmospheric carbon dioxide alone. Hence, it is suspected that the atmosphere of Callisto is actually dominated by molecular oxygen (in amounts 10–100 times greater than CO
    2
    ). [8] 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. [46] At the same time, HST was able to detect condensed oxygen trapped on the surface of Callisto. [47]

    Atomic hydrogen has also been detected in Callisto's atmosphere via recent analysis of 2001 Hubble Space Telescope data. [48] 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. [49]

    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. [22] Therefore, the most favorable model of its formation is a slow accretion in the low-density Jovian subnebula—a disk of the gas and dust that existed around Jupiter after its formation. [21] 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. [21] The allowable timescale of formation of Callisto lies then in the range 0.1 million–10 million years. [21]

    Views of eroding (top) and mostly eroded (bottom) ice knobs (~100 m high), possibly formed from the ejecta of an ancient impact Jagged Hills PIA03455.jpg
    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 after accretion was determined by the balance of the radioactive heating, cooling through thermal conduction near the surface, and solid state or subsolidus convection in the interior. [30] 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. [50] 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. [50] 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. [22] [50] 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. [50] [51] The convection in the interior parts of Callisto may be layered, because under the high pressures found there, water ice exists in different crystalline phases beginning from the ice I on the surface to ice VII in the center. [30] The early onset of subsolidus convection in the Callistoan interior could have prevented large-scale ice melting and any resulting differentiation that would have otherwise formed a large rocky core and 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. [51]

    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). [22] 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. [30] [50] [51] The presence of even small amounts of ammonia—about 1–2% by weight—almost guarantees the liquid's existence because ammonia would lower the melting temperature even further. [22]

    Although Callisto is very similar in bulk properties to Ganymede, it apparently had a much simpler geological history. The surface appears to have been shaped mainly by impacts and other exogenic forces. [18] Unlike neighboring Ganymede with its grooved terrain, there is little evidence of tectonic activity. [15] 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, [52] the greater tidal heating experienced by Ganymede, [53] and the more numerous and energetic impacts that would have been suffered by Ganymede during the Late Heavy Bombardment. [54] [55] [56] The relatively simple geological history of Callisto provides planetary scientists with a reference point for comparison with other more active and complex worlds. [15]

    Potential habitability

    It is speculated that there could be life in Callisto's subsurface ocean. Like Europa and Ganymede, as well as Saturn's moons Enceladus, Mimas, Dione and Titan, a possible subsurface ocean might be composed of salt water.

    It is possible that halophiles could thrive in the ocean. [57] As with Europa and Ganymede, the idea has been raised that habitable conditions and even extraterrestrial microbial life may exist in the salty ocean under the Callistoan surface. [23] 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. [23] Scientist Torrence Johnson said the following about comparing the odds of life on Callisto with the odds on other Galilean moons: [58]

    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 microbial life. [23] [59]

    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. [5] 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. [5] A second round of exploration lasted from 1994 to 2003, when the Galileo 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. [18] In 2000, the Cassini spacecraft en route to Saturn acquired high-quality infrared spectra of the Galilean satellites including Callisto. [34] In February–March 2007, the New Horizons probe on its way to Pluto obtained new images and spectra of Callisto. [60]

    The next planned mission to the Jovian system is the European Space Agency's Jupiter Icy Moon Explorer (JUICE), due to launch in 2022. [61] Several close flybys of Callisto are planned during the mission. [61]

    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's moons. In February 2009 it was announced that ESA/NASA had given this mission priority ahead of the Titan Saturn System Mission. [62] ESA's contribution still faced funding competition from other ESA projects. [63] EJSM consisted of the NASA-led Jupiter Europa Orbiter, the ESA-led Jupiter Ganymede Orbiter, and possibly a JAXA-led Jupiter Magnetospheric Orbiter.

    Potential colonization

    Artist's impression of a base on Callisto Callisto base.PNG
    Artist's impression of a base on Callisto

    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. [24] [65]

    The study proposed a possible surface base on Callisto that would produce rocket propellant for further exploration of the Solar System. [64] 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, 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. [24]

    In December 2003, NASA reported that a manned mission to Callisto might be possible in the 2040s. [66]

    See also

    Notes

    1. Periapsis is derived from the semimajor axis (a) and eccentricity (e): .
    2. Apoapsis is derived from the semimajor axis (a) and eccentricity (e): .
    3. Surface area derived from the radius (r): .
    4. Volume derived from the radius (r): .
    5. Surface gravity derived from the mass (m), the gravitational constant (G) and the radius (r): .
    6. Escape velocity derived from the mass (m), the gravitational constant (G) and the radius (r): .
    7. 1 2 The leading hemisphere is the hemisphere facing the direction of the orbital motion; the trailing hemisphere faces the reverse direction.
    8. The dimensionless moment of inertia referred to is , where I is the moment of inertia, m the mass, and r the maximal radius. It is 0.4 for a homogenous spherical body, but less than 0.4 if density increases with depth.
    9. In the case of icy satellites, palimpsests are defined as bright circular surface features, probably old impact craters [18]
    10. To embay means to shut in, or shelter, as in a bay.

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