CALLISTO (MOON)
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:''This article is about the moon of Jupiter. For other meanings, see Callisto.''
:''There is also an asteroid named '204 Kallisto'.''
'Callisto' ''(kə-lis'-toe,'' ; Greek ''Καλλιστώ)'' is a moon of the planet Jupiter, discovered in 1610 by Galileo Galilei.Galilei, G.; ''Sidereus Nuncius'' (March 13 1610) It is the third largest moon in the Solar System, about 99% the size of the planet Mercury but much less massive. The radius of its orbit is about 1,880,000 kilometers, and Callisto is the fourth Galilean moon of Jupiter by distance from the planet. It is not part of the orbital resonance of the three inner Galilean satellites, which results in the absence of tidal heating. However, Callisto is locked into synchronous rotation with its orbital period, so the same face is always turned toward Jupiter. Because of the distance of Callisto from Jupiter, its surface is less affected by the planetary magnetosphere.
The mean density of Callisto is about 1.83 g/cm³, and it is composed of approximately equal amounts of rocks and ices. The surface of the moon shows spectral signatures of water ice, carbon dioxide, silicates, organics and other compounds. The investigation by Galileo spacecraft revealed that, while Callisto is only partially differentiated, it has a subsurface ocean of liquid water at depths greater than 100 kilometers, and may have a small silicate core in the center.
The surface of Callisto is heavily cratered and extremely old. It does not show any signatures of endogenic processes such as plate tectonics, earthquakes and volcanoes, and is thought to evolve mainly under the influence of impacts. The prominent surface features are multi-ring structures, variously shaped impact craters, and chains of craters – catenae and associated scarps, ridges and deposits. At a small scale the surface is inhomogeneous and consists of small bright frost deposits at the tops of elevations, surrounded by a low-lying, smooth blanket of dark material. Such a surface is thought to result from the sublimation-driven degradation of small landforms. This 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 a thin atmosphere made from carbon dioxide and by a rather thick ionosphere. The atmosphere may also contain significant amounts of molecular oxygen. Callisto is thought to have formed by slow accretion in the disk of the gas and dust that surrounded Jupiter after its formation. The slow accretion and lack of the tidal heating prevented rapid differentiation, and the creation of a rocky core, and icy mantle. The slow convection in the interior of Callisto, which commenced soon after the formation, led to the partial differentiation and formation of the subsurface ocean at the depth 100-150 kilometers.
The presence of the ocean in Callisto indicates that it can or could harbor life. However this is less likely than on Europa. The exploration of Callisto included various space probes from Pioneers10-11 to Galileo and Cassini spacecrafts. Callisto has long been considered as the most suitable place for a human base in the future exploration of the system of Jupiter.
'Callisto' ''(kə-lis'-toe,'' ; Greek ''Καλλιστώ)'' is a moon of the planet Jupiter, discovered in 1610 by Galileo Galilei. Callisto is named after Callisto, one of Zeus's many lovers in Greek mythology.
The name "Callisto" was suggested by Simon Marius soon after the moon's discovery,[2] who attributed the suggestion to Johannes Kepler. Satellites of Jupiter However this name and the names of the other Galilean satellites fell into disfavour 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 simply referred to by its Roman numeral designation — a system introduced by Galileo — 'Jupiter IV' or as "the fourth satellite of Jupiter". Discovery and Observation of a Fifth Satellite to Jupiter, , E. E., Barnard, Astronomical Journal, 1892
In scientific writing, the adjectival form of the name is ''Callistoan'', Geological Evidence for an Ocean on Callisto or ''Callistan''. The form used in this article is ''Callistoan''.
Callisto is the fourth Galilean moon of Jupiter by the distance from the planet. It circles Jupiter at a radius of approximately 1,880,000 km (26.3 Rj, where Rj=71,398 km is the radius of Jupiter), Planetary Satellite Mean Orbital Parameters which is significantly larger than the orbital radius—1,070,000 km—of the previous Galilean satellite Ganymede. As a result of its large orbital radius, Callisto does not participate in the mean motion resonance, which the three inner Galilean satellites—Io, Europa, and Ganymede— are locked in, and probably has never participated. Numerical Simulations of the Orbits of the Galilean Satellites, , Susanna, Musotto, , 2002
Like other close planetary moons, Callisto is locked in the synchronous rotation. The length of the callistoan day is about 16.7 earth days and is equal to the orbital period. Callisto's orbit is slightly eccentric and inclined to the Jovian equator. The eccentricity and inclination of the orbit change periodically under the influence of solar and planetary disturbances; the ranges of orbital change are 0.0072-0.0076 and 0.2-0.06°, respectively.. The orbital variations cause the obliquity (the angle between rotational and orbital axes) to vary between 0.4 and 1.6°; these changes are measured by the scale of centuries. Free and forced obliquities of the Galilean satellites of Jupiter, , Bruce G., Bills, , 2005
The dynamical isolation of Callisto means that it has never been tidally heated, which has had important consequences for its internal structure and evolution. The remoteness of Callisto from Jupiter also means that charged particles flux from the planetary magnetosphere at its surface is relatively low, about 300 times lower than that at Europa. The charged particle irradiation has had a relatively minor effect on the callistoan surface. Energetic Ion and Electron Irradiation of the Icy Galilean Satellites, , John F., Cooper, , 2001
The average density of Callisto – 1.83 g/cm³ implies that it is composed of a mixture of rocky material and water ice with some addition of volatile ices such as ammonia. Internal structure of Europa and Callisto, , O.L., Kuskov, , 2005 The mass fraction of ices lies in the interval 49-55%. The exact composition of rock is not known, but is probably close to the composition of L/LL type ordinary chondrites, which are characterized by less total iron, less metalic iron and more iron oxide. The weight ratio of iron to silicon is 0.9-1.3 in Callisto, whereas the solar ratio is around 1.8.
The composition of the dark (albedo about 20%) callistoan surface generally resembles its bulk composition. Near-infrared spectroscopy showed the presence of water ice absorption bands at the wavelengths: 1.04, 1.25, 1.5, 2.0 and 3.0 micrometers. The water ice seems to be ubiquitous on the surface of Callisto, as the mass fraction of the water ice on the surface is 25-50%. The Galilean Satellites, , Adam P., Showman, Science, 1999 The analysis of the 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, carbon dioxide, sulfur dioxide, Detection of SO2 on Callisto with the Hubble Space Telescope and possibly ammonia and various organic compounds. Spectral data indicate that the planet's surface is extremely inhomogeneous 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 hemisphere — the hemisphere facing the direction of the orbital motion — Leading hemisphere is a hemisphere looking in the direction of the orbital motion, the trailing hemisphere looks in the reverse direction is darker than trailing one. This distinguishes Callisto from other Galilean satellites, where the reverse is true. The trailing hemisphere of Callisto appears enriched in carbon dioxide, while the leading hemisphere has more sulfur dioxide. Distributions of CO2 and SO2 on the Surface of Callisto Many fresh impact craters like Lofn also show enrichment in the carbon dioxide. Overall the chemical composition of the surface, especially in the dark areas, may be close to the D-type asteroids, which are made of carbonaceous material.
Callisto's battered surface lies on top of the cold and stiff icy lithosphere that is about 80-150 kilometers thick. A salty ocean as deep as 50-200 kilometers may lie beneath the crust. Its presence is indicated by studies of the magnetic fields around Jupiter and its moons.[3][4] It was found that Callisto responds to the varying background magnetic field generated by Jupiter like a perfectly conducting sphere, i.e. the field can not penetrate inside the moon. This suggests a presence of a layer of a highly conductive fluid within it with thickness at least 10 km. The existence of the ocean seems to be more likely if water contains a small amount of ammonia (or other antifreeze) up to 5% by weight.[5] In the latter case the ocean can be as thick as 250-300 km.
Beneath the ocean, Callisto seems to have an interior that is not entirely uniform but does not vary dramatically. ''Galileo'' orbiter data, especially the dimensionless moment of inertia – 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.[6] In other words Callisto is only partially differentiated. The callistoan density and moment of inertia are compatible with the existence of a small silicate core in the center of the satellite. The radius of the core may be no more than 600 km, and the density may lie in the interval 3.1-3.6 g/cm³.
The surface of Callisto is very old and appears to be one of the most heavily cratered in the solar system.[7] In fact, the crater density is close to saturation, i.e., any new crater will erase an older one. The large scale geology of Callisto is relatively simple. There are no large callistoan mountains, volcanoes and other endogenic tectonic features.[8] The impact craters, multi-ring structures together with associated fractures, scarps and deposits are the only features to be found on the callistoan surface.
The surface of Callisto can be subdivided into several geologically different parts: cratered plains, light plains, bright smooth plains and various units associated with multi-ring structures and impact craters.[9] The cratered plains constitute most of the surface area and represent the ancient lithosphere consisting of mixture of ice and rocky material. The light plains include bright impact craters like Burr and Lofn, which are termed palimpsests, central parts of multi-ring structures, and isolated patches in the cratered plains. They 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 trough zones of Valhalla and Asgard formations and as isolated spots in the cratered plains. They were supposed 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 square less than 10000 km², which appear to embay (''embay'': to shut in, or shelter, as in a bay) surrounding terrain. They are possible cryovolcanic deposits. Both light and smooth plains are somewhat younger and less cratered.[10]
The surface of Callisto is populated by numerous impact craters. Their diameters range from 0.1 km, which is the limit defined by the imaging resolution, to more than 100 km, not counting the multi-ring structures. The small craters with diameters less than 5 km have simple bowl or flat floored shapes. The craters, which are 5-40 km across, usually have a central peak in the center. The even larger impact features with diameters in the range 25-100 km have central pits instead of peaks, like Tindr crater. The largest craters with diameters more than 60 km can demonstrate central domes, which are thought to result from the central uplift following the impact. The examples are Doh and Har craters. A small number of very large – more 100 km in diameter – and bright impact craters show anomalous dome geometry. They are unusually shallow and may be a transitional landform to the multi-ring structures, which can be in the case of Lofn impact feature. The callistoan craters are generally shallower than those on the Moon.
The largest impact features on the callistoan surface are multi-ring basins. Two of them are enormous. Valhalla is the largest one, with a bright central region that is 600 kilometers in diameter, and rings extending as far as 1800 kilometers from the center (see figure).[11] The second largest multi-ring basin is Asgard, measuring about 1600 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, ocean. Another interesting feature of the callistoan surface is catenae (for example Gomul Catena) – 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, much like Comet Shoemaker-Levy 9) was, before the impacts or by very oblique impacts.
As mentioned above, the surface of Callisto consists of small patches of pure water ice with the albedo as high as 80% surrounded by much darker material. On the high resolution Galileo images the bright patches are predominately found on the elevated surface features: crater rims, scarps, ridges and knobs. They are likely to be thin water frost deposits. The dark material usually lies in the lowlands surrounding and mantling bright features and appears to be smooth. It often forms smooth patches up to 5 km across within the crater floors and in the intercrater depressions.
At the small scale the surface of Callisto is more degraded than the surfaces of other icy galilean moons. It usually exhibits a deficit of the small impact craters with diameters less than 1 km as compared with, for instance, the dark plains on Ganymede. Instead of small craters the almost ubiquitous surface features are small knobs and pits. The knobs are thought to represent remnants of the craters degraded by an undetermined process. Mass Movement and Landform Degradation on the Icy Galilean Satellites: Results of the Galileo Nominal Mission, , Jeffrey M., Moore, , 1999 The most likely candidate is ice the sublimation caused by the relatively high surface temperature, which can reach 165 kelvins at the subsolar point. The sublimation of water or other volatiles from the dirty ice causes decomposition of the icy bedrock. 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 resembles certain martian surface features. In this 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 covered predominantly icy surface.
The relative ages of the different surface units on Callisto can be determined by counting the number of impact craters on them. The older is the surface, the denser is the crater population. Populations of Small Craters on Europa, Ganymede, and Callisto: Initial Galileo Imaging Results The absolute ages of surface features on Callisto are not known. 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 cratering rates and are estimated by different authors to vary between 1 and 4 billion years.
Callisto has a very tenuous atmosphere composed of carbon dioxide.[12] It was detected by the ''Galileo'' Near Infrared Mapping Spectrometer (NIMS) by its absorption feature near the wavelength 4.2 micrometers. The surface pressure is estimated to be 7.5 pbar and density – 4 cm-3. Because such a thin atmosphere would be lost in ~4 days ''(see atmospheric escape)'', scientists believe that it is constantly replenished, possibly by slow sublimation of carbon dioxide ice from the satellite's icy crust.
Callisto has an ionosphere, which was detected during ''Galileo'' flybys.[13] It appears to have rather a high electron density of 7-17 cm-3, which cannot be explained by the photoionization of carbon dioxide alone. So it is possible that the atmosphere of Callisto contains significant (10-100 times more than ) amounts of molecular oxygen.[14]
However oxygen has not been directly detected yet in the atmosphere of Callisto. The observations of the Hubble Space Telescope (HST) only placed an upper limit on its possible concentration in the atmosphere, which is higher that follows from the ionospheric measurements. Hubble Space Telescope Space Telescope Imaging Spectrograph Search for an Atmosphere on Callisto: a Jovian Unipolar Inductor, , Darrell F., Strobel, , 2002 At the same time HST was able to detect condensed oxygen trapped in the surface of Callisto. Condensed O2 on Europa and Callisto, , John R., Spencer, , 2002
The partial differentiation of Callisto means that it has never been heated considerably. Therefore the most favorable model of its formation is a slow accretion in the low density Jupiter’s subnebula — a disk of the gas and dust that existed around Jupiter after its formation. The possible timescale of formation of Callisto lies in the range 0.1-10 million years. The prolonged accretion stage prevented accumulation of the gravitational energy inside the moon, its heating and fast differentiation. Formation of the Galilean Satellites: Conditions of Accretion, , Robin M., Canup, , 2002
The evolution of Callisto after accretion was determined by the balance of the radioactive heating and cooling through the thermal conduction and solid state or subsolidus convection. Non-Newtonian stagnant lid convection and the thermal evolution of Ganymede and Callisto, , J., Freeman, , 2006 The subsolidus convection in the ice is the main source of uncertanty in the models of all icy moons. It is known to develop when the temperature is sufficiently close to the melting point, which is explained by temperature dependence of the ice viscosity. The convection in icy bodies is slow process with speeds of ice motions of order of 1 cm/year and is, in fact, a very effective cooling mechanism. On convection in ice I shells of outer Solar System bodies, with detailed application to Callisto, , William B., McKinnon, , 2006 The convection in the icy moons is thought to proceed in stagnant lid regime, when a cold outer layer of the moon, with thickness about 100 km in the case of Callisto, is conductive, not convective. This layer corresponds to the cold and rigid lithosphere and explains 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 a high pressure water ice exists in different modifications beginning from the ice I on the surface to ice VII in the center. The early onset of subsolidus convection in the callistoan interior prevented large scale ice melting and rapid differentiation with formation of a rocky core and icy mantle. Separation of rocks and ices inside Callisto has been proceeding slowly on the timescale of billion years and may be continuing even now. A model for the interior structure, evolution, and differentiation of Callisto, , K.a, Nagel, , 2004
The current understanding of the evolution of Callisto allows existence of a layer of liquid water in its interior. This is connected with anomalous behavior of the Ice I melting temperature, which decreases with pressure, achieving as low as 251 kelvins at 207 megapascals. In all realistic models of Callisto the temperature in the layer between 100-200 km in depth is very close or exceeds lightly the melting temperature. The presence of even small amounts of ammonia – about 1-2 weight% – almost guarantees its existence.
While Callisto is very similar in bulk properties to Ganymede, it apparently had a much simpler geological history. The surface of Callisto seems to have formed mainly under the influence of impacts and other exogenic forces. Unlike neighbouring Ganymede with its grooved terrain, there is little evidence of the tectonic activity. The different geological histories of the two satellites has been an important problem for planetary scientists. "Simple" Callisto is a good reference for comparison with other more complex worlds.
Like with Europa and Ganymede, the idea has been brought up that extraterrestrial microbial life may exist in the salty ocean under the Callistoan surface. Astrobiology of Jupiter’s Icy Moons, , Jere H., Lipps, , 2004 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 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: Callisto makes a big splash
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. Exo-Astrobiological Aspects of Europa and Titan: from Observations to speculations, , Raulin, François, , 2005
The Pioneer 10 and Pioneer 11 Jupiter encounters in the early 1970s contributed little new information about Callisto. The real breakthrough happened later with Voyager 1&2 flybys in 1979-1980. They imaged more than half of the callistoan surface with resolution 1-2 km, measured its temperature, mass and shape. From 1994 to 2003 Galileo spacecraft had eight close encounters with Callisto, the last one during C30 orbit in 2001 was as close as 138 km. Galileo orbiter completed the global imaging of the surface and delivered a number of pictures with resolution as high as 15 meters of the selected areas of Callisto. Unfortunately Callisto wasn’t a primary target of Galileo, which was handicapped in its ability to transmit high resolution images. In 2000 Cassini spacecraft enroute to Saturn acquired high quality infrared spectra of Galilean satellites including Callisto. Observations with the Visual and Infrared Mapping Spectrometer (VIMS) during Cassini’s Flyby of Jupiter, , R. H., Brown, Icarus, 2003 In February-March 2007 New Horizons probe on its way to Pluto obtained new images and spectra of Callisto. Ring Leader, , F., Morring, Aviation Week&Space Technology,
In 2003 NASA performed a study called HOPE (Human Outer Planets Exploration) regarding the future exploration of the outer solar system. The target chosen was Callisto. Revolutionary Concepts for Human Outer Planet Exploration(HOPE) The mission deals with the idea of human hibernation. Scientists study space hibernation It could be possible to build a surface base that would produce fuel for further exploration of the solar system.[15] The radiation is low at the distance of Callisto and it is also geologically very stable. So it could be a suitable place for a base. This base would also be a centre for exploration of the Jovian system, for example remote exploration of Europa. It would also be the ideal location for a Jovian system way station that could service spacecraft headed farther into the outer Solar System, using a gravity assist from a close fly-by of Jupiter after departing Callisto.
★ List of craters on Callisto
★ List of geological features on Callisto
★ Colonization of Callisto
★ Callisto in fiction.''
1. Shape, mean radius, gravity field and interior structure of Callisto, , J. D., Anderson, Icarus, 2001
2. Marius, S.; ''Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici'' (1614)
3. Induced magnetic fields as evidence for subsurface oceans in Europa and Callisto, , K. K., Khurana, Nature, 1998
4. Subsurface Oceans on Europa and Callisto: Constraints from Galileo Magnetometer Observations, , C., Zimmer, Icarus, 2000
5. Oceans in the icy Galilean satellites of Jupiter?, , T., Spohn, Icarus, 2003
6. Distribution of Rock, Metals and Ices in Callisto, , J. D., Anderson, Science, 1998
7. Cratering Rates on the Galilean Satellites, , K., Zahnle, Icarus, 1998
8.
9. Galileo views of the geology of Callisto, , R., Greeley, Planetary and Space Science, 2000
10.
11.
12. A Tenuous Carbon Dioxide Atmosphere on Jupiter's Moon Callisto, , R. W., Carlson, Science, 1999
13. Ionosphere of Callisto from Galileo radio occultation observations, , A. J., Kliore, Journal of Geophysics Research, 2002
14. Atmosphere of Callisto, , M. C., Liang, Journal of Geophysics Research, 2005
15. Vision for Space Exploration
★ Callisto Profile by NASA's Solar System Exploration
''... | Ganymede | 'Callisto' | Themisto | ...''
:''There is also an asteroid named '204 Kallisto'.''
'Callisto' ''(kə-lis'-toe,'' ; Greek ''Καλλιστώ)'' is a moon of the planet Jupiter, discovered in 1610 by Galileo Galilei.Galilei, G.; ''Sidereus Nuncius'' (March 13 1610) It is the third largest moon in the Solar System, about 99% the size of the planet Mercury but much less massive. The radius of its orbit is about 1,880,000 kilometers, and Callisto is the fourth Galilean moon of Jupiter by distance from the planet. It is not part of the orbital resonance of the three inner Galilean satellites, which results in the absence of tidal heating. However, Callisto is locked into synchronous rotation with its orbital period, so the same face is always turned toward Jupiter. Because of the distance of Callisto from Jupiter, its surface is less affected by the planetary magnetosphere.
The mean density of Callisto is about 1.83 g/cm³, and it is composed of approximately equal amounts of rocks and ices. The surface of the moon shows spectral signatures of water ice, carbon dioxide, silicates, organics and other compounds. The investigation by Galileo spacecraft revealed that, while Callisto is only partially differentiated, it has a subsurface ocean of liquid water at depths greater than 100 kilometers, and may have a small silicate core in the center.
The surface of Callisto is heavily cratered and extremely old. It does not show any signatures of endogenic processes such as plate tectonics, earthquakes and volcanoes, and is thought to evolve mainly under the influence of impacts. The prominent surface features are multi-ring structures, variously shaped impact craters, and chains of craters – catenae and associated scarps, ridges and deposits. At a small scale the surface is inhomogeneous and consists of small bright frost deposits at the tops of elevations, surrounded by a low-lying, smooth blanket of dark material. Such a surface is thought to result from the sublimation-driven degradation of small landforms. This 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 a thin atmosphere made from carbon dioxide and by a rather thick ionosphere. The atmosphere may also contain significant amounts of molecular oxygen. Callisto is thought to have formed by slow accretion in the disk of the gas and dust that surrounded Jupiter after its formation. The slow accretion and lack of the tidal heating prevented rapid differentiation, and the creation of a rocky core, and icy mantle. The slow convection in the interior of Callisto, which commenced soon after the formation, led to the partial differentiation and formation of the subsurface ocean at the depth 100-150 kilometers.
The presence of the ocean in Callisto indicates that it can or could harbor life. However this is less likely than on Europa. The exploration of Callisto included various space probes from Pioneers10-11 to Galileo and Cassini spacecrafts. Callisto has long been considered as the most suitable place for a human base in the future exploration of the system of Jupiter.
Discovery and naming
'Callisto' ''(kə-lis'-toe,'' ; Greek ''Καλλιστώ)'' is a moon of the planet Jupiter, discovered in 1610 by Galileo Galilei. Callisto is named after Callisto, one of Zeus's many lovers in Greek mythology.
The name "Callisto" was suggested by Simon Marius soon after the moon's discovery,[2] who attributed the suggestion to Johannes Kepler. Satellites of Jupiter However this name and the names of the other Galilean satellites fell into disfavour 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 simply referred to by its Roman numeral designation — a system introduced by Galileo — 'Jupiter IV' or as "the fourth satellite of Jupiter". Discovery and Observation of a Fifth Satellite to Jupiter, , E. E., Barnard, Astronomical Journal, 1892
In scientific writing, the adjectival form of the name is ''Callistoan'', Geological Evidence for an Ocean on Callisto or ''Callistan''. The form used in this article is ''Callistoan''.
Orbit and rotation
Callisto is the fourth Galilean moon of Jupiter by the distance from the planet. It circles Jupiter at a radius of approximately 1,880,000 km (26.3 Rj, where Rj=71,398 km is the radius of Jupiter), Planetary Satellite Mean Orbital Parameters which is significantly larger than the orbital radius—1,070,000 km—of the previous Galilean satellite Ganymede. As a result of its large orbital radius, Callisto does not participate in the mean motion resonance, which the three inner Galilean satellites—Io, Europa, and Ganymede— are locked in, and probably has never participated. Numerical Simulations of the Orbits of the Galilean Satellites, , Susanna, Musotto, , 2002
Like other close planetary moons, Callisto is locked in the synchronous rotation. The length of the callistoan day is about 16.7 earth days and is equal to the orbital period. Callisto's orbit is slightly eccentric and inclined to the Jovian equator. The eccentricity and inclination of the orbit change periodically under the influence of solar and planetary disturbances; the ranges of orbital change are 0.0072-0.0076 and 0.2-0.06°, respectively.. The orbital variations cause the obliquity (the angle between rotational and orbital axes) to vary between 0.4 and 1.6°; these changes are measured by the scale of centuries. Free and forced obliquities of the Galilean satellites of Jupiter, , Bruce G., Bills, , 2005
The dynamical isolation of Callisto means that it has never been tidally heated, which has had important consequences for its internal structure and evolution. The remoteness of Callisto from Jupiter also means that charged particles flux from the planetary magnetosphere at its surface is relatively low, about 300 times lower than that at Europa. The charged particle irradiation has had a relatively minor effect on the callistoan surface. Energetic Ion and Electron Irradiation of the Icy Galilean Satellites, , John F., Cooper, , 2001
Physical characteristics
Composition
Near-infrared spectrum of a cratered palins area (Courtesy NASA/JPL-Caltech)
The average density of Callisto – 1.83 g/cm³ implies that it is composed of a mixture of rocky material and water ice with some addition of volatile ices such as ammonia. Internal structure of Europa and Callisto, , O.L., Kuskov, , 2005 The mass fraction of ices lies in the interval 49-55%. The exact composition of rock is not known, but is probably close to the composition of L/LL type ordinary chondrites, which are characterized by less total iron, less metalic iron and more iron oxide. The weight ratio of iron to silicon is 0.9-1.3 in Callisto, whereas the solar ratio is around 1.8.
The composition of the dark (albedo about 20%) callistoan surface generally resembles its bulk composition. Near-infrared spectroscopy showed the presence of water ice absorption bands at the wavelengths: 1.04, 1.25, 1.5, 2.0 and 3.0 micrometers. The water ice seems to be ubiquitous on the surface of Callisto, as the mass fraction of the water ice on the surface is 25-50%. The Galilean Satellites, , Adam P., Showman, Science, 1999 The analysis of the 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, carbon dioxide, sulfur dioxide, Detection of SO2 on Callisto with the Hubble Space Telescope and possibly ammonia and various organic compounds. Spectral data indicate that the planet's surface is extremely inhomogeneous 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 hemisphere — the hemisphere facing the direction of the orbital motion — Leading hemisphere is a hemisphere looking in the direction of the orbital motion, the trailing hemisphere looks in the reverse direction is darker than trailing one. This distinguishes Callisto from other Galilean satellites, where the reverse is true. The trailing hemisphere of Callisto appears enriched in carbon dioxide, while the leading hemisphere has more sulfur dioxide. Distributions of CO2 and SO2 on the Surface of Callisto Many fresh impact craters like Lofn also show enrichment in the carbon dioxide. Overall the chemical composition of the surface, especially in the dark areas, may be close to the D-type asteroids, which are made of carbonaceous material.
Internal structure
Interior of Callisto (Courtesy NASA/JPL-Caltech)
Callisto's battered surface lies on top of the cold and stiff icy lithosphere that is about 80-150 kilometers thick. A salty ocean as deep as 50-200 kilometers may lie beneath the crust. Its presence is indicated by studies of the magnetic fields around Jupiter and its moons.[3][4] It was found that Callisto responds to the varying background magnetic field generated by Jupiter like a perfectly conducting sphere, i.e. the field can not penetrate inside the moon. This suggests a presence of a layer of a highly conductive fluid within it with thickness at least 10 km. The existence of the ocean seems to be more likely if water contains a small amount of ammonia (or other antifreeze) up to 5% by weight.[5] In the latter case the ocean can be as thick as 250-300 km.
Beneath the ocean, Callisto seems to have an interior that is not entirely uniform but does not vary dramatically. ''Galileo'' orbiter data, especially the dimensionless moment of inertia – 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.[6] In other words Callisto is only partially differentiated. The callistoan density and moment of inertia are compatible with the existence of a small silicate core in the center of the satellite. The radius of the core may be no more than 600 km, and the density may lie in the interval 3.1-3.6 g/cm³.
Surface features
Cratered plains on Callisto (Courtesy NASA/JPL-Caltech)
The surface of Callisto is very old and appears to be one of the most heavily cratered in the solar system.[7] In fact, the crater density is close to saturation, i.e., any new crater will erase an older one. The large scale geology of Callisto is relatively simple. There are no large callistoan mountains, volcanoes and other endogenic tectonic features.[8] The impact craters, multi-ring structures together with associated fractures, scarps and deposits are the only features to be found on the callistoan surface.
The surface of Callisto can be subdivided into several geologically different parts: cratered plains, light plains, bright smooth plains and various units associated with multi-ring structures and impact craters.[9] The cratered plains constitute most of the surface area and represent the ancient lithosphere consisting of mixture of ice and rocky material. The light plains include bright impact craters like Burr and Lofn, which are termed palimpsests, central parts of multi-ring structures, and isolated patches in the cratered plains. They 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 trough zones of Valhalla and Asgard formations and as isolated spots in the cratered plains. They were supposed 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 square less than 10000 km², which appear to embay (''embay'': to shut in, or shelter, as in a bay) surrounding terrain. They are possible cryovolcanic deposits. Both light and smooth plains are somewhat younger and less cratered.[10]
Impact crater Har on Callisto with a central dome (Courtesy NASA/JPL-Caltech)
The surface of Callisto is populated by numerous impact craters. Their diameters range from 0.1 km, which is the limit defined by the imaging resolution, to more than 100 km, not counting the multi-ring structures. The small craters with diameters less than 5 km have simple bowl or flat floored shapes. The craters, which are 5-40 km across, usually have a central peak in the center. The even larger impact features with diameters in the range 25-100 km have central pits instead of peaks, like Tindr crater. The largest craters with diameters more than 60 km can demonstrate central domes, which are thought to result from the central uplift following the impact. The examples are Doh and Har craters. A small number of very large – more 100 km in diameter – and bright impact craters show anomalous dome geometry. They are unusually shallow and may be a transitional landform to the multi-ring structures, which can be in the case of Lofn impact feature. The callistoan craters are generally shallower than those on the Moon.
Valhalla multi-ring structure, on Callisto (Courtesy NASA/JPL-Caltech)
The largest impact features on the callistoan surface are multi-ring basins. Two of them are enormous. Valhalla is the largest one, with a bright central region that is 600 kilometers in diameter, and rings extending as far as 1800 kilometers from the center (see figure).[11] The second largest multi-ring basin is Asgard, measuring about 1600 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, ocean. Another interesting feature of the callistoan surface is catenae (for example Gomul Catena) – 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, much like Comet Shoemaker-Levy 9) was, before the impacts or by very oblique impacts.
As mentioned above, the surface of Callisto consists of small patches of pure water ice with the albedo as high as 80% surrounded by much darker material. On the high resolution Galileo images the bright patches are predominately found on the elevated surface features: crater rims, scarps, ridges and knobs. They are likely to be thin water frost deposits. The dark material usually lies in the lowlands surrounding and mantling bright features and appears to be smooth. It often forms smooth patches up to 5 km across within the crater floors and in the intercrater depressions.
Landslides termed debris aprons and small knobs on the surface of Callisto (Courtesy NASA/JPL-Caltech)
At the small scale the surface of Callisto is more degraded than the surfaces of other icy galilean moons. It usually exhibits a deficit of the small impact craters with diameters less than 1 km as compared with, for instance, the dark plains on Ganymede. Instead of small craters the almost ubiquitous surface features are small knobs and pits. The knobs are thought to represent remnants of the craters degraded by an undetermined process. Mass Movement and Landform Degradation on the Icy Galilean Satellites: Results of the Galileo Nominal Mission, , Jeffrey M., Moore, , 1999 The most likely candidate is ice the sublimation caused by the relatively high surface temperature, which can reach 165 kelvins at the subsolar point. The sublimation of water or other volatiles from the dirty ice causes decomposition of the icy bedrock. 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 resembles certain martian surface features. In this 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 covered predominantly icy surface.
The relative ages of the different surface units on Callisto can be determined by counting the number of impact craters on them. The older is the surface, the denser is the crater population. Populations of Small Craters on Europa, Ganymede, and Callisto: Initial Galileo Imaging Results The absolute ages of surface features on Callisto are not known. 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 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.[12] It was detected by the ''Galileo'' Near Infrared Mapping Spectrometer (NIMS) by its absorption feature near the wavelength 4.2 micrometers. The surface pressure is estimated to be 7.5 pbar and density – 4 cm-3. Because such a thin atmosphere would be lost in ~4 days ''(see atmospheric escape)'', scientists believe that it is constantly replenished, possibly by slow sublimation of carbon dioxide ice from the satellite's icy crust.
Callisto has an ionosphere, which was detected during ''Galileo'' flybys.[13] It appears to have rather a high electron density of 7-17 cm-3, which cannot be explained by the photoionization of carbon dioxide alone. So it is possible that the atmosphere of Callisto contains significant (10-100 times more than ) amounts of molecular oxygen.[14]
However oxygen has not been directly detected yet in the atmosphere of Callisto. The observations of the Hubble Space Telescope (HST) only placed an upper limit on its possible concentration in the atmosphere, which is higher that follows from the ionospheric measurements. Hubble Space Telescope Space Telescope Imaging Spectrograph Search for an Atmosphere on Callisto: a Jovian Unipolar Inductor, , Darrell F., Strobel, , 2002 At the same time HST was able to detect condensed oxygen trapped in the surface of Callisto. Condensed O2 on Europa and Callisto, , John R., Spencer, , 2002
Origin and evolution
Knobby terrain on Callisto (credit: NASA/JPL/Arizona State University)
The partial differentiation of Callisto means that it has never been heated considerably. Therefore the most favorable model of its formation is a slow accretion in the low density Jupiter’s subnebula — a disk of the gas and dust that existed around Jupiter after its formation. The possible timescale of formation of Callisto lies in the range 0.1-10 million years. The prolonged accretion stage prevented accumulation of the gravitational energy inside the moon, its heating and fast differentiation. Formation of the Galilean Satellites: Conditions of Accretion, , Robin M., Canup, , 2002
The evolution of Callisto after accretion was determined by the balance of the radioactive heating and cooling through the thermal conduction and solid state or subsolidus convection. Non-Newtonian stagnant lid convection and the thermal evolution of Ganymede and Callisto, , J., Freeman, , 2006 The subsolidus convection in the ice is the main source of uncertanty in the models of all icy moons. It is known to develop when the temperature is sufficiently close to the melting point, which is explained by temperature dependence of the ice viscosity. The convection in icy bodies is slow process with speeds of ice motions of order of 1 cm/year and is, in fact, a very effective cooling mechanism. On convection in ice I shells of outer Solar System bodies, with detailed application to Callisto, , William B., McKinnon, , 2006 The convection in the icy moons is thought to proceed in stagnant lid regime, when a cold outer layer of the moon, with thickness about 100 km in the case of Callisto, is conductive, not convective. This layer corresponds to the cold and rigid lithosphere and explains 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 a high pressure water ice exists in different modifications beginning from the ice I on the surface to ice VII in the center. The early onset of subsolidus convection in the callistoan interior prevented large scale ice melting and rapid differentiation with formation of a rocky core and icy mantle. Separation of rocks and ices inside Callisto has been proceeding slowly on the timescale of billion years and may be continuing even now. A model for the interior structure, evolution, and differentiation of Callisto, , K.a, Nagel, , 2004
The current understanding of the evolution of Callisto allows existence of a layer of liquid water in its interior. This is connected with anomalous behavior of the Ice I melting temperature, which decreases with pressure, achieving as low as 251 kelvins at 207 megapascals. In all realistic models of Callisto the temperature in the layer between 100-200 km in depth is very close or exceeds lightly the melting temperature. The presence of even small amounts of ammonia – about 1-2 weight% – almost guarantees its existence.
While Callisto is very similar in bulk properties to Ganymede, it apparently had a much simpler geological history. The surface of Callisto seems to have formed mainly under the influence of impacts and other exogenic forces. Unlike neighbouring Ganymede with its grooved terrain, there is little evidence of the tectonic activity. The different geological histories of the two satellites has been an important problem for planetary scientists. "Simple" Callisto is a good reference for comparison with other more complex worlds.
Possibility of life in the ocean
Like with Europa and Ganymede, the idea has been brought up that extraterrestrial microbial life may exist in the salty ocean under the Callistoan surface. Astrobiology of Jupiter’s Icy Moons, , Jere H., Lipps, , 2004 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 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: Callisto makes a big splash
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. Exo-Astrobiological Aspects of Europa and Titan: from Observations to speculations, , Raulin, François, , 2005
Exploration
The Pioneer 10 and Pioneer 11 Jupiter encounters in the early 1970s contributed little new information about Callisto. The real breakthrough happened later with Voyager 1&2 flybys in 1979-1980. They imaged more than half of the callistoan surface with resolution 1-2 km, measured its temperature, mass and shape. From 1994 to 2003 Galileo spacecraft had eight close encounters with Callisto, the last one during C30 orbit in 2001 was as close as 138 km. Galileo orbiter completed the global imaging of the surface and delivered a number of pictures with resolution as high as 15 meters of the selected areas of Callisto. Unfortunately Callisto wasn’t a primary target of Galileo, which was handicapped in its ability to transmit high resolution images. In 2000 Cassini spacecraft enroute to Saturn acquired high quality infrared spectra of Galilean satellites including Callisto. Observations with the Visual and Infrared Mapping Spectrometer (VIMS) during Cassini’s Flyby of Jupiter, , R. H., Brown, Icarus, 2003 In February-March 2007 New Horizons probe on its way to Pluto obtained new images and spectra of Callisto. Ring Leader, , F., Morring, Aviation Week&Space Technology,
In 2003 NASA performed a study called HOPE (Human Outer Planets Exploration) regarding the future exploration of the outer solar system. The target chosen was Callisto. Revolutionary Concepts for Human Outer Planet Exploration(HOPE) The mission deals with the idea of human hibernation. Scientists study space hibernation It could be possible to build a surface base that would produce fuel for further exploration of the solar system.[15] The radiation is low at the distance of Callisto and it is also geologically very stable. So it could be a suitable place for a base. This base would also be a centre for exploration of the Jovian system, for example remote exploration of Europa. It would also be the ideal location for a Jovian system way station that could service spacecraft headed farther into the outer Solar System, using a gravity assist from a close fly-by of Jupiter after departing Callisto.
See also
★ List of craters on Callisto
★ List of geological features on Callisto
★ Colonization of Callisto
★ Callisto in fiction.''
Notes and references
1. Shape, mean radius, gravity field and interior structure of Callisto, , J. D., Anderson, Icarus, 2001
2. Marius, S.; ''Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici'' (1614)
3. Induced magnetic fields as evidence for subsurface oceans in Europa and Callisto, , K. K., Khurana, Nature, 1998
4. Subsurface Oceans on Europa and Callisto: Constraints from Galileo Magnetometer Observations, , C., Zimmer, Icarus, 2000
5. Oceans in the icy Galilean satellites of Jupiter?, , T., Spohn, Icarus, 2003
6. Distribution of Rock, Metals and Ices in Callisto, , J. D., Anderson, Science, 1998
7. Cratering Rates on the Galilean Satellites, , K., Zahnle, Icarus, 1998
8.
9. Galileo views of the geology of Callisto, , R., Greeley, Planetary and Space Science, 2000
10.
11.
12. A Tenuous Carbon Dioxide Atmosphere on Jupiter's Moon Callisto, , R. W., Carlson, Science, 1999
13. Ionosphere of Callisto from Galileo radio occultation observations, , A. J., Kliore, Journal of Geophysics Research, 2002
14. Atmosphere of Callisto, , M. C., Liang, Journal of Geophysics Research, 2005
15. Vision for Space Exploration
External links
★ Callisto Profile by NASA's Solar System Exploration
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