Callisto , or Jupiter IVis 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 that may not be properly distinguished. Callisto was discovered in 1610 by Galileo Galilei. At 4821 km in diameter, Callisto has about 99% of the diameter of the planet Mercury, but only about one-third of its mass. It is Jupiter’s fourth Galilean moon in the distance, with an orbital radius of about 1 883 000 km .  It is not in orbital resonance like the other three Galilean satellites – Io, Europa, and Ganymede – and is therefore not significantly heated.  Callisto’s rotation is tidally locked to its orbit around Jupiter, so that the same hemisphere always points inward. For this reason, there is a subjovian point on Callisto’s surface where Jupiter appears to hang directly above it. It is less affected by Jupiter’s magnetosphere than the other inner satellites because it is farther out of Jupiter’s main radiation belt.  
Callisto consists of approximately equal amounts of rock and ice with a density of about 1.83 g / cm 3 , the lowest density and surface gravity of Jupiter’s main moons. Compounds detected spectroscopically on the surface include water ice,  carbon dioxide, silicates, and organic compounds. Investigation by the Galileo Spacecraft revealed that Callisto may have a small silicate core and possibly a subsurface ocean of liquid water  at depths greater than 100 km .  
Callisto’s surface is the oldest and most cratered in the solar system.  Its surface is completely covered with impact craters.  It shows no signatures of subsurface processes such as plate tectonics or volcanism, with no evidence that geologic activity in general has ever occurred, and is thought to have developed predominantly under the influence of shocks.  Prominent surface features include multi-ring structures, differently shaped impact craters, and crater chains (Catenae) and associated scarps, ridges, and deposits.  On a small scale, the surface is diverse, consisting of small, sparkling frost deposits at the tops of high places surrounded by a low-lying, smooth blanket of dark material.  This is thought to be due to sublimation-induced degradation of small landforms, supported by the general deficit of small impact craters and the presence of numerous small knobs thought to be their remnants.  The absolute age of the landforms is not known.
Callisto is surrounded by an extremely thin atmosphere of carbon dioxide  and probably molecular oxygen,  as well as by a fairly intense ionosphere.  Callisto is thought to have formed from the disk by slow accretion of gas and dust that surrounded Jupiter after its formation.  Callisto’s gradual accretion and lack of tidal heating meant that there was insufficient heat available for rapid differentiation. Slow convection in Callisto’s interior, which began shortly after formation, led to partial differentiation and possibly the formation of a subsurface ocean at a depth of 100-150 km and a small rocky core. 
The likely presence of an ocean in Callisto leaves open the possibility that it could harbor life. However, conditions are believed to be less favorable than in nearby Europa.  Various space probes of Pioneers 10 and 11 To Galileo and Cassini had studied Callisto. Because of its low radiation, Callisto has long been considered the most suitable site for a human base for future exploration of the Jupiter system. 
Callisto was discovered by Galileo in January 1610 along with the three other major Jovian moons – Ganymede, Io, and Europa. 
Callisto is named after one of the many lovers of Zeus in Greek mythology. Callisto was a nymph (or according to some sources, the daughter of Lycaon) associated with the goddess of the hunt, Artemis.  The name was suggested by Simon Marius shortly after Callisto’s discovery.  Marius attributed the proposal to Johannes Kepler. 
… autem celeantur tres fœminæ virgins, quarum furtivo amore Iupiter captus& positus est … Calisto Lycaonis … filia … à me vocatur … Quartus denique Calisto … [Io,] Europa, Ganimedes Puer, Atque Calisto, Lascivo Nimium Perplacuere Jovi.
… three young women captured by Jupiter for secret love are to be honored, [including] Callisto, the daughter of Lycaon … At last the fourth [moon] is named by me Callisto … Io, Europa, the boy Ganymede, and Callisto rejoiced greatly in the lustful Jupiter. 
However, the names of the Galilean satellites fell out of favor for a long time, and were not used until the mid-20th century. Callistan was revived in common usage at the beginning of the nineteenth 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.". 
There is no established English adjectival form of the name. The adjectival form of the Greek Καλλιστῴ Kallistōi Καλλιστῴος is Kallistōi-os, from which one might expect Latin Callistōius and English * Callistoian, parallel to Sapphoian for Sapphōᵢ  and Letoian for Lētōᵢ.  In such Greek names, however, the iota index is often omitted (cf. Inoan  of Īnōᵢ  and Argoan [ 33 ] from Argōᵢ  ) and, in fact, the analogous form Callistoan found.   
A second oblique stem appears in Virgil in Latin: Callistōn-,  but the corresponding Callistonian Has rarely appeared in English.  One also sees Ad hoc Shapes such as Callistan,  Callistian  and Callistean.  
Orbit and rotation[ edit]
Callisto is the outermost of the four Galilean moons of Jupiter. It orbits at a distance of about 1 880 000 km (26.3 times the radius of 71 492 km of Jupiter itself).  This is significantly larger than the orbital radius (1 070 000 km) of the nearest Galilean satellite, Ganymede. As a result of this relatively distant orbit, Callisto does not participate in the resonance of the mean motion in which the three inner Galilean satellites are included, and probably never did so. 
Like most other regular planetary moons, Callisto’s rotation is locked to be synchronous with its orbit.  The length of Callisto’s day, at the same time its orbital period, is about 16.7 Earth days. Its orbit is very slightly eccentric and inclined to Jupiter’s equator, with the eccentricity and inclination changing quasi-periodically on a timescale of centuries due to gravitational perturbations of the Sun and the planet. The ranges of change are 0.0072-0.0076, respectively. 0,20-0,60 °.  These orbital variations cause the axial inclination (the angle between the rotation and orbital axes) to vary between 0.4 and 1.6 °. 
Callisto’s dynamic isolation means that it has never been significantly heated, which has important consequences for its internal structure and evolution.  The distance from Jupiter also means that the flux of charged particles from Jupiter’s magnetosphere to its surface is relatively low – about 300 times lower than, say, Europa’s. Unlike the other Galilean moons, irradiation with charged particles therefore had a relatively small effect on the surface of Callisto.  The radiation level at the surface of Callisto corresponds to a dose of about 0.01 rem (0.1 mSv) per day, which is more than ten times the average background radiation of the Earth.  
Physical properties[ edit]
Callisto’s average density is 1.83 g / cm 3 ,  suggests a composition of approximately equal parts of rocky material and water ice with some additional volatile ice such as ammonia.  The mass fraction of ice is 49-55%.   The exact composition of the Callisto rock component is not known, but is probably close to the composition of ordinary L / LL-type chondrites.  characterized by less total iron, less metallic iron, and more iron oxide than H chondrites. The weight ratio of iron to silicon in Callisto is 0.9-1.3, while the solar ratio is 1: 8. 
Callisto’s surface has an albedo of about 20%.  It is believed that its surface composition is broadly similar to its overall composition. Near-infrared spectroscopy has shown the presence of water ice absorption bands at wavelengths of 1.04, 1.25, 1.5, 2.0, and 3.0 microns.  Water ice appears to be ubiquitous on the surface of Callisto with a mass fraction of 25-50%.  Analysis of high-resolution near-infrared and UV spectra obtained by the Galileo Spacecraft and from the ground have detected various non-ice materials: magnesium and iron-containing hydrated silicates,  carbon dioxide,  sulfur dioxide,  and possibly ammonia and various organic compounds.   Spectral data show that the surface of Callisto is extremely heterogeneous on a small scale. Small, bright patches of pure water ice are mixed with patches of a rock-ice mixture and extensive dark areas of non-ice material.  
The callistoan surface is asymmetric: the anterior hemisphere is darker than the posterior one. This differs from other Galilean satellites where the opposite is true.  Callisto’s posterior hemisphere appears to be enriched in carbon dioxide, while the leading hemisphere contains more sulfur dioxide.  Many fresh impact craters such as Lofn also show enrichment in carbon dioxide.  Overall, the chemical composition of the surface, especially in the dark areas, may be close to that observed for D-type asteroids.  whose surfaces are composed of carbonaceous material.
Internal structure[ edit ]
Callisto’s battered surface lies on a cold, stiff, and icy lithosphere that is between 80 and 150 km thick.   A salty ocean with a depth of 150 to 200 km may lie beneath the crust.   indicated by studies of the magnetic fields around Jupiter and its moons.   Callisto has been found to respond to Jupiter’s changing background magnetic field like a perfectly conducting sphere; That is, the field cannot penetrate Callisto, suggesting a layer of highly conductive fluid at least 10 km thick.  The existence of an ocean is more likely when water contains a small amount of ammonia or other antifreeze, up to 5 wt .-%.  In this case the water + ice layer can be up to 250-300 km thick.  In the absence of an ocean, the icy lithosphere can be slightly thicker, up to 300 km.
Beneath the lithosphere and presumed ocean, Callisto’s interior appears to be neither completely uniform nor particularly variable. Galileo Orbiter data  (especially the dimensionless moment of inertia [h] – 0.3549 ± 0.0042 (determined during close flybys) suggest that Callisto is composed of compressed rock and ice in hydrostatic equilibrium, with the amount of rock increasing with depth due to partial deposition of its constituents.   In other words, Callisto can only be partially differentiated. The density and moment of inertia of a Callisto equilibrium are consistent with the existence of a small silicate core at the center of Callisto. The radius of such a core may not exceed 600 km, and the density may range from 3.1 to 3.6 g / cm 3 .   In this case, Callisto’s interior would contrast sharply with that of Ganymede, which appears to be completely differentiated.  
However, a re-analysis of Galileo data in 2011 suggests that Callisto is not in hydrostatic equilibrium. Its S22 coefficient from gravity data is anomalous 10% of its C22 value, which is inconsistent with a body in hydrostatic equilibrium and thus significantly increases the error bars in the Callisto moment of inertia. Furthermore, an undifferentiated Callisto is not consistent with the presence of a substantial inner ocean, as indicated by magnetic data, and it would be difficult for an object as large as Callisto to be undifferentiated at any point.  In this case, gravity data may be more consistent with a more thoroughly differentiated Callisto with a hydrated silicate core. 
Surface features[ edit ]
Callisto ancient surface is one of the most cratered in the solar system.  In fact, crater density is close to saturation: Each new crater tends to erase an older one. The large-scale geology is relatively simple; there are no large mountains on Callisto, volcanoes, or other endogenous tectonic features.  Impact craters and multi-ring structures, along with associated fractures, scarps, and deposits, are the only major features found on the surface.  
Callisto’s surface can be divided into several geologically distinct parts: Crater plains, bright plains, bright and dark smooth plains, and various units associated with specific multi-ring structures and impact craters.   The crater plains make up most of the surface and represent ancient lithosphere, a mixture of ice and rocky material. Bright plains include bright impact craters such as Burr and Lofn, and the vanished remains of ancient large craters called palimpsests. [i] the central parts of multi-ring structures and isolated patches in the crater plains.  These light levels are thought to be icy impact deposits. The bright, smooth plains make up a small part of Callisto’s surface and are found in the ridge and trough zones of the Valhalla and Asgard formations, and as isolated patches in the crater plains. They have been assumed to be associated with endogenous activity, but with the high resolution Galileo Images showed that the bright, smooth planes correlate with highly fractured and gnarled terrain and show no signs of surface renewal.  The Galileo The images also showed small, dark, smooth areas with a total coverage of less than 10.000 km 2 , which appear to beautify [j] the surrounding terrain. They are possible cryovolcanic deposits.  Both the bright and the various smooth plains are somewhat younger and less cratered than the cratered background plains.  
Impact crater diameters range from 0.1 km – a limit defined by image resolution – to more than 100 km, excluding multi-ring structures.  Small craters with diameters of less than 5 km have simple bowl or flat-bottom shapes. These 5 to 40 km usually have a central peak. Larger impact features with diameters in the range of 25 to 100 km have central pits instead of peaks like Tindr crater.  The largest craters, with diameters over 60 km, may have central domes that are thought to result from central tectonic uplift after impact.  Examples include Doh and Har craters. A small number of very large craters greater than 100 km in diameter and bright impact craters exhibit anomalous dome geometry. These are unusually flat and may be a transitional form to multi-ring structures, as in the Lofn impact feature.  Callisto’s craters are generally shallower than those on the Moon.
The largest impacts on Callisto’s surface are multi-ring basins.   Two are enormous. Valhalla is the largest, with a bright central region 600 kilometers in diameter and rings that extend up to 1.Extending 800 kilometers from the center (see figure).  The second largest is Asgard with a diameter of about 1.600 kilometers.  Multi-ring structures probably formed as a result of concentric fracturing of the lithosphere after impact on a layer of soft or liquid material, possibly an ocean.  The catenae – for example, Gomul Catena – are long chains of impact craters arranged in straight lines across the surface. They were probably created by objects disturbed by tides near Jupiter before impacting Callisto, or by very oblique impacts.  A historical example of a fault was Comet Shoemaker-Levy 9.
As noted above, on the surface of Callisto are small patches of pure water ice with an albedo of up to 80%, surrounded by much darker material.  High resolution Galileo Images showed that the bright patches were predominantly on elevated surface features: Crater rims, scarps, ridges, and knobs.  These are probably thin water frost deposits. Dark material usually lies in the lowlands surrounding and enveloping bright features, and appears to be smooth. It often forms patches up to 5 km in diameter within crater floors and in intercrater depressions. 
On the sub-kilometer scale, the surface of Callisto is more affected than the surface of other icy Galilean moons.  Typically, there is a deficit of small impact craters with diameters less than 1 km compared to, for example, the dark plains on Ganymede.  Instead of small craters, the nearly ubiquitous surface features are small knobs and pits.  Buttons are thought to represent remnants of crater rims that have been degraded by a still uncertain process.  The most likely candidate process is the slow sublimation of ice, enabled by a temperature of up to 165 K reached at a subsolar point.  Such sublimation of water or other volatiles from the dirty ice that is the bedrock causes its decomposition. The non-ice remnants form debris avalanches that descend from the slopes of the crater walls.  Such avalanches are often observed near and within impact craters and are referred to as "debris aprons".    Sometimes crater walls are cut by sinuous valley-like incisions called "canyons" that resemble certain features of the Martian surface.  In the ice sublimation hypothesis, the low-lying dark material is interpreted as a blanket of mainly non-ice debris derived from the degraded rims of craters that covered a predominantly ice bedrock.
The relative ages of the various surface units on Callisto can be determined from the density of impact craters on them. The older the surface is, the denser the crater population is.  Absolute dating has not been done, but theoretical considerations suggest that the crater plains are about 4.5 billion years old and date back almost to the formation of the solar system. The age of multi-ring structures and impact craters depends on the background cratering rates chosen and is estimated by various authors to be between 1 and 4 billion years old.  
Atmosphere and ionosphere[ edit ]
Callisto has a very weak atmosphere of carbon dioxide.  It has been observed by Galileo Near Infrared Mapping Spectrometer (NIMS) from its absorption feature near the 4.2 micron wavelength. The surface pressure is estimated to be 7.5 picobars (0.75 uPa) and the particle density is estimated to be 4 × 10 8 cm -3 . For such a thin atmosphere would be formed in only ca. 4 days are lost (see atmospheric escape.)it must be constantly replenished, possibly by slow sublimation of carbon dioxide ice from Callisto’s ice crust.  This would be consistent with the sublimation-degradation hypothesis for the formation of the surface knobs.
Callisto’s ionosphere was first discovered during Galileo Callisto flybys;  its high electron density of 7-17 × 10 4 cm -3 cannot be explained solely by photoionization of atmospheric carbon dioxide. Therefore, Callisto’s atmosphere is thought to be actually dominated by molecular oxygen (in amounts 10-100 times higher than CO
2 ).  However, oxygen has not yet been directly detected in Callisto’s atmosphere. Hubble Space Telescope (HST) observations put an upper limit on its possible concentration in the atmosphere, since no detection was available that is still compatible with ionospheric measurements.  At the same time, HST was able to detect condensed oxygen on the surface of Callisto. 
Atomic hydrogen was also detected in Callisto’s atmosphere by a recent analysis of the 2001 Hubble Space Telescope data.  The on 15. and 24. Spectral images taken in December 2001 were re-examined and showed a weak scattered light signal indicative of a hydrogen corona. The observed brightness of the scattered sunlight in Callisto’s hydrogen corona is about twice as large when the leading hemisphere is observed. This asymmetry may stem from a difference in hydrogen abundance in both the anterior and posterior hemispheres. However, this hemispheric difference in the brightness of Callisto’s hydrogen corona is likely due to the extinction of the signal in Earth’s geocorona, which is greater when the trailing hemisphere is observed. 
Origin and evolution[ edit ]
The partial differentiation of Callisto (z. B. inferred from moment-of-inertia measurements) implies that it was never heated enough to melt its ice component.  The most favorable model for its formation is therefore a slow accretion in Jupiter’s low-density subnebula – a disk of gas and dust that existed around Jupiter after its formation.  Such a prolonged accretion stage would allow cooling to largely keep pace with heat accumulation caused by collisions, radioactive decay, and contraction, preventing melting and rapid differentiation.  The allowable time span for the formation of Callisto is then in the range of 0.1 to 10 million years. 
100 m high), which may have originated from the ejecta of an ancient impact
The further evolution of Callisto after accretion was determined by the balance between radioactive heating, cooling by thermal conduction near the surface, and solid-state or subsolidus convection in the interior.  Details of subsolidus convection in the ice are the main source of uncertainty in models of all icy moons. It is known to evolve due to the temperature dependence of ice viscosity when the temperature is sufficiently close to the melting point.  Subsolidus convection in icy bodies is a slow process with ice movements on the order of 1 centimeter per year, but actually a very effective cooling mechanism on long time scales.  It is thought to run in the so-called stagnant lid regime, where a stiff, cold outer layer of Callisto conducts heat without convection, while the underlying ice convects in the subsolidus regime.   For Callisto, the outer conducting layer corresponds to the cold and rigid lithosphere with a thickness of about 100 km. Its presence would explain the absence of any sign of endogenous activity on the callistoan surface.   Convection in the inner parts of Callisto may be stratified, as under the high pressures there, water ice is present in various crystalline phases, ranging from ice I on the surface to ice VII in the center.  The early onset of subsolidus convection in the Callistoan interior could have prevented large-scale ice melting and the resulting differentiation that would otherwise have formed a large rocky core and icy mantle. Because of the convection process, the very slow and partial separation and differentiation of rocks and ice in Callisto has continued for billions of years and may continue to this day. 
Current understanding of Callisto’s evolution allows for the existence of a layer or "ocean" of liquid water in its interior. This is related to the anomalous behavior of the melting temperature of the ice I phase, which decreases with pressure and reaches temperatures as low as 251 K at 2.070 bar (207 MPa) reached.  In all realistic models of Callisto, the temperature in the layer between 100 and 200 km depth is very close to or slightly exceeds this anomalous melting temperature.    The presence of even small amounts of ammonia – about 1-2 wt .-% – almost guarantees the existence of the liquid, since ammonia would lower the melting temperature even further. 
Although Callisto is very similar to Ganymede in its bulk properties, it apparently had a much simpler geologic history. The surface seems to have been formed mainly by impacts and other exogenous forces.  In contrast to neighboring Ganymede with its grooved terrain, there is little evidence of tectonic activity.  Explanations that have been proposed for the contrasts in internal heating and resulting differentiation and geologic activity between Callisto and Ganymede include differences in formation conditions,  the greater tidal heating of Ganymede,  and the more numerous and energetic impacts that Ganymede would have suffered during late heavy bombardment.    Callisto’s relatively simple geologic history provides planetary scientists with a reference point for comparison with other more active and complex worlds. 
Possible habitability[ edit ]
It is speculated that there may be life in Callisto’s subsurface ocean. Like Europa and Ganymede, as well as Saturn’s moons Enceladus, Dione, and Titan, and Neptune’s moon Triton,  a possible subsurface ocean could be composed of salt water.
It is possible that halophiles thrive in the ocean. 
As with Europa and Ganymede, the idea has been expressed that habitable conditions and even extraterrestrial microbial life may exist in the salty ocean beneath the Callistoan surface.  However, the environmental conditions necessary for life appear to be less favorable on Callisto than on Europa. The main reasons are the lack of contact with rocky material and the lower heat flux from the interior of Callisto.  Scientist Torrence Johnson said the following about comparing the odds of life on Callisto to the odds on other Galilean moons: 
The basic ingredients for life – what we call "prebiotic chemistry" – are abundant in many solar system objects such as comets, asteroids and icy moons. Biologists believe that liquid water and energy will then be needed to actually support life. It is therefore exciting to find another place where we may have liquid water. Energy is another matter, however, and currently Callisto’s ocean is heated only by radioactive elements, while Europa also has tidal energy due to its closer proximity to Jupiter.
Based on the above considerations and other scientific observations, Europa is thought to have the greatest chance of supporting microbial life of all Jupiter’s moons.  
Exploration[ edit ]
The Pioneer 10 and Pioneer 11 Jupiter encounters in the early 1970s provided little new information about Callisto compared to what was already known from observations on Earth.  The real breakthrough happened later with the Voyager 1 and Voyager 2 flyby in 1979. They imaged more than half of Callistoan surface at 1-2 km resolution and accurately measured its temperature, mass and shape.  A second round of exploration lasted from 1994 to 2003, when the Galileo The spacecraft had eight close encounters with Callisto, the last flyby during the C30 orbit in 2001 coming within 138 km of the surface. The Galileo The orbiter completed global imaging of the surface and provided a series of images with a resolution of up to 15 meters of selected areas of Callisto.  In 2000 Cassini Spacecraft en route to Saturn took high-quality infrared spectra of Galilean satellites including Callisto.  In February to March 2007, the New horizons Probe on its way to Pluto received new images and spectra of Callisto. 
The next planned mission in the Jupiter system is the European Space Agency’s Jupiter Icy Moon Explorer (JUICE), scheduled for launch in 2022.  Several close flybys of Callisto are planned during the mission. 
Old proposals[ edit ]
The Europa Jupiter System Mission (EJSM), previously proposed for launch in 2020, was a joint proposal by NASA and ESA to explore Jupiter’s moons. In February 2009, it was announced that ESA / NASA had prioritized this mission ahead of the Titan Saturn System mission.  At that time, ESA’s contribution was still subject to funding competition from other ESA projects.  EJSM consisted of the NASA-led Jupiter Europa orbiter, the ESA-led Jupiter Ganymede orbiter, and possibly a JAXA-led Jupiter Magnetospheric Orbiter.
Possible humanization[ edit ]
In 2003, NASA conducted a conceptual study entitled Human Outer Planets Exploration (HOPE) on future human exploration of the outer solar system. The target to be considered in detail was Callisto.  
The study suggested a possible surface base on Callisto that would produce rocket fuel for further solar system exploration.  The advantages of a base on Callisto include low radiation (due to its distance from Jupiter) and geological stability. Such a base could facilitate remote Europa exploration or be an ideal location for a Jupiter system waystation serving spacecraft venturing further into the outer solar system, using gravity assist from a close flyby of Jupiter after leaving Callisto. 
In December 2003, NASA reported that a manned mission to Callisto could be possible in the 2040s.