# Callisto (moon)

Last updated

Callisto
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
Pronunciation [2]
Named after
Καλλιστώ Kallistō
Jupiter IV
etc. (see text)
Orbital characteristics
Periapsis 1869000 km [lower-alpha 1]
Apoapsis 1897000 km [lower-alpha 2]
1 882 700 km [3]
Eccentricity 0.0074 [3]
16.6890184  d [3]
Average orbital speed
8.204 km/s
Inclination 2.017° (to the ecliptic)
0.192° (to local Laplace planes) [3]
Satellite of Jupiter
Group Galilean moon
Physical characteristics
2410.3±1.5 km (0.378 Earths) [4]
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) [4]
Mean density
1.8344±0.0034 g/cm3 (0.333 Earths) [4]
1.235  m/s2 (0.126 g ) [lower-alpha 5]
0.3549±0.0042 [5]
2.440 km/s [lower-alpha 6]
synchronous [4]
zero [4]
Albedo 0.22 (geometric) [6]
Surface temp. minmeanmax
K [6] 80±5134±11165±5
5.65 (opposition) [7]
Atmosphere
Surface pressure
0.75 μPa (7.40×10−12 atm) [8]
Composition by volume 4×108 molecules/cm3 carbon dioxide; [8]
up to 2×1010 molecules/cm3 molecular oxygen(O2) [9]

Callisto, or 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 that may not 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. [3] 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. Because of this, there is a sub-Jovian point on Callisto's surface, from which Jupiter would appear to hang directly overhead. It is less affected by Jupiter's magnetosphere than the other inner satellites because of its more remote orbit, located just outside Jupiter's main radiation belt. [11] [12]

## 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]

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. [6] 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.

Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide [8] and probably molecular oxygen, [9] 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]

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]

## History

### Discovery

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

### Name

Callisto is named after one of Zeus's many lovers or other sexual partners 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]

... autem celebrantur tres fœminæ Virgines, 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 who were captured by Jupiter for secret love shall be honoured, [including] Callisto, the daughter of Lycaon... Finally, the fourth [moon] is called by me Callisto... Io, Europa, the boy Ganymede, and Callisto greatly pleased lustful Jupiter. [27]

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". [28]

There's no established English adjectival form of the name. The adjectival form of Greek Καλλιστῴ Kallistōi is Καλλιστῴος Kallistōi-os, from which one might expect Latin Callistōius and English *Callistóian, parallel to Sapphóian for Sapphōi [29] and Letóian for Lētōi . [30] However, the iota subscript is often omitted from such Greek names (cf. Inóan [31] from Īnōi [32] and Argóan [33] from Argōi [34] ), and indeed the analogous form Callistoan is found. [35] [36] [37] In Virgil, a second oblique stem appears in Latin: Callistōn-, [38] but the corresponding Callistonian has rarely appeared in English. [39] One also sees ad hoc forms, such as Callistan, [19] Callistian [40] and Callistean. [41] [42]

## Orbit and rotation

Callisto is the outermost of the four Galilean moons of Jupiter. It orbits at a distance of approximately 1 880 000 km (26.3 times the 71 492 km radius of Jupiter itself). [3] 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. [4] 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°. [43]

The dynamical isolation of Callisto means that it has never been appreciably tidally heated, which has important consequences for its internal structure and evolution. [44] 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. [45] [46]

## Physical characteristics

### Composition

The average density of Callisto, 1.83 g/cm3, [4] 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%. [6] 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. [6] 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, [6] carbon dioxide, [48] sulfur dioxide, [49] and possibly ammonia and various organic compounds. [15] [6] 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. [6] [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. [6] The trailing hemisphere [lower-alpha 7] of Callisto appears to be enriched in carbon dioxide, whereas the leading hemisphere has more sulfur dioxide. [50] Many fresh impact craters like Lofn also show enrichment in carbon dioxide. [50] 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

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. [51] [52] 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. [52] 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 [4] (especially the dimensionless moment of inertia [lower-alpha 8] —0.3549 ± 0.0042—determined during close flybys) suggest that, if Callisto is in hydrostatic equilibrium, 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] [53] In other words, Callisto may be only partially differentiated. The density and moment of inertia for an equilibrium Callisto are compatible with the existence of a small silicate 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. [4] [14] In this case, Callisto's interior would be in stark contrast to that of Ganymede, which appears to be fully differentiated. [15] [54]

However, a 2011 reanalysis of Galileo data suggests that Callisto is not in hydrostatic equilibrium; its S22 coefficient from gravity data is an anomalous 10% of its C22 value, which is not consistent with a body in hydrostatic equilibrium and thus significantly increases the error bars on Callisto's moment of inertia. Further, an undifferentiated Callisto is inconsistent with the presence of a substantial internal ocean as inferred by magnetic data, and it would be difficult for an object as large as Callisto to fail to differentiate at any point. [55] In that case, the gravity data may be more consistent with a more thoroughly differentiated Callisto with a hydrated silicate core. [56]

### Surface features

The ancient surface of Callisto is one of the most heavily cratered in the Solar System. [57] 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. [58] 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] [58]

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] [58] 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] [59]

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 than 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.

The largest impact features on Callisto's surface are multi-ring basins. [18] [58] 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). [60] The second largest is Asgard, measuring about 1,600 kilometers in diameter. [60] 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. [35] 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. [6] High-resolution Galileo images showed the bright patches to be predominately located on elevated surface features: crater rims, scarps, ridges and knobs. [6] 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. [6]

On a sub-kilometer scale the surface of Callisto is more degraded than the surfaces of other icy Galilean moons. [6] 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. [6] 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. [6] 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". [6] [18] [19] Sometimes crater walls are cut by sinuous valley-like incisions called "gullies", which resemble certain Martian surface features. [6] 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. [61] 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] [57]

### Atmosphere and ionosphere

Callisto has a very tenuous atmosphere composed of carbon dioxide. [8] 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, [8] 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
). [9] 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. [62] At the same time, HST was able to detect condensed oxygen trapped on the surface of Callisto. [63]

Atomic hydrogen has also been detected in Callisto's atmosphere via recent analysis of 2001 Hubble Space Telescope data. [64] 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. [65]

## 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]

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. [44] 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. [66] 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. [66] 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] [66] 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. [66] [67] 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. [44] 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. [67]

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. [44] [66] [67] 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, [68] the greater tidal heating experienced by Ganymede, [69] and the more numerous and energetic impacts that would have been suffered by Ganymede during the Late Heavy Bombardment. [70] [71] [72] 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, Dione and Titan and Neptune's moon Triton, [73] a possible subsurface ocean might be composed of salt water.

It is possible that halophiles could thrive in the ocean. [74] 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: [74]

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] [75]

## 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. [6] 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. [6] 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. [48] In February–March 2007, the New Horizons probe on its way to Pluto obtained new images and spectra of Callisto. [76]

The next planned mission to the Jovian system is the European Space Agency's Jupiter Icy Moon Explorer (JUICE), due to launch in 2023. [77] Several close flybys of Callisto are planned during the mission. [78]

### 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. [79] At the time ESA's contribution still faced funding competition from other ESA projects. [80] EJSM consisted of the NASA-led Jupiter Europa Orbiter, the ESA-led Jupiter Ganymede Orbiter, and possibly a JAXA-led Jupiter Magnetospheric Orbiter.

## Potential human exploration

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] [82]

The study proposed a possible surface base on Callisto that would produce rocket propellant for further exploration of the Solar System. [81] 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. [83]

## Notes

1. Periapsis is derived from the semimajor axis (a) and eccentricity (e): ${\displaystyle a(1-e)}$.
2. Apoapsis is derived from the semimajor axis (a) and eccentricity (e): ${\displaystyle a(1+e)}$.
3. Surface area derived from the radius (r): ${\displaystyle 4\pi r^{2}}$.
4. Volume derived from the radius (r): ${\displaystyle {\frac {4}{3}}\pi r^{3}}$.
5. Surface gravity derived from the mass (m), the gravitational constant (G) and the radius (r): ${\displaystyle {\frac {Gm}{r^{2}}}}$.
6. Escape velocity derived from the mass (m), the gravitational constant (G) and the radius (r): ${\displaystyle \textstyle {\sqrt {\frac {2Gm}{r}}}}$.
7. 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 ${\displaystyle I/(mr^{2})}$, 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.

## Related Research Articles

The Galilean moons are the four largest moons of Jupiter—Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei in December 1609 or January 1610, and recognized by him as satellites of Jupiter in March 1610. They were the first objects found to orbit a planet other than the Earth.

Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth the mass of the Sun. Jupiter is the third-brightest natural object in the Earth's night sky after the Moon and Venus. It has been observed since pre-historic times and is named after the Roman god Jupiter, the king of the gods, because of its observed size.

Europa, or Jupiter II, 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.

Amalthea is a moon of Jupiter. It has the third closest orbit around Jupiter among known moons and was the fifth moon of Jupiter to be discovered, so it is also known as Jupiter V. It is also the fifth largest moon of Jupiter, after the four Galilean Moons. Edward Emerson Barnard discovered the moon on 9 September 1892 and named it after Amalthea of Greek mythology. It was the last natural satellite to be discovered by direct visual observation; all later moons were discovered by photographic or digital imaging.

Ganymede, a satellite of Jupiter, is the largest and most massive of the Solar System's moons. The ninth-largest object of the Solar System, it is the largest without a substantial atmosphere. It has a diameter of 5,268 km (3,273 mi), making it 26% larger than the planet Mercury by volume, although it is 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.

Oberon, also designated Uranus IV, is the outermost major moon of the planet Uranus. It is the second-largest and second most massive of the Uranian moons, and the ninth most massive moon in the Solar System. Discovered by William Herschel in 1787, Oberon is named after the mythical king of the fairies who appears as a character in Shakespeare's A Midsummer Night's Dream. Its orbit lies partially outside Uranus's magnetosphere.

Thebe, also known as Jupiter XIV, is the fourth of Jupiter's moons by distance from the planet. It was discovered by Stephen P. Synnott in images from the Voyager 1 space probe taken on March 5, 1979, while making its flyby of Jupiter. In 1983, it was officially named after the mythological nymph Thebe.

Icy moons are a class of natural satellites with surfaces composed mostly of ice. An icy moon may harbor an ocean underneath the surface, and possibly include a rocky core of silicate or metallic rocks. It is thought that they may be composed of ice II or other polymorph of water ice. The prime example of this class of object is Europa.

There are 80 known moons of Jupiter, not counting a number of moonlets likely shed from the inner moons. All together, they form a satellite system which is called the Jovian system. The most massive of the moons are the four Galilean moons: Io; Europa; Ganymede; and Callisto, 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. Much more recently, beginning in 1892, dozens of far smaller Jovian moons have been detected 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 76 known moons and the rings together composing just 0.003% of the total orbiting mass.

Located on Jupiter's moon Callisto, Valhalla is the largest multi-ring impact crater in the Solar System. It is named after Valhalla, the hall where warriors are taken after death in Norse mythology.

Io, or Jupiter I, is the innermost and third-largest of the four Galilean moons of the planet Jupiter. Slightly larger than the Moon, Io is the fourth-largest moon in the Solar System, has the highest density of any moon, and has the lowest amount of water 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's lovers.

A crater chain is a line of craters along the surface of an astronomical body. The descriptor term for crater chains is catena, plural catenae, as specified by the International Astronomical Union's rules on planetary nomenclature.

The exploration of Jupiter has been conducted via close observations by automated spacecraft. It began with the arrival of Pioneer 10 into the Jovian system in 1973, and, as of 2016, has continued with eight further spacecraft missions. All of these missions were undertaken by the National Aeronautics and Space Administration (NASA), and all but two were flybys taking detailed observations without landing or entering orbit. These probes make Jupiter the most visited of the Solar System's outer planets as all missions to the outer Solar System have used Jupiter flybys. On 5 July 2016, spacecraft Juno arrived and entered the planet's orbit—the second craft ever to do so. Sending a craft to Jupiter is difficult, mostly due to large fuel requirements and the effects of the planet's harsh radiation environment.

The habitability of natural satellites is a measure of the potential of natural satellites to have environments hospitable to life. Habitable environments do not necessarily harbor life. Natural satellite habitability is an emerging field which is considered important to astrobiology for several reasons, foremost being that natural satellites are predicted to greatly outnumber planets and it is hypothesized that habitability factors are likely to be similar to those of planets. There are, however, key environmental differences which have a bearing on moons as potential sites for extraterrestrial life.

The exploration of Io, Jupiter's innermost Galilean and third-largest moon, began with its discovery in 1610 and continues today with Earth-based observations and visits by spacecraft to the Jupiter system. Italian astronomer Galileo Galilei was the first to record an observation of Io on January 8, 1610, though Simon Marius may have also observed Io at around the same time. During the 17th century, observations of Io and the other Galilean satellites helped with the measurement of longitude by map makers and surveyors, with validation of Kepler's Third Law of planetary motion, and with measurement of the speed of light. Based on ephemerides produced by astronomer Giovanni Cassini and others, Pierre-Simon Laplace created a mathematical theory to explain the resonant orbits of three of Jupiter's moons, Io, Europa, and Ganymede. This resonance was later found to have a profound effect on the geologies of these moons. Improved telescope technology in the late 19th and 20th centuries allowed astronomers to resolve large-scale surface features on Io as well as to estimate its diameter and mass.

A planetary-mass moon is a planetary-mass object that is also a natural satellite. They are large and ellipsoidal in shape. Two moons in the Solar System are larger than the planet Mercury : Ganymede and Titan, and seven are larger and more massive than the dwarf planet Pluto.

The Jupiter Icy Moons Explorer (JUICE) is an interplanetary spacecraft in development by the European Space Agency (ESA) with Airbus Defence and Space as the main contractor. The mission will study three of Jupiter's Galilean moons: Ganymede, Callisto, and Europa all of which are thought to have significant bodies of liquid water beneath their surfaces, making them potentially habitable environments.

Planetary oceanography also called exo-oceanography is the study of oceans on planets and moons other than Earth. Unlike other planetary sciences like astrobiology, astrochemistry and planetary geology, it only began after the discovery of underground oceans in Saturn's Titan and Jupiter's Europa. This field remains speculative until further missions reach the oceans beneath the rock or ice layer of the moons. There are many theories about oceans or even ocean worlds of celestial bodies in the Solar System, from oceans made of diamond in Neptune to a gigantic ocean of liquid hydrogen that may exist underneath Jupiter's surface.

## References

1. Galilei, G. (13 March 1610). Sidereus Nuncius.
2. "Callisto". Lexico UK English Dictionary. Oxford University Press. n.d.
3. "Planetary Satellite Mean Orbital Parameters". Jet Propulsion Laboratory, California Institute of Technology.
4. Anderson, J. D.; Jacobson, R. A.; McElrath, T. P.; Moore, W. B.; Schubert, G.; Thomas, P. C. (2001). "Shape, mean radius, gravity field and interior structure of Callisto". Icarus. 153 (1): 157–161. Bibcode:2001Icar..153..157A. doi:10.1006/icar.2001.6664. S2CID   120591546.
5. Schubert, G.; Anderson, J. D.; Spohn, T.; McKinnon, W. B. (2004). "Interior composition, structure and dynamics of the Galilean satellites". In Bagenal, F.; Dowling, T. E.; McKinnon, W. B. (eds.). Jupiter : the planet, satellites, and magnetosphere. New York: Cambridge University Press. pp. 281–306. ISBN   978-0521035453. OCLC   54081598.
6. Moore, Jeffrey M.; Chapman, Clark R.; Bierhaus, Edward B.; et al. (2004). "Callisto" (PDF). In Bagenal, Fran; Dowling, Timothy E.; McKinnon, William B. (eds.). Jupiter: The planet, Satellites and Magnetosphere. Cambridge University Press.
7. "Classic Satellites of the Solar System". Observatorio ARVAL. Archived from the original on 9 July 2011. Retrieved 13 July 2007.
8. Carlson, R. W.; et al. (1999). "A Tenuous Carbon Dioxide Atmosphere on Jupiter's Moon Callisto" (PDF). Science. 283 (5403): 820–821. Bibcode:1999Sci...283..820C. CiteSeerX  . doi:10.1126/science.283.5403.820. PMID   9933159. Archived from the original (PDF) on 3 October 2008. Retrieved 10 July 2007.
9. Liang, M. C.; Lane, B. F.; Pappalardo, R. T.; et al. (2005). "Atmosphere of Callisto". Journal of Geophysical Research. 110 (E2): E02003. Bibcode:2005JGRE..11002003L. doi:.
10. Musotto, Susanna; Varadi, Ferenc; Moore, William; Schubert, Gerald (2002). "Numerical Simulations of the Orbits of the Galilean Satellites". Icarus. 159 (2): 500–504. Bibcode:2002Icar..159..500M. doi:10.1006/icar.2002.6939.
11. Cooper, John F.; Johnson, Robert E.; Mauk, Barry H.; Garrett, Garry H.; Gehrels, Neil (2001). "Energetic Ion and Electron Irradiation of the Icy Galilean Satellites" (PDF). Icarus. 139 (1): 133–159. Bibcode:2001Icar..149..133C. doi:10.1006/icar.2000.6498. Archived from the original (PDF) on 16 January 2012. Retrieved 25 October 2011.
12. "Exploring Jupiter – JIMO – Jupiter Icy Moons Orbiter – the moon Callisto". Space Today Online.
13. Chang, Kenneth (12 March 2015). "Suddenly, It Seems, Water Is Everywhere in Solar System". The New York Times . Retrieved 12 March 2015.
14. Kuskov, O.L.; Kronrod, V.A. (2005). "Internal structure of Europa and Callisto". Icarus. 177 (2): 550–369. Bibcode:2005Icar..177..550K. doi:10.1016/j.icarus.2005.04.014.
15. Showman, A. P.; Malhotra, R. (1 October 1999). "The Galilean Satellites". Science. 286 (5437): 77–84. doi:10.1126/science.286.5437.77. PMID   10506564. S2CID   9492520.
16. "Callisto – Overview – Planets – NASA Solar System Exploration". NASA Solar System Exploration. Archived from the original on 28 March 2014.
17. Glenday, Craig (2013). Guinness Book of World Records 2014. Guinness World Records Limited. p.  187. ISBN   978-1-908843-15-9.
18. Greeley, R.; Klemaszewski, J. E.; Wagner, L.; et al. (2000). "Galileo views of the geology of Callisto". Planetary and Space Science. 48 (9): 829–853. Bibcode:2000P&SS...48..829G. doi:10.1016/S0032-0633(00)00050-7.
19. Moore, Jeffrey M.; Asphaug, Erik; Morrison, David; Spencer, John R.; Chapman, Clark R.; Bierhaus, Beau; Sullivan, Robert J.; Chuang, Frank C.; Klemaszewski, James E.; Greeley, Ronald; Bender, Kelly C.; Geissler, Paul E.; Helfenstein, Paul; Pilcher, Carl B. (1999). "Mass Movement and Landform Degradation on the Icy Galilean Satellites: Results of the Galileo Nominal Mission". Icarus. 140 (2): 294–312. Bibcode:1999Icar..140..294M. doi:10.1006/icar.1999.6132.
20. Kliore, A. J.; Anabtawi, A.; Herrera, R. G.; et al. (2002). "Ionosphere of Callisto from Galileo radio occultation observations" (PDF). Journal of Geophysical Research. 107 (A11): 1407. Bibcode:2002JGRA.107kSIA19K. doi:. hdl:2027.42/95670.
21. Canup, Robin M.; Ward, William R. (2002). "Formation of the Galilean Satellites: Conditions of Accretion" (PDF). The Astronomical Journal. 124 (6): 3404–3423. Bibcode:2002AJ....124.3404C. doi:10.1086/344684.
22. Spohn, T.; Schubert, G. (2003). "Oceans in the icy Galilean satellites of Jupiter?" (PDF). Icarus. 161 (2): 456–467. Bibcode:2003Icar..161..456S. doi:10.1016/S0019-1035(02)00048-9.
23. Lipps, Jere H.; Delory, Gregory; Pitman, Joe; et al. (2004). Hoover, Richard B; Levin, Gilbert V; Rozanov, Alexei Y (eds.). "Astrobiology of Jupiter's Icy Moons" (PDF). Proc. SPIE. Instruments, Methods, and Missions for Astrobiology VIII. 5555: 10. Bibcode:2004SPIE.5555...78L. doi:10.1117/12.560356. S2CID   140590649. Archived from the original (PDF) on 20 August 2008.
24. Trautman, Pat; Bethke, Kristen (2003). "Revolutionary Concepts for Human Outer Planet Exploration (HOPE)" (PDF). NASA. Archived from the original (PDF) on 19 January 2012.
25. "Satellites of Jupiter". The Galileo Project. Retrieved 31 July 2007.
26. Marius, Simon (1614). Mundus Iovialis: anno MDCIX detectus ope perspicilli Belgici, hoc est, quatuor Jovialium planetarum, cum theoria, tum tabulæ. Nuremberg: Sumptibus & Typis Iohannis Lauri. p. B2, recto and verso (images 35 and 36), with erratum on last page (image 78). Retrieved 30 June 2020.
27. Barnard, E. E. (1892). "Discovery and Observation of a Fifth Satellite to Jupiter". Astronomical Journal. 12: 81–85. Bibcode:1892AJ.....12...81B. doi:10.1086/101715.
28. The Thistle, January 1903, vol. I, no. 2, p. 4
29. E. Alan Roberts (2013) The Courage of Innocence: (The Virgin of Phileros), p. 191
30. George Stuart (1882) The Eclogues, Georgics, and Moretum of Virgil, p. 271
31. Ino . Charlton T. Lewis and Charles Short. A Latin Dictionary on Perseus Project .
32. Noah Webster (1832) A Dictionary of the English Language
33. Argo . Charlton T. Lewis and Charles Short. A Latin Dictionary on Perseus Project .
34. Klemaszewski, J.A.; Greeley, R. (2001). "Geological Evidence for an Ocean on Callisto" (PDF). Lunar and Planetary Science XXXI. p. 1818.
35. Steven Croft (1985) "Ripple Ring Basins on Ganymede and Callisto", [ibid] p. 206
36. David M. Harland (2000) Jupiter Odyssey: The Story of NASA's Galileo Mission, p. 165
37. Genitive Callistūs or Callistōnis. Callisto . Charlton T. Lewis and Charles Short. A Latin Dictionary on Perseus Project .
38. P. Leonardi (1982), Geological results of twenty years of space enterprises: Satellites of Jupiter and Saturn, in Geologica romana, p. 468.
39. Pierre Thomas & Philippe Mason (1985) "Tectonics of the Vahalla Structure on Callisto", Reports of Planetary Geology and Geophysics Program – 1984, NASA Technical Memorandum 87563, p. 535
40. Jean-Pierre Burg & Mary Ford (1997) Orogeny Through Time, p. 55
41. Bills, Bruce G. (2005). "Free and forced obliquities of the Galilean satellites of Jupiter". Icarus. 175 (1): 233–247. Bibcode:2005Icar..175..233B. doi:10.1016/j.icarus.2004.10.028.
42. Freeman, J. (2006). "Non-Newtonian stagnant lid convection and the thermal evolution of Ganymede and Callisto" (PDF). Planetary and Space Science. 54 (1): 2–14. Bibcode:2006P&SS...54....2F. doi:10.1016/j.pss.2005.10.003. Archived from the original (PDF) on 24 August 2007.
43. United Nations Scientific Committee on the Effects of Atomic Radiation. New York: United Nations. 2008. p. 4. ISBN   978-92-1-142274-0.
44. Frederick A. Ringwald (29 February 2000). "SPS 1020 (Introduction to Space Sciences)". California State University, Fresno. Archived from the original on 25 July 2008. Retrieved 4 July 2009.
45. Clark, R. N. (10 April 1981). "Water frost and ice: the near-infrared spectral reflectance 0.65–2.5 μm". Journal of Geophysical Research . 86 (B4): 3087–3096. Bibcode:1981JGR....86.3087C. doi:10.1029/JB086iB04p03087 . Retrieved 3 March 2010.
46. Brown, R. H.; Baines, K. H.; Bellucci, G.; Bibring, J-P.; Buratti, B. J.; Capaccioni, F.; Cerroni, P.; Clark, R. N.; Coradini, A.; Cruikshank, D. P.; Drossart, P.; Formisano, V.; Jaumann, R.; Langevin, Y.; Matson, D. L.; McCord, T. B.; Mennella, V.; Nelson, R. M.; Nicholson, P. D.; Sicardy, B.; Sotin, C.; Amici, S.; Chamberlain, M. A.; Filacchione, G.; Hansen, G.; Hibbitts, K.; Showalter, M. (2003). "Observations with the Visual and Infrared Mapping Spectrometer (VIMS) during Cassini's Flyby of Jupiter". Icarus. 164 (2): 461–470. Bibcode:2003Icar..164..461B. doi:10.1016/S0019-1035(03)00134-9.
47. Noll, K.S. (1996). "Detection of SO2 on Callisto with the Hubble Space Telescope" (PDF). Lunar and Planetary Science XXXI. p. 1852. Archived from the original (PDF) on 4 June 2016. Retrieved 25 July 2007.
48. Hibbitts, C.A.; McCord, T. B.; Hansen, G.B. (1998). "Distributions of CO2 and SO2 on the Surface of Callisto" (PDF). Lunar and Planetary Science XXXI. p. 1908. Archived from the original (PDF) on 4 June 2016. Retrieved 10 July 2007.
49. Khurana, K. K.; Kivelson, M. G.; Stevenson, D. J.; Schubert, G.; Russell, C. T.; Walker, R. J.; Polanskey, C. (1998). "Induced magnetic fields as evidence for subsurface oceans in Europa and Callisto" (PDF). Nature. 395 (6704): 777–780. Bibcode:1998Natur.395..777K. doi:10.1038/27394. PMID   9796812. S2CID   4424606.
50. Zimmer, C.; Khurana, K. K.; Kivelson, Margaret G. (2000). "Subsurface Oceans on Europa and Callisto: Constraints from Galileo Magnetometer Observations" (PDF). Icarus. 147 (2): 329–347. Bibcode:2000Icar..147..329Z. CiteSeerX  . doi:10.1006/icar.2000.6456.
51. Anderson, J. D.; Schubert, G.; Jacobson, R. A.; Lau, E. L.; Moore, W. B.; Sjo Gren, W. L. (1998). "Distribution of Rock, Metals and Ices in Callisto" (PDF). Science. 280 (5369): 1573–1576. Bibcode:1998Sci...280.1573A. doi:10.1126/science.280.5369.1573. PMID   9616114. Archived from the original (PDF) on 26 September 2007.
52. Sohl, F.; Spohn, T.; Breuer, D.; Nagel, K. (2002). "Implications from Galileo Observations on the Interior Structure and Chemistry of the Galilean Satellites". Icarus. 157 (1): 104–119. Bibcode:2002Icar..157..104S. doi:10.1006/icar.2002.6828.
53. Monteux, J.; Tobie, G.; Choblet, G.; Le Feuvre, M. (2014). "Can large icy moons accrete undifferentiated?" (PDF). Icarus. 237: 377–387. Bibcode:2014Icar..237..377M. doi:10.1016/j.icarus.2014.04.041.
54. Castillo-Rogez, J. C.; et al. (2011). "How differentiated is Callisto" (PDF). 42nd Lunar and Planetary Science Conference: 2580. Retrieved 2 January 2020.
55. Zahnle, K.; Dones, L.; Levison, Harold F. (1998). "Cratering Rates on the Galilean Satellites" (PDF). Icarus. 136 (2): 202–222. Bibcode:1998Icar..136..202Z. doi:10.1006/icar.1998.6015. PMID   11878353. Archived from the original (PDF) on 27 February 2008.
56. Bender, K. C.; Rice, J. W.; Wilhelms, D. E.; Greeley, R. (1997). "Geological map of Callisto". Abstracts of the 25th Lunar and Planetary Science Conference. 25: 91. Bibcode:1994LPI....25...91B. Archived from the original on 24 January 2015. Retrieved 28 August 2017.
57. Wagner, R.; Neukum, G.; Greeley, R; et al. (12–16 March 2001). Fractures, Scarps, and Lineaments on Callisto and their Correlation with Surface Degradation (PDF). 32nd Annual Lunar and Planetary Science Conference.
58. Controlled Photomosaic Map of Callisto JC 15M CMN (Map) (2002 ed.). U.S. Geological Survey.
59. Chapman, C.R.; Merline, W.J.; Bierhaus, B.; et al. (1997). "Populations of Small Craters on Europa, Ganymede, and Callisto: Initial Galileo Imaging Results" (PDF). Lunar and Planetary Science XXXI. p. 1221.
60. Strobel, Darrell F.; Saur, Joachim; Feldman, Paul D.; et al. (2002). "Hubble Space Telescope Space Telescope Imaging Spectrograph Search for an Atmosphere on Callisto: a Jovian Unipolar Inductor". The Astrophysical Journal. 581 (1): L51–L54. Bibcode:2002ApJ...581L..51S. doi:.
61. Spencer, John R.; Calvin, Wendy M. (2002). "Condensed O2 on Europa and Callisto" (PDF). The Astronomical Journal. 124 (6): 3400–3403. Bibcode:2002AJ....124.3400S. doi:10.1086/344307.
62. Roth, Lorenz; et al. (27 May 2017). "Detection of a hydrogen corona at Callisto". Journal of Geophysical Research: Planets. 122 (5): 1046–1055. Bibcode:2017JGRE..122.1046R. doi:10.1002/2017JE005294.
63. Alday, Juan; Roth, Lorenz; Ivchenko, Nickolay; Retherford, Kurt D; Becker, Tracy M; Molyneux, Philippa; Saur, Joachim (15 November 2017). "New constraints on Ganymede's hydrogen corona: Analysis of Lyman-α emissions observed by HST/STIS between 1998 and 2014". Planetary and Space Science. 148: 35–44. Bibcode:2017P&SS..148...35A. doi:10.1016/j.pss.2017.10.006. ISSN   0032-0633.
64. McKinnon, William B. (2006). "On convection in ice I shells of outer Solar System bodies, with detailed application to Callisto". Icarus. 183 (2): 435–450. Bibcode:2006Icar..183..435M. doi:10.1016/j.icarus.2006.03.004.
65. Nagel, K.a; Breuer, D.; Spohn, T. (2004). "A model for the interior structure, evolution, and differentiation of Callisto". Icarus. 169 (2): 402–412. Bibcode:2004Icar..169..402N. doi:10.1016/j.icarus.2003.12.019.
66. Barr, A. C.; Canup, R. M. (3 August 2008). "Constraints on gas giant satellite formation from the interior states of partially differentiated satellites". Icarus . 198 (1): 163–177. Bibcode:2008Icar..198..163B. doi:10.1016/j.icarus.2008.07.004.
67. Showman, A. P.; Malhotra, R. (March 1997). "Tidal evolution into the Laplace resonance and the resurfacing of Ganymede". Icarus . 127 (1): 93–111. Bibcode:1997Icar..127...93S. doi:10.1006/icar.1996.5669. S2CID   55790129.
68. Baldwin, E. (25 January 2010). "Comet impacts explain Ganymede-Callisto dichotomy". Astronomy Now . Retrieved 1 March 2010.
69. Barr, A. C.; Canup, R. M. (March 2010). Origin of the Ganymede/Callisto dichotomy by impacts during an outer solar system late heavy bombardment (PDF). 41st Lunar and Planetary Science Conference (2010). Houston. Retrieved 1 March 2010.
70. Barr, A. C.; Canup, R. M. (24 January 2010). "Origin of the Ganymede–Callisto dichotomy by impacts during the late heavy bombardment" (PDF). Nature Geoscience . 3 (March 2010): 164–167. Bibcode:2010NatGe...3..164B. doi:10.1038/NGEO746.
71. Nimmo, Francis (15 January 2015). "Powering Triton's recent geological activity by obliquity tides: Implications for Pluto geology" (PDF). Icarus. 246: 2–10. Bibcode:2015Icar..246....2N. doi:10.1016/j.icarus.2014.01.044.
72. Phillips, Tony (23 October 1998). "Callisto makes a big splash". NASA. Retrieved 15 August 2015.
73. François, Raulin (2005). "Exo-Astrobiological Aspects of Europa and Titan: from Observations to speculations". Space Science Reviews. 116 (1–2): 471–487. Bibcode:2005SSRv..116..471R. doi:10.1007/s11214-005-1967-x. S2CID   121543884.
74. Morring, F. (7 May 2007). "Ring Leader". Aviation Week & Space Technology: 80–83.
75. "ESA Science & Technology - JUICE". ESA. 8 November 2021. Retrieved 10 November 2021.
76. Amos, Jonathan (2 May 2012). "Esa selects 1bn-euro Juice probe to Jupiter". BBC News Online . Retrieved 2 May 2012.
77. Rincon, Paul (20 February 2009). "Jupiter in space agencies' sights". BBC News. Retrieved 20 February 2009.
78. "Cosmic Vision 2015–2025 Proposals". ESA. 21 July 2007. Retrieved 20 February 2009.
79. "Vision for Space Exploration" (PDF). NASA. 2004.
80. Troutman, Patrick A.; Bethke, Kristen; Stillwagen, Fred; Caldwell, Darrell L. Jr.; Manvi, Ram; Strickland, Chris; Krizan, Shawn A. (28 January 2003). "Revolutionary Concepts for Human Outer Planet Exploration (HOPE)". AIP Conference Proceedings. 654: 821–828. Bibcode:2003AIPC..654..821T. doi:10.1063/1.1541373. hdl:.
81. "High Power MPD Nuclear Electric Propulsion (NEP) for Artificial Gravity HOPE Missions to Callisto" (PDF). NASA. 2003. Archived from the original (PDF) on 2 July 2012. Retrieved 25 June 2009.