Callisto (/kəˈlɪstoʊ/kə-LIST-oh) is the second-largestmoon of Jupiter, afterGanymede. It is also thethird-largest moon in the Solar System, following Ganymede andSaturn's moonTitan, and nearly as large as the planetMercury. With a diameter of4,821 km, Callisto is roughly a third larger than Earth'sMoon and orbits Jupiter on average at a distance of 1.883 million km, which is about five times further out than the Moon orbiting Earth. It is the outermost of the four largeGalilean moons of Jupiter,[3] which were discovered in 1610 with one of thefirst telescopes, and is today visible from Earth with commonbinoculars.
The surface of Callisto is the oldest and most heavilycratered in the Solar System.[11] Its surface is completely covered with impact craters.[12] It does not show any signatures ofsubsurface processes such asplate tectonics orvolcanism, with no signs that geological activity in general has ever occurred, and is thought to have evolved predominantly under the influence ofimpacts.[13] Prominent surface features includemulti-ring structures, variously shapedimpact craters, and chains of craters calledcatenae and associatedscarps, ridges and deposits.[13] At a small scale, the surface is varied and made up of small, sparkly frostdeposits at the tips of high spots, surrounded by a low-lying, smooth blanket of dark material.[7] This is thought to result from thesublimation-driven degradation of smalllandforms, which is supported by the general deficit of small impact craters and the presence of numerous small knobs, considered to be their remnants.[14] The absolute ages of the landforms are not known.Callisto is composed of approximately equal amounts ofrock andice, with adensity of about1.83 g/cm3, the lowest density and surface gravity of Jupiter's major moons. Compounds detectedspectroscopically on the surface includewater ice,[15]carbon dioxide,silicates andorganic compounds. Investigation by theGalileo spacecraft revealed that Callisto may have a smallsilicatecore and possibly asubsurface ocean of liquidwater[15] at depths greater than100 km.[16][17]
It is not in anorbital resonance like the three other Galilean satellites—Io,Europa andGanymede—and is thus not appreciablytidally heated.[18] Callisto's rotation istidally locked to its orbit around Jupiter, so that it always faces the same direction, making Jupiter appear to hang directly overhead over its near-side. It is less affected by Jupiter'smagnetosphere than the otherinner satellites because of its more remote orbit, located just outside Jupiter's main radiation belt.[19][20] Callisto is surrounded by an extremely thinatmosphere composed ofcarbon dioxide[9] and probablymolecular oxygen,[10] as well as by a rather intenseionosphere.[21] Callisto is thought to have formed by slowaccretion from the disk of the gas and dust that surrounded Jupiter after its formation.[22] Callisto's gradual accretion and the lack of tidal heating meant that not enough heat was available for rapiddifferentiation. The slowconvection 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, rockycore.[23]
The likely presence of an ocean within Callisto leaves open the possibility that it could harborlife. However, conditions are thought to be less favorable than on nearbyEuropa.[24] Various space probes fromPioneers 10 and11 toGalileo andCassini have studied Callisto. Because of its lowradiation levels, Callisto has long been considered the most suitable to base possible future crewed missions on to study the Jovian system.[25]
Callisto, like all of Jupiter's moons, is named after one ofZeus's many lovers or other sexual partners inGreek mythology.Callisto was a nymph (or, according to some sources, the daughter ofLycaon) who was associated with the goddess of the hunt,Artemis.[26] The name was suggested bySimon Marius soon after Callisto's discovery.[27] Marius attributed the suggestion toJohannes Kepler.[26]
Jupiter is much blamed by the poets on account of his irregular loves. Three maidens are especially mentioned as having been clandestinely courted by Jupiter with success. Io, daughter of the River Inachus, Callisto of Lycaon, Europa of Agenor. Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell... I think, therefore, that I shall not have done amiss if the First is called by me Io, the Second Europa, the Third, on account of its majesty of light, Ganymede, the Fourth Callisto...[28][29]
However, the names of theGalilean satellites fell into disfavor for a considerable time, and were not revived in common use until the mid-20th century. In much of the earlier astronomical literature, Callisto is referred to by its Roman numeral designation, a system introduced by Galileo, asJupiter IV or as "the fourth satellite of Jupiter".[30]
There is no established English adjectival form of the name. The adjectival form of Greek ΚαλλιστῴKallistōi is ΚαλλιστῴοςKallistōi-os, from which one might expect LatinCallistōius and English *Callistóian (with 5 syllables), parallel to Sapphóian (4 syllables) forSapphōi[31] and Letóian forLētōi.[32] However, theiota subscript is often omitted from such Greek names (cf.Inóan[33] fromĪnōi[34] andArgóan[35] fromArgōi[36]), and indeed the analogous formCallistoan is found.[37][38][39]In Virgil, a secondoblique stem appears in Latin:Callistōn-,[40] but the correspondingCallistonian has rarely appeared in English.[41] One also seesad hoc forms, such asCallistan,[14]Callistian[42] andCallistean.[43][44]
Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of thedwarf planet symbols, proposed a Greekkappa (the initial of Callisto) combined with the cross-bar of the Jupiter symbol as the symbol of Callisto (). This symbol is not widely used.[45]
Galilean moons around Jupiter Jupiter·Io·Europa·Ganymede· Callisto
Callisto is the outermost of the four Galilean moons of Jupiter. It orbits at a distance of approximately 1.88 million km (26.3 times the 71,492 km radius of Jupiter itself).[3] This is significantly larger than the orbital radius—1.07 million km—of the next-closest Galilean satellite, Ganymede. As a result of this relatively distant orbit, Callisto does not participate in anymean-motion resonance—in which the three inner Galilean satellites are locked—and probably never has.[18] Callisto is expected to be captured into the resonance in about 1.5 billion years, completing the 1:2:4:8 chain.[46]
Like most other regular planetary moons, Callisto's rotation is locked to besynchronous with its orbit.[4] The length of Callisto's day, simultaneously itsorbital period, is about 16.7 Earth days. Its orbit is very slightly eccentric and inclined to the Jovianequator, with theeccentricity andinclination changingquasi-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.[18] These orbital variations cause theaxial tilt (the angle between the rotational and orbital axes) to vary between 0.4 and 1.6°.[47]
The dynamical isolation of Callisto means that it has never been appreciablytidally heated, which has important consequences for its internal structure andevolution.[48] Its distance from Jupiter also means that thecharged-particleflux from Jupiter'smagnetosphere at its surface is relatively low—about 300 times lower than, for example, that atEuropa. Hence, unlike the other Galilean moons, charged-particleirradiation has had a relatively minor effect on Callisto's surface.[19] The radiation level at Callisto's surface is equivalent to a dose of about 0.01rem (0.1mSv) per day, which is just over ten times higher than Earth's average background radiation,[49][50] but less than inLow Earth Orbit or onMars.
The averagedensity of Callisto, 1.83 g/cm3,[4] suggests a composition of approximately equal parts of rocky material andwater ice, with some additional volatile ices such asammonia.[16] The mass fraction of ices is 49–55%.[16][23] The exact composition of Callisto'srock component is not known, but is probably close to the composition of L/LL typeordinary chondrites,[16] which are characterized by less totaliron, less metallic iron and moreiron oxide thanH chondrites. The weight ratio of iron tosilicon is 0.9–1.3 in Callisto, whereas thesolar ratio is around 1:8.[16]
Callisto's surface has analbedo of about 20%.[7] Its surface composition is thought to be broadly similar to its composition as a whole. Near-infraredspectroscopy has revealed the presence of water iceabsorption bands at wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0 micrometers.[7] Water ice seems to be ubiquitous on the surface of Callisto, with a mass fraction of 25–50%.[17] The analysis of high-resolution,near-infrared andUVspectra obtained by theGalileo spacecraft and from the ground has revealed various non-ice materials:magnesium- andiron-bearing hydratedsilicates,[7]carbon dioxide,[52]sulfur dioxide,[53] and possiblyammonia and variousorganic compounds.[7][17] 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.[7][13]
The Callistoan surface is asymmetric: the leading hemisphere[g] is darker than the trailing one. This is different from otherGalilean satellites, where the reverse is true.[7] The trailing hemisphere[g] of Callisto appears to be enriched incarbon dioxide, whereas the leading hemisphere has moresulfur dioxide.[54] Many freshimpact craters likeLofn also show enrichment in carbon dioxide.[54] Overall, the chemical composition of the surface, especially in the dark areas, may be close to that seen onD-type asteroids,[13] whose surfaces are made ofcarbonaceous material.
Model of Callisto's internal structure showing a surface ice layer, a possible liquid water layer, and an ice–rock interior
Callisto's battered surface lies on top of a cold, stiff and icylithosphere that is between 80 and 150 km thick.[16][23] A salty ocean 150–200 km deep may lie beneath thecrust,[16][23] indicated by studies of themagnetic fields around Jupiter and its moons.[55][56] It was found that Callisto responds to Jupiter's varying background magnetic field like a perfectlyconducting 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.[56] The existence of an ocean is more likely if water contains a small amount ofammonia or otherantifreeze, up to 5% by weight.[23] In this case the water-and-ice layer can be as thick as 250–300 km.[16] 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 dimensionlessmoment of inertia[h]—0.3549 ± 0.0042—determined during close flybys) suggest that, if Callisto is inhydrostatic equilibrium, its interior is composed of compressedrocks andices, with the amount of rock increasing with depth due to partial settling of its constituents.[16][57] In other words, Callisto may be only partiallydifferentiated. The density and moment of inertia for an equilibrium Callisto are compatible with the existence of a smallsilicate 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][16] In this case, Callisto's interior would be in stark contrast tothat of Ganymede, which appears to be fully differentiated.[17][58]
However, a 2011 reanalysis of Galileo data suggests that Callisto is not in hydrostatic equilibrium.[59] In that case, the gravity data may be more consistent with a more thoroughly differentiated Callisto with a hydrated silicate core.[60]
Galileo image of cratered plains, illustrating the pervasive local smoothing of Callisto's surface
The ancient surface of Callisto is one of the most heavily cratered in the Solar System.[61] In fact, thecrater density is close tosaturation: any new crater will tend to erase an older one. The large-scalegeology is relatively simple; on Callisto there are no large mountains, volcanoes or otherendogenictectonic features.[62] The impact craters and multi-ring structures—together with associatedfractures,scarps anddeposits—are the only large features to be found on the surface.[13][62]
Callisto's surface can be divided into several geologically different parts: cratered plains, light plains, bright and dark smooth plains, and various units associated with particular multi-ring structures and impact craters.[13][62] The cratered plains make up most of the surface area and represent the ancient lithosphere, a mixture of ice and rocky material. The light plains include bright impact craters likeBurr andLofn, as well as the effaced remnants of old large craters calledpalimpsests,[i] the central parts of multi-ring structures, and isolated patches in the cratered plains.[13] These light plains are thought to be icy impact deposits. The bright, smooth plains make up a small fraction of Callisto's surface and are found in the ridge andtrough zones of theValhalla andAsgard formations and as isolated spots in the cratered plains. They were thought to be connected with endogenic activity, but the high-resolutionGalileo images showed that the bright, smooth plains correlate with heavily fractured and knobby terrain and do not show any signs of resurfacing.[13] TheGalileo images also revealed small, dark, smooth areas with overall coverage less than 10,000 km2, which appear to embay[j] the surrounding terrain. They are possiblecryovolcanic deposits.[13] Both the light and the various smooth plains are somewhat younger and less cratered than the background cratered plains.[13][63]
Impact craterHár with a central dome.Chains ofsecondary craters from formation of the more recent craterTindr at upper right crosscut the terrain
Impact crater diameters seen range from 0.1 km—a limit defined by theimaging resolution—to over 100 km, not counting the multi-ring structures.[13] 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 asTindr crater.[13] The largest craters with diameters over 60 km can have central domes, which are thought to result from centraltectonic uplift after an impact;[13] examples includeDoh andHár 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 transitionallandform to the multi-ring structures, as with theLofn impact feature.[13] Callisto's craters are generally shallower than those on theMoon.
The largest impact features on Callisto's surface are multi-ring basins.[13][62] Two are enormous.Valhalla is the largest, with a bright central region 600 km in diameter, and rings extending as far as 1,800 km from the center (see figure).[64] The second largest isAsgard, measuring about 1,600 km in diameter.[64] Multi-ring structures probably originated as a result of a post-impactconcentric fracturing of the lithosphere lying on a layer of soft or liquid material, possibly an ocean.[37] The catenae—for exampleGomul 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 veryoblique impacts.[13] A historical example of a disruption wasComet Shoemaker–Levy 9.
As mentioned above, small patches of pure water ice with analbedo as high as 80% are found on the surface of Callisto, surrounded by much darker material.[7] High-resolutionGalileo images showed the bright patches to be predominately located on elevated surface features:crater rims,scarps, ridges and knobs.[7] They are likely to be thin waterfrostdeposits. 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.[7]
On a sub-kilometer scale the surface of Callisto is more degraded than the surfaces of other icyGalilean moons.[7] Typically there is a deficit of small impact craters with diameters less than 1 km as compared with, for instance, the dark plains onGanymede.[13] Instead of small craters, the almost ubiquitous surface features are small knobs and pits.[7] The knobs are thought to represent remnants of crater rims degraded by an as-yet uncertain process.[14] The most likely candidate process is the slowsublimation of ice, which is enabled by a temperature of up to 165 K, reached at a subsolar point.[7] Such sublimation of water or othervolatiles from the dirty ice that is thebedrock causes its decomposition. The non-ice remnants formdebris avalanches descending from the slopes of the crater walls.[14] Such avalanches are often observed near and inside impact craters and termed "debris aprons".[7][13][14] Sometimes crater walls are cut by sinuous valley-like incisions called "gullies", which resemble certainMartian surface features.[7] 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.[65] 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 theSolar System. The ages of multi-ring structures and impact craters depend on chosen background cratering rates and are estimated by different authors to vary between 1 and 4 billion years.[13][61]
Callisto has a very tenuous atmosphere composed ofcarbon dioxide[9] and probably oxygen. It was detected by theGalileo 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 (0.75μPa) and particle density 4 × 108 cm−3. Because such a thin atmosphere would be lost in only about four years thoughatmospheric escape, it must be constantly replenished, possibly by slow sublimation of carbon dioxide ice from Callisto's icy crust,[9] which would be compatible with the sublimation–degradation hypothesis for the formation of the surface knobs.
Callisto's ionosphere was first detected duringGalileo flybys;[21] its high electron density of 7–17 × 104 cm−3 cannot be explained by the photoionization of the atmosphericcarbon dioxide alone. Hence, it is suspected that the atmosphere of Callisto is actually dominated bymolecular oxygen (in amounts 10–100 times greater thanCO 2).[10] However,oxygen has not yet been directly detected in the atmosphere of Callisto. Observations with theHubble 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.[66] At the same time, HST was able to detectcondensed oxygen trapped on the surface of Callisto.[67]
Atomic hydrogen has also been detected in Callisto's atmosphere via analysis of 2001 Hubble Space Telescope data.[68] 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 the 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 Earth'sgeocorona, which is greater when the trailing hemisphere is observed.[69]
The partialdifferentiation of Callisto (inferred e.g. from moment of inertia measurements) means that it has never been heated enough to melt its ice component.[23] Therefore, the most favorable model of its formation is a slowaccretion in the low-density Joviansubnebula—a disk of the gas and dust that existed around Jupiter after its formation.[22] 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.[22] The allowable timescale for the formation of Callisto lies then in the range 0.1 million–10 million years.[22]
Views of eroding (top) and mostly eroded (bottom) ice knobs (~100 m high), possibly formed from theejecta of an ancientimpact
The further evolution of Callisto afteraccretion was determined by the balance of theradioactive heating, cooling throughthermal conduction near the surface, and solid state or subsolidusconvection in the interior.[48] Details of the subsolidus convection in the ice is the main source of uncertainty in the models of allicy moons. It is known to develop when the temperature is sufficiently close to themelting point, due to the temperature dependence of iceviscosity.[70] 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.[70] 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.[23][70] For Callisto, the outer conductive layer corresponds to the cold and rigidlithosphere with a thickness of about 100 km. Its presence would explain the lack of any signs of theendogenic activity on the Callistoan surface.[70][71] The convection in the interior parts of Callisto may be layered, because under the high pressures found there, waterice exists in different crystalline phases beginning from theice I on the surface toice VII in the center.[48] The early onset of subsolidus convection in the Callistoan interior could have prevented large-scale ice melting and any resultingdifferentiation that would have otherwise formed a large rockycore and icymantle. 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.[71]
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 withpressure, achieving temperatures as low as 251 K at 2,070 bar (207 MPa).[23] 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.[48][70][71] The presence of even small amounts ofammonia—about 1–2% by weight—almost guarantees the liquid's existence because ammonia would lower the melting temperature even further.[23]
Although Callisto is very similar in bulk properties toGanymede, it apparently had a much simplergeological history. The surface appears to have been shaped mainly by impacts and otherexogenic forces.[13] Unlike neighboring Ganymede with its grooved terrain, there is little evidence oftectonic activity.[17] 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,[72] the greater tidal heating experienced by Ganymede,[73] and the more numerous and energetic impacts that would have been suffered by Ganymede during theLate Heavy Bombardment.[74][75][76] The relatively simple geological history of Callisto provides planetary scientists with a reference point for comparison with other more active and complex worlds.[17]
It is possible thathalophiles could thrive in the ocean.[78]As withEuropa andGanymede, the idea has been raised thathabitable conditions and evenextraterrestrial microbial life may exist in the saltyocean under the Callistoan surface.[24] 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.[24] Callisto's ocean is heated only by radioactive decay, while Europa's is also heated by tidal energy, as it is much closer to Jupiter.[78] It is thought that of all of Jupiter's moons, Europa has the greatest chance of supportingmicrobial life.[24][79]
ThePioneer 10 andPioneer 11 Jupiter encounters in the early 1970s contributed little new information about Callisto in comparison with what was already known from Earth-based observations.[7] The real breakthrough happened later with theVoyager 1 andVoyager 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.[7] A second round of exploration lasted from 1994 to 2003, when theGalileo 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. TheGalileo orbiter completed the global imaging of the surface and delivered a number of pictures with a resolution as high as 15 meters of selected areas of Callisto.[13] In 2000, theCassini spacecraft en route toSaturn acquired high-quality infrared spectra of the Galilean satellites including Callisto.[52] In February–March 2007, theNew Horizons probe on its way to Pluto obtained new images and spectra of Callisto.[82]
In 2003NASA conducted a conceptual study called Human Outer Planets Exploration (HOPE) regarding the future human exploration of theouter Solar System. The target chosen to consider in detail was Callisto.[25][92]
The study proposed a possible surface base on Callisto that would producerocket propellant for further exploration of the Solar System.[91] 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 ofEuropa, or be an ideal location for a Jovian system waystation servicing spacecraft heading farther into the outer Solar System, using agravity assist from a close flyby of Jupiter after departing Callisto.[25]
In December 2003, NASA reported that a crewed mission to Callisto might be possible in the 2040s.[93]
^Surface gravity derived from the mass (m), thegravitational constant (G) and the radius (r):.
^Escape velocity derived from the mass (m), thegravitational constant (G) and the radius (r):.
^abThe leading hemisphere is the hemisphere facing the direction of the orbital motion; the trailing hemisphere faces the reverse direction.
^The dimensionless moment of inertia referred to is, whereI is the moment of inertia,m the mass, andr the maximal radius. It is 0.4 for a homogenous spherical body, but less than 0.4 if density increases with depth.
^In the case of icy satellites, palimpsests are defined as bright circular surface features, probably old impact craters.[13]
^Toembay means to shut in, or shelter, as in a bay.
^abcdefghijklmnopqrsMoore, 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.Archived(PDF) from the original on 9 October 2022.
^abcMusotto, 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.
^Van Helden, Albert (August 1994)."Naming the Satellites of Jupiter and Saturn"(PDF).The Newsletter of the Historical Astronomy Division of the American Astronomical Society (32).Archived(PDF) from the original on 7 December 2022. Retrieved10 March 2023.
^P. Leonardi (1982), Geological results of twenty years of space enterprises: Satellites of Jupiter and Saturn, inGeologica romana, p. 468.
^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
^Jean-Pierre Burg & Mary Ford (1997)Orogeny Through Time, p. 55
^Bala, Gavin Jared; Miller, Kirk (7 March 2025)."Phobos and Deimos symbols"(PDF).unicode.org. The Unicode Consortium. Retrieved14 March 2025.
^abBrown, 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.
^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.
^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.ISSN0032-0633.
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