A planetary core consists of the innermost layers of a planet . Cores may be entirely liquid, or a mixture of solid and liquid layers as is the case in the Earth. In the Solar System , core sizes range from about 20% (the Moon ) to 85% of a planet's radius ( Mercury ).
144-485: Callisto ( / k ə ˈ l ɪ s t oʊ / kə- LIST -oh ), or Jupiter IV , is the second-largest moon of Jupiter , after Ganymede . In the Solar System it is the third-largest moon after Ganymede and Saturn 's largest moon Titan , and nearly as large as the smallest planet Mercury . Callisto is, with a diameter of 4,821 km , roughly a third larger than Earth's Moon and orbits Jupiter on average at
288-618: A magnetic field around Ganymede . Then the Cassini probe to Saturn flew by Jupiter in 2000 and collected data on interactions of the Galilean moons with Jupiter's extended atmosphere. The New Horizons spacecraft flew by Jupiter in 2007 and made improved measurements of its satellites' orbital parameters. In 2016, the Juno spacecraft imaged the Galilean moons from above their orbital plane as it approached Jupiter orbit insertion, creating
432-501: A spheroid are highlighted in bold. These are the four Galilean moons , which are comparable in size to the Moon . The other moons are much smaller. The Galilean moon with the smallest amount of mass is greater than 7,000 times more massive than the most massive of the other moons. The irregular captured moons are shaded light gray and orange when prograde and yellow, red, and dark gray when retrograde . The orbits and mean distances of
576-479: A "reddish star". However, the first certain observations of Jupiter's satellites were those of Galileo Galilei in 1609. By January 1610, he had sighted the four massive Galilean moons with his 20× magnification telescope , and he published his results in March 1610. Simon Marius had independently discovered the moons one day after Galileo, although he did not publish his book on the subject until 1614. Even so,
720-456: A 1:2:4 resonance. Ganymede's larger mass means that it would have migrated inward at a faster rate than Europa or Io. Tidal dissipation in the Jovian system is still ongoing and Callisto will likely be captured into the resonance in about 1.5 billion years, creating a 1:2:4:8 chain. The outer, irregular moons are thought to have originated from captured asteroids , whereas the protolunar disk
864-411: A 5–10% mass deficit for the entire core and a density deficit from 4–5% for the inner core. The Fe/Ni value of the core is well constrained by chondritic meteorites. Sulfur, carbon, and phosphorus only account for ~2.5% of the light element component/mass deficit. No geochemical evidence exists for including any radioactive elements in the core. However, experimental evidence has found that potassium
1008-464: A Metal World,” is aiming to studying a body that could possibly be a remnant planetary core. As the field of exoplanets grows as new techniques allow for the discovery of both diverse exoplanets, the cores of exoplanets are being modeled. These depend on initial compositions of the exoplanets, which is inferred using the absorption spectra of individual exoplanets in combination with the emission spectra of their star. A chthonian planet results when
1152-478: A chondritic reference frame, show mild depletion in bulk silicate Earth and the moon. Pallasites are thought to form at the core-mantle boundary of an early planetesimal, although a recent hypothesis suggests that they are impact-generated mixtures of core and mantle materials. Dynamo theory is a proposed mechanism to explain how celestial bodies like the Earth generate magnetic fields. The presence or lack of
1296-457: A composition of approximately equal parts of rocky material and water ice , with some additional volatile ices such as ammonia . The mass fraction of ices is 49–55%. The exact composition of Callisto's rock component is not known, but is probably close to the composition of L/LL type ordinary chondrites , which are characterized by less total iron , less metallic iron and more iron oxide than H chondrites . The weight ratio of iron to silicon
1440-817: A core. Conservation of energy calculations as well as magnetic field measurements can also constrain composition, and surface geology of the planets can characterize differentiation of the body since its accretion. Mercury, Venus, and Mars’ cores are about 75%, 50%, and 40% of their radius respectively. Planetary systems form from flattened disks of dust and gas that accrete rapidly (within thousands of years) into planetesimals around 10 km in diameter. From here gravity takes over to produce Moon to Mars-sized planetary embryos (10 – 10 years) and these develop into planetary bodies over an additional 10–100 million years. Jupiter and Saturn most likely formed around previously existing rocky and/or icy bodies, rendering these previous primordial planets into gas-giant cores. This
1584-618: A crewed mission to Callisto might be possible in the 2040s. Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion Arm → Milky Way → Milky Way subgroup → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster → Local Hole → Observable universe → Universe Each arrow ( → ) may be read as "within" or "part of". Moon of Jupiter There are 95 moons of Jupiter with confirmed orbits as of 5 February 2024 . This number does not include
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#17327865770871728-761: A diameter of only 1–2 km (0.62–1.2 mi), making it one of the faintest and smallest confirmed moons of Jupiter even as of 2023 . Meanwhile, in September 2011, Scott Sheppard, now a faculty member of the Carnegie Institution for Science , discovered two more irregular moons using the institution's 6.5-meter (21 ft) Magellan Telescopes at Las Campanas Observatory , raising Jupiter's known moon count to 67. Although Sheppard's two moons were followed up and confirmed by 2012, both became lost due to insufficient observational coverage. In 2016, while surveying for distant trans-Neptunian objects with
1872-546: A diamond. PSR J1719-1438 is a 5.7 millisecond pulsar found to have a companion with a mass similar to Jupiter but a density of 23 g/cm , suggesting that the companion is an ultralow mass carbon white dwarf , likely the core of an ancient star. Exoplanets with moderate densities (more dense than Jovian planets, but less dense than terrestrial planets) suggests that such planets like GJ1214b and GJ436 are composed of primarily water. Internal pressures of such water-worlds would result in exotic phases of water forming on
2016-714: A distance of 1,883,000 km , which is about six times further out than the Moon orbiting Earth. It is the outermost of the four large Galilean moons of Jupiter, which were discovered in 1610 with one of the first telescopes , being visible from Earth with common binoculars . The surface of Callisto is the oldest and most heavily cratered in the Solar System. Its surface is completely covered with impact craters. 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
2160-469: 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
2304-399: A first-order calculation of the components that make up the interior of a planetary body. The structure of rocky planets is constrained by the average density of a planet and its moment of inertia . The moment of inertia for a differentiated planet is less than 0.4, because the density of the planet is concentrated in the center. Mercury has a moment of inertia of 0.346, which is evidence for
2448-494: A functional dynamo, the Martian core was initially hotter by 150 K than the mantle (agreeing with the differentiation history of the planet, as well as the impact hypothesis), and with a liquid core potassium-40 would have had opportunity to partition into the core providing an additional source of heat. The model further concludes that the core of mars is entirely liquid, as the latent heat of crystallization would have driven
2592-543: A gas giant has its outer atmosphere stripped away by its parent star, likely due to the planet's inward migration. All that remains from the encounter is the original core. Carbon planets , previously stars, are formed alongside the formation of a millisecond pulsar . The first such planet discovered was 18 times the density of water, and five times the size of Earth. Thus the planet cannot be gaseous, and must be composed of heavier elements that are also cosmically abundant like carbon and oxygen; making it likely crystalline like
2736-435: A heat source for the outer layers of a planet. In the Earth, the heat flux over the core mantle boundary is 12 terawatts. This value is calculated from a variety of factors: secular cooling, differentiation of light elements, Coriolis forces , radioactive decay , and latent heat of crystallization. All planetary bodies have a primordial heat value, or the amount of energy from accretion. Cooling from this initial temperature
2880-651: 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 . At the time ESA's contribution still faced funding competition from other ESA projects. EJSM consisted of the NASA-led Jupiter Europa Orbiter ,
3024-490: A liquid outer layer that makes up 60% of the volume of the core, with a solid inner core. The cores of the rocky planets were initially characterized by analyzing data from spacecraft, such as NASA's Mariner 10 that flew by Mercury and Venus to observe their surface characteristics. The cores of other planets cannot be measured using seismometers on their surface, so instead they have to be inferred based on calculations from these fly-by observation. Mass and size can provide
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#17327865770873168-407: A longer-lasting (greater than one billion years) dynamo. If the core of Mars is liquid, the lower bound for sulfur would be five weight %. Ganymede has an observed magnetic field generated within its metallic core. Jupiter has an observed magnetic field generated within its core , indicating some metallic substance is present. Its magnetic field is the strongest in the Solar System after
3312-410: A magnetic field can help constrain the dynamics of a planetary core. Refer to Earth's magnetic field for further details. A dynamo requires a source of thermal and/or compositional buoyancy as a driving force. Thermal buoyancy from a cooling core alone cannot drive the necessary convection as indicated by modelling, thus compositional buoyancy (from changes of phase ) is required. On Earth the buoyancy
3456-502: A major source of heat powering the early Martian dynamo. Core merging between proto-Mars and another differentiated planetoid could have been as fast as 1000 years or as slow as 300,000 years (depending on the viscosity of both cores and mantles). Impact-heating of the Martian core would have resulted in stratification of the core and kill the Martian dynamo for a duration between 150 and 200 million years. Modelling done by Williams, et al. 2004 suggests that in order for Mars to have had
3600-469: A mass fraction of 25–50%. 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 , carbon dioxide , sulfur dioxide , and possibly ammonia and various organic compounds . Spectral data indicate that Callisto's surface is extremely heterogeneous at
3744-439: A metal core including the crystallization of perovskite . Crystallization of perovskite in an early magma ocean is an oxidation process and may drive the production and extraction of iron metal from an original silicate melt. Impacts between planet-sized bodies in the early Solar System are important aspects in the formation and growth of planets and planetary cores. The giant impact hypothesis states that an impact between
3888-522: A number of meter-sized moonlets thought to be shed from the inner moons , nor hundreds of possible kilometer-sized outer irregular moons that were only briefly captured by telescopes. All together, Jupiter's moons form a satellite system 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
4032-445: A nymph (or, according to some sources, the daughter of Lycaon ) who was associated with the goddess of the hunt, Artemis . The name was suggested by Simon Marius soon after Callisto's discovery. Marius attributed the suggestion to Johannes Kepler . 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
4176-411: A phase change would be on the order of 10 joules; equivalent to the total energy release due to earthquakes through geologic time . Such an event could explain the asteroid belt . Such phase changes would only occur at specific mass to volume ratios, and an example of such a phase change would be the rapid formation or dissolution of a solid core component. All of the rocky inner planets, as well as
4320-429: A rather intense ionosphere . Callisto is thought to have formed by slow accretion from the disk of the gas and dust that surrounded Jupiter after its formation. 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
4464-433: A reference point for comparison with other more active and complex worlds. 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 , a possible subsurface ocean might be composed of salt water . It is possible that halophiles could thrive in the ocean. As with Europa and Ganymede ,
Callisto (moon) - Misplaced Pages Continue
4608-673: A result, many could not be reliably tracked and ended up becoming lost. Beginning in 2009, a team of astronomers, namely Mike Alexandersen, Marina Brozović, Brett Gladman, Robert Jacobson, and Christian Veillet, began a campaign to recover Jupiter's lost irregular moons using the CFHT and Palomar Observatory 's 5.1-meter (17 ft) Hale Telescope . They discovered two previously unknown Jovian irregular moons during recovery efforts in September 2010, prompting further follow-up observations to confirm these by 2011. One of these moons, S/2010 J 2 (now Jupiter LII), has an apparent magnitude of 24 and
4752-538: A second oblique stem appears in Latin: Callistōn-, but the corresponding Callistonian has rarely appeared in English. One also sees ad hoc forms, such as Callistan , Callistian and Callistean . 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). This is significantly larger than
4896-518: A small amount of ammonia or other antifreeze , up to 5% by weight. In this case the water+ice layer can be as thick as 250–300 km. 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 (especially the dimensionless moment of inertia —0.3549 ± 0.0042—determined during close flybys) suggest that, if Callisto
5040-413: A solid silicate crust and mantle overlying a solid metallic outer core layer, followed by a deeper liquid core layer, and then a possible solid inner core making a third layer. The composition of the iron-rich core remains uncertain, but it likely contains nickel, silicon and perhaps sulfur and carbon, plus trace amounts of other elements. The composition of Venus ' core varies significantly depending on
5184-503: A stiff, cold outer layer of Callisto conducts heat without convection, whereas the ice beneath it convects in the subsolidus regime. 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. The convection in the interior parts of Callisto may be layered, because under
5328-458: A substantial fraction (several tens of a percent) of the mass of Jupiter captured from the solar nebula was passed through it. However, only 2% of the proto-disk mass of Jupiter is required to explain the existing satellites. Thus, several generations of Galilean-mass satellites may have been in Jupiter's early history. Each generation of moons might have spiraled into Jupiter, because of drag from
5472-841: A survey conducted with the Mauna Kea Observatory 's 2.2-meter (88 in) UH88 telescope in November 2000, discovering eleven new irregular moons of Jupiter including the previously lost Themisto with the aid of automated computer algorithms. From 2001 onward, Sheppard and Jewitt alongside other collaborators continued surveying for Jovian irregular moons with the 3.6-meter (12 ft) Canada-France-Hawaii Telescope (CFHT), discovering an additional eleven in December 2001, one in October 2002, and nineteen in February 2003. At
5616-399: A theoretical Mars-sized planet Theia and the early Earth formed the modern Earth and Moon. During this impact the majority of the iron from Theia and the Earth became incorporated into the Earth's core. Core merging between the proto-Mars and another differentiated planetoid could have been as fast as 1000 years or as slow as 300,000 years (depending on viscosity of both cores). Using
5760-767: A time-lapse movie of their motion. With a mission extension, Juno has since begun close flybys of the Galileans, flying by Ganymede in 2021 followed by Europa and Io in 2022. It flew by Io again in late 2023 and once more in early 2024. Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion Arm → Milky Way → Milky Way subgroup → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster → Local Hole → Observable universe → Universe Each arrow ( → ) may be read as "within" or "part of". Planetary core Gas giants also have cores, though
5904-476: A transitional landform to the multi-ring structures, as with the Lofn impact feature. Callisto's craters are generally shallower than those on the Moon . The largest impact features on Callisto's surface are multi-ring basins. 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). The second largest
Callisto (moon) - Misplaced Pages Continue
6048-429: A year later, taking low-resolution images of the four Galilean moons and returning data on their atmospheres and radiation belts. The Voyager 1 and Voyager 2 probes visited Jupiter in 1979, discovering the volcanic activity on Io and the presence of water ice on the surface of Europa . Ulysses further studied Jupiter's magnetosphere in 1992 and then again in 2000. The Galileo spacecraft
6192-460: Is Asgard , measuring about 1,600 km in diameter. Multi-ring structures probably originated as a result of a post-impact concentric fracturing of the lithosphere lying on a layer of soft or liquid material, possibly an ocean. 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
6336-514: Is siderophile element . Thus if metal segregation (between the Earth's core and mantle) occurred in under 45 million years, silicate reservoirs develop positive Hf/W anomalies, and metal reservoirs acquire negative anomalies relative to undifferentiated chondrite material. The observed Hf/W ratios in iron meteorites constrain metal segregation to under 5 million years, the Earth's mantle Hf/W ratio places Earth's core as having segregated within 25 million years. Several factors control segregation of
6480-462: Is tidally 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's magnetosphere than the other inner satellites because of its more remote orbit, located just outside Jupiter's main radiation belt. Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide and probably molecular oxygen , as well as by
6624-416: Is 0.9–1.3 in Callisto, whereas the solar ratio is around 1:8. Callisto's surface has an albedo of about 20%. 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. Water ice seems to be ubiquitous on the surface of Callisto, with
6768-513: Is around 600 +600 −300 within a factor of 2. Although the team considers their characterized candidates to be likely moons of Jupiter, they all remain unconfirmed due to insufficient observation data for determining reliable orbits. The true population of Jovian irregular moons is likely complete down to magnitude 23.2 at diameters over 3 km (1.9 mi) as of 2020 . The moons of Jupiter are listed below by orbital period. Moons massive enough for their surfaces to have collapsed into
6912-740: Is attributed to lighter elements that should be cosmically abundant and are iron-soluble; H, O, C, S, P, and Si. Earth's core contains half the Earth's vanadium and chromium , and may contain considerable niobium and tantalum . Earth's core is depleted in germanium and gallium . Sulfur is strongly siderophilic and only moderately volatile and depleted in the silicate earth; thus may account for 1.9 weight % of Earth's core. By similar arguments, phosphorus may be present up to 0.2 weight %. Hydrogen and carbon, however, are highly volatile and thus would have been lost during early accretion and therefore can only account for 0.1 to 0.2 weight % respectively. Silicon and oxygen thus make up
7056-826: Is believed that these are at least partially collisional families that were created when larger (but still small) parent bodies were shattered by impacts from asteroids captured by Jupiter's gravitational field. These families bear the names of their largest members. The identification of satellite families is tentative, but the following are typically listed: Based on their survey discoveries in 2000–2003, Sheppard and Jewitt predicted that Jupiter should have approximately 100 irregular satellites larger than 1 km (0.6 mi) in diameter, or brighter than magnitude 24. Survey observations by Alexandersen et al. in 2010–2011 agreed with this prediction, estimating that approximately 40 Jovian irregular satellites of this size remained undiscovered in 2012. In September 2020, researchers from
7200-405: Is called secular cooling, and in the Earth the secular cooling of the core transfers heat into an insulating silicate mantle. As the inner core grows, the latent heat of crystallization adds to the heat flux into the mantle. Small planetary cores may experience catastrophic energy release associated with phase changes within their cores. Ramsey (1950) found that the total energy released by such
7344-488: Is derived from crystallization of the inner core (which can occur as a result of temperature). Examples of compositional buoyancy include precipitation of iron alloys onto the inner core and liquid immiscibility both, which could influence convection both positively and negatively depending on ambient temperatures and pressures associated with the host-body. Other celestial bodies that exhibit magnetic fields are Mercury, Jupiter, Ganymede, and Saturn. A planetary core acts as
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#17327865770877488-438: Is difficult to produce in laboratory settings, due to the high pressures needed. Jupiter and Saturn appear to release a lot more energy than they should be radiating just from the sun, which is attributed to heat released by the hydrogen and helium layer. Uranus does not appear to have a significant heat source, but Neptune has a heat source that is attributed to a “hot” formation. The following summarizes known information about
7632-433: Is expected to have about 100 irregular moons larger than 1 km (0.6 mi) in diameter, plus around 500 more smaller retrograde moons down to diameters of 0.8 km (0.5 mi). Of the 87 known irregular moons of Jupiter, 38 of them have not yet been officially given names. The physical and orbital characteristics of the moons vary widely. The four Galileans are all over 3,100 kilometres (1,900 mi) in diameter;
7776-468: Is heated only by radioactive decay, while Europa's is also heated by tidal energy, as it is much closer to Jupiter. It is thought that of all of Jupiter's moons, Europa has the greatest chance of supporting microbial life . 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. The real breakthrough happened later with
7920-456: 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. 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
8064-461: Is likely to originate from the extinction of the signal in Earth's geocorona , which is greater when the trailing hemisphere is observed. Callisto's atmosphere has been modelled to gain better understanding of impact of collisional molecular interactions. Researchers used a kinetic method to model collisions between the constituent elements of Callisto's atmosphere (carbon dioxide, molecular oxygen and molecular hydrogen). The modeling took into account
8208-581: Is little evidence of tectonic activity. 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, the greater tidal heating experienced by Ganymede, and the more numerous and energetic impacts that would have been suffered by Ganymede during the Late Heavy Bombardment . The relatively simple geological history of Callisto provides planetary scientists with
8352-469: Is locked to be synchronous with its orbit. 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. These orbital variations cause
8496-413: Is one of the most heavily cratered in the Solar System. 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; on Callisto there are no large mountains, volcanoes or other endogenic tectonic features. The impact craters and multi-ring structures—together with associated fractures , scarps and deposits —are
8640-450: Is potential for potassium in planetary cores of planets, and therefore potassium-40 as well. Hafnium / tungsten (Hf/W) isotopic ratios, when compared with a chondritic reference frame, show a marked enrichment in the silicate earth indicating depletion in Earth's core. Iron meteorites, believed to be resultant from very early core fractionation processes, are also depleted. Niobium / tantalum (Nb/Ta) isotopic ratios, when compared with
8784-441: Is present within the core (in lower abundances than Jupiter). Saturn has a rock and or ice core 10–30 times the mass of the Earth, and this core is likely soluble in the gas envelope above, and therefore it is primordial in composition. Since the core still exists, the envelope must have originally accreted onto previously existing planetary cores. Thermal contraction/evolution models support the presence of metallic hydrogen within
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#17327865770878928-459: Is strongly siderophile when dealing with temperatures associated with core-accretion, and thus potassium-40 could have provided an important source of heat contributing to the early Earth's dynamo, though to a lesser extent than on sulfur rich Mars. The core contains half the Earth's vanadium and chromium, and may contain considerable niobium and tantalum. The core is depleted in germanium and gallium. Core mantle differentiation occurred within
9072-416: Is the planetary core accretion model of planet formation. Planetary differentiation is broadly defined as the development from one thing to many things; homogeneous body to several heterogeneous components. The hafnium-182 / tungsten-182 isotopic system has a half-life of 9 million years, and is approximated as an extinct system after 45 million years. Hafnium is a lithophile element and tungsten
9216-428: Is thought to have evolved predominantly under the influence of impacts . Prominent surface features include multi-ring structures , variously shaped impact craters , and chains of craters ( catenae ) and associated scarps , ridges and deposits. At a small scale, the surface is 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. This
9360-499: 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. The absolute ages of the landforms are not known. Callisto is composed of approximately equal amounts of rock and ice , with a density of about 1.83 g/cm , the lowest density and surface gravity of Jupiter's major moons. Compounds detected spectroscopically on
9504-522: Is very close to, or exceeds slightly, this anomalous melting temperature. 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. 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. Unlike neighboring Ganymede with its grooved terrain, there
9648-460: Is Καλλιστῴος Kallistōi-os , from which one might expect Latin Callistōius and English *Callistóian (with 5 syllables), parallel to Sapphóian (4 syllables) for Sapphō i and Letóian for Lētō i . However, the iota subscript is often omitted from such Greek names (cf. Inóan from Īnō i and Argóan from Argō i ), and indeed the analogous form Callistoan is found. In Virgil,
9792-502: 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. 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
9936-507: The P-wave shadow zone; the liquid outer core. By 1936 seismologists had determined the size of the overall core as well as the boundary between the fluid outer core and the solid inner core. The internal structure of the Moon was characterized in 1974 using seismic data collected by the Apollo missions of moonquakes . The Moon's core has a radius of 300 km. The Moon's iron core has
10080-500: The University of British Columbia identified 45 candidate irregular moons from an analysis of archival images taken in 2010 by the CFHT. These candidates were mainly small and faint, down to magnitude of 25.7 or above 0.8 km (0.5 mi) in diameter. From the number of candidate moons detected within a sky area of one square degree, the team extrapolated that the population of retrograde Jovian moons brighter than magnitude 25.7
10224-481: The axial tilt (the angle between the rotational and orbital axes) to vary between 0.4 and 1.6°. The dynamical isolation of Callisto means that it has never been appreciably tidally heated , which has important consequences for its internal structure and evolution . 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
10368-418: The crust , indicated by studies of the magnetic fields around Jupiter and its moons. 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. The existence of an ocean is more likely if water contains
10512-430: The first 30 million years of Earth's history. Inner core crystallization timing is still largely unresolved. Mars possibly hosted a core-generated magnetic field in the past. The dynamo ceased within 0.5 billion years of the planet's formation. Hf/W isotopes derived from the martian meteorite Zagami , indicate rapid accretion and core differentiation of Mars; i.e. under 10 million years. Potassium-40 could have been
10656-538: The 1970s. Several different suggestions were made for names of Jupiter's outer satellites, but none were universally accepted until 1975 when the International Astronomical Union 's (IAU) Task Group for Outer Solar System Nomenclature granted names to satellites V–XIII, and provided for a formal naming process for future satellites still to be discovered. The practice was to name newly discovered moons of Jupiter after lovers and favorites of
10800-523: The ESA-led Jupiter Ganymede Orbiter and possibly a JAXA -led Jupiter Magnetospheric Orbiter . 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. The study proposed a possible surface base on Callisto that would produce rocket propellant for further exploration of
10944-601: The Magellan Telescopes, Sheppard serendipitously observed a region of the sky located near Jupiter, enticing him to search for Jovian irregular moons as a detour. In collaboration with Chadwick Trujillo and David Tholen , Sheppard continued surveying around Jupiter from 2016 to 2018 using the Cerro Tololo Observatory 's 4.0-meter (13 ft) Víctor M. Blanco Telescope and Mauna Kea Observatory's 8.2-meter (27 ft) Subaru Telescope . In
11088-714: 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... However,
11232-498: The Solar System formed. Discovery of outer planet moons The Galilean moons of Jupiter ( Io , Europa , Ganymede , and Callisto ) were named by Simon Marius soon after their discovery in 1610. However, these names fell out of favor until the 20th century. The astronomical literature instead simply referred to "Jupiter I", "Jupiter II", etc., or "the first satellite of Jupiter", "Jupiter's second satellite", and so on. The names Io, Europa, Ganymede, and Callisto became popular in
11376-435: The Solar System. 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. In December 2003, NASA reported that
11520-586: The Sun's. Jupiter has a rock and/or ice core 10–30 times the mass of the Earth, and this core is likely soluble in the gas envelope above, and so primordial in composition. Since the core still exists, the outer envelope must have originally accreted onto a previously existing planetary core. Thermal contraction/evolution models support the presence of metallic hydrogen within the core in large abundances (greater than Saturn). Saturn has an observed magnetic field generated within its metallic core . Metallic hydrogen
11664-627: The Sun. The other four regular satellites, known as the inner moons, are much smaller and closer to Jupiter; these serve as sources of the dust that makes up Jupiter's rings. The remainder of Jupiter's moons are outer irregular satellites whose prograde and retrograde orbits are much farther from Jupiter and have high inclinations and eccentricities . The largest of these moons were likely asteroids that were captured from solar orbits by Jupiter before impacts with other small bodies shattered them into many kilometer-sized fragments, forming collisional families of moons sharing similar orbits. Jupiter
11808-475: The accepted belief that the Earth was much denser in its interior. Following the discovery of iron meteorites , Wiechert in 1898 postulated that the Earth had a similar bulk composition to iron meteorites, but the iron had settled to the interior of the Earth, and later represented this by integrating the bulk density of the Earth with the missing iron and nickel as a core. The first detection of Earth's core occurred in 1906 by Richard Dixon Oldham upon discovery of
11952-610: 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. At the same time, HST was able to detect condensed oxygen trapped on the surface of Callisto. Atomic hydrogen has also been detected in Callisto's atmosphere via recent analysis of 2001 Hubble Space Telescope data. Spectral images taken on 15 and 24 December 2001 were re-examined, revealing
12096-816: The background cratered plains. 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. 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. The largest craters with diameters over 60 km can have central domes, which are thought to result from central tectonic uplift after an impact; 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
12240-402: The chondritic reference model and combining known compositions of the crust and mantle , the unknown component, the composition of the inner and outer core, can be determined: 85% Fe, 5% Ni, 0.9% Cr, 0.25% Co, and all other refractory metals at very low concentration. This leaves Earth's core with a 5–10% weight deficit for the outer core, and a 4–5% weight deficit for the inner core; which
12384-415: The composition of these are still a matter of debate and range in possible composition from traditional stony/iron, to ice or to fluid metallic hydrogen . Gas giant cores are proportionally much smaller than those of terrestrial planets, though they can be considerably larger than the Earth's nevertheless; Jupiter 's is 10–30 times heavier than Earth, and exoplanet HD149026 b may have a core 100 times
12528-454: The core in large abundances (but still less than Jupiter). Missions to bodies in the asteroid belt will provide more insight to planetary core formation. It was previously understood that collisions in the solar system fully merged, but recent work on planetary bodies argues that remnants of collisions have their outer layers stripped, leaving behind a body that would eventually become a planetary core. The Psyche mission , titled “Journey to
12672-401: 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. Callisto has a very tenuous atmosphere composed of carbon dioxide and probably oxygen. It
12816-427: The dark plains on Ganymede . Instead of small craters, the almost ubiquitous surface features are small knobs and pits. The knobs are thought to represent remnants of crater rims degraded by an as-yet uncertain process. 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. Such sublimation of water or other volatiles from
12960-445: The density may lie between 3.1 and 3.6 g/cm. In this case, Callisto's interior would be in stark contrast to that of Ganymede , which appears to be fully differentiated. However, a 2011 reanalysis of Galileo data suggests that Callisto is not in hydrostatic equilibrium. In that case, the gravity data may be more consistent with a more thoroughly differentiated Callisto with a hydrated silicate core. The ancient surface of Callisto
13104-443: The direction opposite to Jupiter's rotation ( retrograde motion ). Jupiter's regular satellites are believed to have formed from a circumplanetary disk, a ring of accreting gas and solid debris analogous to a protoplanetary disk . They may be the remnants of a score of Galilean-mass satellites that formed early in Jupiter's history. Simulations suggest that, while the disk had a relatively high mass at any given moment, over time
13248-471: 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. Such avalanches are often observed near and inside impact craters and termed "debris aprons". Sometimes crater walls are cut by sinuous valley-like incisions called "gullies", which resemble certain Martian surface features. In the ice sublimation hypothesis,
13392-535: The discovery of smaller, kilometre-sized moons around Jupiter, the IAU has established an additional convention to limit the naming of small moons with absolute magnitudes greater than 18 or diameters smaller than 1 km (0.6 mi). Some of the most recently confirmed moons have not received names. Some asteroids share the same names as moons of Jupiter: 9 Metis , 38 Leda , 52 Europa , 85 Io , 113 Amalthea , 239 Adrastea . Two more asteroids previously shared
13536-437: The disk, with new moons then forming from the new debris captured from the solar nebula. By the time the present (possibly fifth) generation formed, the disk had thinned so that it no longer greatly interfered with the moons' orbits. The current Galilean moons were still affected, falling into and being partially protected by an orbital resonance with each other, which still exists for Io , Europa , and Ganymede : they are in
13680-535: The effaced remnants of old large craters called palimpsests , the central parts of multi-ring structures, and isolated patches in the cratered plains. 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 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
13824-404: 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 ). In all realistic models of Callisto the temperature in the layer between 100 and 200 km in depth
13968-503: 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 other sexual partners) 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
14112-402: The formation of Callisto lies then in the range 0.1 million–10 million years. 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. Details of the subsolidus convection in the ice is the main source of uncertainty in
14256-457: The formation of a subsurface ocean at a depth of 100–150 km and a small, rocky core . 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 . 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
14400-520: The fortuitous discovery of Callirrhoe by the Spacewatch survey in October 1999. During the 1990s, photographic plates phased out as digital charge-coupled device (CCD) cameras began emerging in telescopes on Earth, allowing for wide-field surveys of the sky at unprecedented sensitivities and ushering in a wave of new moon discoveries. Scott Sheppard , then a graduate student of David Jewitt , demonstrated this extended capability of CCD cameras in
14544-419: The god Jupiter ( Zeus ) and, since 2004, also after their descendants. All of Jupiter's satellites from XXXIV ( Euporie ) onward are named after descendants of Jupiter or Zeus, except LIII ( Dia ), named after a lover of Jupiter. Names ending with "a" or "o" are used for prograde irregular satellites (the latter for highly inclined satellites), and names ending with "e" are used for retrograde irregulars. With
14688-621: 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. 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. The current understanding of
14832-456: The high-resolution Galileo images showed that the bright, smooth plains correlate with heavily fractured and knobby terrain and do not show any signs of resurfacing. The Galileo images also revealed small, dark, smooth areas with overall coverage less than 10,000 km, which appear to embay the surrounding terrain. They are possible cryovolcanic deposits. Both the light and the various smooth plains are somewhat younger and less cratered than
14976-407: The idea has been raised that habitable conditions and even extraterrestrial microbial life may exist in the salty ocean under the Callistoan surface. 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. Callisto's ocean
15120-525: The impact on Callisto, or by very oblique impacts. 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. High-resolution Galileo images showed the bright patches to be predominately located on elevated surface features: crater rims , scarps , ridges and knobs. They are likely to be thin water frost deposits . Dark material usually lies in
15264-809: The irregular moons are highly variable over short timescales due to frequent planetary and solar perturbations , so proper orbital elements which are averaged over a period of time are preferably used. The proper orbital elements of the irregular moons listed here are averaged over a 400-year numerical integration by the Jet Propulsion Laboratory : for the above reasons, they may strongly differ from osculating orbital elements provided by other sources. Otherwise, recently discovered irregular moons without published proper elements are temporarily listed here with inaccurate osculating orbital elements that are italicized to distinguish them from other irregular moons with proper orbital elements. Some of
15408-548: The irregular moons' proper orbital periods in this list may not scale accordingly with their proper semi-major axes due to the aforementioned perturbations. The irregular moons' proper orbital elements are all based on the reference epoch of 1 January 2000. Some irregular moons have only been observed briefly for a year or two, but their orbits are known accurately enough that they will not be lost to positional uncertainties . Nine spacecraft have visited Jupiter. The first were Pioneer 10 in 1973, and Pioneer 11
15552-420: The largest Galilean, Ganymede , is the ninth largest object in the Solar System , after the Sun and seven of the planets , Ganymede being larger than Mercury . All other Jovian moons are less than 250 kilometres (160 mi) in diameter, with most barely exceeding 5 kilometres (3.1 mi). Their orbital shapes range from nearly perfectly circular to highly eccentric and inclined , and many revolve in
15696-484: The leading hemisphere has more sulfur dioxide . Many fresh impact craters like Lofn also show enrichment in carbon dioxide. Overall, the chemical composition of the surface, especially in the dark areas, may be close to that seen on D-type asteroids , whose surfaces are made of carbonaceous material. Callisto's battered surface lies on top of a cold, stiff and icy lithosphere that is between 80 and 150 km thick. A salty ocean 150–200 km deep may lie beneath
15840-445: 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. Absolute dating has not been carried out, but based on theoretical considerations,
15984-423: 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. On a sub-kilometer scale the surface of Callisto is more degraded than the surfaces of other icy Galilean moons . Typically there is a deficit of small impact craters with diameters less than 1 km as compared with, for instance,
16128-438: The mass of the Earth. Planetary cores are challenging to study because they are impossible to reach by drill and there are almost no samples that are definitively from the core. Thus, they are studied via indirect techniques such as seismology, mineral physics, and planetary dynamics. In 1797, Henry Cavendish calculated the average density of the Earth to be 5.48 times the density of water (later refined to 5.53), which led to
16272-485: The mid-20th century, whereas the rest of the moons remained unnamed and were usually numbered in Roman numerals V (5) to XII (12). Jupiter V was discovered in 1892 and given the name Amalthea by a popular though unofficial convention, a name first used by French astronomer Camille Flammarion . The other moons were simply labeled by their Roman numeral (e.g. Jupiter IX) in the majority of astronomical literature until
16416-501: The model used to calculate it, thus constraints are required. The existence of a lunar core is still debated; however, if it does have a core it would have formed synchronously with the Earth's own core at 45 million years post-start of the Solar System based on hafnium-tungsten evidence and the giant impact hypothesis . Such a core may have hosted a geomagnetic dynamo early on in its history. The Earth has an observed magnetic field generated within its metallic core. The Earth has
16560-430: 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 . 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. It is thought to proceed in the so-called stagnant lid regime, where
16704-403: The moon, have an iron-dominant core. Venus and Mars have an additional major element in the core. Venus’ core is believed to be iron-nickel, similarly to Earth. Mars, on the other hand, is believed to have an iron-sulfur core and is separated into an outer liquid layer around an inner solid core. As the orbital radius of a rocky planet increases, the size of the core relative to the total radius of
16848-411: 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. 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. The allowable timescale for
16992-451: The most suitable to base possible future crewed missions on to study the Jovian system. Callisto was discovered independently by Simon Marius and Galileo Galilei in 1610, along with the three other large Jovian moons— Ganymede , Io and Europa . Callisto, like all of Jupiter's moons, is named after one of Zeus 's many lovers or other sexual partners in Greek mythology . Callisto was
17136-493: The names Marius assigned are used today: Ganymede , Callisto , Io , and Europa . No additional satellites were discovered until E. E. Barnard observed Amalthea in 1892. With the aid of telescopic photography with photographic plates , further discoveries followed quickly over the course of the 20th century. Himalia was discovered in 1904, Elara in 1905, Pasiphae in 1908, Sinope in 1914, Lysithea and Carme in 1938, Ananke in 1951, and Leda in 1974. By
17280-555: The names of Jovian moons until spelling differences were made permanent by the IAU: Ganymede and asteroid 1036 Ganymed ; and Callisto and asteroid 204 Kallisto . These have prograde and nearly circular orbits of low inclination and are split into two groups: The irregular satellites are substantially smaller objects with more distant and eccentric orbits. They form families with shared similarities in orbit ( semi-major axis , inclination , eccentricity ) and composition; it
17424-510: 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". There's no established English adjectival form of the name. The adjectival form of Greek Καλλιστῴ Kallistōi
17568-444: The near future. The European Space Agency 's Jupiter Icy Moons Explorer (JUICE), which launched on 14 April 2023, will perform 21 close flybys of Callisto between 2031 and 2034. NASA's Europa Clipper , which launched on 14 October 2024, will conduct nine close flybys of Callisto beginning in 2030. China's CNSA Tianwen-4 is planned to launch to Jupiter around 2030 before entering orbit around Callisto. Formerly proposed for
17712-547: The next two years. Many more irregular moons of Jupiter will inevitably be discovered in the future, especially after the beginning of deep sky surveys by the upcoming Vera C. Rubin Observatory and Nancy Grace Roman Space Telescope in the mid-2020s. The Rubin Observatory's 8.4-meter (28 ft) aperture telescope and 3.5 square-degree field of view will probe Jupiter's irregular moons down to diameters of 1 km (0.6 mi) at apparent magnitudes of 24.5, with
17856-490: The only large features to be found on the surface. 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. 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 like Burr and Lofn , as well as
18000-445: 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 mean-motion resonance —in which the three inner Galilean satellites are locked—and probably never has. Callisto is expected to be captured into the resonance in about 1.5 billion years, completing the 1:2:4:8 chain. Like most other regular planetary moons, Callisto's rotation
18144-474: The other Galilean moons, charged-particle irradiation has had a relatively minor effect on Callisto's surface. The radiation level at Callisto's surface is equivalent to a dose of about 0.01 rem (0.1 mSv ) per day, which is just over ten times higher than Earth's average background radiation, but less than in Low Earth Orbit or on Mars . The average density of Callisto, 1.83 g/cm, suggests
18288-460: The planet decreases. This is believed to be because differentiation of the core is directly related to a body's initial heat, so Mercury's core is relatively large and active. Venus and Mars, as well as the moon, do not have magnetic fields. This could be due to a lack of a convecting liquid layer interacting with a solid inner core, as Venus’ core is not layered. Although Mars does have a liquid and solid layer, they do not appear to be interacting in
18432-401: The planetary cores of given non-stellar bodies. Mercury has an observed magnetic field, which is believed to be generated within its metallic core. Mercury's core occupies 85% of the planet's radius, making it the largest core relative to the size of the planet in the Solar System; this indicates that much of Mercury's surface may have been lost early in the Solar System's history. Mercury has
18576-612: The potential of increasing the known population by up to tenfold. Likewise, the Roman Space Telescope's 2.4-meter (7.9 ft) aperture and 0.28 square-degree field of view will probe Jupiter's irregular moons down to diameters of 0.3 km (0.2 mi) at magnitude 27.7, with the potential of discovering approximately 1,000 Jovian moons above this size. Discovering these many irregular satellites will help reveal their population's size distribution and collisional histories, which will place further constraints to how
18720-911: The process, Sheppard's team recovered several lost moons of Jupiter from 2003 to 2011 and reported two new Jovian irregular moons in June 2017. Then in July 2018, Sheppard's team announced ten more irregular moons confirmed from 2016 to 2018 observations, bringing Jupiter's known moon count to 79. Among these was Valetudo , which has an unusually distant prograde orbit that crosses paths with the retrograde irregular moons. Several more unidentified Jovian irregular satellites were detected in Sheppard's 2016–2018 search, but were too faint for follow-up confirmation. From November 2021 to January 2023, Sheppard discovered twelve more irregular moons of Jupiter and confirmed them in archival survey imagery from 2003 to 2018, bringing
18864-471: The remaining 91 known moons and the rings together comprising just 0.003% of the total orbiting mass. Of Jupiter 's moons, eight are regular satellites with prograde and nearly circular orbits that are not greatly inclined with respect to Jupiter's equatorial plane. The Galilean satellites are nearly spherical in shape due to their planetary mass , and are just massive enough that they would be considered major planets if they were in direct orbit around
19008-439: The remaining mass deficit of Earth's core; though the abundances of each are still a matter of controversy revolving largely around the pressure and oxidation state of Earth's core during its formation. No geochemical evidence exists to include any radioactive elements in Earth's core. Despite this, experimental evidence has found potassium to be strongly siderophilic at the temperatures associated with core formation, thus there
19152-461: The same time, another independent team led by Brett J. Gladman also used the CFHT in 2003 to search for Jovian irregular moons, discovering four and co-discovering two with Sheppard. From the start to end of these CCD-based surveys in 2000–2004, Jupiter's known moon count had grown from 17 to 63. All of these moons discovered after 2000 are faint and tiny, with apparent magnitudes between 22–23 and diameters less than 10 km (6.2 mi). As
19296-421: The same way that Earth's liquid and solid components interact to produce a dynamo. Current understanding of the outer planets in the solar system, the ice and gas giants, theorizes small cores of rock surrounded by a layer of ice, and in Jupiter and Saturn models suggest a large region of liquid metallic hydrogen and helium. The properties of these metallic hydrogen layers is a major area of contention because it
19440-421: The small scale. Small, bright patches of pure water ice are intermixed with patches of a rock–ice mixture and extended dark areas made of a non-ice material. The Callistoan surface is asymmetric: the leading hemisphere is darker than the trailing one. This is different from other Galilean satellites , where the reverse is true. The trailing hemisphere of Callisto appears to be enriched in carbon dioxide , whereas
19584-498: The sublimation–degradation hypothesis for the formation of the surface knobs. Callisto's ionosphere was first detected during Galileo flybys; its high electron density of 7–17 × 10 cm 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 ). However, oxygen has not yet been directly detected in
19728-454: 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 . It is not in an orbital resonance like the three other Galilean satellites— Io , Europa and Ganymede —and is thus not appreciably tidally heated . Callisto's rotation
19872-556: 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. In 2000, the Cassini spacecraft en route to Saturn acquired high-quality infrared spectra of the Galilean satellites including Callisto. In February–March 2007, the New Horizons probe on its way to Pluto obtained new images and spectra of Callisto. Callisto will be visited by three spacecraft in
20016-691: The thermal desorption of these compounds due to solar exposure and the resulting variations in temperature on the surface. The simulation showed that the density of Callisto's atmosphere could be explained by the trapping of hydrogen by the heavier gases, carbon dioxide and oxygen. The model shows how kinetic interactions between molecules affect the atmosphere, although it has limitations in terms of variables considered. The simulated densities correlate with expected thresholds for experimental detection. 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. Therefore,
20160-472: The time that the Voyager space probes reached Jupiter, around 1979, thirteen moons had been discovered, not including Themisto , which had been observed in 1975, but was lost until 2000 due to insufficient initial observation data. The Voyager spacecraft discovered an additional three inner moons in 1979: Metis , Adrastea , and Thebe . No additional moons were discovered until two decades later, with
20304-542: The total count to 92. Among these was S/2018 J 4 , a highly inclined prograde moon that is now known to be in same orbital grouping as the moon Carpo , which was previously thought to be solitary. On 22 February 2023, Sheppard announced three more moons discovered in a 2022 survey, now bringing Jupiter's total known moon count to 95. In a February 2023 interview with NPR , Sheppard noted that he and his team are currently tracking even more moons of Jupiter, which should place Jupiter's moon count over 100 once confirmed over
20448-601: 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 pico bar (0.75 μPa ) and particle density 4 × 10 cm. Because such a thin atmosphere would be lost in only about four years (see atmospheric escape ) , it must be constantly replenished, possibly by slow sublimation of carbon dioxide ice from Callisto's icy crust, which would be compatible with
20592-447: Was still massive enough to absorb much of their momentum and thus capture them into orbit. Many are believed to have been broken up by mechanical stresses during capture, or afterward by collisions with other small bodies, producing the moons we see today. Chinese historian Xi Zezong claimed that the earliest record of a Jovian moon (Ganymede or Callisto) was a note by Chinese astronomer Gan De of an observation around 364 BC regarding
20736-414: Was the first to enter orbit around Jupiter, arriving in 1995 and studying it until 2003. During this period, Galileo gathered a large amount of information about the Jovian system, making close approaches to all of the Galilean moons and finding evidence for thin atmospheres on three of them, as well as the possibility of liquid water beneath the surfaces of Europa, Ganymede, and Callisto. It also discovered
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