Ganymede (moon)

Template:Short description Template:Distinguish Template:Featured article Template:Use mdy dates {{#invoke:infobox|infoboxTemplate | class = vcard | titleclass = fn org | title = Ganymede | image = {{#invoke:InfoboxImage|InfoboxImage|image=Ganymede - Perijove 34 Composite.png|upright={{#if:||1.1}}|alt=Surface with swatches of light and dark brown. The bright crater on the lower left is the Tros crater.}} | caption = Ganymede in true color as imaged by the Juno spacecraft in June 2021Template:Efn<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> | headerstyle = {{#if:Wheat|background-color:Wheat|background-color:#E0CCFF}} | labelstyle = max-width:{{#if:||11em}}; | autoheaders = y

| header1 = Discovery<ref name="SidereusNuncius">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="NASA">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

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| label11 = Template:Longitem | data11 = | label12 = Pronunciation | data12 = Template:IPAc-en
Template:Respell<ref>Template:Cite OED
Template:Cite Merriam-Webster</ref> | label13 = Template:Longitem | data13 = {{#invoke:Lang|lang}}, Template:Transliteration | label14 = Template:Longitem | data14 = Jupiter Template:Rn | label15 = Template:Longitem | data15 = | label16 = Adjectives | data16 = Ganymedian,<ref>Quinn Passey & E. M. Shoemaker (1982) "Craters on Ganymede and Callisto", in David Morrison, ed., Satellites of Jupiter, vol. 3, International Astronomical Union, pp. 385–386, 411.</ref>
Ganymedean<ref>Journal of Geophysical Research, v. 95 (1990).</ref><ref>E. M. Shoemaker et al. (1982) "Geology of Ganymede", in David Morrison, ed., Satellites of Jupiter, vol. 3, International Astronomical Union, pp. 464, 482, 496.</ref> (Template:IPAc-en) | label17 = Symbol | data17 =

| header20 = Orbital characteristics{{#ifeq:|yes| (barycentric)}}

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0.20° (to Jupiter's equator)<ref name="orbit" /> | label41 = Template:Longitem | data41 = | label42 = Template:Longitem | data42 = | label43 = Template:Longitem | data43 = | label44 = Template:Longitem | data44 = | label45 = Template:Longitem | data45 = | label46 = Template:Nowrap | data46 = | label47 = Satellite of | data47 = Jupiter | label48 = Group | data48 = Galilean moon | label49 = {{#switch: |yes|true=Satellites |Known satellites}} | data49 = | label50 = Star | data50 = | label51 = Earth MOID | data51 = | label52 = Mercury MOID | data52 = | label53 = Venus MOID | data53 = | label54 = Mars MOID | data54 = | label55 = Jupiter MOID | data55 = | label56 = Saturn MOID | data56 = | label57 = Uranus MOID | data57 = | label58 = Neptune MOID | data58 = | label59 = T_Jupiter | data59 =

| header60 = Proper orbital elements

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({{#expr:365.25*360/1 round 3}} d) }} | label66 = Template:Longitem | data66 = {{#if:|{{{perihelion_rate}}} arcsec Template:\yr }} | label67 = Template:Longitem | data67 = {{#if:|{{{node_rate}}} arcsec Template:\yr}}

| header70 = Template:Anchor{{#if:| Physical characteristics|Physical characteristics}}

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| data100 = {{#if:K°C|

Surface temp.	min	mean	max
K	70<ref name="Delitsky1998" />	110<ref name="Delitsky1998">Template:Cite journal</ref>	152<ref name="Orton1996">Template:Cite journal</ref>
°C	−203	−163	−121
{{{temp_name3}}}
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| label101 = Surface absorbed dose rate | data101 = | label102 = Surface equivalent dose rate | data102 = | label103 = Template:Longitem | data103 = | label104 = Template:Longitem | data104 = | label105 = Template:Longitem | data105 = 4.61 (opposition)<ref name="jplfact" />
4.38 (in 1951)<ref name="horizons">{{#invoke:citation/CS1|citation |CitationClass=web }} (4.38 on 1951-Oct-03).</ref> | label106 = Template:Longitem | data106 = | label107 = Template:Longitem | data107 = Template:Nowrap

| header110 = Atmosphere

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Ganymede is a natural satellite of Jupiter and the largest and most massive in the Solar System. Like Saturn's largest moon Titan, it is larger than the planet Mercury, but has somewhat less surface gravity than Mercury, Io, or the Moon due to its lower density compared to the three.<ref name="nasa.gany">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively.

Ganymede is composed of silicate rock and water in approximately equal proportions. It is a fully differentiated body with an iron-rich, liquid metallic core, giving it the lowest moment of inertia factor of any solid body in the Solar System. Its internal ocean potentially contains more water than all of Earth's oceans combined.<ref name="Ocean Hubble">Template:Cite news</ref><ref name="clubsandwich 2014">Template:Cite news</ref><ref name="Vance">Template:Cite journal</ref><ref name="NASA-20140501c">{{#invoke:citation/CS1|citation |CitationClass=web }}Template:Cbignore</ref>

Ganymede's magnetic field is probably created by convection within its core, and influenced by tidal forces from Jupiter's far greater magnetic field.<ref name="Kivelson2002">Template:Cite journal</ref> Ganymede has a thin oxygen atmosphere that includes O, O₂, and possibly O₃ (ozone).<ref name="Hall1998" /> Atomic hydrogen is a minor atmospheric constituent. Whether Ganymede has an ionosphere associated with its atmosphere is unresolved.<ref name="Eviatar2001">Template:Cite journal</ref>

Ganymede's surface is composed of two main types of terrain, the first of which are lighter regions, generally crosscut by extensive grooves and ridges, dating from slightly less than 4 billion years ago, covering two-thirds of Ganymede. The cause of the light terrain's disrupted geology is not fully known, but may be the result of tectonic activity due to tidal heating. The second terrain type are darker regions saturated with impact craters, which are dated to four billion years ago.<ref name="Showman1999" />

Ganymede's discovery is credited to Simon Marius and Galileo Galilei, who both observed it in 1610,<ref name="SidereusNuncius" />Template:Efn as the third of the Galilean moons, the first group of objects discovered orbiting another planet.<ref name="Planetary Society">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Its name was soon suggested by astronomer Simon Marius, after the mythological Ganymede, a Trojan prince desired by Zeus (the Greek counterpart of Jupiter), who carried him off to be the cupbearer of the gods.<ref name="Naming">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Beginning with Pioneer 10, several spacecraft have explored Ganymede.<ref name="Pioneer 11" /> The Voyager probes, Voyager 1 and Voyager 2, refined measurements of its size, while Galileo discovered its underground ocean and magnetic field. The next planned mission to the Jovian system is the European Space Agency's Jupiter Icy Moons Explorer (JUICE), which was launched in 2023.<ref name="esa-juice">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> After flybys of all three icy Galilean moons, it is planned to enter orbit around Ganymede.<ref name="selection">Template:Cite news</ref>

File:Ganymede, Earth & Moon size comparison.jpg

Size comparison of Earth, the Moon (top left), and Ganymede (bottom left)

HistoryEdit

Chinese astronomical records report that in 365 BC, Gan De detected what might have been a moon of Jupiter, probably Ganymede, with the naked eye.<ref name="ads.793b">Template:Cite journal</ref> However, Gan De reported the color of the companion as reddish, which is puzzling since moons are too faint for their color to be perceived with the naked eye.<ref name=encyc>Template:Cite encyclopedia</ref> Shi Shen and Gan De together made fairly accurate observations of the five major planets.<ref name="Deng2011">Template:Cite book</ref><ref>Template:Cite journal</ref>

On January 7, 1610, Galileo Galilei used a telescope to observe what he thought were three stars near Jupiter, including what turned out to be Ganymede, Callisto, and one body that turned out to be the combined light from Io and Europa; the next night he noticed that they had moved. On January 13, he saw all four at once for the first time, but had seen each of the moons before this date at least once. By January 15, Galileo concluded that the stars were actually bodies orbiting Jupiter.<ref name="SidereusNuncius" /><ref name="NASA" />Template:Efn

NameEdit

Galileo claimed the right to name the moons he had discovered. He considered "Cosmian Stars" and settled on "Medicean Stars", in honor of Cosimo II de' Medici.<ref name="Naming" />

The French astronomer Nicolas-Claude Fabri de Peiresc suggested individual names from the Medici family for the moons, but his proposal was not taken up.<ref name="Naming" /> Simon Marius, who had originally claimed to have found the Galilean satellites,<ref name="College">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> tried to name the moons the "Saturn of Jupiter", the "Jupiter of Jupiter" (this was Ganymede), the "Venus of Jupiter", and the "Mercury of Jupiter", another nomenclature that never caught on. Later on, after finding out about a suggestion from Johannes Kepler, Marius agreed with Kepler's proposal and so he then proposed a naming system based on Greek mythology instead. This final Kepler/Marius proposal was ultimately successful.<ref name="Naming" />

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This name and those of the other Galilean satellites fell into disfavor for a considerable time, and were not in common use until the mid-20th century. In much of the earlier astronomical literature, Ganymede is referred to instead by its Roman numeral designation, Template:Nowrap (a system introduced by Galileo), in other words "the third satellite of Jupiter". Following the discovery of moons of Saturn, a naming system based on that of Kepler and Marius was used for Jupiter's moons.<ref name="Naming" /> Ganymede is the only Galilean moon of Jupiter named after a male figure—like Io, Europa, and Callisto, he was a lover of Zeus.

In English, the Galilean satellites Io, Europa and Callisto have the Latin spellings of their names, but the Latin form of Ganymede is Ganymēdēs, which would be pronounced Template:IPAc-en.<ref>Merriam-Webster's Encyclopedia of Literature, 1995.</ref> However, the final syllable is dropped in English, perhaps under the influence of French Ganymède ({{#invoke:IPA|main}}).

Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of the dwarf planet symbols, proposed a Greek gamma (the initial of Ganymede) combined with the cross-bar of the Jupiter symbol as the symbol of Ganymede (File:Ganymede symbol (fixed width).svg). This symbol is not widely used.<ref name=moons>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Orbit and rotationEdit

File:Galilean moon Laplace resonance animation 2.gif

Laplace resonance of Ganymede, Europa, and Io (conjunctions are highlighted by color changes)

Ganymede orbits Jupiter at a distance of Template:Convert, third among the Galilean satellites,<ref name="Planetary Society" /> and completes a revolution every seven days and three hours (7.155 days<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>). Like most known moons, Ganymede is tidally locked, with one side always facing toward the planet, hence its day is also seven days and three hours.<ref name="The Grand Tour">Template:Cite book</ref> 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 0.0009–0.0022 and 0.05–0.32°, respectively.<ref name="Musotto2002">Template:Cite journal</ref> These orbital variations cause the axial tilt (the angle between the rotational and orbital axes) to vary between 0 and 0.33°.<ref name="Bills2005" />

Ganymede participates in orbital resonances with Europa and Io: for every orbit of Ganymede, Europa orbits twice and Io orbits four times.<ref name="Musotto2002" /><ref name="SPACE.com">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Conjunctions (alignment on the same side of Jupiter) between Io and Europa occur when Io is at periapsis and Europa at apoapsis. Conjunctions between Europa and Ganymede occur when Europa is at periapsis.<ref name="Musotto2002" /> The longitudes of the Io–Europa and Europa–Ganymede conjunctions change at the same rate, making triple conjunctions impossible. Such a complicated resonance is called the Laplace resonance.<ref name="Showman1997a">Template:Cite journal</ref> The current Laplace resonance is unable to pump the orbital eccentricity of Ganymede to a higher value.<ref name="Showman1997a" /> The value of about 0.0013 is probably a remnant from a previous epoch, when such pumping was possible.<ref name="SPACE.com" /> The Ganymedian orbital eccentricity is somewhat puzzling; if it is not pumped now it should have decayed long ago due to the tidal dissipation in the interior of Ganymede.<ref name="Showman1997a" /> This means that the last episode of the eccentricity excitation happened only several hundred million years ago.<ref name="Showman1997a" /> Because Ganymede's orbital eccentricity is relatively low—on average 0.0015<ref name="SPACE.com" />—tidal heating is negligible now.<ref name="Showman1997a" /> However, in the past Ganymede may have passed through one or more Laplace-like resonances<ref name="laplaceres" group=lower-alpha /> that were able to pump the orbital eccentricity to a value as high as 0.01–0.02.<ref name="Showman1999" /><ref name="Showman1997a" /> This probably caused a significant tidal heating of the interior of Ganymede; the formation of the grooved terrain may be a result of one or more heating episodes.<ref name="Showman1999" /><ref name="Showman1997a" />

There are two hypotheses for the origin of the Laplace resonance among Io, Europa, and Ganymede: that it is primordial and has existed from the beginning of the Solar System;<ref name="Peale2002">Template:Cite journal</ref> or that it developed after the formation of the Solar System. A possible sequence of events for the latter scenario is as follows: Io raised tides on Jupiter, causing Io's orbit to expand (due to conservation of momentum) until it encountered the 2:1 resonance with Europa; after that, the expansion continued, but some of the angular moment was transferred to Europa as the resonance caused its orbit to expand as well; the process continued until Europa encountered the 2:1 resonance with Ganymede.<ref name="Showman1997a" /> Eventually the drift rates of conjunctions between all three moons were synchronized and locked in the Laplace resonance.<ref name="Showman1997a" />

Physical characteristicsEdit

File:Noaa ganymede.jpg

Depiction of Ganymede centered over 45° W. longitude; dark areas are Perrine (upper) and Nicholson (lower) regions; prominent craters are Tros (upper right) and Cisti (lower left).

File:Ganymede - Voyager 2 (26670869304).png

Three high-resolution views of Ganymede taken by Voyager 1 near closest approach on July 9, 1979

SizeEdit

Template:See also With a diameter of about Template:Convert and a mass of Template:Convert, Ganymede is the largest and most massive moon in the Solar System.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> It is slightly more massive than the second most massive moon, Saturn's satellite Titan, and is more than twice as massive as the Earth's Moon. It is larger than the planet Mercury, which has a diameter of Template:Convert but is only 45 percent of Mercury's mass. Ganymede is the ninth-largest object in the solar system, but the tenth-most massive.

CompositionEdit

The average density of Ganymede, 1.936 g/cm³ (a bit greater than Callisto's), suggests a composition of about equal parts rocky material and mostly water ices.<ref name="Showman1999" /> Some of the water is liquid, forming an underground ocean.<ref name="NYT-20150315" /> The mass fraction of ices is between 46 and 50 percent, which is slightly lower than that in Callisto.<ref name="Kuskov2005">Template:Cite journal</ref> Some additional volatile ices such as ammonia may also be present.<ref name="Kuskov2005" /><ref name="Spohn2003" /> The exact composition of Ganymede's rock is not known, but is probably close to the composition of L/LL type ordinary chondrites,<ref name="Kuskov2005" /> which are characterized by less total iron, less metallic iron and more iron oxide than H chondrites. The weight ratio of iron to silicon ranges between 1.05 and 1.27 in Ganymede, whereas the solar ratio is around 1.8.<ref name="Kuskov2005" />

Surface featuresEdit

File:Tros Crater, Ganymede - PJ34-1 - Detail - Map Projected.png

Tros crater (Juno; June 7, 2021)

File:Ganymede-moon.jpg

CitationClass=web }}</ref> The crater Tashmetum's prominent rays are at lower right, and the large ejecta field of Hershef at upper right. Part of dark Nicholson Regio is at lower left, bounded on its upper right by Harpagia Sulcus.

File:PIA26075-JupiterMoon-GanymedeTerrain-20231030.jpg

Ganymede grooved terrain
(Juno; June 7, 2021)

Ganymede's surface has an albedo of about 43 percent.<ref name="Calvin1995">Template:Cite journal</ref> Water ice seems to be ubiquitous on its surface, with a mass fraction of 50–90 percent,<ref name="Showman1999" /> significantly more than in Ganymede as a whole. Near-infrared spectroscopy has revealed the presence of strong water ice absorption bands at wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0 μm.<ref name="Calvin1995" /> The grooved terrain is brighter and has a more icy composition than the dark terrain.<ref name="RESA">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The analysis of high-resolution, near-infrared and UV spectra obtained by the Galileo spacecraft and from Earth observations has revealed various non-water materials: carbon dioxide, sulfur dioxide and, possibly, cyanogen, hydrogen sulfate and various organic compounds.<ref name="Showman1999" /><ref name="McCord1998">Template:Cite journal</ref> Galileo results have also shown magnesium sulfate (MgSO₄) and, possibly, sodium sulfate (Na₂SO₄) on Ganymede's surface.<ref name="The Grand Tour" /><ref name="McCord2001">Template:Cite journal</ref> These salts may originate from the subsurface ocean.<ref name="McCord2001" />

File:Craters on Ganymede.jpg

The craters Gula and Achelous (bottom), in the grooved terrain of Ganymede, with ejecta "pedestals" and ramparts

The Ganymedian surface albedo is very asymmetric; the leading hemisphere<ref name="hemispherecomment" group=lower-alpha /> is brighter than the trailing one.<ref name="Calvin1995" /> This is similar to Europa, but the reverse for Callisto.<ref name="Calvin1995" /> The trailing hemisphere of Ganymede appears to be enriched in sulfur dioxide.<ref name="Domingue1996">Template:Cite journal</ref><ref name="Domingue1998">Template:Cite journal</ref> The distribution of carbon dioxide does not demonstrate any hemispheric asymmetry, but little or no carbon dioxide is observed near the poles.<ref name="McCord1998" /><ref name="Hibbitts2003">Template:Cite journal</ref> Impact craters on Ganymede (except one) do not show any enrichment in carbon dioxide, which also distinguishes it from Callisto. Ganymede's carbon dioxide gas was probably depleted in the past.<ref name="Hibbitts2003" /> Ganymede's surface is a mix of two types of terrain: very old, highly cratered, dark regions and somewhat younger (but still ancient), lighter regions marked with an extensive array of grooves and ridges. The dark terrain, which comprises about one-third of the surface,<ref name="Patterson2007">Template:Cite journal</ref> contains clays and organic materials that could indicate the composition of the impactors from which Jovian satellites accreted.<ref name="Pappalardo2001">Template:Cite journal</ref>

The heating mechanism required for the formation of the grooved terrain on Ganymede is an unsolved problem in the planetary sciences. The modern view is that the grooved terrain is mainly tectonic in nature.<ref name="Showman1999" /> Cryovolcanism is thought to have played only a minor role, if any.<ref name="Showman1999" /> The forces that caused the strong stresses in the Ganymedian ice lithosphere necessary to initiate the tectonic activity may be connected to the tidal heating events in the past, possibly caused when the satellite passed through unstable orbital resonances.<ref name="Showman1999" /><ref name="Showman1997b">Template:Cite journal</ref> The tidal flexing of the ice may have heated the interior and strained the lithosphere, leading to the development of cracks and horst and graben faulting, which erased the old, dark terrain on 70 percent of the surface.<ref name="Showman1999" /><ref name="Bland2007">Template:Cite journal</ref> The formation of the grooved terrain may also be connected with the early core formation and subsequent tidal heating of Ganymede's interior, which may have caused a slight expansion of Ganymede by one to six percent due to phase transitions in ice and thermal expansion.<ref name="Showman1999" /> During subsequent evolution deep, hot water plumes may have risen from the core to the surface, leading to the tectonic deformation of the lithosphere.<ref name="Barr">Template:Cite journal</ref> Radiogenic heating within the satellite is the most relevant current heat source, contributing, for instance, to ocean depth. Research models have found that if the orbital eccentricity were an order of magnitude greater than currently (as it may have been in the past), tidal heating would be a more substantial heat source than radiogenic heating.<ref name="gra.6">Template:Cite journal</ref>

Cratering is seen on both types of terrain, but is especially extensive on the dark terrain: it appears to be saturated with impact craters and has evolved largely through impact events.<ref name="Showman1999" /> The brighter, grooved terrain contains many fewer impact features, which have been only of minor importance to its tectonic evolution.<ref name="Showman1999" /> The density of cratering indicates an age of 4 billion years for the dark terrain, similar to the highlands of the Moon, and a somewhat younger age for the grooved terrain (but how much younger is uncertain).<ref name="Zahnle1998">Template:Cite journal</ref> Ganymede may have experienced a period of heavy cratering 3.5 to 4 billion years ago similar to that of the Moon.<ref name="Zahnle1998" /> If true, the vast majority of impacts happened in that epoch, whereas the cratering rate has been much smaller since.<ref name="nineplanets.org-Ganymede">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Craters both overlay and are crosscut by the groove systems, indicating that some of the grooves are quite ancient. Relatively young craters with rays of ejecta are also visible.<ref name="nineplanets.org-Ganymede" /><ref name="Ganymede">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Ganymedian craters are flatter than those on the Moon and Mercury. This is probably due to the relatively weak nature of Ganymede's icy crust, which can (or could) flow and thereby soften the relief. Ancient craters whose relief has disappeared leave only a "ghost" of a crater known as a palimpsest.<ref name="nineplanets.org-Ganymede" />

One significant feature on Ganymede is a dark plain named Galileo Regio, which contains a series of concentric grooves, or furrows, likely created during a period of geologic activity.<ref name="Casacchia">Template:Cite journal</ref>

Ganymede also has polar caps, likely composed of water frost. The frost extends to 40° latitude.<ref name="The Grand Tour" /> These polar caps were first seen by the Voyager spacecraft. Theories on the formation of the caps include the migration of water to higher latitudes and the bombardment of the ice by plasma. Data from Galileo suggests the latter is correct.<ref name="Polar caps" /> The presence of a magnetic field on Ganymede results in more intense charged particle bombardment of its surface in the unprotected polar regions; sputtering then leads to redistribution of water molecules, with frost migrating to locally colder areas within the polar terrain.<ref name="Polar caps">Template:Cite journal</ref>

A crater named Anat provides the reference point for measuring longitude on Ganymede. By definition, Anat is at 128° longitude.<ref name="iau.table2">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The 0° longitude directly faces Jupiter, and unless stated otherwise longitude increases toward the west.<ref name="targetcoordsys">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Internal structureEdit

Ganymede appears to be fully differentiated, with an internal structure consisting of an iron-sulfide–iron core, a silicate mantle, and outer layers of water ice and liquid water.<ref name="Showman1999" /><ref name="Sohl2002">Template:Cite journal</ref> <ref name="Bhatia2017">Template:Cite journal</ref> The precise thicknesses of the different layers in the interior of Ganymede depend on the assumed composition of silicates (fraction of olivine and pyroxene) and amount of sulfur in the core.<ref name="Kuskov2005" /><ref name="Sohl2002" /><ref name="Kuskov2005b">Template:Cite journal</ref><ref name="Kuskov2005c">Template:Cite journal</ref> Ganymede has the lowest moment of inertia factor, 0.31,<ref name="Showman1999" /> among the solid Solar System bodies. This is a consequence of its substantial water content and fully differentiated interior.

Subsurface oceansEdit

File:Ganymede diagram.svg

Artist's cut-away representation of the internal structure of Ganymede. Layers drawn to scale.

In the 1970s, NASA scientists first suspected that Ganymede had a thick ocean between two layers of ice, one on the surface and one beneath a liquid ocean and atop the rocky mantle.<ref name="Showman1999" /><ref name="clubsandwich 2014" /><ref name="Sohl2002" /><ref name="Freeman2006">Template:Cite journal</ref><ref name="amount of water in ocean">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> In the 1990s, NASA's Galileo mission flew by Ganymede, and found indications of such a subsurface ocean.<ref name="NYT-20150315">Template:Cite news</ref> An analysis published in 2014, taking into account the realistic thermodynamics for water and effects of salt, suggests that Ganymede might have a stack of several ocean layers separated by different phases of ice, with the lowest liquid layer adjacent to the rocky mantle.<ref name="clubsandwich 2014" /><ref name="Vance" /><ref name="NASA-20140501c" /><ref name="Hubble 2015">Template:Cite news</ref> Water–rock contact may be an important factor in the origin of life.<ref name="clubsandwich 2014" /> The analysis also notes that the extreme depths involved (~800 km to the rocky "seafloor") mean that temperatures at the bottom of a convective (adiabatic) ocean can be up to 40 K higher than those at the ice–water interface.

In March 2015, scientists reported that measurements with the Hubble Space Telescope of how the aurorae moved confirmed that Ganymede has a subsurface ocean.<ref name="NYT-20150315"/> A large saltwater ocean affects Ganymede's magnetic field, and consequently, its aurorae.<ref name="Ocean Hubble"/><ref name="Hubble 2015"/><ref name="sciencedaily1503">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="sswater1">Template:Cite journal</ref> The evidence suggests that Ganymede's oceans might be the largest in the entire Solar System.<ref name='Sci Am 2017'>Template:Cite news</ref> These observations were later supported by Juno, which detected various salts and other compounds on Ganymede's surface, including hydrated sodium chloride, ammonium chloride, sodium bicarbonate, and possibly aliphatic aldehydes. These compounds were potentially deposited from Ganymede's ocean in past resurfacing events and were discovered to be most abundant in Ganymede's lower latitudes, shielded by its small magnetosphere.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> As a result of these findings, there is increasing speculation on the potential habitability of Ganymede's ocean.<ref name="amount of water in ocean"/><ref name="subsurface ocean found">Template:Cite news</ref>

CoreEdit

The existence of a liquid, iron–nickel-rich core<ref name="Bhatia2017" /> provides a natural explanation for the intrinsic magnetic field of Ganymede detected by Galileo spacecraft.<ref name="Hauk2006">Template:Cite journal</ref> The convection in the liquid iron, which has high electrical conductivity, is the most reasonable model of magnetic field generation.<ref name="Kivelson2002" /> The density of the core is 5.5–6 g/cm³ and the silicate mantle is 3.4–3.6 g/cm³.<ref name="Kuskov2005" /><ref name="Sohl2002" /><ref name="Kuskov2005b" /><ref name="Hauk2006" /> The radius of this core may be up to 500 km.<ref name="Hauk2006" /> The temperature in the core of Ganymede is probably 1500–1700 K and pressure up to Template:Convert.<ref name="Sohl2002" /><ref name="Hauk2006" />

Atmosphere and ionosphereEdit

In 1972, a team of Indian, British and American astronomers working in Java, Indonesia and Kavalur, India claimed that they had detected a thin atmosphere during an occultation, when it and Jupiter passed in front of a star.<ref name="Carlson1973">Template:Cite journal</ref> They estimated that the surface pressure was around 0.1 Pa (1 microbar).<ref name="Carlson1973" /> However, in 1979, Voyager 1 observed an occultation of the star κ Centauri during its flyby of Jupiter, with differing results.<ref name="Broadfoot1981">Template:Cite journal</ref> The occultation measurements were conducted in the far-ultraviolet spectrum at wavelengths shorter than 200 nm, which were much more sensitive to the presence of gases than the 1972 measurements made in the visible spectrum. No atmosphere was revealed by the Voyager data. The upper limit on the surface particle number density was found to be Template:Nowrap, which corresponds to a surface pressure of less than 2.5 μPa (25 picobar).<ref name="Broadfoot1981" /> The latter value is almost five orders of magnitude less than the 1972 estimate.<ref name="Broadfoot1981" />

File:Map of temparatureof ganymede.jpg

False-color temperature map of Ganymede

Despite the Voyager data, evidence for a tenuous oxygen atmosphere (exosphere) on Ganymede, very similar to the one found on Europa, was found by the Hubble Space Telescope (HST) in 1995.<ref name="Hall1998" /><ref name="JPLAtmosphere">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> HST actually observed airglow of atomic oxygen in the far-ultraviolet at the wavelengths 130.4 nm and 135.6 nm. Such an airglow is excited when molecular oxygen is dissociated by electron impacts,<ref name="Hall1998" /> which is evidence of a significant neutral atmosphere composed predominantly of O₂ molecules. The surface number density probably lies in the Template:Nowrap range, corresponding to the surface pressure of Template:Nowrap.<ref name="Hall1998" /><ref name="surfacedensitynumber" group=lower-alpha /> These values are in agreement with Voyager's upper limit set in 1981. The oxygen is not evidence of life; it is thought to be produced when water ice on Ganymede's surface is split into hydrogen and oxygen by radiation, with the hydrogen then being more rapidly lost due to its low atomic mass.<ref name="JPLAtmosphere" /> The airglow observed over Ganymede is not spatially homogeneous like that observed over Europa. HST observed two bright spots located in the northern and southern hemispheres, near ± 50° latitude, which is exactly the boundary between the open and closed field lines of the Ganymedian magnetosphere (see below).<ref name="Feldman2000">Template:Cite journal</ref> The bright spots are probably polar auroras, caused by plasma precipitation along the open field lines.<ref name="Johnson1997">Template:Cite journal</ref>

The existence of a neutral atmosphere implies that an ionosphere should exist, because oxygen molecules are ionized by the impacts of the energetic electrons coming from the magnetosphere<ref name="Paranicas1999">Template:Cite journal</ref> and by solar EUV radiation.<ref name="Eviatar2001" /> However, the nature of the Ganymedian ionosphere is as controversial as the nature of the atmosphere. Some Galileo measurements found an elevated electron density near Ganymede, suggesting an ionosphere, whereas others failed to detect anything.<ref name="Eviatar2001" /> The electron density near the surface is estimated by different sources to lie in the range 400–2,500 cm⁻³.<ref name="Eviatar2001" /> As of 2008, the parameters of the ionosphere of Ganymede were not well constrained.

Additional evidence of the oxygen atmosphere comes from spectral detection of gases trapped in the ice at the surface of Ganymede. The detection of ozone (O₃) bands was announced in 1996.<ref name="Noll1996">Template:Cite journal</ref> In 1997 spectroscopic analysis revealed the dimer (or diatomic) absorption features of molecular oxygen. Such an absorption can arise only if the oxygen is in a dense phase. The best candidate is molecular oxygen trapped in ice. The depth of the dimer absorption bands depends on latitude and longitude, rather than on surface albedo—they tend to decrease with increasing latitude on Ganymede, whereas O₃ shows an opposite trend.<ref name="Oxygen97">Template:Cite journal</ref> Laboratory work has found that O₂ would not cluster or bubble but would dissolve in ice at Ganymede's relatively warm surface temperature of 100 K (−173.15 °C).<ref name="sci.5320">Template:Cite journal</ref>

A search for sodium in the atmosphere, just after such a finding on Europa, turned up nothing in 1997. Sodium is at least 13 times less abundant around Ganymede than around Europa, possibly because of a relative deficiency at the surface or because the magnetosphere fends off energetic particles.<ref name="ic.126.1">Template:Cite journal</ref> Another minor constituent of the Ganymedian atmosphere is atomic hydrogen. Hydrogen atoms were observed as far as 3,000 km from Ganymede's surface. Their density on the surface is about Template:Nowrap.<ref name="Barth1997">Template:Cite journal</ref>

In 2021, water vapour was detected in the atmosphere of Ganymede.<ref>Water vapor detected on huge Jupiter moon Ganymede for 1st time Template:Webarchive, Space.com</ref>

MagnetosphereEdit

File:Ganymede magnetic field.svg

Magnetic field of the Jovian satellite Ganymede, which is embedded into the magnetosphere of Jupiter. Closed field lines are marked with green color.

The Galileo craft made six close flybys of Ganymede from 1995 to 2000 (G1, G2, G7, G8, G28 and G29)<ref name="Kivelson2002" /> and discovered that Ganymede has a permanent (intrinsic) magnetic moment independent of the Jovian magnetic field.<ref name="Kivelson1997">Template:Cite journal</ref> The value of the moment is about Template:Nowrap,<ref name="Kivelson2002" /> which is three times larger than the magnetic moment of Mercury. The magnetic dipole is tilted with respect to the rotational axis of Ganymede by 176°, which means that it is directed against the Jovian magnetic moment.<ref name="Kivelson2002" /> Its north pole lies below the orbital plane. The dipole magnetic field created by this permanent moment has a strength of 719 ± 2 nT at Ganymede's equator,<ref name="Kivelson2002" /> which should be compared with the Jovian magnetic field at the distance of Ganymede—about 120 nT.<ref name="Kivelson1997" /> The equatorial field of Ganymede is directed against the Jovian field, meaning reconnection is possible. The intrinsic field strength at the poles is two times that at the equator—1440 nT.<ref name="Kivelson2002" />

File:15-33i2-JupiterMoon-Ganymede-Aurora-20150312.png

Aurorae on Ganymede—auroral belt shifting may indicate a subsurface saline ocean.

The permanent magnetic moment carves a part of space around Ganymede, creating a tiny magnetosphere embedded inside that of Jupiter; it is the only moon in the Solar System known to possess the feature.<ref name="Kivelson1997" /> Its diameter is 4–5 Ganymede radii.<ref name="Kivelson1998">Template:Cite journal</ref> The Ganymedian magnetosphere has a region of closed field lines located below 30° latitude, where charged particles (electrons and ions) are trapped, creating a kind of radiation belt.<ref name="Kivelson1998" /> The main ion species in the magnetosphere is single ionized oxygen<ref name="Eviatar2001" /> (O⁺) which fits well with Ganymede's tenuous oxygen atmosphere. In the polar cap regions, at latitudes higher than 30°, magnetic field lines are open, connecting Ganymede with Jupiter's ionosphere.<ref name="Kivelson1998" /> In these areas, the energetic (tens and hundreds of kiloelectronvolt) electrons and ions have been detected,<ref name="Paranicas1999" /> which may cause the auroras observed around the Ganymedian poles.<ref name="Feldman2000" /> In addition, heavy ions precipitate continuously on Ganymede's polar surface, sputtering and darkening the ice.<ref name="Paranicas1999" />

The interaction between the Ganymedian magnetosphere and Jovian plasma is in many respects similar to that of the solar wind and Earth's magnetosphere.<ref name="Kivelson1998" /><ref name="Volwerk1999">Template:Cite journal</ref> The plasma co-rotating with Jupiter impinges on the trailing side of the Ganymedian magnetosphere much like the solar wind impinges on the Earth's magnetosphere. The main difference is the speed of plasma flow—supersonic in the case of Earth and subsonic in the case of Ganymede. Because of the subsonic flow, there is no bow shock off the trailing hemisphere of Ganymede.<ref name="Volwerk1999" />

In addition to the intrinsic magnetic moment, Ganymede has an induced dipole magnetic field.<ref name="Kivelson2002" /> Its existence is connected with the variation of the Jovian magnetic field near Ganymede. The induced moment is directed radially to or from Jupiter following the direction of the varying part of the planetary magnetic field. The induced magnetic moment is an order of magnitude weaker than the intrinsic one. The field strength of the induced field at the magnetic equator is about 60 nT—half of that of the ambient Jovian field.<ref name="Kivelson2002" /> The induced magnetic field of Ganymede is similar to those of Callisto and Europa, indicating that Ganymede also has a subsurface water ocean with a high electrical conductivity.<ref name="Kivelson2002" />

Given that Ganymede is completely differentiated and has a metallic core,<ref name="Showman1999" /><ref name="Hauk2006" /> its intrinsic magnetic field is probably generated in a similar fashion to the Earth's: as a result of conducting material moving in the interior.<ref name="Kivelson2002" /><ref name="Hauk2006" /> The magnetic field detected around Ganymede is likely to be caused by compositional convection in the core,<ref name="Hauk2006" /> if the magnetic field is the product of dynamo action, or magnetoconvection.<ref name="Kivelson2002" /><ref name="Hauck2002">Template:Cite journal</ref>

Despite the presence of an iron core, Ganymede's magnetosphere remains enigmatic, particularly given that similar bodies lack the feature.<ref name="Showman1999" /> Some research has suggested that, given its relatively small size, the core ought to have sufficiently cooled to the point where fluid motions, hence a magnetic field would not be sustained. One explanation is that the same orbital resonances proposed to have disrupted the surface also allowed the magnetic field to persist: with Ganymede's eccentricity pumped and tidal heating of the mantle increased during such resonances, reducing heat flow from the core, leaving it fluid and convective.<ref name="Bland2007" /> Another explanation is a remnant magnetization of silicate rocks in the mantle, which is possible if the satellite had a more significant dynamo-generated field in the past.<ref name="Showman1999" />

Radiation environmentEdit

The radiation level at the surface of Ganymede is considerably lower than on Europa, being 50–80 mSv (5–8 rem) per day, an amount that would cause severe illness or death in human beings exposed for two months.<ref name= "Podzolko">Template:Cite conference</ref>

Origin and evolutionEdit

File:Ganymede terrain.jpg

A sharp boundary divides the ancient dark terrain of Nicholson Regio from the younger, finely striated bright terrain of Harpagia Sulcus.

Ganymede probably formed by an accretion in Jupiter's subnebula, a disk of gas and dust surrounding Jupiter after its formation.<ref name="Canup2002">Template:Cite journal</ref> The accretion of Ganymede probably took about 10,000 years,<ref name="Mosqueira2003">Template:Cite journal</ref> much shorter than the 100,000 years estimated for Callisto. The Jovian subnebula may have been relatively "gas-starved" when the Galilean satellites formed; this would have allowed for the lengthy accretion times required for Callisto.<ref name="Canup2002" /> In contrast, Ganymede formed closer to Jupiter, where the subnebula was denser, which explains its shorter formation timescale.<ref name="Mosqueira2003" /> This relatively fast formation prevented the escape of accretional heat, which may have led to ice melt and differentiation: the separation of the rocks and ice. The rocks settled to the center, forming the core.<ref name="Bhatia2017"/> In this respect, Ganymede is different from Callisto, which apparently failed to melt and differentiate early due to loss of the accretional heat during its slower formation.<ref name="McKinnon2006">Template:Cite journal</ref> This hypothesis explains why the two Jovian moons look so dissimilar, despite their similar mass and composition.<ref name="Freeman2006" /><ref name="McKinnon2006" /> Alternative theories explain Ganymede's greater internal heating on the basis of tidal flexing<ref name="Showman2">Template:Cite journal</ref> or more intense pummeling by impactors during the Late Heavy Bombardment.<ref name="Baldwin">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name = "Phys.Org2010">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="LPI1158">Template:Cite conference</ref><ref name="Barr3">Template:Cite journal</ref> In the latter case, modeling suggests that differentiation would become a runaway process at Ganymede but not Callisto.<ref name="LPI1158" /><ref name="Barr3" />

After formation, Ganymede's core largely retained the heat accumulated during accretion and differentiation, only slowly releasing it to the ice mantle.<ref name="McKinnon2006" /> The mantle, in turn, transported it to the surface by convection.<ref name="Freeman2006" /> The decay of radioactive elements within rocks further heated the core, causing increased differentiation: an inner, iron–iron-sulfide core and a silicate mantle formed.<ref name="Hauk2006" /><ref name="McKinnon2006" /> With this, Ganymede became a fully differentiated body.<ref name="Bhatia2017" /> By comparison, the radioactive heating of undifferentiated Callisto caused convection in its icy interior, which effectively cooled it and prevented large-scale melting of ice and rapid differentiation.<ref name="Nagel2004">Template:Cite journal</ref> The convective motions in Callisto have caused only a partial separation of rock and ice.<ref name="Nagel2004" /> Today, Ganymede continues to cool slowly.<ref name="Hauk2006" /> The heat being released from its core and silicate mantle enables the subsurface ocean to exist,<ref name="Spohn2003">Template:Cite journal</ref> whereas the slow cooling of the liquid Fe–FeS core causes convection and supports magnetic field generation.<ref name="Hauk2006" /> The current heat flux out of Ganymede is probably higher than that out of Callisto.<ref name="McKinnon2006" />

A study from 2020 by Hirata, Suetsugu and Ohtsuki suggests that Ganymede probably was hit by a massive asteroid 4 billion years ago; an impact so violent that may have shifted the moon's axis. The study came to this conclusion analyzing images of the furrows system in the satellite's surface.<ref>Template:Cite journal</ref>

ExplorationEdit

Several spacecraft have performed close flybys of Ganymede: two Pioneer and two Voyager spacecraft made a single flyby each between 1973 and 1979; the Galileo spacecraft made six passes between 1996 and 2000; and the Juno spacecraft performed two flybys in 2019 and 2021.<ref name="NYT-20210608">Template:Cite news Template:Cbignore</ref> No spacecraft has yet orbited Ganymede, but the JUICE mission, which launched in April 2023, intends to do so.

Completed flybysEdit

File:Ganymede from Pioneer 10 19.jpg

Ganymede from Pioneer 10 (1973)

The first spacecraft to approach close to Ganymede was Pioneer 10, which performed a flyby in 1973 as it passed through the Jupiter system at high speed. Pioneer 11 made a similar flyby in 1974.<ref name="Pioneer 11">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Data sent back by the two spacecraft was used to determine the moon's physical characteristics<ref name="Terraformers">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and provided images of the surface with up to Template:Convert resolution.<ref name="chap6">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Pioneer 10's closest approach was 446,250 km, about 85 times Ganymede's diameter.<ref name="dmu.p10">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Voyager 1 and Voyager 2 both studied Ganymede when passing through the Jupiter system in 1979. Data from those flybys were used to refine the size of Ganymede, revealing it was larger than Saturn's moon Titan, which was previously thought to have been bigger.<ref name="Voyager">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Images from the Voyagers provided the first views of the moon's grooved surface terrain.<ref name="Voyager Mission">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

The Pioneer and Voyager flybys were all at large distances and high speeds, as they flew on unbound trajectories through the Jupiter system. Better data can be obtained from a spacecraft which is orbiting Jupiter, as it can encounter Ganymede at a lower speed and adjust the orbit for a closer approach. In 1995, the Galileo spacecraft entered orbit around Jupiter and between 1996 and 2000 made six close flybys of Ganymede.<ref name="The Grand Tour" /> These flybys were denoted G1, G2, G7, G8, G28 and G29.<ref name="Kivelson2002" /> During the closest flyby (G2), Galileo passed just 264 km from the surface of Ganymede (five percent of the moon's diameter),<ref name="Kivelson2002" /> which remains the closest approach by any spacecraft. During the G1 flyby in 1996, Galileo instruments detected Ganymede's magnetic field.<ref name="Magnetic Field Discovery">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Data from the Galileo flybys was used to discover the sub-surface ocean, which was announced in 2001.<ref name="Kivelson2002" /><ref name="The Grand Tour" /> High spatial resolution spectra of Ganymede taken by Galileo were used to identify several non-ice compounds on the surface.<ref name="McCord1998" />

The New Horizons spacecraft also observed Ganymede, but from a much larger distance as it passed through the Jupiter system in 2007 (en route to Pluto). The data were used to perform topographic and compositional mapping of Ganymede.<ref name="New Horizons">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Grundy2007">Template:Cite journal</ref>

Like Galileo, the Juno spacecraft orbited Jupiter. On 2019 December 25, Juno performed a distant flyby of Ganymede during its 24th orbit of Jupiter, at a range of Template:Convert. This flyby provided images of the moon's polar regions.<ref name="Juno">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Inaf">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> In June 2021, Juno performed a second flyby, at a closer distance of Template:Convert.<ref name="NYT-20210608" /><ref>Template:Cite news</ref> This encounter was designed to provide a gravity assist to reduce Juno's orbital period from 53 days to 43 days. Additional images of the surface were collected.<ref name="NYT-20210608" />

Future missionsEdit

The Jupiter Icy Moons Explorer (JUICE) will be the first to enter orbit around Ganymede itself. JUICE was launched on April 14, 2023.<ref name="esa-20221202">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> It is intended to perform its first flyby of Ganymede in 2031, then enter orbit of the moon in 2032. When the spacecraft consumes its propellant, JUICE is planned to be deorbited and impact Ganymede in February 2034.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

In addition to JUICE, NASA's Europa Clipper, which was launched in October 2024, will conduct 4 close flybys of Ganymede beginning in 2030.<ref>Tour Design Techniques for the Europa Clipper Mission – Campagnola et al. (2019)</ref> It may also crash into Ganymede at the end of its mission to aid JUICE in studying the surface's geochemistry.<ref name="LPI OPAG 2022">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Waldek 2022">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Cancelled proposalsEdit

Several other missions have been proposed to flyby or orbit Ganymede, but were either not selected for funding or cancelled before launch.

The Jupiter Icy Moons Orbiter would have studied Ganymede in greater detail.<ref name="JIMO">Template:Cite encyclopedia</ref> However, the mission was canceled in 2005.<ref name="n.050207">Template:Cite journal</ref> Another old proposal was called The Grandeur of Ganymede.<ref name="Pappalardo2001" />

A Ganymede orbiter based on the Juno probe was proposed in 2010 for the Planetary Science Decadal Survey.<ref name="ssb">{{#invoke:citation/CS1|citation |CitationClass=web }} {{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The mission was not supported, with the Decadal Survey preferring the Europa Clipper mission instead.<ref>Template:Cite book</ref>

The Europa Jupiter System Mission had a proposed launch date of 2020, and was a joint NASA and ESA proposal for exploration of many of Jupiter's moons including Ganymede. In February 2009 it was announced that ESA and NASA had given this mission priority ahead of the Titan Saturn System Mission.<ref name="bbc.7897585">Template:Cite news</ref> The mission was to consist of the NASA-led Jupiter Europa Orbiter, the ESA-led Jupiter Ganymede Orbiter, and possibly a JAXA-led Jupiter Magnetospheric Orbiter. The NASA and JAXA components were later cancelled, and ESA's appeared likely to be cancelled too,<ref name="esa.41177">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> but in 2012 ESA announced it would go ahead alone. The European part of the mission became the Jupiter Icy Moon Explorer (JUICE).<ref name="juiceL1">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

The Russian Space Research Institute proposed a Ganymede lander astrobiology mission called Laplace-P,<ref name="Lander workshop">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> possibly in partnership with JUICE.<ref name="Lander workshop" /><ref name="bbc.20407902">Template:Cite news</ref> If selected, it would have been launched in 2023. The mission was cancelled due to a lack of funding in 2017.<ref name=":1">Template:Cite news</ref>

NotesEdit

Template:Reflist

ReferencesEdit