View source for Galilean moons

{{short description|Four largest moons of Jupiter}}
[[File:Jupiter and the Galilean Satellites (True Color).png|thumb|Montage of [[Jupiter]]'s four Galilean moons in a composite image depicting part of Jupiter and their relative sizes (positions are illustrative, not actual). From top to bottom: [[Io (moon)|Io]], [[Europa (moon)|Europa]], [[Ganymede (moon)|Ganymede]], [[Callisto (moon)|Callisto]].]]
The '''Galilean moons''' ({{IPAc-en|ˌ|g|æ|l|ᵻ|'|l|eɪ|.|ə|n}}),<ref>{{OED|Galilean}}</ref> or '''Galilean satellites''', are the four largest [[moons of Jupiter]]. They are, in descending-size order, [[Ganymede (moon)|Ganymede]], [[Callisto (moon)|Callisto]], [[Io (moon)|Io]], and [[Europa (moon)|Europa]]. They are the most [[apparent magnitude|readily visible]] [[Solar System object]]s after [[Saturn]], the dimmest of the [[classical planet]]s; though their closeness to bright Jupiter makes naked-eye observation very difficult, they are readily seen with common [[binoculars]], even under [[night sky]] [[Bortle scale|conditions]] of high [[light pollution]]. The invention of the [[telescope]] allowed astronomers to discover the moons in 1610. Through this, they became the first [[Timeline of discovery of Solar System planets and their moons|Solar System objects discovered]] since humans have started tracking the classical planets, and the first objects to be found to orbit any [[planet]] beyond Earth.

They are [[planetary-mass moon]]s and among the [[List of Solar System objects by size|largest objects]] in the [[Solar System]]. All four, along with [[Titan (moon)|Titan]], [[Triton (moon)|Triton]], and Earth's [[Moon]], are larger than any of the Solar System's [[dwarf planet]]s. The largest, Ganymede, is the largest moon in the Solar System and surpasses the planet [[Mercury (planet)|Mercury]] in size (though not mass). Callisto is only slightly smaller than Mercury in size; the smaller ones, Io and Europa, are about the size of the Moon. The three inner moons — Io, Europa, and Ganymede — are in a 4:2:1 [[orbital resonance]] with each other. While the Galilean moons are spherical, all of [[Jupiter]]'s remaining moons have irregular forms because they are too small for their [[self-gravitation]] to pull them into spheres.

The Galilean moons are named after [[Galileo Galilei]], who observed them in either December 1609 or January 1610, and recognized them as satellites of Jupiter in March 1610;<ref>{{Cite book |last=Drake |first=Stillman |title=Galileo at Work: His Scientific Biography |date=1978 |publisher=[[University of Chicago Press]] |isbn=978-0-226-16226-3 |location=Chicago}}</ref> they remained the only known moons of Jupiter until the discovery of the fifth largest moon of Jupiter [[Amalthea (moon)|Amalthea]] in 1892.<ref>{{Cite web|url=https://solarsystem.nasa.gov/moons/jupiter-moons/amalthea/in-depth|title=In Depth {{!}} Amalthea|website=NASA Solar System Exploration|access-date=2019-11-17|archive-date=2019-08-25|archive-url=https://web.archive.org/web/20190825223006/https://solarsystem.nasa.gov/moons/jupiter-moons/amalthea/in-depth/|url-status=live}}</ref> Galileo initially named his discovery the '''Cosmica Sidera''' ("[[Cosimo II de' Medici, Grand Duke of Tuscany|Cosimo]]'s stars") or '''Medicean Stars''', but the names that eventually prevailed were chosen by [[Simon Marius]]. Marius discovered the moons independently at nearly the same time as Galileo, 8 January 1610, and gave them their present individual names, after mythological characters that [[Zeus]] seduced or [[List of rape victims from ancient history and mythology|abducted]], which were suggested by [[Johannes Kepler]] in his ''Mundus Jovialis'', published in 1614.<ref>{{cite journal
 |author = Pasachoff, Jay M.
 |title=Simon Marius's Mundus Iovialis: 400th Anniversary in Galileo's Shadow
 |journal=Journal for the History of Astronomy|date=2015|volume=46
 |issue=2|pages=218–234
 |bibcode=2015AAS...22521505P |doi = 10.1177/0021828615585493
|s2cid=120470649
 }}</ref> Their discovery showed the importance of the telescope as a tool for astronomers by proving that there were objects in space that cannot be seen by the naked eye. The discovery of celestial bodies orbiting something other than Earth dealt a serious blow to the then-accepted (among educated Europeans) [[Geocentrism|Ptolemaic world system]], a geocentric theory in which everything orbits around Earth.

== History ==

=== Pre-discovery ===
[[File:Jupiter taken with Nikon P900 - 2019-06-16 (2).jpg|thumb|Photograph of Jupiter and its Galilean moons, visible with binoculars]]
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">{{cite journal |bibcode=1981BAAS...13..793B |title=Ancient Astronomy in Modern China |journal=Bulletin of the Astronomical Society |volume=13 |page=793 |last=Brecher |first=K. |date=1981 }}</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>{{cite encyclopedia
 | first=Huang | last=Yi-Long | editor=Helaine Selin |editor-link=Helaine Selin | title=Gan De | encyclopedia=Encyclopaedia of the history of science, technology, and medicine in non-western cultures
 | page=342 | publisher=Springer | date=1997 | url=https://books.google.com/books?id=GzjpCAAAQBAJ&pg=PA342 | isbn=978-0-7923-4066-9 }}</ref> [[Shi Shen]] and Gan De together made fairly accurate observations of the five major planets.<ref name="Deng2011">{{cite book|author=Yinke Deng|title=Ancient Chinese Inventions|url=https://books.google.com/books?id=EeVPT6UAk3EC&pg=PA68|date=March 3, 2011|publisher=Cambridge University Press|isbn=978-0-521-18692-6|page= 68}}</ref><ref>{{cite journal | last = Xi | first = Ze-zong | title = The Discovery of Jupiter's Satellite Made by Gan De 2000 Years Before Galileo | journal = [[Acta Astrophysica Sinica]] | volume = 1 | issue = 2 | pages = 87 | date = 1981 | bibcode = 1981AcApS...1...85X | url = http://en.cnki.com.cn/Article_en/CJFDTOTAL-TTWL198102000.htm | access-date = March 22, 2017 | archive-date = November 4, 2020 | archive-url = https://web.archive.org/web/20201104160900/http://en.cnki.com.cn/Article_en/CJFDTOTAL-TTWL198102000.htm | url-status = dead }}</ref>

=== Discovery ===
[[File:Satélites de Júpiter - Galileo Galilei.png|thumb|Galileo Galilei's observations of 13 January 1610 in a manuscript noting the positions of the Jovian moons, with modern depiction of their positions]]

As a result of [[Galilean telescope|improvements]] that [[Galileo Galilei]] made to the [[telescope]], with a magnifying capability of 20×,<ref>{{cite journal|first=Albert|last=Van Helden|title=The Telescope in the Seventeenth Century|journal=Isis|volume=65|issue=1|date=March 1974|pages=38–58|jstor=228880|doi=10.1086/351216|s2cid=224838258 }}</ref> he was able to see celestial bodies more distinctly than was previously possible. This allowed Galileo to observe in January 1610 what came to be known as the Galilean moons.<ref name="Galileo89">{{Cite book |last1=Galilei |first1=Galileo |author-link1=Galileo Galilei |title=Sidereus Nuncius, or The Sidereal Messenger |title-link=Sidereus Nuncius |last2=Van Helden |first2=Albert |date=1989 |publisher=[[University of Chicago Press]] |isbn=978-0-226-27902-2 |location=Chicago |pages=14–16 |language=en, la}}</ref><ref>{{cite book|title=The Starry Messenger|last=Galilei|first=Galileo|date=1610|location=Venice|url=https://archive.org/details/starrymessengerb00sisp|quote=On the seventh day of January in this present date 1610....|isbn=978-0-374-37191-3|url-access=registration}}</ref>

On 7 January 1610, Galileo wrote a letter containing the first mention of Jupiter's moons. At the time, he saw only three of them, and he believed them to be fixed stars near Jupiter. It turned out to be Ganymede, [[Callisto (moon)|Callisto]], and the combined light from [[Io (moon)|Io]] and [[Europa (moon)|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">{{cite web|last=Galilei |first=Galileo |url=http://hsci.cas.ou.edu/images/barker/5990/Sidereus-Nuncius-whole.pdf |date=March 1610 |title=Sidereus Nuncius |author2=translated by Edward Carlos |editor-first=Peter |editor-last=Barker |publisher=University of Oklahoma History of Science |access-date=January 13, 2010 |url-status=dead |archive-url=https://web.archive.org/web/20051220010647/http://hsci.cas.ou.edu/images/barker/5990/Sidereus-Nuncius-whole.pdf |archive-date=December 20, 2005 }}</ref><ref name="NASA">{{cite web |url=https://solarsystem.nasa.gov/moons/jupiter-moons/ganymede/in-depth/ |title=In Depth {{!}} Ganymede |website=NASA Solar System Exploration |access-date=June 16, 2021 |archive-date=July 28, 2018 |archive-url=https://web.archive.org/web/20180728113750/https://solarsystem.nasa.gov/moons/jupiter-moons/ganymede/in-depth/ |url-status=live }}</ref> He continued to observe these [[Celestial spheres|celestial orbs]] to 2 March 1610.

Galileo's discovery proved the importance of the telescope as a tool for astronomers by showing that there were objects in space to be discovered that until then had remained unseen by the naked eye. More importantly, the discovery of celestial bodies orbiting something other than Earth dealt a blow to the then-accepted [[Geocentrism|Ptolemaic world system]], which held that Earth was at the center of the universe and all other celestial bodies revolved around it.<ref>{{cite web|url=https://galileo.rice.edu/sci/observations/jupiter_satellites.html|title=Satellites of Jupiter|access-date=9 August 2007|work=The Galileo Project|publisher=[[Rice University]]|date=1995|url-status=live|archive-url=https://web.archive.org/web/20120211140650/https://galileo.rice.edu/sci/observations/jupiter_satellites.html|archive-date=11 February 2012}}</ref> Galileo's 13 March 1610, ''[[Sidereus Nuncius]]'' (''Starry Messenger''), which announced celestial observations through his telescope, does not explicitly mention [[Copernican heliocentrism]], a theory that placed the [[Sun]] at the center of the universe. Nevertheless, Galileo accepted the Copernican theory.<ref name=Galileo89/>

A Chinese historian of astronomy, [[Xi Zezong]], has claimed that a "small reddish star" observed near Jupiter in 364 BCE by Chinese astronomer [[Gan De]] may have been Ganymede. If true, this might predate Galileo's discovery by around two millennia.<ref>{{Cite journal |last=Ze-zong |first=Xi |date=June 1981 |title=The sighting of Jupiter's satellite by Gan De 2000 years before Galileo |journal=Chinese Astronomy and Astrophysics |language=en |volume=5 |issue=2 |pages=242–243 |doi=10.1016/0275-1062(81)90039-4|bibcode=1981ChA&A...5..242X }}</ref>

[[Simon Marius]] claimed another observation, and he later reported observing the moons in November 1609, with a first written record for 29 December 1609.<ref name="solarviews">{{Cite web |title=The Discovery of the Galilean Satellites |url=https://solarviews.com/eng/galdisc.htm |url-status=live |archive-url=https://web.archive.org/web/20191119090237/http://solarviews.com/eng/galdisc.htm |archive-date=2019-11-19 |access-date=2019-11-17 |website=solarviews.com}}</ref> However, because he did not publish these findings until after Galileo, there is a degree of uncertainty around his records.<ref name="solarviews" /> Later, it turned out that he had used the old style [[Julian calendar]], while Galilei used the modern style [[Gregorian calendar|Gregorian one]]. So, in fact, Marius observed the moons first on 8 January 1610, one day after Galilei.<ref name="Pasachoff">{{cite journal |last=Pasachoff |first=Jay M. |date=May 2015 |title=Simon Marius's Mundus Iovialis: 400th Anniversary in Galileo's Shadow |journal=Journal for the History of Astronomy |volume=46 |issue=2 |pages=218–234 |bibcode=2015JHA....46..218P |doi=10.1177/0021828615585493 |s2cid=120470649}}</ref>

=== Names ===
[[File:Sidereus Nuncius Medicean Stars.jpg|thumb|right|upright|The Medician stars in the ''[[Sidereus Nuncius]]'' (the 'starry messenger'), 1610. The moons are drawn in changing positions.]]
[[File:Apparatus to demonstrate the motion of Jupiter's satellites in Putnam Gallery, 2009-11-24.jpg|thumb|A ''[[Jovilabe]]'':<ref>{{cite web | url=https://catalogue.museogalileo.it/object/Jovilabe.html | title=Jovilabe | publisher=[[Museo Galileo]] | access-date=15 April 2015 | url-status=live | archive-url=https://archive.wikiwix.com/cache/20150416155028/https://catalogue.museogalileo.it/object/Jovilabe.html | archive-date=16 April 2015 }}</ref> an apparatus from the mid-18th century for demonstrating the orbits of Jupiter's satellites]]

In 1605, Galileo had been employed as a mathematics tutor for [[Cosimo II de' Medici, Grand Duke of Tuscany|Cosimo de' Medici]]. In 1609, Cosimo became Grand Duke Cosimo II of [[Tuscany]]. Galileo, seeking patronage from his now-wealthy former student and his powerful family, used the discovery of Jupiter's moons to gain it.<ref name=Galileo89/> On 13 February 1610, Galileo wrote to the Grand Duke's secretary:<ref name=Galileo89/>

{{blockquote|God graced me with being able, through such a singular sign, to reveal to my Lord my devotion and the desire I have that his glorious name live as equal among the stars, and since it is up to me, the first discoverer, to name these new planets, I wish, in imitation of the great sages who placed the most excellent heroes of that age among the stars, to inscribe these with the name of the Most Serene Grand Duke.}}

Galileo initially called his discovery the '''Cosmica Sidera''' ("Cosimo's stars"), in honour of Cosimo alone.{{Efn|''[[wikt:Cosimo|Cosimo]]'' is the Italian form of the Greek name ''[[wikt:Cosmas|Cosmas]]'' itself deriving from ''[[wikt:κόσμος#Ancient Greek|cosmos]]'' (whence the [[Grammatical gender|neuter]] plural adjective ''cosmica''). ''Sidera'' is the plural form of the Latin noun ''[[wikt:sidus#Latin|sidus]]'' "star, constellation".}} Cosimo's secretary suggested to change the name to '''Medicea Sidera''' ("the '''Medician stars'''"), honouring all four Medici brothers (Cosimo, Francesco, [[Carlo de' Medici (cardinal)|Carlo]], and Lorenzo).<ref name=Galileo89/> The discovery was announced in the ''[[Sidereus Nuncius]]'' ("Starry Messenger"), published in [[Venice]] in March 1610, less than two months after the first observations.

On 12 March 1610, Galileo wrote his dedicatory letter to the Duke of Tuscany, and the next day sent a copy to the Grand Duke, hoping to obtain the Grand Duke's support as quickly as possible. On 19 March, he sent the telescope he had used to first view Jupiter's moons to the Grand Duke, along with an official copy of ''[[Sidereus Nuncius]]'' (''The Starry Messenger'') that, following the secretary's advice, named the four moons the Medician Stars.<ref name=Galileo89/> In his dedicatory introduction, Galileo wrote:<ref name=Galileo89/>

{{blockquote|Scarcely have the immortal graces of your soul begun to shine forth on earth than bright stars offer themselves in the heavens which, like tongues, will speak of and celebrate your most excellent virtues for all time. Behold, therefore, four stars reserved for your illustrious name{{nbsp}}... which{{nbsp}}... make their journeys and orbits with a marvelous speed around the star of Jupiter{{nbsp}}... like children of the same family{{nbsp}}... Indeed, it appears the Maker of the Stars himself, by clear arguments, admonished me to call these new planets by the illustrious name of Your Highness before all others.}}

Other names put forward include:
* I. ''Principharus'' (for the "prince" of Tuscany), II. ''Victripharus'' (after [[Vittoria della Rovere]]), III. ''Cosmipharus'' (after [[Cosimo I de' Medici, Grand Duke of Tuscany|Cosimo de' Medici]]) and IV. ''Fernipharus'' (after Duke [[Ferdinando II de' Medici, Grand Duke of Tuscany|Ferdinando de' Medici]])<ref>{{cite book|title=Annuaire de l'Observatoire royal de Bruxelles|url=https://books.google.com/books?id=eF4LAQAAIAAJ&pg=PA263|year=1879|publisher=L'Académie royale des sciences, des lettres et des beaux-arts de Belgique|page=263|access-date=2016-03-03|archive-date=2016-04-29|archive-url=https://web.archive.org/web/20160429062938/https://books.google.com/books?id=eF4LAQAAIAAJ&pg=PA263|url-status=live}}</ref> – by [[Giovanni Battista Hodierna]], a disciple of Galileo and author of the first [[Ephemeris|ephemerides]] (''Medicaeorum Ephemerides'', 1656);
* ''Circulatores Jovis'', or ''Jovis Comites'' – by [[Johannes Hevelius]];
* ''Gardes'', or ''Satellites'' (from the Latin ''satelles, satellitis'', meaning "escorts") – by [[Jacques Ozanam]].

The names that eventually prevailed were chosen by [[Simon Marius]], who had discovered the moons independently at about the same time as Galileo. At the suggestion of [[Johannes Kepler]], he named them after four lovers of the god Zeus (the Greek equivalent of Jupiter) in his ''Mundus Jovialis'' (published in 1614):<ref name=Helden>{{cite journal |last1=Van Helden |first1=Albert |title=Naming the Satellites of Jupiter and Saturn |journal=The Newsletter of the Historical Astronomy Division of the American Astronomical Society |date=August 1994 |issue=32 |url=https://had.aas.org/sites/had.aas.org/files/HADN32.pdf |access-date=10 March 2023}}</ref>

{{blockquote|Jupiter is much blamed by the poets on account of his irregular loves. Three maidens are especially mentioned as having been clandestinely courted by Jupiter with success. Io, daughter of the River Inachus, Callisto of Lycaon, Europa of Agenor. Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell{{nbsp}}... 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{{nbsp}}... This fancy, and the particular names given, were suggested to me by Kepler, Imperial Astronomer, when we met at Ratisbon fair in October 1613. So if, as a jest, and in memory of our friendship then begun, I hail him as joint father of these four stars, again I shall not be doing wrong.}}

Galileo steadfastly refused to use Marius's names and invented as a result the numbering scheme that is still used nowadays, in parallel with proper moon names. The numbers run from Jupiter outward, thus I, II, III and IV for Io, Europa, Ganymede, and Callisto respectively.<ref name="marazzini"/> Galileo used this system in his notebooks but never actually published it.<ref name=Helden/> The numbered names (Jupiter ''x'') were used until the mid-20th century when other inner moons were discovered, and Marius's names became widely used.<ref name="marazzini">{{cite journal|last=Marazzini|first= C.|date=2005|title=The names of the satellites of Jupiter: from Galileo to Simon Marius|journal=Lettere Italiana|volume=57|issue= 3|pages=391–407}}</ref>

<gallery caption="The Galilean moons' namesakes">
File:Io Argos MAN Napoli Inv9556.jpg|Io (left) watched by [[Argus Panoptes]] (right) on Hera's orders
File:Wall painting - Europa and the bull - Pompeii (IX 5 18-21) - Napoli MAN 111475 - 02.jpg|Europa on the back of Zeus turned into a bull
File:Zeus abducts Ganymede, large terracotta, before 470 BC, AM Olympia, Olym26.jpg|Ganymede (left) abducted by Zeus (right)
File:Wall painting - Artemis and Kallisto - Pompeii (VII 12 26) - Napoli MAN 111441.jpg|Callisto (leftmost) with Eros and other nymphs, with Artemis seated
</gallery>

=== Determination of longitude ===
{{main|History of longitude#Methods of determining longitude|l1=History of longitude}}
[[File:Carte de France corrigée par ordre du Roy.jpg|thumb|Map of France presented in 1684, showing the outline of a previous map (Sanson, light outline) compared to the new survey by Cassini and Picard using the moons of Jupiter as timing reference (heavier, shaded outline). The King of France reportedly quipped that the astronomers had taken more territory from him than his enemies.<ref name =rgm />]]
Galileo's discovery had practical applications. Safe navigation required accurately determining a ship's position at sea. While [[latitude]] could be measured well enough by local astronomical observations, determining [[longitude]] required knowledge of the time of each observation synchronized to the time at a reference longitude. The [[History of longitude|longitude problem]] was so important that large prizes were offered for its solution at various times by Spain, The Netherlands, and The United Kingdom.

Galileo proposed determining longitude based on the timing of the orbits of the Galilean moons.<ref>{{Cite book |last=Howse |first=Derek |url=https://archive.org/details/greenwichtimedis0000hows |title=Greenwich time and the discovery of the longitude |date=1980 |publisher=[[Oxford University Press]] |isbn=978-0-19-215948-9 |location=Oxford; New York |pages=12}}</ref> The times of the eclipses of the moons could be precisely calculated in advance and compared with local observations on land or on ship to determine the local time and hence longitude. Galileo applied in 1616 for the Spanish prize of 6,000 gold [[ducat]]s with a lifetime pension of 2,000 a year, and almost two decades later for the Dutch prize, but by then he was under house arrest for [[Galileo affair|possible heresy]].<ref name="danson-wtw">{{Cite book |last=Danson |first=Edwin |title=Weighing the World: The Quest to Measure the Earth |date=2006 |publisher=[[Oxford University Press]] |isbn=978-0-19-518169-2 |location=Oxford; New York, N.Y}}</ref>{{rp|15–16}}

The main problem with the Jovian moon technique was that it was difficult to observe the Galilean moons through a telescope on a moving ship, a problem that Galileo tried to solve with the invention of the [[celatone]]. Others suggested improvements, but without success.<ref name =rgm>{{cite web|url=https://www.rmg.co.uk/stories/blog/solving-longitude-jupiters-moons|title=Solving Longitude: Jupiter's Moons|date=16 October 2014|publisher=[[Royal Museums Greenwich]]}}</ref>

Land mapping surveys had the same problem determining longitude, though with less severe observational conditions. The method proved practical and was used by [[Giovanni Domenico Cassini]] and [[Jean Picard]] to re-map [[France]].<ref>{{cite book | last=Howse | first=Derek | title=Greenwich Time and the Longitude | publisher=Philip Wilson | year=1997 | pages=26, 31}}</ref>

== Members ==
{{further|Moons of Jupiter}}
Some models predict that there may have been several generations of Galilean satellites in Jupiter's early history. Each generation of moons to have formed would have spiraled into Jupiter and been destroyed, due to tidal interactions with Jupiter's [[proto-satellite disk]], with new moons forming from the remaining debris. By the time the present generation formed, the gas in the proto-satellite disk had thinned out to the point that it no longer greatly interfered with the moons' orbits.<ref name="arxiv0812"/><ref name="newsci">{{cite web|url=https://www.newscientist.com/article/mg20126984.300-cannibalistic-jupiter-ate-its-early-moons.html|title=Cannibalistic Jupiter ate its early moons|last=Chown|first=Marcus|date=7 March 2009|work=[[New Scientist]]|access-date=18 March 2009|url-status=live|archive-url=https://web.archive.org/web/20090323013754/https://www.newscientist.com/article/mg20126984.300-cannibalistic-jupiter-ate-its-early-moons.html|archive-date=23 March 2009}}</ref>

Other models suggest that Galilean satellites formed in a proto-satellite disk, in which formation timescales were comparable to or shorter than orbital migration timescales.<ref name=dangelo_podolak_2015>{{cite journal |doi=10.1088/0004-637X/806/2/203 |arxiv=1504.04364 |bibcode=2015ApJ...806..203D |title=Capture and Evolution of Planetesimals in Circumjovian Disks |journal=The Astrophysical Journal |volume=806 |issue=2 |pages=203 |year=2015 |last1=d'Angelo |first1=Gennaro |last2=Podolak |first2=Morris |s2cid=119216797 }}</ref>  Io is [[anhydrous]] and likely has an interior of rock and metal.<ref name="arxiv0812">{{cite book |author1=Canup, Robin M. |author1-link=Robin Canup |author2=Ward, William R. |title=Origin of Europa and the Galilean Satellites |publisher=University of Arizona Press |date=2008-12-30 |arxiv=0812.4995|bibcode = 2009euro.book...59C |page=59|isbn=978-0-8165-2844-8}}</ref> Europa is thought to contain 8% ice and water by mass with the remainder rock.<ref name="arxiv0812"/> These moons are, in increasing order of distance from Jupiter:

{| class="sortable wikitable"
|+
|-
! Name<br>
! class="unsortable" |Image
! class="unsortable" |Model of interior 
! Diameter<br>(km)
! Mass<br>(kg)
! Density<br>(g/cm<sup>3</sup>)
! <small>[[Semi-major axis]]<br>(km)<ref>Computed using the [https://archive.today/20110520031937/https://www.minorplanetcenter.org/iau/NatSats/NaturalSatellites.html IAU-MPC Satellites Ephemeris Service] μ value</ref></small>
! <small>[[Orbital period]] ([[day]]s)<ref>{{Cite web |title=Galilean Moons of Jupiter |url=https://www.nasa.gov/wp-content/uploads/2009/12/moons_of_jupiter_lithograph.pdf |url-status=live |archive-url=https://web.archive.org/web/20170608134945/https://www.nasa.gov/sites/default/files/files/Moons_of_Jupiter_Lithograph.pdf |archive-date=8 June 2017 |website=NASA.gov}}</ref> (relative to Io)</small>
! [[Inclination]]<br>([[Degree (angle)|°]])<ref>Computed from [https://www.hnsky.org/iau-iag.htm IAG Travaux 2001] {{webarchive|url=https://web.archive.org/web/20110807210105/https://www.hnsky.org/iau-iag.htm |date=2011-08-07 }}.</ref>
! <small>[[Orbital eccentricity|Eccentricity]]</small>
|-
| style="text-align:center;" | '''[[Io (moon)|Io]]''' <br>''Jupiter I''
| style="background:black; text-align:center;" |[[File:Io highest resolution true color.jpg|135px]]
| style="background:black; text-align:center;" |[[File:Io_diagram.svg|215px]]
| style="text-align:center;" | {{val|3660.0}}<br>{{val|p=×&thinsp;|3637.4}}<br>{{val|p=×&thinsp;|3630.6}}
| style="text-align:center;" | {{sort|0893|{{val|8.93|e=22}}}}
| style="text-align:center;" | {{val|3.528}}
| style="text-align:center;" | {{val|421800}}
| style="text-align:center;" | 1.769 <br>(1)
| style="text-align:center;" | 0.050
| style="text-align:center;" | 0.0041
|-
| style="text-align:center;" | '''[[Europa (moon)|Europa]]''' <br>''Jupiter II''
| style="background:black; text-align:center;" |[[File:Europa in natural color.png|135px]]
| style="background:black; text-align:center;" |[[File:Europa_poster.svg|215px]]
| style="text-align:center;" | {{val|3121.6}}
| style="text-align:center;" | {{sort|0480|{{val|4.8|e=22}}}}
| style="text-align:center;" | 3.014
| style="text-align:center;" | {{val|671100}}
| style="text-align:center;" | 3.551 <br>(2.0)
| style="text-align:center;" | 0.471
| style="text-align:center;" | 0.0094
|-
| style="text-align:center;" | '''[[Ganymede (moon)|Ganymede]]''' <br>''Jupiter III''
| style="background:black; text-align:center;" |[[File:Ganymede - Perijove 34 Composite.png|135px]]
| style="background:black; text-align:center;" |[[File:Ganymede_diagram.svg|215px]]
| style="text-align:center;" | {{val|5268.2}}
| style="text-align:center;" | {{sort|1480|{{val|1.48|e=23}}}}
| style="text-align:center;" | 1.942
| style="text-align:center;" | {{val|1070400}}
| style="text-align:center;" | 7.155 <br>(4.0)
| style="text-align:center;" | 0.204
| style="text-align:center;" | 0.0011
|-
| style="text-align:center;" | '''[[Callisto (moon)|Callisto]]''' <br>''Jupiter IV''
| style="background:black; text-align:center;" |[[File:Callisto - July 8 1979 (38926064465).jpg|135px]]
| style="background:black; text-align:center;" |[[File:Callisto_diagram.svg|215px]]
| style="text-align:center;" | {{val|4820.6}}
| style="text-align:center;" | {{sort|1080|{{val|1.08|e=23}}}}
| style="text-align:center;" | 1.834
| style="text-align:center;" | {{val|1882700}}
| style="text-align:center;" | 16.689 <br>(9.4)
| style="text-align:center;" | 0.205
| style="text-align:center;" | 0.0074
|}

=== Io ===
{{main|Io (moon)}}
[[File:Io Tupan Patera.jpg|thumb|[[Tupan Patera]] on Io]]
Io (Jupiter I) is the innermost of the four Galilean moons of Jupiter; with a diameter of 3642&nbsp;kilometers, it is the [[List of moons by diameter|fourth-largest moon]] in the Solar System, and is only marginally larger than [[Moon|Earth's moon]]. It was named after [[Io (mythology)|Io]], a priestess of [[Hera]] who became one of the lovers of [[Zeus]]. It was referred to as "Jupiter I", or "The first satellite of Jupiter" until the mid-20th century.<ref name="marazzini"/>

With over 400 active volcanos, Io is the most geologically active object in the Solar System.<ref name=Lopes2004>{{cite journal |doi=10.1016/j.icarus.2003.11.013 |title=Lava lakes on Io: Observations of Io's volcanic activity from Galileo NIMS during the 2001 fly-bys |journal=Icarus |volume=169 |issue=1 |pages=140–74 |year=2004 |last1=Lopes |first1=Rosaly M.C |author1-link=Rosaly Lopes |last2=Kamp |first2=Lucas W |last3=Smythe |first3=William D |last4=Mouginis-Mark |first4=Peter |last5=Kargel |first5=Jeff |last6=Radebaugh |first6=Jani |last7=Turtle |first7=Elizabeth P |last8=Perry |first8=Jason |last9=Williams |first9=David A |last10=Carlson |first10=R.W |last11=Douté |first11=S. |bibcode=2004Icar..169..140L |author12=the Galileo NIMS |author13=SSI Teams }}</ref> Its surface is dotted with more than 100 mountains, some of which are taller than Earth's [[Mount Everest]].<ref name=Schenk2001>{{cite journal |doi=10.1029/2000JE001408 |title=The mountains of Io: Global and geological perspectives from Voyager and Galileo |journal=Journal of Geophysical Research: Planets |volume=106 |issue=E12 |pages=33201–22 |year=2001 |last1=Schenk |first1=Paul |last2=Hargitai |first2=Henrik |last3=Wilson |first3=Ronda |last4=McEwen |first4=Alfred |last5=Thomas |first5=Peter |bibcode=2001JGR...10633201S |doi-access=free }}</ref> Unlike most satellites in the outer Solar System (which have a thick coating of ice), Io is primarily composed of silicate rock surrounding a molten iron or iron sulfide core.<ref name="Anderson1996">{{cite journal
|last=Anderson
|first=J. D.
|display-authors=etal
|title=Galileo Gravity Results and the Internal Structure of Io
|journal=Science
|volume=272
|issue=5262
|pages=709–712
|date=1996
|doi=10.1126/science.272.5262.709
|pmid=8662566
|bibcode=1996Sci...272..709A
|s2cid=24373080
}}
</ref>

Although not proven, data from the Galileo orbiter indicates that Io might have its own magnetic field.<ref>{{cite journal |doi=10.1126/science.1079462 |pmid=12624258 |title=Cassini Imaging of Jupiter's Atmosphere, Satellites, and Rings |journal=Science |volume=299 |issue=5612 |pages=1541–7 |year=2003 |last1=Porco |first1=C. C. |last2=West |first2=Robert A. |last3=McEwen |first3=Alfred |last4=Del Genio |first4=Anthony D. |last5=Ingersoll |first5=Andrew P. |last6=Thomas |first6=Peter |last7=Squyres |first7=Steve |last8=Dones |first8=Luke |last9=Murray |first9=Carl D. |last10=Johnson |first10=Torrence V. |last11=Burns |first11=Joseph A. |last12=Brahic |first12=Andre |last13=Neukum |first13=Gerhard |last14=Veverka |first14=Joseph |last15=Barbara |first15=John M. |last16=Denk |first16=Tilmann |last17=Evans |first17=Michael |last18=Ferrier |first18=Joseph J. |last19=Geissler |first19=Paul |last20=Helfenstein |first20=Paul |last21=Roatsch |first21=Thomas |last22=Throop |first22=Henry |last23=Tiscareno |first23=Matthew |last24=Vasavada |first24=Ashwin R. |bibcode=2003Sci...299.1541P |s2cid=20150275 |url=https://www.ciclops.org/sci/docs/porco-etal-cassini-jupiter-science-2003.pdf |url-status=live |archive-url=https://web.archive.org/web/20170922010218/https://www.ciclops.org/sci/docs/porco-etal-cassini-jupiter-science-2003.pdf |archive-date=2017-09-22 }}</ref> Io has an extremely thin atmosphere made up mostly of [[sulfur dioxide]] (SO<sub>2</sub>).<ref>{{cite journal |doi=10.1126/science.281.5373.87 |pmid=9651251 |title=High-Temperature Silicate Volcanism on Jupiter's Moon Io |journal=Science |volume=281 |issue=5373 |pages=87–90 |year=1998 |last1=McEwen |first1=A. S. |last2=Keszthelyi |first2=L. |last3=Spencer |first3=J. R. |last4=Schubert |first4=G. |last5=Matson |first5=D. L. |last6=Lopes-Gautier |first6=R. |last7=Klaasen |first7=K. P. |last8=Johnson |first8=T. V. |last9=Head |first9=J. W. |last10=Geissler |first10=P. |last11=Fagents |first11=S. |last12=Davies |first12=A. G. |last13=Carr |first13=M. H. |last14=Breneman |first14=H. H. |last15=Belton |first15=M. J. S. |bibcode=1998Sci...281...87M |s2cid=28222050 |url=https://pdfs.semanticscholar.org/3d23/d4126eace55e4e525da42c1af8131c030d5b.pdf |archive-url=https://web.archive.org/web/20200923092649/https://pdfs.semanticscholar.org/3d23/d4126eace55e4e525da42c1af8131c030d5b.pdf |url-status=dead |archive-date=2020-09-23 }}</ref> If a surface data or collection vessel were to land on Io in the future, it would have to be extremely tough (similar to the [[tank]]-like bodies of the Soviet [[Venera]] landers) to survive the radiation and magnetic fields that originate from Jupiter.<ref>{{cite journal |doi=10.1126/science.186.4167.922 |pmid=17730914 |title=Io: A Surface Evaporite Deposit? |journal=Science |volume=186 |issue=4167 |pages=922–5 |year=1974 |last1=Fanale |first1=F. P. |last2=Johnson |first2=T. V. |last3=Matson |first3=D. L. |bibcode=1974Sci...186..922F |s2cid=205532 }}</ref>

=== Europa ===
{{main|Europa (moon)}}
[[File:PIA25696-Europa-JupiterMoon-20220929.jpg|thumb|Closeup of Europan lineae]]
Europa (Jupiter II), the second of the four Galilean moons, is the second closest to Jupiter and the smallest at 3121.6 kilometers in diameter, which is slightly smaller than [[Moon|Earth's Moon]]. The name comes from a mythical [[Phoenicia]]n noblewoman, [[Europa (consort of Zeus)|Europa]], who was courted by [[Zeus]] and became the queen of [[Crete]], though the name did not become widely used until the mid-20th century.<ref name="marazzini"/>

It has a smooth and bright surface,<ref>{{cite web|url=https://solarsystem.nasa.gov/planets/europa/indepth|title=Europa: In Depth|last=Hefler|first=Michael|date=2001|publisher=[[NASA]], Jet Propulsion Laboratory|access-date=9 August 2007|work=NASA, Solar system Exploration|url-status=live|archive-url=https://web.archive.org/web/20151114055431/https://solarsystem.nasa.gov/planets/europa/indepth|archive-date=14 November 2015}}</ref> with a layer of water surrounding the mantle of the planet, thought to be 100 kilometers thick.<ref>{{Cite book |last1=Schenk |first1=Paul M. |url=https://lasp.colorado.edu/mop/files/2015/08/jupiter_ch18-1.pdf |title=Jupiter: the planet, satellites, and magnetosphere |last2=Chapman |first2=Clark R. |last3=Zahnle |first3=Kevin |last4=Moore |first4=Jeffrey M. |date=2004 |publisher=[[Cambridge University Press]] |isbn=978-0-521-81808-7 |editor-last=Bagenal |editor-first=Fran |series=Cambridge planetary science |location=Cambridge, UK; New York |chapter=Ages and Interiors: the Cratering Record of the Galilean Satellites |bibcode=2004jpsm.book..427S |editor-last2=Dowling |editor-first2=Timothy E. |editor-last3=McKinnon |editor-first3=William B.}}</ref> The smooth surface includes a layer of ice, while the bottom of the ice is theorized to be liquid water.<ref name="EuropaAlbedo">{{cite web|url=https://www.solarviews.com/eng/europa.htm|author=Hamilton, C. J.|title=Jupiter's Moon Europa|url-status=live|archive-url=https://web.archive.org/web/20120124013342/https://www.solarviews.com/eng/europa.htm|archive-date=2012-01-24}}</ref> The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it, which could conceivably serve as an abode for [[extraterrestrial life]].<ref>{{cite web|url=https://people.msoe.edu/~tritt/sf/europa.life.html |title=Possibility of Life on Europa |last=Tritt |first=Charles S. |access-date=10 August 2007 |publisher=Milwaukee School of Engineering |date=2002 |url-status=dead |archive-url=https://web.archive.org/web/20070609150109/https://people.msoe.edu/~tritt/sf/europa.life.html |archive-date=9 June 2007 }}</ref> Heat energy from [[tidal flexing]] ensures that the ocean remains liquid and drives geological activity.<ref name=geology>{{cite web| url=https://geology.asu.edu/~glg_intro/planetary/p8.htm |title=Tidal Heating|access-date=2007-10-20|work=geology.asu.edu |archive-url = https://web.archive.org/web/20060329000051/https://geology.asu.edu/~glg_intro/planetary/p8.htm |archive-date = 2006-03-29}}</ref> Life may exist in Europa's under-ice ocean. So far, there is no evidence that life exists on Europa, but the likely presence of liquid water has spurred calls to send a probe there.<ref>{{cite web|last=Phillips|first=Cynthia|author-link=Cynthia B. Phillips|url=https://www.space.com/2954-time-europa.html|title=Time for Europa|publisher=Space.com|date=28 September 2006|access-date=5 January 2014|url-status=live|archive-url=https://web.archive.org/web/20111211091904/https://www.space.com/2954-time-europa.html|archive-date=11 December 2011}}</ref>

[[File:Hubble sees recurring plume erupting from Europa.jpg|thumb|Recurring plume erupting from Europa.<ref>{{cite web|title=Hubble sees recurring plume erupting from Europa|url=https://www.spacetelescope.org/images/opo1717a/|website=www.spacetelescope.org|access-date=24 April 2017|url-status=live|archive-url=https://web.archive.org/web/20170425031144/https://www.spacetelescope.org/images/opo1717a/|archive-date=25 April 2017}}</ref>]]

The prominent markings that criss-cross the moon seem to be mainly [[albedo feature]]s, which emphasize low topography. There are few [[Impact crater|craters]] on Europa because its surface is tectonically active and young.<ref>{{cite journal |doi=10.1006/icar.1998.5986 |title=Tectonic Processes on Europa: Tidal Stresses, Mechanical Response, and Visible Features |journal=Icarus |volume=135 |issue=1 |pages=64–78 |year=1998 |last1=Greenberg |first1=Richard |last2=Geissler |first2=Paul |last3=Hoppa |first3=Gregory |last4=Tufts |first4=B.Randall |last5=Durda |first5=Daniel D. |last6=Pappalardo |first6=Robert |last7=Head |first7=James W. |last8=Greeley |first8=Ronald |last9=Sullivan |first9=Robert |last10=Carr |first10=Michael H. |bibcode=1998Icar..135...64G |s2cid=7444898 |url=https://pdfs.semanticscholar.org/fad2/aceb725eec9e700ac791063633c99796d903.pdf |archive-url=https://web.archive.org/web/20200412144704/https://pdfs.semanticscholar.org/fad2/aceb725eec9e700ac791063633c99796d903.pdf |url-status=dead |archive-date=2020-04-12 }}</ref> Some theories suggest that Jupiter's gravity is causing these markings, as one side of Europa is constantly facing Jupiter. Volcanic water eruptions splitting the surface of Europa and even geysers have also been considered as causes. The reddish-brown color of the markings is theorized to be caused by sulfur, but because no data collection devices have been sent to Europa, scientists cannot yet confirm this.<ref>{{cite journal|title=Distribution of hydrate on Europa: Further evidence for sulfuric acid hydrate|doi=10.1016/j.icarus.2005.03.026|first1=R.W.|last1=Carlson|author2=M.S. Anderson |date=2005|bibcode=2005Icar..177..461C|volume=177|issue=2|journal=Icarus|pages=461–471}}</ref> Europa is primarily made of [[silicate]] rock and likely has an [[iron]] core. It has a tenuous atmosphere composed primarily of [[oxygen]].<ref>{{Cite web|url=https://phys.org/news/2015-06-moons-jupiter.html|title=The moons of Jupiter|access-date=2020-02-26|archive-date=2020-02-26|archive-url=https://web.archive.org/web/20200226071614/https://phys.org/news/2015-06-moons-jupiter.html|url-status=live}}</ref>

=== Ganymede ===
{{main|Ganymede (moon)}}
[[File:PIA24682-Ganymede-DarkSide-JupiterMoon-20210607.jpg|thumb|Ancient tectonic features on Ganymede]]
Ganymede (Jupiter III), the third Galilean moon, is named after the mythological [[Ganymede (mythology)|Ganymede]], cupbearer of the [[Greek mythology|Greek gods]] and [[Zeus]]'s beloved.<ref>{{cite web| url = https://galileo.rice.edu/sci/observations/jupiter_satellites.html| title = Satellites of Jupiter| work = The Galileo Project| access-date = 2007-11-24| url-status = live| archive-url = https://web.archive.org/web/20120211140650/https://galileo.rice.edu/sci/observations/jupiter_satellites.html| archive-date = 2012-02-11}}</ref> Ganymede is the [[List of natural satellites by diameter|largest natural satellite]] in the Solar System at 5262.4 kilometers in diameter, which makes it larger than the planet [[Mercury (planet)|Mercury]] – although only at about half of its mass<ref name="nineplanets.org-Ganymede">{{cite web|publisher=nineplanets.org|title=Ganymede|date=October 31, 1997|url=https://www.nineplanets.org/ganymede.html|access-date=2008-02-27|url-status=live|archive-url=https://web.archive.org/web/20120208061056/https://nineplanets.org/ganymede.html|archive-date=February 8, 2012}}</ref> since Ganymede is an icy world. It is the only satellite in the Solar System known to possess a [[magnetosphere]], likely created through [[convection]] within the liquid iron core.<ref>{{cite journal |doi=10.1006/icar.2002.6834 |title=The Permanent and Inductive Magnetic Moments of Ganymede |journal=Icarus |volume=157 |issue=2 |pages=507–22 |year=2002 |last1=Kivelson |first1=M.G. |last2=Khurana |first2=K.K. |last3=Volwerk |first3=M. |bibcode=2002Icar..157..507K |hdl=2060/20020044825 |s2cid=7482644 |url=https://pdfs.semanticscholar.org/7c21/9bca2f2a0e98f1ee870903ef563c8c5d20ca.pdf |archive-url=https://web.archive.org/web/20200412144652/https://pdfs.semanticscholar.org/7c21/9bca2f2a0e98f1ee870903ef563c8c5d20ca.pdf |url-status=dead |archive-date=2020-04-12 |hdl-access=free }}</ref>

Ganymede is composed primarily of [[silicate|silicate rock]] and water ice, and a salt-water ocean is believed to exist nearly 200&nbsp;km below Ganymede's surface, sandwiched between layers of ice.<ref>{{cite web|url=https://www.jpl.nasa.gov/releases/2000/aguganymederoundup.html|title=Solar System's largest moon likely has a hidden ocean|access-date=2008-01-11|date=2000-12-16|work=Jet Propulsion Laboratory|publisher=NASA|url-status=live|archive-url=https://web.archive.org/web/20120117100414/https://www.jpl.nasa.gov/releases/2000/aguganymederoundup.html|archive-date=2012-01-17}}</ref> The metallic core of Ganymede suggests a greater heat at some time in its past than had previously been proposed. The surface is a mix of two types of terrain—highly cratered dark regions and younger, but still ancient, regions with a large array of grooves and ridges. Ganymede has a high number of craters, but many are gone or barely visible due to its icy crust forming over them. The satellite has a thin [[oxygen]] [[atmosphere]] that includes O, O<sub>2</sub>, and possibly O<sub>3</sub> ([[ozone]]), and some [[atomic hydrogen]].<ref>{{cite journal |doi=10.1086/305604 |title=The Far-Ultraviolet Oxygen Airglow of Europa and Ganymede |journal=The Astrophysical Journal |volume=499 |issue=1 |pages=475–481 |year=1998 |last1=Hall |first1=D. T. |last2=Feldman |first2=P. D. |last3=McGrath |first3=M. A. |last4=Strobel |first4=D. F. |bibcode=1998ApJ...499..475H |doi-access= }}</ref><ref>{{cite journal |doi=10.1016/S0032-0633(00)00154-9 |title=The ionosphere of Ganymede |journal=Planetary and Space Science |volume=49 |issue=3–4 |pages=327–36 |year=2001 |last1=Eviatar |first1=Aharon |last2=m. Vasyliūnas |first2=Vytenis |last3=a. Gurnett |first3=Donald |bibcode=2001P&SS...49..327E }}</ref>

=== Callisto ===
{{main|Callisto (moon)}}
[[File:Valhalla crater on Callisto.jpg|thumb|Callisto's [[Valhalla (crater)|Valhalla impact crater]] in enhanced color as seen by Voyager]]
Callisto (Jupiter IV) is the fourth and last Galilean moon, and is the second-largest of the four, and at 4820.6 kilometers in diameter, it is [[List of moons by diameter|the third largest moon]] in the Solar System, and barely smaller than Mercury, though only a third of the latter's mass. It is named after the Greek mythological nymph [[Callisto (mythology)|Callisto]], a lover of Zeus who was a daughter of the Arkadian King Lykaon and a hunting companion of the goddess Artemis. The moon does not form part of the [[orbital resonance]] that affects three inner Galilean satellites and thus does not experience appreciable [[tidal heating]].<ref>{{cite journal |doi=10.1006/icar.2002.6939 |title=Numerical Simulations of the Orbits of the Galilean Satellites |journal=Icarus |volume=159 |issue=2 |pages=500–4 |year=2002 |last1=Musotto |first1=S |last2=Varadi |first2=Ferenc |last3=Moore |first3=William |last4=Schubert |first4=Gerald |bibcode=2002Icar..159..500M }}</ref> Callisto is composed of approximately equal amounts of [[rock (geology)|rock]] and [[Volatile (astrogeology)|ices]], which makes it the least dense of the Galilean moons. It is one of the most heavily cratered satellites in the Solar System, and one major feature is a basin around 3000&nbsp;km wide called [[Valhalla (crater)|Valhalla]].<ref>{{Cite web|url=https://abyss.uoregon.edu/~js/ast121/lectures/lec13.html|title=Galilean Satellites|access-date=2022-02-20|archive-date=2021-12-20|archive-url=https://web.archive.org/web/20211220235547/http://abyss.uoregon.edu/~js/ast121/lectures/lec13.html|url-status=live}}</ref>

Callisto is surrounded by an extremely thin atmosphere composed of [[carbon dioxide]]<ref>{{cite journal |doi=10.1126/science.283.5403.820 |pmid=9933159 |title=A Tenuous Carbon Dioxide Atmosphere on Jupiter's Moon Callisto |journal=Science |volume=283 |issue=5403 |pages=820–1 |year=1999 |last1=Carlson |first1=R. W. |bibcode=1999Sci...283..820C |hdl=2014/16785 |url=https://trs-new.jpl.nasa.gov/dspace/bitstream/2014/16785/1/99-0186.pdf |url-status=dead |archive-url=https://web.archive.org/web/20081003231710/https://trs-new.jpl.nasa.gov/dspace/bitstream/2014/16785/1/99-0186.pdf |archive-date=2008-10-03 |citeseerx=10.1.1.620.9273 }}</ref> and probably [[molecular oxygen]].<ref>{{cite journal |doi=10.1029/2004JE002322 |title=Atmosphere of Callisto |journal=Journal of Geophysical Research |volume=110 |issue=E2 |pages=E02003 |year=2005 |last1=Liang |first1=Mao-Chang |last2=Lane |first2=Benjamin F. |last3=Pappalardo |first3=Robert T. |last4=Allen |first4=Mark |last5=Yung |first5=Yuk L. |bibcode=2005JGRE..110.2003L |s2cid=8162816 |doi-access=free |url=https://authors.library.caltech.edu/records/hknxa-mce17/files/jgre1905.pdf?download=1 }}</ref> Investigation revealed that Callisto may possibly have a subsurface ocean of liquid water at depths less than 300 kilometres.<ref>{{cite journal |doi=10.1006/icar.2000.6456 |title=Subsurface Oceans on Europa and Callisto: Constraints from Galileo Magnetometer Observations |journal=Icarus |volume=147 |issue=2 |pages=329–47 |year=2000 |last1=Zimmer |first1=C |last2=Khurana |first2=Krishan K. |last3=Kivelson |first3=Margaret G. |bibcode=2000Icar..147..329Z |url=https://www.igpp.ucla.edu/people/mkivelson/Publications/ICRUS147329.pdf |url-status=live |archive-url=https://web.archive.org/web/20090327052124/https://www.igpp.ucla.edu/people/mkivelson/Publications/ICRUS147329.pdf |archive-date=2009-03-27 |citeseerx=10.1.1.366.7700 }}</ref> The likely presence of an ocean within Callisto indicates that it can or could harbour [[extraterrestrial life|life]]. However, this is less likely than on nearby [[Europa (moon)|Europa]].<ref>{{Cite book |last1=Lipps |first1=Jere H. |last2=Delory |first2=Gregory |last3=Pitman |first3=Joseph T. |last4=Rieboldt |first4=Sarah |title=Instruments, Methods, and Missions for Astrobiology VIII |chapter=Astrobiology of Jupiter's icy moons |date=November 2004 |publisher=SPIE |isbn=978-0-8194-5493-5 |editor-last=Hoover |editor-first=Richard B. |series=SPIE proceedings series |volume=5555 |location=Bellingham, Wash |pages=78–92 |bibcode=2004SPIE.5555...78L |doi=10.1117/12.560356 |oclc=ocm57077468 |editor-last2=Levin |editor-first2=Gilbert V. |editor-last3=Rozanov |editor-first3=A. Y. |s2cid=140590649}}</ref> Callisto has long been considered the most suitable place for a human base for future exploration of the Jupiter system since it is furthest from the intense radiation of Jupiter's magnetic field.<ref>{{cite web|title=Revolutionary Concepts for Human Outer Planet Exploration(HOPE)|last1=Trautman|first1=Pat|author2=Bethke, Kristen|publisher=NASA|date=2003|url=https://www.nasa-academy.org/soffen/travelgrant/bethke.pdf|url-status=dead|archive-url=https://web.archive.org/web/20120119170143/https://www.nasa-academy.org/soffen/travelgrant/bethke.pdf|archive-date=2012-01-19}}</ref>

== Comparative structure ==
[[File:Jupiter.moons1.jpg|thumb|600px|center|Surface features of the four members at different levels of zoom in each row]]

<div style="float:right; margin:2px;">
{| class=wikitable style="text-align:center; font-size:11px"
|+ Jovian radiation
! Moon !! [[Röntgen equivalent man|rem]]/day
|-
| Io || 3600<ref name="ringwald">{{cite web |date=29 February 2000 |title=SPS 1020 (Introduction to Space Sciences) |publisher=California State University, Fresno |last=Ringwald |first=Frederick A. |url=https://zimmer.csufresno.edu/~fringwal/w08a.jup.txt |url-status=dead |access-date=5 January 2014 |archive-url=https://web.archive.org/web/20080725050708/https://zimmer.csufresno.edu/~fringwal/w08a.jup.txt |archive-date=25 July 2008 }}</ref>
|-
| Europa || 540<ref name="ringwald"/>
|-
|Ganymede || 8<ref name="ringwald"/>
|-
| Callisto || 0.01<ref name="ringwald"/>
|-
! Earth (Max) !!  0.07
|-
! Earth (Avg) !! 0.0007

|}</div>

Fluctuations in the orbits of the moons indicate that their mean density decreases with distance from Jupiter. Callisto, the outermost and least dense of the four, has a density intermediate between ice and rock whereas Io, the innermost and densest moon, has a density intermediate between rock and iron. Callisto has an ancient, heavily cratered and unaltered ice surface and the way it rotates indicates that its density is equally distributed, suggesting that it has no rocky or metallic core but consists of a homogeneous mix of rock and ice. This may well have been the original structure of all the moons. The rotation of the three inner moons, in contrast, indicates differentiation of their interiors with denser matter at the core and lighter matter above. They also reveal significant alteration of the surface. Ganymede reveals past tectonic movement of the ice surface which required partial melting of subsurface layers. Europa reveals more dynamic and recent movement of this nature, suggesting a thinner ice crust. Finally, Io, the innermost moon, has a sulfur surface, active volcanism and no sign of ice. All this evidence suggests that the nearer a moon is to Jupiter the hotter its interior. The current model is that the moons experience tidal heating as a result of the gravitational field of Jupiter in inverse proportion to the square of their distance from the giant planet. In all but Callisto this will have melted the interior ice, allowing rock and iron to sink to the interior and water to cover the surface. In Ganymede a thick and solid ice crust then formed. In warmer Europa a thinner more easily broken crust formed. In Io the heating is so extreme that all the rock has melted and water has long ago boiled out into space.

===Size===

[[File:Moons of solar system v7.jpg|thumb|900px|center|Galilean moons compared with moons of other planets (and with Earth; the scale is changed to 1 pixel = 94 km at this resolution).]]

===Latest flyby===
{{multiple image
| width     = 100
| footer    = <div style="text-align: center;">Jupiter and Galilean moons circa 2007, imaged by ''[[New Horizons]]'' during flyby. (greyscale colour)</div>
| align     = center
| image1    = Jupiter taken by New Horizons probe (2007-01-08).jpg
| alt1      =
| caption1  = [[w:Jupiter|Jupiter]] and [[w:Io (moon)|Io]]
| image2    = NewHorizonsIo.jpg
| alt2      =
| caption2  = [[w:Io (moon)|Io]]
| image3    = PIA09246 Europa.jpg
| alt3      =
| caption3  = [[w:Europa (moon)|Europa]]
| image4    = PIA09245 Ganymede.jpg
| alt4      =
| caption4  = [[w:Ganymede (moon)|Ganymede]]
| image5    = NewHorizonsCallisto.jpg
| alt5      =
| caption5  = [[w:Callisto (moon)|Callisto]]
}}
{{clear}}

==Origin and evolution==
[[File:Relative Masses of Jovian Satellites.png|thumb|right|300px|The relative masses of the Jovian moons. Those smaller than Europa are not visible at this scale, and combined would only be visible at 100× magnification.]]

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]].<ref name=Canup2009>{{cite book |last1=Canup |first1=Robert M. |last2=Ward |first2=William R. |title=Europa |publisher=University of Arizona Press |date=2009 |chapter=Origin of Europa and the Galilean Satellites |chapter-url={{Google books|Jpcz2UoXejgC|page=59|plainurl=yes}} |pages=59–83 |isbn=978-0-8165-2844-8}}</ref><ref name=Alibert2005>{{cite journal|last1=Alibert|first1=Y. |last2=Mousis|first2=O. |last3=Benz|first3=W. |title=Modeling the Jovian subnebula I. Thermodynamic conditions and migration of proto-satellites|date=2005|journal=Astronomy & Astrophysics|volume=439|issue=3|pages=1205–13|bibcode=2005A&A...439.1205A|doi=10.1051/0004-6361:20052841|arxiv = astro-ph/0505367 |s2cid=2260100 }}</ref> They may be the remnants of a score of Galilean-mass satellites that formed early in Jupiter's history.<ref name="newsci"/><ref name=Canup2009/>

Simulations suggest that, while the disk had a relatively high mass at any given moment, over time a substantial fraction (several tenths of a percent) of the mass of Jupiter captured from the Solar nebula was processed through it. However, the disk mass of only 2% that of Jupiter is required to explain the existing satellites.<ref name=Canup2009/> Thus there may have been several generations of Galilean-mass satellites in Jupiter's early history. Each generation of moons would have spiraled into Jupiter, due to drag from the disk, with new moons then forming from the new debris captured from the Solar nebula.<ref name=Canup2009/> By the time the present (possibly fifth) generation formed, the disk had thinned out to the point that it no longer greatly interfered with the moons' orbits.<ref name="newsci"/> The current Galilean moons were still affected, falling into and being partially protected by an [[orbital resonance]] which still exists for Io, Europa, and Ganymede. Ganymede's larger mass means that it would have migrated inward at a faster rate than Europa or Io.<ref name=Canup2009/> Tidal dissipation in the Jovian system is still ongoing and [[Callisto (moon)|Callisto]] will likely be captured into the resonance in about 1.5 billion years, creating a 1:2:4:8 chain.<ref>{{cite journal |last1=Lari |first1=Giacomo |last2=Saillenfest |first2=Melaine |first3=Marco |last3=Fenucci |date=2020 |title=Long-term evolution of the Galilean satellites: the capture of Callisto into resonance |url=https://www.aanda.org/articles/aa/full_html/2020/07/aa37445-20/aa37445-20.html |journal=Astronomy & Astrophysics |volume=639 |pages=A40 |doi=10.1051/0004-6361/202037445 |arxiv=2001.01106 |bibcode=2020A&A...639A..40L |s2cid=209862163 |access-date=1 August 2022}}</ref>

==Visibility==
All four Galilean moons are bright enough to be viewed from Earth without a [[optical telescope|telescope]], if only they could appear farther away from Jupiter. (They are, however, easily distinguished with even low-powered [[binoculars]].) They have [[apparent magnitude]]s between 4.6 and 5.6 when Jupiter is in [[opposition (planets)|opposition]] with the Sun,<ref name=jplfact>{{cite web |last=Yeomans |first=Donald K. |date=2006-07-13 |title=Planetary Satellite Physical Parameters |publisher=JPL Solar System Dynamics |url=https://ssd.jpl.nasa.gov/?sat_phys_par |access-date=2008-08-23 |url-status=live |archive-url=https://web.archive.org/web/20100527091333/https://ssd.jpl.nasa.gov/?sat_phys_par |archive-date=2010-05-27 }}</ref> and are about one unit of magnitude dimmer when Jupiter is in [[conjunction (astronomy)|conjunction]]. The main difficulty in observing the moons from Earth is their proximity to Jupiter, since they are obscured by its brightness.<ref>Jupiter is about 750 times brighter than Ganymede and about 2000 times brighter than Callisto.<br>Ganymede: (5th root of 100)^(4.4 Ganymede [[apparent magnitude|APmag]] − (−2.8 Jup APmag)) = 758<br>Callisto: (5th root of 100)^(5.5 Callisto APmag − (−2.8 Jup APmag)) = 2089</ref> The maximum [[angular distance|angular separations]] of the moons are between 2 and 10 [[minute and second of arc|arcminutes]] from Jupiter,<ref>Jupiter near [[apsis|perihelion]] 2010-Sep-19: 656.7 (Callisto angular separation arcsec) − 24.9 (jup angular [[radius]] arcsec) = 631 arcsec = 10 arcmin</ref> which is close to the limit of human [[visual acuity]]. Ganymede and Callisto, at their maximum separation, are the likeliest targets for potential naked-eye observation.<ref>{{cite journal |last1=Dutton |first1=Denis |date=December 1976 |title=Naked-Eye Observations of Jupiter's Moons |url=http://www.denisdutton.com/jupiter_moons.htm |journal=Sky & Telescope |volume= |issue= |pages=482–4 |doi= |access-date=1 August 2022 |archive-date=12 November 2020 |archive-url=https://web.archive.org/web/20201112024151/http://www.denisdutton.com/jupiter_moons.htm |url-status=dead }}</ref>

<gallery mode=packed heights=160>
File:Jupiter-moons.jpg|Jupiter and all of the Galilean moons as seen through a {{convert|25|cm|in|0|abbr=on}} amateur telescope ([[Meade LX200]]).
File:Jupiter.mit.Io.Ganymed.Europa.Calisto.Vollmond.10.4.2017.jpg|Jupiter with the Galilean moons and the [[full moon|full Moon]] as seen around [[conjunction (astronomy)|conjunction]] on 10 April 2017
File:Galilean satellite triple conjunction 2015-01-24.jpg|Two [[Hubble Space Telescope]] views of a rare triple transit of Jupiter by Europa, Callisto and Io (24 January 2015)|alt=Small satellites visible against the vastness of the largest planet in the solar system
</gallery>

==Orbit animations==
GIF animations depicting the Galilean moon orbits and the resonance of Io, Europa, and Ganymede
{{multiple image |direction=horizontal |align=left |total_width=700
 |image1=Galilean moon Laplace resonance animation 2.gif |caption1=The [[Laplace resonance]] of Io, Europa and Ganymede (conjunctions are highlighted by color changes)
 |image2=Galilean moons around Jupiter.gif |caption2=The Galilean moons orbiting Jupiter<br />{{legend2|Lime|Jupiter}}{{·}}{{legend2|OrangeRed|Io}}{{·}}{{legend2|RoyalBlue|Europa}}{{·}}{{legend2|Gold|Ganymede}}{{·}}{{legend2|Cyan|Callisto}}
}}
{{Clear}}

== See also ==
* [[Jupiter's moons in fiction]]
*[[Space colonization#Jovian moons|Colonization of the Jovian System]]
{{clear}}

== Notes ==
{{notelist}}

== References ==
{{Reflist}}

==External links==

{{commons category|Galilean moons}}

* [https://skyandtelescope.org/wp-content/plugins/observing-tools/jupiter_moons/jupiter.html Sky & Telescope utility for identifying Galilean moons]
* [https://thehappykoala.github.io/Harmony-of-the-Spheres/#/category/Solar%20System/scenario/The%20Jovian%20System Interactive 3D visualisation of Jupiter and the Galilean moons]
* [https://www.youtube.com/watch?v=Xo6w5QVrABc NASA's Stunning Discoveries on Jupiter's Largest Moons | Our Solar System's Moons]
* [https://thebigbangoptics.com/jupiters-moons/ A Beginner's Guide to Jupiter's Moons]
* Dominic Ford: [https://in-the-sky.org/jupiter.php The Moons of Jupiter]. With a chart of the current position of the Galilean moons.
{{Moons of Jupiter}}
{{Solar System moons (compact)}}
{{Jupiter}}
{{Solar System}}
{{Galileo Galilei}}
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