Editing Galilean moons (section)

== 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 (mythology)|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>