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{{Английская Википедия/Панель перехода}}
{{Short description|Innermost of the four Galilean moons of Jupiter}}
{{Redirect|Jupiter I|other uses|Jupiter 1 (disambiguation){{!}}Jupiter 1}}
{{Featured article}}
{{Use dmy dates|date=October 2018}}
{{Infobox planet
| name = Io
| alt_names = Jupiter {{rn|I}}
| image = Io highest resolution true color.jpg
| image_alt = The dark spot just left of the center is the erupting volcano [[Prometheus (volcano)|Prometheus]]. The whitish plains on either side of it are coated with volcanically deposited [[sulfur dioxide]] frost, whereas the yellower regions contain a higher proportion of [[sulfur]].
| caption = Enhanced color image of Io from the ''[[Galileo (spacecraft)|Galileo]]'' spacecraft, taken in 1999. 
| bgcolour = #a0ffa0
| discoverer = [[Galileo Galilei]]
| discovered = 8 January 1610<ref name="IAUMoonDiscoveries"/>
| pronounced = {{IPAc-en|ˈ|aɪ|.|oʊ}}<ref>{{Cite dictionary |url=http://www.lexico.com/definition/Io |archive-url=https://web.archive.org/web/20200229215137/https://www.lexico.com/definition/io |url-status=dead |archive-date=29 February 2020 |title=Io |dictionary=[[Lexico]] UK English Dictionary |publisher=[[Oxford University Press]]}}<br/>{{MW|Io}}</ref> or as Greco-Latin ''Īō'' (approximated as {{IPAc-en|ˈ|i:|.|oʊ}}){{cn|date=February 2022}}
| adjectives = Ionian {{IPAc-en|aɪ|ˈ|oʊ|n|i|ə|n}}<ref>S. W. Kieffer (1982) "Ionian Volcanism", in David Morrison, ed., ''Satellites of Jupiter'', vol. 3, International Astronomical Union</ref><ref>"Electron Beams and Ion Composition Measured at Io and in Its Torus", ''Science'', 1996 October 18</ref>
| named_after = [[Io (mythology)|Ἰώ]] ''Īō''
| mean_orbit_radius = {{val|421700|u=km}} ({{val|0.002819|ul=AU}})
| eccentricity = {{val|0.0040313019}}
| periapsis = {{val|420000|u=km}} ({{val|0.002808|u=AU}})
| apoapsis = {{val|423400|u=km}} ({{val|0.002830|u=AU}})
| period = {{val|1.769137786|u=d}} ({{val|152853.5047|u=s}}, {{val|42.45930686|u=h}})
| avg_speed = {{val|17.334|u=km/s}}
| inclination = 0.05° (to Jupiter's equator)<br />2.213° (to the [[ecliptic]])
| satellite_of = [[Jupiter]]
| group = [[Galilean moon]]
| mean_radius = {{val|1821.6|.5|u=km}} (0.28592 Earths)<ref name="jplfact"/>
| dimensions = 3,660.0 × 3,637.4 × 3,630.6&nbsp;km<ref name="Thomas1998">{{cite journal |last=Thomas |first=P. C. |display-authors=etal |date=1998 |title=The Shape of Io from Galileo Limb Measurements |journal=Icarus |volume=135 |issue=1 |pages=175–180 |doi=10.1006/icar.1998.5987 |bibcode=1998Icar..135..175T|doi-access=free }}
</ref>
| surface_area = {{val|41698064.7357|u=km2}} (0.082 Earths)
| volume = {{val|2.5319064907|e=10|u=km3}} (0.023 Earths)
| mass = {{val|8.931938|.000018|e=22|u=kg}} (0.015 Earths)<ref name="jplfact"/>
| density = {{val|3.528|.006|u=g/cm3}} (0.639 Earths)<ref name="jplfact"/>
| surface_grav = {{val|1.796502844|ul=m/s2}} (0.1831923077 [[g-force|g]])
| moment_of_inertia_factor = {{val|0.37824|0.00022}}<ref name="Schubert2004">{{cite book|last1=Schubert|first1=G.|last2=Anderson|first2=J. D.|last3= Spohn|first3= T.|last4= McKinnon|first4=W. B.|editor1-last= Bagenal| editor1-first= F.| editor2-last= Dowling| editor2-first= T. E.| editor3-last= McKinnon| editor3-first= W. B.|title=Jupiter : the planet, satellites, and magnetosphere|chapter= Interior composition, structure and dynamics of the Galilean satellites|date= 2004|publisher= Cambridge University Press|location=New York|url= https://books.google.com/books?id=aMERHqj9ivcC|chapter-url = https://books.google.com/books?id=aMERHqj9ivcC&pg=PA281|isbn= 978-0521035453|pages= 281–306|oclc= 54081598}}</ref>
| escape_velocity = 2,558.3174910781&nbsp;m/s
| albedo = {{val|.63|.02}}<ref name="jplfact">{{cite web |last=Yeomans |first=Donald K. |date=13 July 2006 |title=Planetary Satellite Physical Parameters |publisher=JPL Solar System Dynamics |url=http://ssd.jpl.nasa.gov/?sat_phys_par}}</ref>
| magnitude = 5.02 ([[Opposition (astronomy)|opposition]])<ref name="magnitude">{{cite web |title=Classic Satellites of the Solar System |url=http://www.oarval.org/ClasSaten.htm |publisher=Observatorio ARVAL |access-date=28 September 2007 |archive-url=https://web.archive.org/web/20110709203915/http://www.oarval.org/ClasSaten.htm |archive-date=9 July 2011 |url-status=dead}}</ref>
| angular_size = {{nowrap |1.2 [[minute and second of arc|arcseconds]]}}<ref name="www.eso.org 2001">{{cite web | title=Io | website=www.eso.org | date=2001-12-05 | url=https://www.eso.org/public/images/eso0204b/ | access-date=2022-05-21}}</ref>
| rotation = [[synchronous rotation|synchronous]]
| rot_velocity = 271&nbsp;km/h
| temp_name1 = Surface
| min_temp_1 = 90&nbsp;[[Kelvin|K]]
| mean_temp_1 = 110&nbsp;[[Kelvin|K]]
| max_temp_1 = 130&nbsp;K<ref name="Rathbun2004">{{cite journal |last1=Rathbun |first1=J. A. |last2=Spencer |date=2004 |first2=J.R. |last3=Tamppari |first3=L.K. |last4=Martin |first4=T.Z. |last5=Barnard |first5=L. |last6=Travis |first6=L.D. |title=Mapping of Io's thermal radiation by the Galileo photopolarimeter-radiometer (PPR) instrument |journal=Icarus |volume=169 |issue=1 |pages=127–139 |doi=10.1016/j.icarus.2003.12.021 |bibcode=2004Icar..169..127R}}
</ref>
| atmosphere = yes (extremely faint, exosphere only)
| surface_pressure = {{convert|0.5 to 4|mPa|atm|sigfig=3|abbr=on}}
| atmosphere_composition = 90%&nbsp;[[sulfur dioxide]]
}}

'''Io''' ({{IPAc-en|ˈ|aɪ|.|oʊ}}), or '''Jupiter I''', is the innermost and second-smallest of the four [[Galilean moons]] of the planet [[Jupiter]]. Slightly larger than [[Earth]]'s [[moon]], Io is the [[list of natural satellites|fourth-largest moon]] in the [[Solar System]], has the highest density of any moon, the strongest surface gravity of any moon, and the lowest amount of water by [[atomic ratio]] of any known [[astronomical object]] in the Solar System. It was discovered in 1610 by [[Galileo Galilei]] and was named after the mythological character [[Io (mythology)|Io]], a priestess of [[Hera]] who became one of [[Zeus]]'s lovers.

With over 400 active [[volcano]]es, Io is the most geologically active object in the Solar System.<ref name="book">{{cite book |title=Encyclopedia of the Solar System |chapter=Io: The Volcanic Moon |author=Rosaly MC Lopes |publisher=Academic Press |date=2006 |editor1=Lucy-Ann McFadden |editor2=Paul R. Weissman |editor3=Torrence V. Johnson |pages=[https://archive.org/details/encyclopediaofso0000unse_u6d1/page/419 419–431] |isbn=978-0-12-088589-3 |chapter-url=https://archive.org/details/encyclopediaofso0000unse_u6d1/page/419 }}</ref><ref name="Lopes2004">{{cite journal |title=Lava lakes on Io: Observations of Io's volcanic activity from Galileo NIMS during the 2001 fly-bys |journal=Icarus |last=Lopes |first=R. M. C. |display-authors=etal |pages=140–174 |volume=169 |issue=1 |date=2004 |doi=10.1016/j.icarus.2003.11.013 |bibcode=2004Icar..169..140L}}</ref><ref name="NYT-20190626" /> This extreme geologic activity is the result of [[Tidal heating of Io|tidal heating]] from [[friction]] generated within Io's interior as it is pulled between Jupiter and the other Galilean moons—[[Europa (moon)|Europa]], [[Ganymede (moon)|Ganymede]] and [[Callisto (moon)|Callisto]]. Several volcanoes produce plumes of [[sulfur]] and [[sulfur dioxide]] that climb as high as {{convert|500|km|sigfig=1|abbr=on}} above the surface. Io's surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of Io's [[silicate]] crust. Some of these peaks are taller than [[Mount Everest]], the highest point on Earth's surface.<ref name="Schenk2001">{{cite journal |last=Schenk |first=P. |display-authors=etal |date=2001 |title=The Mountains of Io: Global and Geological Perspectives from ''Voyager'' and ''Galileo'' |journal=Journal of Geophysical Research |volume=106 |issue=E12 |pages=33201–33222 |doi=10.1029/2000JE001408 |bibcode=2001JGR...10633201S|doi-access=free }}</ref> Unlike most moons in the outer Solar System, which are mostly composed of water [[ice]], Io is primarily composed of [[silicate]] rock surrounding a molten [[iron]] or [[Pyrite|iron sulfide]] core. Most of Io's surface is composed of extensive plains with a frosty coating of [[sulfur]] and [[sulfur dioxide]].

Io's volcanism is responsible for many of its unique features. Its volcanic plumes and lava flows produce large surface changes and paint the surface in various subtle shades of yellow, red, white, black, and green, largely due to [[Allotropes of sulfur|allotropes]] and compounds of sulfur. Numerous extensive lava flows, several more than {{convert|500|km|sigfig=1|abbr=on}} in length, also mark the surface. The materials produced by this volcanism make up Io's thin, patchy atmosphere, and they also greatly affect the nature and radiation levels of Jupiter's extensive [[Magnetosphere of Jupiter|magnetosphere]]. Io's volcanic ejecta also produce a large [[Gas torus|plasma torus]] around Jupiter.

Io played a significant role in the development of astronomy in the 17th and 18th centuries; discovered in January 1610 by Galileo Galilei, along with the other [[Galilean satellites]], this discovery furthered the adoption of the [[Heliocentrism|Copernican model]] of the Solar System, the development of [[Johannes Kepler|Kepler's]] laws of motion, and [[Rømer's determination of the speed of light|the first measurement of the speed of light]]. Viewed from Earth, Io remained just a point of light until the late 19th and early 20th centuries, when it became possible to resolve its large-scale surface features, such as the dark-red polar and bright equatorial regions. In 1979, the two ''[[Voyager program|Voyager]]'' spacecraft revealed Io to be a geologically active world, with numerous volcanic features, large mountains, and a young surface with no obvious impact craters. The ''[[Galileo (spacecraft)|Galileo]]'' spacecraft performed several close flybys in the 1990s and early 2000s, obtaining data about Io's interior structure and surface composition. These spacecraft also revealed the relationship between Io and Jupiter's [[magnetosphere]] and the existence of a belt of high-energy radiation centered on Io's orbit. Io receives about 3,600 [[Röntgen equivalent man|rem]] (36 [[sievert|Sv]]) of [[ionizing radiation]] per day.<ref>{{cite web |url=http://zimmer.csufresno.edu/~fringwal/w08a.jup.txt |title=2000 February 29, SPS 1020 (Introduction to Space Sciences) |work=CSUFresno.edu |date=29 February 2000 |url-status=dead |archive-url=https://web.archive.org/web/20080725050708/http://zimmer.csufresno.edu/~fringwal/w08a.jup.txt |archive-date=25 July 2008}}</ref>

Further observations have been made by ''[[Cassini–Huygens]]'' in 2000, ''[[New Horizons]]'' in 2007, and ''[[Juno (spacecraft)|Juno]]'' since 2017, as well as from Earth-based [[telescope]]s and the [[Hubble Space Telescope]].

==Nomenclature==
{{main|Galilean moons#Names}}
{{See also|List of regions on Io|List of volcanic features on Io|List of mountains on Io}}
[[File:Io_comparison_with_Earth_and_Moon.png|left|thumb|Size comparison between Io (lower left), the [[Moon]] (upper left) and Earth]]

Although [[Simon Marius]] is not credited with the sole discovery of the Galilean satellites, his names for the moons were adopted.<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> In his 1614 publication ''Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici'', he proposed several alternative names for the innermost of the large moons of Jupiter, including "The Mercury of Jupiter" and "The First of the Jovian Planets".<ref name=Helden/><ref name="Marius">{{cite journal |last=Marius |first=S. |author-link=Simon Marius |date=1614 |title=Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici |journal=The Observatory |volume=39 |pages=367 |trans-title=The World of Jupiter discovered in the year 1609 by Means of a Belgian spy-glass |bibcode=1916Obs....39..367. }}</ref> Based on a suggestion from Johannes Kepler in October 1613, he also devised a naming scheme whereby each moon was named for a lover of the [[Greek mythology|Greek god]] [[Zeus]] or his [[Roman mythology|Roman]] equivalent, [[Jupiter (mythology)|Jupiter]]. He named the innermost large moon of Jupiter after the Greek [[Io (mythology)|Io]]:<ref>{{cite web|last=Van Helden|first=Al|date=1995 |title=Satellites of Jupiter |url=http://galileo.rice.edu/sci/observations/jupiter_satellites.html}}</ref><ref name=Helden/>

{{quote|Jupiter is much blamed by the poets on account of his irregular loves. Three maidens are especially mentioned as having been clandestinely courted by Jupiter with success. Io, daughter of the River Inachus, Callisto of Lycaon, Europa of Agenor. Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell... I think, therefore, that I shall not have done amiss if the First is called by me Io, the Second Europa, the Third, on account of its majesty of light, Ganymede, the Fourth Callisto...<ref name=Helden/>}}

Marius's names were not widely adopted until centuries later (mid-20th century).<ref name="marazzini">{{cite journal |title=I nomi dei satelliti di Giove: da Galileo a Simon Marius |trans-title=The names of the satellites of Jupiter: from Galileo to Simon Marius |journal=Lettere Italiane |last=Marazzini |first=Claudio |volume=57 |issue=3 |pages=391–407 |date=2005 |jstor=26267017 |url=https://www.jstor.org/stable/26267017 |lang=it}}</ref> In much of the earlier astronomical literature, Io was generally referred to by its [[Roman numerals|Roman numeral]] designation (a system introduced by Galileo) as "{{nowrap|Jupiter I}}",<ref>{{cite web |url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jup_Io |archive-url=https://web.archive.org/web/20030211022640/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jup_Io |url-status=dead |archive-date=11 February 2003 |title=Io: Overview |publisher=NASA |access-date=5 March 2012}}</ref> or as "the first satellite of Jupiter".<ref name="Barnard1894"/><ref name="Barnard1891"/>

The customary English pronunciation of the name is {{IPAc-en|'|aɪ|oʊ}},<ref>{{OED|Io2}}<br/>{{Cite dictionary |url=http://www.lexico.com/definition/Io |archive-url=https://web.archive.org/web/20200229215137/https://www.lexico.com/definition/io |url-status=dead |archive-date=29 February 2020 |title=Io |dictionary=[[Lexico]] UK English Dictionary UK English Dictionary |publisher=[[Oxford University Press]]}}<br/>{{MW|Io}}<br/>{{Dict.com|Io}}</ref> though sometimes people attempt a more 'authentic' pronunciation, {{IPAc-en|'|iː|oʊ}}.<ref>{{Dict.com|Io}}</ref> The name has two competing stems in Latin: ''Īō'' and (rarely) ''Īōn''.<ref>genitives ''Iūs'' and ''Iōnis'': {{L&S|Io2|ref}}</ref> The latter is the basis of the English adjectival form, ''Ionian.''<ref>Morrison & Matthews (1982) ''Satellites of Jupiter'', part 1, p. 649</ref><ref>McEwen et al., 'Lithosphere and Surface of Io', Schubert et al., 'Satellite Interiors', and Schenk et al., 'Ages and Interiors', in Bagenal et al. eds. (2007) ''Jupiter: The Planet, Satellites and Magnetosphere''</ref><ref>Outside astronomy, "Ionian" would likely be misunderstood as referring to [[Ionia]], but an adjective based on the other stem, "Ioan" {{IPAc-en|aɪ|'|oʊ|ə|n}}, is not found.</ref>

Features on Io are named after characters and places from the Io myth, as well as deities of fire, volcanoes, the Sun, and thunder from various myths, and characters and places from [[Dante Alighieri|Dante's]] ''[[Divine Comedy|Inferno]]'': names appropriate to the volcanic nature of the surface.<ref name="NameCategories">{{cite web |url=http://planetarynames.wr.usgs.gov/append6.html |title=Categories for Naming Features on Planets and Satellites |publisher=U.S. Geological Survey |first=Jennifer |last=Blue |access-date=12 September 2013}}</ref> Since the surface was first seen up close by ''[[Voyager 1]]'', the [[International Astronomical Union]] has approved 249 names for Io's volcanoes, mountains, plateaus, and large albedo features. The approved feature categories used for Io for different types of volcanic features include ''patera'' ('saucer'; volcanic depression), ''[[fluctus]]'' ('flow'; lava flow), ''[[Vallis (planetary geology)|vallis]]'' ('valley'; lava channel), and active eruptive center (location where plume activity was the first sign of volcanic activity at a particular volcano). Named mountains, plateaus, layered [[terrain]], and shield volcanoes include the terms ''mons'', ''mensa'' ('table'), ''planum'', and ''[[tholus]]'' ('rotunda'), respectively.<ref name="NameCategories"/> Named, bright albedo regions use the term ''regio''. Examples of named features are [[Prometheus (volcano)|Prometheus]], Pan Mensa, [[Tvashtar Paterae]], and Tsũi Goab Fluctus.<ref name="Featurenames">{{cite web |url=http://planetarynames.wr.usgs.gov/jsp/FeatureTypes2.jsp?system=Jupiter&body=Io&systemID=5&bodyID=7&sort=AName&show=Fname&show=Lat&show=Long&show=Diam&show=Stat&show=Orig |title=Io Nomenclature Table of Contents |publisher=U.S. Geological Survey |first=Jennifer |last=Blue |date=14 June 2007 |archive-url=https://web.archive.org/web/20070629200803/http://planetarynames.wr.usgs.gov/jsp/FeatureTypes2.jsp?system=Jupiter&body=Io&systemID=5&bodyID=7&sort=AName&show=Fname&show=Lat&show=Long&show=Diam&show=Stat&show=Orig |archive-date=29 June 2007}}</ref>
{{clear left}}

==Observational history==
{{Main|Exploration of Io}}
[[File:Galileo.arp.300pix.jpg|thumb|upright|right|[[Galileo Galilei]], the discoverer of Io]]
The first reported observation of Io was made by [[Galileo Galilei]] on 7 January 1610 using a [[Galilean telescope|20x-power, refracting telescope]] at the [[University of Padua]]. However, in that observation, Galileo could not separate Io and [[Europa (moon)|Europa]] due to the low power of his telescope, so the two were recorded as a single point of light. Io and Europa were seen for the first time as separate bodies during Galileo's observations of the Jovian system the following day, 8 January 1610 (used as the discovery date for Io by the [[International Astronomical Union|IAU]]).<ref name="IAUMoonDiscoveries">{{cite web |last=Blue |first=Jennifer |date=9 November 2009 |url=http://planetarynames.wr.usgs.gov/append7.html |title=Planet and Satellite Names and Discoverers |publisher=USGS}}</ref> The discovery of Io and the other Galilean satellites of Jupiter was published in Galileo's ''[[Sidereus Nuncius]]'' in March 1610.<ref name="IobookChap2">{{cite book |last1=Cruikshank |first1=D. P. |last2=Nelson |first2=R. M. |editor-last=Lopes |editor-first=R. M. C. |editor2-last=Spencer |editor2-first=J. R. |title=Io after Galileo |date=2007 |publisher=Springer-Praxis |isbn=978-3-540-34681-4 |pages=5–33 |chapter=A history of the exploration of Io}}</ref> In his ''Mundus Jovialis'', published in 1614, Simon Marius claimed to have discovered Io and the other moons of Jupiter in 1609, one week before Galileo's discovery. Galileo doubted this claim and dismissed the work of Marius as plagiarism. Regardless, Marius's first recorded observation came from 29 December 1609 in the [[Julian calendar]], which equates to 8 January 1610 in the [[Gregorian calendar]], which Galileo used.<ref name="GaliloProjectMarius">{{cite web |last=Van Helden |first=Albert |url=http://galileo.rice.edu/sci/marius.html |date=14 January 2004 |title=The Galileo Project / Science / Simon Marius |publisher=Rice University}}</ref> Given that Galileo published his work before Marius, Galileo is credited with the discovery.<ref name="JPLDiscovery">{{cite web |last=Baalke |first=Ron |url=http://www2.jpl.nasa.gov/galileo/ganymede/discovery.html |archive-url=https://web.archive.org/web/19970106010432/http://www.jpl.nasa.gov/galileo/ganymede/discovery.html |url-status=dead |archive-date=6 January 1997 |title=Discovery of the Galilean Satellites |publisher=Jet Propulsion Laboratory |access-date=7 January 2010}}</ref>

For the next two and a half centuries, Io remained an unresolved, 5th-magnitude point of light in astronomers' telescopes. During the 17th century, Io and the other Galilean satellites served a variety of purposes, including early methods to determine [[longitude]],<ref>{{cite web |last1=O'Connor |first1=J. J. |last2=Robertson |first2=E. F. |date=February 1997 |url=http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Longitude1.html |title=Longitude and the Académie Royale |publisher=University of St. Andrews |access-date=14 June 2007 |archive-date=2 September 2011 |archive-url=https://web.archive.org/web/20110902144831/http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Longitude1.html |url-status=dead }}</ref> validating Kepler's [[Kepler's third law|third law of planetary motion]], and determining the time required for [[Rømer's determination of the speed of light|light to travel]] between Jupiter and Earth.<ref name="IobookChap2"/> Based on [[Ephemeris|ephemerides]] produced by astronomer [[Giovanni Domenico Cassini|Giovanni Cassini]] and others, [[Pierre-Simon Laplace]] created a mathematical theory to explain the [[orbital resonance|resonant orbits]] of Io, [[Europa (moon)|Europa]], and [[Ganymede (moon)|Ganymede]].<ref name="IobookChap2"/> This resonance was later found to have a profound effect on the geologies of the three moons.<ref name="Iobook2Chap4">{{cite book |last1=Keane |first1=J. T. |last2=Matsuyama |first2=Isamu |last3=Bierson |first3=C. J. |last4=Trinh |first4=A. |editor-last=Lopes |editor-first=R. M. C. |editor2-last=de Kleer |editor2-first=K. |editor3-last=Keane |editor3-first=J. T. |title=Io: A New View of Jupiter's Moon |date=2023 |publisher=Springer|isbn=978-3-031-25669-1 |pages=95–146 |chapter=Tidal Heating and the Interior Structure of Io}}</ref>

Improved telescope technology in the late 19th and 20th centuries allowed astronomers to [[Optical resolution|resolve]] (that is, see as distinct objects) large-scale surface features on Io. In the 1890s, [[Edward Emerson Barnard|Edward E. Barnard]] was the first to observe variations in Io's brightness between its equatorial and polar regions, correctly determining that this was due to differences in color and [[albedo]] between the two regions and not due to Io being egg-shaped, as proposed at the time by fellow astronomer [[William Henry Pickering|William Pickering]], or two separate objects, as initially proposed by Barnard.<ref name="Barnard1894">{{cite journal |last=Barnard |first=E. E. |author-link=Edward Emerson Barnard |date=1894 |title=On the Dark Poles and Bright Equatorial Belt of the First Satellite of Jupiter |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=54 |issue=3 |pages=134–136 |bibcode=1894MNRAS..54..134B |doi=10.1093/mnras/54.3.134|doi-access=free }}</ref><ref name="Barnard1891">{{cite journal |last=Barnard |first=E. E. |author-link=Edward Emerson Barnard |date=1891 |title=Observations of the Planet Jupiter and his Satellites during 1890 with the 12-inch Equatorial of the Lick Observatory |journal=Monthly Notices of the Royal Astronomical Society |volume=51 |issue=9 |pages=543–556 |bibcode=1891MNRAS..51..543B |doi=10.1093/mnras/51.9.543|doi-access=free }}</ref><ref name="Dobbins">{{cite journal |last1=Dobbins |first1=T. |last2=Sheehan |first2=W. |date=2004 |title=The Story of Jupiter's Egg Moons |journal=Sky & Telescope |volume=107 |issue=1 |pages=114–120|bibcode=2004S&T...107a.114D }}</ref> Later telescopic observations confirmed Io's distinct reddish-brown polar regions and yellow-white equatorial band.<ref name="Minton1973">{{cite journal |last=Minton |first=R. B. |date=1973 |title=The Red Polar Caps of Io |journal=Communications of the Lunar and Planetary Laboratory |volume=10 |pages=35–39 |bibcode=1973CoLPL..10...35M}}</ref>

Telescopic observations in the mid-20th century began to hint at Io's unusual nature. Spectroscopic observations suggested that Io's surface was devoid of water ice (a substance found to be plentiful on the other Galilean satellites).<ref name="Lee1972">{{cite journal |last=Lee |first=T. |date=1972 |title=Spectral Albedos of the Galilean Satellites |journal=Communications of the Lunar and Planetary Laboratory |volume=9 |issue=3 |pages=179–180 |bibcode=1972CoLPL...9..179L}}</ref> The same observations suggested a surface dominated by evaporates composed of [[sodium]] salts and [[sulfur]].<ref name="Fanale1974">{{cite journal |title=Io: A Surface Evaporite Deposit? |journal=[[Science (journal)|Science]] |last=Fanale |first=F. P. |display-authors=etal |pages=922–925 |volume=186 |issue=4167 |date=1974 |doi=10.1126/science.186.4167.922 |pmid=17730914 |bibcode=1974Sci...186..922F|s2cid=205532 }}</ref> Radiotelescopic observations revealed Io's influence on the Jovian [[magnetosphere]], as demonstrated by [[Decametre|decametric]] [[wavelength]] bursts tied to the orbital period of Io.<ref name="Bigg1964">{{cite journal |last=Bigg |first=E. K. |date=1964 |title=Influence of the Satellite Io on Jupiter's Decametric Emission |journal=Nature |volume=203 |issue=4949 |pages=1008–1010 |doi=10.1038/2031008a0 |bibcode=1964Natur.203.1008B|s2cid=12233914 }}</ref>

===''Pioneer''===
The first spacecraft to pass by Io were the ''[[Pioneer 10]]'' and ''[[Pioneer 11|11]]'' probes on 3 December 1973 and 2 December 1974, respectively.<ref name="PioneerChap5">{{cite web |url=https://history.nasa.gov/SP-349/ch5.htm |title=First into the Outer Solar System |access-date=5 June 2007 |last=Fimmel |first=R. O. |display-authors=etal |date=1977 |work=Pioneer Odyssey |publisher=NASA}}</ref> Radio tracking provided an improved estimate of Io's mass, which, along with the best available information of its size, suggested it had the highest density of the Galilean satellites, and was composed primarily of silicate rock rather than water ice.<ref name="Anderson1974">{{cite journal |last=Anderson |first=J. D. |display-authors=etal |title=Gravitational parameters of the Jupiter system from the Doppler tracking of Pioneer&nbsp;10 |journal=Science |volume=183 |issue=4122 |pages=322–323 |date=1974 |doi=10.1126/science.183.4122.322 |pmid=17821098 |bibcode=1974Sci...183..322A|s2cid=36510719 }}</ref> The ''Pioneer''s also revealed the presence of a thin atmosphere and intense radiation belts near the orbit of Io. The camera on board ''Pioneer&nbsp;11'' took the only good image of the moon obtained by either spacecraft, showing its north polar region and its yellow tint.<ref name="Pioneer11image">{{cite web |url=http://www2.jpl.nasa.gov/galileo/io/pioio.html |archive-url=https://web.archive.org/web/19970409095549/http://www.jpl.nasa.gov/galileo/io/pioio.html |url-status=dead |archive-date=9 April 1997 |title=''Pioneer&nbsp;11'' Images of Io |work=Galileo Home Page |access-date=21 April 2007}}</ref> Close-up images were planned during ''Pioneer 10''{{'}}s encounter, but those were lost because of the high-radiation environment.<ref name="PioneerChap5"/>

===''Voyager''===
[[File:Io VGR South polar color mosaic.jpg|thumb|upright|''[[Voyager 1]]'' mosaic covering Io's south polar region. This includes [[List of tallest mountains in the Solar System|two of Io's ten highest peaks]], the Euboea Montes at upper extreme left and Haemus Mons at bottom.]]
When the twin probes ''[[Voyager 1|Voyager&nbsp;1]]'' and ''[[Voyager 2|Voyager&nbsp;2]]'' passed by Io in 1979, their more advanced imaging systems allowed for far more detailed images. ''Voyager&nbsp;1'' flew past Io on 5 March 1979 from a distance of {{convert|20600|km|sigfig=3|abbr=on}}.<ref name="VoyagerDesc">{{cite web |url=http://pds-rings.seti.org/voyager/mission/ |title=Voyager Mission Description |date=19 February 1997 |work=NASA PDS Rings Node }}</ref> The images returned during the approach revealed a strange, multi-colored landscape devoid of impact craters.<ref name="Smith1979">{{cite journal |title=The Jupiter system through the eyes of Voyager 1 |journal=Science |last=Smith |first=B. A. |display-authors=etal |pages=951–972 |volume=204 |issue=4396 |date=1979 |doi=10.1126/science.204.4396.951 |pmid=17800430 |bibcode=1979Sci...204..951S|s2cid=33147728 }}</ref><ref>{{cite news |url=https://news.google.com/newspapers?id=YY5QAAAAIBAJ&pg=4875,944849&dq=jupiter&hl=en |title=Jupiter moon shows color, erosion signs |work=The Milwaukee Sentinel |agency=United Press International |page=2 |date=6 March 1979 }}{{Dead link|date=February 2022 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> The highest-resolution images showed a relatively young surface punctuated by oddly shaped pits, mountains taller than Mount Everest, and features resembling volcanic lava flows.<ref name="Smith1979"/><ref name="Carr1980">{{cite journal |title=Volcanic Features on Io |journal=Nature |last=Carr |first=M. H. |display-authors=etal |pages=729–733 |volume=280 |date=1980 |doi=10.1038/280729a0 }}</ref>

Shortly after the encounter, ''Voyager'' navigation engineer [[Linda A. Morabito]] noticed a plume emanating from the surface in one of the images.<ref name="Morabito1979">{{cite journal |last=Morabito |first=L. A. |display-authors=etal |title=Discovery of currently active extraterrestrial volcanism |journal=Science |volume=204 |issue=4396 |page=972 |date=1979 |doi=10.1126/science.204.4396.972 |pmid=17800432 |bibcode=1979Sci...204..972M|s2cid=45693338 }}</ref> Analysis of other ''Voyager 1'' images showed nine such plumes scattered across the surface, proving that Io was volcanically active.<ref name="Strom1979">{{cite journal |title=Volcanic eruption plumes on Io |journal=[[Nature (journal)|Nature]] |last=Strom |first=R. G. |display-authors=etal |pages=733–736 |volume=280 |issue=5725 |date=1979 |doi=10.1038/280733a0 |bibcode=1979Natur.280..733S|s2cid=8798702 |doi-access=free }}</ref> This conclusion was predicted in a paper published shortly before the ''Voyager&nbsp;1'' encounter by [[Stanton J. Peale|Stan Peale]], Patrick Cassen, and R. T. Reynolds. The authors calculated that Io's interior must experience significant tidal heating caused by its orbital resonance with Europa and Ganymede (see the "[[#Tidal heating|Tidal heating]]" section for a more detailed explanation of the process).<ref name="Peale1979a">{{cite journal |title=Melting of Io by Tidal Dissipation |journal=Science |last=Peale |first=S. J. |display-authors=etal |pages=892–894 |volume=203 |issue=4383 |date=1979 |doi=10.1126/science.203.4383.892 |pmid=17771724 |bibcode=1979Sci...203..892P|s2cid=21271617 |url=https://pdfs.semanticscholar.org/49cf/72fc28e4bdab61efb05b8c43a31e867def6e.pdf |archive-url=https://web.archive.org/web/20200211222246/https://pdfs.semanticscholar.org/49cf/72fc28e4bdab61efb05b8c43a31e867def6e.pdf |url-status=dead |archive-date=2020-02-11 }}</ref> Data from this flyby showed that the surface of Io is dominated by sulfur and [[sulfur dioxide]] frosts. These compounds also dominate its thin [[#Atmosphere|atmosphere]] and the [[#Interaction with Jupiter's magnetosphere|torus of plasma]] centered on Io's orbit (also discovered by ''Voyager'').<ref name="Soderblom1980">{{cite journal |last=Soderblom |first=L. A. |display-authors=etal |title=Spectrophotometry of Io: Preliminary Voyager 1 results |journal=Geophys. Res. Lett. |volume=7 |issue=11 |pages=963–966 |date=1980 |doi=10.1029/GL007i011p00963 |bibcode=1980GeoRL...7..963S}}</ref><ref name="Pearl1979">{{cite journal |last=Pearl |first=J. C. |display-authors=etal |title=Identification of gaseous {{chem|SO|2}} and new upper limits for other gases on Io |journal=Nature |volume=288 |issue= 5725 |pages=757–758 |date=1979 |doi=10.1038/280755a0 |bibcode=1979Natur.280..755P |s2cid=4338190 |doi-access=free }}</ref><ref name="Broadfoot1979">{{cite journal |last=Broadfoot |first=A. L. |display-authors=etal |title=Extreme ultraviolet observations from ''Voyager 1'' encounter with Jupiter |journal=Science |volume=204 |issue=4396 |pages=979–982 |date=1979 |doi=10.1126/science.204.4396.979 |pmid=17800434 |bibcode=1979Sci...204..979B|s2cid=1442415 }}</ref>

''Voyager&nbsp;2'' passed Io on 9 July 1979 at a distance of {{convert|1130000|km|sigfig=2|abbr=on}}. Though it did not approach nearly as close as ''Voyager&nbsp;1'', comparisons between images taken by the two spacecraft showed several surface changes that had occurred in the four months between the encounters. In addition, observations of Io as a crescent as ''Voyager&nbsp;2'' departed the Jovian system revealed that seven of the nine plumes observed in March were still active in July 1979, with only the volcano [[Pele (volcano)|Pele]] shutting down between flybys.<ref name="Strom1982">{{cite book |last1=Strom |first1=R. G. |last2=Schneider |first2=N. M. |editor=Morrison, D. |title=Satellites of Jupiter |date=1982 |publisher=University of Arizona Press |isbn=0-8165-0762-7 |pages=[https://archive.org/details/satellitesofjupi0000unse/page/598 598–633] |chapter=Volcanic eruptions on Io |chapter-url=https://archive.org/details/satellitesofjupi0000unse/page/598 }}</ref>

===''Galileo''===
[[File:PIA01667-Io's Pele Hemisphere After Pillan Changes.jpg|thumb|upright|Enhanced-color ''Galileo'' image showing a dark spot (just lower-left of center, interrupting the red ring of short-chain [[Allotropes of sulfur|sulfur allotropes]] deposited by [[Pele (volcano)|Pele]]) produced by a major eruption at [[Pillan Patera]] in 1997]]

The [[Galileo (spacecraft)|''Galileo'']] spacecraft arrived at Jupiter in 1995 after a six-year journey from Earth to follow up on the discoveries of the two ''Voyager'' probes and the ground-based observations made in the intervening years. Io's location within one of Jupiter's most intense radiation belts precluded a prolonged close flyby, but ''Galileo'' did pass close by shortly before entering orbit for its two-year, primary mission studying the Jovian system. Although no images were taken during the close flyby on 7 December 1995, the encounter did yield significant results, such as the discovery of a large iron core, similar to that found on the rocky planets of the inner Solar System.<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>

Despite the lack of close-up imaging and mechanical problems that greatly restricted the amount of data returned, several significant discoveries were made during ''Galileo''{{'s}} primary mission. ''Galileo'' observed the effects of a major eruption at Pillan Patera and confirmed that volcanic eruptions are composed of silicate magmas with magnesium-rich [[mafic]] and [[Ultramafic rock|ultramafic]] compositions.<ref name="Mcewen1998b">{{cite journal |title=High-temperature silicate volcanism on Jupiter's moon Io |journal=Science |last=McEwen |first=A. S. |display-authors=etal |pages=87–90 |volume=281 |issue=5373 |date=1998 |doi=10.1126/science.281.5373.87 |pmid=9651251 |bibcode=1998Sci...281...87M|s2cid=28222050 |url=http://pdfs.semanticscholar.org/3d23/d4126eace55e4e525da42c1af8131c030d5b.pdf |archive-url=https://web.archive.org/web/20200923092649/http://pdfs.semanticscholar.org/3d23/d4126eace55e4e525da42c1af8131c030d5b.pdf |url-status=dead |archive-date=2020-09-23 }}</ref> Distant imaging of Io was acquired for almost every orbit during the primary mission, revealing large numbers of active volcanoes (both thermal emission from cooling magma on the surface and volcanic plumes), numerous mountains with widely varying morphologies, and several surface changes that had taken place both between the ''Voyager'' and ''Galileo'' eras and between ''Galileo'' orbits.<ref name="IobookChap3">{{cite book |last=Perry |first=J. |display-authors=etal |editor=Lopes, R. M. C. |editor2=Spencer, J. R. |title=Io after Galileo |date=2007 |publisher=Springer-Praxis |isbn=978-3-540-34681-4 |pages=35–59 |chapter=A Summary of the Galileo mission and its observations of Io}}</ref>

The ''Galileo'' mission was twice extended, in 1997 and 2000. During these extended missions, the probe flew by Io three times in late 1999 and early 2000, and three times in late 2001 and early 2002. Observations during these encounters revealed the geologic processes occurring at Io's volcanoes and mountains, excluded the presence of a magnetic field, and demonstrated the extent of volcanic activity.<ref name="IobookChap3"/>

===''Cassini''===
[[File:PIA02879 - A New Year for Jupiter and Io.jpg|thumb|upright|The ''[[Cassini-Huygens]]'' mission's view of Io and Jupiter on January 1, 2001]]
In December 2000, the [[Cassini–Huygens|''Cassini'']] spacecraft had a distant and brief encounter with the Jovian system en route to [[Saturn]], allowing for joint observations with ''Galileo''. These observations revealed a new plume at [[Tvashtar Paterae]] and provided insights into Io's [[aurora]]e.<ref name="Porco2003">{{cite journal |last=Porco |first=C. C. |author-link=Carolyn Porco |display-authors=etal |title=Cassini imaging of Jupiter's atmosphere, satellites, and rings |journal=Science |volume=299 |issue=5612 |pages=1541–1547 |date=2003 |url= https://authors.library.caltech.edu/36622/7/Porco.SOM.pdf|doi=10.1126/science.1079462 |pmid=12624258 |bibcode=2003Sci...299.1541P|s2cid=20150275 }}</ref>

===''New Horizons''===
The ''[[New Horizons]]'' spacecraft, en route to [[Pluto]] and the [[Kuiper belt]], flew by the Jovian system and Io on 28 February 2007. During the encounter, numerous distant observations of Io were obtained. These included images of a large plume at Tvashtar, providing the first detailed observations of the largest class of Ionian volcanic plume since observations of Pele's plume in 1979.<ref name="Spencer2007">{{cite journal |title=Io Volcanism Seen by New Horizons: A Major Eruption of the Tvashtar Volcano |journal=Science |last=Spencer |first=J. R. |display-authors=etal |pages=240–243 |volume=318 |issue=5848 |date=2007 |doi=10.1126/science.1147621 |pmid=17932290 |bibcode=2007Sci...318..240S|s2cid=36446567 |url=http://pdfs.semanticscholar.org/8a93/6fc39a8bab739fa3e128f05600d46e918d5d.pdf |archive-url=https://web.archive.org/web/20200412142851/http://pdfs.semanticscholar.org/8a93/6fc39a8bab739fa3e128f05600d46e918d5d.pdf |url-status=dead |archive-date=2020-04-12 }}</ref> ''New Horizons'' also captured images of a volcano near ''Girru Patera'' in the early stages of an eruption, and several volcanic eruptions that have occurred since ''Galileo''.<ref name="Spencer2007"/>

===''Juno''===
[[File:Image of Io from Juno JunoCam from December 2023.png|thumb|right|Global image of Jupiter's moon Io acquired by Juno's JunoCam camera on December 30, 2023]]
The ''[[Juno (spacecraft)|Juno]]'' spacecraft was launched in 2011 and entered orbit around Jupiter on July 5, 2016. ''Juno''{{'}}s mission is primarily focused on improving our understanding of Jupiter's interior, magnetic field, aurorae, and polar atmosphere.<ref name="JunoOverview">{{cite web |title=Juno – Mission Overview |url=https://www.missionjuno.swri.edu/orbit/ |work=[[NASA]] |access-date=March 11, 2024  }}</ref> ''Juno''{{'}}s 54-day orbit is highly inclined and highly eccentric in order to better characterize Jupiter's polar regions and to limit its exposure to the planet's harsh inner radiation belts, limiting close encounters with Jupiter's moons. The closest approach to Io during the initial, prime mission occurred in February 2020 at a distance of 195,000 kilomters.<ref name="Mura2020">{{cite journal |title=Infrared observations of Io from Juno |journal=Icarus |last=Mura |first=A. |display-authors=etal |volume=341 |date=2020 |page=113607 |doi=10.1016/j.icarus.2019.113607 |bibcode=2020Icar..34113607M |s2cid=213970081 }}</ref> Juno's extended mission, begun in June 2021, allowed for closer encounters with Jupiter's Galilean satellites due to ''Juno''{{'}}s orbital precession.<ref name="JunoEMPress">{{cite web |url=https://www.missionjuno.swri.edu/news/nasa-s-juno-mission-expands-into-the-future |title=NASA's Juno Mission Expands into the Future |date=January 13, 2021 |access-date=February 1, 2021}}</ref> After a series of increasingly closer encounters with Io in 2022 and 2023, ''Juno'' performed a pair of close flybys on December 30, 2023,<ref name="NYT-20240104">{{cite news |last=Miller |first=Katrina |title=New Images of Jupiter's Moon Io Capture Infernal Volcanic Landscape - Juno, a NASA mission designed to study Jupiter's origins, sent back new views of the most eruptive world in the solar system. |url=https://www.nytimes.com/2024/01/04/science/nasa-jupiter-io-moon-pictures.html |date=4 January 2024 |work=[[The New York Times]] |url-status=live |archiveurl=https://archive.today/20240104173647/https://www.nytimes.com/2024/01/04/science/nasa-jupiter-io-moon-pictures.html |archivedate=4 January 2024 |accessdate=4 January 2024 }}</ref> and February 3, 2024,<ref name="Feb3flyby">{{cite web |title=Juno to Get Second Close Look at Jupiter's Volcanic Moon Io on Feb. 3 |url=https://www.missionjuno.swri.edu/news/feb-3-close-look-at-jupiters-volcanic-moon-io |access-date=March 11, 2024 |date=February 3, 2024}}</ref> both with altitudes of 1,500 kilometers.  The primary goal of these encounters were to improve our understanding of Io's gravity field using doppler tracking and to image Io's surface to look for surface changes since Io was last seen up-close in 2007.<ref name="JunoOPAG">{{cite web |url=https://www.lpi.usra.edu/opag/meetings/opag2020fall/presentations/Bolton_6011.pdf |title=Juno OPAG Report |first=Scott |last=Bolton |date=September 2, 2020 |access-date=August 31, 2020}}</ref>

During several orbits, ''Juno'' has observed Io from a distance using JunoCAM, a wide-angle, visible-light camera, to look for volcanic plumes and JIRAM, a near-infrared spectrometer and imager, to monitor thermal emission from Io's volcanoes.<ref name="ChalybesPlumeArticle">{{cite web |last=Anderson |first=Paul Scott |title=New Juno images of Io's fiery volcanoes |url=https://earthsky.org/space/juno-spacecraft-io-volcanoes-from-a-distance |date=January 6, 2019 |work=[[EarthSky]] |access-date=February 14, 2020}}</ref><ref name="Mura2020"/>  JIRAM near-infrared spectroscopy has so far allowed for the coarse mapping of sulfur dioxide frost across Io's surface as well as mapping minor surface components weakly absorbing sunlight at 2.1 and 2.65&nbsp;µm.<ref name="Tosi2020">{{cite journal |title=Mapping Io's Surface Composition With Juno/JIRAM |journal=Journal of Geophysical Research: Planets |last=Tosi |first=F. |display-authors=et al |date=2020 |volume=125 |issue=11 |pages=e06522 |doi=10.1029/2020JE006522 |bibcode=2020JGRE..12506522T |s2cid=225456943 }}</ref>

===Future missions===
There are two forthcoming missions planned for the Jovian system. The [[Jupiter Icy Moon Explorer]] (JUICE) is a planned [[European Space Agency]] mission to the Jovian system that is intended to end up in Ganymede orbit.<ref name="JUICEannouncement">{{cite news |title=Esa selects 1bn-euro Juice probe to Jupiter |author=Jonathan Amos |url=https://www.bbc.co.uk/news/science-environment-17917102 |newspaper=BBC News |date=2 May 2012}}</ref> JUICE launched in April 2023, with arrival at Jupiter planned for July 2031.<ref>{{cite conference |last1=Witasse |first1=O. |last2=Altobelli |first2=N. |last3=Andres |first3=R. |last4=Atzei |first4=A. |last5=Boutonnet |first5=A. |last6=Budnik |first6=F. |last7=Dietz |first7=A. |last8=Erd |first8=C. |last9=Evill |first9=R. |last10=Lorente |first10=R. |last11=Munoz |first11=C. |last12=Pinzan |first12=G. |last13=Scharmberg |first13=C. |last14=Suarez |first14=A. |last15=Tanco |first15=I. |last16=Torelli |first16=F. |last17=Torn |first17=B. |last18=Vallat |first18=C. |author19=JUICE Science Working Team |url=https://meetingorganizer.copernicus.org/EPSC2021/EPSC2021-358.html |title=JUICE (Jupiter Icy Moon Explorer): Plans for the cruise phase |conference=Europlanet Science Congress (EPSC) 2021 |date=July 2021 |access-date=28 August 2021 |doi=10.5194/epsc2021-358 |doi-access= free}}</ref><ref name="JUICEYellowBook">{{Citation |year=2012 |title=JUICE assessment study report (Yellow Book) |publisher=ESA |url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=49837}}</ref> JUICE will not fly by Io, but it will use its instruments, such as a narrow-angle camera, to monitor Io's volcanic activity and measure its surface composition during the two-year Jupiter-tour phase of the mission prior to Ganymede orbit insertion. [[Europa Clipper]] is a planned NASA mission to the Jovian system focused on Jupiter's moon Europa.  Like JUICE, Europa Clipper will not perform any flybys of Io, but distant volcano monitoring is likely. ''Europa Clipper'' has a planned launch in 2024 with an arrival at Jupiter in 2030.<ref name="EuropaClipperNASA">{{cite web |url=https://europa.nasa.gov/mission/about/ |title=Europa Clipper Mission Overview |publisher=Jet Propulsion Laboratory |access-date=22 December 2022}}</ref>

The ''[[Io Volcano Observer]]'' (IVO) was a proposal to NASA for a low-cost, Discovery-class mission selected for a Phase A study along with three other missions in 2020. IVO would launch in January 2029 and perform ten flybys of Io while in orbit around Jupiter beginning in the early 2030s.<ref name="McEwenLPSC2015">{{cite conference |url=https://www.hou.usra.edu/meetings/lpsc2021/pdf/1352.pdf |title=The Io Volcano Observer (IVO) |conference=52nd [[Lunar and Planetary Science Conference]]. 15–19 March 2020. |first1=A. S. |last1=McEwen |author2=IVO Team |date=2021 |id=Abstract #2548}}</ref><ref name="DiscoveryDownselect">{{cite news |title=NASA Selects Four Possible Missions to Study the Secrets of the Solar System |url=https://www.nasa.gov/press-release/nasa-selects-four-possible-missions-to-study-the-secrets-of-the-solar-system |newspaper=NASA |date=13 Feb 2020}}</ref> However, the Venus missions [[DAVINCI+]] and [[VERITAS (spacecraft)|VERITAS]] were selected in favor of those.<ref name="veritasselection">{{cite news |title=Two new NASA missions will uncover the secrets of Venus |newspaper=CNN |date=June 3, 2021 |url=https://www.cnn.com/2021/06/02/world/venus-nasa-discovery-missions-scn/index.html |last=Strickland |first=Ashley}}</ref>

==Orbit and rotation==
[[File:Galilean moon Laplace resonance animation 2.gif|thumb|right|Animation of the [[Laplace resonance]] of Io, Europa and Ganymede (conjunctions are highlighted by color changes)]]
{{See also|Tidal heating}}
Io orbits Jupiter at a distance of {{convert|421700|km|sigfig=3|abbr=on}} from Jupiter's center and {{convert|350000|km|sigfig=3|abbr=on}} from its cloudtops. It is the innermost of the Galilean satellites of Jupiter, its orbit lying between those of [[Thebe (moon)|Thebe]] and [[Europa (moon)|Europa]]. Including Jupiter's inner satellites, Io is the fifth moon out from Jupiter. It takes Io about 42.5 hours (1.77 days) to complete one orbit around Jupiter (fast enough for its motion to be observed over a single night of observation). Io is in a 2:1 mean-motion [[orbital resonance]] with Europa and a 4:1 mean-motion orbital resonance with [[Ganymede (moon)|Ganymede]], completing two orbits of Jupiter for every one orbit completed by Europa, and four orbits for every one completed by Ganymede. This resonance helps maintain Io's [[orbital eccentricity]] (0.0041), which in turn provides the primary heating source for its geologic activity.<ref name="Peale1979a"/> Without this forced eccentricity, Io's orbit would circularize through [[Tidal acceleration|tidal dissipation]], leading to a less geologically active world.<ref name="Peale1979a"/>

Like the other [[Galilean moons|Galilean satellites]] and the [[Moon]], Io rotates [[Tidal locking|synchronously]] with its orbital period, keeping one face nearly pointed toward Jupiter. This synchrony provides the definition for Io's longitude system. Io's [[prime meridian]] intersects the equator at the sub-Jovian point. The side of Io that always faces Jupiter is known as the subjovian hemisphere, whereas the side that always faces away is known as the antijovian hemisphere. The side of Io that always faces in the direction that Io travels in its orbit is known as the leading hemisphere, whereas the side that always faces in the opposite direction is known as the trailing hemisphere.<ref name="Lopes2005">{{cite journal |title=Io after ''Galileo'' |journal=[[Reports on Progress in Physics]] |last1=Lopes |first1=R. M. C. |first2=D. A. |last2=Williams |pages=303–340 |volume=68 |issue=2 |date=2005 |doi=10.1088/0034-4885/68/2/R02 |bibcode=2005RPPh...68..303L|s2cid=44208045 }}</ref>

From the surface of Io, Jupiter would subtend an arc of 19.5°, making Jupiter appear 39 times the apparent diameter of Earth's Moon.{{cn|date=February 2022}}

==Interaction with Jupiter's magnetosphere==
[[File:Jupiter magnetosphere schematic.jpg|thumb|250px|Schematic of Jupiter's magnetosphere and the components influenced by Io (near the center of the image): the plasma torus (in red), the neutral cloud (in yellow), the flux tube (in green), and magnetic field lines (in blue).<ref name="SpencerGraphic">{{cite web |url=http://www.boulder.swri.edu/~spencer/digipics.html |title=John Spencer's Astronomical Visualizations |access-date=25 May 2007 |last=Spencer |first=J. }}</ref>]]
Io plays a significant role in shaping [[Magnetosphere of Jupiter|Jupiter's magnetic field]], acting as an electric generator that can develop 400,000 [[volt]]s across itself and create an electric current of 3 million amperes, releasing ions that give Jupiter a magnetic field inflated to more than twice the size it would otherwise have.<ref name="nasa-jup-io">{{cite web |url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jup_Io |archive-url=https://web.archive.org/web/20030211022640/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jup_Io |url-status=dead |archive-date=11 February 2003 |title=Io: Overview |publisher=NASA |series=Solar System Exploration |access-date=29 October 2014}}</ref> The magnetosphere of Jupiter sweeps up gases and dust from Io's thin atmosphere at a rate of 1 [[tonne]] per second.<ref name="IobookChap11">{{cite book |last1=Schneider |first1=N. M. |last2=Bagenal |first2=F. |editor-last=Lopes |editor-first=R. M. C. |editor2-last=Spencer |editor2-first=J. R. |title=Io after Galileo |date=2007 |publisher=Springer-Praxis |isbn=978-3-540-34681-4 |pages=265–286 |chapter=Io's neutral clouds, plasma torus, and magnetospheric interactions}}</ref> This material is mostly composed of [[ion]]ized and atomic sulfur, oxygen and chlorine; atomic sodium and potassium; molecular [[sulfur dioxide]] and sulfur; and [[sodium chloride]] dust.<ref name="IobookChap11" /><ref name="Postberg2006">{{cite journal |last=Postberg |first=F. |display-authors=etal |title=Composition of jovian dust stream particles |journal=Icarus |volume=183 |issue=1 |pages=122–134 |date=2006 |doi=10.1016/j.icarus.2006.02.001 |bibcode=2006Icar..183..122P}}</ref> These materials originate from Io's volcanic activity, with the material that escapes to Jupiter's magnetic field and into interplanetary space coming directly from Io's atmosphere. These materials, depending on their ionized state and composition, end up in various neutral (non-ionized) clouds and radiation belts in Jupiter's [[magnetosphere]] and, in some cases, are eventually ejected from the Jovian system.<ref name="Iobook2Chap9">{{cite book |last1=Bagenal |first1=F. |last2=Dols |first2=V. |editor-last=Lopes |editor-first=R. M. C. |editor2-last=de Kleer |editor2-first=K. |editor3-last=Keane |editor3-first=J. T. |title=Io: A New View of Jupiter's Moon |date=2023 |publisher=Springer|isbn=978-3-031-25669-1 |pages=291–322 |chapter=Space Environment of Io}}</ref>

[[File:PIA24524-Jupiter-Io-Interaction-Artwork-20210715.jpg|thumb|250px|left|The Jupiter - Io  System and Interaction<br />(artwork; 15 July 2021)]]
Surrounding Io (at a distance of up to six Io radii from its surface) is a cloud of neutral sulfur, oxygen, sodium, and potassium atoms. These particles originate in Io's upper atmosphere and are excited by collisions with ions in the [[plasma (physics)|plasma]] [[torus]] (discussed below) and by other processes into filling Io's [[Hill sphere]], which is the region where Io's gravity is dominant over Jupiter's. Some of this material escapes Io's gravitational pull and goes into orbit around Jupiter. Over a 20-hour period, these particles spread out from Io to form a banana-shaped, neutral cloud that can reach as far as six Jovian radii from Io, either inside Io's orbit and ahead of it or outside Io's orbit and behind it.<ref name="IobookChap11" /> The collision process that excites these particles also occasionally provides sodium ions in the plasma torus with an electron, removing those new "fast" neutrals from the torus. These particles retain their velocity (70 km/s, compared to the 17 km/s orbital velocity at Io), and are thus ejected in jets leading away from Io.<ref name="Burger1999">{{cite journal |last=Burger |first=M. H. |display-authors=etal |title=Galileo's close-up view of Io sodium jet |journal=Geophys. Res. Lett. |volume=26 |issue=22 |pages=3333–3336 |date=1999 |doi=10.1029/1999GL003654 |bibcode=1999GeoRL..26.3333B|doi-access=free }}</ref>

Io orbits within a belt of intense radiation known as the Io plasma torus. The plasma in this [[doughnut]]-shaped ring of ionized sulfur, oxygen, sodium, and chlorine originates when neutral atoms in the "cloud" surrounding Io are ionized and carried along by the Jovian magnetosphere.<ref name="IobookChap11" /> Unlike the particles in the neutral cloud, these particles co-rotate with Jupiter's magnetosphere, revolving around Jupiter at 74 km/s. Like the rest of Jupiter's magnetic field, the plasma torus is tilted with respect to Jupiter's equator (and Io's orbital plane), so that Io is at times below and at other times above the core of the plasma torus. As noted above, these ions' higher velocity and energy levels are partly responsible for the removal of neutral atoms and molecules from Io's atmosphere and more extended neutral clouds. The torus is composed of three sections: an outer, "warm" torus that resides just outside Io's orbit; a vertically extended region known as the "ribbon", composed of the neutral source region and cooling plasma, located at around Io's distance from Jupiter; and an inner, "cold" torus, composed of particles that are slowly spiraling in toward Jupiter.<ref name="IobookChap11" /> After residing an average of 40 days in the torus, particles in the "warm" torus escape and are partially responsible for Jupiter's unusually large [[magnetosphere]], their outward pressure inflating it from within.<ref name="Krimigis2002">{{cite journal |last=Krimigis |first=S. M. |display-authors=etal |title=A nebula of gases from Io surrounding Jupiter |journal=[[Nature (journal)|Nature]] |volume=415 |issue=6875 |pages=994–996 |date=2002 |doi=10.1038/415994a |pmid=11875559 |bibcode=2002Natur.415..994K|doi-access=free }}</ref> Particles from Io, detected as variations in magnetospheric plasma, have been detected far into the long magnetotail by ''New Horizons''. To study similar variations within the plasma torus, researchers measured the [[ultraviolet]] light it emits. Although such variations have not been definitively linked to variations in Io's volcanic activity (the ultimate source for material in the plasma torus), this link has been established in the neutral sodium cloud.<ref name="Mendillo2004">{{cite journal |last=Medillo |first=M. |display-authors=etal |title=Io's volcanic control of Jupiter's extended neutral clouds |journal=Icarus |volume=170 |issue=2 |pages=430–442 |date=2004 |doi=10.1016/j.icarus.2004.03.009 |bibcode=2004Icar..170..430M}}</ref>

During an encounter with Jupiter in 1992, the [[Ulysses (spacecraft)|''Ulysses'']] spacecraft detected a stream of dust-sized particles being ejected from the Jovian system.<ref name="Grun1993">{{cite journal |last=Grün |first=E. |display-authors=etal |title=Discovery of Jovian dust streams and interstellar grains by the ULYSSES spacecraft |journal=Nature |volume=362 |issue=6419 |pages=428–430 |date=1993 |url=https://zenodo.org/record/1233143 |doi=10.1038/362428a0 |bibcode=1993Natur.362..428G|s2cid=4315361 }}</ref> The dust in these discrete streams travels away from Jupiter at speeds upwards of several hundred kilometers per second, has an average particle size of 10&nbsp;[[micrometre|μm]], and consists primarily of sodium chloride.<ref name="Postberg2006" /><ref name="Zook1996">{{cite journal |last=Zook |first=H. A. |display-authors=etal |title=Solar Wind Magnetic Field Bending of Jovian Dust Trajectories |journal=Science |volume=274 |issue=5292 |pages=1501–1503 |date=1996 |doi=10.1126/science.274.5292.1501 |pmid=8929405 |bibcode=1996Sci...274.1501Z|s2cid=25816078 }}</ref> Dust measurements by ''Galileo'' showed that these dust streams originated on Io, but exactly how these form, whether from Io's volcanic activity or material removed from the surface, is unknown.<ref name="Grun1996">{{cite journal |last=Grün |first=E. |display-authors=etal |title=Dust Measurements During Galileo's Approach to Jupiter and Io Encounter |journal=Science |volume=274 |issue=5286 |pages=399–401 |date=1996 |doi=10.1126/science.274.5286.399 |bibcode=1996Sci...274..399G|s2cid=119868296 }}</ref>

Jupiter's [[magnetic field]], which Io crosses, couples Io's atmosphere and neutral cloud to Jupiter's polar upper atmosphere by [[Faraday's law of induction|generating]] an electric current known as the Io [[flux tube]].<ref name="IobookChap11" /> This current produces an auroral glow in Jupiter's polar regions known as the Io footprint, as well as aurorae in Io's atmosphere. Particles from this auroral interaction darken the Jovian polar regions at visible wavelengths. The location of Io and its auroral footprint with respect to Earth and Jupiter has a strong influence on Jovian [[radio]] emissions from our vantage point: when Io is visible, radio signals from Jupiter increase considerably.<ref name=Bigg1964/><ref name="IobookChap11" /> The ''[[Juno (spacecraft)|Juno]]'' mission, currently in orbit around Jupiter, should help shed light on these processes. The Jovian magnetic field lines that do get past Io's ionosphere also induce an electric current, which in turn creates an induced magnetic field within Io's interior. Io's induced magnetic field is thought to be generated within a partially molten, silicate magma ocean 50 kilometers beneath Io's surface.<ref name=Khurana>{{cite journal |last1=Khurana |first1=Krishan K. |last2=Jia |first2=Xianzhe |last3=Kivelson |first3=Margaret G. |last4=Nimmo |first4=Francis |last5=Schubert |first5=Gerald |last6=Russell |first6=Christopher T. |title=Evidence of a Global Magma Ocean in Io's Interior |journal=Science |date=3 June 2011 |volume=332 |issue=6034 |pages=1186–1189 |doi=10.1126/science.1201425 |pmid=21566160 |bibcode=2011Sci...332.1186K|s2cid=19389957 |doi-access=free }}</ref> Similar induced fields were found at the other Galilean satellites by ''Galileo'', possibly generated within liquid water oceans in the interiors of those moons.<ref name="Khurana1998">{{cite journal |last=Khurana |first=K. K. |display-authors=etal |title=Induced magnetic fields as evidence for subsurface oceans in Europa and Callisto |journal=Nature |volume=395 |pages=777–780 |date=1998 |doi=10.1038/27394  }}</ref>

==Geology==
Io is slightly larger than Earth's [[Moon]]. It has a mean radius of {{convert|1821.3|km|sigfig=5|abbr=on}} (about 5% greater than the Moon's) and a mass of 8.9319{{e|22}} kg (about 21% greater than the Moon's). It is a slight [[ellipsoid]] in shape, with its longest axis directed toward Jupiter. Among the [[Galilean moons|Galilean satellites]], in both mass and volume, Io ranks behind [[Ganymede (moon)|Ganymede]] and [[Callisto (moon)|Callisto]] but ahead of [[Europa (moon)|Europa]].<ref name="Iobook2Chap2">{{cite book |last1=Schneider |first1=N. M. |last2=Spencer |first2=J. R. |editor-last=Lopes |editor-first=R. M. C. |editor2-last=de Kleer |editor2-first=K. |editor3-last=Keane |editor3-first=J. T. |title=Io: A New View of Jupiter's Moon |date=2023 |publisher=Springer|isbn=978-3-031-25669-1 |pages=9–40 |chapter=Understanding Io: Four Centuries of Study and Surprise}}</ref>

===Interior===
[[File:Io diagram.svg|thumb|upright=2|Model of the possible interior composition of Io with various features labelled.]]
Composed primarily of [[silicate]] [[rock (geology)|rock]] and [[iron]], Io and Europa are closer in bulk composition to terrestrial planets than to other satellites in the outer Solar System, which are mostly composed of a mix of water ice and silicates. Io has a density of {{val|3.5275|u=g/cm3}}, the highest of any regular moon in the [[Solar System]]; significantly higher than the other Galilean satellites (Ganymede and Callisto in particular, whose densities are around {{val|1.9|u=g/cm3}}) and slightly higher (~5.5%) than the Moon's {{val|3.344|u=g/cm3}} and Europa's  {{val|2.989|u=g/cm3}}.<ref name="Schubert2004" /> Models based on the ''Voyager'' and ''Galileo'' measurements of Io's mass, radius, and quadrupole gravitational coefficients (numerical values related to how mass is distributed within an object) suggest that its interior is differentiated between a silicate-rich [[Crust (geology)|crust]] and [[Mantle (geology)|mantle]] and an iron- or [[Pyrite|iron-sulfide]]-rich [[Planetary core|core]].<ref name=Anderson1996/> Io's metallic core makes up approximately 20% of its mass.<ref name="Anderson2001">{{cite journal |last=Anderson |first=J. D. |display-authors=etal |title=Io's gravity field and interior structure |journal=J. Geophys. Res. |volume=106 |issue=E12 |pages=32963–32969 |date=2001 |doi=10.1029/2000JE001367 |bibcode=2001JGR...10632963A|doi-access= }}</ref> Depending on the amount of sulfur in the core, the core has a radius between {{convert|350|and(-)|650|km|sigfig=2|abbr=on}} if it is composed almost entirely of iron, or between {{convert|550|and(-)|900|km|sigfig=2|abbr=on}} for a core consisting of a mix of iron and sulfur. ''Galileo''{{'s}} [[magnetometer]] failed to detect an internal, intrinsic magnetic field at Io, suggesting that the core is not [[Convection|convecting]].<ref name="Kivelson2001">{{cite journal |last=Kivelson |first=M. G. |display-authors=etal |title=Magnetized or Unmagnetized: Ambiguity persists following Galileo's encounters with Io in 1999 and 2000 |journal=J. Geophys. Res. |volume=106 |issue=A11 |pages=26121–26135 |date=2001 |doi=10.1029/2000JA002510 |bibcode=2001JGR...10626121K|doi-access=free }}</ref>

Modeling of Io's interior composition suggests that the mantle is composed of at least 75% of the magnesium-rich mineral [[forsterite]], and has a bulk composition similar to that of [[L chondrite|L-chondrite]] and [[LL chondrite|LL-chondrite]] [[meteorite]]s, with higher iron content (compared to [[silicon]]) than the Moon or Earth, but lower than Mars.<ref name="Sohl2002">{{cite journal |last=Sohl |first=F. |display-authors=etal |title=Implications from Galileo observations on the interior structure and chemistry of the Galilean satellites |journal=Icarus |volume=157 |issue=1 |pages=104–119 |date=2002 |doi=10.1006/icar.2002.6828 |bibcode=2002Icar..157..104S}}</ref><ref name="Kuskov2001">{{cite journal |last1=Kuskov |first1=O. L. |first2=V. A. |last2=Kronrod |title=Core sizes and internal structure of the Earth's and Jupiter's satellites |journal=Icarus |volume=151 |issue=2 |pages=204–227 |date=2001 |doi=10.1006/icar.2001.6611 |bibcode=2001Icar..151..204K}}</ref> To support the heat flow observed on Io, 10–20% of Io's mantle may be molten, though regions where high-temperature volcanism has been observed may have higher melt fractions.<ref name="IobookChap5">{{cite book |last=Moore |first=W. B. |display-authors=etal |editor=R. M. C. Lopes |editor2=J. R. Spencer |title=Io after Galileo |date=2007 |publisher=Springer-Praxis |pages=89–108 |isbn=978-3-540-34681-4 |chapter=The Interior of Io.}}</ref> However, re-analysis of ''Galileo'' magnetometer data in 2009 revealed the presence of an induced magnetic field at Io, requiring a magma ocean {{convert|50|km|sigfig=2|abbr=on}} below the surface.<ref name=Khurana/> Further analysis published in 2011 provided direct evidence of such an ocean.<ref>{{cite news |title=NASA's Galileo Reveals Magma 'Ocean' Beneath Surface of Jupiter's Moon |url=https://www.sciencedaily.com/releases/2011/05/110512150723.htm |date=12 May 2011 |publisher=Science Daily}}</ref> This layer is estimated to be 50 km thick and to make up about 10% of Io's mantle. It is estimated that the temperature in the magma ocean reaches 1,200 °C. It is not known if the 10–20% partial melting percentage for Io's mantle is consistent with the requirement for a significant amount of molten silicates in this possible magma ocean.<ref name="PerryInducedMagmaOcean">{{cite web |last=Perry |first=J. |title=Science: Io's Induced Magnetic Field and Mushy Magma Ocean |date=21 January 2010 |url=http://gishbar.blogspot.com/2010/01/science-discovery-of-ios-induced.html |work=The Gish Bar Times}}</ref> The [[lithosphere]] of Io, composed of basalt and sulfur deposited by Io's extensive volcanism, is at least {{convert|12|km|abbr=on}} thick, and likely less than {{convert|40|km|sigfig=2|abbr=on}} thick.<ref name="Anderson2001" /><ref name="Jaeger2003">{{cite journal |last=Jaeger |first=W. L. |display-authors=etal |title=Orogenic tectonism on Io |journal=J. Geophys. Res. |volume=108 |issue=E8 |pages=12–1 |date=2003 |doi=10.1029/2002JE001946 |bibcode=2003JGRE..108.5093J|doi-access=free }}</ref>

===Tidal heating===
{{Main|Tidal heating of Io}}
Unlike Earth and the Moon, Io's main source of internal heat comes from [[Tidal force|tidal]] dissipation rather than radioactive [[isotope]] decay, the result of Io's orbital resonance with Europa and Ganymede.<ref name=Peale1979a/> Such heating is dependent on Io's distance from Jupiter, its orbital eccentricity, the composition of its interior, and its physical state.<ref name="IobookChap5" /> Its [[orbital resonance|Laplace resonance]] with Europa and Ganymede maintains Io's eccentricity and prevents tidal dissipation within Io from [[tidal circularization|circularizing]] its orbit. The resonant orbit also helps to maintain Io's distance from Jupiter; otherwise tides raised on Jupiter would cause Io to slowly spiral outward from its parent planet.<ref name="Yoder1979">{{cite journal |title=How tidal heating in Io drives the Galilean orbital resonance locks |journal=Nature |first=C. F. |last=Yoder |display-authors=etal |volume=279 |issue=5716 |pages=767–770 |date=1979 |doi=10.1038/279767a0 |bibcode=1979Natur.279..767Y|s2cid=4322070 }}</ref> The tidal forces experienced by Io are about 20,000 times stronger than the tidal forces Earth experiences due to the Moon, and the vertical differences in its tidal bulge, between the times Io is at [[Apsis|periapsis]] and [[Apsis|apoapsis]] in its orbit, could be as much as {{convert|100|m|sigfig=2|abbr=on}}.<ref>{{Cite web |url=https://science.nasa.gov/science-news/science-at-nasa/2000/ast04may_1m |title=Interplanetary Low Tide - NASA Science Mission Directorate |access-date=16 October 2019 |archive-date=4 June 2023 |archive-url=https://web.archive.org/web/20230604014510/https://science.nasa.gov/science-news/science-at-nasa/2000/ast04may_1m |url-status=dead }}</ref> The friction or tidal dissipation produced in Io's interior due to this varying tidal pull, which, without the resonant orbit, would have gone into circularizing Io's orbit instead, creates significant tidal heating within Io's interior, melting a significant amount of Io's mantle and core. The amount of energy produced is up to 200 times greater than that produced solely from [[radioactive decay]].<ref name="book" /> This heat is released in the form of volcanic activity, generating its observed high [[Heat transfer|heat flow]] (global total: 0.6 to 1.6×10<sup>14</sup> [[watt|W]]).<ref name="IobookChap5" /> Models of its orbit suggest that the amount of tidal heating within Io changes with time; however, the current amount of tidal dissipation is consistent with the observed heat flow.<ref name="IobookChap5" /><ref name="Lainey2009">{{cite journal |title=Strong tidal dissipation in Io and Jupiter from astrometric observations |journal=Nature |first=V. |last=Lainey |display-authors=etal |volume=459 |issue=7249 |pages=957–959 |date=2009 |doi=10.1038/nature08108 |bibcode=2009Natur.459..957L |pmid=19536258|s2cid=205217186 }}</ref> Models of tidal heating and convection have not found consistent planetary viscosity profiles that simultaneously match tidal energy dissipation and mantle convection of heat to the surface.<ref name="Lainey2009" /><ref name="Moore2003geophys">{{cite journal |title=Tidal heating and convection in Io |journal=Journal of Geophysical Research |first=W. B. |last=Moore |volume=108 |issue=E8 |page=5096 |date=August 2003 |doi=10.1029/2002JE001943 |bibcode=2003JGRE..108.5096M|s2cid=53443229 |doi-access=free }}</ref>

Although there is general agreement that the origin of the heat as manifested in Io's many volcanoes is tidal heating from the pull of gravity from [[Jupiter]] and its moon [[Europa (moon)|Europa]], the volcanoes are not in the positions predicted with tidal heating. They are shifted 30 to 60 degrees to the east.<ref name="Steigerwald2015">{{cite web |url=http://www.nasa.gov/content/goddard/io-volcano-tides |title=Underground Magma Ocean Could Explain Io's 'Misplaced' Volcanoes |publisher=NASA |first=William |last=Steigerwald |date=10 September 2015 |access-date=19 September 2015}}</ref> A study published by Tyler ''et al.'' (2015) suggests that this eastern shift may be caused by an ocean of molten rock under the surface. The movement of this magma would generate extra heat through friction due to its [[viscosity]]. The study's authors believe that this subsurface ocean is a mixture of molten and solid rock.<ref name="Tyler2015">{{cite journal |title=Tidal Heating in a Magma Ocean within Jupiter's Moon Io |journal=The Astrophysical Journal Supplement Series |first1=Robert H. |last1=Tyler |first2=Wade G. |last2=Henning |first3=Christopher W. |last3=Hamilton |volume=218 |issue=2 |at=22 |date=June 2015 |doi=10.1088/0067-0049/218/2/22 |bibcode=2015ApJS..218...22T|doi-access=free |hdl=11603/28862 |hdl-access=free }}</ref>

Other moons in the Solar System are also tidally heated, and they too may generate additional heat through the friction of subsurface magma or water oceans. This ability to generate heat in a subsurface ocean increases the chance of life on bodies like Europa and [[Enceladus]].<ref name="space20150914">{{cite news |url=http://www.space.com/30530-jupiter-moon-io-magma-volcano-mystery.html |title=Magma Oceans on Jupiter's Moon Io May Solve Volcano Mystery |work=Space.com |first=Sarah |last=Lewin |date=14 September 2015 |access-date=19 September 2015}}</ref><ref name="nasajpl20150915">{{cite web |url=http://www.jpl.nasa.gov/news/news.php?feature=4718 |title=Cassini Finds Global Ocean in Saturn's Moon Enceladus |publisher=NASA{{\}}Jet Propulsion Laboratory |date=15 September 2015 |access-date=19 September 2015}}</ref>

===Surface===
[[File:Io from Galileo and Voyager missions.jpg|thumb|right|300px|Io's surface map]]

Based on their experience with the ancient surfaces of the Moon, Mars, and Mercury, scientists expected to see numerous [[impact crater]]s in ''Voyager 1''{{'s}} first images of Io. The density of impact craters across Io's surface would have given clues to Io's age. However, they were surprised to discover that the surface was almost completely lacking in impact craters, but was instead covered in smooth plains dotted with tall mountains, pits of various shapes and sizes, and volcanic lava flows.<ref name=Smith1979/> Compared to most worlds observed to that point, Io's surface was covered in a variety of colorful materials (leading Io to be compared to a rotten [[orange (fruit)|orange]] or to [[pizza]]) from various sulfurous compounds.<ref name="Britt2000">{{cite news |url=http://www.space.com/scienceastronomy/solarsystem/galileo_io_volcanoes_000316.html |title=Pizza Pie in the Sky: Understanding Io's Riot of Color |publisher=[[Space.com]] |last=Britt |first=Robert Roy |date=16 March 2000 |archive-url=https://web.archive.org/web/20000818092821/http://space.com/scienceastronomy/solarsystem/galileo_io_volcanoes_000316.html |archive-date=18 August 2000}}</ref><ref name="Calder2005">{{cite book |url=https://archive.org/details/magicuniversegra0000cald |url-access=registration |title=Magic Universe: A Grand Tour of Modern Science |publisher=Oxford University Press |first=Nigel |last=Calder |date=2005 |page=[https://archive.org/details/magicuniversegra0000cald/page/215 215] |isbn=978-0-19-280669-7}}</ref> The lack of impact craters indicated that Io's surface is geologically young, like the terrestrial surface; volcanic materials continuously bury craters as they are produced. This result was spectacularly confirmed as at least nine active volcanoes were observed by ''Voyager 1''.<ref name=Strom1979/>

====Surface composition====
Io's colorful appearance is the result of materials deposited by its extensive volcanism, including [[silicate]]s (such as [[Pyroxene|orthopyroxene]]), [[sulfur]], and [[sulfur dioxide]].<ref name="IobookChap9">{{cite book |last=Carlson |first=R. W. |display-authors=etal |editor=Lopes, R. M. C. |editor2=Spencer, J. R. |title=Io after Galileo |date=2007 |publisher=Springer-Praxis |isbn=978-3-540-34681-4 |pages=194–229 |chapter=Io's surface composition}}</ref> Sulfur dioxide frost is ubiquitous across the surface of Io, forming large regions covered in white or grey materials. Sulfur is also seen in many places across Io, forming yellow to yellow-green regions. Sulfur deposited in the mid-latitude and polar regions is often damaged by radiation, breaking up the normally stable [[Cyclooctasulfur|cyclic 8-chain sulfur]]. This radiation damage produces Io's red-brown polar regions.<ref name=Barnard1894/>

[[File:First Geologic Map of Jupiter’s Moon Io.jpg|thumb|left|Geological map of Io]]

[[Volcanism on Io#Plumes|Explosive volcanism]], often taking the form of umbrella-shaped plumes, paints the surface with sulfurous and silicate materials. Plume deposits on Io are often colored red or white depending on the amount of sulfur and sulfur dioxide in the plume. Generally, plumes formed at volcanic vents from degassing lava contain a greater amount of {{chem2|S2}}, producing a red "fan" deposit, or in extreme cases, large (often reaching beyond {{convert|450|km|sigfig=2|disp=or|abbr=on}} from the central vent) red rings.<ref name="Spencer2000b">{{cite journal |title=Discovery of Gaseous S<sub>2</sub> in Io's Pele Plume |journal=Science |last=Spencer |first=J. |display-authors=etal |pages=1208–1210 |volume=288 |issue=5469 |date=2000 |doi=10.1126/science.288.5469.1208 |pmid=10817990 |bibcode=2000Sci...288.1208S}}</ref> A prominent example of a red-ring plume deposit is located at Pele. These red deposits consist primarily of sulfur (generally 3- and 4-chain molecular sulfur), sulfur dioxide, and perhaps [[sulfuryl chloride]].<ref name="IobookChap9" /> Plumes formed at the margins of silicate lava flows (through the interaction of lava and pre-existing deposits of sulfur and sulfur dioxide) produce white or gray deposits.<ref name="Kieffer2000">{{cite journal |title=Prometheus: Io's Wandering Plume |journal=Science |last=Kieffer |first=S. W. |display-authors=etal |pages=1204–1208 |volume=288 |issue=5469 |date=2000 |doi=10.1126/science.288.5469.1204 }}</ref>

Compositional mapping and Io's high density suggest that Io contains little to no [[water]], though small pockets of water ice or [[Mineral hydration|hydrated minerals]] have been tentatively identified, most notably on the northwest flank of the mountain [[Gish Bar Mons]].<ref name="Doute2004">{{cite journal |last=Douté |first=S. |display-authors=etal |title=Geology and activity around volcanoes on Io from the analysis of NIMS |journal=Icarus |volume=169 |issue=1 |pages=175–196 |date=2004 |doi=10.1016/j.icarus.2004.02.001 |bibcode=2004Icar..169..175D}}</ref> Io has the least amount of water of any known body in the Solar System.<ref name="Seeds2012">{{cite book |title=The Solar System |publisher=Cengage Learning |first1=Michael A. |last1=Seeds |first2=Dana E. |last2=Backman |edition=8th |page=514 |date=2012 |isbn=9781133713685}}</ref> This lack of water is likely due to Jupiter being hot enough early in the [[Formation and evolution of the Solar System|evolution of the Solar System]] to drive off [[Volatile (astrogeology)|volatile materials]] like water in the vicinity of Io, but not hot enough to do so farther out.<ref name="astrobio20140306">{{cite news |url=http://www.astrobio.net/news-exclusive/alien-moons-could-bake-dry-from-young-gas-giants-hot-glow/ |title=Alien Moons Could Bake Dry from Young Gas Giants' Hot Glow |work=Astrobiology Magazine |first=Adam |last=Hadhazy |date=6 March 2014 |access-date=28 October 2014 |archive-url=https://web.archive.org/web/20150831232122/http://www.astrobio.net/news-exclusive/alien-moons-could-bake-dry-from-young-gas-giants-hot-glow/ |archive-date=2015-08-31 |url-status=usurped}}</ref>
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====Volcanism====
{{Main|Volcanism on Io}}
{{See also|List of volcanic features on Io}}
[[File:Tvastarpic2.jpg|thumb|300px|Active lava flows in volcanic region [[Tvashtar Paterae]] (blank region represents saturated areas in the original data). Images taken by ''Galileo'' in November 1999 and February 2000.]]
The tidal heating produced by Io's forced [[orbital eccentricity]] has made it the most volcanically active world in the Solar System, with hundreds of volcanic centers and extensive [[Lava|lava flows]].<ref name="NYT-20190626">{{cite news |last=Sokol |first=Joshua |title=This World Is a Simmering Hellscape. They've Been Watching Its Explosions. - Researchers have released a five-year record of volcanic activity on Io, a moon of Jupiter, hoping others will find more patterns. |url=https://www.nytimes.com/2019/06/26/science/io-volcanic-moon.html |date=26 June 2019 |work=[[The New York Times]] |access-date=26 June 2019 }}</ref> During a major eruption, lava flows tens or even hundreds of kilometers long can be produced, consisting mostly of [[basalt]] silicate lavas with either [[mafic]] or [[Ultramafic rock|ultramafic]] (magnesium-rich) compositions. As a by-product of this activity, sulfur, sulfur dioxide gas and silicate [[Pyroclastic rock|pyroclastic]] material (like ash) are blown up to {{convert|200|km|sigfig=2|abbr=on}} into space, producing large, umbrella-shaped plumes, painting the surrounding terrain in red, black, and white, and providing material for Io's patchy atmosphere and Jupiter's extensive magnetosphere.<ref name="Iobook2Chap7">{{cite book |last1=Keszthelyi |first1=L. P. |last2=Suer |first2=T. |editor-last=Lopes |editor-first=R. M. C. |editor2-last=de Kleer |editor2-first=K. |editor3-last=Keane |editor3-first=J. T. |title=Io: A New View of Jupiter's Moon |date=2023 |publisher=Springer|isbn=978-3-031-25669-1 |pages=211–232 |chapter=The Composition of Io}}</ref><ref name="Iobook2Chap9">{{cite book |last1=Bagenal |first1=F. |last2=Dols |first2=V. |editor-last=Lopes |editor-first=R. M. C. |editor2-last=de Kleer |editor2-first=K. |editor3-last=Keane |editor3-first=J. T. |title=Io: A New View of Jupiter's Moon |date=2023 |publisher=Springer|isbn=978-3-031-25669-1 |pages=291–322 |chapter=Space Environment of Io}}</ref>

Io's surface is dotted with volcanic depressions known as ''paterae'' which generally have flat floors bounded by steep walls.<ref name="Radebaugh2001">{{cite journal |title=Paterae on Io: A new type of volcanic caldera? |journal=J. Geophys. Res. |last=Radebaugh |first=D. |display-authors=etal |pages=33005–33020 |volume=106 |issue=E12 |date=2001 |doi=10.1029/2000JE001406 |bibcode=2001JGR...10633005R|url=http://www.lpl.arizona.edu/%7Ejani/janijgr2001.pdf |doi-access=free }}</ref> These features resemble terrestrial [[caldera]]s, but it is unknown if they are produced through collapse over an emptied lava chamber like their terrestrial cousins. One hypothesis suggests that these features are produced through the exhumation of volcanic [[Sill (geology)|sills]], and the overlying material is either blasted out or integrated into the sill.<ref name="Keszthelyi2004">{{cite journal |title=A Post-Galileo view of Io's Interior |journal=Icarus |last=Keszthelyi |first=L. |display-authors=etal |pages=271–286 |volume=169 |issue=1 |date=2004 |doi=10.1016/j.icarus.2004.01.005 |bibcode=2004Icar..169..271K|url=https://zenodo.org/record/1259017 }}</ref> Examples of paterae in various stages of exhumation have been mapped using ''Galileo'' images of the [[Chaac-Camaxtli region]].<ref name=Williams>{{cite journal |last1=Williams |first1=David |last2=Radebaugh |first2=Jani |last3=Keszthelyi |first3=Laszlo P. |last4=McEwen |first4=Alfred S. |last5=Lopes |first5=Rosaly M. C. |last6=Douté |first6=Sylvain |last7=Greeley |first7=Ronald |title=Geologic mapping of the Chaac-Camaxtli region of Io from Galileo imaging data |journal=Journal of Geophysical Research |year=2002 |volume=107 |issue=E9 |page=5068 |bibcode=2002JGRE..107.5068W |doi=10.1029/2001JE001821|s2cid=41607277 |doi-access= }}</ref> Unlike similar features on Earth and Mars, these depressions generally do not lie at the peak of [[shield volcano]]es and are normally larger, with an average diameter of {{convert|41|km|abbr=on}}, the largest being [[Loki Patera]] at {{convert|202|km|sigfig=3|abbr=on}}.<ref name="Radebaugh2001" /> Loki is also consistently the strongest volcano on Io, contributing on average 25% of Io's global heat output.<ref>{{Cite book |title=Astronomy Encyclopedia |publisher=Oxford University Press |location=New York |editor-first=Patrick |editor-last=Moore |page=[https://archive.org/details/astronomyencyclo0000unse/page/232 232] |date=2002 |isbn=0-19-521833-7 |url-access=registration |url=https://archive.org/details/astronomyencyclo0000unse/page/232 }}</ref> Whatever the formation mechanism, the morphology and distribution of many paterae suggest that these features are structurally controlled, with at least half bounded by faults or mountains.<ref name="Radebaugh2001" /> These features are often the site of volcanic eruptions, either from lava flows spreading across the floors of the paterae, as at an eruption at [[Gish Bar Patera]] in 2001, or in the form of a [[lava lake]].<ref name="Lopes2004"/><ref name="Perry2003">{{cite conference |title=Gish Bar Patera, Io: Geology and Volcanic Activity, 1997–2001 |work=[[Lunar and Planetary Science Conference|LPSC XXXIV]] |location=[[Clear Lake City (Greater Houston)]] |last=Perry |first=J. E. |display-authors=etal |date=2003 |id=Abstract #1720 |url=http://www.lpi.usra.edu/meetings/lpsc2003/pdf/1720.pdf }}</ref> Lava lakes on Io either have a continuously overturning lava crust, such as at Pele, or an episodically overturning crust, such as at Loki.<ref name="Radebaugh2004">{{cite journal |title=Observations and temperatures of Io's Pele Patera from Cassini and Galileo spacecraft images |journal=Icarus |last=Radebaugh |first=J. |display-authors=etal |pages=65–79 |volume=169 |issue=1 |date=2004 |doi=10.1016/j.icarus.2003.10.019 |bibcode=2004Icar..169...65R}}</ref><ref name="Howell2007">{{cite journal |title=The nature of the volcanic activity at Loki: Insights from Galileo NIMS and PPR data |journal=Icarus |last1=Howell |first1=R. R. |last2=Lopes |first2=R. M. C. |pages=448–461 |volume=186 |issue=2 |date=2007 |doi=10.1016/j.icarus.2006.09.022 |bibcode=2007Icar..186..448H}}</ref>

[[File:PIA25888-JupiterMoon-Io-VolcanoActivity-20230515.jpg|thumb|left|250px|Jupiter moon Io volcanic activity<br />(12/14/2022/left and 03/01/2023)]]
Lava flows represent another major volcanic terrain on Io. Magma erupts onto the surface from vents on the floor of paterae or on the plains from fissures, producing inflated, compound lava flows similar to those seen at [[Kīlauea|Kilauea]] in Hawaii. Images from the ''Galileo'' spacecraft revealed that many of Io's major lava flows, like those at [[Prometheus (volcano)|Prometheus]] and [[Amirani (volcano)|Amirani]], are produced by the build-up of small breakouts of lava flows on top of older flows.<ref name="Keszthelyi2001">{{cite journal |title=Imaging of volcanic activity on Jupiter's moon Io by Galileo during the Galileo Europa Mission and the Galileo Millennium Mission |journal=J. Geophys. Res. |last=Keszthelyi |first=L. |display-authors=etal |pages=33025–33052 |volume=106 |issue=E12 |date=2001 |doi=10.1029/2000JE001383 |bibcode=2001JGR...10633025K|doi-access= }}</ref> Larger outbreaks of lava have also been observed on Io. For example, the leading edge of the Prometheus flow moved {{convert|75|to|95|km|sigfig=2|abbr=on}} between ''Voyager'' in 1979 and the first ''Galileo'' observations in 1996. A major eruption in 1997 produced more than {{convert|3500|km2|sigfig=2|abbr=on}} of fresh lava and flooded the floor of the adjacent Pillan Patera.<ref name="Mcewen1998b"/>

Analysis of the ''Voyager'' images led scientists to believe that these flows were composed mostly of various compounds of molten sulfur. However, subsequent Earth-based [[infrared]] studies and measurements from the ''Galileo'' spacecraft indicate that these flows are composed of basaltic lava with mafic to ultramafic compositions.<ref name="Battaglia2019">{{Cite conference |title= A Jökulhlaup-like Model for Secondary Sulfur Flows on Io |conference=50th Lunar and Planetary Science Conference. 18–22 March 2019. The Woodlands, Texas. |first=Steven M. |last=Battaglia |date=March 2019 |id=LPI Contribution No. 1189 |bibcode=2019LPI....50.1189B}}</ref> This hypothesis is based on temperature measurements of Io's "hotspots", or thermal-emission locations, which suggest temperatures of at least 1,300&nbsp;K and some as high as 1,600&nbsp;K.<ref name="Keszthelyi2007">{{cite journal |title=New estimates for Io eruption temperatures: Implications for the interior |journal=Icarus |last=Keszthelyi |first=L. |display-authors=etal |pages=491–502 |volume=192 |issue=2 |date=2007 |doi=10.1016/j.icarus.2007.07.008 |bibcode=2007Icar..192..491K|url=https://zenodo.org/record/1259031 }}</ref> Initial estimates suggesting eruption temperatures approaching 2,000&nbsp;K<ref name="Mcewen1998b"/> have since proven to be overestimates because the wrong thermal models were used to model the temperatures.<ref name="Keszthelyi2007" /><ref name="Battaglia2019" />

[[File:Tvashtarvideo.gif|right|thumb|Five-image sequence of ''[[New Horizons]]'' images showing Io's volcano Tvashtar spewing material 330&nbsp;km above its surface]]
The discovery of plumes at the volcanoes [[Pele (volcano)|Pele]] and [[Loki (volcano)|Loki]] were the first sign that Io is geologically active.<ref name="Morabito1979"/> Generally, these plumes are formed when volatiles like sulfur and sulfur dioxide are ejected skyward from Io's volcanoes at speeds reaching {{convert|1|km/s|abbr=on}}, creating umbrella-shaped clouds of gas and dust. Additional material that might be found in these volcanic plumes include sodium, [[potassium]], and [[chlorine]].<ref name="Roesler1999">{{cite journal |title=Far-Ultraviolet Imaging Spectroscopy of Io's Atmosphere with HST/STIS |journal=Science |first1=F. L. |last1=Roesler |first2=H. W. |last2=Moos |first3=R. J. |last3=Oliversen |first4=R. C. |last4=Woodward, Jr. |first5=K. D. |last5=Retherford |first6=F. |last6=Scherb |first7=M. A. |last7=McGrath |first8=W. H. |last8=Smyth |first9=P. D. |last9=Feldman |first10=D. F. |last10=Strobel |display-authors=5 |volume=283 |issue=5400 |pages=353–357 |date=January 1999 |doi=10.1126/science.283.5400.353 |pmid=9888844 |bibcode=1999Sci...283..353R}}</ref><ref name="Geissler1999">{{cite journal |title=Galileo Imaging of Atmospheric Emissions from Io |journal=Science |first1=P. E. |last1=Geissler |first2=A. S. |last2=McEwen |first3=W. |last3=Ip |first4=M. J. S. |last4=Belton |first5=T. V. |last5=Johnson |first6=W. H. |last6=Smyth |first7=A. P. |last7=Ingersoll |display-authors=5 |volume=285 |issue=5429 |pages=870–874 |date=August 1999 |doi=10.1126/science.285.5429.870 |pmid=10436151 |bibcode=1999Sci...285..870G|s2cid=33402233 |url=http://pdfs.semanticscholar.org/2234/0ec6c1741fc267ddef24675bcbf33f7e2cf2.pdf |archive-url=https://web.archive.org/web/20190220100620/http://pdfs.semanticscholar.org/2234/0ec6c1741fc267ddef24675bcbf33f7e2cf2.pdf |url-status=dead |archive-date=2019-02-20 }}</ref> These plumes appear to be formed in one of two ways.<ref name="McEwen1983">{{cite journal |title=Two classes of volcanic plume on Io |journal=Icarus |last1=McEwen |first1=A. S. |last2=Soderblom |first2=L. A. |pages=197–226 |volume=55 |issue=2 |date=August 1983 |doi=10.1016/0019-1035(83)90075-1 |bibcode=1983Icar...55..191M}}</ref> Io's largest plumes, such as those emitted by [[Pele (volcano)|Pele]], are created when dissolved sulfur and sulfur dioxide gas are released from erupting magma at volcanic vents or lava lakes, often dragging silicate pyroclastic material with them.<ref name="Battaglia2014">{{cite journal |title=Io's theothermal (sulfur) - Lithosphere cycle inferred from sulfur solubility modeling of Pele's magma supply |journal=Icarus |first1=Steven M. |last1=Battaglia |first2=Michael A. |last2=Stewart |first3=Susan W. |last3=Kieffer |volume=235 |pages=123–129 |date=June 2014 |doi=10.1016/j.icarus.2014.03.019 |bibcode=2014Icar..235..123B}}</ref> These plumes form red (from the short-chain sulfur) and black (from the silicate pyroclastics) deposits on the surface. Plumes formed in this manner are among the largest observed at Io, forming red rings more than {{convert|1000|km|sigfig=2|abbr=on}} in diameter. Examples of this plume type include Pele, Tvashtar, and [[Dazhbog Patera|Dazhbog]]. Another type of plume is produced when encroaching lava flows vaporize underlying sulfur dioxide frost, sending the sulfur skyward. This type of plume often forms bright circular deposits consisting of sulfur dioxide. These plumes are often less than {{convert|100|km|sigfig=2|abbr=on}} tall, and are among the most long-lived plumes on Io. Examples include [[Prometheus (volcano)|Prometheus]], [[Amirani (volcano)|Amirani]], and [[Masubi (volcano)|Masubi]]. The erupted sulfurous compounds are concentrated in the upper crust from a decrease in sulfur solubility at greater depths in Io's lithosphere and can be a determinant for the eruption style of a hot spot.<ref name="Battaglia2014"/><ref name="Battaglia2015">{{Cite conference |title=Io: The role of Sulfide Droplet Nucleation in Pele-Type Volcanism |conference=46th Lunar and Planetary Science Conference. 16–20 March 2015. The Woodlands, Texas. |first=Steven M. |last=Battaglia |date=March 2015 |id=LPI Contribution No. 1832 |bibcode=2015LPI....46.1044B}}</ref><ref name="Battaglia2018">{{Cite conference |title=Does Io have a Lopsided Asthenosphere? Insights from Katla's Magma Plumbing System, Iceland |conference=49th Lunar and Planetary Science Conference. 19–23 March 2018. The Woodlands, Texas. |first=Steven M. |last=Battaglia |date=March 2018 |id=LPI Contribution No. 1047 |bibcode=2018LPI....49.1047B}}</ref> 
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====Mountains====
{{Main|Mountains of Io}}
{{See also|List of mountains on Io}}
[[File:Tohil Mons.jpg|right|thumb|''[[Galileo (spacecraft)|Galileo]]'' greyscale image of [[Tohil Mons]], a 5.4-km-tall mountain]]
Io has 100 to 150 mountains. These structures average {{Convert|6|km|abbr=on}} in height and reach a maximum of {{convert|17.5|±|1.5|km|1|abbr=on}} at South ''Boösaule Montes''.<ref name="Schenk2001"/> Mountains often appear as large (the average mountain is {{convert|157|km|0|disp=or|abbr=on}} long), isolated structures with no apparent global tectonic patterns outlined, in contrast to the case on Earth.<ref name=Schenk2001/> To support the tremendous topography observed at these mountains requires compositions consisting mostly of silicate rock, as opposed to sulfur.<ref name="Clow1980">{{cite journal |title=Stability of sulfur slopes on Io |journal=Icarus |first1=G. D. |last1=Clow |first2=M. H. |last2=Carr |pages=268–279 |volume=44 |issue=2 |date=1980 |doi=10.1016/0019-1035(80)90022-6 |bibcode=1980Icar...44..268C}}</ref>

Despite the extensive volcanism that gives Io its distinctive appearance, nearly all of its mountains are tectonic structures, and are not produced by volcanoes. Instead, most Ionian mountains form as the result of compressive stresses on the base of the lithosphere, which uplift and often tilt chunks of Io's crust through [[thrust fault]]ing.<ref name="SchenkBulmer1998">{{cite journal |last1=Schenk |first1=P. M. |last2=Bulmer |first2=M. H. |title=Origin of mountains on Io by thrust faulting and large-scale mass movements |journal=Science |volume=279 |issue=5356 |pages=1514–1517 |date=1998 |url=http://pdfs.semanticscholar.org/136b/6ffa06a36d3131948a9ef322cec92465c07b.pdf |archive-url=https://web.archive.org/web/20190219004441/http://pdfs.semanticscholar.org/136b/6ffa06a36d3131948a9ef322cec92465c07b.pdf |url-status=dead |archive-date=2019-02-19 |doi=10.1126/science.279.5356.1514 |pmid=9488645 |bibcode=1998Sci...279.1514S|s2cid=8518290 }}</ref> The compressive stresses leading to mountain formation are the result of [[subsidence]] from the continuous burial of volcanic materials.<ref name="SchenkBulmer1998" /> The global distribution of mountains appears to be opposite that of volcanic structures; mountains dominate areas with fewer volcanoes and vice versa.<ref name="McKinnon2001">{{cite journal |last=McKinnon |first=W. B. |display-authors=etal |title=Chaos on Io: A model for formation of mountain blocks by crustal heating, melting, and tilting |journal=Geology |volume=29 |issue=2 |pages=103–106 |date=2001 |doi=10.1130/0091-7613(2001)029<0103:COIAMF>2.0.CO;2 |bibcode=2001Geo....29..103M|s2cid=140149197 |url=https://pdfs.semanticscholar.org/6651/f84d9443fc9125392c09ce64230e5e3a40fd.pdf |archive-url=https://web.archive.org/web/20200211021753/https://pdfs.semanticscholar.org/6651/f84d9443fc9125392c09ce64230e5e3a40fd.pdf |url-status=dead |archive-date=2020-02-11 }}</ref> This suggests large-scale regions in Io's lithosphere where compression (supportive of mountain formation) and extension (supportive of patera formation) dominate.<ref name="Tackley2001">{{cite journal |last=Tackley |first=P. J. |title=Convection in Io's asthenosphere: Redistribution of nonuniform tidal heating by mean flows |journal=J. Geophys. Res. |volume=106 |issue=E12 |pages=32971–32981 |date=2001 |doi=10.1029/2000JE001411 |bibcode=2001JGR...10632971T|doi-access= |s2cid=130527846 }}</ref> Locally, however, mountains and paterae often abut one another, suggesting that magma often exploits faults formed during mountain formation to reach the surface.<ref name="Radebaugh2001" />

Mountains on Io (generally, structures rising above the surrounding plains) have a variety of morphologies. [[Plateau]]s are most common.<ref name="Schenk2001"/> These structures resemble large, flat-topped [[mesa]]s with rugged surfaces. Other mountains appear to be tilted crustal blocks, with a shallow slope from the formerly flat surface and a steep slope consisting of formerly sub-surface materials uplifted by compressive stresses. Both types of mountains often have steep [[Escarpment|scarps]] along one or more margins. Only a handful of mountains on Io appear to have a volcanic origin. These mountains resemble small [[shield volcano]]es, with steep slopes (6–7°) near a small, central [[caldera]] and shallow slopes along their margins.<ref name="Schenk2004">{{cite journal |last1=Schenk |first1=P. M. |last2=Wilson |first2=R. R. |last3=Davies |first3=A. G. |title=Shield volcano topography and the rheology of lava flows on Io |journal=Icarus |volume=169 |issue=1 |pages=98–110 |date=2004 |doi=10.1016/j.icarus.2004.01.015 |bibcode=2004Icar..169...98S}}</ref> These volcanic mountains are often smaller than the average mountain on Io, averaging only {{convert|1|to|2|km|6=1|abbr=on}} in height and {{convert|40|to|60|km|sigfig=2|abbr=on}} wide. Other shield volcanoes with much shallower slopes are inferred from the morphology of several of Io's volcanoes, where thin flows radiate out from a central patera, such as at [[Ra Patera]].<ref name="Schenk2004" />

Nearly all mountains appear to be in some stage of degradation. Large [[landslide]] deposits are common at the base of Ionian mountains, suggesting that [[mass wasting]] is the primary form of degradation. Scalloped margins are common among Io's mesas and plateaus, the result of sulfur dioxide [[Groundwater sapping|sapping]] from Io's crust, producing zones of weakness along mountain margins.<ref name="Moore2001">{{cite journal |last=Moore |first=J. M. |display-authors=etal |title=Landform degradation and slope processes on Io: The Galileo view |journal=J. Geophys. Res. |volume=106 |issue=E12 |pages=33223–33240 |date=2001 |url=http://planetary.brown.edu/pdfs/2559.pdf |doi=10.1029/2000JE001375 |bibcode=2001JGR...10633223M |access-date=25 August 2019 |archive-date=18 July 2019 |archive-url=https://web.archive.org/web/20190718150342/http://www.planetary.brown.edu/pdfs/2559.pdf |url-status=dead }}</ref>

==Atmosphere==
{{Main article|Atmosphere of Io}}
[[File:Io Aurorae color.jpg|thumb|right|Auroral glows in Io's upper atmosphere. Different colors represent emission from different components of the atmosphere (green comes from emitting sodium, red from emitting oxygen, and blue from emitting volcanic gases like sulfur dioxide). Image taken while Io was in eclipse.]]

Io has an extremely thin [[atmosphere]] consisting mainly of [[sulfur dioxide]] ({{chem|SO|2}}), with minor constituents including [[sulfur monoxide]] ({{chem|SO}}), [[sodium chloride]] ({{chem|NaCl}}), and atomic [[sulfur]] and [[oxygen]].<ref name="IobookChap10">{{cite book |last=Lellouch |first=E. |display-authors=etal |editor=Lopes, R. M. C. |editor2=and Spencer, J. R. |title=Io after Galileo |date=2007 |publisher=Springer-Praxis |isbn=978-3-540-34681-4 |pages=231–264 |chapter=Io's atmosphere}}</ref> The atmosphere has significant variations in density and temperature with time of day, latitude, volcanic activity, and surface frost abundance. The maximum atmospheric pressure on Io ranges from 3.3{{e-sp|−5}}&nbsp;to&nbsp;3{{e-sp|−4}}&nbsp;[[Pascal (unit)|pascals]] (Pa) or 0.3&nbsp;to&nbsp;3&nbsp;[[Bar (unit)|nbar]], spatially seen on Io's anti-Jupiter hemisphere and along the equator, and temporally in the early afternoon when the temperature of surface frost peaks.<ref name="IobookChap10" /><ref name="Walker2010">{{cite journal |last=Walker |first=A. C. |display-authors=etal |title=A Comprehensive Numerical Simulation of Io's Sublimation-Driven Atmosphere |journal=Icarus |volume=press |issue=1 |series=in |pages=409–432 |date=2010 |doi=10.1016/j.icarus.2010.01.012 |bibcode=2010Icar..207..409W}}</ref><ref name="Spencer2005">{{cite journal |last=Spencer |first=A. C. |display-authors=etal |title=Mid-infrared detection of large longitudinal asymmetries in Io's {{chem|SO|2}} atmosphere |journal=Icarus |volume=176 |issue=2 |pages=283–304 |date=2005 |url=http://www.boulder.swri.edu/~spencer/spencer2005_accepted_fmt.pdf |doi=10.1016/j.icarus.2005.01.019 |bibcode=2005Icar..176..283S }}</ref> Localized peaks at volcanic plumes have also been seen, with pressures of 5{{e-sp|−4}} to 40 {{e-sp|−4}}&nbsp;Pa (5 to 40&nbsp;nbar).<ref name=Pearl1979/> Io's atmospheric pressure is lowest on Io's night side, where the pressure dips to 0.1{{e-sp|−7}}&nbsp;to&nbsp;1{{e-sp|−7}}&nbsp;Pa (0.0001 to 0.001&nbsp;nbar).<ref name="IobookChap10" /><ref name="Walker2010" /> Io's atmospheric temperature ranges from the temperature of the surface at low altitudes, where sulfur dioxide is in vapor pressure equilibrium with frost on the surface, to 1,800 K at higher altitudes where the lower atmospheric density permits heating from plasma in the Io plasma torus and from Joule heating from the Io flux tube.<ref name="IobookChap10" /><ref name="Walker2010" /> The low pressure limits the atmosphere's effect on the surface, except for temporarily redistributing sulfur dioxide from frost-rich to frost-poor areas, and to expand the size of plume deposit rings when plume material re-enters the thicker dayside atmosphere.<ref name="IobookChap10" /><ref name="Walker2010" />

Gas in Io's atmosphere is stripped by [[Magnetosphere of Jupiter|Jupiter's magnetosphere]], escaping to either the neutral cloud that surrounds Io, or the Io plasma torus, a ring of [[ion]]ized particles that shares Io's orbit but co-rotates with the magnetosphere of Jupiter.<ref name="Krimigis2002" /> Approximately one ton of material is removed from the atmosphere every second through this process so that it must be constantly replenished.<ref name="IobookChap11" /> The most dramatic source of {{chem|SO|2}} are volcanic plumes, which pump {{10^|4}} kg of sulfur dioxide per second into Io's atmosphere on average, though most of this condenses back onto the surface.<ref name=autogenerated2>{{cite book |last1=Geissler |first1=P. E. |last2=Goldstein |first2=D. B. |editor-last=Lopes |editor-first=R. M. C. |editor2-last=Spencer |editor2-first=J. R. |title=Io after Galileo |date=2007 |publisher=Springer-Praxis |isbn=978-3-540-34681-4 |pages=163–192 |chapter=Plumes and their deposits}}</ref> Much of the sulfur dioxide in Io's atmosphere is sustained by sunlight-driven [[Sublimation (phase transition)|sublimation]] of {{chem|SO|2}} frozen on the surface.<ref name="Moullet2010">{{cite journal |last=Moullet |first=A. |display-authors=etal |title=Simultaneous mapping of SO2, SO, NaCl in Io's atmosphere with the Submillimeter Array |journal=Icarus |volume=press |issue=1 |pages=353–365 |date=2010 |doi=10.1016/j.icarus.2010.02.009 |bibcode=2010Icar..208..353M}}</ref> The day-side atmosphere is largely confined to within 40° of the equator, where the surface is warmest and most active volcanic plumes reside.<ref name="Feaga2009">{{cite journal |last=Feaga |first=L. M. |display-authors=etal |title=Io's dayside {{chem|SO|2}} atmosphere |journal=Icarus |volume=201 |issue=2 |pages=570–584 |date=2009 |doi=10.1016/j.icarus.2009.01.029 |bibcode=2009Icar..201..570F }}</ref> A sublimation-driven atmosphere is also consistent with observations that Io's atmosphere is densest over the anti-Jupiter hemisphere, where {{chem|SO|2}} frost is most abundant, and is densest when Io is closer to the Sun.<ref name="IobookChap10" /><ref name="Moullet2010" /><ref name="AlohaIo">{{cite web |last=Spencer |first=John |url=http://www.planetary.org/blog/article/00001980/ |title=Aloha, Io |work=The Planetary Society Blog |publisher=The Planetary Society |date=8 June 2009}}</ref> However, some contributions from volcanic plumes are required as the highest observed densities have been seen near volcanic vents.<ref name="IobookChap10" /> Because the density of sulfur dioxide in the atmosphere is tied directly to surface temperature, Io's atmosphere partially collapses at night, or when Io is in the shadow of Jupiter (with an ~80% drop in column density<ref name="Tsang2016">{{cite journal |last1=Tsang |first1=C. C. C. |last2=Spencer |first2=J. R. |last3=Lellouch |first3=E. |last4=Lopez-Valverde |first4=M. A. |last5=Richter |first5=M. J. |title=The collapse of Io's primary atmosphere in Jupiter eclipse |journal=Journal of Geophysical Research: Planets |date=2 August 2016 |doi=10.1002/2016JE005025 |volume=121 |issue=8 |pages=1400–1410 |bibcode=2016JGRE..121.1400T|hdl=10261/143708 |s2cid=19544014 |url=https://hal.archives-ouvertes.fr/hal-02329995 |hdl-access=free }}</ref>). The collapse during eclipse is limited somewhat by the formation of a diffusion layer of sulfur monoxide in the lowest portion of the atmosphere, but the atmosphere pressure of Io's nightside atmosphere is two to four orders of magnitude less than at its peak just past noon.<ref name="Walker2010" /><ref name="Moore2009">{{cite journal |last=Moore |first=C. H. |display-authors=etal |title=1-D DSMC simulation of Io's atmospheric collapse and reformation during and after eclipse |journal=Icarus |volume=201 |issue=2 |pages=585–597 |date=2009 |doi=10.1016/j.icarus.2009.01.006 |bibcode=2009Icar..201..585M}}</ref> The minor constituents of Io's atmosphere, such as {{chem|NaCl}}, {{chem|SO}}, {{chem|O}}, and {{chem|S}} derive either from: direct volcanic outgassing; [[photodissociation]], or chemical breakdown caused by solar ultraviolet radiation, from {{chem|SO|2}}; or the [[sputtering]] of surface deposits by charged particles from Jupiter's magnetosphere.<ref name="Moullet2010" />

Various researchers have proposed that the atmosphere of Io [[deposition (phase transition)|freezes onto the surface]] when it passes into the shadow of Jupiter. Evidence for this is a "post-eclipse brightening", where the moon sometimes appears a bit brighter as if covered with frost immediately after eclipse. After about 15 minutes the brightness returns to normal, presumably because the frost has disappeared through [[sublimation (phase transition)|sublimation]].<ref>{{cite journal |title=Io: Could SO<sub>2</sub> condensation/sublimation cause the sometimes reported post-eclipse brightening? |journal=Geophysical Research Letters |first1=F. P. |last1=Fanale |first2=W. B. |last2=Banerdt |first3=D. P. |last3=Cruikshank |display-authors=1 |volume=8 |issue=6 |pages=625–628 |date=June 1981 |doi=10.1029/GL008i006p00625 |bibcode=1981GeoRL...8..625F}}</ref><ref>{{cite journal |title=The Brightness of Jupiter's Satellite Io Following Emergence from Eclipse: Selected Observations, 1981–1989 |journal=Icarus |first1=Robert M. |last1=Nelson |first2=Arthur L. |last2=Lane |first3=Michael E. |last3=Morrill |first4=Brad D. |last4=Wallis |first5=James |last5=Gibson |first6=William D. |last6=Smythe |first7=Linda J. |last7=Horn |first8=Bonnie J. |last8=Buratti |display-authors=1 |volume=101 |issue=2 |pages=223–233 |date=February 1993 |doi=10.1006/icar.1993.1020 |bibcode=1993Icar..101..223N}}</ref><ref>{{cite journal |title=Voyager search for posteclipse brightening on Io |journal=Icarus |first1=J. |last1=Veverka |first2=D. |last2=Simonelli |first3=P. |last3=Thomas |first4=D. |last4=Morrison |first5=T. V. |last5=Johnson |display-authors=1 |volume=47 |issue=1 |pages=60–74 |date=July 1981 |doi=10.1016/0019-1035(81)90091-9 |bibcode=1981Icar...47...60V}}</ref><ref>{{cite journal |title=A Hubble Space Telescope study of posteclipse brightening and albedo changes on Io |journal=Icarus |first1=James J. |last1=Secosky |first2=Michael |last2=Potter |volume=111 |issue=1 |pages=73–78 |date=September 1994 |doi=10.1006/icar.1994.1134 |bibcode=1994Icar..111...73S}}</ref> Besides being seen through ground-based telescopes, post-eclipse brightening was found in near-infrared wavelengths using an instrument aboard the ''Cassini'' spacecraft.<ref>{{cite journal |title=Cassini/VIMS observation of an Io post-eclipse brightening event |journal=Icarus |first1=Giancarlo |last1=Bellucci |first2=E. |last2=D'Aversa |first3=V. |last3=Formisano |first4=D. |last4=Cruikshank |first5=R. M. |last5=Nelson |first6=R. N. |last6=Clark |first7=K. H. |last7=Baines |first8=D. |last8=Matson |first9=R. H. |last9=Brown |first10=T. B. |last10=McCord |first11=B. J. |last11=Buratti |first12=P. D. |last12=Nicholson |display-authors=1 |volume=172 |issue=1 |pages=141–148 |date=November 2004 |doi=10.1016/j.icarus.2004.05.012 |bibcode=2004Icar..172..141B}}</ref> Further support for this idea came in 2013 when the [[Gemini Observatory]] was used to directly measure the collapse of Io's {{chem2|SO2}} atmosphere during, and its reformation after, eclipse with Jupiter.<ref>{{cite web |url=https://www.swri.org/press-release/swri-space-scientists-observe-io%E2%80%99s-atmospheric-collapse-during-eclipse |title=SwRI Space Scientists Observe Io's Atmospheric Collapse During Eclipse |publisher=Southwest Research Institute |first=Robert |last=Crowe |date=2 August 2016 |access-date=4 October 2018}}</ref><ref>{{cite journal |title=The collapse of Io's primary atmosphere in Jupiter eclipse |journal=Journal of Geophysical Research: Planets |first1=Constantine C. C. |last1=Tsang |first2=John R. |last2=Spencer |first3=Emmanuel |last3=Lellouch |first4=Miguel A. |last4=Lopez-Valverde |first5=Matthew J. |last5=Richter |display-authors=1 |volume=121 |issue=8 |pages=1400–1410 |date=August 2016 |doi=10.1002/2016JE005025 |bibcode=2016JGRE..121.1400T|url=https://digital.csic.es/bitstream/10261/143708/1/IAA_2016_LopezValverde_AGU-PLANETS.pdf |hdl=10261/143708 |s2cid=19544014 |hdl-access=free }}</ref>

High-resolution images of Io acquired when Io is experiencing an eclipse reveal an [[Aurora (astronomy)|aurora]]-like glow.<ref name="Geissler1999"/> As on Earth, this is due to [[particle radiation]] hitting the atmosphere, though in this case the charged particles come from Jupiter's magnetic field rather than the [[solar wind]]. Aurorae usually occur near the magnetic poles of planets, but Io's are brightest near its equator. Io lacks an intrinsic magnetic field of its own; therefore, electrons traveling along Jupiter's magnetic field near Io directly impact Io's atmosphere. More electrons collide with its atmosphere, producing the brightest aurora, where the field lines are tangent to Io (i.e. near the equator), because the column of gas they pass through is the longest there. Aurorae associated with these tangent points on Io are observed to rock with the changing orientation of Jupiter's tilted [[magnetic dipole]].<ref name="Retherford2000">{{cite journal |last=Retherford |first=K. D. |display-authors=etal |title=Io's Equatorial Spots: Morphology of Neutral UV Emissions |journal=J. Geophys. Res. |volume=105 |issue=A12 |pages=27,157–27,165 |date=2000 |doi=10.1029/2000JA002500 |bibcode=2000JGR...10527157R|doi-access=free }}</ref> Fainter aurora from oxygen atoms along the limb of Io (the red glows in the image at right), and sodium atoms on Io's night-side (the green glows in the same image) have also been observed.<ref name="Geissler1999" />

==See also==
{{portal|Solar System|Outer space|Astronomy}}
{{div col|colwidth=20em}}
* [[Atmosphere of Io]]
* [[Exploration of Io]]
* [[Jupiter]]
* [[Moons of Jupiter]]
* [[Galilean moons]] (the four biggest moons of Jupiter)
* [[Jupiter's moons in fiction#Io|Jupiter's moons in fiction]]
* [[List of natural satellites]]
* [[Planetary geology]]
{{div col end}}

==References==
{{reflist|30em}}

==External links==
{{Commons category|Io}}
{{colbegin|colwidth=60em}}

===General information===
* [https://web.archive.org/web/20180726045625/https://solarsystem.nasa.gov/moons/jupiter-moons/io/overview/ Io profile] at [https://solarsystem.nasa.gov/ NASA's Solar System Exploration site]
* [https://nineplanets.org/io.html Bill Arnett's Io webpage] from [https://nineplanets.org/ The Nine Planets website]
* [https://www.windows2universe.org/jupiter/moons/io.html Io overview] from the University of Michigan's [https://www.windows2universe.org/ Windows to the Universe]
* [http://solarviews.com/eng/io.htm Calvin Hamilton's Io page] from the [http://solarviews.com/eng/ Views of the Solar System website]

===Movies===
* [http://stereomoons.blogspot.com/2009/10/galileo-4-moons-at-400-years.html Paul Schenk's 3D images and flyover videos of Io and other outer solar system satellites]
* [https://www.flickr.com/photos/136797589@N04/34330010470/in/photostream/ High resolution video] simulation of rotating Io by Seán Doran

===Images===
* [https://photojournal.jpl.nasa.gov/target/Io Catalog of NASA images of Io]
* [https://pirlwww.lpl.arizona.edu/wiki/missions/Galileo/releases/ ''Galileo'' images of Io]
* [http://pluto.jhuapl.edu/Galleries/Featured-Images/view.php?gallery_id=2&page=1&bytopic=31 ''New Horizons'' images of Io]
** [http://pluto.jhuapl.edu/soc/ ''New Horizons'' LORRI Raw Images, includes numerous Io images]
* [https://www.flickr.com/photos/lunarandplanetaryinstitute/5075141107/in/photostream Io through Different ''New Horizons'' Imagers]

===Maps===
* [https://astrogeology.usgs.gov/maps/io-voyager-galileo-global-mosaics Io global basemaps] at the USGS Astrogeology Science Center based on ''Galileo'' and ''Voyager'' images
* [https://planetarynames.wr.usgs.gov/Page/IO/target Io nomenclature] and [https://planetarynames.wr.usgs.gov/images/io_comp_color.pdf map with feature names] from the [https://planetarynames.wr.usgs.gov/ USGS planetary nomenclature page]
* [https://www.google.com/maps/space/io/ Interactive map of Io] by Google Maps

===Additional references===

* [http://www.phy6.org/earthmag/wio.htm Io dynamo] from educational website [http://www.phy6.org/Education/Intro.html The Exploration of the Earth's Magnetosphere]

* [https://www.youtube.com/watch?v=Xo6w5QVrABc NASA's Stunning Discoveries on Jupiter's Largest Moons | Our Solar System's Moons]

* [https://www.cosis.net/abstracts/EAE03/07912/EAE03-J-07912.pdf The Conundrum Posed by Io's Minimum Surface Temperatures]
* [http://planetologia.elte.hu/io/name.html Io Mountain Database]
* [https://astrogeology.usgs.gov/geology/io-cassini-visible-aurorae ''Cassini'' Observations of Io's Visible Aurorae] at the USGS Astrogeology Science Center
* [http://www.gishbartimes.org/ The Gish Bar Times], Jason Perry's Io-related blog
{{colend}}

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