Io (moon) 1051892 move=:edit= 225023585 2008-07-11T14:44:46Z Thijs!bot 1392310 robot Adding: [[nds:Io (Maand)]] <!-- Scroll down to edit the contents of this page. Additional parameters for this template are available at [[Template:Infobox Planet]]. --> {{Infobox Planet | name = Io | image = [[Image:Io highest resolution true color.jpg|250px|True-color image taken by the ''Galileo'' probe.]] | caption = Click image for description | bgcolour = #a0ffa0 | discovery = yes | discoverer = [[Galileo Galilei]] | discovered = [[January 7]], [[1610]] | mean_orbit_radius = 421,700&nbsp;[[kilometre|km]] (0.002819 [[astronomical unit|AU]]) | eccentricity = 0.0041 | periapsis = 420,000&nbsp;km (0.002807 AU) | apoapsis = 423,400&nbsp;km (0.002830 AU) | period = 1.769137786&nbsp;d (152,853.5047&nbsp;s, 42&nbsp;h) | avg_speed = 17.334&nbsp;km/s | inclination = 2.21° (to the [[ecliptic]])<br/>0.05° (to Jupiter's equator) | satellite_of = [[Jupiter]] | physical_characteristics = yes | mean_radius = 1821.3 km (0.286 Earths)<ref name=Thomas1998> {{cite journal |last=Thomas |first=P. C. |coauthors=''et al.'' |date=1998 |title=The Shape of Io from Galileo Limb Measurements |journal=Icarus |volume=135 |issue=1 |pages=175–180 |url=http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998Icar..135..175T&db_key=AST&link_type=ABSTRACT&high=45f9f8ee3002809 |doi=10.1006/icar.1998.5987 }} </ref> | dimensions = 3660.0 × 3637.4 × 3630.6&nbsp;km<ref name=Thomas1998/> | surface_area = 41,910,000&nbsp;[[square kilometre|km<sup>2</sup>]] (0.082 Earths) | volume = 2.53{{e|10}}&nbsp;[[cubic metre|km<sup>3</sup>]] (0.023 Earths) | mass = 8.9319{{e|22}}&nbsp;[[kilogram|kg]] (0.015 Earths) | density = 3.528&nbsp;[[gram|g]]/[[cubic centimetre|cm<sup>3</sup>]] | surface_grav = 1.796&nbsp;[[acceleration|m/s<sup>2</sup>]] (0.183 ''[[g-force|''g'']]'') | escape_velocity = 2.558&nbsp;km/s | albedo = 0.63 ± 0.02<ref name=jplfact>{{cite web|last=Yeomans|first=Donald K.|date=[[July 13]], [[2006]]|title=Planetary Satellite Physical Parameters|publisher=JPL Solar System Dynamics|url=http://ssd.jpl.nasa.gov/?sat_phys_par|accessdate=2007-11-05}}</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|accessdate=2007-09-28}}</ref> | rotation = [[synchronous rotation|synchronous]] | rot_velocity = 271&nbsp;km/h | temperatures = yes | temp_name1 = Surface | min_temp 1 = 90&nbsp;[[kelvin|K]] | mean_temp_1 = 130&nbsp;K | max_temp_1 = 200&nbsp;K | adjectives = Ionian | atmosphere = yes | surface_pressure = trace | atmosphere_composition = 90%&nbsp;[[sulfur dioxide]] }} '''Io''' ({{pronEng|ˈaɪoʊ}} ''<small>EYE</small>-oe,'' or as [[Greek language|Greek]] ''Ῑώ)'' is the innermost of the four [[Galilean moons|Galilean]] [[natural satellite|moons]] of [[Jupiter]] and, with a [[diameter]] of 3,642&nbsp;[[Kilometre|kilometers]], the [[List of moons by diameter|fourth-largest moon]] in the [[Solar System]]. It was named after [[Io (mythology)|Io]], a priestess of [[Hera]] that became one of the lovers of [[Zeus]]. With over 400 active volcanoes, Io is the most geologically active object in the Solar System.<ref name=book /><ref name=Lopes2004>{{cite journal |last=Lopes |first=R. M. C. |coauthors=''et al.'' |date=2004 |title=Lava Lakes on Io: Observations of Io's Volcanic Activity from Galileo NIMS During the 2001 Fly-bys |journal=Icarus |volume=169 |issue=1 |pages=140–174 |url=http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004Icar..169..140L&db_key=AST&link_type=ABSTRACT&high=45f9f8ee3003884 |doi=10.1016/j.icarus.2003.11.013 }}</ref> This extreme geologic activity is the result of tidal heating from friction generated within Io's interior by Jupiter's varying pull. Several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500&nbsp;km (310&nbsp;mi). Io's surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of the moon's silicate crust. Some of these peaks are taller than Earth's [[Mount Everest]].<ref name=Schenk2001>{{cite journal |last=Schenk |first=P. |coauthors=''et al.'' |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 |url=http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001JGR...10633201S&db_key=AST&link_type=ABSTRACT&high=45f9f8ee3004779 |doi=10.1029/2000JE001408 }}</ref> Unlike most satellites in the outer Solar System (which have a thick coating of ice), Io is primarily composed of silicate rock surrounding a molten iron or iron sulfide core. Most of Io's surface is characterized by extensive plains coated with sulfur and sulfur dioxide frost. Io's volcanism is responsible for many of that satellite's unique features. Its volcanic plumes and lava flows produce large surface changes and paint the surface in various shades of red, yellow, white, black, and green, largely due to the sulfurous compounds. Numerous extensive lava flows, several longer than {{convert|500|km|mi|0}} in length, also mark the surface. These volcanic processes have given rise to a comparison of the visual appearance of Io's surface to a pizza. The materials produced by this volcanism provide material for Io's thin, patchy atmosphere and Jupiter's extensive magnetosphere. Io played a significant role in the development of astronomy in the 17th and 18th centuries. It was discovered in 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 the first measurement of the speed of light. From Earth, Io remained nothing more than 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 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 the satellite and Jupiter's magnetosphere and the existence of a belt of radiation centered on Io's orbit. The exploration of Io continued in the early months of 2007 with a distant flyby by Pluto-bound ''[[New Horizons]]''. ==Name== {{See Also|List of geological features on Io|List of paterae on Io|List of mountains on Io}} While [[Simon Marius]] is not credited with the sole discovery of the Galilean satellites, his names for the moons have stuck. In his 1614 publication ''[[Simon Marius|Mundus Jovialis]]'', he named the innermost large moon of Jupiter after the [[Greek mythology|Greek mythological]] figure [[Io (mythology)|Io]], one of the many lovers of [[Zeus]] (who is also known as [[Jupiter (mythology)|Jupiter]] in [[Roman mythology]]).<ref> {{cite journal |last=Marius |first=S. |authorlink=Simon Marius |date=1614 |title=Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici |url=http://galileo.rice.edu/sci/marius.html }} (in which he [http://galileo.rice.edu/sci/observations/jupiter_satellites.html attributes the suggestion] to Johannes Kepler) </ref> Marius' names fell out of favor, and were not revived in common use until the mid-20th century. In much of the earlier astronomical literature, Io is simply referred to by its [[Roman numerals|Roman numeral]] designation (a system introduced by Galileo) as "'''{{nowrap|Jupiter I}}'''", or simply as "the first satellite of Jupiter". The most common adjectival form of the name is ''Ionian''. 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 | last=Blue | first=Jennifer | date=October 16, 2006 |url=http://planetarynames.wr.usgs.gov/append6.html | title=Categories for Naming Features on Planets and Satellites | publisher=USGS | accessdate= 2007-06-14 }}</ref> Since the surface was first seen up close by ''[[Voyager 1]]'' the [[International Astronomical Union]] has approved 225 names for Io's volcanoes, mountains, plateaus, and large albedo features. The approved feature names used for Io include ''patera'' (volcanic depression), ''mons'', ''mensa'', ''planum'', and ''tholus'' (various types of mountain, with morphologic characteristics such as size, shape, and height determining the term used), ''fluctus'' (lava flow), ''vallis'' (lava channel), ''regio'' (large-scale albedo feature), and ''active eruptive center'' (location where plume activity was the first sign of volcanic activity at a particular volcano).<ref name=NameCategories/> Examples of named features include Prometheus, Pan Mensa, [[Tvashtar Paterae]], and Tsũi Goab Fluctus.<ref name=Featurenames>{{cite web | last=Blue | first=Jennifer | date=[[June 14]], [[2007]] |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=USGS | accessdate= 2007-06-14 }}</ref> ==Observational history== [[Image:Galileo.arp.300pix.jpg|thumb|150 px|[[Galileo Galilei]], the discoverer of Io]] The first reported observation of Io was made by [[Galileo Galilei]] on [[7 January]] [[1610]]. 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 |last=Cruikshank |first=D. P. |coauthors= and Nelson, R. M. |editor=Lopes, R. M. C.; and Spencer, J. R. |title=Io after Galileo |year=2007 |publisher=Springer-Praxis |isbn=3-540-34681-3 |pages=pp. 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. Given that Galileo published his work before Marius, Galileo is credited with the discovery. 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, such as helping mariners determine their [[longitude]],<ref>{{cite web | author=O'Connor, J. J.; Robertson, 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 | accessdate= 2007-06-14 }}</ref> validating Kepler's [[Kepler's laws of planetary motion#Third_law|Third Law of planetary motion]], and determining the time for 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 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. Improved telescope technology in the late 19th and 20th centuries allowed astronomers to [[Optical resolution|resolve]] (that is, see) 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. |authorlink=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 |url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1894MNRAS..54..134B&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=45f9f8ee3007558 }} </ref><ref name=Dobbins>{{cite journal |last=Dobbins |first=T. |authorlink= |coauthors=and Sheehan, W. |date=2004 |title=The Story of Jupiter's Egg Moons |journal=Sky & Telescope |volume=107 |issue=1 |pages=114–120 }} </ref><ref name=Barnard1891>{{cite journal |last=Barnard |first=E. E. |authorlink=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 |url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1891MNRAS..51..543B&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=45f9f8ee3010624 }} </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 Polar Caps of Io |journal=Communications of the Lunar and Planetary Laboratory |volume=10 |pages=35–39 |url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1973CoLPL..10...35M&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=45f9f8ee3013123 }} </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 |url=http://ntrs.nasa.gov/search.jsp?R=91295&id=3&qs=No%3D50%26Ne%3D20%26Ntt%3Dlee%26Ntk%3DAuthorList%26Ntx%3Dmode%2520matchall%26N%3D222%26Ns%3DHarvestDate%7C1 }} </ref> The same observations suggested a surface dominated by evaporates composed of [[sodium]] salts and [[sulfur]].<ref name=Fanale1974> {{cite journal |last=Fanale |first=F. P. |coauthors=''et al.'' |date=1974 |title=Io: A Surface Evaporite Deposit? |journal=Science |volume=186 |issue=4167 |pages=pp. 922–925 |url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1974Sci...186..922F&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=45f9f8ee3017014 |doi=10.1126/science.186.4167.922 |pmid=17730914 }} </ref> Radio telescopic 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 |pages=1008–1010 |url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1964Natur.203.1008B&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=45f9f8ee3018594 |doi=10.1038/2031008a0 }} </ref> ===''Pioneer''=== The first spacecraft to pass by Io were the twin ''[[Pioneer 10]]'' and ''[[Pioneer 11|11]]'' probes on [[December 3]], [[1973]] and [[December 2]], [[1974]] respectively.<ref name=PioneerChap5>{{cite web |url=http://history.nasa.gov/SP-349/ch5.htm |title=First into the Outer Solar System |accessdate=2007-06-05 |last=Fimmel |first=R. O. |coauthors=''et al.'' |date=1977 |year= |month= |format= |work=Pioneer Odyssey |publisher=NASA |pages= |language= |archiveurl= |archivedate= |quote= }}</ref> Radio tracking provided an improved estimate of Io's mass, which, along with the best available information of Io's size, suggested that Io had the highest density of the four Galilean satellites, and was composed primarily of silicate rock rather than water ice.<ref name=Anderson1974>{{cite journal | last=Anderson |first=J. D. |coauthors=''et al.'' |title=Gravitational parameters of the Jupiter system from the Doppler tracking of Pioneer&nbsp;10 |journal=Science |volume=183 |issue= |pages=322–323 |date=1974 |url= |doi = }}</ref> The ''Pioneers'' also revealed the presence of a thin atmosphere at Io and intense radiation belts near the orbit of Io. The camera on board ''Pioneer&nbsp;11'' took the only good image of Io obtained by either spacecraft, showing its north polar region.<ref name=Pioneer11image>{{cite web |url=http://www2.jpl.nasa.gov/galileo/io/pioio.html |title=''Pioneer&nbsp;11'' Images of Io |work=Galileo Home Page |accessdate=2007-04-21 }}</ref> Close-up images were planned during ''Pioneer 10''<nowiki>'</nowiki>s encounter with Io, but those observations were lost due to the high-radiation environment.<ref name=PioneerChap5/> ===''Voyager''=== [[Image:Io VGR South polar color mosaic.jpg|thumb|200px|left|Mosaic of ''[[Voyager 1]]'' images covering Io's south polar region]] When the twin probes ''[[Voyager 1|Voyager&nbsp;1]]'' and ''[[Voyager 2|Voyager&nbsp;2]]'' passed by Io in 1979, their more advanced imaging system allowed for far more detailed images. ''Voyager&nbsp;1'' flew past the satellite on [[March 5]], [[1979]] from a distance of 20,600&nbsp;km (12,800&nbsp;mi).<ref name=VoyagerDesc>{{cite web |url=http://pds-rings.seti.org/voyager/mission/ |title=Voyager Mission Description |accessdate=2007-04-21 |last= |first= |coauthors= |date= 1997-02-19 |year= |month= |format= |work=NASA PDS Rings Node |publisher= |pages= |language= |archiveurl= |archivedate= |quote= }}</ref> The images returned during the approach revealed a strange, multi-colored landscape devoid of impact craters.<ref name=Smith1979>{{cite journal | last=Smith |first=B. A. |coauthors=''et al.'' |title=The Jupiter system through the eyes of Voyager&nbsp;1 |journal=Science |volume=204 |issue= |pages=951–972 |date=1979 |url= |doi = }}</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. 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. |coauthors=''et al.'' |title=Discovery of currently active extraterrestrial volcanism |journal=Science |volume=204 |issue= |pages=972 |date=1979 |url= |doi = }}</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 | last=Strom |first=R. G. |coauthors=''et al.'' |title=Volcanic eruption plumes on Io |journal=Nature |volume=280 |issue= |pages=733–736 |date=1979 |url= |doi = }}</ref> This conclusion was predicted in a paper published shortly before the ''Voyager 1'' encounter by Stan J. 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 | last=Peale |first=S. J. |coauthors=''et al.'' |title=Melting of Io by Tidal Dissipation |journal=Science |volume=203 |issue= |pages=892–894 |date=1979 |url= |doi = }}</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. |coauthors=''et al.'' |title=Spectrophotometry of Io: Preliminary Voyager 1 results |journal=Geophys. Res. Lett. |volume=7 |issue= |pages=963–966 |date=1980 |url= |doi = }}</ref><ref name=Pearl1979>{{cite journal | last=Pearl |first=J. C. |coauthors=''et al.'' |title=Identification of gaseous {{chem|SO|2}} and new upper limits for other gases on Io |journal=Nature |volume=288 |issue= |pages=757–758 |date=1979 |url= |doi = }}</ref><ref name=Broadfoot1979>{{cite journal | last=Broadfoot |first=A. L. |coauthors=''et al.'' |title=Extreme ultraviolet observations from ''Voyager'' encounter with Jupiter |journal=Science |volume=204 |issue= |pages=979–982 |date=1979 |url= |doi = }}</ref> ''Voyager&nbsp;2'' passed Io on [[July 9]], [[1979]] at a distance of 1,130,000&nbsp;km (702,150&nbsp;mi). 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 five months between the encounters. In addition, observations of Io as a crescent as ''Voyager 2'' departed the Jovian system revealed that eight 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 |last=Strom |first=R. G. | coauthors=Schneider, N. M. |editor=Morrison, D. |title=Satellites of Jupiter |year=2007 |publisher=University of Arizona Press |isbn= |pages=pp. 598–633 |chapter=Volcanic eruptions on Io }}</ref> ===''Galileo''=== {| class="toccolours" style="float:right; font-size:75%; margin:0 0 1em 1em; " cellpadding="0" cellspacing="0" |- ! bgcolor="#a0ffa0" colspan="2" | ''Galileo'' encounters with Io |- !Date ! Distance (km) |- |[[December 7]], [[1995]] |align="right"| 897 |- |[[November 4]], [[1996]] |align="right"| 244,000 |- |[[March 29]], [[1998]] |align="right"| 252,000 |- |[[June 30]], [[1999]] |align="right"| 127,000 |- |[[October 11]], [[1999]] |align="right"| 611 |- |[[November 26]], [[1999]] |align="right"| 301 |- |[[February 22]], [[2000]] |align="right"| 198 |- |[[August 6]], [[2001]] |align="right"| 194 |- |[[October 16]], [[2001]] |align="right"| 184 |- |[[January 17]], [[2002]] |align="right"| 102 |- |[[November 7]], [[2002]] |align="right"| 45,800 |} 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 ground-based observations taken 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. While no images were taken during the close flyby on [[December 7]], [[1995]], the encounter did yield significant results, such as the discovery of a large iron core, similar to that found in the rocky planets of the inner solar system.<ref name=Anderson1996>{{cite journal | last=Anderson |first=J. D. |coauthors=''et al.'' |title=Galileo Gravity Results and the Internal Structure of Io |journal=Science |volume=272 |issue= |pages=709–712 |date=1996 |url= |doi = }}</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''<nowiki>'</nowiki>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 with sulfur and sulfur dioxide serving a similar role to water and [[carbon dioxide]] on Earth.<ref name=Mcewen1998>{{cite journal | last=McEwen |first=A. S. |coauthors=''et al.'' |title=High-temperature silicate volcanism on Jupiter's moon Io |journal=Science |volume=281 |issue= |pages=87–90 |date=1998 |url= |doi = }}</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.; ''et al.'' |editor=Lopes, R. M. C.; and Spencer, J. R. |title=Io after Galileo |year=2007 |publisher=Springer-Praxis |isbn=3-540-34681-3 |pages=pp. 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/> In December 2000, the [[Cassini–Huygens|''Cassini'']] spacecraft had a distant and brief encounter with the Jupiter 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 aurorae.<ref name=Porco2003>{{cite journal | last=Porco |first=C. C. |authorlink=Carolyn Porco |coauthors=''et al.'' |title=Cassini imaging of Jupiter's atmosphere, satellites, and rings |journal=Science |volume=299 |issue= |pages=1541–1547 |date=2003 |url= |doi = }}</ref> ===Subsequent observations=== [[Image:Iosurface.jpg|thumb|300 px|Changes in surface features in the eight years between ''Galileo'' and ''New Horizons'' observations]] Following ''Galileo''<nowiki>'</nowiki>s fiery demise in Jupiter's atmosphere in September 2003, new observations of Io's volcanism came from Earth-based telescopes. In particular, [[adaptive optics]] imaging from the [[W. M. Keck Observatory|Keck telescope]] in [[Hawaii]] and imaging from the Hubble telescope have allowed astronomers to monitor Io's active volcanoes.<ref name=Marchis2005>{{cite journal | last=Marchis |first=F. |coauthors=''et al.'' |title=Keck AO survey of Io global volcanic activity between 2 and 5 µm |journal=Icarus |volume=176 |issue= |pages=96–122 |date=2005 |url= |doi = }}</ref><ref name=SpencerBlog02232007>{{cite web |url=http://planetary.org/blog/article/00000874/ |title=Here We Go! |accessdate=2007-06-03 |last=Spencer |first=John |coauthors= |date=[[2007-02-23]] |year= |month= |format= |work= |publisher= |pages= |language= |archiveurl= |archivedate= |quote= }}</ref> This imaging has allowed scientists to monitor volcanic activity on Io, even without a spacecraft in the Jupiter system. The ''[[New Horizons]]'' spacecraft, en route to [[Pluto]] and the [[Kuiper belt]], flew by the Jupiter system and Io on [[February 28]], [[2007]]. During the encounter, numerous distant observations of Io were obtained. Early results include 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=NHTvashtarimage>{{cite web |url=http://pluto.jhuapl.edu/gallery/missionPhotos/pages/031307.html |title=A Midnight Plume |accessdate=2007-04-21 |last= |first= |coauthors= |date=[[2007-03-13]] |year= |month= |format= |work= |publisher= |pages= |language= |archiveurl= |archivedate= |quote= }}</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''. The only forthcoming mission planned for the Jupiter system, ''[[Juno (spacecraft)|Juno]]'', does not have an imaging system powerful enough to perform Io surface science. The Europa/Jupiter System Mission, a joint NASA/ESA project currently in the concept study phase, would be able to study Io from a distance as well as during as many as four close flybys. If approved by the two space agencies, the two spacecraft would arrive in the 2021-2024 timeframe.<ref name=GreeleyOPAG>{{cite web |url=http://www.lpi.usra.edu/opag/march_08_meeting/presentations/greeley.pdf |title=Preliminary Report of the Joint Jupiter SDT |accessdate=2008-04-10 |last=Greeley |first=R. |coauthors= |date=[[2008-03-31]] |year= |month= |format= |work= |publisher=Outer Planets Assessment Group |pages= |language= |archiveurl= |archivedate= |quote= }}</ref> Another possible mission, called the ''Io Volcanic Observer'', would launch in 2013 as a Discovery-class mission and involve multiple flybys of Io while in orbit around Jupiter, however at [[As of 2008|present]], this project is also in the concept study phase.<ref name=DudinskiOPAG>{{cite web |url=http://www.lpi.usra.edu/opag/march_08_meeting/presentations/dudzinski.pdf |title=Radioisotope Power for NASA's Space Science Missions |accessdate=2008-04-10 |last=Dudzinski |first=L. A. |coauthors= |date=[[2008-03-31]] |year= |month= |format= |work= |publisher=Outer Planets Assessment Group |pages= |language= |archiveurl= |archivedate= |quote= }}</ref> ==Orbit== [[Image:Galilean moon Laplace resonance animation.gif|thumb|150px|right|Animation showing Io's Laplace resonance with Europa and Ganymede]] Io orbits Jupiter at a distance of 421,700&nbsp;km (262,000&nbsp;mi) from the planet's center and 350,000&nbsp;km (217,000&nbsp;mi) 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 42.5 hours to revolve once (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 (see the "[[#Tidal heating|Tidal heating]]" section for a more detailed explanation of the process).<ref name=Peale1979a/> Without this forced eccentricity, Io's orbit would circularize through [[Tidal acceleration|tidal dissipation]], leading to a geologically less active world. Like the other Galilean satellites of Jupiter and the Earth's [[Moon]], Io rotates [[Tidal locking|synchronously]] with its orbital period, keeping one face nearly pointed toward Jupiter. ==Interaction with Jupiter's magnetosphere== [[Image:Jupiter magnetosphere schematic.jpg|thumb|250px|left|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 |accessdate=2007-05-25 |last=Spencer |first=J. |coauthors= |date= |year= |month= |format= |work= |publisher= |pages= |language= |archiveurl= |archivedate= |quote= }}</ref>]] Io plays a significant role in shaping the Jovian magnetic field. The magnetosphere of Jupiter sweeps up gases and dust from Io's thin atmosphere at a rate of 1&nbsp;[[tonne]] per second.<ref name=IobookChap11>{{cite book |last=Schneider |first=N. M. |coauthors=Bagenal, F. |editor=Lopes, R. M. C.; and Spencer, J. R. |title=Io after Galileo |year=2007 |publisher=Springer-Praxis |isbn=3-540-34681-3 |pages=pp. 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. |coauthors=''et al.'' |title=Composition of jovian dust stream particles |journal=Icarus |volume=183 |issue= |pages=122–134 |date=2006 |url= |doi = }}</ref> These materials ultimately have their origin from Io's volcanic activity, but the material that escapes to Jupiter's magnetic field and into interplanetary space comes directly from Io's atmosphere. These materials, depending on their ionized state and composition, ultimately 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. Surrounding Io (up to a distance of 6 Io radii from the moon's surface) is a cloud of neutral sulfur, oxygen, sodium, and potassium atoms. These particles originate in Io's upper atmosphere but are excited from collisions with ions in the [[plasma (physics)|plasma]] torus (discussed below) and other processes into filling Io's [[Hill sphere]], which is the region where the moon's gravity is predominant over Jupiter. 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 6 Jovian radii from Io, either inside Io's orbit and ahead of the satellite or outside Io's orbit and behind the satellite.<ref name=IobookChap11/> The collisional 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. However, these particles still retain their velocity (70&nbsp;km/s, compared to the 17&nbsp;km/s orbital velocity at Io), leading these particles to be ejected in jets leading away from Io.<ref name=Burger1999>{{cite journal | last=Burger |first=M. H. |coauthors=''et al.'' |title=Galileo's close-up view of Io sodium jet |journal=Geophys. Res. Let. |volume=26 |issue=22 |pages=3333–3336 |date=1999 |url= |doi = }}</ref> Io orbits within a belt of intense radiation known as the Io plasma torus. The plasma in this donut-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&nbsp;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), meaning 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 cloud. 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. |coauthors=''et al.'' |title=A nebula of gases from Io surrounding Jupiter |journal=[[Nature (journal)|Nature]] |volume=415 |issue= |pages=994–996 |date=2002 |url=http://www.nature.com/nature/journal/v415/n6875/full/415994a.html |doi = }}</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 measure the [[ultraviolet]]-wavelength light it emits. While 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. |coauthors=''et al.'' |title=Io's volcanic control of Jupiter's extended neutral clouds |journal=Icarus |volume=170 |issue= |pages=430–442 |date=2004 |url= |doi = }}</ref> During an encounter with Jupiter in 1992, the [[Ulysses (spacecraft)|''Ulysses'']] spacecraft detected a stream of dust-sized particles being ejected from the Jupiter system.<ref name=Grun1993>{{cite journal | last=Grün |first=E. |coauthors=''et al.'' |title=Discovery of Jovian dust streams and interstellar grains by the ULYSSES spacecraft |journal=Nature |volume=362 |issue= |pages=428–430 |date=1993 |url= |doi = }}</ref> The dust in these discrete streams travel away from Jupiter at speeds upwards of several hundred kilometers per second, have an average size of 10&nbsp;[[micrometre|μm]], and consist primarily of sodium chloride.<ref name=Zook1996>{{cite journal | last=Zook |first=H. A. |coauthors=''et al.'' |title=Solar Wind Magnetic Field Bending of Jovian Dust Trajectories |journal=Science |volume=274 |issue=5292 |pages=1501–1503 |date=1996 |url= |doi = }}</ref><ref name=Postberg2006/> Dust measurements by ''Galileo'' showed that these dust streams originate from Io, but the exact mechanism for 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. |coauthors=''et al.'' |title=Dust measurements during ''Galileo's'' approach to Jupiter and Io encounter |journal=Science |volume=274 |issue= |pages=399–401 |date=1996 |url= |doi = }}</ref> Jupiter's [[magnetic field]] lines, which Io crosses, couples Io to Jupiter's polar upper atmosphere through the [[Faraday's law of induction|generation]] of 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 act to darken the Jovian polar regions at visible wavelengths. The location of Io and its auroral footprint with respect to the 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=IobookChap11/><ref name=Bigg1964/> The ''Juno'' mission, planned for the next decade, should help to shed light on these processes. ==Structure== Io is slightly larger than Earth's [[Moon]]. It has a mean radius of 1,821.3&nbsp;km (about five percent greater than the Moon's) and a mass of 8.9319{{e|22}}&nbsp;[[kilogram|kg]] (about 21 percent greater than the Moon's). 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]]. ===Interior=== [[Image:PIA01129 Interior of Io.jpg|thumb|240px|left|Model of the possible interior composition of Io with an inner iron or iron sulfide core (in gray), an outer silicate crust (in brown), and a partially molten silicate mantle in between (in orange)]] Composed primarily of [[silicate]] [[rock (geology)|rock]] and [[iron]], Io is closer in bulk composition to the 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 3.5275 g/cm<sup>3</sup>, the highest of any moon in the [[Solar System]]; significantly higher than the other Galilean satellites and higher than the Earth's moon.<ref name=Schubert2004>{{cite book |last=Schubert |first=J. ''et al.'' |editor=F. Bagenal ''et al.'' |title=Jupiter: The Planet, Satellites, and Magnetosphere |year=2004 |publisher=Cambridge University Press |pages=281–306 |chapter=Interior composition, structure, and dynamics of the Galilean satellites. }}</ref> Models based on the ''Voyager'' and ''Galileo'' measurements of the moon'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 an outer, silicate-rich [[Crust (geology)|crust]] and [[Mantle (geology)|mantle]] and an inner, iron- or [[Pyrite|iron sulfide]]–rich [[Planetary core|core]].<ref name=Anderson1996/> The metallic core makes up approximately 20% of Io's mass.<ref name=Anderson2001>{{cite journal | last=Anderson |first=J. D. |coauthors=''et al.'' |title=Io's gravity field and interior structure |journal=J. Geophys. Res. |volume=106 |issue= |pages=32,963–32,969 |date=2001 |url= |doi = }}</ref> Depending on the amount of sulfur in the core, the core has a radius between 350 and 650&nbsp;km (220 to 400&nbsp;mi) if it is composed almost entirely of iron, or between 550 and 900&nbsp;km (310 to 560&nbsp;mi) for a core consisting of a mix of iron and sulfur. ''Galileo''<nowiki>'</nowiki>s [[magnetometer]] failed to detect an internal magnetic field at Io, suggesting that the core is not [[Convection|convecting]].<ref name=Kivelson2001>{{cite journal | last=Kivelson |first=M. G. |coauthors=''et al.'' |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 |url= |doi = }}</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. |coauthors=''et al.'' |title=Implications from Galileo observations on the interior structure and chemistry of the Galilean satellites |journal=Icarus |volume=157 |issue= |pages=104–119 |date=2002 |url= |doi = }}</ref><ref name=Kuskov2001>{{cite journal | last=Kuskov |first=O. L. |coauthors=V. A. Kronrod |title=Core sizes and internal structure of the Earth's and Jupiter's satellites |journal=Icarus |volume=151 |issue= |pages=204–227 |date=2001 |url= |doi = }}</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. ''et al.'' |editor=R. M. C. Lopes and J. R. Spencer |title=Io after Galileo |year=2007 |publisher=Springer-Praxis |pages=89–108 |chapter=The Interior of Io. }}</ref> The [[lithosphere]] of Io, composed of basalt and sulfur deposited by Io's extensive volcanism, is at least 12&nbsp;km (7&nbsp;mi) thick, but is likely to be less than 40&nbsp;km (25&nbsp;mi) thick.<ref name=Jaeger2003>{{cite journal | last=Jaeger |first=W. L. |coauthors=''et al.'' |title=Orogenic tectonism on Io |journal=J. Geophys. Res. |volume=108 |issue= | pages=12–1|date=2003 |url= |doi=10.1029/2002JE001946}}</ref><ref name=Anderson2001/> ===Tidal heating=== Unlike the 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 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 | last=Yoder |first=C. F. |coauthors=''et al.'' |title=How tidal heating in Io drives the Galilean orbital resonance locks |journal=Nature |volume=279 |issue= |pages=767–770 |date=1979 |url= |doi = }}</ref> The vertical differences in Io's tidal bulge, between the times Io is at [[Apsis|periapsis]] and [[Apsis|apoapsis]] in its orbit, could be as much as 100&nbsp;m (330&nbsp;ft). 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 the moon'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>{{cite book|title=Encyclopedia of the Solar System|chapter=Io: The Volcanic Moon|author=Rosaly MC Lopes||publisher=Academic Press |year=2007|editor=Lucy-Ann McFadden, Paul R. Weissman, Torrence V. Johnson|pages=419, 423}}</ref> 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>&nbsp;[[watt|W]]).<ref name=IobookChap5/> Models of its orbit suggest that the amount of tidal heating within Io changes with time, and that the current heat flow is not representative of the long-term average.<ref name=IobookChap5/> [[Image:Iorotateing1day.ogg|thumb|250px|right|Rotating image of Io's surface, the large red ring is around the volcano [[Pele (volcano)|Pele]].]] ===Surface=== 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''<nowiki>'</nowiki>s first images of Io. The density of impact craters across Io's surface would have given clues to the moon'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| last=Britt | first=Robert Roy | title=Pizza Pie in the Sky: Understanding Io's Riot of Color | publisher=[[Space.com]] | date=[[16 March]] [[2000]] | url=http://www.space.com/scienceastronomy/solarsystem/galileo_io_volcanoes_000316.html | accessdate=2007-07-25 }}</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/> In addition to volcanoes, Io's surface includes non-volcanic mountains, numerous lakes of molten sulfur, [[caldera]]s up to several kilometers deep, and extensive flows of low-viscosity fluid (possibly some form of molten sulfur or silicate), which stretch for hundreds of kilometers.<ref>{{cite news | author=Staff | title=A Volcanic Flashback | publisher=Science at NASA | date=[[5 November]] [[1999]] | url=http://science.nasa.gov/newhome/headlines/ast19nov99_1.htm | accessdate=2007-06-14 }}</ref> ====Surface composition==== Io's colorful appearance is the result of various materials produced by its extensive volcanism. These materials include [[silicate]]s (such as [[Pyroxene|orthopyroxene]]), [[sulfur]], and [[sulfur dioxide]].<ref name=IobookChap9>{{cite book |last=Carlson |first=R. W.; ''et al.'' |editor=Lopes, R. M. C.; and Spencer, J. R. |title=Io after Galileo |year=2007 |publisher=Springer-Praxis |isbn=3-540-34681-3 |pages=pp. 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 the satellite, forming yellow to yellow-green regions. Sulfur deposited in the mid-latitude and polar regions is often radiation damaged, breaking up normally stable 8-chain sulfur. This radiation damage produces Io's red-brown polar regions.<ref name=Barnard1894/> [[Image:Io from Galileo and Voyager Orbiters.jpg|thumb|250px|left|Io's surface map.]] [[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 S<sub>2</sub>, producing a red "fan" deposit, or in extreme cases, large (often reaching beyond 450&nbsp;km (280&nbsp;mi) from the central vent) red rings.<ref name=Spencer2000b>{{cite journal | last=Spencer |first=J. |coauthors=''et al.'' |title=Discovery of Gaseous S<sub>2</sub> in Io's Pele Plume |journal=Science |volume=288 |issue= |pages=1208–1210 |date=2000 |url= |doi = }}</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 Cl<sub>2</sub>SO<sub>2</sub>.<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. 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. |coauthors=''et al.'' |title=Geology and activity around volcanoes on Io from the analysis of NIMS |journal=Icarus |volume=169 |issue= |pages=175–196 |date=2004 |url= |doi = }}</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 [[Volatiles|volatile materials]] like water in the vicinity of Io, but not hot enough to do so farther out. ====Volcanism==== {{Main|Volcanism on Io}} {{See Also|List of paterae on Io}} [[Image:Tvastarpic2.jpg|thumb|300 px|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 led the moon to become one of the most volcanically active worlds in the solar system, with hundreds of volcanic centers and extensive [[Lava|lava flows]]. 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 500&nbsp;km (310&nbsp;mi) 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. Io's surface is dotted with volcanic depressions known as ''paterae''.<ref name=Radebaugh2001>{{cite journal | last=Radebaugh |first=J. |coauthors=''et al.'' |title=Paterae on Io: A new type of volcanic caldera? |journal=J. Geophys. Res. |volume=106 |issue= |pages=33005–33020 |date=2001 |url= |doi = }}</ref> Paterae generally have flat floors bounded by steep walls. 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 | last=Keszthelyi |first=L. |coauthors=''et al.'' |title=A Post-Galileo view of Io's Interior |journal=Icarus |volume=169 |issue= |pages=271–286 |date=2004 |url= |doi = }}</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 41&nbsp;km (25&nbsp;mi), the largest being [[Loki Patera]] at 202&nbsp;km (126&nbsp;mi).<ref name=Radebaugh2001/> 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=Perry2003>{{cite conference |last=Perry |first=J. E. |coauthors =''et al.'' |title=Gish Bar Patera, Io: Geology and Volcanic Activity, 1997–2001 |booktitle=LPSC XXXIV |date=2003 |location =Clear Lake, TX | url= |id=Abstract#1720 |accessdate= }}</ref><ref name=Lopes2004/> 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 | last=Radebaugh |first=J. |coauthors=''et al.'' |title=Observations and temperatures of Io’s Pele Patera from Cassini and Galileo spacecraft images |journal=Icarus |volume=169 |issue= |pages=65–79 |date=2004 |url= |doi = }}</ref><ref name=Howell2007>{{cite journal | last=Howell |first=R. R. |coauthors=Lopes, R. M. C. |title=The nature of the volcanic activity at Loki: Insights from Galileo NIMS and PPR data |journal=Icarus |volume=186 |issue= |pages=448–461 |date=2007 |url= |doi = }}</ref> 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 | last=Keszthelyi |first=L. |coauthors=''et al.'' |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. |volume=106 |issue= |pages=33025–33052 |date=2001 |url= |doi = }}</ref> Larger outbreaks of lava have also been observed on Io. For example, the leading edge of the Prometheus flow moved 75 to 95&nbsp;km (47 to 59&nbsp;mi) between ''Voyager'' in 1979 and the first ''Galileo'' observations in 1996. A major eruption in 1997 produced more than 3,500&nbsp;km<sup>2</sup> (1,350&nbsp;sq&nbsp;mi) of fresh lava and flooded the floor of the adjacent Pillan Patera.<ref name=Mcewen1998/> [[Image:Tvashtarvideo.gif|left|thumb|200 px|Five-image sequence of ''[[New Horizons]]'' images showing Io's volcano Tvashtar spewing material 330&nbsp;km above its surface.]] 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. 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 | last=Keszthelyi |first=L. |coauthors=''et al.'' |title=New Estimates for Io Eruption Temperatures: Implications for the Interior |journal=Icarus |volume= 192|issue= | pages=491|date=2007 |url= |doi=10.1016/j.icarus.2007.07.008}}</ref> Initial estimates suggesting eruption temperatures approaching 2,000&nbsp;K<ref name=Mcewen1998/> have since proven to be overestimates since the wrong thermal models were used to model the temperatures.<ref name=Keszthelyi2007/> The discovery of plumes at the volcanoes Pele and 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 1&nbsp;km/s (0.6&nbsp;mps). Additional material that might be found in these volcanic plumes include sodium, [[potassium]], and [[chlorine]].<ref name=Roesler1999>{{cite journal | last=Roesler |first=F. L. |coauthors=''et al.'' |title=Far-Ultraviolet Imaging Spectroscopy of Io's Atmosphere with HST/STIS |journal=[[Science (journal)|Science]] |volume=283 |issue=5400 |pages=353–357 |date=1999 |url=http://www.sciencemag.org/cgi/content/full/283/5400/353?ijkey=2e836ace96ce3f47c37ce6a444fed5fd7b8f0dd8 |doi = }}</ref><ref name=Geissler1999>{{cite journal | last=Geissler |first=P. E. |coauthors=''et al.'' |title=Galileo Imaging of Atmospheric Emissions from Io |journal=Science |volume=285 |issue=5429 |pages=448–461 |date=1999 |url=http://www.sciencemag.org/cgi/content/full/285/5429/870#RF4 |doi = }}</ref> These plumes appear to be formed in one of two ways.<ref name=McEwen1983>{{cite journal | last=McEwen |first=A. S. |coauthors=Soderblom, L. A. |title=Two classes of volcanic plume on Io |journal=Icarus |volume=58 |issue= |pages=197–226 |date=1983 |url= |doi = }}</ref> Io's largest plumes are created when sulfur and sulfur dioxide gas dissolve from erupting magma at volcanic vents or lava lakes, often dragging silicate pyroclastic material with them. 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 1,000&nbsp;km (620&nbsp;mi) 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 100&nbsp;km (62&nbsp;mi) tall, and are among the most long-lived plumes on Io. Examples include Prometheus, Amirani, and [[Masubi (volcano)|Masubi]]. ====Mountains==== {{See Also|List of mountains on Io}} [[Image:Tohil Mons.jpg|right|thumb|''[[Galileo Galilei|Galileo]]'' greyscale image of [[Tohil Mons]], a 5.4 km tall mountain]] Io has 100 to 150 mountains. These structures average 6&nbsp;km (4&nbsp;mi) in height and reach a maximum of 17.5&nbsp;±&nbsp;1.5&nbsp;km (10.9&nbsp;±&nbsp;1&nbsp;mi) at South [[Boosaule Montes|Boösaule Montes]].<ref name=Schenk2001>{{cite journal | last=Schenk |first=P. M. |coauthors=''et al.'' |title=The mountains of Io: Global and geological perspectives from Voyager and Galileo |journal=J. Geophys. Res. |volume=106 |issue= |pages=33201–33222 |date=2001 |url= |doi = }}</ref> Mountains often appear as large (the average mountain is 157&nbsp;km (98&nbsp;mi) long), isolated structures with no apparent global tectonic patterns outlined, as is 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 | last=Clow |first=G. D. |coauthors=Carr, M. H. |title=Stability of sulfur slopes on Io |journal=Icarus |volume=44 |issue= |pages=729–733 |date=1980 |url= |doi = }}</ref> Despite the extensive volcanism that gives Io its distinctive appearance, nearly all 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 | last=Schenk |first=P. M. |coauthors=Bulmer, M. H. |title=Origin of mountains on Io by thrust faulting and large-scale mass movements |journal=Science |volume=279 |issue= |pages=1514–1517 |date=1998 |url= |doi = }}</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. |coauthors=''et al.'' |title=Chaos on Io: A model for formation of mountain blocks by crustal heating, melting, and tilting |journal=Geology |volume=29 |issue= |pages=103–106 |date=2001 |url= |doi = }}</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= |pages=pp. 32971–32981 |date=2001 |url= |doi = }}</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|scarp]]s 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 | last=Schenk |first=P. M. |coauthors=''et al.'' |title=Shield volcano topography and the rheology of lava flows on Io |journal=Icarus |volume=169 |issue= |pages=98–110 |date=2004 |url= |doi = }}</ref> These volcanic mountains are often smaller than the average mountain on Io, averaging only 1 to 2&nbsp;km (0.6 to 1.2&nbsp;mi) in height and 40 to 60&nbsp;km (25 to 37&nbsp;mi) 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. |coauthors=''et al.'' |title=Landform degradation and slope processes on Io: The Galileo view |journal=J. Geophys. Res. |volume=106 |issue= |pages=33223–33240 |date=2001 |url= |doi = }}</ref> [[Image:Io Aurorae color.jpg|thumb|240px|right|Auroral glows in Io's upper atmosphere. Different colors represent emission from different components of the atmosphere (green comes from emitting sodium atoms, red from emitting oxygen atoms, and blue from emitting volcanic gases like sulfur dioxide). Image taken while Io was in eclipse.]] ===Atmosphere=== Io has an extremely thin [[atmosphere]] consisting mainly of sulfur dioxide ({{chem|SO|2}}) with a pressure of a billionth of an [[atmosphere (unit)|atmosphere]].<ref name=Pearl1979/> The thin Ionian atmosphere means any future landing probes sent to investigate Io will not need to be encased in an aeroshell-style heatshield, but instead will require [[retrorocket]]s for a soft [[landing]]. The thin atmosphere also necessitates a rugged lander capable of enduring the strong Jovian [[radiation]], which a thicker atmosphere would attenuate. The same radiation (in the form of a [[magnetosphere|plasma]]) strips the atmosphere so that it must be constantly replenished.<ref name=IobookChap10>{{cite book |last=Lellouch |first=E.; ''et al.'' |editor=Lopes, R. M. C.; and Spencer, J. R. |title=Io after Galileo |year=2007 |publisher=Springer-Praxis |isbn=3-540-34681-3 |pages=pp. 231–264 |chapter=Io's atmosphere }}</ref> The most dramatic source of {{chem|SO|2}} is volcanism, but the atmosphere is largely sustained by sunlight-driven sublimation of {{chem|SO|2}} frozen on the surface. The atmosphere is largely confined to the equator, where the surface is warmest and most active volcanic plumes reside.<ref name=Feldman2000>{{cite journal | last=Feldman |first=P. D. |coauthors=''et al.'' |title=Lyman-α imaging of the {{chem|SO|2}} distribution on Io |journal=Geophys. Res. Lett. |volume=27 |issue= |pages=1787–1790 |date=1979 |url= |doi = }}</ref> Other variations also exist, with the highest densities near volcanic vents (particularly at sites of volcanic plumes) and on Io's anti-Jovian hemisphere (the side that faces away from Jupiter, where {{chem|SO|2}} frost is most abundant).<ref name=IobookChap10/> High-resolution images of Io acquired while the satellite is experiencing an eclipse reveal an [[Aurora (astronomy)|aurora]]-like glow. As on Earth, this is due to radiation hitting the atmosphere. Aurorae usually occur near the magnetic poles of planets, but Io's are brightest near its equator. Io lacks a magnetic field of its own; therefore, electrons traveling along Jupiter's magnetic field near Io directly impact the satellite's atmosphere. More electrons collide with the atmosphere, producing the brightest aurora, where the field lines are tangent to the satellite (i.e., near the equator), since the column of gas they pass through is longer 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. |coauthors=''et al.'' |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 |url=http://www.agu.org/journals/ja/ja0012/2000JA002500/pdf/2000JA002500.pdf |format=PDF| doi = }}</ref> <!--- A summary of Io's atmosphere is contained in the chapter by McGrath et al. in the book ''Jupiter: The Planet, Satellites and Magnetosphere'' (2004). ---> ==See also== {{portal|Solar System|Solar system.jpg}} * [[Jupiter's moons in fiction]] * [[Moons of Jupiter]] {{clear}} ==References== {{reflist|2}} ==External links== *General information **[http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jup_Io Io Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration] **[http://www.nineplanets.org/io.html Bill Arnett's Io webpage] **[http://www.windows.ucar.edu/tour/link=/jupiter/moons/io.html&edu=high Io overview] from the University of Michigan's [http://www.windows.ucar.edu/ Windows to the Universe] **[http://www.solarviews.com/eng/io.htm Calvin Hamilton's Io page] **[http://www.planetary.org/explore/topics/jupiter/io.html The Planetary Society: Io information] *Images **[http://photojournal.jpl.nasa.gov/target/Io Catalog of NASA images of Io] **[http://pirlwww.lpl.arizona.edu/missions/Galileo/releases/ Galileo Image Releases] **[http://pluto.jhuapl.edu/soc/ New Horizons LORRI Raw Images, includes numerous Io images] **[http://pluto.jhuapl.edu/gallery/missionPhotos.html New Horizons Image Releases] **[http://planetarynames.wr.usgs.gov/images/io_comp_color.pdf Map of Io with labels] **[http://astrogeology.usgs.gov/Projects/JupiterSatellites/io.html USGS Io basemaps using Galileo and Voyager images] *Additional References **[http://www.martiana.org/mars/jupiter/jupifrm.htm The Calendars of Jupiter] **[http://www.cosis.net/abstracts/EAE03/07912/EAE03-J-07912.pdf The Conundrum Posed by Io's Minimum Surface Temperatures] **[http://planetologia.elte.hu/io/ Io Mountain Database] **[http://astrogeology.usgs.gov/Projects/IoAurorae/ Cassini observations of Io's visible aurorae] {{Moons of Jupiter}} {{Solar System moons (compact)}} {{Jupiter}} {{Solar System}} {{featured article}} <!--Categories--> [[Category:Jupiter's moons]] [[Category:Io (moon)]] {{Link FA|de}} {{Link FA|pt}} <!--Interwiki--> [[als:Io (Mond)]] [[frp:Io (satèlite)]] [[ast:Ío (lluna)]] [[br:Io (loarenn)]] [[bg:Йо (спътник)]] [[ca:Ió (satèl·lit)]] [[cs:Io (měsíc)]] [[co:Io]] [[da:Io (måne)]] [[de:Io (Mond)]] [[et:Io (Jupiter)]] [[el:Ιώ (δορυφόρος)]] [[es:Ío (luna)]] [[eo:Ioo]] [[eu:Io]] [[fr:Io (lune)]] [[ga:Io (gealach)]] [[gl:Ío (satélite)]] [[zh-classical:木衛一]] [[ko:이오 (위성)]] [[hr:Ija (mjesec)]] [[is:Íó]] [[it:Io (astronomia)]] [[he:איו (ירח)]] [[ka:იო (თანამგზავრი)]] [[la:Io (satelles)]] [[lv:Jo (pavadonis)]] [[lt:Ijo (palydovas)]] [[hu:Io]] [[mk:Ио (месечина)]] [[mr:आयो उपग्रह]] [[ms:Io (bulan)]] [[nl:Io (maan)]] [[ja:イオ (衛星)]] [[no:Io (måne)]] [[nn:Jupitermånen Io]] [[nds:Io (Maand)]] [[pl:Io (księżyc)]] [[pt:Io]] [[ro:Io (satelit)]] [[ru:Ио (спутник)]] [[simple:Io (moon)]] [[sk:Io (mesiac)]] [[sl:Io (luna)]] [[sr:Ио]] [[fi:Io (kuu)]] [[sv:Io (måne)]] [[tl:Io (buwan)]] [[vi:Io (vệ tinh)]] [[tg:Ио (радиф)]] [[tr:İo (uydu)]] [[uk:Іо (супутник)]] [[zh:木卫一]]