Io_(moon) - Wikiparency

# Io (moon)  **Microsim — three.js (Wikitube framework):** *Io in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=Io&embed=1" data-title="Io in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=Io`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.*  *Try: press l to label the moons, then set the speed to 1 day/s and watch Io circle Jupiter in under two seconds, lapping Europa twice and Ganymede four times per orbit of each; press space to pause, then scroll to zoom in until Io's orbit fills the view and compare how close it runs to the planet.* **Io** is the innermost of the four [[Galilean_moons|Galilean moons]] of [[Jupiter]] and the most volcanically active body in the [[PORTAL_Solar_System|Solar System]], with more than 400 active volcanoes.[^lopes2007][^lopes2004] Slightly larger than Earth's [[Moon]], with a mean radius of 1,821.3 km and a mass of 8.93 × 10²² kg, it is the densest moon in the Solar System and is made of silicate rock around an iron-rich core rather than the ice that dominates most outer-system moons.[^schneider2023][^schubert2004] Io's heat comes from [[Tide|tides]]. Its orbit, 421,700 km from Jupiter's centre, is held slightly eccentric by a resonance with [[Europa_(moon)|Europa]] and [[Ganymede_(moon)|Ganymede]], so Jupiter's pull flexes the moon every 1.77 days; friction from that flexing drives eruptions of basaltic lava and plumes of [[Sulfur|sulfur]] and sulfur dioxide, and more than 100 mountains, some taller than Mount Everest, rise from its plains.[^peale1979][^schenk2001] Volcanic gases form a thin atmosphere and feed a torus of [[Plasma_(physics)|plasma]] around Jupiter that makes Io's orbit one of the harshest radiation environments in the Solar System.[^schneider2007][^ringwald2000] Voyager 1 revealed the volcanism in 1979, the Galileo orbiter studied it for eight years, and Juno passed within 1,500 km of the surface in 2023 and 2024.[^morabito1979][^perry2007][^nasa-juno] The explorer at the top of this page opens in Jupiter's own frame, with Io circling at its true distance and period alongside the other Galilean moons; the moons' starting phases are ILLUSTRATIVE.[^jpl-satphys] ## Nomenclature Galileo designated the moon by number, and it was long known in the literature as Jupiter I, the first satellite of [[Jupiter]].[^barnard1894][^marazzini2005] Its name comes from Simon Marius, who in his *Mundus Iovialis* of 1614 adopted [[Johannes_Kepler|Johannes Kepler]]'s suggestion of October 1613 to name Jupiter's moons after lovers of Zeus, and called the innermost after Io, a priestess of Hera whom Zeus pursued. Marius also offered alternatives such as "the Mercury of Jupiter".[^vanhelden1994] His names came into general use only in the mid-20th century.[^marazzini2005] Surface features are named under rules of the International Astronomical Union, which draw on characters and places from the Io myth, deities of fire, volcanoes, the [[Sun]] and thunder from many cultures, and figures and places from Dante's *Inferno*, choices that suit the moon's volcanic surface.[^usgs-categories] Since Voyager 1, 249 names have been approved for volcanoes, mountains, plateaus and bright regions. The feature types include patera, a volcanic depression; fluctus, a lava flow; vallis, a lava channel; mons, a mountain; mensa, a flat-topped plateau; tholus, a dome; and regio, a large bright area.[^usgs-categories] Examples include Prometheus, Pan Mensa, Tvashtar Paterae and Tsũi Goab Fluctus.[^usgs-gazetteer] ## Observational history Galileo first recorded Io on 7 January 1610 with a telescope magnifying about 20 times, but could not separate it from [[Europa_(moon)|Europa]]; the two were seen as separate points the next night, 8 January, which the IAU uses as Io's discovery date.[^usgs-names] Marius's first recorded observation, 29 December 1609 in the Julian calendar, falls on the same 8 January in Galileo's Gregorian reckoning, and Galileo is credited because he published first.[^vanhelden2004] For more than two centuries Io remained an unresolved fifth-magnitude point, yet it was useful: its eclipses gave a proposed method for finding longitude, the Galilean moons confirmed [[Kepler's_laws_of_planetary_motion|Kepler's third law]] on a small scale, and timings of their eclipses first showed that light takes time to cross the Solar System.[^cruikshank2007] Pierre-Simon Laplace worked out the mathematics of the [[Resonance|resonance]] linking Io, Europa and [[Ganymede_(moon)|Ganymede]].[^cruikshank2007] In the 1890s Edward Barnard saw that Io's poles are darker than its equatorial band and correctly attributed the difference to colour rather than an elongated shape.[^barnard1894] Spectra in the early 1970s showed no water ice on the surface, suggesting instead salts and sulfur, and in 1964 Bigg found that Io's position controls bursts of decametric radio emission from [[Jupiter]].[^lee1972][^fanale1974][^bigg1964] ### Pioneer Pioneer 10 and Pioneer 11 passed Io, on their way past [[Jupiter]], on 3 December 1973 and 2 December 1974. Radio tracking refined its mass, showing it to be the densest Galilean moon and therefore rocky; the spacecraft also found a thin atmosphere and intense radiation belts, and Pioneer 11 returned the only good image, of the yellowish north polar region.[^fimmel1977][^anderson1974] ### Voyager [[Voyager_1|Voyager 1]] passed 20,600 km from Io on 5 March 1979 and revealed a multicoloured surface with no impact craters, pits, lava flows and mountains higher than Everest.[^pds-voyager][^smith1979] Days later the navigation engineer Linda Morabito noticed a plume rising above the limb in an image, and nine plumes were soon identified, proving active volcanism.[^morabito1979][^strom1979] Stanton Peale, Patrick Cassen and Ray Reynolds had predicted this in a paper published just before the encounter, from the tidal heating that the resonance should cause.[^peale1979] Sulfur and sulfur dioxide dominated the surface, the atmosphere and the plasma torus that Voyager also found.[^soderblom1980] Voyager 2, passing 1,130,000 km away on 9 July 1979, saw that seven of the nine plumes were still erupting, with only Pele shut down.[^strom1982] ### Galileo The Galileo orbiter flew close to Io on 7 December 1995, just before entering Jupiter orbit, and its gravity measurements revealed a large [[Iron|iron]] core.[^anderson1996] During its primary mission it saw the aftermath of a major eruption at Pillan Patera and established that Io's lavas are magnesium-rich silicates.[^mcewen1998] In extended missions it made six close flybys between late 1999 and early 2002, studying volcanoes and mountains up close.[^perry2007] ### Cassini The Cassini spacecraft passed Jupiter in December 2000 on its way to [[Saturn]], observing Io jointly with Galileo; the observations revealed a new plume at Tvashtar Paterae and shed light on Io's aurorae.[^porco2003] ### New Horizons New Horizons flew past Jupiter on 28 February 2007 on its way to [[Pluto]]. It imaged a large plume from Tvashtar, the first detailed view of the largest class of Io plume since Voyager saw Pele's in 1979, and an eruption near Girru Patera in its early stages.[^spencer2007] ### Juno Juno entered Jupiter orbit in July 2016. Its polar orbit initially kept it far from Io, with a closest approach of 195,000 km in 2020, but the orbit's slow evolution brought the moons within reach.[^mura2020][^nasa-juno] It passed about 1,500 km from Io's surface on 30 December 2023 and 3 February 2024, measuring the gravity field and imaging changes since 2007.[^nasa-juno][^bolton2020] Infrared data from its JIRAM instrument confirmed continuing activity at volcanoes including Amirani, Prometheus, Seth, Culann, Tvashtar and Girru, some apparently erupting for more than 45 years, and found a flow field about 130 by 40 km near Seth Patera that did not exist in Galileo images.[^lopes2025] ### Future missions ESA's JUICE, launched in April 2023 and due at Jupiter in July 2031, and NASA's Europa Clipper, launched in October 2024 and due in 2030, will not fly by Io, but both can monitor its volcanoes from a distance.[^nasa-juno][^nasa-clipper] A dedicated Discovery-class Io Volcano Observer, planned to make ten flybys from Jupiter orbit, was one of four concepts NASA selected for further study in 2020, but in 2021 the agency chose two [[Venus]] missions instead.[^mcewen2021][^nasa2020][^strickland2021] ## Orbit and rotation Io [[Orbit|orbits]] 421,700 km from [[Jupiter]]'s centre, about 350,000 km above the cloud tops, between the small moon Thebe and Europa; counting the four small inner moons it is the fifth moon outward.[^jpl-satphys] One orbit takes 42.5 hours, fast enough that its motion can be followed over a single night, and the orbital speed is about 17.3 km/s (derived from 2π × 421,700 km / 1.769 days).[^jpl-satphys] Io completes two orbits for each one of [[Europa_(moon)|Europa]] and four for each one of [[Ganymede_(moon)|Ganymede]]. Those regularly repeated pulls, always at the same points in the orbit, force an eccentricity of 0.0041 that tidal friction would otherwise erase, and that eccentricity is the root of Io's heating.[^peale1979][^yoder1979] Io keeps one face toward Jupiter, rotating once per orbit as the [[Moon]] does for [[Earth]]. This fixes its coordinate system: the prime meridian passes through the point facing Jupiter, dividing the moon into a sub-Jovian and an anti-Jovian hemisphere, and into leading and trailing hemispheres relative to its direction of motion.[^lopes2005] In the explorer's local frame, Io's short period is plain to see: at 1 day/s it completes an orbit in under two seconds, while [[Callisto_(moon)|Callisto]] takes more than sixteen.[^jpl-satphys] ## Interaction with Jupiter's magnetosphere Io is the main source of [[Plasma_(physics)|plasma]] in [[Jupiter]]'s magnetosphere. Gas escaping its atmosphere, about a tonne per second, includes [[Sulfur|sulfur]], [[Oxygen|oxygen]], chlorine, [[Sodium|sodium]] and [[Potassium|potassium]]; once ionised it is swept up by Jupiter's rotating magnetic field.[^schneider2007] The ions form the Io plasma torus, a doughnut of charged particles along Io's orbit that corotates with the planet at about 74 km/s, far faster than Io's own 17 km/s, so the torus constantly overtakes the moon and strips its atmosphere.[^schneider2007] Ions stay in the torus for about 40 days on average before escaping outward, and their pressure inflates Jupiter's magnetosphere well beyond its size without them.[^krimigis2002] Moving through the planet's field, Io acts as a generator, developing some 400,000 volts across itself and driving a current of about 3 million amperes along the magnetic flux tube that links it to Jupiter's poles, where it produces an auroral footprint.[^nasa-io-overview][^schneider2007] Neutral atoms that escape Io's gravity form clouds around the moon and a banana-shaped cloud along its orbit, and some sodium atoms leave in fast jets after exchanging charge with torus ions.[^schneider2007] Ulysses discovered in 1992 streams of fine dust leaving the Jovian system at hundreds of kilometres per second; Galileo traced them to Io, and the grains are mostly sodium chloride.[^grun1993][^postberg2006] Io's position also governs Jupiter's decametric radio bursts, which increase when Io is at particular points relative to [[Earth]].[^bigg1964] Galileo's magnetometer found no field generated inside Io, but a re-analysis in 2011 identified an induced field and argued for a partly molten "magma ocean" about 50 km beneath the surface.[^kivelson2001][^khurana2011] Juno's gravity measurements challenged this: Park and colleagues found in 2024 that Io's tidal response rules out a shallow global magma ocean, leaving a mostly solid mantle.[^park2024] ## Geology Io's radius is about 5% larger than the [[Moon]]'s and its mass about 21% greater; it is slightly elongated toward [[Jupiter]] by the planet's [[Tide|tide]].[^schneider2023] Its surface gravity is about 1.80 m/s², slightly more than the Moon's (derived from GM/r²).[^schneider2023] ### Interior Io's density of 3.53 g/cm³ is the highest of any regular moon in the Solar System.[^schubert2004] Gravity data show it is differentiated into a silicate crust and mantle over an iron or iron-sulfide core that holds about 20% of its mass, with a radius of roughly 350–650 km depending on its sulfur content.[^anderson1996][^anderson2001] Models give a mantle rich in the magnesium silicate forsterite, with a bulk composition resembling L and LL chondrite meteorites.[^sohl2002] To carry the observed heat, perhaps 10–20% of the mantle is molten.[^moore2007] The lithosphere, or rigid [[Crust_(geology)|crust]] and upper mantle, of basalt and sulfur is at least 12 km thick and probably less than 40 km.[^anderson2001] ### Tidal heating Unlike [[Earth]] and the [[Moon]], Io is heated mainly by tidal dissipation rather than [[Radioactive_decay|radioactive decay]].[^peale1979] The tidal forces it feels are about 20,000 times those the Moon raises on Earth, and the solid tidal bulge rises and falls by as much as 100 m over each orbit as Io's distance from Jupiter changes.[^nasa-lowtide] The resulting heat, up to 200 times what radioactive decay alone would supply, escapes as a global heat flow of 0.6–1.6 × 10¹⁴ W; spread over Io's surface area of 4.17 × 10⁷ km², that is 1.4–3.8 W/m² (derived).[^lopes2007][^moore2007] The resonance keeps the energy coming: without it, tides would circularise Io's orbit, and tides raised on Jupiter would slowly push Io outward.[^yoder1979] Where the heat is released is not fully understood. The volcanoes are displaced 30–60° east of where models of tidal heating in a solid mantle put the maximum, which Tyler and colleagues attributed to heat generated by flow in a subsurface layer of molten rock.[^steigerwald2015][^tyler2015] ### Surface Scientists expected [[Voyager_1|Voyager]] to show a cratered, ancient surface. Instead Io had almost no impact craters: its volcanoes bury them faster than they form, making it one of the youngest surfaces in the Solar System.[^smith1979][^strom1979] Its colours, yellow, red, white, black and green, come from sulfur compounds and silicates. Sulfur dioxide frost covers large white and grey areas, yellow sulfur is widespread, and radiation-damaged sulfur gives the poles their reddish-brown tone.[^carlson2007][^barnard1894] Io appears to contain almost no [[Water|water]], probably because the young Jupiter was hot enough to drive volatiles away from Io's orbit.[^doute2004][^hadhazy2014] The volcanism takes several forms. Paterae, steep-walled depressions averaging 41 km across, are the commonest volcanic landform; the largest, Loki Patera, is 202 km across and alone produces about a quarter of Io's heat output.[^radebaugh2001][^oxford-astro2002] Some contain lava lakes whose crusts overturn continuously, as at Pele, or episodically, as at Loki.[^lopes2004] Lava flows hundreds of kilometres long spread across the plains; a 1997 eruption at Pillan covered more than 3,500 km².[^mcewen1998] Measured eruption temperatures of at least 1,300 K and up to about 1,600 K indicate basaltic, magnesium-rich lavas rather than the molten sulfur once suspected.[^keszthelyi2007] Umbrella-shaped plumes loft sulfur, sulfur dioxide and ash hundreds of kilometres high, coating the ground around them in red and white rings.[^keszthelyi2023][^bagenal2023] Io's mountains, averaging about 6 km high and reaching about 17.5 km at Boösaule Montes, are mostly not volcanoes but blocks of crust thrust up by compression at the base of the lithosphere.[^schenk2001] ## Atmosphere Io's atmosphere is extremely thin, mostly [[Sulfur|sulfur]] dioxide with sulfur monoxide, sodium chloride and atomic sulfur and oxygen; its dayside pressure is about 0.3–3 nanobars (3 × 10⁻⁵ to 3 × 10⁻⁴ Pa), densest near the equator and over the anti-Jovian hemisphere.[^lellouch2007][^walker2010] Most of it is sustained by light from the [[Sun]] evaporating sulfur dioxide frost, with volcanic plumes contributing locally.[^moullet2010][^lellouch2007] Because frost supply depends on surface temperature, the atmosphere partly collapses at night and in [[Jupiter]]'s shadow: during eclipse its column density falls by about 80%, and it re-forms within minutes of sunlight's return.[^tsang2016][^swri2016] Its temperature ranges from that of the surface frost near the ground to about 1,800 K at altitude, where the plasma torus heats it.[^lellouch2007] Stripped continuously by the magnetosphere, the atmosphere must be replenished at the tonne-per-second rate at which it is lost.[^schneider2007] Images taken during eclipse show an aurora-like glow, brightest near the equator rather than the poles, because Io has no field of its own and Jupiter's field lines are tangent to the atmosphere there.[^geissler1999][^retherford2000] ## See also - [[Galilean_moons]] · [[Europa_(moon)]] · [[Ganymede_(moon)]] · [[Callisto_(moon)]] - [[Jupiter]] - [[Enceladus]] · [[Triton_(moon)]] - [[Tide]] ## Notes Derived values: orbital speed 2π × 421,700 km / (1.769 × 86,400 s) ≈ 17.3 km/s; surface gravity GM/r² with G = 6.674 × 10⁻¹¹ m³ kg⁻¹ s⁻², M = 8.9319 × 10²² kg and r = 1,821.3 km gives 1.80 m/s²; surface area 4πr² ≈ 4.17 × 10¹³ m², so 0.6–1.6 × 10¹⁴ W corresponds to 1.4–3.8 W/m²; 1 nbar = 10⁻⁴ Pa. ## References [^lopes2007]: Lopes, R. 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"Io's equatorial spots: morphology of neutral UV emissions". *Journal of Geophysical Research* 105: 27157–27165. https://doi.org/10.1029/2000JA002500 ## External links ### General information - NASA Science: Io. https://science.nasa.gov/jupiter/jupiter-moons/io/ - JPL Solar System Dynamics: planetary satellites. https://ssd.jpl.nasa.gov/sats/ ### Movies - NASA Science: Juno mission, including the Io flyby animations of 2023–2024. https://science.nasa.gov/mission/juno/ ### Images - NASA Photojournal: Io. https://photojournal.jpl.nasa.gov/target/io ### Maps - USGS Gazetteer of Planetary Nomenclature: Io feature names and maps. https://planetarynames.wr.usgs.gov/ ### Additional references - Lopes, R. M. C.; Spencer, J. R., eds. (2007). *Io after Galileo: A New View of Jupiter's Volcanic Moon*. Springer-Praxis. ISBN 978-3-540-34681-4. - Lopes, R. M. C.; de Kleer, K.; Keane, J. T., eds. (2023). *Io: A New View of Jupiter's Moon*. Springer. ISBN 978-3-031-25669-1. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Io_(moon)) : [Wikitube](https://en.wikitube.io/wiki/Io_(moon)) · pinned revision [1375514130](https://en.wikipedia.org/w/index.php?oldid=1375514130) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-057 · explorer state `?obj=Io`.*