Io
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Discovery
Discovered byGalileo Galilei
Discovery dateJanuary 7, 1610
Periapsis420,000 km (0. Galileo Galilei (15 February 1564 &ndash 8 January 1642 was a Tuscan ( Italian) Physicist, Mathematician, Astronomer, and Philosopher Events 1325 - Alfonso IV becomes King of Portugal. 1558 - France takes Calais, the last continental In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from 002807 AU)
Apoapsis423,400 km (0. In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from 002830 AU)
Mean orbit radius421,700 km (0. Remote Authentication Dial In User Service ( RADIUS) is a networking protocol that provides centralized access authorization and accounting management for people or computers The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand 002819 AU)
Eccentricity0. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape 0041
Orbital period1. The orbital period is the time taken for a given object to make one complete Orbit about another object 769137786 d (152,853. 5047 s, 42 h)
Average orbital speed17. The orbital speed of a body generally a Planet, a Natural satellite, an artificial satellite, or a Multiple star, is the speed at which it 334 km/s
Inclination2. Inclination in general is the Angle between a Reference plane and another plane or axis of direction 21° (to the ecliptic)
0. The ecliptic is the apparent path that the Sun traces out in the sky during the year 05° (to Jupiter's equator)
Satellite ofJupiter
Physical characteristics
Dimensions3660. A natural satellite or moon is a Celestial body that Orbits a Planet or smaller body which is called the primary. 0 × 3637. 4 × 3630. 6 km[1]
Mean radius1821. 3 km (0. 286 Earths)[1]
Surface area41,910,000 km2 (0. Equation A spheroid centered at the origin and rotated about the z axis is defined by the implicit equation \left(\frac{x}{a}\right^2+\left(\frac{y}{a}\right^2+\left(\frac{z}{b}\right^2 Square Kilometre ( US spelling square kilometer) symbol km2, is a decimal multiple of the SI unit of 082 Earths)
Volume2. The volume of any solid plasma vacuum or theoretical object is how much three- Dimensional space it occupies often quantified numerically 53×1010 km3 (0. CM3 redirects here If you were looking for the 3rd game in the Cooking Mama series abbreviated as CM3 see here. 023 Earths)
Mass8. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object 9319×1022 kg (0. 015 Earths)
Mean density3. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different 528 g/cm3
Equatorial surface gravity1. For other uses of the words gram or gramme see Gram (disambiguation. A cubic centimetre or cubic centimeter (symbol cm3 —the abbreviation cc, though widely used is deprecated is a commonly used unit of Volume The surface gravity, g, of an astronomical or other object is the Gravitational acceleration experienced at its surface 796 m/s2 (0. 183 g)
Escape velocity2. g-force (also G-force, g-load) is a measurement of an object's Acceleration expressed in g s In Physics, escape velocity is the speed where the Kinetic energy of an object is equal to the magnitude of its Gravitational potential energy 558 km/s
Rotation periodsynchronous
Equatorial rotation velocity271 km/h
Albedo0. The rotation period of an astronomical object is the time it takes to complete one revolution around its Axis of rotation relative to the background stars In Astronomy, synchronous rotation is a planetological term describing a body orbiting another where the orbiting body takes as long to rotate The albedo of an object is the extent to which it diffusely reflects light from the sun 63 ± 0. 02[2]
Surface temp.
Surface
minmeanmax
130 K200 K
Apparent magnitude5. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature The apparent magnitude ( m) of a celestial body is a measure of its Brightness as seen by an observer on Earth, normalized to the value 02 (opposition)[3]
Atmosphere
Surface pressuretrace
Composition90% sulfur dioxide

Io (pronounced /ˈaɪoʊ/ EYE-oe, or as Greek Ῑώ) is the innermost of the four Galilean moons of Jupiter and, with a diameter of 3,642 kilometers, the fourth-largest moon in the Solar System. Opposition is a term used in Positional astronomy and Astrology to indicate when one celestial body is on the opposite side of the sky when viewed from Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei. A natural satellite or moon is a Celestial body that Orbits a Planet or smaller body which is called the primary. Geometry, a diameter of a Circle is any straight Line segment that passes through the center of the circle and whose Endpoints are on the The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand The tables below of Natural satellites in the Solar System are ordered from largest to smallest by average diameter The Solar System consists of the Sun and those celestial objects bound to it by Gravity. It was named after Io, a priestess of Hera that became one of the lovers of Zeus. In Greek mythology, Io (ˈaɪoʊ or /ˈiːoʊ/ World Book «EYE oh», in Ancient Greek Ἰώ) was a priestess of Hera in Argos In the Olympian pantheon of classical Greek Mythology, Hera (ˈhɪərə or /ˈhɛrə/ Greek) or Here ( in Ionic and Homer Zeus (zjuːs in Greek: nominative: Zeús /zdeús/ genitive: Diós; Modern Greek /'zefs/ in Greek mythology

With over 400 active volcanoes, Io is the most geologically active object in the Solar System. [4][5] 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 km (310 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. Mount Everest, also called Sagarmatha (सगरमाथा meaning Head of the Sky) or Chomolungma, Qomolangma or Zhumulangma (in [6] 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 500 kilometres (311 mi) 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. Galileo Galilei (15 February 1564 &ndash 8 January 1642 was a Tuscan ( Italian) Physicist, Mathematician, Astronomer, and Philosopher This discovery furthered the adoption of the Copernican model of the Solar System, the development of Kepler's laws of motion, and the first measurement of the speed of light. In Astronomy, heliocentrism is the theory that the Sun is at the center of the Solar System. Johannes Kepler (ˈkɛplɚ ( December 27 1571 &ndash November 15 1630) was a German Mathematician, Astronomer 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 spacecraft revealed Io to be a geologically active world, with numerous volcanic features, large mountains, and a young surface with no obvious impact craters. See also Voyager 1 and Voyager 2. The Voyager program consists of a pair of unmanned scientific probes Voyager 1 and 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. New Horizons is a Robotic spacecraft mission by NASA currently underway

## Name

See also: List of geological features on Io, List of paterae on Io, and 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. This is a list of named geological features on Io. See also the List of mountains on Io and the List of paterae on Io. This is a list of Paterae (shallow craters on the surface of Io. Io's surface is covered in volcanoes and Mountains The larger ones are named after prominent figures from the mythological travels of Io, or Simon Marius ( Latinized from German Simon Mayr) ( January 10, 1573 &ndash December 26, 1624) was a German In his 1614 publication Mundus Jovialis, he named the innermost large moon of Jupiter after the Greek mythological figure Io, one of the many lovers of Zeus (who is also known as Jupiter in Roman mythology). Simon Marius ( Latinized from German Simon Mayr) ( January 10, 1573 &ndash December 26, 1624) was a German Greek mythology is the body of stories belonging to the ancient Greeks concerning their gods and Heroes the nature of the world and the origins and significance In Greek mythology, Io (ˈaɪoʊ or /ˈiːoʊ/ World Book «EYE oh», in Ancient Greek Ἰώ) was a priestess of Hera in Argos Zeus (zjuːs in Greek: nominative: Zeús /zdeús/ genitive: Diós; Modern Greek /'zefs/ in Greek mythology In Roman mythology, Jupiter was the king of the gods and the god of Sky and Thunder. Roman mythology, or more appropriately Latin mythology, refers to the mythological beliefs of the Italic people inhabiting the region of Latium and its [7] 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 numeral designation (a system introduced by Galileo) as "Jupiter I", or simply as "the first satellite of Jupiter". Roman numerals are a Numeral system originating in ancient Rome, adapted from Etruscan numerals. 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's Inferno, names appropriate to the volcanic nature of the surface. The Divine Comedy [8] 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 Voyager 1 spacecraft is a 722-kilogram robotic Space probe of the outer Solar system and beyond launched September 5, 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). [8] Examples of named features include Prometheus, Pan Mensa, Tvashtar Paterae, and Tsũi Goab Fluctus. Tvashtar Paterae compose an active volcanic region of Jupiter 's moon Io located near the moon's north pole [9]

## Observational history

Galileo Galilei, the discoverer of Io

The first reported observation of Io was made by Galileo Galilei on 7 January 1610. Galileo Galilei (15 February 1564 &ndash 8 January 1642 was a Tuscan ( Italian) Physicist, Mathematician, Astronomer, and Philosopher Galileo Galilei (15 February 1564 &ndash 8 January 1642 was a Tuscan ( Italian) Physicist, Mathematician, Astronomer, and Philosopher Events 1325 - Alfonso IV becomes King of Portugal. 1558 - France takes Calais, the last continental The discovery of Io and the other Galilean satellites of Jupiter was published in Galileo's Sidereus Nuncius in March 1610. Sidereus Nuncius (usually translated into English as Sidereal Messenger, although Starry Messenger and Sidereal Message are [10] 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,[11] validating Kepler's Third Law of planetary motion, and determining the time for light to travel between Jupiter and Earth. Longitude (ˈlɒndʒɪˌtjuːd or ˈlɒŋgɪˌtjuːd symbolized by the Greek character Lambda (λ is the east-west Geographic coordinate measurement In Astronomy, Kepler's Laws of Planetary Motion are three mathematical laws that describe the motion of Planets in the Solar System. [10] Based on ephemerides produced by astronomer Giovanni Cassini and others, Pierre-Simon Laplace created a mathematical theory to explain the resonant orbits of Io, Europa, and Ganymede. An ephemeris (plural ephemerides; from the Greek word ἐφήμερος ephemeros "daily" is a table of values that gives the positions of Giovanni Domenico Cassini ( June 8, 1625 &ndash September 14, 1712) was an Italian Mathematician, Astronomer TemplateInfobox Planet.--> Europa (jʊˈroʊpə; or as TemplateInfobox Planet.--> Ganymede (ˈgænɨmiːd, or as Greek [10] 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 resolve (that is, see) large-scale surface features on Io. Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged In the 1890s, 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 Pickering, or two separate objects, as initially proposed by Barnard. Edward Emerson Barnard ( December 16, 1857 – February 6, 1923) was an American Astronomer. The albedo of an object is the extent to which it diffusely reflects light from the sun William Henry Pickering ( February 15, 1858 &ndash January 17, 1938) was an American Astronomer, brother of Edward [12][13][14] Later telescopic observations confirmed Io's distinct reddish-brown polar regions and yellow-white equatorial band. [15]

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). [16] The same observations suggested a surface dominated by evaporates composed of sodium salts and sulfur. Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 Sulfur or sulphur (ˈsʌlfɚ see spelling below) is the Chemical element that has the Atomic number 16 [17] Radio telescopic observations revealed Io's influence on the Jovian magnetosphere, as demonstrated by decametric wavelength bursts tied to the orbital period of Io. A magnetosphere' is a highly magnetized region around and possessed by an Astronomical object. A decametre also dekametre ( American spelling: dekameter, earlier decameter symbol dam) is a very rarely used unit of In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. [18]

### Pioneer

The first spacecraft to pass by Io were the twin Pioneer 10 and 11 probes on December 3, 1973 and December 2, 1974 respectively. Pioneer 10 ( Pioneer-F) was the first Spacecraft to travel through the Asteroid belt, which it entered on July 15, 1972 Pioneer 11 was the second mission (after its sister probe Pioneer 10) to investigate Jupiter and the Outer solar system and the first to explore the planet Events 1800 - War of the Second Coalition: Battle of Hohenlinden, French Year 1973 ( MCMLXXIII) was a Common year starting on Monday (link will display full calendar of the 1973 Gregorian calendar. Events 1409 - The University of Leipzig opens 1755 - The second Eddystone Lighthouse is destroyed by fire Year 1974 ( MCMLXXIV) was a Common year starting on Tuesday (link will display full calendar of the 1974 Gregorian calendar. [19] 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. [20] 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 11 took the only good image of Io obtained by either spacecraft, showing its north polar region. [21] Close-up images were planned during Pioneer 10's encounter with Io, but those observations were lost due to the high-radiation environment. [19]

### Voyager

Mosaic of Voyager 1 images covering Io's south polar region

When the twin probes Voyager 1 and Voyager 2 passed by Io in 1979, their more advanced imaging system allowed for far more detailed images. The Voyager 1 spacecraft is a 722-kilogram robotic Space probe of the outer Solar system and beyond launched September 5, The Voyager 1 spacecraft is a 722-kilogram robotic Space probe of the outer Solar system and beyond launched September 5, Voyager 2 is an unmanned Interplanetary Spacecraft launched on August 20, 1977. Voyager 1 flew past the satellite on March 5, 1979 from a distance of 20,600 km (12,800 mi). Events 363 - Roman Emperor Julian moves from Antioch with an army of 90000 to attack the Sassanid Empire, in a Year 1979 ( MCMLXXIX) was a Common year starting on Monday (link displays the 1979 Gregorian calendar) [22] The images returned during the approach revealed a strange, multi-colored landscape devoid of impact craters. [23] 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. [24] Analysis of other Voyager 1 images showed nine such plumes scattered across the surface, proving that Io was volcanically active. [25] 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" section for a more detailed explanation of the process). [26] 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 and the torus of plasma centered on Io's orbit (also discovered by Voyager). [27][28][29]

Voyager 2 passed Io on July 9, 1979 at a distance of 1,130,000 km (702,150 mi). Events 455 - Roman military commander Avitus is proclaimed Emperor of the Western Roman Empire. Year 1979 ( MCMLXXIX) was a Common year starting on Monday (link displays the 1979 Gregorian calendar) Though it did not approach nearly as close as Voyager 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 shutting down between flybys. Pele is a volcano on the surface of Jupiter 's moon Io. It is named after the Hawaiian volcano goddess Pele. [30]

### Galileo

Galileo encounters with Io
DateDistance (km)
December 7, 1995897
November 4, 1996244,000
March 29, 1998252,000
June 30, 1999127,000
October 11, 1999611
November 26, 1999301
February 22, 2000198
August 6, 2001194
October 16, 2001184
January 17, 2002102
November 7, 200245,800

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 compositions with sulfur and sulfur dioxide serving a similar role to water and carbon dioxide on Earth. Mafic is an adjective describing a Silicate mineral or rock that is rich in magnesium and iron the term was derived by contracting "magnesium" and "ferric" Ultramafic (also referred to as ultrabasic) rocks are igneous and meta -igneous rocks with very low Silica content (less than 45% generally Carbon dioxide ( Chemical formula:) is a Chemical compound composed of two Oxygen Atoms covalently bonded to a single [32] 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. [33]

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. [33] In December 2000, the Cassini spacecraft had a distant and brief encounter with the Jupiter system en route to Saturn, allowing for joint observations with Galileo. Cassini–Huygens is a joint NASA / ESA / ASI Robotic spacecraft mission currently studying the planet Saturn and its These observations revealed a new plume at Tvashtar Paterae and provided insights into Io's aurorae. Tvashtar Paterae compose an active volcanic region of Jupiter 's moon Io located near the moon's north pole [34]

### Subsequent observations

Changes in surface features in the eight years between Galileo and New Horizons observations

Following Galileo'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 Keck telescope in Hawaii and imaging from the Hubble telescope have allowed astronomers to monitor Io's active volcanoes. Adaptive optics (AO is a Technology used to improve the performance of optical systems by reducing the effects of rapidly changing optical distortion The W M Keck Observatory is a two-telescope Astronomical observatory at the 4145 meter (13600 ft summit of Mauna Kea in Hawai'i. The State of Hawaii ( or həˈwaɪʔiː Hawaiian: Mokuāina o Hawaii) is a state in the United States located on an Archipelago in the [35][36] 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. New Horizons is a Robotic spacecraft mission by NASA currently underway The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending Events 202 BC - coronation ceremony of Liu Bang as Emperor Gaozu of Han takes place initiating four centuries of the Han Dynasty 's rule Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. 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. [37] 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, does not have an imaging system powerful enough to perform Io surface science. Juno is a NASA mission to Jupiter, slated to cost about USD \$700 million (FY03 and scheduled to launch in August 2011 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. [38] 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 present, this project is also in the concept study phase. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common [39]

## Orbit

Animation showing Io's Laplace resonance with Europa and Ganymede

Io orbits Jupiter at a distance of 421,700 km (262,000 mi) from the planet's center and 350,000 km (217,000 mi) from its cloudtops. It is the innermost of the Galilean satellites of Jupiter, its orbit lying between those of Thebe and Europa. TemplateInfobox Planet.--> Thebe (, or as in Greek Θήβη TemplateInfobox Planet.--> Europa (jʊˈroʊpə; or as 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, completing two orbits of Jupiter for every one orbit completed by Europa, and four orbits for every one completed by Ganymede. In Celestial mechanics, an orbital resonance occurs when two Orbiting bodies exert a regular periodic gravitational influence on each other usually due to their TemplateInfobox Planet.--> Ganymede (ˈgænɨmiːd, or as Greek This resonance helps maintain Io's orbital eccentricity (0. In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape 0041), which in turn provides the primary heating source for its geologic activity (see the "Tidal heating" section for a more detailed explanation of the process). [26] Without this forced eccentricity, Io's orbit would circularize through tidal dissipation, leading to a geologically less active world. Tidal acceleration is an effect of the Tidal forces between an orbiting Natural satellite ( i Like the other Galilean satellites of Jupiter and the Earth's Moon, Io rotates synchronously with its orbital period, keeping one face nearly pointed toward Jupiter. A separate article treats the phenomenon of Tidal resonance in Oceanography.

## Interaction with Jupiter's magnetosphere

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). [40]

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 tonne per second. This article is about the tonne or metric ton For other tons see Ton. [41] This material is mostly composed of ionized and atomic sulfur, oxygen and chlorine; atomic sodium and potassium; molecular sulfur dioxide and sulfur; and sodium chloride dust. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge For sodium chloride in the diet see Salt. Sodium chloride, also known as common salt, table salt, or Halite, is a [41][42] 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. A magnetosphere' is a highly magnetized region around and possessed by an Astronomical object.

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 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. In Physics and Chemistry, plasma is an Ionized Gas, in which a certain proportion of Electrons are free rather than being bound A Hill sphere is roughly the volume around an Astronomical body (such as a Planet) where it dominates in attraction of Satellites to that body rather 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. [41] 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 km/s, compared to the 17 km/s orbital velocity at Io), leading these particles to be ejected in jets leading away from Io. [43]

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. [41] 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), 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. [41] 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. A magnetosphere' is a highly magnetized region around and possessed by an Astronomical object. [44] 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. Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays 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. [45]

During an encounter with Jupiter in 1992, the Ulysses spacecraft detected a stream of dust-sized particles being ejected from the Jupiter system. Ulysses is a robotic Space probe designed to study the Sun at all latitudes [46] 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 μm, and consist primarily of sodium chloride. A micrometre ( American spelling: micrometer; symbol µm) is one millionth of a Metre, or equivalently one thousandth of a Millimetre [47][42] 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. [48]

Jupiter's magnetic field lines, which Io crosses, couples Io to Jupiter's polar upper atmosphere through the generation of an electric current known as the Io flux tube. In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges Faraday's law of induction describes an important basic law of electromagnetism which is involved in the working of Transformers Inductors and many forms of Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. A flux tube is a generally tube-like ( cylindrical) region of space containing a Magnetic field, such that the field at the side surfaces is parallel to them [41] 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. Radio is the transmission of signals by Modulation of electromagnetic waves with frequencies below those of visible Light. [41][18] 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 km (about five percent greater than the Moon's) and a mass of 8. 9319×1022 kg (about 21 percent greater than the Moon's). Among the Galilean satellites, in both mass and volume, Io ranks behind Ganymede and Callisto but ahead of Europa. The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei. TemplateInfobox Planet.--> Ganymede (ˈgænɨmiːd, or as Greek TemplateInfobox Planet.--> Callisto (kəˈlɪstoʊ, or as Greek TemplateInfobox Planet.--> Europa (jʊˈroʊpə; or as

### Interior

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 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. For the Artificial intelligence Androids of the 1990s Science fiction series Space Above and Beyond, see Silicate (AI In Geology, rock is a naturally occurring aggregate of Minerals and/or Mineraloids The Earth's outer solid layer the ‘ Lithosphere Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 Io has a density of 3. 5275 g/cm3, the highest of any moon in the Solar System; significantly higher than the other Galilean satellites and higher than the Earth's moon. The Solar System consists of the Sun and those celestial objects bound to it by Gravity. [49] 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 and mantle and an inner, iron- or iron sulfide–rich core. In Geology, a crust is the outermost solid shell of a planet or moon The mantle is a part of an Astronomical object. The interior of the Earth, similar to the other Terrestrial planets, is Chemically divided The planetary core consists of the innermost layer(s of a Planet. [31] The metallic core makes up approximately 20% of Io's mass. [50] Depending on the amount of sulfur in the core, the core has a radius between 350 and 650 km (220 to 400 mi) if it is composed almost entirely of iron, or between 550 and 900 km (310 to 560 mi) for a core consisting of a mix of iron and sulfur. Galileo's magnetometer failed to detect an internal magnetic field at Io, suggesting that the core is not convecting. A magnetometer is a scientific instrument used to measure the strength and/or direction of the Magnetic field in the vicinity of the instrument Convection in the most general terms refers to the movement of molecules within Fluids (i [51]

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 and LL-chondrite meteorites, with higher iron content (compared to silicon) than the Moon or Earth, but lower than Mars. Forsterite (Mg2SiO4 is the magnesium rich end-member of the Olivine Solid solution series The L type Ordinary chondrites are the second most common type of Meteorite, accounting for approximately 35% of all those catalogued and 40% of the ordinary chondrites The LL chondrites are the least abundant group of the Ordinary chondrites accounting for about 10-11% of observed ordinary-chondrite falls and 8-9% of all meteorite falls (see A meteorite is a natural object originating in Outer space that survives an impact with the Earth 's surface Silicon (ˈsɪlɪkən or /ˈsɪlɪkɒn/ silicium is the Chemical element that has the symbol Si and Atomic number 14 EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 [52][53] 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. [54] The lithosphere of Io, composed of basalt and sulfur deposited by Io's extensive volcanism, is at least 12 km (7 mi) thick, but is likely to be less than 40 km (25 mi) thick. The lithosphere (IPA, from the Greek λίθος for "rocky" + σφαίρα for "sphere" is the solid outermost shell of a rocky Planet. [55][50]

### Tidal heating

Unlike the Earth and the Moon, Io's main source of internal heat comes from tidal dissipation rather than radioactive isotope decay, the result of Io's orbital resonance with Europa and Ganymede. The tidal force is a secondary effect of the Force of Gravity and is responsible for the Tides It arises because the gravitational acceleration experienced Isotopes (Greek isos = "equal" tópos = "site place" are any of the different types of atoms ( Nuclides [26] Such heating is dependent on Io's distance from Jupiter, its orbital eccentricity, the composition of its interior, and its physical state. [54] Its Laplace resonance with Europa and Ganymede maintains Io's eccentricity and prevents tidal dissipation within Io from circularizing its orbit. In Celestial mechanics, an orbital resonance occurs when two Orbiting bodies exert a regular periodic gravitational influence on each other usually due to their 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. [56] The vertical differences in Io's tidal bulge, between the times Io is at periapsis and apoapsis in its orbit, could be as much as 100 m (330 ft). In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from 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. Radioactive decay is the process in which an unstable Atomic nucleus loses energy by emitting ionizing particles and Radiation. [4] This heat is released in the form of volcanic activity, generating its observed high heat flow (global total: 0. In thermal physics, heat transfer is the passage of Thermal energy from a hot to a colder body 6 to 1. 6×1014 W). The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. [54] 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. [54]

Rotating image of Io's surface, the large red ring is around the volcano Pele. Pele is a volcano on the surface of Jupiter 's moon Io. It is named after the Hawaiian volcano goddess Pele.

### Surface

Based on their experience with the ancient surfaces of the Moon, Mars, and Mercury, scientists expected to see numerous impact craters in Voyager 1's first images of Io. In the broadest sense the term impact crater can be applied to any depression natural or manmade resulting from the high velocity impact of a projectile with larger body 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. [23] 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 or to pizza) from various sulfurous compounds. An orange —specifically the sweet orange —is the Citrus fruit Citrus sinensis ( syn Pizza (ˈpiːtsə, in Italian:) is a popular dish made with an Oven -baked flat generally round Bread that is covered with tomatoes or a tomato-based [57] 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. [25]

In addition to volcanoes, Io's surface includes non-volcanic mountains, numerous lakes of molten sulfur, calderas 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. A caldera is a cauldron-like volcanic feature formed by the collapse of land following a volcanic eruption [58]

#### Surface composition

Io's colorful appearance is the result of various materials produced by its extensive volcanism. These materials include silicates (such as orthopyroxene), sulfur, and sulfur dioxide. For the Artificial intelligence Androids of the 1990s Science fiction series Space Above and Beyond, see Silicate (AI The pyroxenes are a group of important rock-forming Silicate minerals found in many Igneous and metamorphic rocks. Sulfur or sulphur (ˈsʌlfɚ see spelling below) is the Chemical element that has the Atomic number 16 [59] 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. [12]

Io's surface map.

Explosive volcanism, often taking the form of umbrella-shaped plumes, paints the surface with sulfurous and silicate materials. This is a sub-article to Io (moon Volcanism on Io, a moon of Jupiter, produces Lava flows volcanic pits and plumes 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 S2, producing a red "fan" deposit, or in extreme cases, large (often reaching beyond 450 km (280 mi) from the central vent) red rings. [60] 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 Cl2SO2. [59] 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 hydrated minerals have been tentatively identified, most notably on the northwest flank of the mountain Gish Bar Mons. Water is a common Chemical substance that is essential for the survival of all known forms of Life. Mineral hydration is an inorganic chemical reaction where water is added to the Crystal structure of a Mineral, usually creating a new mineral usually called a [61] This lack of water is likely due to Jupiter being hot enough early in the evolution of the solar system to drive off volatile materials like water in the vicinity of Io, but not hot enough to do so farther out. The formation and evolution of the Solar System is estimated to have begun In Planetary science, volatiles, are that group of elements and compounds with low boiling points (see volatile) that are associated with a planet's or moon's

#### Volcanism

Main article: Volcanism on Io
Active lava flows in volcanic region Tvashtar Paterae (blank region represents saturated areas in the original data). This is a sub-article to Io (moon Volcanism on Io, a moon of Jupiter, produces Lava flows volcanic pits and plumes This is a list of Paterae (shallow craters on the surface of Io. Tvashtar Paterae compose an active volcanic region of Jupiter 's moon Io located near the moon's north pole 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 flows. In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape Lava is molten rock expelled by a Volcano during an eruption When first expelled from a volcanic vent it is a Liquid at Temperatures 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 (magnesium-rich) compositions. Basalt (bəˈsɔːlt ˈbeisɔːlt ˈbæsɔːlt is a common Extrusive Volcanic rock. Mafic is an adjective describing a Silicate mineral or rock that is rich in magnesium and iron the term was derived by contracting "magnesium" and "ferric" Ultramafic (also referred to as ultrabasic) rocks are igneous and meta -igneous rocks with very low Silica content (less than 45% generally As a by-product of this activity, sulfur, sulfur dioxide gas and silicate pyroclastic material (like ash) are blown up to 500 km (310 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. Pyroclastic rocks or pyroclastics (derived from the Greek πῦρ, meaning fire and κλαστός, meaning broken are Clastic rocks

Io's surface is dotted with volcanic depressions known as paterae. [62] Paterae generally have flat floors bounded by steep walls. These features resemble terrestrial calderas, but it is unknown if they are produced through collapse over an emptied lava chamber like their terrestrial cousins. A caldera is a cauldron-like volcanic feature formed by the collapse of land following a volcanic eruption One hypothesis suggests that these features are produced through the exhumation of volcanic sills, and the overlying material is either blasted out or integrated into the sill. In Geology, a sill is a tabular Pluton that has intruded between older layers of Sedimentary rock, beds of Volcanic [63] Unlike similar features on Earth and Mars, these depressions generally do not lie at the peak of shield volcanoes and are normally larger, with an average diameter of 41 km (25 mi), the largest being Loki Patera at 202 km (126 mi). A shield volcano is a large Volcano with shallow-sloping sides Loki Patera is the largest crater on Jupiter 's moon Io, 202 km in diameter [62] 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. [62] 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. Gish Bar Patera is a patera or a complex crater with scalloped edges on Jupiter 's moon Io. Lava lakes are large volumes of molten Lava, usually Basaltic, contained in a vent Volcanic crater, or broad depression [64][5] Lava lakes on Io either have a continuously overturning lava crust, such as at Pele, or an episodically overturning crust, such as at Loki. [65][66]

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 Kilauea in Hawaii. Kīlauea (kiːlauea is an active Volcano in the Hawaiian Islands, one of five Shield volcanoes that together form the Island of Hawai{{okina}}i Images from the Galileo spacecraft revealed that many of Io's major lava flows, like those at Prometheus and Amirani, are produced by the build-up of small breakouts of lava flows on top of older flows. [67] Larger outbreaks of lava have also been observed on Io. For example, the leading edge of the Prometheus flow moved 75 to 95 km (47 to 59 mi) between Voyager in 1979 and the first Galileo observations in 1996. A major eruption in 1997 produced more than 3,500 km2 (1,350 sq mi) of fresh lava and flooded the floor of the adjacent Pillan Patera. [32]

Five-image sequence of New Horizons images showing Io's volcano Tvashtar spewing material 330 km above its surface. New Horizons is a Robotic spacecraft mission by NASA currently underway

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. Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of This hypothesis is based on temperature measurements of Io's "hotspots", or thermal-emission locations, which suggest temperatures of at least 1,300 K and some as high as 1,600 K. [68] Initial estimates suggesting eruption temperatures approaching 2,000 K[32] have since proven to be overestimates since the wrong thermal models were used to model the temperatures. [68]

The discovery of plumes at the volcanoes Pele and Loki were the first sign that Io is geologically active. [24] Generally, these plumes are formed when volatiles like sulfur and sulfur dioxide are ejected skyward from Io's volcanoes at speeds reaching 1 km/s (0. 6 mps). Additional material that might be found in these volcanic plumes include sodium, potassium, and chlorine. Potassium (pəˈtæsiəm is a Chemical element. It has the symbol K (kalium from qalīy Atomic number 19 and Atomic mass 39 Chlorine (ˈklɔriːn from the Greek word 'χλωρóς' ( khlôros, meaning 'pale green' is the Chemical element with Atomic number 17 and [69][70] These plumes appear to be formed in one of two ways. [71] 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 km (620 mi) in diameter. Examples of this plume type include Pele, Tvashtar, and Dazhbog. Dazhbog Patera is a Volcanic feature on Jupiter's moon Io. Dazhbog has a Diameter of 118 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 km (62 mi) tall, and are among the most long-lived plumes on Io. Examples include Prometheus, Amirani, and Masubi.

#### Mountains

Galileo greyscale image of Tohil Mons, a 5. Io's surface is covered in volcanoes and Mountains The larger ones are named after prominent figures from the mythological travels of Io, or Galileo Galilei (15 February 1564 &ndash 8 January 1642 was a Tuscan ( Italian) Physicist, Mathematician, Astronomer, and Philosopher Tohil Mons is a Mountain on the moon of Io, one of Jupiter 's moons 4 km tall mountain

Io has 100 to 150 mountains. These structures average 6 km (4 mi) in height and reach a maximum of 17. 5 ± 1. 5 km (10. 9 ± 1 mi) at South Boösaule Montes. [6] Mountains often appear as large (the average mountain is 157 km (98 mi) long), isolated structures with no apparent global tectonic patterns outlined, as is the case on Earth. [6] To support the tremendous topography observed at these mountains requires compositions consisting mostly of silicate rock, as opposed to sulfur. [72]

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 faulting. A thrust fault is a type of fault, or break in the Earth's crust with resulting movement of each side against the other in which a lower stratigraphic position is pushed up [73] The compressive stresses leading to mountain formation are the result of subsidence from the continuous burial of volcanic materials. In Geology, Engineering, and Surveying, subsidence is the motion of a surface (usually the Earth's surface as it shifts downward relative to [73] The global distribution of mountains appears to be opposite that of volcanic structures; mountains dominate areas with fewer volcanoes and vice versa. [74] This suggests large-scale regions in Io's lithosphere where compression (supportive of mountain formation) and extension (supportive of patera formation) dominate. [75] Locally, however, mountains and paterae often abut one another, suggesting that magma often exploits faults formed during mountain formation to reach the surface. [62]

Mountains on Io (generally, structures rising above the surrounding plains) have a variety of morphologies. Plateaus are most common. In Geology and Earth science, a plateau, also called a high plateau or tableland, is an area of highland, usually consisting [6] These structures resemble large, flat-topped mesas with rugged surfaces. A mesa ( Spanish, Portuguese and Arabic for "table" is an elevated area of land with a flat top and sides that are usually steep cliffs 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 scarps along one or more margins. In Geomorphology, an escarpment is a transition zone between different physiogeographic provinces that involves a sharp steep Elevation differential characterized Only a handful of mountains on Io appear to have a volcanic origin. These mountains resemble small shield volcanoes, with steep slopes (6–7°) near a small, central caldera and shallow slopes along their margins. A shield volcano is a large Volcano with shallow-sloping sides A caldera is a cauldron-like volcanic feature formed by the collapse of land following a volcanic eruption [76] These volcanic mountains are often smaller than the average mountain on Io, averaging only 1 to 2 km (0. 6 to 1. 2 mi) in height and 40 to 60 km (25 to 37 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. [76]

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. A landslide is a geological phenomenon which includes a wide range of ground movement such as rock falls deep failure of slopes and shallow debris flows which can occur Mass wasting, also known as slope movement, is the geomorphic process by which Soil, Regolith, and rock move downslope under Scalloped margins are common among Io's mesas and plateaus, the result of sulfur dioxide sapping from Io's crust, producing zones of weakness along mountain margins. Groundwater sapping is the geomorphic process in which Groundwater exits a bank or hillslope laterally as seeps and springs and erodes [77]

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 (SO2) with a pressure of a billionth of an atmosphere. An atmosphere (from Greek ατμός - atmos, " Vapor " + σφαίρα - sphaira, " Sphere " The Standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of Pressure (symbol atm [28] 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 retrorockets for a soft landing. A retrorocket (short for retrograde rocket) is a Rocket engine used to provide Thrust opposing the motion of a Spacecraft, thereby causing it Landing is the last part of a Flight, where a flying Animal, Aircraft, or Spacecraft returns to the ground The thin atmosphere also necessitates a rugged lander capable of enduring the strong Jovian radiation, which a thicker atmosphere would attenuate. Radiation, as in Physics, is Energy in the form of waves or moving Subatomic particles emitted by an atom or other body as it changes from a higher energy

The same radiation (in the form of a plasma) strips the atmosphere so that it must be constantly replenished. A magnetosphere' is a highly magnetized region around and possessed by an Astronomical object. [78] The most dramatic source of SO2 is volcanism, but the atmosphere is largely sustained by sunlight-driven sublimation of SO2 frozen on the surface. The atmosphere is largely confined to the equator, where the surface is warmest and most active volcanic plumes reside. [79] 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 SO2 frost is most abundant). [78]

High-resolution images of Io acquired while the satellite is experiencing an eclipse reveal an 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. [80]

## References

1. ^ a b Thomas, P. Jupiter 's extensive system of natural satellites &ndash in particular the four large Galilean moons (Io Europa Ganymede and Callisto &ndash has been a common Jupiter has 62 confirmed moons, giving it the largest retinue of moons with "reasonably secure" orbits of any planet in the Solar System C. ; et al. (1998). "The Shape of Io from Galileo Limb Measurements". Icarus 135 (1): 175–180.
2. ^ Yeomans, Donald K. (2006-Jul-13). Planetary Satellite Physical Parameters. JPL Solar System Dynamics. Retrieved on 2007-11-05. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1499 - Publication of the Catholicon in Treguier ( Brittany)
3. ^ Classic Satellites of the Solar System. Observatorio ARVAL. Retrieved on 2007-09-28. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 48 BC - Pompey the Great is assassinated on orders of King Ptolemy of Egypt after landing in Egypt.
4. ^ a b Rosaly MC Lopes (2007). "Io: The Volcanic Moon", in Lucy-Ann McFadden, Paul R. Weissman, Torrence V. Johnson: Encyclopedia of the Solar System. Academic Press, 419, 423.
5. ^ a b Lopes, R. M. C. ; et al. (2004). "Lava Lakes on Io: Observations of Io's Volcanic Activity from Galileo NIMS During the 2001 Fly-bys". Icarus 169 (1): 140–174.
6. ^ a b c d Schenk, P. ; et al. (2001). "The Mountains of Io: Global and Geological Perspectives from Voyager and Galileo". Journal of Geophysical Research 106 (E12): 33201–33222.
7. ^ Marius, S. (1614). Simon Marius ( Latinized from German Simon Mayr) ( January 10, 1573 &ndash December 26, 1624) was a German "Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici".   (in which he attributes the suggestion to Johannes Kepler)
8. ^ a b Blue, Jennifer (October 16, 2006). Categories for Naming Features on Planets and Satellites. USGS. Retrieved on 2007-06-14. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1276 - While taking exile in Fuzhou in southern China, away from the advancing Mongol invaders, the remnants of the
9. ^ Blue, Jennifer (June 14, 2007). Events 1276 - While taking exile in Fuzhou in southern China, away from the advancing Mongol invaders, the remnants of the Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Io Nomenclature Table of Contents. USGS. Retrieved on 2007-06-14. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1276 - While taking exile in Fuzhou in southern China, away from the advancing Mongol invaders, the remnants of the
10. ^ a b c Cruikshank, D. P. ; and Nelson, R. M. (2007). "A history of the exploration of Io", in Lopes, R. M. C. ; and Spencer, J. R. : Io after Galileo. Springer-Praxis, pp. 5–33. ISBN 3-540-34681-3.
11. ^ O'Connor, J. J. ; Robertson, E. F. (February 1997). Longitude and the Académie Royale. University of St. Andrews. Retrieved on 2007-06-14. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1276 - While taking exile in Fuzhou in southern China, away from the advancing Mongol invaders, the remnants of the
12. ^ a b Barnard, E. E. (1894). Edward Emerson Barnard ( December 16, 1857 – February 6, 1923) was an American Astronomer. "On the Dark Poles and Bright Equatorial Belt of the First Satellite of Jupiter". Monthly Notices of the Royal Astronomical Society 54 (3): 134–136.
13. ^ Dobbins, T. ; and Sheehan, W. (2004). "The Story of Jupiter's Egg Moons". Sky & Telescope 107 (1): 114–120.
14. ^ Barnard, E. E. (1891). Edward Emerson Barnard ( December 16, 1857 – February 6, 1923) was an American Astronomer. "Observations of the Planet Jupiter and his Satellites during 1890 with the 12-inch Equatorial of the Lick Observatory". Monthly Notices of the Royal Astronomical Society 51 (9): 543–556.
15. ^ Minton, R. B. (1973). "The Polar Caps of Io". Communications of the Lunar and Planetary Laboratory 10: 35–39.
16. ^ Lee, T. (1972). "Spectral Albedos of the Galilean Satellites". Communications of the Lunar and Planetary Laboratory 9 (3): 179–180.
17. ^ Fanale, F. P. ; et al. (1974). "Io: A Surface Evaporite Deposit?". Science 186 (4167): pp. 922–925.
18. ^ a b Bigg, E. K. (1964). "Influence of the Satellite Io on Jupiter's Decametric Emission". Nature 203: 1008–1010.
19. ^ a b Fimmel, R. O. ; et al. (1977). First into the Outer Solar System. Pioneer Odyssey. NASA. Retrieved on 2007-06-05. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 70 - Titus and his Roman Legions breach the middle wall of Jerusalem in the Siege of Jerusalem
20. ^ Anderson, J. D. ; et al. (1974). "Gravitational parameters of the Jupiter system from the Doppler tracking of Pioneer 10". Science 183: 322–323.
21. ^ Pioneer 11 Images of Io. Galileo Home Page. Retrieved on 2007-04-21. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 753 BC - Romulus and Remus found Rome ( traditional date)
22. ^ Voyager Mission Description. NASA PDS Rings Node (1997-02-19). Year 1997 ( MCMXCVII) was a Common year starting on Wednesday (link will display full 1997 Gregorian calendar Events 197 - Roman Emperor Septimius Severus defeats usurper Clodius Albinus in the Battle of Lugdunum Retrieved on 2007-04-21. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 753 BC - Romulus and Remus found Rome ( traditional date)
23. ^ a b Smith, B. A. ; et al. (1979). "The Jupiter system through the eyes of Voyager 1". Science 204: 951–972.
24. ^ a b Morabito, L. A. ; et al. (1979). "Discovery of currently active extraterrestrial volcanism". Science 204: 972.
25. ^ a b Strom, R. G. ; et al. (1979). "Volcanic eruption plumes on Io". Nature 280: 733–736.
26. ^ a b c Peale, S. J. ; et al. (1979). "Melting of Io by Tidal Dissipation". Science 203: 892–894.
27. ^ Soderblom, L. A. ; et al. (1980). "Spectrophotometry of Io: Preliminary Voyager 1 results". Geophys. Res. Lett. 7: 963–966.
28. ^ a b Pearl, J. C. ; et al. (1979). "Identification of gaseous SO2 and new upper limits for other gases on Io". Nature 288: 757–758.
29. ^ Broadfoot, A. L. ; et al. (1979). "Extreme ultraviolet observations from Voyager encounter with Jupiter". Science 204: 979–982.
30. ^ Strom, R. G. ; Schneider, N. M. (2007). "Volcanic eruptions on Io", in Morrison, D. : Satellites of Jupiter. University of Arizona Press, pp. 598–633.
31. ^ a b Anderson, J. D. ; et al. (1996). "Galileo Gravity Results and the Internal Structure of Io". Science 272: 709–712.
32. ^ a b c McEwen, A. S. ; et al. (1998). "High-temperature silicate volcanism on Jupiter's moon Io". Science 281: 87–90.
33. ^ a b Perry, J. ; et al. (2007). "A Summary of the Galileo mission and its observations of Io", in Lopes, R. M. C. ; and Spencer, J. R. : Io after Galileo. Springer-Praxis, pp. 35–59. ISBN 3-540-34681-3.
34. ^ Porco, C. C.; et al. Carolyn Porco (born March 6 1953 in New York City) is an American planetary scientist known for (2003). "Cassini imaging of Jupiter's atmosphere, satellites, and rings". Science 299: 1541–1547.
35. ^ Marchis, F. ; et al. (2005). "Keck AO survey of Io global volcanic activity between 2 and 5 µm". Icarus 176: 96–122.
36. ^ Spencer, John (2007-02-23). Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1455 - Traditional date for the publication of the Gutenberg Bible, the first Western Book printed from Movable Here We Go!. Retrieved on 2007-06-03. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 350 - Roman usurper Nepotianus, of the Constantinian dynasty, proclaims himself Roman Emperor, entering
37. ^ A Midnight Plume (2007-03-13). Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1138 - Cardinal Gregorio Conti is elected Antipope as Victor IV, succeeding Anacletus II. Retrieved on 2007-04-21. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 753 BC - Romulus and Remus found Rome ( traditional date)
38. ^ Greeley, R. (2008-03-31). 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 307 - After divorcing his wife Minervina, Constantine marries Fausta, the daughter of the retired Roman Emperor Preliminary Report of the Joint Jupiter SDT. Outer Planets Assessment Group. Retrieved on 2008-04-10. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 879 - Louis III becomes King of the Western Franks. 1407 - the lama
39. ^ Dudzinski, L. A. (2008-03-31). 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 307 - After divorcing his wife Minervina, Constantine marries Fausta, the daughter of the retired Roman Emperor Radioisotope Power for NASA's Space Science Missions. Outer Planets Assessment Group. Retrieved on 2008-04-10. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 879 - Louis III becomes King of the Western Franks. 1407 - the lama
40. ^ Spencer, J. . John Spencer's Astronomical Visualizations. Retrieved on 2007-05-25. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1085 - Alfonso VI of Castile takes Toledo Spain back from the Moors.
41. ^ a b c d e f g Schneider, N. M. ; Bagenal, F. (2007). "Io's neutral clouds, plasma torus, and magnetospheric interactions", in Lopes, R. M. C. ; and Spencer, J. R. : Io after Galileo. Springer-Praxis, pp. 265–286. ISBN 3-540-34681-3.
42. ^ a b Postberg, F. ; et al. (2006). "Composition of jovian dust stream particles". Icarus 183: 122–134.
43. ^ Burger, M. H. ; et al. (1999). "Galileo's close-up view of Io sodium jet". Geophys. Res. Let. 26 (22): 3333–3336.
44. ^ Krimigis, S. M. ; et al. (2002). "A nebula of gases from Io surrounding Jupiter". Nature 415: 994–996. Nature is a prominent Scientific journal, first published on 4 November 1869
45. ^ Medillo, M. ; et al. (2004). "Io's volcanic control of Jupiter's extended neutral clouds". Icarus 170: 430–442.
46. ^ Grün, E. ; et al. (1993). "Discovery of Jovian dust streams and interstellar grains by the ULYSSES spacecraft". Nature 362: 428–430.
47. ^ Zook, H. A. ; et al. (1996). "Solar Wind Magnetic Field Bending of Jovian Dust Trajectories". Science 274 (5292): 1501–1503.
48. ^ Grün, E. ; et al. (1996). "Dust measurements during Galileo's approach to Jupiter and Io encounter". Science 274: 399–401.
49. ^ Schubert, J. et al. (2004). "Interior composition, structure, and dynamics of the Galilean satellites. ", in F. Bagenal et al. : Jupiter: The Planet, Satellites, and Magnetosphere. Cambridge University Press, 281–306.
50. ^ a b Anderson, J. D. ; et al. (2001). "Io's gravity field and interior structure". J. Geophys. Res. 106: 32,963–32,969.
51. ^ Kivelson, M. G. ; et al. (2001). "Magnetized or Unmagnetized: Ambiguity persists following Galileo's encounters with Io in 1999 and 2000". J. Geophys. Res. 106 (A11): 26121–26135.
52. ^ Sohl, F. ; et al. (2002). "Implications from Galileo observations on the interior structure and chemistry of the Galilean satellites". Icarus 157: 104–119.
53. ^ Kuskov, O. L. ; V. A. Kronrod (2001). "Core sizes and internal structure of the Earth's and Jupiter's satellites". Icarus 151: 204–227.
54. ^ a b c d Moore, W. B. et al. (2007). "The Interior of Io. ", in R. M. C. Lopes and J. R. Spencer: Io after Galileo. Springer-Praxis, 89–108.
55. ^ Jaeger, W. L. ; et al. (2003). "Orogenic tectonism on Io". J. Geophys. Res. 108: 12-1. doi:10.1029/2002JE001946. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
56. ^ Yoder, C. F. ; et al. (1979). "How tidal heating in Io drives the Galilean orbital resonance locks". Nature 279: 767–770.
57. ^ Britt, Robert Roy. "Pizza Pie in the Sky: Understanding Io's Riot of Color", Space.com, 16 March 2000. Spacecom is a Space and Astronomy news website Its stories are often syndicated to other media outlets including CNN, MSNBC Events 597 BC - Babylonians capture Jerusalem, replace Jehoiachin with Zedekiah as king 2000 ( MM) was a Leap year that started on Saturday of the Common Era, in accordance with the Gregorian calendar. Retrieved on 2007-07-25. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 285 - Diocletian appoints Maximian as Caesar, co-ruler
58. ^ Staff. "A Volcanic Flashback", Science at NASA, 5 November 1999. Events 1499 - Publication of the Catholicon in Treguier ( Brittany) Year 1999 ( MCMXCIX) was a Common year starting on Friday (link will display full 1999 Gregorian calendar) Retrieved on 2007-06-14. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1276 - While taking exile in Fuzhou in southern China, away from the advancing Mongol invaders, the remnants of the
59. ^ a b Carlson, R. W. ; et al. (2007). "Io's surface composition", in Lopes, R. M. C. ; and Spencer, J. R. : Io after Galileo. Springer-Praxis, pp. 194–229. ISBN 3-540-34681-3.
60. ^ Spencer, J. ; et al. (2000). "Discovery of Gaseous S2 in Io's Pele Plume". Science 288: 1208–1210.
61. ^ Douté, S. ; et al. (2004). "Geology and activity around volcanoes on Io from the analysis of NIMS". Icarus 169: 175–196.
62. ^ a b c d Radebaugh, J. ; et al. (2001). "Paterae on Io: A new type of volcanic caldera?". J. Geophys. Res. 106: 33005–33020.
63. ^ Keszthelyi, L. ; et al. (2004). "A Post-Galileo view of Io's Interior". Icarus 169: 271–286.
64. ^ Perry, J. E. ; et al. (2003). "Gish Bar Patera, Io: Geology and Volcanic Activity, 1997–2001". LPSC XXXIV. Abstract#1720.
65. ^ Radebaugh, J. ; et al. (2004). "Observations and temperatures of Io’s Pele Patera from Cassini and Galileo spacecraft images". Icarus 169: 65–79.
66. ^ Howell, R. R. ; Lopes, R. M. C. (2007). "The nature of the volcanic activity at Loki: Insights from Galileo NIMS and PPR data". Icarus 186: 448–461.
67. ^ Keszthelyi, L. ; et al. (2001). "Imaging of volcanic activity on Jupiter's moon Io by Galileo during the Galileo Europa Mission and the Galileo Millennium Mission". J. Geophys. Res. 106: 33025–33052.
68. ^ a b Keszthelyi, L. ; et al. (2007). "New Estimates for Io Eruption Temperatures: Implications for the Interior". Icarus: 491. doi:10.1016/j.icarus.2007.07.008. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
69. ^ Roesler, F. L. ; et al. (1999). "Far-Ultraviolet Imaging Spectroscopy of Io's Atmosphere with HST/STIS". Science 283 (5400): 353–357. Science is the Academic journal of the American Association for the Advancement of Science and is considered one of the world's most prestigious Scientific
70. ^ Geissler, P. E. ; et al. (1999). "Galileo Imaging of Atmospheric Emissions from Io". Science 285 (5429): 448–461.
71. ^ McEwen, A. S. ; Soderblom, L. A. (1983). "Two classes of volcanic plume on Io". Icarus 58: 197–226.
72. ^ Clow, G. D. ; Carr, M. H. (1980). "Stability of sulfur slopes on Io". Icarus 44: 729–733.
73. ^ a b Schenk, P. M. ; Bulmer, M. H. (1998). "Origin of mountains on Io by thrust faulting and large-scale mass movements". Science 279: 1514–1517.
74. ^ McKinnon, W. B. ; et al. (2001). "Chaos on Io: A model for formation of mountain blocks by crustal heating, melting, and tilting". Geology 29: 103–106.
75. ^ Tackley, P. J. (2001). "Convection in Io's asthenosphere: Redistribution of nonuniform tidal heating by mean flows". J. Geophys. Res. 106: pp. 32971–32981.
76. ^ a b Schenk, P. M. ; et al. (2004). "Shield volcano topography and the rheology of lava flows on Io". Icarus 169: 98–110.
77. ^ Moore, J. M. ; et al. (2001). "Landform degradation and slope processes on Io: The Galileo view". J. Geophys. Res. 106: 33223–33240.
78. ^ a b Lellouch, E. ; et al. (2007). "Io's atmosphere", in Lopes, R. M. C. ; and Spencer, J. R. : Io after Galileo. Springer-Praxis, pp. 231–264. ISBN 3-540-34681-3.
79. ^ Feldman, P. D. ; et al. (1979). "Lyman-α imaging of the SO2 distribution on Io". Geophys. Res. Lett. 27: 1787–1790.
80. ^ Retherford, K. D. ; et al. (2000). "Io's Equatorial Spots: Morphology of Neutral UV Emissions" (PDF). J. Geophys. Res. 105 (A12): 27,157–27,165.