Galileo spacecraft image of Io. The dark spot just left of center is the erupting volcano Prometheus. Whitish plains on either side of it are coated with volcanically emplaced sulfur dioxide frost, while yellower regions are encrusted with a higher proportion of sulfur.
|Discovered by||Galileo Galilei|
|Discovery date||January 8, 1610 |
|Alternative names||Jupiter I|
|Periapsis||420,000 km (0.002 807 AU)|
|Apoapsis||423,400 km (0.002 830 AU)|
|Mean orbit radius||421,700 km (0.002 819 AU)|
|Orbital period||1.769 137 786 d (152 853.504 7 s, 42.459 306 86 h)|
|Average orbital speed||17.334 km/s|
|Inclination||2.21° (to the ecliptic)
0.05° (to Jupiter's equator)
|Dimensions||3,660.0 × 3,637.4 × 3,630.6 km|
|Mean radius||1,821.3 km (0.286 Earths)|
|Surface area||41,910,000 km2 (0.082 Earths)|
|Volume||2.53×1010 km3 (0.023 Earths)|
|Mass||8.9319×1022 kg (0.015 Earths)|
|Mean density||3.528 g/cm3|
|Equatorial surface gravity||1.796 m/s2 (0.183 g)|
|Escape velocity||2.558 km/s|
|Equatorial rotation velocity||271 km/h|
|Albedo||0.63 ± 0.02|
|Apparent magnitude||5.02 (opposition)|
|Composition||90% sulfur dioxide|
Io pron.: // is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of 3,642 kilometres (2,263 mi), the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus.
With over 400 active volcanoes, Io is the most geologically active object in the Solar System. This extreme geologic activity is the result of tidal heating from friction generated within Io's interior as it is pulled between Jupiter and the other Galilean satellites—Europa, Ganymede and Callisto. Several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km (300 mi) above the surface. Io's surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of the moon's silicate crust. Some of these peaks are taller than Earth's Mount Everest. Unlike most satellites in the outer Solar System, which are mostly composed of water 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 the satellite's unique features. Its volcanic plumes and lava flows produce large surface changes and paint the surface in various shades of yellow, red, white, black, and green, largely due to allotropes and compounds of sulfur. Numerous extensive lava flows, several more than 500 km (300 mi) in length, also mark the surface. The materials produced by this volcanism provide material for Io's thin, patchy atmosphere and Jupiter's extensive magnetosphere. Io's volcanic ejecta also produce a large plasma torus around Jupiter.
Io played a significant role in the development of astronomy in the 17th and 18th centuries. It was discovered in 1610 by Galileo Galilei, along with the other Galilean satellites. This discovery furthered the adoption of the Copernican model of the Solar System, the development of Kepler's laws of motion, and the first measurement of the speed of light. From Earth, Io remained nothing more than a point of light until the late 19th and early 20th centuries, when it became possible to resolve its large-scale surface features, such as the dark red polar and bright equatorial regions. In 1979, the two Voyager spacecraft revealed Io to be a geologically active world, with numerous volcanic features, large mountains, and a young surface with no obvious impact craters. The Galileo spacecraft performed several close flybys in the 1990s and early 2000s, obtaining data about Io's interior structure and surface composition. These spacecraft also revealed the relationship between the satellite and Jupiter's magnetosphere and the existence of a belt of radiation centered on Io's orbit. Io receives about 3,600 rem (36 Sv) of radiation per day.
While Simon Marius is not credited with the sole discovery of the Galilean satellites, his names for the moons were adopted. In his 1614 publication Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici, he proposed several possible names for the innermost of the large moons of Jupiter, including The Mercury of Jupiter or The First of the "Jovian Planets". Based on a suggestion from Johannes Kepler in October 1613, he also generated a naming scheme so that each moon was given its own name based on the lovers of the Greek mythological Zeus or his Roman equivalent, Jupiter. In this case, he named the innermost large moon of Jupiter after the Greek mythological figure Io. Marius' names were not widely adopted until centuries later, and in much of the earlier astronomical literature, Io was generally referred to by its Roman numeral designation (a system introduced by Galileo) as "Jupiter I", or as "the first satellite of Jupiter".
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. Since the surface was first seen up close by Voyager 1 the International Astronomical Union has approved 225 names for Io's volcanoes, mountains, plateaus, and large albedo features. The approved feature categories used for Io for different types of volcanic features include patera (volcanic depression), fluctus (lava flow), vallis (lava channel), and active eruptive center (location where plume activity was the first sign of volcanic activity at a particular volcano). Named mountains, plateaus, layered terrain, and shield volcanoes use the terms mons, mensa, planum, tholus, respectively. Named, bright albedo regions use the term regio. Examples of named features include Prometheus, Pan Mensa, Tvashtar Paterae, and Tsũi Goab Fluctus.
Observational history 
The first reported observation of Io was made by Galileo Galilei on January 7, 1610 using a 20x-power, refracting telescope at the University of Padua. However, in that observation, Galileo could not separate Io and Europa due to the low power of his telescope, so the two were recorded as a single point of light. Io and Europa were seen for the first time as separate bodies during Galileo's observations of the Jupiter system the following day, January 8, 1610 (used as the discovery date for Io by the IAU). The discovery of Io and the other Galilean satellites of Jupiter was published in Galileo's Sidereus Nuncius in March 1610. In his Mundus Jovialis, published in 1614, Simon Marius claimed to have discovered Io and the other moons of Jupiter in 1609, one week before Galileo's discovery. Galileo doubted this claim and dismissed the work of Marius as plagiarism. Regardless, Marius' first recorded observation came from December 29, 1609 in the Julian calendar, which equates to January 8, 1610 in the Gregorian calendar, which Galileo used. 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, including early methods to determine longitude, validating Kepler's Third Law of planetary motion, and determining the time required for light to travel between Jupiter and Earth. 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. 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 as distinct objects) large-scale surface features on Io. 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. Later telescopic observations confirmed Io's distinct reddish-brown polar regions and yellow-white equatorial band.
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). The same observations suggested a surface dominated by evaporates composed of sodium salts and sulfur. Radio telescopic observations revealed Io's influence on the Jovian magnetosphere, as demonstrated by decametric wavelength bursts tied to the orbital period of Io.
The first spacecraft to pass by Io were the twin Pioneer 10 and 11 probes on December 3, 1973 and December 2, 1974 respectively. 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. 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. Close-up images were planned during Pioneer 10's encounter with Io, but those observations were lost because of the high-radiation environment.
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. Voyager 1 flew past the satellite on March 5, 1979 from a distance of 20,600 km (12,800 mi). The images returned during the approach revealed a strange, multi-colored landscape devoid of impact craters. 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. Analysis of other Voyager 1 images showed nine such plumes scattered across the surface, proving that Io was volcanically active. 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). 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).
Voyager 2 passed Io on July 9, 1979 at a distance of 1,130,000 km (702,000 mi). 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 four months between the encounters. In addition, observations of Io as a crescent as Voyager 2 departed the Jovian system revealed that seven of the nine plumes observed in March were still active in July 1979, with only the volcano Pele shutting down between flybys.
The Galileo spacecraft arrived at Jupiter in 1995 after a six-year journey from Earth to follow up on the discoveries of the two Voyager probes and ground-based observations taken in the intervening years. Io's location within one of Jupiter's most intense radiation belts precluded a prolonged close flyby, but Galileo did pass close by shortly before entering orbit for its two-year, primary mission studying the Jovian system. While no images were taken during the close flyby on December 7, 1995, the encounter did yield significant results, such as the discovery of a large iron core, similar to that found in the rocky planets of the inner Solar System.
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 role similar to water and carbon dioxide on Earth. 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.
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. 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. These observations revealed a new plume at Tvashtar Paterae and provided insights into Io's aurorae.
Subsequent observations 
Following Galileo's deliberate 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. This imaging has allowed scientists to monitor volcanic activity on Io, even without a spacecraft in the Jupiter system.
The New Horizons spacecraft, en route to Pluto and the Kuiper belt, flew by the Jupiter system and Io on February 28, 2007. During the encounter, numerous distant observations of Io were obtained. These included images of a large plume at Tvashtar, providing the first detailed observations of the largest class of Ionian volcanic plume since observations of Pele's plume in 1979. 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.
There are currently two forthcoming missions planned for the Jupiter system. Juno, launched on August 5, 2011, has limited imaging capabilities, but it could provide monitoring of Io's volcanic activity using its near-infrared spectrometer, JIRAM. The Jupiter Icy Moon Explorer (JUICE) is an European Space Agency mission to the Jupiter system that will end up in Ganymede orbit. JUICE has a launch scheduled for 2022 with an arrival at Jupiter planned for January 2030. JUICE will not flyby Io, but it will use its various instruments, such as a narrow-angle camera, to monitor Io's volcanic activity and measure its surface composition during the two-year, Jupiter-tour phase of the mission prior to Ganymede Orbit Insertion. The Io Volcano Observer was a proposal for a Discovery-class mission that would launch in 2015. It involved multiple flybys of Io while in orbit around Jupiter; however, this mission was not selected for Phase A study by NASA, and remains a mission concept.
Orbit and rotation 
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. Including Jupiter's inner satellites, Io is the fifth moon out from Jupiter. It takes 42.5 hours to complete one orbit (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. This resonance helps maintain Io's orbital eccentricity (0.0041), which in turn provides the primary heating source for its geologic activity (see the "Tidal heating" section for a more detailed explanation of the process). Without this forced eccentricity, Io's orbit would circularize through tidal dissipation, leading to a geologically less active world.
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. This synchronicity provides the definition for Io's longitude system. Io's prime meridian intersects the north and south poles, and the equator at the sub-Jovian point. The side of Io that always faces Jupiter is known as the subjovian hemisphere, while the side that always faces away is known as the antijovian hemisphere. The side of Io that always faces in the direction that the moon travels in its orbit is known as the leading hemisphere, while the side that always faces in the opposite direction is known as the trailing hemisphere.
Interaction with Jupiter's magnetosphere 
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 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. These materials ultimately have their origin from Io's volcanic activity, but the material that escapes to Jupiter's magnetic field and into interplanetary space comes directly from Io's atmosphere. These materials, depending on their ionized state and composition, ultimately end up in various neutral (non-ionized) clouds and radiation belts in Jupiter's magnetosphere and, in some cases, are eventually ejected from the Jovian system.
Surrounding Io (up to a distance of 6 Io radii from the moon's surface) is a cloud of neutral sulfur, oxygen, sodium, and potassium atoms. These particles originate in Io's upper atmosphere but are excited from collisions with ions in the plasma torus (discussed below) and other processes into filling Io's Hill sphere, which is the region where the moon's gravity is predominant over Jupiter. Some of this material escapes Io's gravitational pull and goes into orbit around Jupiter. Over a 20-hour period, these particles spread out from Io to form a banana-shaped, neutral cloud that can reach as far as 6 Jovian radii from Io, either inside Io's orbit and ahead of the satellite or outside Io's orbit and behind the satellite. 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.
Io orbits within a belt of intense radiation known as the Io plasma torus. The plasma in this doughnut-shaped ring of ionized sulfur, oxygen, sodium, and chlorine originates when neutral atoms in the "cloud" surrounding Io are ionized and carried along by the Jovian magnetosphere. 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. 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. Particles from Io, detected as variations in magnetospheric plasma, have been detected far into the long magnetotail by New Horizons. To study similar variations within the plasma torus, researchers measure the ultraviolet-wavelength light it emits. While such variations have not been definitively linked to variations in Io's volcanic activity (the ultimate source for material in the plasma torus), this link has been established in the neutral sodium cloud.
During an encounter with Jupiter in 1992, the Ulysses spacecraft detected a stream of dust-sized particles being ejected from the Jupiter system. The dust in these discrete streams travel away from Jupiter at speeds upwards of several hundred kilometres per second, have an average size of 10 μm, and consist primarily of sodium chloride. 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.
Jupiter's magnetic field lines, which Io crosses, couples Io's atmosphere and neutral cloud to Jupiter's polar upper atmosphere through the generation of an electric current known as the Io flux tube. 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. The Juno mission, planned for the next decade, should help to shed light on these processes. The Jovian magnetic field lines that do get past Io's ionosphere also induce an electric current, which in turn creates an induced magnetic field, within Io's interior. Io's induced magnetic field is thought to be generated within a partially molten, silicate magma ocean 50 kilometers beneath the moon's surface. Similar induced fields were found at the other Galilean satellites by Galileo, generated within liquid water oceans in the interiors of those moons.
Io is slightly larger than Earth's Moon. It has a mean radius of 1,821.3 km (1,131.7 mi) (about five percent greater than the Moon's) and a mass of 8.9319×1022 kg (about 21 percent greater than the Moon's). It is a slight ellipsoid in shape, with its longest axis directed toward Jupiter. Among the Galilean satellites, in both mass and volume, Io ranks behind Ganymede and Callisto but ahead of Europa.
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. 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. 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. The metallic core makes up approximately 20% of Io's mass. Depending on the amount of sulfur in the core, the core has a radius between 350 and 650 km (220 and 400 mi) if it is composed almost entirely of iron, or between 550 and 900 km (340 and 560 mi) for a core consisting of a mix of iron and sulfur. Galileo's magnetometer failed to detect an internal, intrinsic magnetic field at Io, suggesting that the core is not convecting.
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. 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. However, re-analysis of Galileo magnetometer data in 2009 revealed the presence of an induced magnetic field at Io, requiring a magma ocean 50 km (31 mi) below the surface. Further analysis published in 2011 provided direct evidence of such an ocean. This layer is estimated to be 50 km thick and makes up approximatively 10% of Io's mantle. Temperatures in the magma ocean reach an estimated 1,200 degrees Celsius. It is not known if the 10-20% partial melting percentage for Io's mantle is consistent with the requirement for a significant amount of molten silicates in this possible magma ocean. 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.
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. Such heating is dependent on Io's distance from Jupiter, its orbital eccentricity, the composition of its interior, and its physical state. Its Laplace resonance with Europa and Ganymede maintains Io's eccentricity and prevents tidal dissipation within Io from circularizing its orbit. The resonant orbit also helps to maintain Io's distance from Jupiter; otherwise tides raised on Jupiter would cause Io to slowly spiral outward from its parent planet. 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). 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. This heat is released in the form of volcanic activity, generating its observed high heat flow (global total: 0.6 to 1.6×1014 W). Models of its orbit suggest that the amount of tidal heating within Io changes with time, however the current amount of tidal dissipation is consistent with the observed heat flow. Models of tidal heating and convection have not found consistent planetary viscosity profiles which simultaneously match tidal energy dissipation and mantle convection of heat to the 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. 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. 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. 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.
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. 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 cyclic 8-chain sulfur. This radiation damage produces Io's red-brown polar regions.
Explosive volcanism, often taking the form of umbrella-shaped plumes, paints the surface with sulfurous and silicate materials. Plume deposits on Io are often colored red or white depending on the amount of sulfur and sulfur dioxide in the plume. Generally, plumes formed at volcanic vents from degassing lava contain a greater amount of S2, producing a red "fan" deposit, or in extreme cases, large (often reaching beyond 450 km or 280 mi from the central vent) red rings. 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. 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. 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 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 centres and extensive lava flows. During a major eruption, lava flows tens or even hundreds of kilometres long can be produced, consisting mostly of basalt silicate lavas with either mafic or ultramafic (magnesium-rich) compositions. As a by-product of this activity, sulfur, sulfur dioxide gas and silicate pyroclastic material (like ash) are blown up to 200 km (120 mi) into space, producing large, umbrella-shaped plumes, painting the surrounding terrain in red, black, and white, and providing material for Io's patchy atmosphere and Jupiter's extensive magnetosphere.
Io's surface is dotted with volcanic depressions known as paterae. 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. 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. 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). 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. 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. Lava lakes on Io either have a continuously overturning lava crust, such as at Pele, or an episodically overturning crust, such as at Loki.
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. 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. 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 (1,400 sq mi) of fresh lava and flooded the floor of the adjacent Pillan Patera.
Analysis of the Voyager images led scientists to believe that these flows were composed mostly of various compounds of molten sulfur. However, subsequent Earth-based infrared studies and measurements from the Galileo spacecraft indicate that these flows are composed of basaltic lava with mafic to ultramafic compositions. This hypothesis is based on temperature measurements of Io's "hotspots", or thermal-emission locations, which suggest temperatures of at least 1300 K and some as high as 1600 K. Initial estimates suggesting eruption temperatures approaching 2000 K have since proven to be overestimates since the wrong thermal models were used to model the temperatures.
The discovery of plumes at the volcanoes Pele and Loki were the first sign that Io is geologically active. 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), creating umbrella-shaped clouds of gas and dust. Additional material that might be found in these volcanic plumes include sodium, potassium, and chlorine. These plumes appear to be formed in one of two ways. Io's largest plumes are created when dissolved sulfur and sulfur dioxide gas are released from erupting magma at volcanic vents or lava lakes, often dragging silicate pyroclastic material with them. 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. 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.
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 ± 0.9 mi) at South Boösaule Montes. Mountains often appear as large (the average mountain is 157 km or 98 mi long), isolated structures with no apparent global tectonic patterns outlined, as is the case on Earth. To support the tremendous topography observed at these mountains requires compositions consisting mostly of silicate rock, as opposed to sulfur.
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. The compressive stresses leading to mountain formation are the result of subsidence from the continuous burial of volcanic materials. The global distribution of mountains appears to be opposite that of volcanic structures; mountains dominate areas with fewer volcanoes and vice versa. This suggests large-scale regions in Io's lithosphere where compression (supportive of mountain formation) and extension (supportive of patera formation) dominate. Locally, however, mountains and paterae often abut one another, suggesting that magma often exploits faults formed during mountain formation to reach the surface.
Mountains on Io (generally, structures rising above the surrounding plains) have a variety of morphologies. Plateaus are most common. These structures resemble large, flat-topped mesas with rugged surfaces. Other mountains appear to be tilted crustal blocks, with a shallow slope from the formerly flat surface and a steep slope consisting of formerly sub-surface materials uplifted by compressive stresses. Both types of mountains often have steep scarps along one or more margins. 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. 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.
Nearly all mountains appear to be in some stage of degradation. Large landslide deposits are common at the base of Ionian mountains, suggesting that mass wasting is the primary form of degradation. Scalloped margins are common among Io's mesas and plateaus, the result of sulfur dioxide sapping from Io's crust, producing zones of weakness along mountain margins.
Io has an extremely thin atmosphere consisting mainly of sulfur dioxide (SO2), with minor constituents including sulfur monoxide (SO), sodium chloride (NaCl), and atomic sulfur and oxygen. The atmosphere has significant variations in density and temperature with time of day, latitude, volcanic activity, and surface frost abundance. The maximum atmospheric pressure on Io ranges from 3.3 × 10−5 to 3 × 10−4 pascals (Pa) or 0.3 to 3 nbar, spatially seen on Io's anti-Jupiter hemisphere and along the equator, and temporally in the early afternoon when the temperature of surface frost peaks. Localized peaks at volcanic plumes have also been seen, with pressures of 5 × 10−4 to 40 × 10−4 Pa (5 to 40 nbar). Io's atmospheric pressure is lowest on the moon's night-side, where the pressure dips to 0.1 × 10−7 to 1 × 10−7 Pa (0.0001 to 0.001 nbar). Io's atmospheric temperature ranges from the temperature of the surface at low altitudes, where sulfur dioxide is in vapor pressure equilibrium with frost on the surface, to 1800 K at higher altitudes where the thinner atmospheric density permits heating from plasma in the Io plasma torus and from Joule heating from the Io flux tube. The low pressure limits the atmosphere's effect on the surface, except for temporarily redistributing sulfur dioxide from frost-rich to frost-poor areas, and to expand the size of plume deposit rings when plume material re-enters the thicker dayside atmosphere. The thin Ionian atmosphere also 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. The thin atmosphere also necessitates a rugged lander capable of enduring the strong Jovian radiation, which a thicker atmosphere would attenuate.
Gas in Io's atmosphere is stripped by Jupiter's magnetosphere, escaping to either the neutral cloud that surrounds Io, or the Io plasma torus, a ring of ionized particles that shares Io's orbit but co-rotates with the magnetosphere of Jupiter. Approximately one ton of material is removed from the atmosphere every second through this process so that it must be constantly replenished. The most dramatic source of SO2 are volcanic plumes, which pump 104 kg of sulfur dioxide per second into Io's atmosphere on average, though most of this condenses back onto the surface. Much of the sulfur dioxide in Io's atmosphere sustained by sunlight-driven sublimation of SO2 frozen on the surface. The day-side atmosphere is largely confined to within 40° of the equator, where the surface is warmest and most active volcanic plumes reside. A sublimation-driven atmosphere is also consistent with observations that Io's atmosphere is densest over the anti-Jupiter hemisphere, where SO2 frost is most abundant, and is densest when Io is closer to the Sun. However, some contributions from volcanic plumes are required as the highest observed densities have been seen near volcanic vents. Because the density of sulfur dioxide in the atmosphere is tied directly to surface temperature, Io's atmosphere partially collapses at night or when the satellite is in the shadow of Jupiter. The collapse during eclipse is limited somewhat by the formation of a diffusion layer of sulfur monoxide in the lowest portion of the atmosphere, but the atmosphere pressure of Io's nightside atmosphere is two to four orders of magnitude less than at its peak just past noon. The minor constituents of Io's atmosphere, such as NaCl, SO, O, and S derive either from: direct volcanic outgassing; photodissociation, or chemical breakdown caused by solar ultraviolet radiation, from SO2; or the sputtering of surface deposits by charged particles from Jupiter's magnetosphere.
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, though in this case the charged particles come from Jupiter's magnetic field rather than the solar wind. Aurorae usually occur near the magnetic poles of planets, but Io's are brightest near its equator. Io lacks an intrinsic magnetic field of its own; therefore, electrons traveling along Jupiter's magnetic field near Io directly impact 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. Fainter aurora from oxygen atoms along the limb of Io (the red glows in the image at right), and sodium atoms on Io's night-side (the green glows in the same image) have also been observed.
In fiction 
- Blue, Jennifer (November 9, 2009). "Planet and Satellite Names and Discoverers". USGS. Retrieved 2010-01-13.
- Thomas, P. C.; et al. (1998). "The Shape of Io from Galileo Limb Measurements". Icarus 135 (1): 175–180. Bibcode:1998Icar..135..175T. doi:10.1006/icar.1998.5987.
- Yeomans, Donald K. (July 13, 2006). "Planetary Satellite Physical Parameters". JPL Solar System Dynamics. Retrieved 2007-11-05.
- "Classic Satellites of the Solar System". Observatorio ARVAL. Retrieved 2007-09-28.
- Rathbun, J. A.; Spencer, J.R.; Tamppari, L.K.; Martin, T.Z.; Barnard, L.; Travis, L.D. (2004). "Mapping of Io's thermal radiation by the Galileo photopolarimeter-radiometer (PPR) instrument". Icarus 169 (1): 127–139. Bibcode:2004Icar..169..127R. doi:10.1016/j.icarus.2003.12.021.
- In US dictionary transcription, ī′·ō, or as Greek: Ἰώ
- Rosaly MC Lopes (2006). "Io: The Volcanic Moon". In Lucy-Ann McFadden, Paul R. Weissman, Torrence V. Johnson. Encyclopedia of the Solar System. Academic Press. pp. 419–431. ISBN 978-0-12-088589-3.
- 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. Bibcode:2004Icar..169..140L. doi:10.1016/j.icarus.2003.11.013.
- 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. Bibcode:2001JGR...10633201S. doi:10.1029/2000JE001408.
- Marius, S. (1614). Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici [The World of Jupiter discovered in the year 1609 by Means of a Belgian spy-glass].
- Marius, S. (1614). Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici. (in which he attributes the suggestion to Johannes Kepler)
- "Io: Overview". NASA. Retrieved March 5, 2012.
- Barnard, E. E. (1894). "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. Bibcode:1894MNRAS..54..134B.
- Barnard, E. E. (1891). "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. Bibcode:1891MNRAS..51..543B.
- Blue, Jennifer (October 16, 2006). "Categories for Naming Features on Planets and Satellites". USGS. Retrieved 2007-06-14.
- Blue, Jennifer (June 14, 2007). "Io Nomenclature Table of Contents". USGS. Retrieved 2007-06-14.
- 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.
- Van Helden, Albert (14). "The Galileo Project / Science / Simon Marius". Rice University. Retrieved 2010-01-07.
- Baalke, Ron. "Discovery of the Galilean Satellites". Jet Propulsion Laboratory. Retrieved 2010-01-07.
- O'Connor, J. J.; Robertson, E. F. (February 1997). "Longitude and the Académie Royale". University of St. Andrews. Retrieved 2007-06-14.
- Dobbins, T.; and Sheehan, W. (2004). "The Story of Jupiter's Egg Moons". Sky & Telescope 107 (1): 114–120.
- Minton, R. B. (1973). "The Red Polar Caps of Io". Communications of the Lunar and Planetary Laboratory 10: 35–39. Bibcode:1973CoLPL..10...35M.
- Lee, T. (1972). "Spectral Albedos of the Galilean Satellites". Communications of the Lunar and Planetary Laboratory 9 (3): 179–180. Bibcode:1972CoLPL...9..179L.
- Fanale, F. P.; et al. (1974). "Io: A Surface Evaporite Deposit?". Science 186 (4167): 922–925. Bibcode:1974Sci...186..922F. doi:10.1126/science.186.4167.922. PMID 17730914.
- Bigg, E. K. (1964). "Influence of the Satellite Io on Jupiter's Decametric Emission". Nature 203 (4949): 1008–1010. Bibcode:1964Natur.203.1008B. doi:10.1038/2031008a0.
- Fimmel, R. O.; et al. (1977). "First into the Outer Solar System". Pioneer Odyssey. NASA. Retrieved 2007-06-05.
- Anderson, J. D.; et al. (1974). "Gravitational parameters of the Jupiter system from the Doppler tracking of Pioneer 10". Science 183 (4122): 322–323. Bibcode:1974Sci...183..322A. doi:10.1126/science.183.4122.322. PMID 17821098.
- "Pioneer 11 Images of Io". Galileo Home Page. Retrieved 2007-04-21.
- "Voyager Mission Description". NASA PDS Rings Node. 1997-02-19. Retrieved 2007-04-21.
- Smith, B. A.; et al. (1979). "The Jupiter system through the eyes of Voyager 1". Science 204 (4396): 951–972. Bibcode:1979Sci...204..951S. doi:10.1126/science.204.4396.951. PMID 17800430.
- The Milwaukee Sentinel, Pasadena, Calif.--UPI, Jupiter moon shows color, erosion signs, Mar. 6, 1979, page 2.
- Morabito, L. A.; et al. (1979). "Discovery of currently active extraterrestrial volcanism". Science 204 (4396): 972. Bibcode:1979Sci...204..972M. doi:10.1126/science.204.4396.972. PMID 17800432.
- Strom, R. G.; et al. (1979). "Volcanic eruption plumes on Io". Nature 280 (5725): 733–736. Bibcode:1979Natur.280..733S. doi:10.1038/280733a0.
- Peale, S. J.; et al. (1979). "Melting of Io by Tidal Dissipation". Science 203 (4383): 892–894. Bibcode:1979Sci...203..892P. doi:10.1126/science.203.4383.892. PMID 17771724.
- Soderblom, L. A.; et al. (1980). "Spectrophotometry of Io: Preliminary Voyager 1 results". Geophys. Res. Lett. 7 (11): 963–966. Bibcode:1980GeoRL...7..963S. doi:10.1029/GL007i011p00963.
- Pearl, J. C.; et al. (1979). "Identification of gaseous SO2 and new upper limits for other gases on Io". Nature 288 (5725): 757–758. Bibcode:1979Natur.280..755P. doi:10.1038/280755a0.
- Broadfoot, A. L.; et al. (1979). "Extreme ultraviolet observations from Voyager 1 encounter with Jupiter". Science 204 (4396): 979–982. Bibcode:1979Sci...204..979B. doi:10.1126/science.204.4396.979. PMID 17800434.
- Strom, R. G.; Schneider, N. M. (1982). "Volcanic eruptions on Io". In Morrison, D. Satellites of Jupiter. University of Arizona Press. pp. 598–633. ISBN 0-8165-0762-7.
- Anderson, J. D.; et al. (1996). "Galileo Gravity Results and the Internal Structure of Io". Science 272 (5262): 709–712. Bibcode:1996Sci...272..709A. doi:10.1126/science.272.5262.709. PMID 8662566.
- McEwen, A. S.; et al. (1998). "High-temperature silicate volcanism on Jupiter's moon Io". Science 281 (5373): 87–90. Bibcode:1998Sci...281...87M. doi:10.1126/science.281.5373.87. PMID 9651251.
- 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.
- Porco, C. C.; et al. (2003). "Cassini imaging of Jupiter's atmosphere, satellites, and rings". Science 299 (5612): 1541–1547. Bibcode:2003Sci...299.1541P. doi:10.1126/science.1079462. PMID 12624258.
- Marchis, F.; et al. (2005). "Keck AO survey of Io global volcanic activity between 2 and 5 μm". Icarus 176 (1): 96–122. Bibcode:2005Icar..176...96M. doi:10.1016/j.icarus.2004.12.014.
- Spencer, John (2007-02-23). "Here We Go!". Retrieved 2007-06-03.
- Spencer, J. R.; et al. (2007). "Io Volcanism Seen by New Horizons: A Major Eruption of the Tvashtar Volcano". Science 318 (5848): 240–243. Bibcode:2007Sci...318..240S. doi:10.1126/science.1147621. PMID 17932290.
- Jonathan Amos (2 May 2012). "Esa selects 1bn-euro Juice probe to Jupiter". BBC News. Retrieved 2 May 2012.
- JUICE assessment study report (Yellow Book), ESA, 2012, retrieved 2 May 2012
- McEwen, A. S.; the IVO Team (2008). "Io Volcano Observer (IVO)" (PDF). Io Workshop 2008. Berkeley, California.
- Lopes, R. M. C.; D. A. Williams (2005). "Io after Galileo". Reports on Progress in Physics 68 (2): 303–340. Bibcode:2005RPPh...68..303L. doi:10.1088/0034-4885/68/2/R02.
- Spencer, J. "John Spencer's Astronomical Visualizations". Retrieved 2007-05-25.
- 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.
- Postberg, F.; et al. (2006). "Composition of jovian dust stream particles". Icarus 183 (1): 122–134. Bibcode:2006Icar..183..122P. doi:10.1016/j.icarus.2006.02.001.
- Burger, M. H.; et al. (1999). "Galileo's close-up view of Io sodium jet". Geophys. Res. Let. 26 (22): 3333–3336. Bibcode:1999GeoRL..26.3333B. doi:10.1029/1999GL003654.
- Krimigis, S. M.; et al. (2002). "A nebula of gases from Io surrounding Jupiter". Nature 415 (6875): 994–996. doi:10.1038/415994a. PMID 11875559.
- Medillo, M.; et al. (2004). "Io's volcanic control of Jupiter's extended neutral clouds". Icarus 170 (2): 430–442. Bibcode:2004Icar..170..430M. doi:10.1016/j.icarus.2004.03.009.
- Grün, E.; et al. (1993). "Discovery of Jovian dust streams and interstellar grains by the ULYSSES spacecraft". Nature 362 (6419): 428–430. Bibcode:1993Natur.362..428G. doi:10.1038/362428a0.
- Zook, H. A.; et al. (1996). "Solar Wind Magnetic Field Bending of Jovian Dust Trajectories". Science 274 (5292): 1501–1503. Bibcode:1996Sci...274.1501Z. doi:10.1126/science.274.5292.1501. PMID 8929405.
- Grün, E.; et al. (1996). "Dust Measurements During Galileo's Approach to Jupiter and Io Encounter". Science 274 (5286): 399–401. Bibcode:1996Sci...274..399G. doi:10.1126/science.274.5286.399.
- Kerr, R. A. (2010). "Magnetics Point to Magma 'Ocean' at Io". Science 327 (5964): 408–409. doi:10.1126/science.327.5964.408-b. PMID 20093451.
- 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. pp. 281–306. ISBN 978-0-521-81808-7.
- Anderson, J. D.; et al. (2001). "Io's gravity field and interior structure". J. Geophys. Res. 106 (E12): 32963–32969. Bibcode:2001JGR...10632963A. doi:10.1029/2000JE001367.
- 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. Bibcode:2001JGR...10626121K. doi:10.1029/2000JA002510.
- Sohl, F.; et al. (2002). "Implications from Galileo observations on the interior structure and chemistry of the Galilean satellites". Icarus 157 (1): 104–119. Bibcode:2002Icar..157..104S. doi:10.1006/icar.2002.6828.
- Kuskov, O. L.; V. A. Kronrod (2001). "Core sizes and internal structure of the Earth's and Jupiter's satellites". Icarus 151 (2): 204–227. Bibcode:2001Icar..151..204K. doi:10.1006/icar.2001.6611.
- Moore, W. B. et al. (2007). "The Interior of Io.". In R. M. C. Lopes and J. R. Spencer. Io after Galileo. Springer-Praxis. pp. 89–108. ISBN 3-540-34681-3.
- "NASA's Galileo Reveals Magma 'Ocean' Beneath Surface of Jupiter's Moon". Science Daily. May 12, 2011.
- Perry, J. (21 January 2010). "Science: Io's Induced Magnetic Field and Mushy Magma Ocean". The Gish Bar Times. Retrieved 2010-01-22.
- Jaeger, W. L.; et al. (2003). "Orogenic tectonism on Io". J. Geophys. Res. 108 (E8): 12–1. Bibcode:2003JGRE..108.5093J. doi:10.1029/2002JE001946.
- Yoder, C. F.; et al. (1979). "How tidal heating in Io drives the Galilean orbital resonance locks". Nature 279 (5716): 767–770. Bibcode:1979Natur.279..767Y. doi:10.1038/279767a0.
- Lainey, V.; et al. (2009). "Strong tidal dissipation in Io and Jupiter from astrometric observations". Nature 459: 957–959. doi:10.1038/nature08108.
- Moore, W. B. (August 2003). "Tidal heating and convection in Io". Journal of Geophysical Research 108 (E8): 5096. doi:10.1029/2002JE001943.
- Britt, Robert Roy (March 16, 2000). "Pizza Pie in the Sky: Understanding Io's Riot of Color". Space.com. Archived from the original on August 18, 2000.
- 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.
- Spencer, J.; et al. (2000). "Discovery of Gaseous S2 in Io's Pele Plume". Science 288 (5469): 1208–1210. Bibcode:2000Sci...288.1208S. doi:10.1126/science.288.5469.1208. PMID 10817990.
- Douté, S.; et al. (2004). "Geology and activity around volcanoes on Io from the analysis of NIMS". Icarus 169 (1): 175–196. Bibcode:2004Icar..169..175D. doi:10.1016/j.icarus.2004.02.001.
- Radebaugh, D.; et al. (2001). "Paterae on Io: A new type of volcanic caldera?". J. Geophys. Res. 106 (E12): 33005–33020. Bibcode:2001JGR...10633005R. doi:10.1029/2000JE001406.
- Keszthelyi, L.; et al. (2004). "A Post-Galileo view of Io's Interior". Icarus 169 (1): 271–286. Bibcode:2004Icar..169..271K. doi:10.1016/j.icarus.2004.01.005.
- Perry, J. E.; et al. (2003). "Gish Bar Patera, Io: Geology and Volcanic Activity, 1997–2001" (PDF). LPSC XXXIV. Clear Lake City (Greater Houston). Abstract #1720.
- Radebaugh, J.; et al. (2004). "Observations and temperatures of Io's Pele Patera from Cassini and Galileo spacecraft images". Icarus 169 (1): 65–79. Bibcode:2004Icar..169...65R. doi:10.1016/j.icarus.2003.10.019.
- 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 (2): 448–461. Bibcode:2007Icar..186..448H. doi:10.1016/j.icarus.2006.09.022.
- 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 (E12): 33025–33052. Bibcode:2001JGR...10633025K. doi:10.1029/2000JE001383.
- Keszthelyi, L.; et al. (2007). "New estimates for Io eruption temperatures: Implications for the interior". Icarus 192 (2): 491–502. Bibcode:2007Icar..192..491K. doi:10.1016/j.icarus.2007.07.008.
- Roesler, F. L.; et al. (1999). "Far-Ultraviolet Imaging Spectroscopy of Io's Atmosphere with HST/STIS". Science (fee requiredBibcode:1999Sci...283..353R. doi:10.1126/science.283.5400.353. PMID 9888844.) 283 (5400): 353–357.
- Geissler, P. E.; et al. (1999). "Galileo Imaging of Atmospheric Emissions from Io". Science (fee requiredBibcode:1999Sci...285..870G. doi:10.1126/science.285.5429.870. PMID 10436151.) 285 (5429): 870–4.
- McEwen, A. S.; Soderblom, L. A. (1983). "Two classes of volcanic plume on Io". Icarus 55 (2): 197–226. Bibcode:1983Icar...55..191M. doi:10.1016/0019-1035(83)90075-1.
- Clow, G. D.; Carr, M. H. (1980). "Stability of sulfur slopes on Io". Icarus 44 (2): 268–279. Bibcode:1980Icar...44..268C. doi:10.1016/0019-1035(80)90022-6.
- Schenk, P. M.; Bulmer, M. H. (1998). "Origin of mountains on Io by thrust faulting and large-scale mass movements". Science 279 (5356): 1514–1517. Bibcode:1998Sci...279.1514S. doi:10.1126/science.279.5356.1514. PMID 9488645.
- McKinnon, W. B.; et al. (2001). "Chaos on Io: A model for formation of mountain blocks by crustal heating, melting, and tilting". Geology 29 (2): 103–106. Bibcode:2001Geo....29..103M. doi:10.1130/0091-7613(2001)029<0103:COIAMF>2.0.CO;2.
- Tackley, P. J. (2001). "Convection in Io's asthenosphere: Redistribution of nonuniform tidal heating by mean flows". J. Geophys. Res. 106 (E12): 32971–32981. Bibcode:2001JGR...10632971T. doi:10.1029/2000JE001411.
- Schenk, P. M.; Wilson, R. R.; Davies, A. G. (2004). "Shield volcano topography and the rheology of lava flows on Io". Icarus 169 (1): 98–110. Bibcode:2004Icar..169...98S. doi:10.1016/j.icarus.2004.01.015.
- Moore, J. M.; et al. (2001). "Landform degradation and slope processes on Io: The Galileo view". J. Geophys. Res. 106 (E12): 33223–33240. Bibcode:2001JGR...10633223M. doi:10.1029/2000JE001375.
- 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.
- Walker, A. C.; et al. (2010). "A Comprehensive Numerical Simulation of Io's Sublimation-Driven Atmosphere". Icarus. in. press (1): 409. Bibcode:2010Icar..207..409W. doi:10.1016/j.icarus.2010.01.012.
- Spencer, A. C.; et al. (2005). "Mid-infrared detection of large longitudinal asymmetries in Io's SO2 atmosphere". Icarus 176 (2): 283–304. Bibcode:2005Icar..176..283S. doi:10.1016/j.icarus.2005.01.019.
- Moullet, A.; et al. (2010). "Simultaneous mapping of SO2, SO, NaCl in Io's atmosphere with the Submillimeter Array". Icarus. in. press (1): 353. Bibcode:2010Icar..208..353M. doi:10.1016/j.icarus.2010.02.009.
- Feaga, L. M.; et al. (2009). "Io's dayside SO2 atmosphere". Icarus 201 (2): 570–584. Bibcode:2009Icar..201..570F. doi:10.1016/j.icarus.2009.01.029.
- Spencer, John (8 June 2009). "Aloha, Io". The Planetary Society Blog. The Planetary Society. Retrieved 2010-03-07.
- Moore, C. H.; et al. (2009). "1-D DSMC simulation of Io's atmospheric collapse and reformation during and after eclipse". Icarus 201 (2): 585–597. Bibcode:2009Icar..201..585M. doi:10.1016/j.icarus.2009.01.006.
- Retherford, K. D.; et al. (2000). "Io's Equatorial Spots: Morphology of Neutral UV Emissions". J. Geophys. Res. 105 (A12): 27,157–27,165. Bibcode:2000JGR...10527157R. doi:10.1029/2000JA002500.
Media related to Io at Wikimedia Commons
General information 
- Io Profile at NASA's Solar System Exploration site
- Bill Arnett's Io webpage from The
Nine8 Planets website
- Io overview from the University of Michigan's Windows to the Universe
- Calvin Hamilton's Io page from the Views of the Solar System website
- Movie of Io's rotation from the National Oceanic and Atmospheric Administration
- Paul Schenk's 3D images and flyover videos of Io and other outer solar system satellites
- Catalog of NASA images of Io
- Galileo Image Releases
- New Horizons LORRI Raw Images, includes numerous Io images
- New Horizons Image Releases
- Io through Different New Horizons Imagers
- Io global basemaps from the USGS's planetary geology website based on Galileo and Voyager images
- Io nomenclature and Io map with feature names from the USGS planetary nomenclature page
Additional references 
- The Calendars of Jupiter
- Io dynamo from educational website The Exploration of the Earth's Magnetosphere
- The Conundrum Posed by Io's Minimum Surface Temperatures
- Io Mountain Database
- Paul Geissler's research on Cassini observations of Io's visible aurorae
- The Gish Bar Times, Jason Perry's Io-related blog