Io (), or Jupiter I, is the innermost and third-largest of the four Galilean moons of the planet Jupiter. It is the fourth-largest moon in the Solar System, has the highest density of all of them, and has the lowest amount of water (by atomic ratio) of any known astronomical object in the Solar System. It was discovered in 1610 by Galileo Galilei and was named after the mythological character Io, a priestess of Hera who became one of Zeus's lovers. 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 moons—Europa, Ganymede and Callisto.
Introduction
Io (moon)
Io (moon) | Introduction
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Io is the closest of the Galilean satellites to Jupiter. It is almost the same size as the Moon. Because of its position, it is subject to constant tidal flexing and heavy radiation from Jupiter's magnetic field. Since 1979, it has also been known for its abundant volcanism.
Jupiter's moons in fiction | Io
Io (), or Jupiter I, is the innermost and third-largest of the four Galilean moons of the planet Jupiter. It is the fourth-largest moon in the Solar System, has the highest density of all of them, and has the lowest amount of water (by atomic ratio) of any known astronomical object in the Solar System. It was discovered in 1610 by Galileo Galilei and was named after the mythological character Io, a priestess of Hera who became one of Zeus's lovers. 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 moons—Europa, Ganymede and Callisto.
Io (moon) | Introduction
Io is anhydrous and likely has an interior of rock and metal. Europa is thought to contain 8% ice and water by mass with the remainder rock. These moons are, in increasing order of distance from Jupiter:
Galilean moons | Members
Io (Jupiter I) is the innermost of the four Galilean moons of Jupiter and, with a diameter of 3642 kilometers, the fourth-largest moon in the Solar System. It was named after Io, a priestess of Hera who became one of the lovers of Zeus. Nevertheless, it was simply referred to as "Jupiter I", or "The first satellite of Jupiter", until the mid-20th century. With over 400 active volcanos, Io is the most geologically active object in the Solar System. Its surface is dotted with more than 100 mountains, some of which are taller than Earth's Mount Everest. 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.
Galilean moons | Io
Fluctuations in the orbits of the moons indicate that their mean density decreases with distance from Jupiter. Callisto, the outermost and least dense of the four, has a density intermediate between ice and rock whereas Io, the innermost and densest moon, has a density intermediate between rock and iron. Callisto has an ancient, heavily cratered and unaltered ice surface and the way it rotates indicates that its density is equally distributed, suggesting that it has no rocky or metallic core but consists of a homogeneous mix of rock and ice. This may well have been the original structure of all the moons.
Galilean moons | Comparative structure
Aside from the Sun, the most prominent objects in Jupiter's sky are the four Galilean moons. Io, the nearest to the planet, would be slightly larger than the full moon in Earth's sky, though less bright, and would be the largest moon in the Solar System as seen from its parent planet. The higher albedo of Europa would not overcome its greater distance from Jupiter, so it would not outshine Io. In fact, the low solar constant at Jupiter's distance (3.7% Earth's) ensures that none of the Galilean satellites would be as bright as the full moon is on Earth, and neither would any other moon in the Solar System.
Extraterrestrial sky | Jupiter's moons as seen from Jupiter
The rotation of the three inner moons, in contrast, indicates differentiation of their interiors with denser matter at the core and lighter matter above. They also reveal significant alteration of the surface. Ganymede reveals past tectonic movement of the ice surface which required partial melting of subsurface layers. Europa reveals more dynamic and recent movement of this nature, suggesting a thinner ice crust. Finally, Io, the innermost moon, has a sulfur surface, active volcanism and no sign of ice. All this evidence suggests that the nearer a moon is to Jupiter the hotter its interior. The current model is that the moons experience tidal heating as a result of the gravitational field of Jupiter in inverse proportion to the square of their distance from the giant planet.
Galilean moons | Comparative structure
Europa (Jupiter II), the second of the four Galilean moons, is the second closest to Jupiter and the smallest at 3121.6 kilometers in diameter, which is slightly smaller than the Moon. The name comes from a mythical Phoenician noblewoman, Europa, who was courted by Zeus and became the queen of Crete, though the name did not become widely used until the mid-20th century. It has a smooth and bright surface, with a layer of water surrounding the mantle of the planet, thought to be 100 kilometers thick. The smooth surface includes a layer of ice, while the bottom of the ice is theorized to be liquid water.
Galilean moons | Europa
In Weinbaum's Solar System, Jupiter radiates enough heat to create Earthlike environments on the Galilean moons. Io, the innermost Galilean satellite, has a tropical climate, so that two human settlements are located at the poles, Junopolis in the north and Herapolis in the south. Extending partway around the equator are the Idiots' Hills, whose peaks extend beyond Io's "dense but shallow" atmosphere. Weinbaum was ignorant that Io is tidally locked, and therefore showed Jupiter rise and set during the story. Two intelligent races are native to Io: the loonies, a humanoid race of only moderate intelligence with large balloonlike heads upon long, slim necks, and the rodentine, warlike slinkers.
The Mad Moon | Weinbaum's Io
Although not proven, recent data from the Galileo orbiter indicate that Io might have its own magnetic field. Io has an extremely thin atmosphere made up mostly of sulfur dioxide (SO). If a surface data or collection vessel were to land on Io in the future, it would have to be extremely tough (similar to the tank-like bodies of the Soviet Venera landers) to survive the radiation and magnetic fields that originate from Jupiter.
Galilean moons | Io
In all but Callisto this will have melted the interior ice, allowing rock and iron to sink to the interior and water to cover the surface. In Ganymede a thick and solid ice crust then formed. In warmer Europa a thinner more easily broken crust formed. In Io the heating is so extreme that all the rock has melted and water has long ago boiled out into space.
Galilean moons | Comparative structure
The eccentricity of their orbits causes regular flexing of the three moons' shapes, with Jupiter's gravity stretching them out as they approach it and allowing them to spring back to more spherical shapes as they swing away. This tidal flexing heats the moons' interiors by friction. This is seen most dramatically in the extraordinary volcanic activity of innermost Io (which is subject to the strongest tidal forces), and to a lesser degree in the geological youth of Europa's surface (indicating recent resurfacing of the moon's exterior).
Jupiter | Galilean moons
The star furthest to the east from Jupiter turned out to be Callisto while the star to the west of Jupiter was Ganymede. The third star, the closest one to the east of Jupiter, was a combination of the light from Io and Europa as Galileo's telescope, while having a high magnification for a telescope from his time, was too low-powered to separate the two moons into distinct points of light. Galileo observed Jupiter the next evening, January 8, 1610, this time seeing three stars to the west of Jupiter, suggesting that Jupiter had moved to the west of the three stars.
Exploration of Io | Discovery: 1610
Europa is the smallest of the four Galilean satellites and the second closest to Jupiter. It is theorized to have an ocean of liquid water underneath its icy surface; the thickness of the ice is much debated. The probable presence of the water ocean has made it a favored location for modern fictional speculation about extraterrestrial life in the Solar System.
Jupiter's moons in fiction | Europa
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. Although no images were taken during the close flyby on 7 December 1995, the encounter did yield significant results, such as the discovery of a large iron core, similar to that found in the rocky planets of the inner Solar System.
Io (moon) | "Galileo"
The exploration of Io, Jupiter's innermost Galilean and third-largest moon, began with its discovery in 1610 and continues today with Earth-based observations and visits by spacecraft to the Jupiter system. Italian astronomer Galileo Galilei was the first to record an observation of Io on January 8, 1610, though Simon Marius may have also observed Io at around the same time. During the 17th century, observations of Io and the other Galilean satellites helped with the measurement of longitude by map makers and surveyors, with validation of Kepler's Third Law of planetary motion, and with measurement of the speed of light.
Exploration of Io | Introduction
Io orbits Jupiter at a distance of from Jupiter's center and 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 Io about 42.5 hours to complete one orbit around Jupiter (fast enough for its motion to be observed over a single night of observation). Io is in a 2:1 mean-motion orbital resonance with Europa and a 4:1 mean-motion orbital resonance with Ganymede, completing two orbits of Jupiter for every one orbit completed by Europa, and four orbits for every one completed by Ganymede.
Io (moon) | Orbit and rotation
Callisto has long been considered the most suitable place for a human base for future exploration of the Jupiter system since it is furthest from the intense radiation of Jupiter.
Galilean moons | Callisto
The moons discovered by Galileo—Io, Europa, Ganymede, and Callisto—are among the largest satellites in the Solar System. The orbits of three of them (Io, Europa, and Ganymede) form a pattern known as a Laplace resonance; for every four orbits that Io makes around Jupiter, Europa makes exactly two orbits and Ganymede makes exactly one. This resonance causes the gravitational effects of the three large moons to distort their orbits into elliptical shapes, because each moon receives an extra tug from its neighbors at the same point in every orbit it makes. The tidal force from Jupiter, on the other hand, works to circularize their orbits.
Jupiter | Galilean moons
All four Galilean moons are bright enough to be viewed from Earth without a telescope, if only they could appear farther away from Jupiter. (They are, however, easily distinguished with even low-powered binoculars.) They have apparent magnitudes between 4.6 and 5.6 when Jupiter is in opposition with the Sun, and are about one unit of magnitude dimmer when Jupiter is in conjunction. The main difficulty in observing the moons from Earth is their proximity to Jupiter, since they are obscured by its brightness. The maximum angular separations of the moons are between 2 and 10 arcminutes from Jupiter, which is close to the limit of human visual acuity.
Galilean moons | Visibility
The Galilean moons (or Galilean satellites) are the four largest moons of Jupiter—Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei in December 1609 or January 1610, and recognized by him as satellites of Jupiter in March 1610. They were the first objects found to orbit a planet other than the Earth. They are among the largest objects in the Solar System with the exception of the Sun and the eight planets, with radii larger than any of the dwarf planets. Ganymede is the largest moon in the Solar System, and is even bigger than the planet Mercury, though only around half as massive.
Galilean moons | Introduction
Each generation of moons would have spiraled into Jupiter, due to drag from the disk, with new moons then forming from the new debris captured from the Solar nebula. By the time the present (possibly fifth) generation formed, the disk had thinned out to the point that it no longer greatly interfered with the moons' orbits. The current Galilean moons were still affected, falling into and being partially protected by an orbital resonance which still exists for Io, Europa, and Ganymede. Ganymede's larger mass means that it would have migrated inward at a faster rate than Europa or Io.
Galilean moons | Origin and evolution
To study similar variations within the plasma torus, researchers measure the ultraviolet light it emits. Although such variations have not been definitively linked to variations in Io's volcanic activity (the ultimate source for material in the plasma torus), this link has been established in the neutral sodium cloud.
Io (moon) | Interaction with Jupiter's magnetosphere
The four largest moons of Jupiter were in poor positions for observation; the necessary path of the gravity-assist maneuver meant that "New Horizons" passed millions of kilometers from any of the Galilean moons. Still, its instruments were intended for small, dim targets, so they were scientifically useful on large, distant moons. Emphasis was put on Jupiter's innermost Galilean moon, Io, whose active volcanoes shoot out tons of material into Jupiter's magnetosphere, and further. Out of eleven observed eruptions, three were seen for the first time. That of Tvashtar reached an altitude of up to . The event gave scientists an unprecedented look into the structure and motion of the rising plume and its subsequent fall back to the surface.
New Horizons | Jovian moons
The side of Io that always faces in the direction that Io travels in its orbit is known as the leading hemisphere, whereas the side that always faces in the opposite direction is known as the trailing hemisphere. From the surface of Io, Jupiter would subtend an arc of 19.5°, making Jupiter appear 39 times the apparent diameter of Earth's Moon.
Io (moon) | Orbit and rotation
Proportionally, the Sun, Moon and Earth would not visually line up at the size ratios shown in the opening shot, nor would the Galilean moons of Jupiter align as in the shot just before Bowman enters the Star Gate. Kubrick himself was aware of this latter point. (Due to the perfect Laplace resonance of the orbits of the four large moons of Jupiter, the first three never align, and the third moon, Ganymede, is always exactly 90 degrees away from the other two whenever the two innermost moons are in perfect alignment.). Similarly, during the scene in the Dawn of Man, where the sun is seen above the monolith, a crescent moon is depicted close by in the sky.
Technologies in 2001: A Space Odyssey | Inaccuracy
The moons of Jupiter are listed below by orbital period. Moons massive enough for their surfaces to have collapsed into a spheroid are highlighted in bold. These are the four Galilean moons, which are comparable in size to the Moon. The other moons are much smaller, with the least massive Galilean moon being more than 7000 times more massive than the most massive of the other moons. The irregular captured moons are shaded light gray when prograde and dark gray when retrograde. All orbits are based on the estimated orbit on the Julian date 2458200, or 23 March 2018. As several moons of Jupiter are currently lost, these orbital elements may be only rough approximations.
Moons of Jupiter | List
Io is slightly larger than Earth's Moon. It has a mean radius of (about 5% greater than the Moon's) and a mass of 8.9319 kg (about 21% greater than the Moon's). It is a slight ellipsoid in shape, with its longest axis directed toward Jupiter. Among the Galilean satellites, in both mass and volume, Io ranks behind Ganymede and Callisto but ahead of Europa.
Io (moon) | Geology
Although 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 alternative names for the innermost of the large moons of Jupiter, including "The Mercury of Jupiter" and "The First of the Jovian Planets". Based on a suggestion from Johannes Kepler in October 1613, he also devised a naming scheme whereby each moon was named for a lover of the Greek mythological Zeus or his Roman equivalent, Jupiter. He named the innermost large moon of Jupiter after the Greek mythological figure Io: Marius's names were not widely adopted until centuries later (mid-20th century).
Io (moon) | Nomenclature
Following "Galileo" planned destruction 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 Jovian system.
Io (moon) | Subsequent observations
Spots were found to correspond to the Galilean moons: Io, Europa, Ganymede and Callisto. They develop because the co-rotation of the plasma interacts with the moons and is slowed in their vicinity. The brightest spot belongs to Io, which is the main source of the plasma in the magnetosphere (see above). The Ionian auroral spot is thought to be related to Alfvén currents flowing from the Jovian to Ionian ionosphere. Europa's is similar but much dimmer, because it has a more tenuous atmosphere and is a weaker plasma source. Europa's atmosphere is produced by sublimation of water ice from its surfaces, rather than the volcanic activity which produces Io's atmosphere.
Magnetosphere of Jupiter | Aurorae
Of Jupiter's moons, eight are regular satellites with prograde and nearly circular orbits that are not greatly inclined with respect to Jupiter's equatorial plane. The Galilean satellites are nearly spherical in shape due to their planetary mass, and so would be considered at least dwarf planets if they were in direct orbit around the Sun. The other four regular satellites are much smaller and closer to Jupiter; these serve as sources of the dust that makes up Jupiter's rings. The remainder of Jupiter's moons are irregular satellites whose prograde and retrograde orbits are much farther from Jupiter and have high inclinations and eccentricities.
Moons of Jupiter | Introduction
Io is not ideal for colonization, due to its hostile environment. The moon is under influence of high tidal forces, causing high volcanic activity. Jupiter's strong radiation belt overshadows Io, delivering 36 Sv a day to the moon. The moon is also extremely dry. Io is the least ideal place for colonization of the four Galilean moons. Despite this, its volcanoes could be energy resources for the other moons, which are better suited to colonization.
Colonization of the outer Solar System | Io
"Pioneer 11"'s closer encounter with Jupiter allowed the spacecraft to discover Jupiter's intense radiation belts similar to Earth's Van Allen Belts. One of the peaks in charged particle flux was found near the orbit of Io. Radio tracking during the encounters of both Pioneers with Io provided an improved estimate of the moon's mass. This was accomplished by analyzing slight changes in trajectory of the two probes due to the influence of Io's gravity and calculating the mass necessary to produce the deviations. When this estimate was combined with the best available information on Io's size, Io was found to have the highest density of the four Galilean satellites and that the densities of the four Galilean satellites trended downward with increasing distance from Jupiter.
Exploration of Io | "Pioneer" era: 1973–1979
This is a list of named geological features on Io, a moon of Jupiter. See also the list of mountains on Io and the list of paterae on Io.
List of geological features on Io | Introduction
Callisto (Jupiter IV) is the fourth and last Galilean moon, and is the second-largest of the four, and at 4820.6 kilometers in diameter, it is the third largest moon in the Solar System, and barely smaller than Mercury, though only a third of the latter's mass. It is named after the Greek mythological nymph Callisto, a lover of Zeus who was a daughter of the Arkadian King Lykaon and a hunting companion of the goddess Artemis. The moon does not form part of the orbital resonance that affects three inner Galilean satellites and thus does not experience appreciable tidal heating.
Galilean moons | Callisto
Over a 20-hour period, these particles spread out from Io to form a banana-shaped, neutral cloud that can reach as far as six Jovian radii from Io, either inside Io's orbit and ahead of it or outside Io's orbit and behind it. The collision process that excites these particles also occasionally provides sodium ions in the plasma torus with an electron, removing those new "fast" neutrals from the torus. These particles retain their velocity (70 km/s, compared to the 17 km/s orbital velocity at Io), and are thus ejected in jets leading away from Io. Io orbits within a belt of intense radiation known as the Io plasma torus.
Io (moon) | Interaction with Jupiter's magnetosphere
GIF animations depicting the Galilean moon orbits and the resonance of Io, Europa, and Ganymede
Galilean moons | Orbit animations
In much of the earlier astronomical literature, Io was generally referred to by its Roman numeral designation (a system introduced by Galileo) as "", or as "the first satellite of Jupiter". The customary English pronunciation of the name is , though sometimes people attempt a more 'authentic' pronunciation, . The name has two competing stems in Latin: "Īō" and (rarely) "Īōn". The latter is the basis of the English adjectival form, "Ionian."
Io (moon) | Nomenclature
These materials, depending on their ionized state and composition, end up in various neutral (non-ionized) clouds and radiation belts in Jupiter's magnetosphere and, in some cases, are eventually ejected from the Jovian system. Surrounding Io (at a distance of up to six Io radii from its surface) is a cloud of neutral sulfur, oxygen, sodium, and potassium atoms. These particles originate in Io's upper atmosphere and are excited by collisions with ions in the plasma torus (discussed below) and by other processes into filling Io's Hill sphere, which is the region where Io's gravity is dominant over Jupiter's. Some of this material escapes Io's gravitational pull and goes into orbit around Jupiter.
Io (moon) | Interaction with Jupiter's magnetosphere
During this observation, the three stars in a line to the west of Jupiter were (from east to west): Io, Europa, and Ganymede. This was the first time that Io and Europa were observed and recorded as distinct points of light so this date, January 8, 1610 is used as the discovery date for the two moons by the International Astronomical Union. Galileo continued to observe the Jupiter system for the next month and a half. On January 13, Galileo observed all four of what would later be known as the Galilean moons of Jupiter for the first time in a single observation, though he had observed all four at various times in the preceding days.
Exploration of Io | Discovery: 1610
He named the innermost large moon of Jupiter after the Greek mythological figure Io.
Exploration of Io | Discovery: 1610
The "New Horizons" spacecraft, en route to Pluto and the Kuiper belt, flew by the Jovian system and Io on 28 February 2007. During the encounter, numerous distant observations of Io were obtained. These included images of a large plume at Tvashtar, providing the first detailed observations of the largest class of Ionian volcanic plume since observations of Pele's plume in 1979. "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".
Io (moon) | New Horizons
All four Galilean moons stand out because of the swiftness of their motion, compared to the Moon. They are all also large enough to fully eclipse the Sun. Because Jupiter's axial tilt is minimal, and the Galilean moons all orbit in the plane of Jupiter's equator, solar eclipses are quite common.
Extraterrestrial sky | Jupiter's moons as seen from Jupiter
In 1975, the International Astronomical Union gave Jupiter IX the official name Sinope. Asimov describes Jupiter IX as being 89 miles in diameter, but its diameter is now thought to be only 23 miles. Part of the novel is also set on Io, the innermost of the Galilean moons. Io is depicted as having a thin atmosphere of methane, and fields of ammonia snow and ice, as well as rivers of liquid ammonia.
Lucky Starr and the Moons of Jupiter | Setting
None of Jupiter's moons have more than traces of atmosphere, so their skies are very nearly black. For an observer on one of the moons, the most prominent feature of the sky by far would be Jupiter. For an observer on Io, the closest large moon to the planet, Jupiter's apparent diameter would be about 20° (38 times the visible diameter of the Moon, covering 5% of Io's sky). An observer on Metis, the innermost moon, would see Jupiter's apparent diameter increased to 68° (130 times the visible diameter of the Moon, covering 18% of Metis's sky). A "full Jupiter" over Metis shines with about 4% of the Sun's brightness (light on Earth from a full moon is 400 thousand times dimmer than sunlight).
Extraterrestrial sky | The skies of Jupiter's moons
The Galilean moons of Jupiter (Io, Europa, Ganymede and Callisto) were named by Simon Marius soon after their discovery in 1610. However, by the late 19th century these names had fallen out of favor, and for a long time it was most common to refer to them in the astronomical literature simply as "Jupiter I", "Jupiter II", etc., or as "the first satellite of Jupiter", "Jupiter's second satellite", etc. By the first decade of the 20th century, the names Io, Europa, Ganymede, and Callisto had once again recovered popularity, but the later-discovered moons, numbered, usually in Roman numerals V (5) through XII (12), remained unnamed.
Naming of moons | Jupiter
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 Io is in the shadow of Jupiter (with an ~80% drop in column density). 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.
Io (moon) | Atmosphere
From these estimates of the orbital periods of Io and the other Galilean moons, astronomers hoped to generate ephemeris tables predicting the positions of each moon with respect to Jupiter, as well as when each moon would transit the face of Jupiter or be eclipsed by it. One benefit of such predictions, particularly those of satellite eclipses by Jupiter since they were subject to less observer error, would be determining an observer's longitude on Earth with respect to the prime meridian. By observing an eclipse of a Jovian satellite, an observer could determine the current time at the prime meridian by looking up the eclipse in an ephemeris table.
Exploration of Io | Io as a tool: 1610–1809
Europa , or Jupiter II, is the smallest of the four Galilean moons orbiting Jupiter, and the sixth-closest to the planet of all the 79 known moons of Jupiter. It is also the sixth-largest moon in the Solar System. Europa was discovered in 1610 by Galileo Galilei and was named after Europa, the Phoenician mother of King Minos of Crete and lover of Zeus (the Greek equivalent of the Roman god Jupiter). Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and has a water-ice crust and probably an iron–nickel core. It has a very thin atmosphere, composed primarily of oxygen.
Europa (moon) | Introduction
Callisto is the outermost of the Galilean satellites. It is a large moon, only slightly smaller than the planet Mercury. It is cold, icy, and heavily cratered, with a very tenuous atmosphere. Despite its size and early discovery, it has not been featured in fiction as much as the other Galilean satellites.
Jupiter's moons in fiction | Callisto
Jupiter's extensive system of natural satellites – in particular the four large Galilean moons (Io, Europa, Ganymede and Callisto) – has been a common science fiction setting.
Jupiter's moons in fiction | Introduction
During several orbits, Juno has observed Io from a distance using JunoCAM, a wide-angle, visible-light camera, to look for volcanic plumes and JIRAM, a near-infrared spectrometer and imager, to monitor thermal emission from Io's volcanoes.
Io (moon) | Juno
In 2011 and online in 2012, USGS released a geologic surface map of Jupiter's moon Io, which is the most volcanically active body in our solar system, about twenty-five times more active than earth. David Williams of Arizona State University was the project lead. The maps are made of the best images from NASA's Voyager 1 and 2 missions (acquired in 1979) as well as the spacecraft Galileo (1995–2003) named for Galileo Galilei who discovered Jupiter's moons in 1610. McNutt said:
Marcia McNutt | Map of Jupiter's moon Io
Io (; ) was, in Greek mythology, one of the mortal lovers of Zeus. An Argive princess, she was an ancestor of many kings and heroes such as Perseus, Cadmus, Heracles, Minos, Lynceus, Cepheus, and Danaus. The astronomer Simon Marius named a moon of Jupiter after Io in 1614. She was sometimes compared to the goddess Isis, whereas her Egyptian husband Telegonus was "Osiris".
Io (mythology) | Introduction
It belongs to the Pasiphae group, irregular retrograde moons orbiting Jupiter at distances ranging between 22.8 and 24.1 Gm, and with inclinations ranging between 144.5° and 158.3°. This moon is currently considered lost.
S/2003 J 23 | Introduction
Each generation of moons might have spiraled into Jupiter, because of drag from the disk, with new moons then forming from the new debris captured from the solar nebula. By the time the present (possibly fifth) generation formed, the disk had thinned so that it no longer greatly interfered with the moons' orbits. The current Galilean moons were still affected, falling into and being partially protected by an orbital resonance with each other, which still exists for Io, Europa, and Ganymede. Ganymede's larger mass means that it would have migrated inward at a faster rate than Europa or Io. The outer, irregular moons are thought to have originated from captured asteroids, whereas the protolunar disk was still massive enough to absorb much of their momentum and thus capture them into orbit.
Moons of Jupiter | Origin and evolution
In addition to the large Galilean moons, Jupiter is orbited by sixty three smaller moons, but only two of them, Amalthea and Thebe, have been imaged at sufficient resolution for surface features to become apparent.
List of geological features on Jupiter's inner moons | Introduction
On January 15, he observed the motions of three of these satellites, including Io, and came to the conclusions that these objects were not background stars, but were in fact, "three stars in the heavens moving about Jupiter, as Venus and Mercury round the Sun." These were the first moons of a planet other than the Earth to be discovered. The discoveries of Io and the other Galilean satellites of Jupiter were published in Galileo's "Sidereus Nuncius" in March 1610. While the Jovian moons he discovered would later be known as the Galilean satellites, after himself, he proposed the name "Medicea Sidera" (Medicean Stars) after his new patrons, the de'Medici family of his native Florence.
Exploration of Io | Discovery: 1610
Mountains are widely distributed across the surface of Io, the innermost large moon of Jupiter. There are about 115 named mountains; the average length is and the average height is . The longest is , and the highest is Boösaule Montes, at , taller than any mountain on Earth. Ionian mountains often appear as large, isolated structures; no global tectonic pattern is evident, unlike on Earth, where plate tectonics is dominant. Io is exceptional for the strong tidal heating it undergoes, caused by the eccentricity of its orbit (which results from its resonance with Europa and Ganymede) in conjunction with the proximity and great mass of Jupiter.
Mountains of Io | Introduction
It belongs to the Pasiphae group, irregular retrograde moons orbiting Jupiter at distances ranging between 22.8 and 24.1 Gm, and with inclinations ranging between 144.5° and 158.3°. This moon has not been seen since its discovery in 2003 and is currently considered lost.
S/2003 J 4 | Introduction
Closer to the Moon () is a 2013 Romanian-American comedy-drama film written and directed by Nae Caranfil, and starring Vera Farmiga, Mark Strong, Harry Lloyd, Joe Armstrong, Tim Plester, Christian McKay, and Anton Lesser. Based on the true story of the Ioanid Gang, it is one of the most expensive productions in Romanian cinema. It had its world premiere at the Making Waves: New Romanian Cinema Festival at the Lincoln Center on November 29, 2013. The film was released in Romania on March 7, 2014, and was given a limited release in the United States on April 17, 2015 by Sundance Selects.
Closer to the Moon | Introduction
Io is the innermost of the four moons of Jupiter discovered by Galileo in January 1610. Rømer and Cassini refer to it as the "first satellite of Jupiter". It orbits Jupiter once every 42½ hours, and the plane of its orbit is very close to the plane of Jupiter's orbit around the sun. This means that it passes some of each orbit in the shadow of Jupiter – an eclipse. Viewed from the Earth, an eclipse of Io is seen in one of two ways. From the Earth, it is not possible to view both the immersion and the emergence for the same eclipse of Io, because one or the other will be hidden (occulted) by Jupiter itself.
Rømer's determination of the speed of light | Eclipses of Io
On 7 January 1610, Galileo observed with his telescope what he described at the time as "three fixed stars, totally invisible by their smallness", all close to Jupiter, and lying on a straight line through it. Observations on subsequent nights showed that the positions of these "stars" relative to Jupiter were changing in a way that would have been inexplicable if they had really been fixed stars. On 10 January, Galileo noted that one of them had disappeared, an observation which he attributed to its being hidden behind Jupiter. Within a few days, he concluded that they were orbiting Jupiter: he had discovered three of Jupiter's four largest moons.
Galileo Galilei | Jupiter's moons
The images returned during the approach revealed a strange, multi-colored landscape devoid of impact craters, unlike the other planetary surfaces imaged to that point such as the Moon, Mars, and Mercury. The dark spots in earlier images resembled volcanic calderas more than they did the impact craters seen on those other worlds. Stunned by the oddity of Io's surface, Voyager imaging scientist Laurence Soderblom at a pre-encounter press conference joked, "this one we got all figured out...[Io] is covered with thin candy shells of anything from sulfates and sulfur and salts to all kinds of strange things." On March 5, 1979, "Voyager 1" performed the closest encounter with Io of the Voyager mission from a distance of over its south pole.
Exploration of Io | "Voyager" era: 1979–1995
Magnetometer data from the encounter, combined with the discovery of an iron core, suggested that Io might have a magnetic field. Jupiter's intense radiation belts near the orbit of Io forced "Galileo" to come no closer than the orbit of Europa until the end of the first extended mission in 1999. Despite the lack of close-up imaging and mechanical problems that greatly restricted the amount of data returned, several significant discoveries at Io were made during "Galileo"'s two-year, primary mission. During the first several orbits, "Galileo" mapped Io in search of surface changes that occurred since the Voyager encounters 17 years earlier.
Exploration of Io | Galileo : era 1995–2003
The Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto) were named by Simon Marius soon after their discovery in 1610. However, these names fell out of favor until the 20th century. The astronomical literature instead simply referred to "Jupiter I", "Jupiter II", etc., or "the first satellite of Jupiter", "Jupiter's second satellite", and so on. The names Io, Europa, Ganymede, and Callisto became popular in the mid-20th century, whereas the rest of the moons remained unnamed and were usually numbered in Roman numerals V (5) to XII (12). Jupiter V was discovered in 1892 and given the name "Amalthea" by a popular though unofficial convention, a name first used by French astronomer Camille Flammarion.
Moons of Jupiter | Naming
They also had spectrometers with a combined spectral range from the far-ultraviolet to the mid-infrared, useful for examining Io's surface and atmospheric composition and to search for thermal emission sources on its surface. "Voyager 1" was first of the two probes to encounter the Jupiter system in March 1979. On approach to Jupiter in late February and early March 1979, "Voyager" imaging scientists noticed that Io appeared distinct from the other Galilean satellites. Its surface was orange in color and marked by dark spots, which were initially interpreted as the sites of impact craters. Among the most intriguing features was a heart-shaped, dark ring across that would later turn out to be the plume deposit of the volcano Pele.
Exploration of Io | "Voyager" era: 1979–1995
The first reported observation of Io was made by Galileo Galilei on 7 January 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 Jovian system the following day, 8 January 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.
Io (moon) | Observational history
By convention these features are identified on lunar maps by placing the letter on the side of the crater midpoint that is closest to Galilaei.
Galilaei (lunar crater) | Satellite craters
This resonance helps maintain Io's orbital eccentricity (0.0041), which in turn provides the primary heating source for its geologic activity. Without this forced eccentricity, Io's orbit would circularize through tidal dissipation, leading to a geologically less active world. Like the other Galilean satellites and the Moon, Io rotates synchronously with its orbital period, keeping one face nearly pointed toward Jupiter. This synchrony provides the definition for Io's longitude system. Io's prime meridian intersects the equator at the sub-Jovian point. The side of Io that always faces Jupiter is known as the subjovian hemisphere, whereas the side that always faces away is known as the antijovian hemisphere.
Io (moon) | Orbit and rotation
Io, a moon of Jupiter, is a small terrestrial planet, its radius is 1821.6±0.5 km, with a size similar to the Moon's. Yet, Io produces a much higher heat flow, 60~160 terawatts (TW), which is 40 times larger than that on Earth. Radioactive decay cannot generate this large amount of heat. Radioactive decay supplies heating on other terrestrial planets. Instead, tidal-generated heat is a better hypothesis as Io is under great tidal influence imposed by Jupiter and other large moons of Jupiter, similar to the Earth and the Moon. The first observation supporting this was the active volcanism found on Io.
Heat-pipe tectonics | Inspiration from Io
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 , the highest of any moon in the Solar System; significantly higher than the other Galilean satellites (Ganymede and Callisto in particular, whose densities are around ) and slightly higher (~5.5%) than the Moon's . Models based on the "Voyager" and "Galileo" measurements of Io's mass, radius, and quadrupole gravitational coefficients (numerical values related to how mass is distributed within an object) suggest that its interior is differentiated between a silicate-rich crust and mantle and an iron- or iron-sulfide-rich core.
Io (moon) | Interior
The "Juno" spacecraft was launched in 2011 and entered orbit around Jupiter on July 5, 2016. Juno's mission is primarily focused on improving our understanding of planet's interior, magnetic field, aurorae, and polar atmosphere. Juno's orbit is highly inclined and highly eccentric in order to better characterize Jupiter's polar regions and to limit its exposure to the planet's harsh inner radiation belts. This orbit also keeps Juno out of the orbital planes of Io and the other major moons of Jupiter. Juno's closest approach to Io to date occurred during Perijove 25 on February 17, 2020, at a distance of 195,000 kilometers, though a pair of flybys at an altitude of 1,500 kilometers are planned for a proposed extended mission in early 2024.
Io (moon) | Juno
The innermost moon of Jupiter, Io is a world emblazoned with the reds, yellows, whites and blacks of sulphur chemistry and the scars of active volcanism. Its features have a wide variety of naming themes: gods, goddesses and heroes associated with fire, sun, thunder, and volcanoes; mythical blacksmiths; people associated with the myth of Io; or people from Dante's Inferno. Names may also be derived from a nearby, more prominent feature; this is a situation where the same name could be used for both a volcano and an adjacent valley. "Montes", mountains, can be named in the Dantean or Greek categories, or for associated features.
List of planetary features with Māori names | Io
Based on ephemerides produced by astronomer Giovanni Cassini and others, Pierre-Simon Laplace created a mathematical theory to explain the resonant orbits of three of Jupiter's moons, Io, Europa, and Ganymede. This resonance was later found to have a profound effect on the geologies of these moons. Improved telescope technology in the late 19th and 20th centuries allowed astronomers to resolve large-scale surface features on Io as well as to estimate its diameter and mass. The advent of unmanned spaceflight in the 1950s and 1960s provided an opportunity to observe Io up-close. In the 1960s the moon's effect on Jupiter's magnetic field was discovered.
Exploration of Io | Introduction
The secretary replied that the latter name would be best.
Galilean moons | Dedication to the Medicis
By occluding Jupiter itself behind a high tree limb perpendicular to the satellites' orbital plane to prevent the planet's glare from obscuring them, one or more of the Galilean moons might be spotted in favorable conditions. However, Gan De reported the color of the companion as reddish, which is puzzling since the moons are too faint for their color to be perceived with the naked eye. Shi and Gan together made fairly accurate observations of the five major planets.
Gan De | Observations
Voyager 2s closest approach to Jupiter occurred at 22:29 UT on July 9, 1979. It came within of the planet's cloud tops. Jupiter's Great Red Spot was revealed as a complex storm moving in a counterclockwise direction. Other smaller storms and eddies were found throughout the banded clouds. Voyager 2 returned images of Jupiter, as well as its moons Amalthea, Io, Callisto, Ganymede, and Europa. During a 10-hour "volcano watch", it confirmed Voyager 1s observations of active volcanism on the moon Io, and revealed how the moon's surface had changed in the four months since the previous visit. Together, the Voyagers observed the eruption of nine volcanoes on Io, and there is evidence that other eruptions occurred between the two Voyager fly-bys.
Voyager 2 | Encounter with Jupiter
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".
Io (moon) | Surface
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), so that Io is at times below and at other times above the core of the plasma torus. As noted above, these ions' higher velocity and energy levels are partly responsible for the removal of neutral atoms and molecules from Io's atmosphere and more extended neutral cloud.
Io (moon) | Interaction with Jupiter's magnetosphere
Intervalla enim quatuor Jovialium a Jove prodit Marius in suo Mundo Joviali ista: 3, 5, 8, 13 (vel 14 Galilaeo) … Periodica vero tempora prodit idem Marius ista: dies 1. h. 18 1/2, dies 3 h. 13 1/3, dies 7 h. 3, dies 16 h. 18: ubique proportio est major quam dupla, major igitur quam intervallorum 3, 5, 8, 13 vel 14, minor tamen quam quadratorum, qui duplicant proportiones intervallorum, sc. 9, 25, 64, 169 vel 196, sicut etiam sescupla sunt majora simplis, minora vero duplis." "Translation" : (4) However, the credibility of this [argument] is proved by the comparison of the four [moons] of Jupiter and Jupiter with the six planets and the Sun.
Godefroy Wendelin | Moons of Jupiter
The three inner moons—Io, Europa, and Ganymede—are in a 4:2:1 orbital resonance with each other. While the Galilean moons are spherical, all of Jupiter's much smaller remaining moons have irregular forms because of their weaker self-gravitation. The Galilean moons were observed in either 1609 or 1610 when Galileo made improvements to his telescope, which enabled him to observe celestial bodies more distinctly than ever. Galileo's observations showed the importance of the telescope as a tool for astronomers by proving that there were objects in space that cannot be seen by the naked eye. The discovery of celestial bodies orbiting something other than Earth dealt a serious blow to the then-accepted Ptolemaic world system, a geocentric theory in which everything orbits around Earth.
Galilean moons | Introduction
"Voyager 2" passed Io on 9 July 1979 at a distance of . 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.
Io (moon) | "Voyager"
Voyager 1 began photographing Jupiter in January 1979. Its closest approach to Jupiter was on March 5, 1979, at a distance of about from the planet's center. Because of the greater photographic resolution allowed by a closer approach, most observations of the moons, rings, magnetic fields, and the radiation belt environment of the Jovian system were made during the 48-hour period that bracketed the closest approach. Voyager 1 finished photographing the Jovian system in April 1979. Discovery of ongoing volcanic activity on the moon Io was probably the greatest surprise. It was the first time active volcanoes had been seen on another body in the Solar System.
Voyager 1 | Flyby of Jupiter
This difference in gravitational forces cause distortion of Io’s shape. Differently from the Earth’s only moon, Jupiter has two other large moons (Europa and Ganymede) that are in an orbital resonance with it. Io is the innermost of this set of resonant moons, and their interactions maintain its orbit in an eccentric (elliptical) state. The varying distance between Jupiter and Io continually changes the degree of distortion of Io's shape and flexes its interior, frictionally heating it. The friction-induced heating drives strong volcanic activities on the surface of Io. Although there is general agreement that the cause of the heat as manifested in Io's many volcanoes is tidal heating from the pull of gravity from Jupiter and its moon Europa, the volcanoes are not in the positions predicted with tidal heating.
Tidal heating of Io | Introduction
Callisto is the outermost of the four Galilean moons of Jupiter. It orbits at a distance of approximately 1 880 000 km (26.3 times the 71 492 km radius of Jupiter itself). This is significantly larger than the orbital radius—1 070 000 km—of the next-closest Galilean satellite, Ganymede. As a result of this relatively distant orbit, Callisto does not participate in the mean-motion resonance—in which the three inner Galilean satellites are locked—and probably never has. Like most other regular planetary moons, Callisto's rotation is locked to be synchronous with its orbit. The length of Callisto's day, simultaneously its orbital period, is about 16.7 Earth days.
Callisto (moon) | Orbit and rotation
It may belong to the Pasiphae group, though its orbit is not known well enough to confirm the assignment. Its orbit has a semi-major axis ~ and inclination ~160°. The limits of Jupiter's gravitational influence are defined by its Hill sphere, whose radius is . Retrograde moons with semi-major axes up to 67% of the Hill radius are believed to be stable. Consequently, it is possible that even more distant moons of Jupiter may be discovered. This moon has not been seen since its discovery in 2003 and is currently considered lost.
S/2003 J 2 | Introduction
He discovered the fourth on 13 January. Galileo named the group of four the "Medicean stars", in honour of his future patron, Cosimo II de' Medici, Grand Duke of Tuscany, and Cosimo's three brothers. Later astronomers, however, renamed them "Galilean satellites" in honour of their discoverer. These satellites were independently discovered by Simon Marius on 8 January 1610 and are now called Io, Europa, Ganymede, and Callisto, the names given by Marius in his "Mundus Iovialis" published in 1614. Galileo's observations of the satellites of Jupiter caused a revolution in astronomy: a planet with smaller planets orbiting it did not conform to the principles of Aristotelian cosmology, which held that all heavenly bodies should circle the Earth, and many astronomers and philosophers initially refused to believe that Galileo could have discovered such a thing.
Galileo Galilei | Jupiter's moons
The first close-up investigation of Io using high-resolution imaging was performed by the twin probes, "Voyager 1" and "Voyager 2", launched on September 5 and August 20, 1977, respectively. These two spacecraft were part of NASA and JPL's Voyager program to explore the giant outer planets through a series of missions in the late 1970s and 1980s. This was a scaled-down version of the earlier Planetary Grand Tour concept. Both probes contained more sophisticated instrumentation than the previous "Pioneer" missions, including a camera capable of taking much higher resolution images. This was important for viewing the geologic features of Jupiter's Galilean moons as well as the cloud features of Jupiter itself.
Exploration of Io | "Voyager" era: 1979–1995
Surrounding Jupiter is a faint planetary ring system and a powerful magnetosphere. Jupiter has 79 known moons, including the four large Galilean moons discovered by Galileo Galilei in 1610. Ganymede, the largest of these, has a diameter greater than that of the planet Mercury. "Pioneer 10 was" the first spacecraft to visit Jupiter, making its closest approach to the planet on December 4, 1973; "Pioneer 10" identified plasma in Jupiter's magnetic field and also found that Jupiter's magnetic tail was nearly 800 million kilometers long, covering the entire distance to Saturn. Jupiter has been explored on a number of occasions by robotic spacecraft, beginning with the "Pioneer" and "Voyager" flyby missions from 1973 to 1979, and later by the "Galileo" orbiter, which arrived at Jupiter in 1995.
Jupiter | Introduction
Planning for the next NASA mission to Jupiter began in 1977, just as the two Voyager probes were launched. Rather than performing a flyby of the Jupiter system like all the missions preceding it, the "Galileo" spacecraft would orbit Jupiter to perform close-up observations of the planet and its many moons, including Io, as well as deliver a Jovian atmospheric probe. Originally scheduled to be launched via the Space Shuttle in 1982, delays resulting from development issues with the shuttle and upper-stage motor pushed the launch back, and in 1986 the "Challenger" disaster delayed "Galileo's" launch even further. Finally, on October 18, 1989, "Galileo began" its journey aboard the shuttle "Atlantis".
Exploration of Io | Galileo : era 1995–2003
The trend in the densities of the four Galilean satellites found by "Pioneer 10" suggested that the satellites formed as part of a collapsing nebula, like a miniature version of what took place in the Solar System as a whole. The initial hot Jupiter prevented the condensation of water at the orbits of Io and Europa, leading those bodies to have higher densities than the outer two moons. Spectroscopic measurements of the light reflected from Io and its surrounding space were made with increasing spectral resolution during the 1970s, providing new insights into its surface composition. Other observations suggested that Io had a surface dominated by evaporites composed of sodium salts and sulfur.
Exploration of Io | "Pioneer" era: 1973–1979
It belongs to the Carme group, made up of irregular retrograde moons orbiting Jupiter at a distance ranging between 23 and 24 Gm and at an inclination of about 165°. This moon is currently considered lost.
S/2003 J 9 | Introduction
In his book "Mundus Iovialis" ("The World of Jupiter"), published in 1614, Simon Marius, the court astronomer to the Margraves of Brandenburg-Ansbach, claimed to have discovered Io and the other moons of Jupiter in 1609, one week before Galileo's discovery. According to Marius, he began observing the Jupiter system in late November 1609. He continued to observe the moons of Jupiter through December 1609, but did not record his observations until December 29, 1609 when he came to the conclusion "that these stars moved round Jupiter, just as the five solar planets, Mercury, Venus, Mars, Jupiter, and Saturn revolve round the Sun." However, Marius' observations were dated based on the Julian calendar, which was 10 days behind the Gregorian calendar used by Galileo.
Exploration of Io | Discovery: 1610
The mean angular diameter of the Sun as viewed from Jupiter is 372 arc-seconds, or 6' 12" (about that of the Sun as viewed from Earth), varying slightly from 381" at perihelion to 357" at aphelion. Unlike the near coincidence of the apparent sizes of the Moon and Sun as viewed from Earth, this perspective exaggerates the apparent diameters of all the Galilean moons in comparison to the Sun. Even distant Callisto is over 50% larger, and Io is nearly six times as large. This disparity in angular size makes the moons' shadows on Jupiter more defined than the lunar shadow on Earth during a total solar eclipse, as it narrows the penumbra for a given distance.
Solar eclipses on Jupiter | Visibility from Jupiter
Jupiter's moon Io has an orbital period of 1.769 days, nearly half that of the next satellite Europa (3.551 days). They are in a 2:1 orbit/orbit resonance. This particular resonance has important consequences because Europa's gravity perturbs the orbit of Io. As Io moves closer to Jupiter and then further away in the course of an orbit, it experiences significant tidal stresses resulting in active volcanoes. Europa is also in a 2:1 resonance with the next satellite Ganymede.
Stability of the Solar System | Jovian moon resonance
The physical and orbital characteristics of the moons vary widely. The four Galileans are all over in diameter; the largest Galilean, Ganymede, is the ninth largest object in the Solar System, after the Sun and seven of the planets, Ganymede being larger than Mercury. All other Jovian moons are less than in diameter, with most barely exceeding . Their orbital shapes range from nearly perfectly circular to highly eccentric and inclined, and many revolve in the direction opposite to Jupiter's rotation (retrograde motion). Orbital periods range from seven hours (taking less time than Jupiter does to rotate around its axis), to some three thousand times more (almost three Earth years).
Moons of Jupiter | Characteristics
There are 79 known moons of Jupiter. The most massive of the moons are the four Galilean moons, which were independently discovered in 1610 by Galileo Galilei and Simon Marius and were the first objects found to orbit a body that was neither Earth nor the Sun. Much more recently, beginning in 1892, dozens of far smaller Jovian moons have been detected and have received the names of lovers or daughters of the Roman god Jupiter or his Greek equivalent Zeus. The Galilean moons are by far the largest and most massive objects to orbit Jupiter, with the remaining 75 known moons and the rings together composing just 0.003% of the total orbiting mass.
Moons of Jupiter | Introduction
Despite observations made in the 1990s by the "Galileo" spacecraft, very little is known about the moon's physical characteristics other than its size and the fact that it is tidally locked to Jupiter.
Adrastea (moon) | Introduction