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Orbit: 422,000 km from Jupiter
Orbital period: 42 hours
Gravity: 1.796 m/s²
Diameter:  3639 km
Mass: 8.93e22 kg
Io is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of 3,642 kilometres, the fourth-largest moon in the Solar System.
Discovered by Galile     and Marius in 1610.In contrast to most of the moons in the outer solar system, Io and Europa may be somewhat       similar in bulk composition to the  terrestrial  planets, primarily composed of       molten silicate rock.       Recent data from  Galileo    indicates that Io has a core of iron (perhaps mixed with iron sulfide)       with a radius of at least 900 km. Io’s surface is radically different from any other body in the solar system.
Jupiter's moon Io
It came as a very big surprise to the        Voyager    scientists on the first encounter. They had expected to see impact craters       like those on the other terrestrial bodies and from their number per       unit area to estimate       the age of Io’s surface. But there are very few, if any, impact craters on Io (above).       Therefore, the surface is very young. Instead of craters, Voyager 1 found hundreds of volcanic  calderas.  Some of the volcanoes are active!
Striking photos of actual eruptions       with plumes 300 km high were       sent back by both Voyagers (above) and by Galileo     This may have been the most important single       discovery of the Voyager missions; it was the first real proof that the       interiors of other “terrestrial” bodies are actually hot and active.       The material erupting from Io’s vents appears to be some form of sulfur or       sulfur dioxide. The volcanic eruptions change rapidly. In just four months       between the arrivals of Voyager 1  and Voyager 2  some of them stopped       and others started up. The deposits surrounding the vents also changed       visibly.
Io mountain
Recent images taken with NASA’s Infrared Telescope Facility on Mauna Kea, Hawaii       show a new and very large eruption.      A large new feature near Ra Patera has also been seen by       HST.      Images from Galileo also      show many changes from the time of Voyager’s encounter.      These observations confirm that Io’s surface is very active indeed. Io has an amazing variety of terrains:       calderas up to several kilometers deep,       lakes  of molten sulfur (below right),       mountains which are apparently NOT volcanoes (above),       extensive flows hundreds of kilometers long       of some low viscosity fluid (some form of sulfur?),       and volcanic vents.       Sulfur and its compounds take on a wide range of colors which are       responsible for Io’s variegated appearance.

Analysis of the Voyager images led scientists       to believe that the lava flows on Io’s       surface were composed mostly of various compounds of molten sulfur.       However, subsequent ground-based infra-red studies indicate that they       are too hot for liquid sulfur.  One current idea is that Io’s       lavas are molten silicate rock.  Recent HST  observations  indicate that the       material may be rich in sodium.      Or there may be a variety of different materials in different locations. Some of the hottest spots on Io may reach temperatures as high as 2000 K though the average is much lower,       about 130 K.       These hot spots are the principal mechanism       by which Io loses its heat. Jupiter's moon Io The energy for all this activity probably derives from tidal interactions       between Io, Europa, Ganymede and Jupiter.       These three moons are locked into resonant    orbits such that Io orbits twice       for each orbit of Europa which in turn orbits twice for each orbit of       Ganymede. Though Io, like Earth’s Moon always       faces the same side toward its planet, the effects of Europa and Ganymede       cause it to wobble a bit. This wobbling stretches and bends        Io by as much as 100 meters (a 100 meter tide!)       and generates heat the same way a coat hanger       heats up when bent back and forth. (Lacking another body to  perturb it,   the Moon is not heated by Earth in this way.) Io electric current Io also cuts across Jupiter’s magnetic field lines, generating an       electric current. Though small compared to the tidal heating, this       current may carry more than 1 trillion watts.  It also strips some       material away from Io which forms a torus of intense radiation       around Jupiter.  Particles escaping       from this torus are partially responsible for Jupiter’s unusually   large magnetosphere. Recent data from Galileo indicate that Io may have its own magnetic field as does Ganymede. Io has a thin atmosphere composed of sulfur dioxide and perhaps some other gases. Unlike the other Galilean satellites, Io has little or no water. This is  probably because Jupiter was hot enough early in the evolution of the  solar system to drive off the volatile elements in the vicinity of Io but  not so hot to do so farther out.



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