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Volcanology of Io, a moon of Jupiter, is the scientific study of lava flows, volcanic pits, and volcanism (volcanic activity) on the surface of Io. Its volcanic activity was discovered in 1979 by ''Voyager 1'' imaging scientist Linda Morabito. Observations of Io by passing spacecraft (the ''Voyagers'', ''Galileo'', ''Cassini'', and ''New Horizons'') and Earth-based astronomers have revealed more than 150 active volcanoes. Up to 400 such volcanoes are predicted to exist based on these observations. Io's volcanism makes the satellite one of only four known currently volcanically active worlds in the Solar System (the other three being Earth, Saturn's moon Enceladus, and Neptune's moon Triton). First predicted shortly before the ''Voyager 1'' flyby, the heat source for Io's volcanism comes from tidal heating produced by its forced orbital eccentricity. This differs from Earth's internal heating, which is derived primarily from radioactive isotope decay and primordial heat of accretion. Io's eccentric orbit leads to a slight difference in Jupiter's gravitational pull on the satellite between its closest and farthest points on its orbit, causing a varying tidal bulge. This variation in the shape of Io causes frictional heating in its interior. Without this tidal heating, Io might have been similar to the Moon, a world of similar size and mass, geologically dead and covered with numerous impact craters.〔 Io's volcanism has led to the formation of hundreds of volcanic centres and extensive lava formations, making it the most volcanically active body in the Solar System. Three different types of volcanic eruptions have been identified, differing in duration, intensity, lava effusion rate, and whether the eruption occurs within a volcanic pit (known as a ''patera''). Lava flows on Io, tens or hundreds of kilometres long, have primarily basaltic composition, similar to lavas seen on Earth at shield volcanoes such as Kīlauea in Hawaii. Although most of the lava on Io is made of basalt, a few lava flows consisting of sulfur and sulfur dioxide have been seen. In addition, eruption temperatures as high as were detected, which can be explained by the eruption of high-temperature ultramafic silicate lavas. As a result of the presence of significant quantities of sulfurous materials in Io's crust and on its surface, some eruptions propel sulfur, sulfur dioxide gas, and pyroclastic material up to into space, producing large, umbrella-shaped volcanic plumes. This material paints the surrounding terrain in red, black, and/or white, and provides material for Io's patchy atmosphere and Jupiter's extensive magnetosphere. Spacecraft that have flown by Io since 1979 have observed numerous surface changes as a result of Io's volcanic activity. ==Discovery== Before the ''Voyager 1'' encounter with Io on March 5, 1979, Io was thought to be a dead world much like the Moon. The discovery of a cloud of sodium surrounding Io led to theories that the satellite would be covered in evaporites. Hints of discoveries to come arose from Earth-based infrared observations taken in the 1970s. An anomalously high thermal flux, compared to the other Galilean satellites, was discovered during measurements taken at an infrared wavelength of 10 μm while Io was in Jupiter's shadow. At the time, this heat flux was attributed to the surface having a much higher thermal inertia than Europa and Ganymede. These results were considerably different from measurements taken at wavelengths of 20 μm, which suggested that Io had similar surface properties to the other Galilean satellites.〔 It has since been determined that the greater flux at shorter wavelengths was due to the combined flux from Io's volcanoes and solar heating, whereas solar heating provides a much greater fraction of the flux at longer wavelengths. A sharp increase in Io's thermal emission at 5 μm was observed on February 20, 1978 by Witteborn, ''et al.'' The group considered volcanic activity at the time, in which case the data was fit into a region on Io in size at . However, the authors considered that hypothesis unlikely, and instead focused on emission from Io's interaction with Jupiter's magnetosphere. Shortly before the ''Voyager 1'' encounter, Stan Peale, Patrick Cassen, and R. T. Reynolds published a paper in the journal ''Science'' predicting a volcanically modified surface and a differentiated interior, with distinct rock types rather than a homogeneous blend. They based this prediction on models of Io's interior that took into account the massive amount of heat produced by the varying tidal pull of Jupiter on Io caused by its slightly eccentric orbit. Their calculations suggested that the amount of heat generated for an Io with a homogeneous interior would be three times greater than the amount of heat generated by radioactive isotope decay alone. This effect would be even greater with a differentiated Io.〔 ''Voyager 1'' On March 8, 1979, three days after passing Jupiter, ''Voyager 1'' took images of Jupiter's moons to help mission controllers determine the spacecraft's exact location, a process called optical navigation. Although processing images of Io to enhance the visibility of background stars, navigation engineer Linda Morabito found a tall cloud along its limb.〔 At first, she suspected the cloud to be a moon behind Io, but no suitably sized body would have been in that location. The feature was determined to be a plume generated by active volcanism at a dark depression later named Pele. Following this discovery, seven other plumes were located in earlier ''Voyager'' images of Io.〔 Thermal emission from multiple sources, indicative of cooling lava, were also found. Surface changes were observed when images acquired by ''Voyager 2'' were compared to those taken four months previously by ''Voyager 1'', including new plume deposits at Aten Patera and Surt. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Volcanology of Io」の詳細全文を読む スポンサード リンク
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