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Io
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Since the early 1960's Io has been known to be an important member of the Jovian system because of its control over some of the Jovian decametric radio emissions. However, it wasn't until the discovery of the very high level of volcanic activity by Voyager that the full breadth of the Io influence on the Jovian system was realized. Io spews out literally tons of material (mainly sulfur and oxygen) per second. One of the results of this activity is the formation of a donut-shaped cloud of plasma surrounding Jupiter near Io's orbit called the Io plasma torus. One of the first results of the Galileo plasma wave investigation at Jupiter was to measure the density of the plasma in the Io torus and to detect very high densities very close to Io.
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Io was an Argive princess and the daughter of Inachus, an ancient hero or river god of Argos. She ... had the misfortune to be subjected to the lust of Zeus. Zeus, in an attempt to avoid the rage and jealousy of Hera, his wife, transformed Io into a handsome white heifer. Hera, who knew Zeus was up to no good, asked for the heifer as a present. Zeus could not refuse. Hera deposited Io in the safe keeping of Argus, the watchman with a hundred eyes.
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The Io moth, named for a character in Greek mythology, has a very obvious black eyespot on each hindwing. Males are golden yellow in color, but females are more brown. It has a wingspan of 2.5-3.5 inches. In Canada and northern states, the io moth produces only one brood a year. Farther south in the United States, they sometimes produce two broods a year, depending on springtime weather. Adults emerge and fly in late April to May and then again July and August.
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Io ... seems to have no qualms about mixing the two volcanic "styles." The calderas, which are also less circular than similar structures on Earth, seem to be linked to cracks in the moon's crust. In one area, a string of the structures follows a line across the surface, taking a right-angle bend.
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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.
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This pair of Hubble images of Jupiter's volcanic moon Io shows the surprising emergence of a 320-kilometer (200-mile) diameter, large yellowish-white feature near the center of the moon's disk (photo on the right). Scientists suggest the spot may be a new class of transient feature on the moon. For comparison, the photo on the left was taken in March 1994 before the spot emerged, and shows that Io's surface had undergone only subtle changes since it was last seen by the Voyager 2 probe in 1979. The new spot in the July 1995 Hubble image replaces a smaller whitish spot seen at about the same location in the March 1994 image. "The new spot surrounds the volcano Ra Patera, which was photographed by Voyager, and is probably composed of material, probably frozen gas, ejected from Ra Patera by a large volcanic explosion or fresh lava flows," according to John Spencer of Lowell Observatory in Flagstaff, Arizona. (Credit: J. Spencer, Lowell Observatory/NASA)
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