/ee"oh/, n., pl. ios.
a small hawk, Buteo solitarius, having two plumage phases and occurring only on the island of Hawaii, where it is a rare species and the only living indigenous bird of prey. Also called Hawaiian hawk.
[ < Hawaiian 'io]

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In Greek mythology, the daughter of the river god Argos.

Io drew Hera's jealousy when Zeus fell in love with her. Zeus changed her into a white heifer to protect her. Hera sent the many-eyed creature Argus to watch over the heifer, but Zeus sent Hermes to lull Argus to sleep and kill him. Hera then sent a gadfly to pursue Io, who fled across Europe and crossed the bodies of water later named the Ionian Sea and the Bosporus ("Ford of the Cow") in her honour. When she arrived in Egypt she resumed her original form. She was later identified with the Egyptian goddess Isis.

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      in Greek mythology, daughter of Inachus (the river god of Argos) and the Oceanid Melia. Under the name of Callithyia, Io was regarded as the first priestess of Hera, the wife of Zeus. Zeus fell in love with her and, to protect her from the wrath of Hera, changed her into a white heifer. Hera persuaded Zeus to give her the heifer and sent Argus Panoptes (Argus) (“the All-Seeing”) to watch her. Zeus thereupon sent the god Hermes, who lulled Argus to sleep and killed him. Hera then sent a gadfly to torment Io, who therefore wandered all over the earth, crossed the Ionian Sea, swam the strait that was thereafter known as the Bosporus (meaning Ox-Ford), and at last reached Egypt, where she was restored to her original form and gave birth to Epaphus.

      Io was identified with the Egyptian goddess Isis, and Epaphus with Apis, the sacred bull. Epaphus was said to have been carried off by order of Hera to Byblos in Syria, where he was found again by Io. This part of the legend connects Io with the Syrian goddess Astarte. Both the Egyptian and the Syrian parts, in fact, reflect interchange with the East and the identification of foreign with Greek gods.

▪ satellite of Jupiter
also called  Jupiter I  
 innermost of the four large moons (Galilean satellites) discovered around Jupiter by the Italian astronomer Galileo in 1610. It was probably also discovered independently that same year by the German astronomer Simon Marius (Marius, Simon), who named it after Io of Greek mythology. Io is the most volcanically (volcano) active body in the solar system.

      Io rotates at the same rate that it revolves around Jupiter (1.769 Earth days) and so always keeps the same face to Jupiter. Its nearly circular orbit has an inclination of only 0.04° to Jupiter's equatorial plane and a radius of about 422,000 km (262,000 miles). The orbit is forced to be slightly eccentric by a gravitational resonance between Io and the Jovian moon Europa. The forced eccentricity causes intense tidal heating of Io—heating from internal friction due to continual flexing of the satellite—by Jupiter's powerful gravitational field, which is the source of energy that powers the volcanoes (volcano).

 Io is about 3,630 km (2,260 miles) in diameter, a little larger than Earth's Moon. Its average density of about 3.55 grams per cubic cm is characteristic of rocks but not ices. Io has a very tenuous atmosphere, composed in large part of sulfur dioxide (sulfur oxide). Its surface is a startling, vividly coloured landscape of erupting volcanic vents, pools and solidified flows of lava, and deposits of sulfur and sulfur compounds. There is no evidence of impact craters on this geologically young surface. The volcanic flows are so extensive and frequent that they are resurfacing the entire satellite to a depth of several metres every few thousand years. Beneath the crust lies a layer of molten rock and a core of molten iron and iron sulfide approximately 1,800 km (1,110 miles) in diameter.

 When the Voyager 1 spacecraft flew by Io on March 5, 1979, it observed nine active volcanoes ejecting fountains of fine particles several hundred kilometres into space. Observations at higher resolution by the Galileo spacecraft some 20 years later indicated that as many as 300 volcanoes may be active on the satellite at a given time. The silicate (silicate mineral) lava that is spewing out is extremely hot (approximately 1,900 K [3,000 °F, 1,630 °C]) and resembles lavas produced more than three billion years ago on Earth. Volcanic material ejected from the surface creates a toroidal (doughnut-shaped) cloud of charged particles that follows Io's orbit and wraps part of the way around Jupiter. The ejected material contains mostly ionized atoms of oxygen, sodium, and sulfur with smaller amounts of hydrogen and potassium. As the satellite travels in its orbit, passing through the magnetic field of Jupiter, it produces an electric current of some five million amperes along a flux tube of spiraling electrons that links Io with the giant planet.

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Universalium. 2010.

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