IO

False-colour picture of Io, a satellite of Jupiter. This picture is a False-colour infrared image of the surface of Io, one of the satellites of innermost of Jupiter's four large Galilean satellites and one of the few objects in the solar system definitely known to have active volcanoes. It was discovered by the Italian astronomer Galileo Galilei in 1610 and named by the German astronomer Simon Marius after a figure in Greek mythology. Io has a period of rotation equal to its period of revolution (1.769 Earth days) and so always keeps the same face to Jupiter. Its nearly circular orbit has an inclination of 0.04 to Jupiter's equatorial plane and a radius of about 422,000 km (262,219 miles). Io's orbital position to some extent controls the beaming of bursts of radio noise at decametre wavelengths from Jupiter to Earth. Auroras on Io, a satellite of Jupiter, are visible as glowing patches of green, red, and violet. Io has a mass of 8.92 1022 kg and a diameter of about 3,630 km (2,256 miles). Its average density of approximately 3.5 grams per cubic cm is characteristic of rocks but not ices. Io has a very thin atmosphere, composed in large part of sulfur dioxide. Its surface is smooth, without any evidence of impact craters, and is covered primarily by sulfur and sulfur compounds. Beneath the crust is a layer of molten rock and a core of molten iron and iron sulfide approximately 1,800 km (1,110 miles) in diameter. False-colour composite of the surface of Io, a satellite of Jupiter. Two When the Voyager 1 spacecraft flew by Io on March 5, 1979, nine active volcanoes were observed ejecting fountains of fine particles several hundred kilometres into space. Four months later, during the passage of Voyager 2, one of the volcanoes had stopped erupting, while another had begun. One of the volcanoes, Prometheus, was observed again by the Galileo spacecraft in 1997; Galileo also observed a second large volcano, Pillan Patera. Volcanic material ejected from the surface occupies a toroid that follows Io's orbit and wraps part of the way around Jupiter. The ejected material contains mostly ionized particles of oxygen, sodium, and sulfur with smaller amounts of hydrogen and potassium also present. Black, red, and orange downhill-flow features are observed near some of the volcanic calderas. Io's extraordinary multicoloured surface is believed to be caused by various sulfur compounds undergoing phase changes at different temperatures. The volcanic flows are so extensive and so frequent that the entire satellite is being resurfaced to a depth of several metres every few thousand years. in Greek mythology, daughter of Inachus, the river god of Argos. 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 (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 bother 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 became the mother of Epaphus. Io was thus 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.