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Io (astronomy)

Io (astronomy), large satellite of the planet Jupiter. Io is the fifth known moon in distance from the planet. The moon circles Jupiter in a slightly elliptical (oval-shaped) orbit over Jupiter's equator at an average distance of 422,000 km (262,200 mi). It completes an orbit once every 1.769 Earth days and rotates once in the same period. Io is slightly larger than Earth's moon, with a radius of about 1,820 km (about 1,130 mi). Like Earth's moon, it ranks among the ten largest moons in the solar system.

Io has a large, dense iron core at its center surrounded by a mantle that contains molten compounds of silicon and oxygen. Its crust, the moon's outer layer, is made up mostly of sulfur and sulfur compounds, which color the moon's surface with areas of yellow, orange, red, white, blue, brown, and black. Io is the most volcanically active world in the solar system, with hundreds of volcanoes dotting its surface. Some shoot plumes of molten sulfur and sulfur dioxide gas up to 300 km (186 mi) high. These volcanic vents also release lava hotter than any other planetary surface temperature ever recorded in the solar system—as hot as 1727C (3140F). Astronomers have identified magnesium-rich silicates, a type of lava that only melts at very high temperatures, around these vents.

Most of Io's volcanism is driven by gravitational forces that act on the planet. The gravitational force created by Jupiter's mass would make Io travel in a smooth elliptical path, if the gravitational forces of Ganymede and Europa, two other large satellites of Jupiter, did not also affect Io's orbit. Io is squeezed and stretched by the conflicting attractive forces of Jupiter, Ganymede, and Europa, causing rocks inside the moon to rub together and produce heat. In addition, Io's orbit takes the moon through Jupiter's powerful magnetic field, generating about 1 trillion watts of electricity—and yet more heat—within the satellite.

Io's largest volcanic feature is Ra Patera, a volcano surrounded by lava flows that stretch up to 300 km (186 mi) from the volcano to cover an area about the size of the state of New Jersey. The Haemus Mountains at Io's south pole measure up to 10 km (6 mi) above the surrounding area, making them higher than Mount Everest, the highest mountain on Earth. Io has a thin atmosphere of sulfur dioxide gas emitted by the volcanoes. Sometimes the sulfur dioxide freezes on Io's surface and produces a white frost.

Io was discovered independently in 1610 by Italian astronomer Galileo and German astronomer Simon Marius. Io and the three other largest moons of Jupiter, all found in 1610, are called the Galilean moons. Marius named Io and the other Galilean moons for mythical lovers of the Greek god Zeus, whom the Romans renamed Jupiter. Zeus transformed Io into a cow to protect her from his jealous wife, Hera. Io's volcanoes are named for gods of fire, the sun, and thunder from mythologies from all over the world.

The United States spacecraft Voyager 1 and 2, which passed Jupiter in 1979, and the Galileo spacecraft, which went into orbit around Jupiter in 1995, have provided much of the information astronomers know about Io. In 1999, for example, Galileo took detailed photographs of Io's volcanic eruptions; scientists hope to use the observations to learn more about similar volcanic activity that occurred on Earth eons ago. The Hubble Space Telescope has also made important observations of Io's volcanic activity.

Io, One of Jupiter's Moons

The Voyager 1 spacecraft launched by the United States National Aeronautics and Space Administration (NASA) photographed both hemispheres of Io, the innermost moon of Jupiter, in 1979. The hemisphere shown at left always faces Jupiter because Io's period of revolution around the planet is equal to its rotation around its own axis. The moon's colors depict its many volcanoes and the large lava flows and sulphur-dioxide snow resulting from Io's tremendous volcanic activity. During the three months between the photos of Io taken by Voyager 1 and Voyager 2, the surface of the moon changed dramatically—some volcanos stopped erupting while previously dormant volcanos became active.

Photo Researchers, Inc./US Geological Survey/NASA