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Io, 2,262 mi (3,640 km) in diameter, is the most spectacular of the Galilean moons. Its brilliant colors of red, orange, and yellow set it apart from any other moon or planet. Active volcanoes have been detected on Io, with some plumes extending up to 200 mi (320 km) above the surface. The relative smoothness of Io's surface and its volcanic activity suggest that it has the youngest surface of Jupiter's moons. Its surface is composed of large amounts of sulfur and sulfur-dioxide frost, which account for the primarily yellow-orange surface color.

The volcanoes seem to eject a sufficient amount of sulfur dioxide to form a doughnut-shaped ring (torus) of ionized sulfur and oxygen atoms around Jupiter near Io's orbit. Close-up views taken in 1999 and 2000 showed that Io had more than 100 erupting volcanoes, gigantic lava flows and lava lakes, and towering, collapsing mountains. The eruptions of Loki, the most powerful volcano in the solar system, can be seen by Earth telescopes.

Observations by Galileo during 1998 revealed dozens of volcanic vents on Io where lava is hotter than any surface temperatures recorded on any planetary body in our solar system. At one such volcanic vent, known as Pillan Patera, two of the spacecraft's instruments indicated that the lava temperature may have been 3,140°F.

In 1996, the Galileo spacecraft detected a huge iron core within Io that occupies half the moon's diameter. Galileo also discovered evidence that Io has its own magnetic field.

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