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	Io At a Glance
	Discovery 1610, Galileo Galilei Diameter 2,273 miles 3,660 km Distance from Jupiter 262,000 miles 422,000 km Orbital Period 1.8 days Compare Moons

Robotic probes may someday provide close-up views of some of the most remarkable vistas in the solar system, from the canyons of Mars to the ice-geysers of Triton. For a true hot-spot, they might show us the surface of Io, one of the moons of Jupiter. It is an eerie landscape of active volcanoes, tall mountains, and plains covered with frozen sulfur.

A volcanic eruption sends ash spewing far above Io’s surface.

Io's interior is heated by a tug-of-war between Jupiter and the planet's other big moons. Io is "locked" so that the same hemisphere always faces the planet, just as the same hemisphere of our own moon always faces Earth. But as the other moons move past Io, their gravity tugs at it, too. That heats Io's interior enough to melt some of its rock, which "bubbles" to the surface.

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