Io Data

Just larger than our Moon, Io has a pizza-like surface that is evidence of the fact that it is the most volcanically active body in the solar system (some 3 dozen volcanoes are active). Jupiter's immense gravity is thought to be responsible, tugging at Io and heating the moon's core.

The before-and-after images below, taken by the Galileo spacecraft, show the emergence of a 250-mile-wide grey splotch caused by an eruption of a volcano called Pillan Patera.

There are no impact craters on Io, because the surface is constantly remade by volcanic activity. Any water there might have been on Io probably disappeared billions of years ago when the volcanic activity started, researchers say. Gases from the volcanoes are spewed hundreds of miles into space.

Io is the most dense of all the Galilean satellites, and it is thought to contain a higher percentage of solid silicates inside, and perhaps a solid core.

Data on the Galilean moons

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Photos & images: NASA's Galileo imaging team

Text by Robert Roy Britt

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