The planet Jupiter travels with a large entourage -- more than 60 known moons. The biggest are complex worlds all their own. And of those, the three that are closest to Jupiter have a complex relationship -- one that changes each of them.
In order of their distance from Jupiter, the moons are Io, Europa, and Ganymede. Their orbits are such that for every revolution that Ganymede makes around Jupiter, Europa makes two and Io makes four.
Jupiter's powerful gravity keeps all three moons "locked" so that the same side always faces the planet, just as the same side of our own moon always faces Earth.
But the gravity of the moons tries to change that. As they pass by each other, each moon's gravity tries to turn the others. That twists and heats their interiors, creating some interesting effects.
Io, for example, is the most volcanically active body in the solar system. Its volcanoes belch superhot lava onto the surface, and blast hot gas high into the sky.
Europa isn't as hot as Io, but it is warm enough to melt some of the ice beneath its frozen crust. That may create a hidden ocean -- a possible abode for life.
Ganymede doesn't feel as strong a tug from the other moons, which line up on the same side as Jupiter. So Ganymede's surface is older and quieter. Even so, its interior is still partially molten -- heated up by its sibling moons.
Look for Jupiter beneath our own Moon this evening. It looks like a brilliant star.
Script by Damond Benningfield, Copyright 2009
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