The largest moon in the solar system, Ganymede is bigger than both Pluto and Mercury. This 3,270 mile- (5,262 km-) wide moon has a rocky interior and an upper mantle of ice. Its icy crust has contrasting dark and bright areas.
the planet. These are Jupiter's so-called Galilean moons, observed by the Italian astronomer Galileo Galilei in 1610.
lo circles Jupiter every 42.5 or so hours. It is the innermost of the Galileans, and contrasts sharply with the other three. They are worlds of rock and ice, whereas Io is the most volcanic body in the solar system. The surface is constantly renewed by eruptions of molten material through the moon's hundreds of volcanoes and vents.
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