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The Hubble Space Telescope

By: Samantha Johnson on Oct. 16, 2009, 11:21 p.m.

Photo: NASA, ESA, and E. Karkoschka (University of Arizona)

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Jupiter and Ganymede

Ganymede, Jupiter’s largest moon, is also the largest moon in the entire solar system -- even bigger than the planet Mercury. It is also the only moon in the solar system that has its own magnetic field, most likely because it has a molten iron core like that of Earth. Scientists also believe there is a salt water ocean 200 kilometers below the surface of Ganymede, trapped between layers of ice.