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Life's lucky 7: The places most likely to harbor aliens

By: Bryan Nelson on Feb. 17, 2010, 12:20 p.m.

Photo: National Oceanic and Atmospheric Administration/Wiki Commons

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Ganymede is Jupiter's largest moon as well as the largest moon in our solar system. Its surface is composed mostly of silicate rock and water ice, but a saltwater ocean is believed to exist nearly 200 kilometers below Ganymede's surface, sandwiched between layers of ice. Like Europa and other moons close to the larger planets, Ganymede is warmed by tidal heating, which allows certain layers under the moon's surface to remain hot enough for liquid water to form.