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

Feb. 17, 2010, 12:20 p.m.
Callisto

Photo: Wiki Commons/public domain

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Callisto

As the second-largest moon in Jupiter's orbit, Callisto lies farther away from Jupiter than both Europa and Ganymede, and thus experiences less tidal heating. Even so, there are many who believe a liquid water layer may exist under the surface. Because of its low radiation levels, Callisto has long been considered the most suitable place for a human base (see PDF) for future exploration of the Jovian system.