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Jupiter’s system of moons is replete with oddballs. Io, Jupiter’s closest moon, is tidally locked and structurally stressed by interactions with Jupiter and with other moons, pumping enough heat into the little orb to render molten its interior rocks; Io is the most volcanically active place in the solar system. Jupiter’s moon Europa has enough H2O that its heating mechanism—the same one at work on Io—has melted the subsurface ice, leaving a warmed ocean below. If ever there was a next-best place to look for life, it’s here.
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Neil deGrasse Tyson (Astrophysics for People in a Hurry (Astrophysics for People in a Hurry Series))