(Top row, left to right) Titan, Earth's moon, Europa and Enceladus. (Bottom row, left to right) Callisto, Charon, Ariel and lo. NASA hide caption

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Hot On The Trail Of Alien Moons
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Many of life's building blocks can be found in the objects bombarding Earth from outer space. Does that mean that life, too, developed elsewhere before arriving here? Mary P. Hrybyk-Keith/NASA hide caption

toggle caption Mary P. Hrybyk-Keith/NASA