The Geologic History of Earth. Note the timescales. We are currently in the Holocene, which has been warm and moist and a great time to grow human civilization. But the activity of civilization is now pushing the planet into a new epoch which scientists call the Anthropocene. Ray Troll/Troll Art hide caption

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Ray Troll/Troll Art

(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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NASA

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

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Mary P. Hrybyk-Keith/NASA