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handout_Origins - came cool enough to allow liquid water to...

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HANDOUT FOR CHAPTER 2 Origins The objects comprising our solar system condensed about 5 billion years ago from a thin cloud that had been enriched by heavy elements made in exploding stars. Earth is density stratified. During its formation heavy materials fell inward to form the core, and lighter substances rose to become the outer layers. Ocean and atmosphere are the least dense of these layers. Earth first had a solid surface about 4.6 billion years ago. The ocean formed from clouds of steam and water vap
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Unformatted text preview: came cool enough to allow liquid water to rest on the surface. Life probably arose on Earth shortly after its formation, about 4 billion years ago. Life may have arisen in the deep ocean. Earth’s atmosphere has changed substantially through time. The most dramatic change began about 2 billion years ago when photosynthetic or-ganisms began producing large quantities of oxygen. Other planets and moons may have—or may have had—oceans....
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