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2.1: Introduction
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2.2: Origin of the Elements
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2.3: Formation and evolution of the Earth
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The solar system is believed to have formed about 5 billion years ago as a result of aggregation of cosmic dust and interstellar atoms in a region of space in which the density of such material happened to be greater than average. Over 99.8% of this mass, which consisted mostly of hydrogen, collapsed into a proto-sun; the gravitational energy released in this process raised the temperature sufficiently to initiate the hydrogen fusion reactions discussed above.
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2.4: The Earth's crust
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2.5: Origin of the atmosphere and the oceans
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2.6: Earthlike Planets - Venus and Mars
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Venus and Mars are the nearest planets and the most accessible to spacecraft. Not surprisingly, the greatest effort in planetary exploration has been devoted to these fascinating worlds. In the chapter, we discuss some of the results of more than four decades of scientific exploration of Mars and Venus. Mars is exceptionally interesting, with evidence that points to habitable conditions in the past.
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2.7: Summary
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