Crystal CozartESC1000November 30, 2009Ch. 17 Homework3.) Earth’s primary atmosphere was predominantly hydrogen and helium. After these light elements boiled off into space, bolides carried gases to create a secondary atmosphere. By 4 bya, outgassing combined with reactions of gases with rocks of the geosphere, created an atmosphere rich in hydrogen and carbon dioxide. Outgassingis the release of gases from the interior of Earth. Most scientists postulated that the Hadean atmosphere was composed mainly of carbon dioxide, with smaller amounts of nitrogen, water, and other gases. 4.) Photosynthesis by primitive cyanobacteria began producing oxygen about 2.7 bya. However, for 300 million years, the oxygen concentration did not increase dramatically in the atmosphere due to two processes: 1.) As the oxygen concentration increased, it initially reacted with dissolved iron in sea water to form banded ion formations. Oxygen concentration rose to its present level after nearly all the dissolved iron had been removed by precipitation.
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