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A review paper on the subject of Earth's early atmosphere must, of necessity, be incomplete. A book, or perhaps several books, would be required to do justice to the topic. Here, I focus on four particular subtopics: formation of the atmosphere and ocean, the prebiotic atmosphere, long-term climate evolution, and the rise of O sub 2 levels. Each of these subtopics has seen an influx of new ideas over the past several years, although it would be presumptuous to claim that any of them are well understood as a result. To further limit the scope of this review, I restrict my discussion to the Precambrian Era, that is, the period before ==540 million years ago. Fluctuations in atmospheric CO sub 2 and O sub 2 levels have almost certainly occurred since that time (1), but these are second-order perturbations by comparison to the changes that took place earlier.
FORMATION OF THE ATMOSPHERE AND OCEAN
Theories for how the atmosphere and ocean formed must begin with an idea of how the Earth itself originated. We are now reasonably certain that the terrestrial planets formed by accretion of solid materials that condensed from the solar nebula (2). Any primary, captured atmosphere (if one existed at all) must have been lost, as evidenced by the pronounced depletion of rare gases in Earth's atmosphere compared to cosmic abundances (3). The present, secondary atmosphere was generated from volatile compounds contained within the solid planetesimals from which the Earth formed. Thirty years ago, it was believed that the Earth formed relatively slowly, with a cold interior, and that most of its volatiles were originally trapped inside the planet (4, 5). As time passed, the Earth's interior was heated by radioactive decay, and the trapped gases were gradually released by volcanic outgassing. These volcanic gases would have been highly reduced [containing H sub 2 , methane (CH sub 4 ), and ammonia (NH sub 3 )] until the Earth's core formed, after which time they would have been similar to modern volcanic gases (containing H sub 2 O, CO sub 2 , and N sub 2 , with traces of H sub 2 and CO) (5).
More recent models of planetary accretion (6-8) suggest that the Earth formed in 10 to 100...