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The preponderance of geologic evidence does not support carbon dioxide as the main carbon species degassed from early Earth, nor a carbon dioxide–rich early atmosphere. In fact, there are several problems that cannot be addressed by assuming either of these facets of what has become conventional wisdom about the early atmosphere. A careful examination of the conditions that most likely accompanied late accretion, incorporating the most probable average composition of accreting materials, suggests an early atmosphere produced by degassing of reduced carbon and nitrogen species, followed by photochemical processing to yield a surface environment rich in organic compounds. Recycling of these organics through hydrothermal and volcanic systems would have maintained a level of reduced gases (photochemically unstable as they may be) in the early atmosphere for an extended period, accompanied by a growing carbon dioxide component derived from mantle magmatism. Such a model for the early atmosphere is not only consistent with geological data, it also solves many problems of the early history of Earth.

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