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Comets and the Origin and Evolution of Life

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Series of articles considering the role of comets in the evolution of Earth's biosphere. For example, A. Delsemme writes about The Origin of the Atmosphere and of the Oceans. It is argued that Earth's atmosphere and its oceans, as well as much of the carbon found in its biosphere, was brought to the Earth by comets. Although present-day impacts of comets are modest, in the first billion years, and especially first half billion years, of the Earth's existence, a large bombardment of comets occurred, as reflected by the cratering record observed on the Moon, Mars and Mercury. The cometary delivery of matter would explain the siderophile metals in the Earth's crust, and would explain how an ocean and atmosphere remained despite depletion by large body impacts.