Development of the Hydrosphere and Atmosphere

Developme nt of the Hydrosphere and Atmosphere , with Special Refer ence to Probable Composition of the Early Atmosphere * Abstract A satisfactory hypothesis of the development of the hydrosphere and atmosphere depends upon evidence from many sciences and the solution of many other fundamental problems of earth history. But because it is so closely related to many other problems, any progress toward unravelling the history of the hydrosphere and atmosphere limits the range of permissible speculation about such distantly related questions as the origin of the solar system, continents, mountains, and living organisms. Several hypotheses of the source of the earth’s air and waters are examined for their consistency with established principles and observed geologic evidence, and special attention is given to the probable composition of the early atmosphere. Hypotheses of the origin of the atmosphere and hydrosphere fall into two chief categories: (1) that all air and water of the earth are residual from a dense primitiv e atmosphere that once enveloped a molten globe; or (2) that they have accumulated at the earth’s surface by leakage from the interior. The quantities of water, carbon dioxide, organic carbon, nitrogen, sulfur, etc., that have been or are now part of the earth’s atmosphere and hydrosphere may be estimated within reasonable limits of uncertainty and these “excess” volatiles afford a basis for testing chemical consequences of the alternative hypotheses. Several writers have suggested that the primitive atmosphere may have been composed largely of CH 4 and NH 3 . However, the equilibrium constants for reactions of these and other gases, combined with the evidence of the “excess” volatiles, indicate that CO 2 and N 2 are much more likely. The stabilities of methane and ammonia depend upon the presence of free hydrogen; and the escape rate of hydrogen from the earth is such that methane probably could have persisted in significant amounts in the early atmosphere no more than 10 6 to 10 8 years. For all but a relatively brief period at the very beginning of earth history, the atmosphere probably contai ned CO 2 and N 2 rather than CH 4 and NH 3 . When the consequences of a dense atmosphere of CO 2 and N 2 (but with almost no free O 2 or H 2 ) are examined, it is found that several chemical effects (such as the quantity of rocks that would have to be weathered, of sodium dissolved in sea water, and of CaCO 3 deposited on the sea floor very early in early history) are not

Development of the Hydrosphere and Atmosphere

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