CHEMICAL EVENTS ON THE PRIMITIVE EARTH

Abstract

In this paper the nature of the primitive atmosphere and ocean is considered in the light of geologic and geophysical information. The hypothesis of an early methane-ammonia atmosphere is found to be without solid foundation and indeed is contraindicated. Geologists favor an alternative view-that genesis of air and oceans is a result of planetary outgassing. Some consequences of this view are examined. Volatiles from outgassing interacted with the alkaline crust to form an ocean having a pH 8-9 and tzo produce an atmosphere consisting of CO, C02, N2, and H2. Radiation interacting with such a mixture yields HCN as a principal product. Ultraviolet irradiation of HCN solutions at pH 8-9 yields amino acids and other important substances of biologic interest. The nature of the earth's environment limited the kinds of compounds that might have accumulated in a soup. Arguments concerning feasible components support the view that amino acids and proteins preceded sugars and nucleic acids. If the methane-ammonia hypothesis were correct, there should be geochemical evidence supporting it. What is the evidence for a primitive methane-ammonia atmosphere on earth? The answer is that there is no evidence for it, but much against it. The methane-ammonia hypothesis is in major trouble with respect to the ammonia component, for ammonia on the primitive earth would have quickly disappeared. The effective threshold for degradation by ultraviolet radiation is 2,250 A. A quantity of ammonia equivalent to present atmospheric nitrogen would be destroyed in -30,000 years. Small amounts of ammonia would be reformed, but this process is unimportant in comparison to the destruction. If large amounts of methane had ever been present in the earth's atmosphere, geologic evidence for it should also be available. Laboratory experiments show that one consequence of irradiating a dense, highly reducing atmosphere is the production of hydrophobic organic molecules which are adsorbed by sedimenting clays. The earliest rocks should contain an unusually large proportion of carbon or organic chemicals. This is not the case. The composition of the present atmosphere with respect to the gases neon, argon, krypton, and xenon is crucial. Neon is present on earth to an extent about 10-1O