Oxygen levels in the prebiological atmosphere

Abstract

A photochemical model of the early terrestrial atmosphere is used to find oxygen profiles as a function of the hydrogen and carbon monoxide outgassing rates from volcanoes and the rate of oxidation of the earth's crust. Steady state solutions for the amount of O₂ in the atmosphere are possible only when the combined loss rate from all three processes can balance the production of oxygen from photodissociation of H₂O, followed by escape of hydrogen to space. If limiting flux controls the escape rate, then the production rate of oxygen atoms depends linearly upon the water vapor mixing ratio at the tropopause. Evidence is presented which indicates that the hydrogen outgassing alone was sufficient to overwhelm the production of oxygen, leading to ground level O₂ mixing ratios of the order of 10⁻¹² times the present atmospheric level (PAL) or below. The O₂ mixing ratio increases rapidly with altitude up to a peak of 10⁻⁵ PAL at 60 km owing to the presence of CO₂. Present‐day concentrations of N₂, CO₂, and H₂O are assumed in the calculation. The effect of possible temperature variations upon the H₂O content is discussed.