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.