Oxygen and ozone in the early earth's atmosphere

The precise amount of O2 and O3 in the earth's prebiological paleoatmosphere has been a topic of considerable discussion in the past. Since the photolysis of H2O and CO2, the prebiological mechanisms to produce O2, depends on the ultraviolet flux from the Sun, a reliable quantification of the problem requires detailed knowledge of such flux. Using the most recent astronomical observation of young stars from the International Ultraviolet Explorer, as well as a detailed photochemical model of the paleoatmosphere, it is found that the amount of O2 in the prebiological paleoatmosphere may have been as much as a million times greater than previously estimated. Some of the implications of this new value are discussed.