Abstract
The absorption of ultraviolet radiation across the dissociative band (1500–2100 Å) is computed for the three component atmospheric system, O2, CO2, H2O, for a considerable range of possible primitive atmospheres. The results illustrate clearly the limitation on photodissociation of H2O as a source of oxygen, as a consequence of shadowing of H2O by the oxygen so produced. Thus the Urey self-regulation of the maximum oxygenic concentration in a primitive atmosphere (in the absence of appreciable photosynthetic oxygenic production) is shown as not exceeding about 10−3 of the present oxygenic concentration. The presence of excess carbon dioxide lowers this limit somewhat. These conclusions have important implications with respect to the limiting oxygenic content of the primitive atmosphere of any planet.
