The composition of the earth's early atmospheric will be studied using a one-dimensional photochemical model. Three specific questions will be addressed. The first is whether impacts could have converted a CO2-dominated atmosphere into a CO-dominated one during the first several hundred million years of the earth's history. The oxidation state of the atmosphere, and the C/O ratio in particular, is an important boundary condition on the origin of life. The second question deals with the coupling between ammonia, methane, and sulfur in the early atmosphere. Elemental sulfur vapor could have provided an effective screen against solar near-UV radiation if the early earth was warmer than today. By shielding ammonia and methane from photolysis, such a screen may have allowed much higher concentrations of these gases to exist in the prebiotic atmosphere. This, too, could have affected the origin of life. The third question concerns the transition from a reducing to an oxidizing atmosphere. The photochemical model will be used to simulate atmospheres containing low levels of O2 (about 10-10 to 10-5 times present) and to determine where the oxygen concentration would have stabilized once photosynthesis had overwhelmed the supply of reduced gases.
