Photochemistry of CO and H_2O: Analysis of Laboratory Experiments and Applications to the Prebiotic Earth's Atmosphere

Abstract

The role photochemical reactions in the early Earth's atmosphere played in the prebiotic synthesis of simple organic molecules was examined. We have extended an earlier calculation of formaldehyde production rates to more reduced carbon species, such as methanol, methane, and acetaldehyde. We have simulated the experimental results of Bar-Nun and Chang (1983) as an aid in the construction of our photochemical scheme and as a way of validating our model. Our results indicate that some fraction of CO_2 and H_2 present in the primitive atmosphere could have been converted to simple organic molecules. The exact amount is dependent on the partial pressure of CO_2 and H_2 in the atmosphere and on what assumptions are made concerning the shape of the absorption spectra of CO_2 and H_2O. In particular, the results are most sensitive to the presence or absence of absorption at wavelengths longward of 2000 Å. We also find that small quantities of CH_4 could have been present in the prebiotic Earth's atmosphere as the result of the photoreduction of CO.