Far UV irradiation of model prebiotic atmospheres

UV light has been the most important energy source on the primitive Earth. However, very few experiments have been performed to test directly the possible role of this energy source on the chemical evolution of the primitive atmosphere, mainly on account of experimental difficulties. Experiments are generally performed with other excitations, mainly electric discharge, and it is frequently assumed that UV irradiation would give similar results.As theoretical considerations make this assumption questionable, direct experimental controls have been undertaken: Model primitive atmospheres have been submitted to 147 nm UV light and the gaseous phase has been analysed. Preliminary qualitative results concerning CH₄–NH₃ atmospheres are reported.Irradiation of pure CH₄ gives rise to the synthesis of a large number of hydrocarbons, mainly saturated hydrocarbons but including also unsaturated ones as, C₂H₂, C₂H₄, C₃H₆, C₃H₄. These insaturated hydrocarbons are synthetized at a very low rate when ammonia is present in the medium.Irradiations of CH₄–NH₃ mixtures give rise, in addition to hydrocarbons, to important amounts of HCN (about 0.1%) and to lesser amounts of CH₃CN and C₂H₅CN. No unsaturated nitriles such as acrylonitrile and cyanoacetylene have been detected. Search for amines is in progress.These results evidence that UV irradiation may contribute largely to synthesis of HCN in CH₄–NH₃ atmospheres and, consequently to the synthesis of many biochemical compounds that can be derivated from HCN. However, synthesis of other compounds, such as pyrimidines, which can derivate from other nitriles, such as cyanoacetylene, cannot be initiated only by UV light, contrary to electric discharges. In addition, if electric discharges are very efficient for synthesis of nitriles in CH₄–N₂ atmospheres, there is not yet evidence that UV light is able to do so.Presented at the 2nd ISSOL Meeting and the 5th ICOL in Kyoto, 5–10 April, 1977.