Photochemical abiotic synthesis of amino-acid precursors from simulated planetary atmospheres by vacuum ultraviolet light

Abstract

For the purpose of investigating the photon energy dependence of the photoinduced abiotic synthesis of organic molecules, gas mixtures that simulate typical planetary atmospheres, including a carbon source (CO or CH4), a nitrogen source (N2 or NH3), and H2O, were irradiated with synchrotron radiation through a vacuum-ultraviolet transmitting window. Three kinds of window material, fused silica, synthetic quartz, and MgF2, were used as a high-energy-cutting filter, whose absorption-edge energies are 6.4, 8.1, and 10.5eV, respectively. Three types of gas mixture, Titan-type (CH4–N2–H2O), comet-type (CO–NH3–H2O), and primitive-Earth-type (CO–N2–H2O), were irradiated with vacuum-ultraviolet photons in the three energy ranges. After the irradiation, amino-acid formation yields in the acid-hydrolyzed solution of the product were measured with a high-performance liquid chromatograph method. From the Titan- and comet-type mixtures, amino acids were detected by irradiation with photons lower than 8.1eV. For both mixtures, the averaged quantum yields of glycine generation in the photon energy region of 7–10.5eV were of the order of 10−5, which was larger by about one order than that in the region 5–8eV. On the other hand, from the primitive-Earth-type mixture, amino-acid formation was difficult to detect even with irradiation as high as 10.5eV, even though amino acids were generated in comparable yields from the Titan- and comet-type mixtures by irradiation with soft x rays or proton beam, whose energies are much higher. These results suggest that the vacuum ultraviolet light is a more effective energy source for the generation of the precursors of bioorganic compounds in extraterrestrial environments than in primitive-Earth atmosphere.