Prebiotic synthesis in atmospheres containing CH4, CO, and CO2

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Abstract
The prebiotic synthesis of organic compounds using a spark discharge on various simulated primitive earth atmospheres at 25°C has been studied. Methane mixtures contained H2+CH4+H2O+N2+NH3 with H2/CH4 molar ratios from 0 to 4 and pNH3=0.1 torr. A similar set of experiments without added NH3 was performed. The yields of amino acids (1.2 to 4.7% based on the carbon) are approximately independent of the H2/CH4 ratio and whether NH3 was present, and a wide variety of amino acids are obtained. Mixtures of H2+CO+H2O+N2 and H2+CO2+H2O+N2, with and without added NH3, all gave about 2% yields of amino acids at H2/CO and H2/CO2 ratios of 2 to 4. For a H2/CO2 ratio of 0, the yield of amino acids is extremely low (10−3%). Glycine is almost the only amino acid produced from CO and CO2 model atmospheres. These results show that the maximum yield is about the same for the three carbon sources at high H2/carbon ratios, but that CH4 is superior at low H2/carbon ratios. In addition, CH4 gives a much greater variety of amino acids than either CO or CO2. If it is assumed that an abundance of amino acids more complex than glycine was required for the origin of life, then these results indicate the requirement for CH4 in the primitive atmosphere.