82 



(<1500 A) and even then the yields are very low. The low yields are 

 probably a result of the low yields of HCN produced by UV light. If 

 the gas mixture is modified by adding gases such as H 2 S or formalde- 

 hyde, then reasonable yields of amino acids can be obtained at rela- 

 tively long wavelengths (<2500 A) where considerable energies from 

 the Sun are available. The H 2 S absorbs at these longer wavelenths 

 and is photodissociated to H and HS. The H atoms have a high veloc- 

 ity ("hot atoms") and react with the CH 4 and NH 3 . It is possible, 

 but not demonstrated, that HCN and other molecules are produced, 

 which then form amino acids in the aqueous part of the system. In 

 experiments simulating thermal environments, pyrolysis of CH 4 and 

 NH 3 gives low yields of amino acids. 



A second model of a reducing atmosphere with less hydrogen 

 would consist of CH 4 , N 2 , and traces of NH 3 and H 2 0. This atmo- 

 sphere is more consistent with current models of the primitive 

 Earth, though still consisting of hydrogenous compounds. Large 

 amounts of NH 3 would not have accumulated in the atmosphere 

 because of photodestruction and also because the NH 3 would dis- 

 solve in the ocean. 



This mixture of gases is quite effective with an electric discharge 

 in producing amino acids. The yields are somewhat lower than with 

 CH 4 , NH 3 , and H 2 but the products are more diverse. Hydroxy 

 acids, short aliphatic acids, and dicarboxylic acids are produced 

 along with the amino acids. Ten of the 20 amino acids that occur in 

 proteins are produced directly in this experiment. Methionine is 

 obtained if H 2 S is added to the mixture, while cysteine, another 

 sulfur containing amino acid, was found in the photolysis of CH 4 , 

 NH 3 , H 2 0, and H 2 S. Phenylalanine, tyrosine, and tryptophan can 

 also be synthesized under putative prebiotic conditions. Thus, only 

 the basic amino acids - lysine, arginine, and histidine - have not 

 been produced in prebiotic synthesis of amino acids. There is no 

 fundamental reason that the basic amino acids cannot be synthe- 

 sized, and this problem may be solved before too long. 



Mildly Reducing Atmospheres 



The geochemists, especially W. Rubey, were not happy with the 

 proposal by Urey that the early atmosphere was composed of meth- 



