Evolution of the Earth's Atmosphere 57 



Various opinions have been advanced as to the nature of this secondary 

 atmosphere and the changes undergone by it [3, 11-17]. 



One of the earhest hypotheses (that of Kelvin) was that the main gases of 

 this secondary atmosphere were CO2, N2 and H2S. According to another hypo- 

 thesis the main gases of the secondary atmosphere were CH4 and NH3 (Oparin, 

 Urey and others). The view has been expressed, however, that oxygen was 

 present to a greater or lesser extent in this secondary atmosphere. 



From the point of view of the possibility of the formation of the organic 

 compounds which served as starting materials for the development of living 

 substance, the most favourable circumstance, according to A. I. Oparin, was 

 the reducing conditions and their later transition to oxidizing conditions. 



Recently Poole and Rubey have accepted the idea that the main gases of the 

 atmosphere were CO2 and N2. Starting from the idea of an abundance of such 

 substances as H2O, CO2, N2 etc. on the superficial zone of the Earth, and 

 considering all these substances as an equilibrium mixture, Rubey arrived at 

 the conclusion that if one excludes water vapour, the atmosphere contained 

 about 91% CO2 and 6-4% N2. 



In order to discuss these very complicated problems one must make a more 

 detailed survey of both the process of the formation of gases and the process of 

 their chemical alteration and dispersal as they have occurred ever since the 

 formation of the Earth as a planet. 



Among the processes of gas formation which were taking place in the pre- 

 biological stage of the existence of the Earth one must take note of: 



Radioactive processes involving the formation of He and A as well as Kr 

 and Xe; 



Radiochemical and photochemical processes occurring as a result of the 

 action of radiations on various substances; 



The chemical processes of breakdown and synthesis. 



For a long time during the pre-biological stage of the existence of the Earth, 

 radioactive processes undoubtedly had a great effect on the composition and 

 abundance of the gases of the atmosphere. 



The giving off of a-particles during the radioactive breakdown of U, Th, and 

 other elements provided the conditions required for the appearance of consider- 

 able amounts of hehum, whüe the transformation of K (^-capture) led to the 

 formation of A. 



We shall reckon, in accord with the views of many workers, that the outer 

 envelope of the Earth contains 4 ppm of U, 11-5 ppm of Th and 2-5% of K [18]. 

 After the passage of 3000 million years the amounts of these elements present 

 in the layers of rocky formations of a thickness of, say, 40 km formed 5 X lo^^ 

 metric tons of He and 8 x lo^^ metric tons of ^OA. 



The amount of helium formed in this way was even greater on account of 

 the breakdown of actinouranium and perhaps neptunium and other previously 

 existent radioactive elements. 



The amount of He contained in the atmosphere at present (about 3-5 x 10^ 



