Evolution of the Earth* s Atmosphere 65 



atmosphere consisted of hydrogen and helium and existed at the time of the 

 formation of the Earth from a cold cloud of dust and gas. 



If we assume that the Earth was formed from an accumulation of cosmic dust 

 (hydrogen being absent or only present in small amounts) or if we assume its 

 origin from a cloud of hot gas which would have permitted the rapid loss of 

 hydrogen, then the atmosphere of the Earth in its earliest stages would not have 

 had the reducing character which has been described in our scheme. 



The formation of hydrocarbons and some of their derivatives seems to have 

 taken place at all stages of the existence of the Earth. In zones where the tem- 

 perature is high these compounds are destroyed. However, on cooling to tem- 

 peratures at which these compounds are stable, they are formed again by means 

 of chemical and radiochemical processes. 



The formation of hydrocarbons and their derivatives certainly occurs in 

 magmatic formations. The water contained in these formations provides a 

 source of hydrogen thanks to which reducing processes take place with the 

 formation of hydrocarbons. 



The finding of hydrocarbons in the Khibin massif is suggestive in this con- 

 nection. Methane and hydrogen are found there. The methane is usually accom- 

 panied by ethane, propane and heavier hydrocarbons. 



Organic residues laid down in sedimentary deposits from another source for 

 the formation of hydrocarbons, which have come into being since the develop- 

 ment of life on the Earth. The further evolution of the atmosphere will surely 

 involve an increase in the amount of argon in it and possibly also of oxygen 

 and nitrogen since CO2 and N2 predominate in the composition of the gases 

 which are given off from the interior of the Earth and which replenish the 

 atmosphere. However, further changes in the concentration of gases in the 

 atmosphere, and in the total amoimt of them, wiU depend on the intensity of 

 the escape of gases into space and on processes of chemical binding of O2, N2 

 and CO2 with the materials of the crust of the Earth. Neither of these has 

 received much study. 



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