Evolution of the Earth's Atmosphere 



63 



studies have demonstrated the absence of any noteworthy concentration of 

 hydrocarbons. Such concentrations must be less than iq-^ % [14]. 



As a result of all these processes the atmosphere assumed its present aspect. 

 According to G. P. Kuiper [17] the percentage of No in the contemporary atmos- 

 phere is 78-09, that of O2 20-95, of A 0-93, of CO2 0-03 of Ne o-ooi8, of He 

 0-00052, of Kr o-oooi, of H2 0-00005 and of Xe 0-000008. 



In considering the evolution of the atmosphere of the Earth, we may divide 

 it into several stages (cf. Table i). The first stage began from the period of the 

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



Table i 

 Evolution of the atmosphere of the Earth 



At this period the atmosphere contained only hydrogen and helium with a 

 small admixture of other gases, mainly neon. There was a gradual dissipation of 

 the atmosphere, especially of the hydrogen in it. 



The second stage was associated with the gradual heating of the planet under- 

 going formation, owing to compression and radioactive and other exothermic 

 processes. 



Various gases were given off into the atmosphere from the interior of the 

 Earth owing to their evaporation with the rising temperature and their forma- 

 tion in the coarse of radioactive, radiochemical and chemical processes. 



The abundance of hydrogen in the primary atmosphere must have influenced 

 its later composition. Even after the loss of hydrogen from the atmosphere, 

 compounds of hydrogen must have remained on the Earth, in the first place 

 compoimds with those elements which have the greatest cosmic abimdance and 

 which were present in the original cloud of dust and gas. This was the origin of 

 such compounds as H2O, CH4, NH3, H2S, etc. The emission of water vapour 

 and the formation of the hydrosphere led to a lowering of the concentration of 

 the easily soluble and chemically active gases. 



Although the Earth lost nearly all its hydrogen in the early stage of its de- 

 velopment it, so to speak, inherited a reducing nature passed on by the remains 

 of the hydrogen and compoimds formed from it. These compounds were formed 

 both in the atmosphere and in the crust of the Earth. A loss of the atmosphere 

 occurred but, at the same time, there occurred a continual replenishment of it 

 caused by the formation in, and migration from, the interior of the Earth of, in 



