Evolution of the Earth^s Atmosphere 59 



Among the gases formed by the action of a-rays we must include oxygen, as 

 there is so much of it in the crust of the Earth and a considerable proportion of 

 the a-particles must have struck oxygen-containing compounds such as Si02, 

 AI2O3, H2O, FeoOs and others. 



However, the oxygen liberated during radiochemical reactions will have 

 entered into combination again, owing to its chemical activity, and various 

 oxides will have been formed. In this way many gaseous oxides must also have 

 been formed, e.g. CO, CO2, SO2, SO3, NO, N2O, NO2 etc. Such oxides as 

 SO2, SO3, NO and NO2 would be retained in the rock formations in the pre- 

 sence of water, and would take part in various reactions, while CO and CO2 

 would, to a considerable extent, be given off into the atmosphere. Hydrogen, 

 which does not take part in any reactions, would also be given off into the atmos- 

 phere. The gas which is emitted into the atmosphere during the radiochemical 

 breakdown of water in the rocky formations is enriched in hydrogen because the 

 oxygen oxidizes various substances in the course of its migrations. Part of the 

 hydrogen reacts with the oxygen to form water again. As a result of reactions 

 between hydrogen and CO or CO2 a certain quantity of hydrocarbons is formed. 

 As a result of radiochemical reactions, nitrogen is given off partly in the free 

 form and partly in the form of compounds such as N2O and NH3. 



Thus radiochemical processes provided for the accession to the atmosphere 

 of such gases as N2, H2, CO2, CO, CH4 and heavier hydrocarbons and also 

 N2O, NH3 etc. There was also some accession of oxygen although its chemical 

 activity and its oxidation of the superficial layers of the rocky formations seem 

 to have led to a marked lowering of its concentration in comparison with that 

 of the other gases of the atmosphere. The oxygen which entered the atmosphere 

 was used up in the oxidation of some of the CO, H2 and CHj. Photochemical 

 reactions played a definite part in the oxidative and other processes which took 

 place in the atmosphere (splitting of H2O, etc.). The only gases which could 

 persist in the atmosphere were such as were very stable chemically and would 

 not enter into combination with one another. 



Passing on to the question of chemical processes of gas formation, we must 

 note their limited importance in view of the low temperatures which prevailed 

 in the protoplanetary envelope and during the first period of the formation of 

 the Earth as an aggregate within this envelope. At that time hydrogen pre- 

 dominated in the atmosphere. Hydrogen and other gases were lost from the 

 atmosphere by dissipation but, at the same time, in proportion to the com- 

 paction, aggregation and heating of the Earth during its formation, there also 

 occurred at first, an emission of those gases which, at the low temperature pre- 

 vailing earlier, existed in the soHd state. 



The amounts of these gases given off were directly proportional to their 

 boihng points and vapour pressures. In the first place such gases as Ne, N2, CO, 

 A, CH4 and Kr were given off (with boihng points of from —152-9 °C for Kr 

 to —245-9 °C for Ne). Fluorine and oxygen must also have been given off in 

 this range of temperature, but, very Ukely they were present in the form of 

 compounds in the primary envelope. It must be noted that the rate of some 

 oxidative reactions at very low temperatures may even be greater than it is at, 



