The Formation of the Earth 73 



Unfortunately we lack any more accurate data about the composition of meteoritic 

 organic material. One can only add that the composition of these carbonaceous 

 compounds must reflect the equilibrium between carbon on the one hand and 

 hydrogen, oxygen, nitrogen and sulphur on the other, which was estabhshed at 

 the time when the substance of the meteorite attained its highest temperature. 



While studying the isotopic composition of the carbon and hydrogen from 

 twelve carbonaceous meteorites, Boato [33] found a notable difference between 

 the different meteorites and, in some cases, between different parts of the same 

 meteorite. This is due to a process of fractionation or to chemical reactions 

 taking place at different stages of the formation of the meteorites and the whole 

 planetary system. Data as to the isotopic composition of the carbon and hydrogen 

 will be very valuable in later investigations of these processes and, in particular 

 for a study of the original organic compounds. 



4. Let us come back, at last, to our own Earth. As we have already said, 

 various gases and vapours, as well as organic compounds, must have entered 

 into the composition of the whole thickness of the substance of the Earth during 

 its formation. During the gradual heating of the interior of the Earth owing to 

 the accumulation of heat generated by the decay of radioactive elements, gases 

 and vapours began to seep towards the surface and were given off, forming the 

 atmosphere and hydrosphere [15]. This same process of internal heating led to 

 the gradual development of the crust of the Earth starting about 3000 million 

 years ago when the temperature became high enough for the partial melting of 

 silicates. We must note that A. P. Vinogradov (p. 23) makes a great mistake in 

 assuming that the temperature of the superficial layers of the Earth changed 

 according to a curve corresponding approximately with that for the amount of 

 heat generated, and that at some time it was high. The mean temperature of the 

 surface of the Earth was always low and was always determined by the heat 

 reaching it from the Sun. The temperature of the interior, however, had in 

 general risen throughout the long history of the Earth. 



At present all astronomers, geologists, geophysicists and geochemists are 

 agreed that the contemporary atmosphere and hydrosphere of the Earth are not 

 the residues of some very extensive primary atmosphere, but are of 'secondary' 

 origin, i.e. they have arisen owing to the gradual giving off of gases and vapours 

 from the interior [34-39 and Sokolov, p. 54]. Among other evidence indicating 

 that the hydrosphere and atmosphere arose in this way, an important piece is 

 that concerning the absence of any great fluctuations in the CO2 content of the 

 atmosphere during the course of the whole of geological history. As the amount 

 of carbon laid down in carbonaceous deposits increased in the course of time, 

 there must have been a continual accession of CO2 to the atmosphere. 



The emergence of gases and vapours from the interior into the atmosphere — 

 during volcanic eruptions, through gas springs and by the general 'breathing' 

 of the planet — continued until the present, though, unfortunately to what extent 

 these gases and vapours are juvenile and to what extent they are taking part in 

 cycles involving only the outer layers of the Earth we do not know. Thus, for 

 example, we cannot tell what part of the methane which is present in the atmos- 

 phere or that which is given off by sources of natural gas, is of abiogenic origin. 



