The Formation of the Earth 71 



However, as we have already said, Urey insists that the internal parts of the 

 asteroidal bodies did not undergo any change. They should therefore bring into 

 the composition of the Earth all those organic compounds from the protoplanetary 

 cloud which had been incorporated in them when they accumulated at low 

 temperatures. 



Thus we see that, according to the theories of both Shmidt and Urey the 

 simplest organic compounds must have entered into the composition of the 

 Earth even during the process of its formation. The Enghsh astrophysicist Hoyle, 

 who also beheves that the Earth accumulated from sohd particles, assigns to the 

 organic compounds derived from the protoplanetary cloud, the determining role 

 in the origin of life and the formation of petroleum beds [30], 



Let us see what evidence astronomy provides about these simplest organic 

 compotmds. 



3. Spectroscopic observations have shown that the simplest organic com- 

 pounds exist both within our planetary system and far beyond its bounds. For 

 example, it has been established that the gas— dust medium which occupies the 

 whole of interstellar space contains molecules of CH and CN. These molecules 

 and also molecules of C2 are present in the atmospheres of relatively cold stars. 



The giant planets, Jupiter, Saturn, Uranus and Neptime, contain large 

 amounts of methane (CH4) and ammonia (NH3) which are imdoubtedly of 

 abiogenic origin. 



Spectroscopic observations have revealed the presence of molecules of C2, 

 C3, CN, CH, OH, CO and N2 in the heads and tails of comets. Until recently 

 it was beheved that these chemically unstable radicals made their appearance as 

 a result of the photodissociation of chemically stable parent molecules which 

 were given off as vapours (desorbed) from the nuclei of the comets. It was sup- 

 posed that CH4, NH3, CO2 and H2O acted as such parent molecules. Recently^ 

 however, Donn & Urey [31] basing themselves on the laboratory experiments 

 which have been carried out during recent years by various workers, have put 

 forward the hypothesis that frozen, unsaturated radicals are present in the nuclei 

 of comets. 



Turning to the planets belonging to the same group as the Earth we must 

 note that CO2 is present in the atmospheres of Venus and Mars. The amount 

 of CO2 present on Venus is equivalent to a layer i km thick (at normal tem- 

 perature and pressure) and that on Mars to one 440 cm thick, i.e. twice as thick 

 as that on the Earth. 



Among the bodies formed in the inner zone of the planetary system must be 

 included the meteorites which fall on the Earth. In the regions of the planetary 

 system which are far from the Sun there are probably meteorites made of the 

 'ices' of various gases, i.e. bodies related to the nuclei of comets. However, the 

 meteoric bodies which move through the same region as the Earth, that is, those 

 near to the Sun, are composed of materials of high melting point, silicates and 

 nickel-iron. Among the gases which are present in a sorbed form in all meteorites, 

 without exception, are organic compounds. The gases given off by meteorites 

 when they are heated contain CO2, CO, CH4, H2, N2, H2S and SO2. The 

 presence of hydrocarbons on some meteorites has been well known for a long 



