ORIGIN OF ORGANIC COMPOUNDS 139 



After the Sun had become a kiminescent star and the 

 discoid protoplanetary cloud had been formed, different con- 

 ditions of temperature were set up in different regions of 

 the cloud. As a result of the radiations of the Sun the clouds 

 became warmer till the temperatures at various distances 

 from the Sun became roughly what they are now. 



Urey considers that the combination of particles with one 

 another which took place during the accumulation of dust 

 composing the protoplanetary cloud and the formation of 

 the planetesimals could only have occurred as a result of 

 the coagulating effect of liquids or damp bodies, as occurs 

 ^\ hen snowballs are made from ^vet snow. 



In the formation of the planets water, ammonia and 

 methane acted as the sticky material. On the basis of his 

 own calculations Urey determined the distances from the Sun 

 at which these substances would condense. It seemed that the 

 condensation of water vapour would occur in the zone 

 between Jupiter and the asteroids, and that of ammonia in 

 the neighbourhood of Sattnn but that methane would remain 

 in the gaseous state right out to the orbit of Pluto. In the 

 region of the Earth and Venus, however, the condensation of 

 water and ammonia (especially in the form of nh^oh) might 

 occur in association \vith local falls in temperature, and this 

 would create the optimal conditions for the accumulation 

 of particles of dust here, while in the region of Mars and 

 the asteroids the crystals of ice were already so dry that they 

 could not effect coagulation. 



The planetesimals which were formed in the neighbour- 

 hood of the Earth incorporated all the non-volatile substances 

 of the primaeval cloud of dust, the silicates and their hy- 

 drates, the oxides and sulphides of iron and other metals, 

 and also ammonium chloride, water and ammonia. In this 

 stage in the formation of the protoplanet which was the fore- 

 runner of the Earth it must already have lost a considerable 

 amount of hydrogen, helium and neon while ammonia and 

 the hydrocarbons only escaped partially. Later there occurred 

 adiabatic compression of the gases of the protoplanet leading 

 to an increase in the temperature of its central parts, which 

 rose to nearly 2,000° C. 



As the planetesimals passed through the strongly heated 



