16 The Evolution of the Universe 



Hoyle believed that originally only a few solid planets condensed 

 from this disc of dust and gas surrounding the sun but that these 

 solid planets were too big to hold together after they had acquired 

 a rotation of their own. They would have broken up, the fragments 

 flying to various distances and ultimately evolving into the planets 

 we now see. 



Urey (1952), however, postulated that the planets became dif- 

 ferentiated first as dust-gas clouds and that the earth cloud originally 

 contained chiefly hydrogen, methane and inert gases, and large 

 amounts of silicates, iron compounds, ice, and ammonia. He sug- 

 gested the following course of events. 



After the sun began emitting heat, the temperature of the earth 

 cloud warmed up to the point at which the water and ammonia 

 became liquid. These liquid particles in the cloud caused colliding 

 dust particles to stick together; some larger pieces were formed, 

 and by gravitational attraction the larger ones absorbed the 

 smaller, until the first earth reached a stage (much smaller than 

 it is now) when it was surrounded by a great number of small 

 bodies called planetesimals revolving around it. At this time, the 

 moon had almost reached its ultimate size. Both earth and moon 

 were composed chiefly of siliceous material. The moon remained 

 cold, but the earth, by virtue of its larger size and consequently 

 greater internal pressure and larger total amounts of radioactive 

 compounds, heated up. Under the high temperatures, most of its 

 original gas atmosphere was lost. At some time the circling plan- 

 etesimals fell onto the earth's surface and brought it up to its 

 present size. Most of the earth's iron probably arrived by this 

 means. Relatively few planetesimals fell on the moon, which 

 presumably increased little in size. Those which did were respon- 

 sible for the lines and craters which make up the curious present- 

 day features of the moon's surface. When completely formed, the 

 earth was chiefly composed of about 55 per cent silicates and 45 

 per cent iron, and had an atmosphere of water vapor, methane, 

 ammonia, hydrogen, and some hydrogen sulphide. 



Other theories as to how the planets were formed differ from the 

 two above in various details. In particular some investigators 

 believe that the early earth had no atmosphere and that the present 

 hydrosphere and atmosphere, instead of dating from a primitive 

 atmosphere, have accumulated gradually through geologic time by 

 the escape of volatile substances from the interior of the earth 

 (Rubey, 1955). 



