354 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 64 



would therefore buckle and thrust over itself sideways, piling up 

 material against gravity. 



Attractive and inevitable as this mechanism seemed, detailed calcula- 

 tions showed that it was likely to give far less contraction than would 

 satisfy the geologists. Measured in terms of circumferential contrac- 

 tion of the entire globe, it might lead in the whole age of the Earth to 

 a reduction of a hundred kilometers or so. The geologists need at 

 least a thousand kilometers — some would prefer even two or three times 

 this amount — to allow adequately for all the earlier periods of moun- 

 tain building. 



But even more serious doubt has been thrown on this hypothesis by 

 the gradually emerging conclusion that initially the Earth may have 

 been sufficiently cool to have been solid throughout. When proposals 

 for the origin of the planets were under review a few decades ago, 

 the only possible source for material seemed to lie in the stars, and here 

 all the material was known to be at very high temperatures. Could 

 released stellar material settle down into a compact planetary mass 

 straightway ? It now seems much more likely that material removed 

 from a star by some catastrophic occurrence would expand almost 

 indefinitely, thereby cooling, and instead of giving rise to a planet 

 would produce a gigantic low-density cloud of gas and dust. The 

 heavens, it is now established, are replete with such clouds, which 

 occupy some 10 percent of all galactic space (pi. 1, lower fig.) . It thus 

 becomes necessary to think in terms of planets developing initially 

 from cool material. 



There are a number of mechanisms by which the sun could have 

 acquired sufficient dust and gas to form all the planets. For example, 

 a companion star to the sun may have exploded as a supernova to pro- 

 vide the material ; alternatively, the sun may have nosed sufficiently 

 slowly through one of these clouds to form a dust-and-gas cloud cir- 

 culating round itself. (The clouds themselves would possess slow 

 circulation in the first place.) Once captured, a cloud of gas and dust 

 would settle down into a thin disklike form moving round the sun, 

 somewhat resembling a giant Saturn's ring but on an immensely larger 

 scale and much further out in proportion. Within this disk the 

 planets would have grown by a process of gradual accretion. But for 

 present purposes it is not necessarj^ to go into the details of all this : it 

 is sufficient if we postulate an initially cool and entirely solid Earth, 

 and ask how such a planet would develop. 



SIZE OF AN ALL-SOLID EARTH 



If then we imagine all the material of the Earth initially gathered 

 into a single all-solid body, almost the first question that springs to 

 mind is to ask how big such a planet would be. To answer this with 



