PLANETESIMAL THEORIES OF THE EARTH'S ORIGIN. 197 



the earth's mass the required conditions would probably be reached. 



" Pressure itself is probably incompetent to melt rock substances 

 that shrink in solidifying, but the high temperatures generated by 

 pressure in the deep interior were constantly moving outward into 

 horizons of lower pressures, where the melting-points were lower. 

 As the computed temperature at the center of the adult earth is 

 about 20,000° C, there would seem to be no lack of heat, in the later 

 stages at least. The essence of the problem lies in its redistribution 

 and in its selective action. 



" The material of the interior was originally, by hypothesis, an 

 intimate mixture of planetesimals of various kinds, with such 

 gaseous material as they carried in or entrapped in the process of 

 growth . . . The outward flow of heat in such a mixture must 

 bring some parts to fusibility much before the melting-points of 

 other parts were reached. Local spots of fusion must thus arise. 

 To this fusion the entrapped and occluded gases may be presumed 

 to have contributed and to have joined themselves to the fused 

 masses, and to have aided in giving them fluidity . 



" It is not necessary to the hypothesis to suppose that volcanic 

 action was an essential preliminary to the acquisition of an 

 atmosphere, for the initial atmosphere may have been supplied from 

 external sources. The apparent vigor and the wide prevalence of 

 volcanic action on the moon, if its pitted surface means vulcanism, 

 as well as the glassy material found in meteorites, whose origin is 

 referred preferably to small atmosphereless bodies, favors the view 

 that the internal gases were given forth abundantly before the 

 earth grew to a mass sufficient to hold them. If this were true, an 

 ample source of atmospheric supply was ready and waiting when 

 the earth first acquired sufficient gravity to clothe itself with a 

 gaseous envelope. 



" When the increasing water-vapor of the growing atmosphere 

 reached the point of saturation, it is of course assumed to have 

 taken the liquid form and become a contribution to the 

 hydrosphere. . . , 



" If it be assumed that the earth's growing hydrosphere appeared 

 at the surface when our planet had attained the mass of Mars, 

 whose radius is about 2,100 miles, the subsequent growth would 

 form a shell about 1,900 miles thick. It is not altogether certain 

 that Mars bears water bodies on its surface, but the areas of 

 greenish shades environed by a surface generally ruddy, the 

 polar white caps (' snow caps ') that come and go with the seasons, 

 and the apparent occasional presence of clouds, not to appeal to the 

 evidence of aqueous absorption lines in the spectrum reported by 

 some good observers, but unconfirmed by others, lend some 

 support to the opinion that water is present, though perhaps not in 

 the form of definite water bodies. . . . 



