70 B. YU. LEVIN 



particles were such that they stuck together when they hit one another, then the 

 formation of asteroidal bodies may have happened in this way too. 



Owing to the opacity of the disc of dust, its outer zone was extremely cold 

 (at a temperature of about 3°K) and even volatile compounds condensed there. 

 Only a small inner zone was heated by the Sun and there only solid particles 

 and bodies made of compounds of high melting point could exist [20, 21]. Thus, 

 there developed a markedly zonal differentiation in the abundance and composi- 

 tion of solid material, in particular, of the asteroidal bodies which had accumu- 

 lated by the end of this first stage. 



The second stage of evolution, which began even before completion of the 

 first, consisted in the formation of planets from the swarm of asteroidal bodies 

 (and also from the fragments which were formed when they collided). The 

 original circular orbits of these bodies gradually became elliptical owing to their 

 gravitational attraction and this provided a wide 'feeding zone' for each planet. 

 The natural averaging out of the pecularities of movement of the separate bodies, 

 which occurred when they amalgamated, led to the orbits of the planets being 

 almost circular. Zonal differences in the characteristics and composition of solid 

 matter caused, as we have said, by differences in temperature, are now reflected 

 in differences in mass and composition betw^een the two groups of planets, those 

 belonging to the same group as the Earth and the giant planets. 



According to this concept, the planets of the same group as the Earth, and also 

 the Moon, should be of essentially the same composition. We have no direct 

 evidence of the composition of these bodies and are therefore obhged to judge 

 their composition by their mean densities, in the case of the Earth 5 • 5 g/cm^, in that 

 of the Moon 33 g/cm^. It would seem that, to solve the problem, it is necessary 

 to adopt one view or another as to the nature of the dense core of the Earth. 



For more than a century it was held that the core of the Earth consisted of 

 nickel-iron. At present, however, most geophysicists believe that the core 

 of the Earth may be explained by a discontinuity in the increase in density owing 

 to the effects of high pressure. This idea was first put forward in a general form 

 by V. N. Lodochnikov [22]. In 1948 the English physicist Ramsey [23-24] 

 developed the same hypothesis independently and it soon obtained recognition. 

 Working on the basis of the Lodochnikov-Ramsey hypothesis, it may be shown 

 that the Earth, Venus, Mars and the Moon are of the same composition and only 

 Mercury is made of denser material. This is obviously due to specially strong 

 heating by the Sun's radiation of those parts of the preplanetary cloud from 

 which Mercury was formed [17, 25-28]. 



Urey supports the earlier hypothesis, that the core of the Earth is largely 

 made of iron. In this case the differences in density between the planets belonging 

 to the same group as the Earth have to be interpreted as being a result of their 

 differing contents of metaUic iron. This leads one to look for processes which 

 might have led to variation in the iron : silicate ratio. Superficial heating of 

 the asteroidal bodies so that some of the silicates were vaporized without 

 changing the internal composition seems to Urey [12, 13] the most hkely, 

 although the astronomical and physical aspects of this process remain extremely 

 obscure [29]. 



