206 A]NTS[UAL REPORT SMITHSONIAN INSTITUTION, 195 



THE PLANETS 



Can there be relationships between the other planets similar to the 

 assumed composition relationship between meteorites and the earth? 

 The planets can be readily divided into two main groups — small 

 planets of high density (Mercury, Venus, Earth, and Mars) and large 

 planets of low density (Jupiter, Saturn, Uranus, and Neptune). It 

 seems clear that the smaller planets are composed almost entirely of 

 the easily condensable substances which constitute such a small fraction 

 of stellar material. Although the variations in density among the 

 smaller planets are considerable, it appears that at least among three 

 of them (Mars, Venus, and the Earth) a substantial amount of the 

 density variation is due to increasing compression with increasing 

 mass. The new determination of the mass of Mercury indicates that 

 some variation in the metal to silicate ratio may exist, but this is 

 uncertain. 



Figure 2. — Distribution of elements in Jupiter. 



Recent developments in the theory of the origin of the solar system 

 make it appear probable that the planets were formed by a process of 

 condensation at low temperature from a medium possessing a compo- 

 sition close to that of the present sun. The terrestrial planets were 

 formed from those substances which were least volatile. In the re- 

 gions of the outer (or Jovian) planets, conditions were such that some 

 of the lighter and more abundant materials could condense as well, 

 thus giving rise to much larger planets of considerably lower density. 



If w^e assume the present abundance values for the sun to be the 

 most probable values for the abundances of the elements in the pre- 

 planctary medium, we can assess the most likely chemical forms in 

 which the elements would exist at reasonable temperatures. For each 

 part by weight of easily condensable material (earth-forming ele- 

 ments) we would have approximately 4 parts by weight of a mixture 



