COMPOSITION OF OUR UNIVERSE — BROWN 203 



information accumulated during the last 50 years has served to substan- 

 tiate the thesis that meteorites belong to a single family possessing a 

 common genesis, quite possibly a planet similar to the earth in physico- 

 chemical characteristics. 



Meteorites range in size from dust particles (which are most difficult 

 to collect) to many tons. In general, meteorites fall into two distinct 

 categories — irons and stones. Iron meteorites are fragments of pure 

 metal, consisting primarily of an alloy of iron containing about 8 per- 

 cent nickel and 2 percent minor constituents. Stony meteorites consist 

 primarily of magnesium and iron silicates through which finely divided 

 particles of metallic iron-nickel are dispersed. The average metal- 

 phase content of stony meteorites is approximately 11 percent, but the 

 quantity of metal may vary from nearly zero to well over 50 percent. 

 A third and less abundant meteoritic phase, known as troilite, and 

 composed primarily of ferrous sulfide, exists in both stony and iron 

 meteorites, usually distributed throughout the mass, but frequently 

 collected into pockets of substantial size. 



A comparison of the abundances of elements in meteorites, the 

 earth's crust, and the sun demonstrates that both meteorites and the 

 earth are very deficient in those elements which are most abundant in 

 the sun (hydrogen, helium, carbon, nitrogen, and oxygen). Meteor- 

 ites, in turn, possess considerably larger proportions of iron and mag- 

 nesium and smaller proportions of sodium and potassium than are 

 observed in the earth's crust. 



Meteorites are much more dense than the surface rocks of the earth. 

 In view of the fact that the earth as a whole has a mean density nearly 

 double that of its surface, the assumption that the earth possesses a 

 composition equivalent to the composition of meteorites would appear 

 to be plausible. In this event, it would be necessary to assume that 

 considerable quantities of metallic iron, together with iron and magne- 

 sium silicates, exist below the earth's surface. 



The hypothesis that the earth possesses a composition equivalent to 

 that of meteoritic matter was fortified by the discovery of the seismic 

 discontinuity of first order located at approximately one-half the 

 earth's radius. It appeared reasonable to assume that this disconti- 

 nuity marked the boundary of a core composed of metallic iron-nickel 

 (similar in composition to iron meteorites). The silicate mantle 

 surrounding the core would then possess a composition equivalent to 

 stony meteorites. 



A study of the trace constituents in meteorites demonstrates that 

 elements are distributed between the metallic and silicate phases 

 according to well-recognized chemical laws. Those elements which 

 possess low affinities for oxygen (i. e., gold, palladium, platinum) exist 

 almost entirely in the metallic phase; those elements which possess 



