Computer Science
Scientific paper
Mar 1959
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1959gecoa..15..257r&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 15, Issue 4, pp.257-283
Computer Science
39
Scientific paper
It is assumed that planets have accreted at low temperature from a cloud of dust and gas of cosmic composition. Subsequent melting may cause reduction of metallic oxides by carbonaceous compounds which were trapped during accretion. The metal so produced would segregate and form a core. Chemical evidence indicates that the metallic phase would consist chiefly of iron, nickel and silicon. The higher the reduction temperature, the higher will be the content of silicon in the nickel-iron. The density variation of terrestrial planets can be explained by differing degrees of reduction of the primitive non-volatile material. According to this view, Mars is composed of completely oxidized cosmic nonvolatile material, whereas Mercury is highly reduced, with most of its silicon occurring in the metal phase. Venus, the earth and meteorites exhibit intermediate stages of reduction, and therefore, intermediate densities. Examination of the chemistry and mineralogy of meteorites supports the suggested theory of origin. The mineralogy is similar to blast-furnace assemblages, suggesting analogy between the processes of metal production. PRIOR'S rules, concerning the inverse variation of oxidized and reduced iron in meteorites, and nickel content of chondritic iron, are readily explained. The reduction hypothesis provides a mechanism for chondrule formation. Textural and chemical evidence indicates that they formed within the meteoritic planet by rapid crystallization due to loss of water below 1000°C. If meteorites and the earth are to possess similar non-volatile compositions, the earth's core must contain about 20 per cent of silicon. Occurrence of some silicon in the core would enable explanation of its probable density and elasticity, which are not satisfactorily explained by the properties of pure nickel-iron. When the metal which is produced by reduction near the surface of the earth segregates to form a core it is out of chemical equilibrium with the mantle because the high temperatures and pressures at the core-mantle boundary displace the equilibria. The reactions chiefly concerned are: Ni 0 + Fe ++ = Ni ++ + Fe 0 Si 0 + 2 Fe ++ = Si ++++ + 2 Fe 0 A substantial e.m.f. is therefore generated at the core-mantle boundary. This may be of importance in connexion with the theory of the earth's magnetic field. Accompanying ionic diffusion effects near the boundary may cause density changes and convection in the core. Resultant changes in the composition of the mantle near the boundary may be responsible for the change in gradient of seismic velocities observed by JEFFREYS and GUTENBERG. A further effect of this process is to cause a steady contraction in the volume of the core.
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