Chemical composition and accretion history of terrestrial planets

Abstract

The high concentrations of moderately siderophile elements (Ni, Co, etc.) in the Earth’s mantle and the similarity of their Cl normalized abundances to those of moderately volatile elements (F, Na, K, Rb) and some elements such as In, which under solar nebula conditions are highly volatile, are striking. To account for the observed abundances, inhomogeneous accretion of the Earth from two components has been proposed. In this model accretion started with the highly reduced component A devoid of all elements more volatile than Na, followed by accretion of more and more oxidized material (component B), containing all elements in Cl abundances. Recent observations have brought almost conclusive evidence that SNC meteorites are martian surface rocks ejected by huge impacts. By assuming that Mars is indeed the parent body of SNC meteorites, the bulk composition of Mars is estimated. The data on the composition of Mars obtained in this way clearly show that the two-component model is also valid for Mars. The striking depletion of all elements with chalcophile character in the martian mantle indicates that, contrary to the Earth, Mars accreted almost homogeneously (H. Wanke, Phil. Trans. R. Soc. Lond. A 303, 287 (1981)).