CORE FORMATION IN EARTH MOON, MARS, AND VESTA

Stimulated by new experimental results on metal/silicate partitioning of elements at elevated temperatures and pressures, we have revisited the question of core formation in Earth's Moon, Mars, and Vesta (the p robable source of the eucritic meteorites). Earlier studies suggested metal/silicate equilibrium in Mars, but led to the paradox that Mars a ccreted homogeneously while the Earth accreted heterogeneously. Using new elevated pressure and temperature metal/silicate partition coeffic ients, we show that abundances of the moderately siderophile elements in the mantles of the Moon, Mars and Vesta are consistent with early m agma oceans on these bodies. The most successful model for explaining the lunar mantle siderophile element abundances requires a core of 5% of the mass of the Moon (500-km radius). Siderophile element abundance s in Mars are consistent with intermediate pressure metal-silicate equ ilibrium, as has also been recently suggested for the Earth. The most successful model for explaining the martian siderophile element abunda nces requires a bulk planetary composition that has greater than CI ch ondritic abundances of the moderately siderophile elements, and a core of 30% of the mass of Mars. Siderophile element abundances in the man tle of Vesta are consistent with low pressure liquid metal-liquid sili cate equilibrium, as expected for an asteroid-sized body, and a core o f 10% of the mass of Vesta. Comparison of our best-fit oxygen fugaciti es for Vesta with thermodynamic calculations of oxygen fugacity for si licate-bearing iron meteorites indicates that parts of the inner solar system were homogeneous with respect to redox state at 4.5 Ga, approx imately 2 log fO(2) units below the Fe-FeO buffer-much higher than est imates for the solar nebula. Similar comparisons for Mars and the Eart h indicate that these bodies have undergone oxidation since 4.5 Ga, be cause the oxygen fugacities associated with metal-silicate equilibrium in both Mars and the Earth-Moon system are much lower than those reco rded in martian and terrestrial basaltic and periodotitic samples. The oxidation on Mars is most likely due to atmospheric effects, whereas Earth's much wider range of oxygen fugacities must be due to both atmo spheric and plate tectonic effects. (C) 1996 Academic Press.