Core Formation in Earth's Moon, Mars, and Vesta

Abstract

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 probable source of the eucritic meteorites). Earlier studies suggested metal/silicate equilibrium in Mars, but led to the paradox that Mars accreted homogeneously while the Earth accreted heterogeneously. Using new elevated pressure and temperature metal/silicate partition coefficients, we show that abundances of the moderately siderophile elements in the mantles of the Moon, Mars and Vesta are consistent with early magma 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 abundances in Mars are consistent with intermediate pressure metal–silicate equilibrium, as has also been recently suggested for the Earth. The most successful model for explaining the martian siderophile element abundances requires a bulk planetary composition that has greater than CI chondritic abundances of the moderately siderophile elements, and a core of 30% of the mass of Mars. Siderophile element abundances in the mantle of Vesta are consistent with low pressure liquid metal–liquid silicate equilibrium, as expected for an asteroid-sized body, and a core of 10% of the mass of Vesta.Comparison of our best-fit oxygen fugacities for Vesta with thermodynamic calculations of oxygen fugacity for silicate-bearing iron meteorites indicates that parts of the inner solar system were homogeneous with respect to redox state at 4.5 Ga, approximately 2 log fO2units below the Fe–FeO buffer—much higher than estimates for the solar nebula. Similar comparisons for Mars and the Earth indicate that these bodies have undergone oxidation since 4.5 Ga, because the oxygen fugacities associated with metal–silicate equilibrium in both Mars and the Earth–Moon system are much lower than those recorded 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 atmospheric and plate tectonic effects.