core formation involves both chemical and physical factors that are not well understood, but recent progress in both of Stimulated by new experimental results on metal/silicate parthese areas has prompted reexamination of core formation titioning of elements at elevated temperatures and pressures, we have revisited the question of core formation in Earth's Moon, models. Specifically, the importance of the physical state Mars, and Vesta (the probable source of the eucritic meteorites). (liquid or solid) of both silicate and metallic materials is Earlier studies suggested metal/silicate equilibrium in Mars, but important to the segregation process as a whole (Stevenson led to the paradox that Mars accreted homogeneously while the 1990). In addition, the partitioning of siderophile (or metal-Earth accreted heterogeneously. Using new elevated pressure seeking) elements between metal and silicate liquids is and temperature metal/silicate partition coefficients, we show clearly a function of the variables temperature, pressure, that abundances of the moderately siderophile elements in the oxygen pressure (or fugacity), and silicate and metallic mantles of the Moon, Mars and Vesta are consistent with early liquid composition (e.g., Righter et al. 1996), and thus may magma oceans on these bodies. The most successful model for provide important constraints on the conditions prevailing explaining the lunar mantle siderophile element abundances reduring metal-silicate segregation in the inner solar system.
quires a core of 5% of the mass of the Moon (500-km radius). Rocky planetary objects in the inner solar system have Siderophile element abundances in Mars are consistent with inradii varying from asteroid (Ο½500 km) to Earth-sized termediate pressure metal-silicate equilibrium, as has also been recently suggested for the Earth. The most successful model for (ΟΎ6000 km) and thus a range of possible P-T regimes for explaining the martian siderophile element abundances requires metal-silicate equilibrium. For instance, asteroid Vesta is a bulk planetary composition that has greater than CI chondritic a small object with a radius of 275 km and a central pressure abundances of the moderately siderophile elements, and a core of about 4 kb, the Moon has a radius of 1738 km correof 30% of the mass of Mars. Siderophile element abundances in sponding to a central pressure of about 49 kb, and Mars the mantle of Vesta are consistent with low pressure liquid has a radius of 3397 km and a central pressure in the range metal-liquid silicate equilibrium, as expected for an asteroid-370-450 kb. Thus, partition coefficients determined at elesized body, and a core of 10% of the mass of Vesta. vated temperatures and pressures would clearly be relevant Comparison of our best-fit oxygen fugacities for Vesta with and important to unraveling the details of core formation thermodynamic calculations of oxygen fugacity for silicate-bearin the Moon and Mars.
ing iron meteorites indicates that parts of the inner solar system Accretion of planets in the inner solar system is thought were homogeneous with respect to redox state at 4.5 Ga, approxito have been accomplished with materials of diverse commately 2 log fO 2 units below the Fe-FeO buffer-much higher position and size (see, e.g., S. R. Taylor 1992), some primithan estimates for the solar nebula. Similar comparisons for Mars and the Earth indicate that these bodies have undergone oxidative (metal dispersed throughout) and some having undertion since 4.5 Ga, because the oxygen fugacities associated with gone differentiation (metal concentrated into cores). As metal-silicate equilibrium in both Mars and the Earth-Moon metal is delivered to the surface of a growing body, initial system are much lower than those recorded in martian and terequilibration (if it ever occurs) will be at low temperatures restrial basaltic and periodotitic samples. The oxidation on Mars and pressures. Whether metallic phases reequilibrate at is most likely due to atmospheric effects, whereas Earth's much higher temperatures and pressures as they transit a planewider range of oxygen fugacities must be due to both atmospheric tary mantle to the core is not known. Reequilibration deand plate tectonic effects.