Core Formation

There are few constraints on the early processes that led to the formation of the cores of the terrestrial planets. Our interests are to pursue experiments that illuminate the effects of variations in sulfur fugacity and oxidation state that may be recorded in the silicates that were subjected to a core-forming event. These results are compared to the compositional variations preserved in mantle minerals to see if there remains any signature of the core-forming event, and to see if the conditions of core formation can be inferred.

The variations in partial pressures of sulfur and oxygen species in a planetary interior exercise key controls on the abundances of siderophile elements that partition between the silicate portion of a planet's interior and the metal portion that segregates to form the core. By combining experimental studies and investigation of relative abundances of siderophile elements left in meteorite samples thought to have formed on Mars, it has been possible to set limits on the conditions of core formation and the size of the Martian core. Our estimates place a limit of 20% on the mass of Mars that is of core material with a composition that is ~ 95 % FeNi alloy and ~5 % S. Similar reasoning applied to the Eucrite Parent body (4-Vesta) leads to the conclusion that core formation occurred at low melt fractions in the parent body. The high siderophile element abundances in the Earth's mantle is inconsistent with any simple core-forming processes and its interpretation presents an ongoing challenge.