Formation of Fe–Ni–Si planetary cores

Abstract

THE planets may have accreted in homogeneously materials accumulating on the proto-planets in the sequence in which they condense at high temperatures (1,000–2,000 K) out of the solar nebula¹. The first solids to condense from the Earth, however, are the Ca and Al oxides, silicates and titanates^2,3 and not Fe or Fe-Ni alloy, which is supposed to form the Earth's core. Anderson and Hanks⁴ consider that such a nucleus of oxides enriched in radioactive elements could provide a mechanism for heating and melting the surrounding solid Fe layer which accretes later. Ringwood⁵ considers this model of core formation unsatisfactory since it may produce a layer of oxide rich material around the core for which we have no geophysical evidence. For Fe, or an alloy of Fe with other elements, to form as the first equilibrium solid, the pressure in the solar nebula at 1,800 K must be several atmospheres^3,6. But, the conditions for the formation of a liquid Fe or a liquid Fe-rich alloy have not been determined. If a liquid alloy formed as the first condensate, it would solve many problems in the current models of formation of planetary cores⁵. The pressure required to form a liquid alloy is likely to be an order of magnitude lower than the pressure for the formation of solid Fe because metal liquid solutions of Fe, Ni and Si have a lower free energy of solution than that of an ideal solution. In particular the Fe-liquid and Si-liquid, and Ni-liquid and Si-liquid form binary solutions with a low free energy which stabilises the liquid solutions at pressures lower than that of the solid. According to Podolak and Cameron⁶ the models of Jupiter have an enhanced O–H ratio of ∼25–30. They consider that pressure required to form chondrules will be lowered several orders of magnitude if the O/H ratio is increased. We report here an investigation into the equilibrium condensation of the solar nebular gas at 1,900 K at varying total pressures and varying abundance of hydrogen (A_H).