Nickel helps scientists iron out a core planetary mystery

Lab experiments modelling the extreme pressure and heat of the early solar system are providing clues to why rocky planets do not have identical cores.

The work, by Stephen Elardo and Anat Shahar of the Geophysical Laboratory at  Carnegie Institution of Washington in the US, suggests that the interaction of heat, nickel and iron varies during each planet’s birth – resulting in what the researchers call a range of iron “flavours”.

The term is used to denote the number of neutrons attached to iron atoms. While every atom has a fixed number of protons (iron has 26), neutron numbers can vary. Atoms with different numbers of neutrons are known as isotopes.

In the early days of the solar system, planetary cores began to form, with heavier elements, notably iron, moving into the centre of the developing mass. As it did so, the iron interacted with another element, nickel, which in some circumstances stripped away certain isotopes, resulting in “lighter” flavoured atoms.

The scientists set out to test whether the degree to which this process occurred – mediated by the temperature of the early core – could explain the isotopic differences observed between hardened lava from beneath the Earth’s surface and rock samples from the moon, Mars and an asteroid known as Vesta.

By setting up laboratory experiments to mimic the conditions of core formations, they found that they way nickel is affected by heat was a critical factor in determining whether iron atoms were separated – or “fractionated” – according to their isotopic mass, so that heavier isotopes tended to end up in the planet’s core while lighter isotopes tended to be incorporated into the mantle, the rocky layer surrounding the core. The results are published in Nature Geoscience. 

The experiments indicated that when the moon, Mars and Vesta formed, nickel interacted vigorously with iron, resulting in high concentrations of lighter flavour isotopes being deposited in their mantles. 

The Earth, however, formed under significantly higher temperatures, which blunted nickel’s effects, greatly reducing isotope fractionation. The results explain why the core flavour of Earth has more in common with samples from primitive meteorites than with its own satellite or closest planetary neighbour.