probe

Scientists have captured unprecedented detail of how iron behaves under extreme conditions approaching those of the core – advancing our understanding of planetary dynamics. Published in Physical Review Letters, these are the first experimental results from the new High-Power Laser Facility (HPLF) at the ESRF.

At the heart of our planet, Earth’s core comprises two distinct sections: a molten outer core that begins around 2,900 km beneath our feet, and a solid inner core starting around 5,150 km. Iron accounts for roughly 85% of the core by weight, combining with nickel and lighter elements to form alloys.

But uncertainties remain over the melting point of iron and its alloys under the extreme pressures of deep Earth. Debates also persist over how iron’s crystal structure may change with depth, which influences its physical and chemical properties at larger scales.

Shocked to the core

Fresh insights into these questions are revealed in new experimental work by Sofia Balugani, PhD student at the ESRF within the InnovaXN programme, in collaboration with the Ecole Polytechnique (LULI Laboratory, France), the First Light Fusion company (UK), and the HPLF team. The researchers “shocked” a tiny iron target (3.5 μm-thick) by firing it with a laser pulse, reaching a pressure of 240 GPa. By coupling the laser with X-rays, they recorded a bulk temperature measurement of 5,340K, the first of its kind for iron’s melting plateau under such extreme conditions. A melting point of 6200K was extrapolated for the even higher pressures of the inner core boundary (ICB).

“After three years of PhD research, this work fulfills my long-standing interest in planets, allowing me to study materials crucial to planets and their properties under extreme conditions, such as those on Earth,” says Balugani.

The research helps refine models of the Earth’s core’s behaviour. That’s because HPLF is optimised for this type of X-ray absorption experiment, which enabled the team to simultaneously track temperature alongside changes in the local order of iron. The findings rule out a transition in iron’s lattice structure to a high-temperature bcc (body-centred cubic) phase, which is observed in some other metals under shock compression such as copper and gold.

Instead, iron remains in the denser hcp (hexagonal close-packed) phase. The results may interest astrophysicists searching for exoplanets, given the importance of the core in generating a geomagnetic field and driving plate tectonics – both of which are key to supporting habitable conditions on Earth.

Read more on ESRF website

Image: The High-Power Laser Facility at the ESRF.

Credit: S. Candé.