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First results of the High-Power Laser Facility probe iron at the Earth's core conditions

20-01-2025

​​​​​​​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.

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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.

A new frontier in Warm Dense Matter research

“This first science experiment is a real success and demonstrates the unique capabilities of the HPLF. It is very promising given the facility has much improved since this experiment,” says Jean-Alexis Hernandez, responsible for dynamic compression experiments on HPLF and co-author of the PRL paper.

Hernandez says that other experiments have already been performed on several iron alloys closer to the chemical conditions of the Earth’s core. Future developments aimed at increasing the laser intensity on the samples would offer more flexibility to tune and extend the range towards the conditions of the Earth’s inner core and of large planetary interiors.

One of the most exciting prospects for the HPLF is the ability to study Warm Dense Matter – a state of matter between a solid and a plasma. Such conditions are triggered during the first phase of inertial confinement fusion, recently demonstrated at the National Ignition Facility in the US.

In particular, First Light Fusion – the partner company to Sofia’s Balugani’s project – is attempting to commercialise nuclear fusion as a clean energy alternative using metallic cavities and metallic impactors, which requires precise knowledge of the involved materials at extreme conditions. Experimental work at HPLF therefore could inform the design of robust fusion technology.

Reference:

Balugani, S., et al, Phys. Rev. Lett. 133, 254101. DOI: https://doi.org/10.1103/PhysRevLett.133.254101

Text by James Dacey

 

Top image: The High-Power Laser Facility at the ESRF. Credits: S. Candé.