Scientists say 40-year dispute over Earth’s inner core is finally settled

By Ariella Morris

A 40-year scientific dispute over the Earth’s inner core may finally be over, as researchers reveal new evidence explaining how iron atoms arrange themselves at the centre of the planet.

Writing in the Journal of Geophysical Research: Solid Earth this year, the team reports that iron adopts the hexagonal close-packed (HCP) crystal structure rather than the competing body-centred cubic (BCC) arrangement under inner-core conditions. The finding helps explain why earthquake waves travel faster in certain directions through the Earth’s centre, a puzzle that has complicated efforts to build accurate models of Earth’s interior – models that underpin seismic monitoring systems and hazard maps worldwide.   

What makes this study decisive is its distinction between mechanical and thermodynamic stability. Previous simulations have shown that both HCP and BCC iron structures can physically exist at inner-core conditions, making them mechanically stable. But the UCL team’s improved methods reveal that only HCP iron is thermodynamically stable, meaning it represents nature’s preferred, lowest-energy state. 

“This question has persisted since the 1980s,” says Lidunka Vočadlo, a geophysicist at UCL and a co-author of the study. “The real difficulty has always been simulating the inner core pressures and temperatures in the laboratory.” 

To deduce the inner core’s structure, scientists rely on indirect methods, recreating its conditions in laboratories and simulations until the results match seismic data. This approach is necessary because the inner core exists under extreme conditions, with pressures exceeding 360 gigapascals and temperatures rivalling the Sun’s surface (Sun et al., 2024; S. Blugani et al., 2024). Scientists cannot sample it directly. Under such extremes, HCP and BCC iron produce almost identical seismic signals, making them difficult to distinguish. 

“When the differences are small, the outcome heavily depends on the assumptions,” says John Brodholt, a UCL geophysicist and co-author. “Different approximations can push the answer one way or the other.” 

In an HCP arrangement, iron atoms stack tightly in layers, a configuration that becomes stable under these pressures. In BCC iron, atoms sit at the corners and centre of a cube. Earlier studies reached contradictory conclusions about which structure dominated because they only tested mechanical stability rather than thermodynamic stability, which determines which structure nature actually chooses.  

They found that although BCC iron remains mechanically stable at extreme temperatures, it is thermodynamically less stable than HCP, which also remains solid to higher temperatures - further evidence that HCP is the dominant inner-core phase. The result confirms Vočadlo’s prediction, published more than two decades ago.   

“Everything in nature settles into its most stable, lowest-energy state - whether it’s a table or the planet’s core,” Vočadlo says. “Under these conditions, that state is HCP iron.” 

Some researchers continue to explore alternative explanations, including atomic diffusion at high temperatures or limitations related to simulation size. Vočadlo and Brodholt stress that such work remains valuable - but argue that the focus of the field should now shift. 

“The argument for BCC iron should be closed,” Brodholt says. “What really matters is how impurities, defects and core growth influence the behaviour of the planet,” Vočadlo agrees. 

With this debate settled, the team will examine how light elements in the iron core influence its properties – work that could help reconstruct Earth’s evolution - and refine the models used to understand everything from seismic activity to planetary evolution.  

This article is based on interviews with Professor of Mineral Physics Lidunka Vočadlo and Professor of Mineral Physics John Brodholt. 

Link to publication: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025JB032139