New study reveals sulfur’s role in shaping Earth’s inner core

More than 2,900 kilometres beneath our feet lies Earth’s core – a vast, iron-rich region divided into a liquid outer layer and a crystallising solid inner core. The Earth’s core plays a crucial role in generating Earth’s magnetic field and driving its long-term evolution, yet its exact composition remains uncertain.

Scientists have long known the core is not made of pure iron. Seismic data shows it is slightly less dense than expected, indicating that lighter elements such as silicon, carbon, oxygen or sulfur must be mixed in.

Pinning down exactly which elements are present, and in what amounts, is challenging. The core’s extreme pressures and temperatures reach more than 3.6 million times atmospheric pressure and over 5,000 Kelvin, so researchers rely on laboratory experiments and seismic observations to investigate its composition.

Although sulfur has long been considered a likely ingredient in Earth’s core, its role alongside other elements and its effect on iron’s density under extreme conditions has not been examined.

Previous studies show that iron mixed with silicon behaves quite similarly to pure iron under pressure and researchers know from previous high-pressure experiments that a tiny bit of silicon is needed to explain the properties of the liquid outer core. However, this combination alone does not fully explain the crystallising solid inner core’s observed density with indications showing that sulfur should also be present.

This study aimed to see what effect sulfur has on a core that already has just enough silicon to explain the properties of the liquid outer core.

To investigate this, the team recreated core-like conditions in the lab. They took tiny samples of iron mixed with silicon and small amounts of sulfur, and subjected them to immense pressures using a diamond anvil cell – a specialised device in which the sample is compressed between two diamonds to replicate the pressures found deep inside the Earth. The samples were then heated to thousands of degrees to mimic core temperatures.

Under these conditions, iron adopts a structure known as hexagonal close-packed (hcp) iron, thought to be the dominant form in the solid inner core. The researchers used high-powered X-ray techniques to measure how tightly the atoms were packed together, allowing them to determine the material’s density.

By comparing these measurements with data from seismic waves travelling through Earth’s core, the team could assess whether this sulfur-bearing iron mixture matches the real core.

Their results show that adding around 2% sulfur by weight does not significantly reduce iron’s density under these conditions. In other words, even with sulfur present, the material remains relatively dense – consistent with observations of Earth’s inner core.

This finding has important implications for understanding the contrast between Earth’s inner and outer core. The inner core is solid and slightly denser, while the outer core is liquid and less dense. Any successful model of core composition must explain this difference.

This work demonstrates that an alloy of iron containing sulfur, along with a small amount of silicon, can match the observed density of the inner core. At the same time, a slightly different mixture - with more light elements - can explain the properties of the liquid outer core.

Their proposed model suggests that both parts of the core share broadly similar ingredients, but in different proportions. The inner core appears to contain iron mixed with silicon and sulfur, while the outer core includes additional lighter elements such as oxygen.

Beyond refining estimates of composition, the study also supports a broader theory about how Earth formed. Early in the planet’s history, during a period when much of Earth was molten, heavy elements like iron sank to form the core, while lighter materials remained in the mantle. During this process, elements were distributed between the core and mantle depending on their chemical behaviour.

The new findings are consistent with this idea of “equilibrium partitioning” in a global magma ocean, suggesting that sulfur, silicon and oxygen were incorporated into the core during its formation billions of years ago.

Understanding the core’s composition is about more than simply knowing what lies beneath our feet. The chemistry of the core influences how heat flows through the planet and drives convection in the liquid outer core – the process that generates Earth’s magnetic field. It also affects how the inner core grows over time as the surrounding molten outer core cools.

Despite decades of research, the core remains one of Earth’s least understood regions. Studies like this one are narrowing down the possibilities, bringing scientists closer to a consistent picture of the planet’s deep interior.

Sulfur, it seems, may play a quieter role than once thought – but one that is nonetheless crucial for explaining the structure and evolution of the Earth’s core.