What is it like in the core of Mars?

What is it like in the core of Mars?

Researchers from the University of Bayreuth and the ESRF, the European Synchrotron, have subjected a blend of iron and sulphur to extreme conditions resembling the deep interior of Mars. They observed the formation of a crystal phase, Fe4+xS3, under high pressures and temperatures – raising the possibility that the Red Planet has a solid inner core.

As a young planet, Mars was home to flowing rivers and lakes, but its fate soon diverged from Earth’s as the Martian surface transitioned into the cold, arid world we see today. To better understand how Mars evolved, scientists must explore its deep interior – the focus of a new study at the ESRF.

In recent years, NASA’s InSight Mars Lander has returned Marsquake data, providing clues about the internal structure of Mars. This seismic data indicates the planet has a molten core with a 1,650–1,800 km radius, accounting for over half of Mars’ total radius. However, unlike Earth, there is insufficient data to establish whether the inner core region is solid.

Many geoscientists assumed the iron-rich heart of Mars was too hot to solidify. That’s partly because the Mars core is richer in light elements compared to the Earth’s. One of those elements is likely sulphur, which is abundant in rocks at the Martian surface and seen in meteorites of the material that coalesced to form Mars.

Breakthrough at the ESRF

In a new study, researchers first identified an iron-sulphide phase, Fe4+xS3, which could theoretically crystallise under Martian core conditions.

“Since the late 1990s, scientists have known this phase could exist, but they didn’t have the experimental techniques to identify the structure and stability,” explains Lianjie Man of Bayerisches Geoinstitut, Germany, lead author of the new study conducted at ESRF beamline ID14 and ID15B.

Using diamond anvil cells, the team subjected iron-sulphur samples to the pressures estimated at the Martian core. Simultaneously, laser heating established high temperatures in the sample. And single-crystal diffraction revealed the crystal structure and density of this novel phase.

Complementary experiments confirmed this iron sulphide could crystallise from liquid when temperatures drop below 1960 (±105) K – which falls within the temperature range in geophysical models for the Mars core. In other words, if the Martian core is on the cooler side of predictions, then a solid core seems perfectly feasible.

Ilya Kupenko, ESRF scientist and study co-author, notes that ID14’s ultra-small (1-micron diameter) X-ray beam, enabled the team to see individual Fe4+xS3 crystals within a larger matrix. “Experiments that are possible in other places are much easier to do here – it’s optimised for this type of research,” says Kupenko.

This study, published in Nature Communications, focused on the phase’s structural and chemical properties. But Kupenko and Man are already leveraging the ESRF’s unique capabilities in a follow-up study of the material’s magnetic properties. While Mars’ core temperatures likely exceed the Curie point – where permanent magnetism is lost – it is still crucial to understand the material’s magnetic order and transition temperature.

What Comes Next?

Future experiments could explore more realistic iron-sulphur mixtures, incorporating other light elements like oxygen and hydrogen. A related line of research is to determine the sound velocities in these materials – through projects like LECOR, Light Elements in the Core, an ERC project based on ESRF-EBS- which is vital for comparing candidate core materials with real-world seismological data.

This work could also help solve another puzzle: a suspected molten zone at the base of Mars’ mantle. This 150-kilometre-thick layer was predicted by two independent analyses of InSight seismic data published in Nature in 2023 (Samuel et al. and Khan et al.)

If further evidence does support a solid Martian core, then it would cast doubt on the existence of this molten zone at the mantle base. The core region would be too cold to melt the overlying mantle material. On the flip side, if new evidence supports the existence of this mantle melt zone, then it would spell bad news for the solid core theory – the region would be too hot.

For now, the question remains open. With InSight’s seismic mission complete and no follow-up missions planned, the answer may lie in deeper analysis of existing data. “If we know the temperature inside Mars, it has a lot of implications for the evolution of this planet over time,” says Kupenko.

DOI: 10.1038/s41467-025-56220-2