A team of researchers at the Bayerisches Geoinstitut has conducted high-pressure-temperature laboratory experiments to determine the crystal structure and density of the iron-sulfide phase in the Martian core.
Man et al. show that the high-pressure iron sulfide phase with the formula Fe4+xS3 has a higher density than the liquid Martian core and that a Fe4+xS3 inner core would crystallize if temperatures fall below 1960 K at the center of Mars. Image credit: NASA / JPL-Caltech / University of Maryland.
Similar to Earth’s core, the core of Mars is expected to be dominantly composed of molten iron metal.
However, it is lower in density, indicating that the Martian core must contain a high abundance of additional lighter elements, such as sulfur.
Previously it had been considered that the temperature in the Martian core is likely too high for a solid inner core to crystallize, but the possibility of an iron-sulfide mineral forming the inner core had not been examined in detail.
“Observations from NASA’s InSight mission have revealed that the core of Mars is enriched in light elements, as its density appears to be substantially lower than that of iron-nickel alloy,” said Bayerisches Geoinstitut researcher Lianjie Man and colleagues.
“From cosmochemical perspectives and geochemical considerations, candidate light elements in the Martian core include sulfur, oxygen, carbon, and hydrogen.”
“Sulfur, in particular, is often highlighted as a likely major light element in the Martian core, primarily due to it being the most prevalent moderately volatile element in the Solar Nebula, its ‘iron-loving’ behavior during core-mantle differentiation, and the fact that core formation on Mars was likely not a sufficiently reducing or high-temperature process for silicon or oxygen to be major light elements.”
“Seismic and lander radio science data from the InSight mission have confirmed that Mars has a liquid core, but the presence of a solid inner core cannot be currently excluded on geophysical grounds.”
“If further geophysical observations were to verify the existence, size, and density of a Martian inner core, then combined with the appropriate mineral physical interpretation, this would provide essential constraints on the composition and temperature of the interior, as well as the possible mechanisms that initiated and terminated the magnetic field of early Mars.”
In their study, the scientists conducted high-pressure-temperature lab experiments to determine the crystal structure and density of the iron-sulfide phase in Mars’ core.
They suggest that, should temperatures at the center of Mars fall below approximately 1,960 Kelvin — which is within the estimated range for this area — the iron-sulfide phase could begin to crystallize and form a solid inner core.
Further geophysical measurements would be needed to confirm the actual presence of a solid Martian inner core.
“However, our work supports the potential for a solid inner Martian core today, or in the near future once Mars has undergone further cooling,” the authors said.
Their paper was published in the journal Nature Communications.
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L. Man et al. 2025. The structure and stability of Fe4+xS3 and its potential to form a Martian inner core. Nat Commun 16, 1710; doi: 10.1038/s41467-025-56220-2
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