PI: Lidunka Vocadlo

What Is Mars’ Core Really Made Of?

New theoretical calculations reveal a surprisingly hydrogen-rich core

The core of Mars holds the key to some of the biggest questions in planetary science: why did Mars lose its global magnetic field? How did it cool so quickly? And how did water and volatile elements shape its early evolution? Our new study (van Driel, Vočadlo and Brodholt, 2025) points to a surprising conclusion: Mars’ core is likely extremely rich in hydrogen, and almost devoid of oxygen.

NASA’s InSight mission transformed our knowledge of Mars by recording the first seismic waves from “marsquakes”. These data showed that Mars has a fully liquid metal core, with a surprisingly low density and slow seismic wave speeds. But density and wave speed alone do not uniquely determine composition — many mixtures of iron and light elements can fit the observations. This study uses first-principles quantum mechanical simulations and machine learning techniques to calculate how alloys of iron, sulphur, oxygen, carbon and hydrogen behave under Martian core conditions. Crucially, we include magnetism, which strongly affects the density and elastic properties at the relatively low pressures inside Mars.

We calculated over 700 unique data points and used them to build a machine-learning equation of state. This model predicts how any mixture of Fe–S–O–C–H behaves across the entire Martian core. We then searched systematically for all possible compositions that match the observed seismic density and wave speeds — not just a few hand-picked candidates.

We find that to match Mars’ core, around 50% of all atoms must be light elements, and when realistic geochemical constraints are applied, hydrogen becomes very likely. Furthermore, when cosmochemical limits on sulphur and experimental constraints on element partitioning are included, we find ~0.5 wt% hydrogen (≈27 mol%) is required, while oxygen is almost absent; furthermore, sulphur and carbon alone cannot explain the data.

Such a hydrogen-rich core implies Mars formed from highly volatile-rich material and incorporated vast amounts of hydrogen very early. Hydrogen-rich iron is less dense and has different thermal properties, which may explain why Mars’ dynamo shut down quickly compared to Earth’s. If large amounts of hydrogen entered the core, it strengthens the case that water and volatiles were abundant in early Mars, with direct implications for its past habitability.