Lab Experiments Reveal a Hidden Magma Ocean at Mars’ Core

A molten secret may be lurking at the bottom of Mars. For years, planetary scientists have debated whether a layer of silicate magma could sit at the boundary between the Red Planet’s iron-rich core and its rocky mantle, a possible remnant of a primordial magma ocean that never fully crystallized. Now, new laboratory experiments that squeeze and heat Martian-like materials to the extreme pressures and temperatures found deep inside the planet provide some of the strongest evidence yet that such a basal magma layer could exist, and that it could have survived for billions of years without freezing or mixing away. The findings, published in Nature Geoscience, reshape how researchers interpret the seismic and geophysical signals coming from deep within our neighboring world.

The study rests on a simple but technically demanding question: what happens to molten rock under the conditions that prevail at Mars’ core–mantle boundary, roughly 2,000 kilometers beneath the surface, where pressures reach tens of gigapascals and temperatures climb above 2,000 kelvin? Earlier in the planet’s history, Mars was almost certainly covered by a global magma ocean. As that ocean cooled, minerals crystallized and settled, and the question of what the last dregs of melt looked like—and where they went—has hung over Martian science for decades. If the final melts were dense enough, they could have drained downward and pooled at the base of the mantle, forming a long-lived silicate layer sandwiched above the liquid iron core.

To test that possibility, the researchers performed high-pressure experiments on synthetic compositions modeled after the silicate melts expected to be produced during the crystallization of a Martian magma ocean. Using multi-anvil press apparatus capable of reaching the gigapascal-scale pressures of the deep Martian interior, they equilibrated samples at controlled temperatures, quenched them rapidly to preserve their textures and chemistry, and then analyzed them with electron microscopy and other microanalytical techniques. From these recovered samples, the team determined the density of the melts as a function of pressure, temperature and composition, along with the melting behavior of the deep mantle assemblage.

The central result concerns buoyancy. For a magma layer to persist at the core–mantle boundary, it must be denser than the overlying solid mantle so that it does not rise and disperse, yet it must remain molten rather than freezing solid. The experiments show that late-stage melts enriched in iron and incompatible elements—components that preferentially remain in the liquid as crystals grow—become sufficiently dense under Martian deep-interior conditions to be gravitationally stable at the base of the mantle. In other words, the last liquids of a crystallizing magma ocean would naturally sink, collect and stagnate at the very bottom of the mantle, exactly where a basal magma layer has been hypothesized.

Just as important, the measurements constrain how much heat such a layer would have to exchange with its surroundings to remain molten. A magma layer at the core–mantle boundary sits atop a hot liquid iron core and beneath crystalline mantle rock, so its survival depends on a delicate thermal balance. The experimental data on melting temperatures and melt densities allowed the team to build thermal evolution models of the Martian interior, tracking the layer’s fate over the planet’s 4.5-billion-year history. Those models indicate that, for plausible core temperatures and mantle heat flows, the layer does not necessarily freeze; instead, it can be maintained as a thin, stable reservoir of molten silicate for geologically long timescales, sustained by heat flowing out of the core.

The implications extend directly to seismology. NASA’s InSight lander, which recorded Martian seismic activity until late 2022, produced the first direct glimpses of the planet’s interior, and analyses of its data have hinted at structure near the base of the mantle—some studies even proposed an entirely molten or partially molten layer above the core to explain the observed seismic velocities and attenuation. A basal silicate magma layer offers a physical mechanism for such signals: molten rock attenuates seismic waves strongly and slows shear waves dramatically compared with solid rock. The new experiments provide the compositional and thermodynamic anchor that seismic modelers need, linking a specific melt composition and thickness to specific seismic signatures rather than treating the layer as a free parameter.

The results also speak to the long-term thermal and chemical evolution of Mars. A persistent magma layer at the core–mantle boundary acts as an insulating blanket between the mantle and the core, regulating how efficiently heat escapes the core. That, in turn, influences whether the core can generate a dynamo-driven magnetic field and how the mantle convects over time. Mars lost its global magnetic field billions of years ago, and understanding the thermal insulation provided by a basal melt layer helps explain the timing and efficiency of core cooling. The layer would also sequester heat-producing and incompatible elements—uranium, thorium, potassium and others—concentrating them at the base of the mantle and altering the planet’s internal heat budget in ways that standard models, which assume a chemically uniform mantle, do not capture.

There are caveats, and the authors are careful about them. The experiments constrain the behavior of candidate melt compositions under simplified conditions; the real Martian core–mantle boundary may host heterogeneous materials, partial melting of mantle rock in contact with the core, or mixtures of silicate melt with light elements transferred from the core. The thickness of any surviving layer depends sensitively on the initial sulfur and iron content of the core, the efficiency of mantle convection and the exact crystallization sequence of the ancient magma ocean. Still, the experimental demonstration that deep Martian melts are gravitationally stable at the base of the mantle removes one of the biggest objections to the basal-magma-layer hypothesis: that such melt would have been buoyant and would have risen away long ago.

The work also carries lessons for other planets. Magma ocean crystallization is a universal stage in terrestrial planet formation, and analogous basal melt layers have been proposed for early Earth, where dense iron-rich melts may have accumulated above the core–mantle boundary and influenced the chemistry of plumes that rise from that region today. If a small planet like Mars can retain a molten basal layer for billions of years, the same physics may operate on Venus, Mercury and rocky exoplanets, where the presence or absence of such layers could alter volcanic activity, magnetic-field generation and long-term habitability. Each new laboratory dataset on melt density and melting curves thus becomes a tool for reading the interiors of worlds no spacecraft will ever drill into.

Future observations could settle the question. Additional seismic data, whether from a future Mars geophysical network or from advances in analyzing the existing InSight archive, could be compared directly with the experimentally derived elastic and attenuation properties of the candidate melt compositions. Improved constraints on Mars’ core size and density, its tidal response and its moment of inertia will further narrow the permissible layer thickness. For now, the experiments turn an intriguing speculation into a physically grounded scenario: beneath the cold, dusty surface of Mars, at the deepest reach of its rocky mantle, the last traces of a primordial magma ocean may still be glowing—molten, dense and stubbornly enduring at the edge of the planet’s iron heart.

Subject of Research: Experimental constraints on the existence of a long-lived basal magma layer at the Martian core–mantle boundary

Article Title: Experimental constraints on a long-lived magma layer at the Martian core–mantle boundary

Article References: Pierru, R., Gréaux, S., Dominijanni, S., Man, L., Kono, Y., Kakizawa, S., Higo, Y., Badro, J., Frost, D. J., & Antonangeli, D. (2026). Experimental constraints on a long-lived magma layer at the Martian core–mantle boundary. Nature Geoscience. https://doi.org/10.1038/s41561-026-02104-z

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02104-z

Keywords: Mars, core–mantle boundary, magma ocean, high-pressure experiments, silicate melt density, InSight seismology, planetary interiors, thermal evolution, Martian mantle, basal magma layer, geophysics, Experimental