Experimental evidence supporting a long-lived magma layer at the Martian core–mantle boundary

Geophysical observations from the InSight mission support low seismic wave velocity at the Martian core–mantle boundary (CMB) likely caused by the presence of a molten (or partially molten) layer. Such a layer would have had a key influence on the planet’s thermal evolution. However the relationship between its physical state and its seismic signature remain poorly constrained because experimental data on Fe-rich Martian melts at deep-mantle conditions are lacking. Here we determined the melting phase relations, seismic properties and densities of Fe-rich Martian mantle analogues at CMB pressures and temperatures. Using in situ ultrasonic interferometry combined with X-ray imaging and diffraction, we track phase transitions during progressive melting and directly determine seismic velocities in partially molten assemblages, while tightly constraining a Martian mantle solidus hundred kelvin lower than previously estimated. We find that small melt fractions (3–10 vol%) produce substantial seismic velocity reductions, with decreases of up to ~30% in shear wave speeds and ~15% in compressional wave speeds. Melts generated at pressures ≥14–18 GPa become strongly enriched in iron (Fe# > 0.45), yielding melt densities comparable to or greater than those of surrounding solids and promoting neutral to negative buoyancy. These results link Martian seismic observations to experimentally determined melt properties and support the existence of a long-lived, compositionally dense magma layer at the base of the Martian mantle.