Low Velocity Zones in the Martian Upper Mantle Highlighted by Sound Velocity Measurements

The InSight mission to Mars is currently monitoring the seismic activity of the planet. Interpretation of seismological observations in terms of composition and mineralogy requires the knowledge of density and thermo-elastic properties of constituent materials at pertinent conditions. We thus performed phase equilibria experiments and carried out sound velocity and density measurements on aggregates representative of the Martian mantle over pressures and temperatures directly relevant for Mars’ upper and mid mantle. Our results indicate the stability of magnetite, although in a small amount, in phase assemblages at upper mantle conditions, especially in an oxidized environment. The measured pressure and temperature derivatives of compressional and shear velocities show that the temperature-induced reduction of seismic wave speeds dominates over pressure-induced effects at Mars' shallow mantle conditions for the predicted areotherms and, independently from mineralogy, support the presence of a low-shear-wave-velocity layer between 150 and 350 km depth, in agreement with seismic observations.