Constraining the composition and thermal state of Mars from inversion of geophysical data

Abstract

We invert the most recent determinations of Martian second degree tidal Love number, tidal dissipation factor, mean density and moment of inertia for mantle composition and thermal state using a stochastic sampling algorithm. We employ Gibbs energy minimization to compute the stable mineralogy of the Martian mantle in the model system CaO‐FeO‐MgO‐Al2O3‐SiO2. This procedure yields density and P and S‐wave velocities in the mantle as a function of depth and temperature and permits direct inversion for composition and thermal state. We find a Martian mantle composition resembling the model composition based on geochemical analyses of Martian meteorites. A prominent discontinuity in all physical properties occurs at ∼1100 km depth, marking the onset of the mantle transition zone and coincides with the olivine → wadsleyite + ringwoodite phase transition. A smaller discontinuity in the upper mantle related to the orthopyroxene → C2/c‐pyroxene + garnet is also apparent. A lower mantle discontinuity is not observed, as pressure and temperature conditions at the core mantle boundary (∼20 GPa, ∼1800°C) are insufficient to stabilize perovskite and magnesiowüstite. The most probable core radius is ∼1680 km; core state and composition are most consistent with a liquid metallic core and a density of ∼6.7 g/cm3. This implies a high S content (>20 wt%), assuming that S is the major alloying element. The most probable bulk Fe/Si ratio is ∼1.2, indicating that Mars most probably accreted from material with a nonchondritic (CI) Fe/Si ratio, such as the ordinary chondrites (L and LL), which also have oxygen isotopic ratios matching those in Martian meteorites.