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

• Published in: Journal of Geophysical Research. E. Planets 2008-07, Vol.113 (E7), p.n/a
• Main Authors: Khan, A., Connolly, J. A. D.
• Format: Article
• Language: English
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; geophysics; inversion; mantle and core composition; Mars; thermal state
• ISSN: 0148-0227; 2156-2202
• DOI: 10.1029/2007JE002996

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.