A Geophysical Perspective on the Major Element Composition of Mars’ Mantle

Introduction. The internal structure of a planet provides essential clues about its origin and evolution. At present we can only claim insight into the internal structure of the Earth and Moon. Seismology has provided by far the most specific information on the interior of these two bodies. However, because no seismic data are available for Mars, it is neccessary to look elsewhere for constraints on the Martian interior. Using the available geophysical data, i.e. mean moment of inertia in combination with mean martian density, have been used to place constraints on the mantle density profile for an assumed core composition and size [1-3]. The study by [4] employed compositions of martian meteorites in combination with models of the planet’s radial density distribution to infer the physical structure of the martian mantle. Other attempts, exemplified by [6], started off with a model of the martian mantle and core composition, which had been derived independently of any geophysical constraints [7], to experimentally determine modal mineralogy along a model pressuretemperature profile and then to use it to calculate a mantle density profile. This could then be used to constrain the size of the core. Concerning the latter, recent attempts at retrieving information on its state and size using the second degree tidal Love number, suggested it to be fluid [8,9]. However, the large observed value of can also be interpreted as the mantle being softer than the assumed elastic solid model because of the presence of partial melt at depth [8,9]. Purpose. The aforementioned studies are essentially forward modeling approaches and therefore provide no information on the range of physical models that are actually consistent with the known geophysical parameters for Mars. In view of this limitation, we employ an inverse method as described in [10] to constrain martian composition and thermal state directly from geophysical observations. The method allows composition and temperature to be transforemd directly to parameters such as mineralogy, Mg# (MgO/(MgO+FeO) 100), bulk mantle and core physical properties, which are all fundamental to our understanding of Mars. The data used in the inversion are, mean moment of inertia ( ), mean density ( ), second degree tidal Love number ( ), tidal dissipation factor ( ) and of course mean radius ( ). The observed values of these parameters are, respectively, 0.3635 0.0012, 3935 0.4 kg/m , 0.163 0.017, 92 11 and 3389.5 km [8, 9, 11]. Method of Analysis. Our model of Mars is assumed spherically symmetric and divided into three layers of variable thickness, corresponding to crust, mantle and core. Crust and mantle layers are parameterized using composition , thickness and temperature , whereas the core is modeled using the parameters radius, density, and wave velocity. Mars’ chemical composition is modeled using the model system CaO-FeO-MgO-Al O -SiO . For a given model configuration that specifies mantle composition and thermal state, the inversion procedure consists of the following steps: