Theoretical seismic models of Mars: the importance of the iron content of the mantle

Present-day averaged temperature profiles of the mantle of Mars are computed through numerical convection experiments performed with axisymmetrical geometry, for different values of core radii and different boundary conditions at the core-mantle boundary. Internal heating of the mantle is considered in each case. It is found that the temperature profiles of the mantle are very stable whatever the imposed conditions at the core-mantle boundary. A 300 km thick thermal lithosphere, displaying a temperature gradient equal to 4.4 K km−1 is followed at greater depths by a quasi-isothermal mantle, the temperature of which is found in a 1200–1600 K temperature range. A mean temperature equal to 1400 K is in a good agreement with the low Q of Mars at tidal frequencies. These characteristics, together with the small increase of pressure with depth, of the order of 0.01 GPa km−1, induce the presence of a low-velocity zone similar to the Earthʹs one, down to 300 km depth. Densities and seismic velocities corresponding to these thermodynamical conditions are computed using Grüneisenʹs and third-order finite strain theory for different values of the iron content of mantle minerals. Below 300 km depth, the values of magnitude as within the Earthʹs transition zone. An increase of the iron content of the Martian mantle with respect to the Earthʹs one results (1) in an increase of density, and a decrease of seismic velocities, which can reach more than 2% of the values expected from an Earth like composition, (2) in a homogenization of mantle structure through the smoothing out of seismic discontinuities over a thickness of a few hundred kilometres. This smoothing process is due to the large pressure domains of coexistence between different phases of olivine when the iron content of this latter mineral increases. Plausible domains of core density and core radius are finally checked back for each of the computed models of mantle density. These tests show that the principal moment of inertia ratio of Mars should not be lower than 0.355 if the iron content of the Martian mantle is at least equal to that of the Earth, and that the thickness of the liquid core should be small, of the order of 300–400 km, if a solid core is present at the centre of the planet. This small thickness might explain the weakness (or absence?) of an internally generated magnetic field on Mars.