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

Research areas:
International Workshop on INTERMARSNET, CAPRI, ITALY, SEP 28-30, 1995
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 Earth's one, down to 300
km depth. Densities and seismic velocities corresponding to these
thermodynamical conditions are computed using Gruneisen's and
third-order finite strain theory for different values of the iron
content of mantle minerals. Below 300 km depth, the values of densities
and seismic velocities have the same order of magnitude as within the
Earth's transition zone. An increase of the iron content of the Martian
mantle with respect to the Earth's 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. Copyright (C) 1996 Elsevier Science Ltd