Geodesy constraints on the interior structure and composition of Mars

Research areas:
Knowledge of the interior structure of Mars is of fundamental importance
to the understanding of its past and present state as well as its future
evolution. The most prominent interior structure properties are the
state of the core, solid or liquid, its radius, and its composition in
terms of light elements, the thickness of the mantle, its composition,
the presence of a lower mantle, and the density of the crust. In the
absence of seismic sounding only geodesy data allow reliably
constraining the deep interior of Mars. Those data are the mass, moment
of inertia, and tides. They are related to Mars' composition, to its
internal mass distribution, and to its deformational response to
principally the tidal forcing of the Sun. Here we use the most recent
estimates of the moment of inertia and tidal Love number k(2) in order
to infer knowledge about the interior structure of the Mars.
We have built precise models of the interior structure of Mars that are
parameterized by the crust density and thickness, the volume fractions
of upper mantle mineral phases, the bulk mantle iron concentration, and
the size and the sulfur concentration of the core. From the bulk mantle
iron concentration and from the volume fractions of the upper mantle
mineral phases, the depth dependent mineralogy is deduced by using
experimentally determined phase diagrams. The thermoelastic properties
at each depth inside the mantle are calculated by using equations of
state. Since it is difficult to determine the temperature inside the
mantle of Mars we here use two end-member temperature profiles that have
been deduced from studies dedicated to the thermal evolution of Mars. We
calculate the pressure and temperature dependent thermoelastic
properties of the core constituents by using equations state and recent
data about reference thermoelastic properties of liquid iron, liquid
iron-sulfur, and solid iron. To determine the size of a possible inner
core we use recent data on the melting temperature of iron-sulfur.
Within our model assumptions the geodesy data imply that Mars has no
solid inner core and that the liquid core contains a large fraction of
sulfur. The absence of a solid inner is in agreement with the absence of
a global magnetic field. We estimate the radius of the core to be 1794
+/- 65 km and its core sulfur concentration to be 16 +/- 2 wt\%. We also
show that it is possible for Mars to have a thin layer of perovskite at
the bottom of the mantle if it has a hot mantle temperature. Moreover a
chondritic Fe/Si ratio is shown to be consistent with the geodesy data,
although significantly different value are also possible. Our results
demonstrate that geodesy data alone, even if a mantle temperature is
assumed, can almost not constrain the mineralogy of the mantle and the
crust. In order to obtain stronger constraints on the mantle mineralogy
bulk properties, like a fixed Fe/Si ratio, have to be assumed. (C) 2011
Elsevier Inc. All rights reserved.