Phase stability and density of FeS at high pressures and temperatures: Implications for the interior structure of Mars

The phase diagram of stoichiometric iron sulfide (FeS) was investigated at high pressures and temperatures (15-35 GPa, 1400-2200 K) with a laser-heated diamond anvil cell and synchrotron X-ray diffraction. The NiAs-structured polymorph of FeS is found to be stable within the P-T range of the Martian core. The density of FeS at 1600 K is measured to be 5.96 g/cm³ at 17 GPa and 6.65 g/cm³ at 35 GPa. The density measurements are used to evaluate structural models of Mars containing a core within the Fe-FeS system. The models that satisfy the geophysical constraints proscribe limits on the thickness of the Martian crust, the size and composition of the core, and the thickness of a perovskite-bearing layer close to the Martian core-mantle boundary.