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.