In situ x-ray diffraction measurements revealed that FeS, a possible core material for the terrestrial planets, transforms to a hexagonal NiAs superstructure with axial ratio (c/a) close to the ideal close-packing value of 1.63 at high pressure and high temperature. The high-pressure-temperature phase has shorter Fe-Fe distances than the low-pressure phase. Significant shortening of the Fe-Fe distance would lead to metallization of FeS, resulting in fundamental changes in physical properties of FeS at high pressure and temperature. Calculations using the density of the high-pressure-temperature FeS phase indicate that the martian core-mantle boundary occurs within the silicate perovskite stability field.