Sound velocity measurements in liquid Fe–S at high pressure: Implications for Earthʹs and lunar cores

The P-wave velocity (VP) of liquid Fe57S43 was measured up to 5.4 GPa using an ultrasonic method combined with the synchrotron X-ray technique. The VP of liquid Fe57S43 showed little change with temperature, but increased almost linearly from 3105±11 m/s to 3845±9 m/s with increasing pressure from 2.4 to 5.4 GPa. This can be approximated by VP [m/s]=2664+205.4×P, where P is the pressure in GPa. The VP of liquid Fe57S43 at 2.4–5.4 GPa was significantly lower than that of pure liquid Fe. However, the pressure dependence of VP of the liquid Fe57S43 was markedly higher than that of pure liquid Fe. The marked difference in the pressure dependences of VP between pure liquid Fe and liquid Fe57S43 may cause VP crossover at around 7 GPa. As a result, the VP of liquid Fe57S43 would become higher than that of pure liquid Fe at pressures higher than 7 GPa. Thus, S decreases VP at low pressures such as those of the lunar outer core, but would increase it at the high pressures of the Earthʹs outer core. Assuming the lunar core consists of a liquid Fe–FeS outer core and a solid Fe inner core, the expected VP in the lunar outer core ranges from 3756 to 4230 m/s.