Sound velocity of Fe-S liquids at high pressure: Implications for the Moon's molten outer core

•Sound velocities of Fe and Fe-S liquids were measured up to 8 GPa and 1973 K.•Velocity increases with P, decreases with sulfur content, and is independent of T.•Sulfur content, density, and temperature of the lunar outer core were constrained.•A new lunar structure model incorporating mineral physics data was proposed.•Our model suggests a top-down solidification scenario for the lunar core.

Sound velocities of Fe and Fe-S liquids were determined by combining the ultrasonic measurements and synchrotron X-ray techniques under high pressure-temperature conditions from 1 to 8 GPa and 1573 K to 1973 K. Four different liquid compositions were studied including Fe, Fe-10 wt% S, Fe-20 wt% S, and Fe-27 wt% S. Our data show that the velocity of Fe-rich liquids increases upon compression and decreases with increasing sulfur content, whereas temperature has negligible effect on the velocity of Fe-S liquids. The sound velocity data were combined with ambient-pressure densities to fit the Murnaghan equation of state (EOS). Compared to the lunar seismic model, our velocity data constrain the sulfur content at 4±3wt%, indicating a significantly denser (6.5±0.5g/cm3) and hotter (1870−70+100K) outer core than previously estimated. A new lunar structure model incorporating available geophysical observations points to a smaller core radius. Our model suggests a top-down solidification scenario for the evolution of the lunar core. Such “iron snow” process may have been an important mechanism for the growth of the inner core.