Earth's core is composed primarily of iron (Fe) with about 10% by weight of lighter elements. The lighter elements are progressively enriched in the liquid outer core as the core cools and the inner core crystallizes. Thermodynamic modeling of Fe-O-S liquids shows that immiscible liquids can exist at outer-core pressures (136 to 330 gigapascals) at temperatures below 5200 kelvin and lead to layering in the outer core if the concentrations of the lighter elements are high enough. We found no evidence for layering in the outer core in the travel times and wave forms of P4KP seismic waves that reflect internally in the core. The absence of layers therefore constrains outer-core compositions in the Fe-O-S system to be no richer than 6 ± 1 weight % (wt %) O and 2 to 15 wt % S. A single core liquid composition of 10.5 ± 3.5 wt % S and 1.5 ± 1.5 wt % O is compatible with wave speeds and densities throughout the outer core.
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