Fig. 1

Crystallization scenarios in the Fe–FeS system. Crystallization scenarios in the Fe–FeS system. a Earth-like, Fe-rich inner core grows from the center. Sulfur is enriched in the outer core and drives chemical convection. b Iron snow forms at the CMB, sinks, and remelts at depth and drives chemical convection. The stable snow zone grows in time. When it reaches the center, an inner solid core will form. c Floating FeS crystals form a stable zone growing toward the CMB where eventually a solid FeS layer will form. The fluid below is enriched in iron and unstable to convection. d A solid FeS layer grows from the CMB. Expulsion of Fe results in chemical convection in the fluid below as in (c). e FeS crystals rise from the center and remelt at lower depths. The liquid above the FeS zone is convectively unstable. When the FeS crystal zone reaches the CMB, a solid FeS layer will form. f Fe₃S snow forms at the CMB and the chemically unstable snow zone grows until it comprises the entire core where a solid Fe₃S inner core will form. The layer below is chemically homogeneous but may convect thermally. g A solid Fe₃S inner core grows with time without the release of chemical buoyancy to the outer core and convection above. Red, green, and blue dots indicate solid iron, solid FeS, and Fe₃S, respectively. Short dashes show the direction of sinking or rising. Red solid lines are the core temperature, blue dashed lines the core melting temperature, and black solid lines the concentration of sulfur, respectively. Solid arrows indicate chemical and dashed arrows thermal convection, respectively. For further explanation, see text