Fig. 5.8. Melting temperature of Fe-FeS alloy at P = 15 GPa according to experimental data (Fig. 5.3). For a given composition, the cooling of the fluid will form the first solid iron at point A. As the temperature decreases, the liquid gets enriched in sulfur (from A to B). E is the eutectic composition that may be eventually reached if the planet cools down sufficiently
Chemical convection may play a role in the dynamics of the liquid outer core. As the inner core crystallizes, sulfur is left over. As it is lighter than iron, it is redistributed in the liquid core. Consequently, the gravitational potential is changed and part of it is turned into heat by viscous dissipation. The presence of a solid core may also affect the geometry of iron circulation. If the solid core is too big, it would be difficult to have circulation of liquid iron parallel to the rotation axis of the Earth. The presence of a solid inner core may play an important role for the generation of a magnetic field. Is it required for a dipolar dynamo to exist? The answer is not yet known.
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