How is it that earth's inner core is still solid?

Question

Earth's inner core is made up of an alloy of Iron and Nickel.

The important note here is that it is solid.

The temperature at the inner core is about $5,763K = 5,490\text{ degrees C}$.

Now the melting point of both solid iron and nickel is about $1,500\text{ degrees C}$, and even though they are combined, I still think that $5,490$ is much greater then the melting point of the alloy.

Regardless, how is it that the inner core still stays solid? I would expect that at such higher temperatures, a phase change to liquid, or maybe even gas would occur.

Why does it not?

Answer 1

The melting point of the vast majority of materials increases as more pressure is applied. At https://www.researchgate.net/figure/312054885_fig1_Figure-231-Experimental-pressure-temperature-phase-diagram-of-pure-iron-a is a phase diagram showing how this variation occurs for pure iron:

From https://physics.stackexchange.com/questions/184032/what-is-the-pressure-at-the-center-of-the-earth-or-a-neutron-star the pressure at the centre of the Earth is 365GPa. At this pressure we can see from the graph that the melting point of pure iron is well over 5000k. Thus, given the experimental uncertainties and the that the core is not pure iron, it is easily conceivable that the core of the Earth is solid, or at least a large proportion of it is.

Answer 2

A liquid's boiling point is defined at a standard pressure, so also a solid's melting point is dependent on the applied pressure. Liquid iron, under pressures at the center of the earth, is compressed to similar density to solid iron, and our best theories say that it has to become solid, even at those temperatures, at that pressure.

Iron's temperature-versus-spacing parameter (thermal expansivity) is about 12 * 10^(-6) per Kelvin; its pressure-versus-volume parameter (bulk modulus) is 170 GPa. So, 10,000 degrees heating has an atomic-spacing expansion factor of 1.12, while a center-of-planet pressure, 320 GPa at 5000km, gives a volume compression factor of 2, or a distance compression factor of 1.3, so it makes some sense that the pressure and temperatures in the Earth's core will not let iron expand from a solid to liquid: the pressure is the winning factor here, not the temperature.

Detailed observations DO find a solid center in the planet, but our knowledge of the exact nature of that solid is mainly theoretical, based on heavy computation of the quantum mechanics of atomic ensembles held apart by their electron shells and pushed together by gravity.