Neutron star crust

The crust of neutron stars extends down to about 1 kilometer below the surface, with densities ranging from a few grams per cubic centimeters on the ground up to several hundred thousand billion grams per cubic centimeters at the bottom.

Here is a sketch (the different layers are not to scale) of the structure of neutron star crust, assuming that the matter is in its ground state ("cold catalyzed matter"):

structure of neutron star crust

The outermost layers of a neutron stars are composed of iron. At densities above ten thousand grams per cubic centimeters, the atoms are fully ionized due to the pressure of the upper layers. The free electrons are degenerate and become relativistic at densities beyond a few million grans per cubic centimeters. With increasing densities, the nuclei are more and more neutron rich owing to electron captures which convert protons into neutrons. This neutronization of the matter leads to the existence of a neutron ocean which permeates the inner layers of the crust at densities beyond around four hundred billion grams per cubic centimeters. The crust dissolves into a uniform mixture of neutron, protons and electrons when the density reaches about hundred million billion grams per cubic centimeters. A mantle of nuclear clusters with unusual shapes (slabs, cylinders or bubble like structures), so-called nuclear "pastas", might possibly exist near the crust-core interface.

Living Review in Relativity about the Physics of Neutron Star crusts
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