Earth's Core Should Be Impossible. A New State of Matter Explains It.

TL;DR

Earth's inner core behaves like rubber, not solid iron, and a superionic state where carbon flows freely through a rigid iron lattice may explain why.

Key Points

• 1.Earth's inner core was discovered layer by layer through seismic detective work. Mohorovičić found the crust-mantle boundary in 1909, Gutenberg confirmed a liquid outer core in 1914 using S-wave shadow zones, and Inge Lehmann identified the solid inner core in 1936 by detecting P-waves in supposed shadow zones.
• 2.The inner core's S-waves travel far too slowly for crystalline iron. Earth's inner core has a Poisson's ratio of ~0.45 — close to rubber — meaning it's highly shearable despite being incompressible, while standard crystalline iron should produce fast S-waves in both compression and shear.
• 3.Alloying and grain fragmentation only partially explain the anomaly. Adding light elements like hydrogen, carbon, or silicon to the iron lattice raises the Poisson's ratio but not enough; granular crystal structures with melt between grains can increase squidginess but only within a very narrow parameter window.
• 4.The superionic state is a simultaneous solid-liquid phase where light atoms flow freely inside a rigid crystal lattice. Molecular dynamic simulations show that at Earth's core pressures and temperatures, carbon atoms in an iron-carbon alloy become liquid-like, yielding a Poisson's ratio of ~0.43 — matching observed seismic data.
• 5.A 2024 lab experiment by Huang, He, Zhang et al. physically produced superionic iron-carbon for the first time. Using a light gas gun to fire a projectile at Mach 20+, they shocked an iron-carbon sample to core-like pressures; photon Doppler velocimetry confirmed shear softening consistent with the superionic prediction.
• 6.The superionic hypothesis also explains additional core mysteries. Preferential flow of interstitial carbon along Earth's rotational axis could account for the polar-vs-equatorial wave speed difference, and that carbon flow may even contribute to the geodynamo effect powering Earth's magnetic field.