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

The video explores the ongoing mystery surrounding Earth's inner core, which current seismic data suggests possesses properties that defy our traditional understanding of a solid, crystalline structure. While we've long known about Earth's liquid outer core and a solid inner core, recent seismic wave analysis has revealed anomalies, particularly concerning shear waves (S-waves). The journey to understanding Earth's interior has been a gradual process, relying heavily on seismology – the study of seismic waves generated by earthquakes. Early discoveries in the 20th century, like Andrija Mohorovičić's identification of the crust-mantle boundary in 1909 and Benno Gutenberg's discovery of the liquid outer core in 1914, were groundbreaking. Inge Lehmann's crucial discovery in 1936 of the solid inner core, based on P-waves reflecting off it, further refined our model. This led to the established view of Earth's layered structure: a crust, a mantle, a liquid outer core, and a solid inner core, all primarily composed of iron. This model effectively explained phenomena like volcanism, plate tectonics, and Earth's magnetic field. However, as seismic monitoring became more sophisticated, new data began to present discrepancies. For instance, P-waves travel faster through the inner core in a polar direction compared to the equatorial direction. This observation is often explained by the anisotropic nature of crystalline iron, where wave speed depends on the orientation relative to the crystal lattice, and how this lattice might align with Earth's spin. Furthermore, an east-west hemispheric asymmetry in wave speed has been observed, suggesting large-scale lumpiness or melt regions within the inner core. The most perplexing anomaly, however, relates to S-waves. While S-waves cannot travel through liquids, they *can* be generated within the solid inner core through the conversion of P-waves. What's baffling is that these core-generated S-waves travel much slower than expected for a stiff material like crystalline iron. Moreover, they lose energy more rapidly than anticipated, indicating a less rigid material than pure crystalline iron. This has led to the concept of a "squidgy" core, a term used informally to describe a material that is highly shearable but not necessarily compressible. The Poisson's ratio, a measure of a material's shearability relative to its compressibility, provides a quantitative aspect to this mystery. For typical solids, this ratio is around 0.2 to 0.3. Earth's core, however, exhibits a ratio close to 0.45, comparable to rubber, suggesting it is much more susceptible to shape changes (shear) than expected for a solid. Scientists are investigating several hypotheses to explain this "squidginess." One idea is that alloying iron with lighter elements like hydrogen, carbon, oxygen, or silicon could alter its properties. While these alloys can increase the Poisson's ratio, it's generally believed that alloying alone cannot account for the observed S-wave speeds. Another possibility is that the inner core is not a single, large crystal but rather a granular structure. When molten metal solidifies, it forms grains with misaligned crystal lattices. The boundaries between these grains are weaker and can slide against each other, especially if microscopic films of molten or softened metal exist between them. This granularization and the presence of melt could explain the high Poisson's ratio. However, this model faces challenges: too much melt or too fine a grain size would cause S-waves to lose too much energy, and it might also hinder the global lattice alignment needed to explain the polar-equatorial speed differences. This leads to the most intriguing hypothesis: the superionic state of matter. In this state, a rigid crystal lattice of one element or molecule allows other atoms to move freely within its interstitial spaces, exhibiting liquid-like mobility. Examples include superionic ice, where hydrogen moves within an oxygen lattice. For Earth's core, the proposed model involves a hexagonal close-packed iron lattice (alloyed with nickel), with lighter elements like carbon occupying the spaces between the iron atoms. Molecular dynamics simulations suggest that at the high pressures and temperatures of Earth's inner core, carbon atoms within an iron-carbon alloy can move freely, behaving in a liquid-like manner while the iron lattice remains relatively rigid. These simulations predict a lower shear velocity and a Poisson's ratio close to the observed seismic values for the inner core. Experimental validation of this superionic state has been a significant step. A recent study by Huang He Zhang et al. created an iron-carbon alloy lattice and subjected it to high-speed particle impacts, generating the necessary shock pressures and temperatures to induce the superionic state. Using a technique called photon-Doppler velocimetry, which is akin to a highly precise speed gun for surfaces, the researchers studied the vibrations and inferred the material's properties. The results were consistent with simulations, showing strong shear softening that aligns with the seismic data for Earth's inner core. While the experiment did not replicate the full pressure and temperature of Earth's inner core, it provided crucial experimental evidence for the superionic state in an iron-carbon alloy. If the superionic hypothesis is correct, it could also help explain other anomalies, such as the polar-equatorial speed difference and potentially even contribute to the geodynamo effect that generates Earth's magnetic field through the flow of interstitial carbon. Ultimately, the exploration of Earth's interior, much like the cosmos, relies on deciphering vibrations to build our understanding of these hidden realms.