Through the utilization of a high-velocity cannon to propel microscopic iron-carbon alloy projectiles, researchers have empirically substantiated the existence of a unique state of matter that is simultaneously solid and liquid. This superionic phase, long hypothesized to reside within the Earth's inner core, provides a robust scientific explanation for various seismic anomalies. Specifically, it accounts for the unexpected deceleration of certain waves and corroborates measurements suggesting that the core exhibits a viscosity comparable to butter, rather than the extreme rigidity typically associated with cold steel.
Experimental validation of superionic matter within the Earth's inner core
The research, led by physicist Youjun Zhang of Sichuan University, marks the first experimental demonstration of remarkably low shear velocities in iron-carbon alloys under conditions replicating the Earth's interior. In this extraordinary state, carbon atoms achieve extreme mobility, diffusing through the iron crystal lattice with high fluidity while the iron atoms maintain a stable, ordered structure. This dual nature significantly diminishes the overall stiffness of the alloy, fundamentally altering the mechanical properties of the material at extreme depths.
Our current comprehension of the Earth’s internal architecture is primarily derived from seismic data, which tracks the movement and reflection of acoustic waves through various geological layers. While these observations have provided a detailed map of the planet's interior, the low speed of shear waves through the core has historically presented a paradox. These findings suggested that if the inner core were indeed solid, its structural integrity would have to differ substantially from conventional solid-state physics.
The experimental breakthrough builds upon a 2022 theoretical framework established by geophysicist Yu He of the Chinese Academy of Sciences. The theory posits that the immense pressure exerted by the Earth's mass maintains iron in a solid matrix, yet the concurrent extreme temperatures allow lighter elements to flow with the characteristics of a fluid. This synthesis of states—known as superionicity—effectively resolves long-standing enigmas regarding the core's composition, proving that the heart of our planet operates under a complex interplay of solid structural stability and liquid-like kinetic energy.
Empirical confirmation through dynamic shock compression
Scientific evidence has recently validated the possibility of superionic matter within the planetary interior. Dr. Zhang, Dr. He, and their colleagues employed a sophisticated technique known as dynamic shock compression to subject a small iron-carbon alloy sample to forces that replicate the specific behaviors expected within the Earth's core. To achieve these conditions, the researchers utilized two-stage light-gas guns—high-precision instruments that leverage smokeless gunpowder and compressed gas to propel microscopic particles at extraordinary velocities.
During the experiment, the iron-carbon projectile was launched at speeds exceeding 7 kilometers per second (approximately 4.3 miles per second) toward a target composed of highly compressible lithium fluoride. This impact generated a reverse shock wave, compressing the sample to pressures reaching 140 gigapascals and temperatures nearing 2,600 Kelvin. While these parameters do not fully reach the 330 to 360 gigapascals and 5,000 to 6,000 Kelvin found at the planet's center, they are sufficiently intense to reproduce the essential physical characteristics of the core environment.
The simulated conditions persisted for only a few nanoseconds or microseconds, yet this brief window provided ample time for high-speed lasers and sensors to probe the sample's temperature, density, and acoustic wave propagation. The resulting data aligned precisely with the low shear wave velocities and specific viscosity measurements—mathematically represented by the Poisson’s ratio—consistently observed in global seismic readings of the Earth’s inner core.
The researchers successfully demonstrated that, under such extreme stress, the iron matrix remains firmly locked in a stable crystalline structure, providing the material's solid framework. Simultaneously, the lighter carbon atoms flow fluidly through the available interstitial gaps. This distinct separation of atomic behavior confirms the superionic state, where a rigid structural lattice and a mobile liquid-like component coexist, offering a definitive explanation for the unique geophysical properties of the Earth's heart.
Implications for the Earth's magnetic field
The discovery of the superionic state offers an exceptionally elegant solution to long-standing discrepancies in planetary science. By aligning experimental data—the most direct evidence currently attainable regarding the Earth's interior—with seismic observations, this research resolves protracted debates concerning the behavior of light elements under extreme pressure. This new framework effectively explains why seismic waves exhibit specific patterns, providing a physical basis for the anomalies that have puzzled geophysicists for decades.
The implications of these findings extend beyond the core's physical structure, potentially offering transformative insights into the Earth's magnetic field. This vast magnetosphere, which projects far into space and protects the planet from solar radiation, is generated by the complex interplay of conduction and convection within the deep interior. The identification of a superionic phase suggests that the kinetic "dance" of atoms within the core plays a more sophisticated role in the geodynamo than previously understood.
According to Dr. Zhang, this research marks a significant paradigm shift, moving the scientific community away from a static, rigid model of the inner core in favor of a more dynamic interpretation. By recognizing that the planet's heart is a vibrant environment of moving particles rather than a dormant block of metal, researchers are gaining a more accurate perspective on the fundamental nature of the Earth.
The successful characterization of this hidden state of matter brings humanity closer to unraveling the profound secrets of our own world and other terrestrial planets. As scientists decode the relationship between extreme heat, pressure, and atomic mobility, they establish a blueprint for understanding the structural evolution of Earth-like bodies throughout the universe. This discovery underscores the importance of experimental physics in mapping the invisible frontiers that lie beneath our feet.
The research was published in the National Science Review.
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