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THE INNER CORE OF EARTH IS IN A SUPERIONIC STATE, WHERE LIGHT ELEMENTS MOVE LIKE A LIQUID WITHIN A SOLID IRON LATTICE. THIS DISCOVERY EXPLAINS THE CORE’S SOFTNESS AND LOW SHEAR WAVE

VELOCITY. Earth’s core, the deepest part of our planet, is characterized by extremely high pressure and temperature. It is composed of a liquid outer core and solid inner core. The inner

core is formed and grows due to the solidification of liquid iron at the inner core boundary. The inner core is less dense than pure iron, and some light elements are believed to be present

in the inner core. SUPERIONIC STATE IN EARTH’S INNER CORE A joint research team led by Prof. Yu He from the Institute of Geochemistry of the Chinese Academy of Sciences (IGCAS) has found

that the inner core of the Earth is not a normal solid but is composed of a solid iron sublattice and liquid-like light elements, which is also known as a superionic state. The liquid-like

light elements are highly diffusive in iron sublattices under inner core conditions. This study will be published in the journal _Nature_ today (February 9, 2022). A superionic state, which

is an intermediate state between solid and liquid, widely exists in the interior of planets. Using high-pressure and high-temperature computational simulations based on quantum mechanics

theory, researchers from IGCAS and the Center for High Pressure Science & Technology Advanced Research (HPSTAR) found that some Fe-H, Fe-C, and Fe-O alloys transformed into a superionic

state under inner core conditions. In superionic iron alloys, light elements become disordered and diffuse like a liquid in the lattice, while iron atoms remain ordered and vibrate about

their lattice grid, forming the solid iron framework. The diffusion coefficients of C, H, and O in superionic iron alloys are the same as those in liquid Fe. “It is quite abnormal. The

solidification of iron at the inner core boundary does not change the mobility of these light elements, and the convection of light elements is continuous in the inner core,” said Prof. Yu

He, the first and corresponding author of the study. SEISMIC VELOCITIES AND CORE SOFTNESS One longstanding mystery about the inner core is that it is quite soft, with quite a low shear wave

velocity. The researchers calculated the seismic velocities in these superionic iron alloys and found a significant decrease in shear wave velocity. “Our results fit well with seismological

observations. It is the liquid-like elements that make the inner core soften,” said co-first author SUN Shichuan from IGCAS. Highly diffusive light elements may influence seismic velocities,

offering important clues to other mysteries in the deep core. The superionic model may explain the anisotropic structure, seismic wave attenuations, and structural changes of the inner core

during the previous decades by examining the distribution and convection of these liquid-like materials in the inner core. Reference: “Superionic iron alloys and their seismic velocities in

Earth’s inner core” by Yu He, Shichuan Sun, Duck Young Kim, Bo Gyu Jang, Heping Li and Ho-kwang Mao, 9 February 2022, _Nature_. DOI: 10.1038/s41586-021-04361-x NEVER MISS A BREAKTHROUGH:

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