The Earth's core, or the deepest part of the planet, is known to have very high pressure and temperature. But it turns out that the core of the earth is not only a hot liquid. Scientists found there is a solid object in the deepest part. In short, the outer core is a liquid, and the inner core is a solid. A group of joint research team led by Prof. HE Yu of the Institute of Geochemistry of the Chinese Academy of Sciences (IGCAS) found that the Earth's inner core is not a normal solid but consists of a solid and liquid-like metal (iron/Fe) sublattice. This unique composition is a light element which is also known as the superionic state. Light elements such as liquids are highly diffusive in metallic sublattices under deep core conditions, the study found. The inner core is formed and grows due to the solidification of the molten metal at the boundary of the inner core. While the inner core is not as dense as pure metal, and some light elements are believed to be present in the inner core. Superionic states, which are intermediate states between solid and liquid, are abundant in the interior of the planet.
Using high-pressure and high-temperature computational simulations based on quantum mechanical 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 turn into superionic states in the under inner core conditions. Superionic metal alloys, light elements become disorganized and diffuse like liquids in a lattice, while Fe atoms remain organized and vibrate around the lattice, forming a solid metal framework. The diffusion coefficients of C, H, and O in superionic metal alloys are the same as in molten Fe.
"This is very abnormal. Metal solidification at the inner core boundary does not change the mobility of these light elements, and light element convection continues in the inner core," said Prof. HE Yu, lead author and correspondent of the study.
One of the long-standing mysteries about the inner core is that it is quite malleable, with fairly low shear wave velocities. The researchers calculated seismic velocities in these superionic metal alloys and found a significant decrease in shear wave velocities.
"Our results match with seismological observations. It is a liquid-like element that softens the inner core," said HE Yu's colleague, SUN Shichuan, of IGCAS.
Highly diffused light elements can affect seismic velocities, providing important clues for understanding other mysteries in the inner core.
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