Earth’s interior structure.
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Earth’s inner core, the deepest part of our planet, is characterized by extremely high pressure and temperature, forcing the matter found there in a status somewhere between liquid and solid, a new study suggests.
Traditionally we’ve been taught the Earth has three main layers: the crust, the mantle, and the core. Observations of how seismic waves are reflected and scattered, and experiments based on how minerals react to high pressure and temperatures, show that Earth’s inner structure is more complicated than previously thought.
Earth’s mantle is composed mostly of magnesium, iron, and silicon-dioxide. A transition zone, referred to as D” layer by seismologists, marks the contact to Earth’s core at a depth of 3,000 kilometers.
The outer core, with a radius of approximately 2,200 kilometers, is believed to be composed mostly of a liquid iron-nickel alloy, formed by heavy elements sinking towards the center of the Earth. The inner core, with a radius of 1,300 kilometers, is believed to slowly cool down, solidifying over time. Research published in 2021 even proposes the existence of an “innermost inner core” of unknown composition and structure. Most models, however, assume that Earth’s core is composed of a solid, crystalline iron alloy. But seismic signals show that the core is not entirely behaving like a solid structure.
Despite surpassing 6,000 degrees, Earth’s core can’t melt because the high pressure of 3.4 million atmospheres found there forces the atoms to stick together like in a solid. The pressure could be high enough to form an unusual state of matter, neither liquid nor completely solid.
A cross-section of the Earth, showing the sub-surface layers and mineral composition.
D.Bressan
Using high-pressure and high-temperature computational simulations, a joint research team led by Prof. He Yu 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 can exist in a “superionic” status.
The simulations show that Earth’s core is composed of a solid framework of iron atoms and that ionized (electrically charged) atoms of light elements – like hydrogen, oxygen and carbon – can move freely between the fixed iron.
A “superionic” structure: the white atoms can move freely between the crystalline framework of … [+]
Goran tek-en/CC-by-sa 4.0
“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,” explains Prof. He Yu.
One longstanding mystery about the inner core is that it is quite soft, with quite low shear wave velocities. Shear waves are seismic waves that can’t travel through a liquid, so the core must be solid, however, seismic velocities in the core are lower than expected for a dense iron alloy.
The researchers calculated the seismic velocities in a superionic iron alloy, finding a significant decrease in shear waves 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.
The open access study is published in the journal Nature.
