Study: Earth core merges rare liquid, solid state

The center of the Earth appears to be even weirder than was imagined by French novelist Jules Verne, who in 1864 published Journey to the Center of the Earth－a fantastical odyssey about traveling down to the hidden worlds buried beneath the Earth's surface.

While scientists know that the Earth isn't hollow and filled with fanciful creatures as Verne wrote, they believed for decades that the Earth's inner core was just a solid metal ball made of mostly iron and nickel, given the extreme pressure and that it is encased in a deep ocean of molten liquid alloys known as the outer core.

But Chinese scientists have discovered that the Earth's inner core is actually in a state of matter called superionic that allows it to behave like solid and liquid at the same time, according to a study published in the journal Nature on Thursday.

This discovery has challenged the traditional understanding of the geological structure of the Earth and may spur new research ideas and theories on some of Earth's biggest mysteries, such as the origin of its magnetic field.

In 2018, scientists squeezed a water droplet between two diamond anvil cells and blasted it with powerful lasers. The extreme pressure and temperature rearranged the hydrogen and oxygen atoms from the water into a strange form called superionic ice, according to a paper published in the journal Nature Physics.

This is due to the intense heat melting the chemical bonds between the hydrogen and oxygen atoms. The high pressure keeps the heavier oxygen atoms stacked in an orderly crystal alignment like a solid, while the hydrogen ions flow through the lattice made of oxygen atoms like a liquid.

Scientists suspect this strange ice could make up a large part of the interiors of icy giants like Uranus and Neptune, thus helping to explain their unusually strong and titled magnetic fields.

Since extreme heat and pressure exist under the Earth's deep mantle, scientists hypothesized that such a superionic state may also be present in iron alloys from our planet's interior.

He Yu, the lead author of the paper and a researcher at the Chinese Academy of Sciences' Institute of Geochemistry, said the Earth's inner core contains light elements, including carbon, oxygen and hydrogen. They are some of the most abundant chemicals in the solar system and were likely to have been introduced into the core during its formation.

Through computer simulations and seismic data, He and his team discovered that the iron alloys from the Earth's inner core that contain these light elements were behaving similarly to superionic ice, meaning that the iron atoms were fixed in place like a solid, while the light elements were drifting around like a liquid between the iron lattice.

"This implies that the Earth's inner core is not in a conventional solid state, but is in a superionic phase consisting of solid iron and floating light elements," He said.

"This discovery has refreshed our understanding of the inner core, and may provide new clues for studying the Earth's interior structure, seismic movement, geomagnetic field and other key scientific topics."

For example, the Earth's magnetic field is powered by the churning of the planet's cores. This field plays a vital role in making the planet habitable as it deflects charged solar wind particles that may otherwise strip away our atmosphere and damage living organisms and electronics on the ground, He said.

But scientists have trouble understanding why the Earth's magnetic field is constantly shifting, and why it can completely flip the locations of its magnetic north and south poles. As a result, what causes the Earth's magnetic field to reverse is listed as one of the world's 125 cutting-edge scientific questions by the journal Science.

He said that examining the Earth's cores and exploring deep Earth in general may yield the answers to these big mysteries. Probing the Earth is also important in terms of the discovery and development of natural resources such as fossil fuel and minerals, as well as in improving the ability to predict earthquakes and volcanic eruptions, he added.

"Due to its immense scale, heat and pressure, deep Earth remains the most inaccessible frontier for scientists. We have people flying into space and diving to the deepest parts of the ocean, but the farthest we have ever got into the Earth is a 12-kilometer borehole in Russia. That is barely scratching the surface of the Earth's roughly 6,300 km radius," He said.

"Therefore, deep Earth is an unchartered frontier that has great research, economic and strategic significance."