450-mile-wide solid metal ball forms Earth's innermost core, earthquake waves reveal

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By JoAnna Wendel
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Scientists calculated the diameter of Earth's innermost core using earthquake waves that bounced through the planet 'like ping-pong balls.'

Structure of the Earth's core against a space background.
Structure of the Earth's core. (Image credit: Rost-9D via Getty Images)

Scientists have harnessed powerful waves from earthquakes to measure Earth's innermost layer and found that our planet's center is a 450-mile-wide (725 kilometers) ball of solid iron-nickel alloy.

Previously, many researchers believed that Earth had four distinct layers — the crust, the mantle, a liquid outer core and a solid inner core. But in the past couple of decades, scientists have proposed that the inner core actually consists of two layers, referred to as the inner core and the innermost inner core. 

Now, in a paper published in the journal Nature Communications Tuesday (Feb. 21), researchers looked at earthquake, or seismic, wave data from all over the world to measure this innermost inner core.

When an earthquake strikes, it triggers waves of energy that move through rock. These waves move at different speeds based on the kinds of minerals the rock is made of and whether the rock is more rigid or soft. Certain kinds of seismic waves can't move through liquid, so they bounce off a liquid layer. Studying the way seismic waves move through Earth can reveal what distinct layers exist deep below the planet's surface.

For example, scientists have previously used seismic waves to discover the churning, liquid iron of Earth's outer core, which creates the planet's magnetic field. Seismic waves have also revealed the inner core, which, despite the heat, remains solid under immense pressure.

In the new paper, the researchers "'observed, for the first time, seismic waves bouncing back and forth from a powerful earthquake to the other side of the globe, like ping-pong balls," study's lead author Thanh-Son Pham, a geophysicist at the Australian National University in Canberra, told Live Science in an email. 

In particular, seismic waves from a magnitude 7.9 earthquake that struck near the Solomon Islands in 2017 reverberated across Earth's entire diameter several times. Seismic networks in the Alaskan Peninsula and European Alps helped the researchers see the reverberating waves, and these bouncing waves enabled the researchers to observe the two distinct layers within Earth's inner core.

3D rendering showing a cross section of an earthquake and its epicenter. (Image credit: Naeblys via Getty Images)

The researchers noted that when the earthquake waves traveled through the innermost core, in an area about 450 miles across, they moved at different speeds depending on the angle at which they were traveling. In the outer layer of the inner core, the waves moved the fastest from pole to pole and the slowest in an equatorial direction. In the innermost layer, meanwhile, the waves moved the slowest at an angle about 50 degrees away from Earth's axis. 

The different behavior of the waves moving through the outer layer of the inner core versus the innermost inner core suggests that although they may be chemically identical (made of an iron-nickel alloy), the crystal structures of these layers are different, Pham said.

"This study strengthens the evidence for the existence of an internal metallic ball with a distinct texture from the outer shell of Earth's inner core," Pham said. 

Earth's structure evolved as it cooled after the planet formed around 4.6 billion years ago. As Earth cooled, heavier elements, like iron and nickel, migrated inward, creating the inner and outer cores, while lighter elements — like the silicon that makes up much of the rock at Earth's surface — rose. 

The new view of Earth's innermost inner core could suggest that an event early in the planet's history affected its formation, and that idea could change what we know about when and how the inner core formed, Pham said.

However, there's currently no way to know what kind of event could have created the distinct layer within the inner core, or when, Pham said.. Scientists think that Earth’s core formed about a billion years ago, but the details of the core’s evolution are not well understood. So it’s difficult to say when an event might have occurred that altered the innermost core. But as the global network of seismometers grows, more seismic data will likely help uncover more details about the inner core’s growth.

"The exact timeline of the possible global event is wildly uncertain," Pham said. "Answering those questions could go a long way in understanding the Earth's evolution."

JoAnna Wendel
JoAnna Wendel
Live Science Contributor

JoAnna Wendel is a freelance science writer living in Portland, Oregon. She mainly covers Earth and planetary science but also loves the ocean, invertebrates, lichen and moss. JoAnna's work has appeared in Eos, Smithsonian Magazine, Knowable Magazine, Popular Science and more. JoAnna is also a science cartoonist and has published comics with Gizmodo, NASA, Science News for Students and more. She graduated from the University of Oregon with a degree in general sciences because she couldn't decide on her favorite area of science. In her spare time, JoAnna likes to hike, read, paint, do crossword puzzles and hang out with her cat, Pancake.

2 Comments Comment from the forums
  • bolide2
    Why should there need to have been some special "event" to have formed this solid "inner inner core"? Could that not have formed just in the natural course of the evolution of the Earth's structure, as the planet cooled?
    Reply
  • Hartmann352
    Even though it is hotter than the surface of the Sun, the crystallized iron core of the Earth remains solid.

    A new study from KTH Royal Institute of Technology in Sweden may finally settle a longstanding debate over how that's possible, as well as why seismic waves travel at higher speeds between the planet's poles than through the equator.

    Spinning within Earth's molten core is a crystal ball – actually a mass formation of almost pure crystallized iron – nearly the size of the moon. Understanding this strange, unobservable feature of our planet depends on knowing the atomic structure of these crystals – something scientists have been trying to do for years.

    As with all metals, the atomic-scale crystal structures of iron change depending on the temperatures and pressures the metal is exposed to. Atoms can be packed into variations of cubic, as well as hexagonal formations. At room temperatures and normal atmospheric pressure, iron is in what is known as a body-centered cubic (BCC) phase, which is a crystal architecture with eight corner points and a center point. But at extremely high pressure the crystalline structures transform into 12-point hexagonal forms, or a close packed (HCP) phase.

    At Earth's core, where pressure is 3.5 million times higher than surface pressure – and temperatures are some 6,000 degrees higher – scientists have proposed that the atomic architecture of iron must be hexagonal. Whether body centered cubic phase (BCC) iron exists in the center of the Earth has been debated for the last 30 years, and a recent 2014 study ruled it out, arguing that BCC would be unstable under such conditions.

    However, in a recent study published in Nature Geosciences, researchers at KTH found that iron at Earth's core is indeed in the BCC phase. Anatoly Belonoshko, a researcher in the Department of Physics at KTH, Royal Institute of Technology
    SE-100 44 Stockholm, Sweden, says that when the researchers looked into larger computational samples of iron than studied previously, characteristics of the BCC iron that were thought to render it unstable wound up doing just the opposite.

    "Under conditions in Earth's core, BCC iron exhibits a pattern of atomic diffusion never before observed," Belonoshko says.

    Belonoshko says the data also shows that pure iron likely accounts for 96 percent of the inner core's composition, along with nickel and possibly light elements.

    View: https://youtu.be/lOABzo9II3c
    Even though it is hotter than the surface of the Sun, the crystallized iron core of the Earth remains solid. A new study from KTH Royal Institute of Technology in Sweden may finally settle a longstanding debate over how that’s possible, as well as why seismic waves travel at higher speeds between the planet’s poles than through the equator. Credit: KTH Royal Institute of Technology
    Their conclusions are drawn from laborious computer simulations performed using Triolith, one of the largest Swedish massive parallel supercomputers. These simulations allowed them to reinterpret observations collected three years ago at Livermore Lawrence National Laboratory in California. "It appears that the experimental data confirming the stability of BCC iron in the Core were in front of us – we just did not know what that really meant," he says.

    At low temperature BCC is unstable and crystalline planes slide out of the ideal BCC structure. But at high temperatures, the stabilization of these structures begins much like a card game – with the shuffling of a "deck". Belonoshko says that in the extreme heat of the core, atoms no longer belong to planes because of the high amplitude of atomic motion.

    "The sliding of these planes is a bit like shuffling a deck of cards," he explains. "Even though the cards are put in different positions, the deck is still a deck. Likewise, the BCC iron retains its cubic structure."

    Such a shuffling leads to an enormous increase in the distribution of molecules and energy – which leads to increasing entropy, or the distribution of energy states. That, in turn, makes the BCC stable.

    Normally, diffusion destroys crystal structures turning them into liquid. In this case, diffusion allows iron to preserve the BCC structure. "The BCC phase goes by the motto: 'What does not kill me makes me stronger'," Belonoshko says. "The instability kills the BCC phase at low temperature, but makes the BCC phase stable at high temperature."

    He says that this diffusion also explains why the Earth's core is anisotropic – that is, it has a texture that is directional – like the grain of wood. Anisotropy explains why seismic waves travel faster between the Earth's poles, than through the equator.

    "The unique features of the Fe BCC phase, such as high-temperature self-diffusion even in a pure solid iron, might be responsible for the formation of large-scale anisotropic structures needed to explain the Earth inner core anisotropy," he says. "The diffusion allows easy texturing of iron in response to any stress."

    The prediction opens the path to understanding the interior of the Earth and eventually to predicting Earth's future, Belonoshko says. "The ultimate goal of Earth Sciences is to understand the past, present and future of the Earth - and our prediction allows us to do just that."

    See: Stabilization of body-centred cubic iron under inner core conditions, Nature Geosciences, nature.com/articles/doi:10.1038/ngeo2892
    See: https://phys.org/news/2017-02-theory-earth-core-solid-extreme.html
    3,200 miles beneath Earth's surface lies the inner core, a ball-shaped mass of mostly iron that is responsible for Earth's magnetic field. In the 1950's, researchers suggested the inner core was solid, in contrast to the liquid metal region surrounding it.

    New research led by Rhett Butler, a geophysicist at the University of Hawai'i at Manoa School of Ocean and Earth Science and Technology (SOEST), suggests that Earth's "solid" inner core is, in fact, endowed with a range of liquid, soft, and hard structures which vary across the top 150 miles of the inner core.

    No human, nor machine has been to this region. The depth, pressure and temperature make inner Earth inaccessible. So Butler, a researcher at SOEST's Hawai'i Institute of Geophysics and Planetology, and co-author Seiji Tsuboi, research scientist at the Japan Agency for Marine-Earth Science and Technology, relied on the only means available to probe the innermost Earth -- earthquake waves.

    "Illuminated by earthquakes in the crust and upper mantle, and observed by seismic observatories at Earth's surface, seismology offers the only direct way to investigate the inner core and its processes," said Butler.

    As seismic waves move through various layers of Earth, their speed changes and they may reflect or refract depending on the minerals, temperature and density of that layer.

    In order to infer features of the inner core, Butler and Tsuboi utilized data from seismometers directly opposite of the location where an earthquake was generated. Using Japan's Earth Simulator supercomputer, they assessed five pairings to broadly cover the inner core region: Tonga-Algeria, Indonesia-Brazil, and three between Chile-China.

    "In stark contrast to the homogeneous, soft iron alloys considered in all Earth models of the inner core since the 1970's, our models suggest there are adjacent regions of hard, soft, and liquid or mushy iron alloys in the top 150 miles of the inner core," said Butler. "This puts new constraints upon the composition, thermal history, and evolution of Earth.

    The study of the inner core and discovery of its heterogeneous structure provide important new information about dynamics at the boundary between the inner and outer core, which impact the generation Earth's magnetic field.

    "Knowledge of this boundary condition from seismology may enable better, predictive models of the geomagnetic field which shields and protects life on our planet," said Butler.

    The researchers plan to model the inner core structure in finer detail using the Earth Simulator and compare how that structure compares with various characteristics of Earth's geomagnetic field.

    See: Rhett Butler, Seiji Tsuboi. Antipodal seismic reflections upon shear wave velocity structures within Earth's inner core. Physics of the Earth and Planetary Interiors, Volume 321, December 2021, 106802 DOI: 10.1016/j.pepi.2021.106802
    Earth's inner core is solid even though the temperature is so high because the pressure is also very high. The temperature at the center is 5,700 K5,700 K and the pressure is estimated to be 330330 to 360 GPa360 GPa (∼3⋅106 atm∼3⋅106 atm).

    The phase diagram shown below (taken from this paper) shows the liquid/solid transition, where fcc and hcp are two different crystalline forms of solid iron. You can see clearly from the slope of the line going off toward the upper right that iron should be solid at this temperature and pressure.

    w82eb.png
    For generations, scientists have probed the structure and composition of the planet using seismic wave studies. This consists of measuring shock waves caused by Earthquakes as they penetrate and pass through the Earth’s core region. By noting differences in speed (a process known as anisotropy), scientists can determine which regions are denser than others. These studies have led to the predominant geological model that incorporates four distinct layers: a crust and a mantle (composed largely of silicate minerals) and an outer core and inner core composed of nickel-iron.

    According to seismologists from The Australian National University (ANU), data obtained in a recent study has shed new light on the deepest parts of Earth’s inner core. In a paper that appeared in Nature Communications, the team reports finding evidence for another distinct layer (a solid metal ball) in the center of Earth’s inner core – an “innermost inner core.” These findings could shed new light on the evolution of our planet and lead to revised geological models of Earth that include five distinct layers instead of the traditional four.

    Earths-layers.jpg
    The Earth's layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com

    See: https://www.universetoday.com/160317/the-earth-has-an-even-more-inner-core-and-its-a-ball-of-solid-metal/
    The structure of the earth is currently divided into five major components: the crust, the mantle, the outer core, the inner core and a solid metal ball within the inner core. Each layer has a unique chemical composition, physical state, and can impact life on Earth's surface. Movement in the mantle caused by variations in heat from the core, cause the plates to shift, which can cause earthquakes and volcanic eruptions.
    Hartmann352
    Reply