There's Another Core Within Earth's Core, Scientists Discover

The innermost inner core (IMIC) “could be a fossilized record of a significant global event from the past,” scientists say.
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Scientists have captured unprecedented observations of a mysterious metal ball that sits within Earth’s inner core, revealing a structure that has been speculated about for years but has never been glimpsed in detail before, reports a new study. 

The results reveal that this innermost inner core (IMIC) of Earth has a radius of about 400 miles and is significantly different from the rest of the planet’s center, a discovery that may help explain how Earth became so habitable to life, while also providing new insights about the search for aliens on other worlds.


Earth’s innermost region is separated into two major components: a solid inner core that measures about 750 miles in radius and sits within the outer core, which is a 1,300-mile-thick layer of liquid metal. Due to its remote location 1,800 miles under our feet, the core has long evaded easy observation, though scientists can peer at some of its hidden features by recording seismic waves from earthquakes that pass through this enigmatic area.

Past studies with seismic waves have offered tantalizing hints that the inner core might itself contain a distinct core, though the size and nature of this potential “fifth layer” of Earth has remained a matter of debate. 

Now, Thanh-Son Phạm and Hrvoje Tkalčić, a pair of researchers at The Australian National University, have used “a previously unobserved and unutilized class of seismological observations” to expose the IMIC, which they say “could be a fossilized record of a significant global event from the past,” according to a study published on Tuesday in Nature Communications.

“Earth’s inner core (IC), which accounts for less than 1% of the Earth’s volume, is a time capsule of our planet’s history,” said Phạm and Tkalčić in the study. “As the IC grows, the latent heat and light elements released by the solidification process drive the convection of the liquid outer core, which, in turn, maintains the geodynamo,” referring to the mechanism that generates Earth’s magnetic field.  


“Although the geomagnetic field might have preceded the IC’s birth, detectable changes in the IC’s structures with depth could signify shifts in the geomagnetic field’s operation, which could have profoundly influenced the Earth’s evolution and its ecosystem,” the team continued. “Therefore, probing the innermost part of the IC is critical to further disentangling the time capsule and understanding Earth’s evolution in the distant past.”

To that point, Earth scientists have been treated to a rush of interesting new studies about Earth’s core over the past few weeks, in part due to advances in seismological studies. Last month, a team found that the rotation of Earth’s inner core might have recently paused and reversed direction (relative to Earth’s surface rotation), while another new study has probed “anomalies” at the boundary between the core and the mantle layer above it.

Now, Phạm and Tkalčić have glimpsed the IMIC by examining a specific set of earthquakes that sent seismic waves reverberating all the way through the entire planet, back and forth, up to five times. For instance, waves from an earthquake in Alaska ended up traveling right to the opposite end of the planet, surfacing in the South Atlantic, before bouncing back again, opening up a detailed new window into the interior of our world. With this novel set of observations, the team was able to spot details about the core that have never been seen before.


“To our knowledge, reverberations from more than two passages are hitherto unreported in the seismological literature,” said Phạm and Tkalčić in the study, adding that this technique provides “tools to constrain the IMIC properties because they sample the IMIC in an unprecedented fashion.” 

As a result, the team was able to confirm the existence of an IMIC and provide a new estimate of its size, at about 800 miles in diameter, that is slightly bigger than projections offered in past studies. Waves that travel through this innermost region of Earth have different signatures from those in the outer inner core (OIC), suggesting that the IMIC has a distinct structure from its shell. 

This discovery could help scientists reconstruct the 4.5-billion-year evolution of Earth’s core, which has played a major role in Earth’s capacity to host life. The outer and inner cores together produce the magnetic field that protects our planet from harmful radiation, and serves as a guide for spotting habitable worlds elsewhere in the universe. Understanding these processes will require the development of ever-more sophisticated tools for probing the depths of our planet, Phạm and Tkalčić said.   

“The findings reported here are a consequence of the unprecedentedly growing volume of digital waveform data and will hopefully inspire further scrutiny of existing seismic records for revealing hidden signals that shed light on the Earth’s deep interior,” the researchers noted in the study.

“We now have enough seismological evidence from several different lines of investigation about the existence of IMIC,” they concluded. “Future efforts should be directed toward characterizing the IMIC-OIC transition (its depth and nature).”