Researchers from Australian National University have found evidence of a hidden layer inside Earth’s inner core. The scientists found the layer, an iron-nickel alloy ball around 800 miles (1350 km) wide, by studying the reverberations of seismic waves from large earthquakes through the Earth.
When seismic waves travel through the Earth, they change shape and provide insight into Earth’s internal structure. Previous studies had only looked at single bounces of seismic waves (from one side of the Earth to the other and back).
In this new study, scientists accessed data from waves of 200 earthquakes with magnitudes above 6.0, traveling up to five times across the Earth’s diameter.
“In this study, for the first time, we report observations of seismic waves originating from powerful earthquakes traveling back and forth from one side of the globe to the other up to five times like a ricochet,” study co-author Dr. Thanh-Son Phạm, a seismologist and postdoctoral fellow at the Research School of Earth Sciences at the Australian National University told CNN in an email.
The earthquake waves penetrated different depths and angles near the center, suggesting a different crystalline structure was nestled beneath the surface. The findings were published in the journal Nature Communications.
Earth’s internal structure has four known layers—an outer crust, a rocky mantle, an outer core made of molten-liquid magma and a solid inner metal core. In the 1930s, scientists used the same seismic wave analysis method to discover the Earth’s metallic inner core.
In 2002, researchers hypothesized that the Earth’s inner core concealed another unknown layer. Thanks to the improved accuracy of seismic monitoring, scientists can now verify this hypothesis.
Lead author of the study and observational seismologist Thanh-Son Pham of ANU, told Reuters, “We may know more about the surface of other distant celestial bodies than the deep interior of our planet.”
“We analyzed digital records of ground motion, known as seismograms, from large earthquakes in the last decade. Our study becomes possible thanks to the unprecedented expansion of the global seismic networks, particularly the dense networks in the contiguous U.S., the Alaskan peninsula and over the European Alps.”
Both the inner core’s outer shell and the sphere within it are hot enough to be molten. However, the high pressures found at the Earth’s center render them solid.
Australian National University geophysicist and study co-author Hrvoje Tkalčić told Reuters, “I like to think about the inner core as a planet within the planet. Indeed, it is a solid ball, approximately the size of Pluto and a bit smaller than the moon.”
“If we were somehow able to dismantle the Earth by removing its mantle and the liquid outer core, the inner core would appear shining like a star. Its temperature is estimated to be about 5,500-6,000 degrees (Celsius/9,930-10,830 Fahrenheit), similar to the sun’s surface temperature.”
According to Pham, there does not seem to be a clear line that distinguishes the outer region of the inner core from the sphere nestled within. Instead, the transition is gradual. Scientists could tell the two regions apart because seismic waves behaved differently between them.
“It could be caused by different arrangements of iron atoms at high temperatures and pressures or the preferred alignment of growing crystals.”
As the Earth slowly cools, its inner core grows, while its outer core contracts as molten materials solidify.
The researchers told The Washington Post that the innermost core might hold clues to a “significant global event that occurred in the planet’s past,” allowing scientists to understand the evolution of Earth’s magnetic field by providing something akin to a fossilized record.
“The latent heat released from solidifying the Earth’s inner core drives the convection in the liquid outer core, generating Earth’s geomagnetic field. Life on Earth is protected from harmful cosmic rays and would not be possible without such a magnetic field.”
The discovery may also give “us a glimpse of what might have happened with other planets,” Pham said. “Take Mars as an example. We don’t understand yet why (Mars’ magnetic field) ceased to exist in the past.”
Mars is thought to be devoid of life because it lost its magnetic field some 4 billion years ago, leaving it vulnerable to the solar winds and dust storms that swept away the planet’s atmosphere and oceans.
Moumita Basuroychowdhury is a Contributing Reporter at The National Digest. After earning an economics degree at Cornell University, she moved to NYC to pursue her MFA in creative writing. She enjoys reporting on science, business and culture news. You can reach her at firstname.lastname@example.org.