It is already known that the Earth has an inner core – a solid metal ball made mainly of iron and about 2400 kilometers wide.
But a new study shows there’s another dense sphere in this core — an “inner core” that’s just over 800 miles across.
Researchers say this “middle ball” is solid, but it has a different, as-yet-unknown structure than the inner core that surrounds it.
The presence and size of such a deepest core has long been hypothesized and debated – but research is increasingly showing that it really does exist.
It follows the discovery of a hidden layer of Earth that sits 100 miles below the surface and covers at least 44 percent of the planet.
A new study claims there is another dense ball in the Earth’s inner core – an “inner core” – that is 400 miles across. The inner core exists as a single body, with the outer core surrounding it, the mantle surrounding it, and the crust surrounding the mantle
This illustration shows seismic waves, caused by an earthquake in Alaska, traveling through the Earth’s inner core before ‘ricocheting’ back to Alaska (the origin of the earthquake)
The new study was conducted by geologists at the Australian National University’s Research School of Earth Sciences in Canberra, Australia.
‘Five layers’ of the earth
Crust: To a depth of up to 70 km, this is the outermost layer of the Earth, covering both ocean and land areas.
Coat: Descending to 2,890 km with the lower mantle, this is the thickest layer on the planet and made of silicate rocks richer in iron and magnesium than the crust above our heads.
Outer core: This region runs to a depth of 3,200 miles (5,150 km) and is made of liquid iron and nickel with trace amounts of lighter elements.
Inner core: Descending to a depth of 3,958 miles (6,370 km) in the center of the Earth, this region is believed to be made of solid iron and nickel.
Inner core: Within the inner core, this region is solid iron in a different, but unknown, structure than the inner core.
‘We now have enough seismological evidence from different lines of research on the existence of IMIC (innermost inner core),’ they say in their paper, published in Nature communication.
“The findings … will hopefully inspire further investigation of existing seismic records for revealing hidden signals that shed light on the deep interior of the Earth.”
For hundreds of years, the Earth has been believed to consist of three distinct layers: the crust, the mantle, and the core, which was later separated into “inner” and “outer.”
Both the inner and outer cores are mainly composed of iron and nickel, but the inner core is under intense pressure, which keeps it solid despite high temperatures.
Temperatures in the inner core are likely to range from 3,700°C to 7,700°C (6,700°F to 14,000°F), while the outer core is estimated to be 2,700°C to 4,200°C (4,900°F to 7,600°F).
Scientists know about the planet’s cores by measuring changes in earthquake-generated seismic waves that pass through the inner core.
These wave signals are recorded by probes stationed around the planet.
For the study, the team collected data from existing probes to measure the different arrival times of seismic waves of energy generated by earthquakes as they traveled through the Earth.
Ray paths of quintuple-reflecting waves along Earth’s diameter provide new probe for the apparent internal shell of Earth’s inner core — the “inner inner core”
Experts collected data from existing probes to measure the different arrival times of seismic energy waves generated by earthquakes as they traveled through the Earth. Here, black inverted triangles indicate the seismic stations
They observed for the first time the waves traveling back and forth from one side of the globe to the other ‘like a ricochet’ up to five times.
The travel times of the waves suggest the presence of a distinct internal shell, separated from the outer layer of the inner core.
“What distinguishes two regions of the inner nuclei is a physical property known as anisotropy, not chemical compositions, or liquid or solid states,” study author Thanh-Son Pham told MailOnline.
“We think they are both solid and have similar chemical compositions.
“The difference in the direction in which seismic waves travel slowest through the materials distinguishes the two regions of the Earth’s inner core.”
Danish seismologist Inge Lehmann discovered in 1936 that the Earth has a solid inner core that differs from the liquid outer core.
Earth’s core was thought to be a single molten sphere, but Lehmann deduced the solid inner core by studying earthquake seismograms in New Zealand.
Scientists have already suggested the existence of an inner inner core with different physical properties from the rest of the inner core – although what exactly these properties are is more of a mystery.
In 2021, another team of researchers from the ANU “confirmed” the existence of the inner core based on evidence from seismic waves.
They saw changes in the structure of iron in the inner core that suggested a “boundary line” about 400 miles from the center of the Earth — in other words, a ball 807 miles in diameter right at the center of the planet.
PKIKP is the phase that travels as a P-wave from the source through the mantle, outer core, inner core, outer core, and mantle on its path back to the surface
Earth’s “enigmatic” inner core represents less than 1 percent of Earth’s volume, but it is a “time capsule of our planet’s history,” say the authors of this new study.
Exploring the center of the Earth is critical to understanding the formation and evolution of the Earth in the distant past, so more research will be needed to learn more about the inner core.
“There are still important unknowns regarding the IMIC beam, the nature of the transition to the outer inner core, and its precise anisotropic properties, such as its strength and its fast and slow directions,” the researchers say.
‘These topics continue to give rise to further research.’
THE LIQUID IRON CORE OF THE EARTH CREATES THE MAGNETIC FIELD
Our planet’s magnetic field is believed to be generated deep within the Earth’s core.
No one has ever traveled to the center of the Earth, but by studying earthquake shock waves, physicists have been able to determine its likely structure.
At the heart of the Earth is a solid inner core, two-thirds the size of the Moon, made primarily of iron.
At 5,700°C, this iron is as hot as the surface of the sun, but the crushing pressure caused by gravity prevents it from liquefying.
Surrounding this outer core is a 1,242-mile (2,000 km) thick layer of iron, nickel, and small amounts of other metals.
The metal here is liquid, due to the lower pressure than the inner core.
Differences in temperature, pressure and composition in the outer core cause convection currents in the molten metal as cool, dense matter sinks and warm matter rises.
The ‘Coriolis’ force, caused by the rotation of the earth, also causes swirling eddies.
This flow of liquid iron generates electric currents, which in turn create magnetic fields.
Charged metals passing through these fields continue to create their own electrical currents, and so the cycle continues.
This self-sustaining loop is known as the geodynamo.
The spiraling caused by the Coriolis force means that the individual magnetic fields are roughly aligned in the same direction, with their combined effect adding up to produce one massive magnetic field that engulfs the planet.