What is Earth’s inner core and why do scientists want to study it?

More than 5,000 kilometres beneath the surface is Earth’s inner core, a solid metallic ball, slightly smaller than the moon, that sits at the centre of our planet. Think of it as a time capsule – a fossilised record that takes us back into the deep past and tells us more about the planet’s evolution.  

Not only can the inner core enlighten us about events that happened on Earth hundreds of millions to billions of years ago, it’s an ‘engine room’ integral to sustaining the planet’s magnetic field, which is what makes all life possible.  

Developing new ways to study Earth’s innermost region can even help us learn more about other planets in our solar system. 

Digging deep 

Earth is like a thick, multi-layered cake and the inner core is the bottom layer. It’s the hardest layer to study because it’s located in the centremost region of Earth, under thousands of kilometres of dense rock. Much of what we know about Earth’s deep interior comes from data captured by an extensive network of seismometers – instruments used to record the motion of the ground – placed in all corners of the globe.  

To learn more about the inner core, seismologists analyse seismic waves triggered by earthquakes. These waves, which originate from the energy created by earthquakes, penetrate and pass through the inner core. Seismic waves reveal many properties about the Earth’s interior.  Different types of seismic waves give us different clues.  

P-waves, which are more frequently observed by researchers, and J-waves, which are harder to detect, are types of seismic waves that pass through the inner core. Seismologists are particularly interested in detecting and analysing J-waves as they hold the key to understanding the state and composition of the inner core, which has been continually growing over millions of years.  

Journey to the centre of the Earth 

Seismic waves speed up or slow down depending on the composition and texture of the material they travel through. By observing J-waves and analysing their speed, scientists can unlock clues about the inner core’s material, including whether it is liquid or crystallised, and how rigid it is.  

However, detecting J-waves is difficult because of their weak signals–they are seemingly invisible when employing traditional seismometer observation methods. That’s why ANU researchers have developed an innovative new technique that measures them. They did this by using data from thousands of digital records from seismometers deployed across Earth’s surface. 

The team then created a simulation using the southern hemisphere’s biggest supercomputer, located on the ANU campus, to determine the speeds at which the J-waves travel through Earth’s centre. 

“Detecting J-waves is like finding a needle in a haystack,” Professor Hrvoje Tkalčić says. 

“Although we detected J-waves in 2018, our new technique provides a much better estimate of their speed. It’s almost like looking at the same thing but through a much sharper lens.” 

According to ANU seismologist Dr Thanh-Son Phạm, the ANU-developed technique to study J-waves is similar to how astrophysicists processed their geomagnetic records to produce the first-ever image of a black hole a few years ago. 

Solid but squishy 

By developing a new way to examine J-waves as they pierce the inner core, ANU researchers have confirmed that while the core is solid (a hypothesis first outlined in 1940), it is also softer than previously thought. 

PhD student Thuany Costa de Lima, who works with Tkalčić and Phạm, says the best way to describe the inner core’s texture is “squishy”. 

“Although it is difficult to answer why the inner core is squishy but solid, it could be because it experienced a complex solidification process at some point during Earth’s evolutionary timeline,” she says. 

“Understanding how the material in the inner core is affected by high pressures and temperatures in the labs, combined with seismological measurements of J-wave speed, could prove game-changing in understanding the inner core’s structure and evolution.” 

A brewing, beating ‘heart’

Although the inner core is solid, ANU scientists believe that convection – a process that occurs when heat is transferred from one place to another due to the movement of liquids or gases – is taking place inside the inner core.

Tkalčić says convection is like “making a hot cup of coffee”.

“Hot coffee rises to the surface, cools quickly at the surface due to evaporation, and as it cools, becomes denser and sinks due to gravity. This is convection,” he said.

“Convection is present everywhere in nature and the universe. In a cup of espresso, a pot of boiling soup, layers of the sun and stars and in Earth’s liquid core, which is responsible for generating the planet’s magnetic field.”

Convection inside the inner core has been hypothesised and debated by the scientific community for a long time, but Tkalčić and colleagues have now published research to officially confirm it by developing simulations using data from thousands of seismic waves that pass through Earth’s interior.

Understanding whether convection is taking place inside Earth’s interior is important because it provides scientists with insights into how the planet has evolved into its current state over billions of years.

“What is special and perhaps less intuitive here is that Earth’s inner core is solid, but convection still takes place,” Tkalčić says. “Of course, this is because it happens slowly, over hundreds of thousands and millions of years, rather than seconds or fractions of a second.”

According to ANU geophysicists, confirming that convection is taking place inside the inner core, also known as Earth’s “beating heart”, helps scientists better understand the crystallised structure of the planet’s deep interior as well as the origin, or “birth” of its geomagnetic field.

“In other words, it helps us better understand the heat transportation within our planet, the life cycle of the inner core, how old it is and whether it’s a necessary ingredient for the magnetic field to exist. Only planets with global magnetic fields can harbour life because magnetospheres protect us from cosmic radiation,” Tkalčić says.

Piecing together the puzzle of Earth’s formation 

ANU seismologists, based at the Research School of Earth Sciences, are at the forefront of major discoveries shaping our understanding of Earth and how the world around us came to be. 

Not long ago it was thought Earth’s structure comprised four distinct layers: the crust, the mantle, the outer core and the inner core. Thanks to a team of ANU researchers, including Tkalčić, Phạm and de Lima, we now know there’s a fifth layer referred to as the innermost inner core. 

“The existence of an internal metallic ball within the inner core, known as the innermost inner core, was hypothesised about 20 years ago,” Phạm says. 

“Earlier this year we provided another line of evidence to prove the hypothesis.” 

While the work of ANU seismologists is helping other scientists study the evolution of Earth, their research is also helping us learn more about life beyond our world. 

This includes developing new ways to study the deep interior of planets such as Mars. 

“Our work opens up new avenues for further investigation of our planet’s deep interior and evolution, as well as the evolution of other planets in the solar system such as Mars, and the many moons,” Tkalčić says.