Is there another core hidden in the Earth's inner core?

The Earth is the common home of all living things, and a deep understanding of the Earth's internal structure is of great significance to human beings. I believe that many people have seen the structure diagram of the Earth, which is mainly composed of four parts: the outermost crust, the main part of the mantle, and the outer core and inner core of the core.

But in February 2023, a study released by the Australian National University showed that the Earth's inner core may also contain an innermost sphere about 650 kilometers thick! So how did the researchers discover this hidden structure?

Methods for exploring the structure of the Earth

First of all, we need to know how scientists detect the internal structure of the Earth?

Let's imagine an earthquake. When an earthquake occurs, people usually feel the ground "shaking", which is actually the effect caused by the propagation of seismic waves. If we want to record these so-called "seismic waves", we need to use a seismograph.

What the seismograph records is a curve with different fluctuations, which we can call a seismogram. Such a curve can actually tell us a lot of information. For example, we can analyze the type of waves and the time difference of arrival of different types of waves through the vibration amplitude and density of the seismic waves shown. We can also use the maximum amplitude and period of seismic waves to calculate the magnitude of the earthquake.

In addition, if we have more than two observation stations observing the same earthquake at the same time, we can even calculate the earthquake location and focal depth through the actual time difference between the seismic waves received by the two stations.

But if seismic waves can only read the above content, how can they be used to detect the deep structure of the earth? In fact, seismic waves also have the property of "transformation". The interior of the earth is not homogeneous, so when the medium is different, the nature and behavior of seismic waves may change, which is called "transformation". Generally speaking, the speed of seismic waves increases with the depth from the surface. If the speed of seismic waves is found to suddenly slow down, or abnormal phenomena such as deflection occur, it may mean that the seismic waves may have encountered a mutation zone where the medium changes during their course. In fact, this is the theoretical basis for scientists to continuously discover different stratification states in the deep earth.

The core of the core

The problem is that the inner core is so small, accounting for less than 1% of the Earth's volume, that to study such a small volume, researchers need to put seismometers on the other side of the Earth, at or near the antipodes where earthquakes occur.

In this case, seismic waves start from the surface, then pass through the mantle, outer core, inner core, outer core and mantle in sequence, and finally reach the surface on the other side of the earth. This wave is named PKIKP wave. The propagation time and amplitude of PKIKP wave have become the main short-period tool for inferring spatial distribution characteristics.

To better probe the innermost layer of the inner core (IMIC), in this new study, the researchers demonstrated a completely new seismological method, the composite PKIKP method.

Before we learn more about this method, let's first learn more about seismic waves. According to the nature of the waves, seismic waves are mainly divided into three types: solid waves, surface waves, and coda waves. Solid waves are waves that can propagate in objects, and they can be divided into two types: P waves (longitudinal waves) and S waves (transverse waves). P waves can propagate in solids and liquids, while S waves can only propagate through solids, so only P waves can reach the core of the earth.

After the P wave reaches the antipodal point corresponding to the earthquake source, it may be reflected by the ground and generate reflected waves. If you want to generate multiple reflected waves, you need to encounter a good reflection interface, because the reflection coefficient of the general reflection interface is small, the intensity of the single reflected wave is very weak, and the energy of the wave after multiple reflections is even weaker, and it is generally difficult to distinguish it from noise. Only multiple reflections generated on a reflection interface with a large reflection coefficient (such as an unconformity surface) can form strong multiple waves. This is the application basis of the composite PKIKP method.

In this picture (below), the yellow five-pointed star in the big circle on the right is the signal source, and the blue small triangle is the signal receiver. The blue line represents the path of the seismic wave that received the signal for the first time, and the orange line represents the path of the seismic wave that received the signal for the second time.

How did the researchers reveal the hidden core through these waves bouncing back and forth? The key lies in the "transformation" I just mentioned.

The researchers found that seismic waves have significantly different anisotropy in the outer core and the innermost layer of the inner core. From this picture, we can see that in the outer core and the innermost layer of the inner core, the fastest propagation direction of seismic waves is parallel to the Earth's rotation axis; but the slowest propagation direction is significantly different. In other words, the slowing direction of waves passing through the innermost layer of the inner core is different from the slowing direction of waves passing through the outer core.

Finally, through the two anisotropies of the inner core, the researchers believe that there is a smaller core in the inner core of the Earth.

Impact and significance of the new research

This study provides us with a new way to use seismic data to explore the structure of the deep earth. By re-screening and analyzing the existing data, we may gain some new insights.

There are still many questions worth pondering in the future, such as whether the transformation process between the innermost layer of the inner core and the outer core can serve as a fossil record of major earth events in the past? Is there a better way to explore deep layers? Is there still a separable internal structure in the innermost layer of the inner core?

Images by Drew Whitehouse, Son Phạm and Hrvoje Tkalčic.

This article is a work supported by Science Popularization China Starry Sky Project

Author: Xiao Long

Reviewer: Ren Ye (Senior Engineer of Shanghai Earthquake Administration)

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.