The inside of the Earth is like a big mystery. The more scientists explore it, the more confused they get.

If you quickly look under the Earth’s surface, you’ll find out that there’s a lot we don’t know about what’s going on right beneath us.

We usually pay attention to what’s happening around us and don’t think much about the Earth’s core. Imagine Earth as an apple; the part we live on is as thin as the skin of the apple.

How hot is the Earth’s core?

Think of Earth like an apple; it has a core hidden beneath a layer called the mantle. This core developed about 200 million years after Earth first came together, around 4.5 billion years ago. The Earth’s core is big, almost half the size of Mars, and it experiences intense pressure that makes its temperature as hot as the surface of the Sun.

Just to give you an idea, the temperature of Earth’s core is about 6,000 degrees Celsius! And remember, the core is only about 3,000 kilometers below the surface. If the Sun were that close to us, it would completely melt everything.

What is the Earth’s core made of?

The Earth’s core has two main parts: an inner core and an outer core. After that, there’s the mantle and, finally, the crust.

1. Inner Core

The inner core is a solid structure made of crystallized iron, enduring intense heat and pressure. The crystals are likely hexagonal in shape, and there might be two different crystal structures. These crystals are thought to align in a north-south direction, matching the Earth’s rotation axis and its magnetic field orientation.

2. Outer Core

The outer core is the only part of Earth’s inside that is genuinely liquid. It’s about 2,000 kilometers thick and consists mostly of iron and nickel, with a smaller amount of lighter elements, around five to ten percent. The shift from the inner core to the outer core happens roughly 5,150 kilometers beneath the Earth’s surface.

3. Mantle

The crust and the upper part of the mantle together form the lithosphere. This lithosphere is divided into tectonic plates that move around. When these plates shift, it leads to earthquakes and the movement of continents. The mantle is the biggest part of the Earth, making up 84 percent of its total volume.

4. Crust

The crust has two parts: the oceanic crust, which is at most 10 kilometers (6.2 miles) thick, and the continental crust, which can be as thick as 80 kilometers (49.7 miles) in certain areas. The crust also experiences daily rising and falling, up to 25 centimeters, due to the gravitational pull of the Moon.

How is the Core’s Composition Determined?

Earthquakes have been crucial in helping us comprehend the Earth’s internal structure. The modern seismometer, created in 1880, detects the vibrations caused by earthquakes as they travel through the planet. In the early 20th century, scientists believed that the Earth’s core was entirely molten, and its movements generated the planet’s magnetic field.

In 1936, the Danish seismologist Inge Lehmann made a significant discovery using seismometers. She found that seismic waves were bouncing off something deep within the Earth. She correctly deduced that the Earth’s core had two parts: a solid inner core, kind of like a nested Russian doll, surrounded by a molten outer core.

Recent research suggests that the situation might be a bit more intricate. Dr. Thanh-Son Phạm and Prof. Hrvoje Tkalčić from The Australian National University took a unique approach. According to Phạm, “We claim the detection for the first time of ricocheting seismic waves, which propagate from the earthquake source to the other side of Earth, and back, up to five times.”

“The detection is significant because it allows a new way to probe the very center of Earth, which was very unlikely in the past.” This technique has commonly been used in the search for new minerals but hasn’t been applied extensively to explore the inner structure of the Earth.

Phạm and Tkalčić published their findings in February 2023, where they examined data from the expanding network of seismometers placed worldwide. The key aspect was gathering data from locations near the earthquake epicenters and their exact opposite points on the other side of the planet, referred to as the antipode.

This has been challenging before because earthquakes mostly occur in clusters around an equatorial belt, mainly in oceans and other distant regions.

When an earthquake happens, the resulting vibrations echo inside the Earth for days. It takes about 20 minutes for these vibrations to travel from one side of the Earth to its antipode. Phạm and Tkalčić observed up to five back-and-forth bounces from several magnitude-six earthquakes.

As the waves weakened with each bounce, the scientists employed a technique called stacking to merge them. This method allowed Phạm and Tkalčić to extract more information from the fainter signals. Before their study, only two bounces had been analyzed.

Travel of seismic waves at different speeds

The scientists discovered that seismic waves traveled differently through the innermost part of the inner core compared to the outermost part. When these waves encountered the solid core, they slowed down, but they slowed down in different directions.

Phạm suggests that this indicates the iron crystals forming the core are arranged differently in the inner core. Their estimate suggests that the innermost part of the inner core is about 650 kilometers thick and occupies slightly more than half of the entire inner core.

However, there is still more work to be done. Phạm mentions, “The nature of the transitional layer between the innermost region and the upper layer of the inner core remains to be answered. Hopefully, this question can be addressed in the near future.”

Exploring Earth’s Core for Insights on Mars

Knowing the exact structure is crucial because Earth didn’t always have a solid core; it’s thought to have formed between 600 million and 1.5 billion years ago. Understanding its structure could also provide valuable insights for astronomers and planetary scientists in figuring out what happened to Mars.

Information gathered from Mars rovers suggests a warmer and wetter history for the Red Planet, making it more Earth-like in the past. If Mars’s core fully solidified, its magnetic field would have turned off, exposing it to the solar wind that slowly eroded much of the Martian atmosphere.

What’s Happening to Earth’s Spin?

Earth’s rotation is gradually slowing down, causing our days to become longer over time.

Indeed, a day is not a fixed 24-hour period. It’s determined by the time Earth takes to complete one rotation on its axis, and various factors impact our spin speed. The gravitational pull of the Moon has played a role, slowing down the day from about 19 hours over 1.4 billion years ago to the now familiar 24 hours.

The idea that Earth’s rotation is affected by tidal forces is not just theoretical; it’s supported by the study of fossilized coral that is 430 million years old. The coral grew by adding a new calcium line each day, creating patterns that represent the seasons. Within these patterns, there are 420 lines, indicating 420 days in a year. Since a year is the fixed time for Earth to orbit the Sun, having more days means each day is shorter. At the time the coral stopped growing, there were just under 21 hours in each day.

The melting of the polar caps during the conclusion of Earth’s regular ice ages has contributed to slowing down our planet’s rotation. These are long-term changes. On a shorter timescale, events like the 2010 earthquake in Chile had an impact, speeding up the Earth and slightly shortening the day by 1.26 microseconds. In fact, June 29, 2022, was recorded as the shortest day ever directly measured.

An unusual trend has emerged in short-term observations. Since 2020, the average day has been getting longer, indicating that Earth is slowing down. This contradicts the previous pattern of the average day shortening over the half-century before that.

Prof Xiaodong Song and Yi Yang from Peking University in China propose an explanation, suggesting that Earth’s inner core could be the key to understanding this phenomenon.

Differences in Spin: Core vs. Mantle

The solid inner core is enclosed within the liquid outer core, and it isn’t firmly fixed in position. It can spin independently of the mantle and crust above. Historically, the inner core spun faster than the rest of the planet, but according to Yang and Song, it has recently slowed down and might even be rotating more slowly than the layers above it.

To investigate this, Yang and Song examined seismic events occurring in the same location but many years apart. They focused on earthquakes near the South Sandwich Islands in the Atlantic and studied the seismic detections in Alaska. Their research, titled “Multidecadal variation of the Earth’s inner-core rotation,” also considered seismic waves recorded in Montana, US, resulting from two nuclear tests conducted at Novaya Zemlya, USSR, in 1971 and 1974.

If the Earth’s core remained constant, it would reflect seismic waves in a consistent manner, resulting in nearly identical surface patterns. However, Song and Yang discovered differences in the seismic waves, leading them to conclude that since 2009, the inner core has been slowing down.

By comparing this recent data to older measurements dating back to 1964, they deduce that this behavior is “part of an approximately seven-decade oscillation, with another turning point in the early 1970s.” This suggests a potential repeating pattern of the Earth’s core speeding up and slowing down over time.

Why is the core slowing down?

This discovery is tentative and requires further supporting data. Other researchers have proposed alternative explanations, such as the possibility that the surface of the inner core might not be as smooth as commonly thought. If it’s rougher, it could alter how the inner core reflects seismic waves without necessarily indicating a change in speed. More research is needed to validate and refine these findings.

If the core is indeed slowing down, the reason behind it could be linked to the fact that the inner core isn’t entirely free to move; it’s partially constrained by the gravitational pull of the mantle. The interaction between the inner core and the mantle could play a role in influencing the rotational dynamics of the Earth’s core.

Some geophysicists propose that this establishes a cycle where the inner core experiences periods of slowing down and speeding up. It’s plausible that what we are currently witnessing is just a segment of this cycle, and the inner core might accelerate again in the near future. Understanding this potential cyclical behavior is crucial for gaining a comprehensive grasp of Earth’s dynamic internal processes.

The growing network of seismometers worldwide, continually expanding, offers unprecedented opportunities for obtaining answers. With such a vast array of instruments scattered across the planet, we may not have to wait too long to unravel the mysteries surrounding the Earth’s inner core and its dynamic behavior.

What’s happening with our magnetic field?

Earth’s magnetosphere acts as a protective shield, safeguarding us from the Sun’s radiation. However, recent research suggests that this intricate system might be on the verge of a significant change.

Our planet’s magnetic field is vast, stretching about 65,000 kilometers toward the Sun and extending beyond six million kilometers on the night side. This means that during about a week of its month-long orbit around Earth, the Moon is enveloped within our magnetic field.

The driving force behind this phenomenon is the Earth’s core. As heat escapes from the solid inner core, it moves into the molten outer core, creating convection currents. These currents, in turn, move electrically charged material, generating a magnetic field that extends up through the crust and out into space, where it meets the solar wind – a high-energy stream of particles from the Sun. This interaction causes our magnetosphere to extend significantly on the night side of Earth.

The magnetosphere has played a crucial role in keeping life on Earth safe for billions of years. Explorers have relied on compasses aligned with it for navigation, and animals also use it to find their way. However, despite its apparent constancy, the magnetosphere is not as stable as it may appear.

During the 1970s, scientists observed a phenomenon known as geomagnetic jerks – sudden and unpredictable changes in our magnetic field. However, a deeper understanding of these events only emerged when scientists began to observe Earth from space.

Scientists taking help from computer simulations

In 2019, Julien Aubert from the University of Paris and Prof. Christopher Finlay from the Technical University of Denmark conducted a supercomputer simulation of the outer core. Their findings revealed that waves originating in the inner core propagate into the outer core, causing abrupt changes in the liquid flow beneath the magnetic field. It can take up to 25 years for a rising blob of metal to lead to a geomagnetic jerk.

Additionally, Earth’s magnetic field has the capability to flip. When lava cools, it preserves information about the Earth’s magnetic field direction at that time. Analyzing layers of lava has shown that, on average, the direction of our magnetic field reverses approximately every 200,000 years. The last reversal occurred 780,000 years ago, and there are indications that another reversal may be underway.

Measuring the Strength of Earth’s Magnetic Field

Over the past two centuries, the European Space Agency notes that the global average strength of our magnetic field has decreased by nine percent. Historically, such declines have foreshadowed previous reversals. In certain locations, the reduction has occurred even more rapidly, particularly in the area known as the South Atlantic Anomaly (SAA) over South America. NASA geophysicists Weijia Kuang and Terence Sabaka describe it as a region where geomagnetic intensity is the lowest, and it is undergoing changes.

“The observations have found that the SAA is expanding and moving westward,” note Kuang and Sabaka. The field strength of the SAA has also decreased by eight percent between 1970 and 2020. The cause? “The short answer is that the SAA is due to vigorous convection in Earth’s outer core,” explain Kuang and Sabaka. It is linked to a magnetic reversal in the outer core that counteracts the main magnetic field.

However, there are drawbacks to the expansion of the SAA. Satellites traversing the region have experienced failures due to the intense radiation from space. Astronauts are unable to perform spacewalks in its vicinity. Former astronaut Terry Virts even reported seeing a massive flash of light with his eyes closed when passing over it.

Despite these challenges, the SAA provides valuable insights for geophysicists studying the Earth’s interior. According to Kuang and Sabaka, the SAA can be utilized to map the flow of material in the topmost part of the outer core. Additionally, “The SAA forecast accuracy can [also] be used to estimate the entire core state, which is not observable from Earth’s surface or in space,” they add.

What will happen in the future?

The fate of Earth’s core raises intriguing questions: Will it stop spinning? Will it solidify completely? And what implications will these changes have for those of us living above it?

The Earth’s core has a rich history that dates back to the formation of the planet. When the Sun emerged from a cloud of interstellar gas and dust, a residual material band formed around it known as the protoplanetary disc. This disc contained iron ejected into the Universe by supernovae, marking the end of the lives of massive stars.

Gravity gradually shaped this material into rock and metal clumps called planetesimals, which collided and merged to form planets. These collisions were so intense that the rock and metal melted, and gravity shaped the newly formed objects into spheres. The dense iron settled in the center, while the lighter rock floated to the top.

While a crust formed on the planet’s surface as it cooled, the iron core remained molten. The immense gravitational pressure from the layers above kept the core in this molten state, exerting a crushing force. This dynamic interplay between gravity and temperature has been instrumental in shaping Earth’s internal structure.

Over billions of years, the Earth’s core has been gradually cooling. Dr. Dan Frost, a seismologist at the University of South Carolina, explains, “As the liquid iron in the outer core cools, it slowly freezes into solid iron and becomes the inner core.” This ongoing process of cooling and solidification is a fundamental aspect of the evolution of our planet’s interior.

Impact of Earth’s Cooling on the Inner Core?

The cooling process of the Earth’s inner core involves the continuous freezing of liquid iron into solid iron, adding approximately 8,000 tonnes of iron to the inner core every second. This massive influx of iron is equivalent to the daily mass of the entire human population.

As the inner core cools, the released energy transfers to the outer core, driving convection currents and contributing to the generation of our global magnetic field.

However, recent research led by Dr. Dan Frost suggests that the growth of the inner core is not uniform. The eastern part of the inner core lies beneath Asia and the Western Pacific, while the western part is situated below the Americas and the Atlantic.

Frost’s team embarked on the challenging task of measuring the growth across these geographically distant regions of the planet’s interior. Since direct measurements of inner core growth are not feasible, the researchers looked for evidence of movement within the inner core to infer its changing size.

Seismic waves can tell us what’s going on

Seismic waves traversing the inner core exhibit different velocities based on their direction. These waves move faster when traveling parallel to Earth’s rotation axis (roughly north-south) compared to their speed when moving parallel to the equator. This variation in wave speed provides valuable information about the internal dynamics and structure of the Earth’s inner core.

The observation that seismic waves move faster parallel to Earth’s rotation axis in the inner core suggests that the crystals within the inner core are aligned in a similar direction, according to Dr. Dan Frost. He explains, “The way to get that alignment is if the inner core moves.” This alignment is likened to how sticks dropped into a river align with the direction of the flowing water.

Frost’s research team discovered that the core beneath the Banda Sea near Indonesia is growing faster than the side beneath Brazil. However, this asymmetry is not permanent. Gravity ultimately compels the wider part to return to the center. Frost notes, “That flow of material would cause the crystal alignment that we see.” The dynamic interplay between growth, gravity, and crystal alignment sheds light on the intricate processes occurring within Earth’s inner core.

Is there more inside the Earth’s core than we can see?

Dr. Dan Frost’s research assumes that the inner core consists of only one type of crystallized iron. While other studies suggest a potential difference between the outermost inner core and the innermost inner core, Frost is not entirely persuaded by those conclusions. He argues, “There isn’t a sharp transition between an outermost and innermost inner core.”

Instead, Frost suggests, “It’s more of a smooth transition.” According to him, such a model is consistent with his own findings. The ongoing exploration of Earth’s inner core involves various perspectives and interpretations, and researchers continue to refine their understanding of its complex composition and behavior.

The mysteries surrounding Earth’s solid core indeed remain largely unsolved, considering that its existence has been known for less than a century. As we continue to explore and study, the next century may bring more insights and a deeper understanding of the mechanisms that shield us from the space-borne radiation.

There’s no immediate concern about the core completely solidifying. Its growth is a slow process, with the inner core expanding by only about 2 mm each year. Although relatively fast in geological terms, some estimates suggest that it would take another 91 billion years for the molten outer core to vanish. However, the Earth is not expected to reach that point, as the dying Sun is projected to pose a threat long before this process completes.