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06-15-2024

Earth's inner core rotation is slowing down, making our days a bit longer

Deep beneath our feet, in the heart of our planet, something unexpected is happening. Scientists have discovered that Earth’s inner core, a solid iron-nickel sphere roughly the size of the moon, is slowing down.

This finding challenges previous assumptions about the inner core’s behavior and raises intriguing questions about its impact on our planet.

20-year long debate settled

The movement of the inner core has been a topic of scientific debate for over two decades. While some research suggested that the inner core rotates faster than the Earth‘s surface, the new USC study provides unambiguous evidence to the contrary.

According to John Vidale, Dean’s Professor of Earth Sciences at USC Dornsife College of Letters, Arts, and Sciences, the inner core began to decrease its speed around 2010, moving slower than the Earth’s surface for the first time in approximately 40 years.

“When I first saw the seismograms that hinted at this change, I was stumped,” Vidale admits. “But when we found two dozen more observations signaling the same pattern, the result was inescapable. The inner core had slowed down for the first time in many decades.”

Composition of Earth’s inner core

The extreme pressure and temperature conditions at the center of the Earth cause the iron and nickel metals that compose the core to behave in unique ways, forming a solid inner core despite the incredibly high temperatures.

The inner core’s density is estimated to be about 12.8 to 13.1 grams per cubic centimeter, which is slightly less dense than pure iron at Earth’s surface.

This difference in density suggests that the inner core may contain lighter elements such as sulfur, oxygen, or silicon in addition to iron and nickel.

The Earth’s magnetic field is generated by the motion of the liquid iron outer core surrounding the solid inner core. This process, known as the geodynamo, is driven by the convection currents in the outer core and the Earth’s rotation. The inner core plays a crucial role in stabilizing and sustaining the magnetic field.

Seismic waves traveling through the inner core exhibit anisotropy, meaning they travel at different speeds depending on their direction. This anisotropy suggests that the inner core has a complex structure, possibly with aligned iron crystals or layering.

Scientists believe that the inner core is gradually growing larger over time as the Earth cools and the outer core solidifies. This growth process is estimated to occur at a rate of about 0.5 to 1 millimeter per year, which means the inner core’s age is approximately 1 to 1.5 billion years old.

Repeating earthquakes and seismic waves

To determine the slowdown of Earth’s inner core, Vidale’s team employed a novel methodology. Their approach centered on analyzing seismic data collected from 121 repeating earthquakes near the South Sandwich Islands between 1991 and 2023.

These repeating earthquakes, which originate from the same location and produce identical seismic wave patterns (seismograms), provide a consistent baseline to study the Earth’s interior.

Additionally, the team incorporated historical data from twin Soviet nuclear tests (1971-1974) and repeated French and American nuclear tests.

By comparing the seismic waves generated by these events over time, they were able to detect subtle changes in how long it took for the waves to travel through the Earth.

These variations in wave travel time served as indicators of the inner core’s shifting rotation patterns, ultimately leading to the discovery of its slowdown.

Two possible culprits identified

The exact cause of the inner core’s slowdown remains unclear, but Vidale proposes two primary factors.

First, the turbulent movement of the liquid iron outer core, responsible for generating Earth’s magnetic field, could be exerting a drag force on the inner core, slowing its rotation.

Second, gravitational interactions with the denser regions of the rocky mantle above the outer core could be pulling on the inner core, further contributing to its deceleration.

Earth's inner core began to decrease its speed around 2010, moving slower than the Earth’s surface. Credit: USC/Edward Sotelo
Earth’s inner core began to decrease its speed around 2010, moving slower than the Earth’s surface. Credit: USC/Edward Sotelo

These intricate interactions deep within the Earth highlight the interconnected nature of our planet’s systems.

The inner core’s rotation is not isolated but rather influenced by the complex interplay of forces within the Earth’s interior.

Understanding these dynamics is crucial for comprehending how our planet functions and evolves over time.

Ripple effects on the surface

The implications of this change in the inner core’s movement for Earth’s surface are still a matter of speculation. Vidale notes that the backtracking of the inner core may alter the length of a day by fractions of a second.

“It’s very hard to notice, on the order of a thousandth of a second, almost lost in the noise of the churning oceans and atmosphere,” Vidale adds.

While the immediate effects may be subtle, this discovery opens up a new avenue of research, offering a deeper understanding of the intricate processes that shape our planet.

Deeper into Earth’s inner core

The USC scientists are eager to continue their research and chart the trajectory of the inner core in greater detail.

By unraveling the mysteries of Earth’s heart, they hope to shed light on the complex interplay of forces that drive our planet’s evolution.

“The dance of the inner core might be even more lively than we know so far,” Vidale concludes.

This new discovery reminds us that even the most seemingly solid and unchanging parts of our world are in constant motion, influenced by a complex web of interactions that we are only beginning to understand.

The study is published in the journal Nature.

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