From the ocean tides to raising mountains, the Earth performs giant geological processes every day. These phenomena are all intrinsically connected to one another, exerting their influences in a plethora of ways, and working together to sustain life in diverse conditions.
The Earth’s core plays a vital role in tectonic plate movement and generating the magnetic field. This article will give a background on the layers of the Earth, and explore two recent papers concerning the structure and rotation of the inner core at the center.
Core Concepts
Before going over the discoveries, it first seems reasonable to give a “brief” introduction to the structure of the Earth. The physical structure of the Earth is comparable to an onion with four (maybe five, this will be discussed later) distinct layers: The crust, mantle, outer core, and inner core. Unfortunately for scientists, this “onion” is exceptionally difficult to conceptualize, for “except for the thin crust we live on, Earth's structure is intangible deep beneath our feet” (Kuthunur, 2023).
On the surface of the planet, the lithosphere contains the continental and oceanic crusts, along with the upper part of the mantle. Beneath this - but still in the mantle - is the asthenosphere, where convection currents push at the tectonic plates. Further down, the transition zone gate separates the upper and lower mantle, with the lower mantle being much hotter than the former (Buis, 2021).
Beneath the indomitable depths of the mantle, with a chemical composition of mostly iron and nickel, the Earth’s core is roughly the size of Mars (Kolirin, 2023). Like the mantle, the core is also divided, with two main domains that are starkly different. Separated from the planet’s surface by 3,200 miles of miles of rock, the solid inner core is encased in what Scientific American’s Stephanie Pappas calls a “liquid cocoon”. This “cocoon” is a molten-metal outer core in which “the inner core sits suspended like a ball bearing” (Pappas, 2023). Scientists speculate the inner core is 1,520 miles wide (in diameter), and might be about as hot as the sun’s surface (Andrews, 2023).
Seismic Probing and the Innermost Inner Core
But how do scientists know these measurements about the core? Hrvoje Tkalčić, a geophysicist and seismologist at the Australian National University, answers this question in an email to Space.com. The “inner core is notoriously difficult to probe by seismic waves” he says, but by studying how seismic waves caused by large earthquakes get distorted as they go through the core can help to unearth the mysteries of the Earth’s depths (Kuthunur, 2023). Tkalčić was a co-author of a recently released study in March that found evidence of an “innermost inner core”. This isn’t a new discovery, as the existence of another layer was first theorized in 2002 (Sullivan, 2023).
First authors of this study, Thanh-Son Pham and Tkalčić looked at data from large earthquakes of the past. As the waves reverberate, they lose energy with each pass through the planet. The echoing waves had faint signals which the researchers combined to detect the rebounding waves (Pappas, 2023). Pham said that due to the recent installments of new seismic sensors around the globe, it's increasingly possible to detect these weak seismic signals (Kuthunur, 2023). These waves rippled across the Earth’s diameter five times, the highest reflection rate ever recorded. As the seismic waves pass through this region, the speed is slowed depending on the angle the wave hits the core. They concluded that the iron crystals are organized differently between the two layers of the core. The innermost inner core exhibits an “anisotropic” phenomenon, which allows a material to possess different properties in different directions. They estimate that this layer of the core is 800 miles in diameter, and while the two layers are of similar composition, they have different crystal structures (Patel, 2023). This is just the most recent in a string of observations, most of the Earth's core still remains a mystery. Pham says, “we may know more about the surface of other, distant celestial bodies than the deep interior of our planet” (Sullivan, 2023).
A Slowing Core and the Importance of a Magnetic Field
In late January, a study analyzing seismic wave data was published by researchers at Peking University in China. It concluded that the spin of the inner core’s rotation had stopped in 2009, and then proceeded to reverse direction. Authors of this paper, Yi Yang and Xiaodong Song, believe that this is a part of a 7-decade cycle, with the last rotation change occurring in the early 1970s (Hurst, 2023). The researchers compared two nearly identical earthquakes - in the same location with the same magnitude - only separated by time, and noticed subtle differences in the rebounded waves (Pappas, 2023).
Based on their calculations, they conclude the reason for this switch ultimately comes down to two reasons: the Earth’s magnetic field and gravitational forces acting on the core (Kolirin, 2023). The Earth’s magnetic field - called the magnetosphere - is generated in the outer core, in a mechanism called “geodynamo”, where the heat convection in the fluid outer core causes the iron to make an electrical current. The magnetosphere is essential to protecting life from cosmic radiation and solar winds (Buis, 2021). As for the gravity, the composition of the mantle and inner core are thought to be heterogeneous (mixed material). Consequently, in a process called “gravitational coupling”, the gravity between these domains forces the inner core into a position of a gravitational equilibrium, further inducing spin (Kolirin, 2023). Each of these processes contribute to the rotation of the inner core.
Seismic studies have shown that “each year the core expands by about a millimeter, as some of the molten iron in the outer core solidifies” (Pappas, 2023). Like the convection currents, this solidification also drives the circulation of the outer core, creating the magnetosphere. It’s not yet understood what the observed change in pace could mean for the production of the magnetosphere. If this is just part of a cycle like Yang and Song think, it will likely be insignificant. Tkalčić thinks the cycle takes 20 to 30 years, and this shift is a normal occurrence (Hurst, 2023).
Concluding Thoughts on Misinterpreting Data
It's possible that these two studies could be connected to one another, and the results are being wrongly assessed. It could be that the inner core just doesn’t have a smooth surface, and instead, it's a rough and fluctuating surface. Both of these discoveries might be evidence of the same thing. Lianxing Wen from Stony Brook University believes “the inner core has a shifting topography,” which to him, this interpretation “best explains [the] observed temporal changes of seismic waves” (Pappas, 2023). The difference in the earthquakes could be attributed to a variety of different factors that don't necessarily correlate to a change in spin direction or speed. We may never truly know what's under the Earth’s surface, but human curiosity continues the search forward; “because of its inaccessibility, this abyssal realm may forever elude explanation” (Andrews, 2023).
This article was written for the high school club, Staples STEM Journal.
It would have been more appropriate to publish in March (the month it was written) considering the subject matter is "Recent Discoveries"
As with the last publication, when the full issue is released digitally, I'll be sure to update this post with the link.
References are in the following pdf document...
Link for title image...
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