Seismic Waves Uncover Earth's Innermost Sphere of Iron and Nickel at Core, Revealing Fascinating Secrets

In Jules Verne's renowned novel "Journey to the Center of the Earth," published in 1864, explorers embark on a quest through a volcanic opening in Iceland and find themselves in a subterranean world inhabited by prehistoric creatures. However, the actual composition of the Earth's core is markedly different from this imaginative portrayal and is, in some aspects, even more remarkable. In a recent study, scientists analyzed seismic waves generated by significant earthquakes and discovered a distinct structure within the deep interior of the Earth, confirming the presence of an incredibly hot, solid sphere made of iron and nickel at the very center, spanning around 800 miles (1,350 km) in diameter.

The Earth has a diameter of approximately 7,900 miles (12,750 km) and comprises four distinct layers: an outer rocky crust, a rocky mantle, a liquid outer core, and a solid inner core. The discovery of the metallic inner core, which measures about 1,500 miles (2,440 km) in width, was made in the 1930s through the analysis of seismic waves traveling through the Earth. In 2002, scientists hypothesized that within this inner core lay a separate, smaller section, similar to a Russian Matryoshka doll. Advances in seismic monitoring technology have since enabled this theory to be confirmed.

(Photo: NASA)
View of the Earth as seen by the Apollo 17 crew traveling toward the moon

Secrets of Earth's Realm

When earthquakes occur, they generate seismic waves that propagate through the Earth and can provide valuable insight into the planet's internal structure based on the changes in a wave shape. Reuters mentioned that previously, scientists had only been able to observe these waves bouncing back and forth twice across the Earth's diameter. However, a recent study examined seismic waves from over 200 earthquakes with magnitudes greater than 6.0, which were observed to ricochet like ping-pong balls up to five times within the Earth's interior.

Observational seismologist Thanh-Son Pham, the lead author of a study published in the journal Nature Communications, remarked that our knowledge about the interior of our planet is comparatively limited to what we know about the surfaces of other celestial bodies, highlighting that we have more information about distant astronomical objects than about the deep interior of Earth itself.

According to Pham, the research was made possible by studying seismograms - digital records of ground motion - from significant earthquakes over the past decade. This analysis was made feasible by the remarkable expansion of global seismic networks, particularly emphasizing the dense networks in the contiguous U.S., the Alaskan peninsula, and the European Alps. Pham also confirmed that both the outer shell and the innermost sphere of the Earth's inner core are hot enough to be molten, but they remain solid due to the immense pressure at the center of the planet.

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'Planet Within The Planet'

Hrvoje Tkalčić, a geophysicist at the Australian National University and co-author of the study, likened the Earth's inner core to a "planet within the planet," describing it as a solid sphere approximately the same size as Pluto and slightly smaller than the moon. Tkalčić also noted that if it were possible to remove the Earth's mantle and liquid outer core, the inner core would be visible and appear to shine like a star due to its estimated temperature of 5,500-6,000 degrees Celsius (9,930-10,830 degrees Fahrenheit), which is comparable to the surface temperature of the sun, as reported by Latestly.

Pham stated that the boundary between the outer region of the inner core and the newly confirmed innermost sphere appears to be gradual rather than a sharp boundary. The researchers could distinguish the two regions because the seismic waves behaved differently in each. Pham suggested that this difference could be caused by the unique arrangement of iron atoms at high pressures and temperatures or the preferred alignment of growing crystals.

As Earth slowly cools, the inner core grows larger by solidifying molten materials at the expense of the outer core, a process that has been ongoing since the planet's formation approximately 4.5 billion years ago. Pham added that the release of latent heat resulting from the solidification of Earth's inner core generates convection in the liquid outer core, generating Earth's geomagnetic field. This field is essential for protecting life on Earth from harmful cosmic rays, making life on our planet possible.