Earth's core

27-Jan-2023: Earth's inner core might've stopped spinning faster than its surface

Recent research on Inner Core

Methodology

Analyzed seismic waves from repeating earthquakes over the last six decades
Estimated the rotation of the inner core, which is believed to move independently from the mantle and rest of the planet

Findings

Inner core started rotating slightly faster than the rest of the planet in the early 1970s
Slowed down before coming in sync with Earth's rotation around 2009
Negative trend, meaning inner core is now rotating slower than the surface
Results seem to indicate that the Earth's inner core changes its speed of rotation every 60-70 years on average

Significance

Can motivate researchers to build and test models treating Earth as an integrated dynamic system
Slowdown could change how rapidly the entire planet spins and influence how the core evolves with time

Earth's Layers

Crust
- Outermost layer
- Made of solid rock (mostly basalt and granite)
Mantle
- Lies below the crust
- Up to 2900 km thick
- Consists of hot, dense, iron and magnesium-rich solid rock
Core(Center of the earth) is made up of two parts:
- Liquid outer core: Made of nickel, iron and molten rock
- Solid inner core:
  - Innermost layer of the earth
  - Hot iron ball of the size of Pluto with an average radius of 1220 km
  - Solid due to pressure caused by weight of other top layers
  - Spins independently because it floats in the liquid outer core
  - Temperature ranges between 7,200–8,500ºF (4,000–4,700ºC)

Inner Core Properties

Predicted to have very high thermal and electrical conductivity
Lehman Seismic Discontinuity is the boundary between the inner and outer cores, located at approximately 5150 km below the surface of the Earth.

2018

23-Oct-2018: Earth's inner core is hard but softer than previously thought

A new study by researchers at The Australian National University (ANU) could help us understand how our planet was formed. Associate Professor Hrvoje Tkalčić and PhD Scholar Thanh-Son Phạm are confident they now have direct proof the earth's inner core is solid. They came up with a way to detect shear waves, or "J waves" in the inner core - a type of wave which can only travel through solid objects.

The inner core is indeed solid, but it's also softer than previously thought. If the results are correct - the inner core shares some similar elastic properties with gold and platinum. The inner core is like a time capsule, if we understand it we'll understand how the planet was formed, and how it evolves.

Inner core shear waves are so tiny and feeble they can't be observed directly. In fact, detecting them has been considered the "Holy Grail" of global seismology since scientists first predicted the inner core was solid in the 1930s and 40s. So the researchers had to come up with a creative approach.

Their so-called correlation wavefield method looks at the similarities between the signals at two receivers after a major earthquake, rather than the direct wave arrivals. A similar technique has been used by the same team to measure the thickness of the ice in Antarctica.

Using a global network of stations, scientists take every single receiver pair and every single large earthquake - that's many combinations - and measure the similarity between the seismograms. That's called cross correlation, or the measure of similarity. From those similarities researchers construct a global correlogram - a sort of fingerprint of the earth.

These results can then be used to demonstrate the existence of J waves and infer the shear wave speed in the inner core. While this specific information about shear waves is important, Dr Tkalčić says what this research tells us about the inner core is even more exciting. For instance we don't know yet what the exact temperature of the inner core is, what the age of the inner core is, or how quickly it solidifies, but with these new advances in global seismology, we are slowly getting there.

The understanding of the Earth's inner core has direct consequences for the generation and maintenance of the geomagnetic field, and without that geomagnetic field there would be no life on the Earth's surface.

2017

12-Jan-2017: Silicon identified as ‘missing element’ in the Earth’s core

Experiments are still going on to understand the innermost part of the earth. Earth’s core is mainly composed of iron(85%) and nickel(10%). Japanese scientists claim to have identified the missing 5% of the Earth's core which could be silicon dissolved into the iron-nickel alloys.

Scientists created alloys of iron and nickel and mixed them with silicon. They recreated high temperatures and pressures that exist in the inner core. To their surprise, they discovered that this mixture matched what was seen in the Earth's interior with seismic data.

The discovery could help us to better understand how our world formed. However, more work was needed to confirm the presence of silicon. Scientist, also did not rule out the presence of other elements.

The innermost part of Earth, which is believed to have a radius of about 1,200km (745 miles), is too deep to be investigated directly. Hence, scientists study how seismic waves pass through this region to give them some idea about its composition.

According to Simon Redfern from the University of Cambridge, UK, "These difficult experiments are really exciting because they can provide a window into what Earth's interior was like soon after it first formed, 4.5 billion years ago, when the core first started to separate from the rocky parts of Earth.”

He didn’t rule out the possibility of presence of oxygen in the core as recent studies have suggested. Countering the claim of silicon’s presence, he said that the rest of the planet would have been relatively oxygen-rich if a larger amount of silicon had been incorporated in Earth's core more than four billion years ago.

Redfern stated that though recent results add to the understanding, but it is not the last word on the story.

Source: Downtoearth