Almost all data on the internal structure of the Earth comes from observations of the propagation of seismic waves. Over time, analysis of changes in their velocity revealed that the planet is composed of several concentric layers, starting from the earth’s crust to the core, which is located at a depth of about 2,900 kilometers.
Body waves used in geophysical observations are divided into P waves (longitudinal waves) in which elastic mechanical vibrations occur along the direction of propagation, and S waves with perpendicular vibrations (transverse waves). Each seismic impulse initiates both waves simultaneously, but these waves refract differently at the boundaries of media of different densities. S waves, also called shear waves, only propagate through solid materials, while P waves can pass through all kinds of materials.
ALLEGED TO CONSIST OF TWO SHELLS IN 1936
Danish geophysicist Inge Lehmann proposed in 1936 that the Earth’s core is not homogeneous, consisting of two crusts, the molten outer and solid inner crust. Subsequent studies confirmed that there was indeed a boundary between the two environments in the core, at a depth of about 5,100 kilometers from the surface. This information provided the opportunity to determine the thickness of the fluid outer core at which shear waves are extinguished. Thus, the radius of the inner core was determined to be about 1300 kilometers. However, for a long time there was no definite opinion about the phase structure of this nucleus.
THE MYSTERY OF THE INNER CORE
On the one hand, the nature of the refraction and reflection of longitudinal waves implied that the inner core was mostly solid. Also, it is not possible to imagine that matter could be in a different state at pressures of about 3 million 800 thousand bar at these depths. But to confirm this prediction, it was necessary to detect the propagation of shear waves. This was also a huge problem. Because such waves can hardly pass through the liquid shell of the outer core.
Letters are used to mark different types of waves in the earth layers: P, longitudinal waves in the earth mantle; S is shear waves in the mantle; K is longitudinal waves in the outer core; I, longitudinal waves in the inner core; J, shear waves in the inner core.
Waves that start with a certain seismic impulse and pass through various shells and are detected by seismographs at the exit are given specific codes. For example, if longitudinal vibration is detected in the mantle and outer core at the same time, and transverse vibration is detected in the inner core, it is given the code PKJKP, if no vibration is detected in the inner crust, it is given the code PKIKP.
The scientists’ first goal was to find the J wave inside the PKJKP waves. They achieved this for the first time in 2018. Geophysicists Hrvoje Tkalcic and Thanh-Son Pham of the Australian National University used a special technique, the correlation wave fields method, to detect super-weak waves. The basis of this method is the principle of comparing the direct and reflected signals emitted from the same earthquake and detected by two seismographs placed in opposite parts of the planet.
However, Australian experts deliberately ignored the first 3 hours of the seismograms, thus excluding strong wave signals from the study and only considering the 3 to 10 hours after the earthquake. Thanks to this method, the velocity of transverse waves in the Earth’s inner core could be determined: 3.42 km per second at the boundary with the outer core and 3.58 km per second at the centre.
These values turned out to be slightly lower than they should have been in a solid body made of pure iron or its alloys. Scientists have developed several guesses to explain this. First of all, it was speculated that the inner core is not completely solid and has a certain plasticity, like some metals such as gold or platinum. The observed parameters also point to an alloy of iron, silicon and carbon.
The low velocity of J waves can also be explained by the presence of molten matter in the inner core, in the space between the crystals of solid matter.
A third option is also possible: the inner core is heterogeneous. The difference in the velocities indicated by the seismic waves at its boundary and at its center points to this.
PİNPO’S NIGHT
For the new research, Tkalcic and Pham selected 200 earthquakes of magnitude 6 or greater that have occurred in the past decade. Experts focused on waves like PKIKP. Thanks to a special program, data from a large number of sensors all over the planet were processed to create images of the wave fields covering the entire surface of the Earth for each of the seismic events. These are called global correlograms.
The developed signal amplification method made it possible to reconstruct the paths and velocities of seismic waves in unprecedented detail. In some cases, reverberation of up to 5 reverbs (waves reflecting off the earth’s surface and re-passing through the earth) could be detected.
“It’s like a ping pong ball bouncing around,” Thanh-Son Pham said.
Each reverb takes about 20 minutes to get from one end of the world to the other, and the signal gets weaker with each reberb. In the past, at most one ‘reflection’ of this type could be detected.
DISCOVERING THE MINI-CORE
The obtained data revealed the existence of a hot and dense metal ball with a diameter of about 1300 kilometers in the very center of the planet. This ball, as well as the entire inner core, probably consists of crystal iron with a small nickel doping.
It is noteworthy that the slowing nature of the waves passing through the “nucleus” does not resemble the picture in the crust surrounding it. In the scientists’ view, this is due to the peculiarities of packing the iron atoms or the predominant orientation of growing crystals at different temperatures and pressures.
“The iron crystals in the central part are probably different from those in the outer layer,” Tkalcic noted.
The differences are observed in the level of anisotropy of the iron-nickel alloy forming both regions of the inner core. Its detection was made possible thanks to the analysis of the changes in the velocity of the seismic waves as they enter the mini-core from different angles. Because in some cases, the waves slowed down, while in other cases, on the contrary, they accelerated.
Although the metal ball at the center of the earth makes up no more than one percent of its total volume, this discovery is extremely important for understanding the evolution of the earth.
“The inner core is like a time capsule,” Tkalcic said. “This fossil record opens the door to Earth’s past,” he said.
It is now clear that at the earliest stage in geological history some kind of global event occurred that led to a significant rearrangement of the crystalline structure of the core. This may have played an important role in the emergence of Earth’s magnetic field, which enabled the development of complex life forms.
