For generations, schools have taught that the Earth has four main layers: the crust, the mantle, the outer core, and the inner core. At around 5,000 degrees of temperature, the latter, the true heart of the planet, is a solid, fiery ‘ball’ made primarily of iron and nickel. Its size is smaller than that of the Moon and it barely represents 1% of the total volume of the Earth, but its behavior determines, in many ways, what the world we live in is like (and what it will be like in the future).
A few weeks ago, a study on a ‘slowdown’ in the rotation of the core with respect to the mantle and the earth’s crust came to the forefront, and many media outlets were filled with more or less accurate, even alarmist, headlines about the consequences of the phenomenon.
But the core continues to deliver surprises, and this week the journal Nature offers new insights into an intriguing possibility that has been making its way among scientists for years: Earth’s inner core is made up of two distinct layers. That is to say, that there is ‘another’ nucleus within the nucleus.
The idea has been on the table for at least two decades, but probing Earth’s innermost core has been very difficult due to a lack of probes sensitive enough to sample the planet’s deep interior. In fact, it was not until 2015 that a group of geologists from the universities of Illinois, in the United States, and Nanjing, in China, announced the discovery of this ‘second core’ of the earth thanks to the use of a new interpretation and reading technology. of seismic waves.
Later, in 2021, another team of researchers, this time from the Australian National University, confirmed the existence of this ‘core within a core’ in a paper published in the Journal of Geophysical Research. Joanne Stephenson, lead author of that study, said then that while this new layer is very difficult to observe, its different properties point to a previously unknown major event in Earth’s history. “We found evidence – said the researcher – indicating a change in the structure of the iron, which perhaps suggests two separate cooling events in Earth’s history.” A new piece, then, in the knowledge of the history and evolution of our planet.
iron ball
And now, Thanh-Son Phạm and Hrvoje Tkalčić, also researchers at the Australian National University, provide in Nature Communications the first concrete data on the characteristics of that second Earth core. According to the study, it is an iron ball of about 1,300 km in diameter, located inside the inner core (which has a diameter of about 2,500 km).
During their work, the researchers collected extensive data from existing probes to measure the different arrival times at detectors of seismic waves created by earthquakes as they traveled across the Earth. Phạm and Tkalčić observed how the waves reverberated along the entire diameter of the Earth up to five times, something that is achieved for the first time. And the travel times of the waves suggest the presence of a distinct inner shell, roughly 1,300 km in diameter and clearly separated from the outer shell of the inner core. According to the authors, this inner interface could reflect a past change in the growth of the inner core.
Now, the researchers will focus on the transition layer between this new inner core and the outer part of the inner core, to better understand how Earth’s deep interior works and what it can tell us about both its past and its future.