Earth’s inner core grows more on one side than the other – here’s why the planet doesn’t tip over

More than 5,000 kilometers below us, the solid metallic core of the Earth was not discovered until 1936. Almost a century later, we still struggle to answer fundamental questions about when and how it happened. ‘is formed for the first time.

These are not easy puzzles to solve. We cannot directly sample the inner core, so the key to unraveling its mysteries lies in the collaboration between seismologists, who sample it indirectly with seismic waves, geodynamicists, who create models of its dynamics, and earth physicists. minerals, which study the behavior of iron. alloys at high pressures and temperatures.

By combining these disciplines, scientists have provided an important clue as to what is going on for miles beneath our feet. In a new study, they reveal how the Earth’s inner core is growing faster on one side than the other, which could help explain the age of the inner core and the fascinating history of Earth’s magnetic field.

Primitive earth

The Earth’s core formed very early in our planet’s 4.5 billion year history, in the first 200 million years. Gravity pulled the heaviest iron to the center of the young planet, leaving rocky and silicate minerals to form the mantle and crust.

The formation of the Earth captured a lot of heat inside the planet. The loss of this heat and the warming due to the ongoing radioactive decay have since driven the evolution of our planet. The loss of heat inside the Earth causes the vigorous flux in the outer core of liquid iron, which creates the Earth’s magnetic field. Meanwhile, cooling in the Earth’s deep interior helps fuel plate tectonics, which shapes our planet’s surface.

As the Earth cooled over time, the temperature at the center of the planet eventually dropped below the melting point of iron at extreme pressures, and the inner core began to crystallize. Today, the inner core continues to grow by about 1 mm in radius each year, which is equivalent to the solidification of 8,000 tons of molten iron per second. Billions of years from now, this cooling will eventually lead to the solidification of the entire core, leaving the Earth without its protective magnetic field.

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Read more: Curious Kids: What If Earth’s Core Cools Down?

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Central problem

One might assume that this solidification creates a homogeneous solid sphere, but this is not the case. In the 1990s, scientists realized that the speed of seismic waves passing through the inner core varied unexpectedly. This suggested that something asymmetrical was happening in the inner core.

Specifically, the eastern and western halves of the inner core showed different variations in seismic wave velocity. The eastern part of the inner core lies under Asia, the Indian Ocean, and the Western Pacific Ocean, and the west lies under the Americas, the Atlantic Ocean, and the Eastern Pacific.

The new study probed this mystery, using new seismic observations combined with geodynamic modeling and estimates of the behavior of iron alloys at high pressure. They found that the eastern inner core below the Banda Sea in Indonesia is growing faster than the western side below Brazil.

You can think of this uneven growth as if you were trying to make ice cream in a freezer that only works on one side – ice crystals only form on the side of the ice cream where cooling is effective. On Earth, uneven growth is caused by the rest of the planet sucking heat faster from some parts of the inner core than others.

But unlike ice cream, the solid inner core is subjected to gravitational forces that evenly distribute new growth through a process of creeping inner flow, which maintains the spherical shape of the inner core. This means that the Earth is not in danger of tipping over, although this uneven growth is recorded in the seismic wave velocities in the inner core of our planet.

Dating from the nucleus

So, does this approach help us understand how old the inner core can be? When the researchers matched their seismic observations with their flow models, they found that the inner core – at the center of the entire core that formed much earlier – is likely to be between 500 million and 1,500 million years ago.

The study reports that the younger end of this age group matches the best, although the older end matches an estimate made by measuring changes in the strength of the Earth’s magnetic field. Whichever number turns out to be correct, it’s clear that the inner core is relatively young, somewhere between one-ninth and one-third the age of the Earth itself.

This new work presents a powerful new model of the inner core. However, a number of physical assumptions made by the authors would have to be true for this to be correct. For example, the model only works if the inner core consists of a specific crystal phase of iron, about which there is some uncertainty.

And does our uneven inner core make Earth unusual? It turns out that many planetary bodies have two halves that are somewhat different from each other. On Mars, the surface of the northern half is lower while the southern half is more mountainous. The crust on the near side of the Moon is chemically different from that on the far side. On Mercury and Jupiter it is not the surface that is uneven but the magnetic field, which does not form a mirror image between north and south.

So while the causes of all of these asymmetries vary, Earth appears to be in good company as a slightly asymmetrical planet in a solar system of unbalanced celestial bodies.