Located more than 5,000 kilometers below us, the solid metal inner core of the Earth was not discovered until 1936. Almost a century later, we are still struggling to answer basic questions about when and how it was formed.
They are not easy puzzles to solve. We cannot directly sample the inner core, so the key to unraveling its mysteries lies in collaboration between seismologists, which take indirect samples with seismic waves, geodynamists, who create models of their dynamics, and mineral physicists, 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 happening miles below us. On a new study, 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 intriguing history of Earth’s magnetic field.
The early Earth
The core of the Earth was formed soon in the 4.5 billion years of our planet’s history, in the first 200 million years. Gravity pulled the heavier iron toward the center of the young planet, leaving the rocky and silicate minerals to form the mantle and the crust.
The formation of the Earth captured a lot of heat inside the planet. The loss of this heat and warming from ongoing radioactive decay have driven the evolution of our planet ever since. The loss of heat inside drives the vigorous flow into the outer liquid iron core, which creates the earth’s magnetic field. Meanwhile, the cooling deep inside helps to boost the Tectonic plates, which shapes the surface of our planet.
As the Earth cooled, 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 at a rate of approximately 1 mm in radius each year, which is equivalent to the solidification of 8,000 tons of cast iron every second. In billions of years, this cooling will eventually cause the entire core becomes solid, leaving Earth without its protective magnetic field.
It could be assumed that this solidification creates a homogeneous solid sphere, but it does not. In the 1990s, the scientists they realized that the speed of the seismic waves traveling 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 the speed of seismic waves. The eastern part is under Asia, the Indian Ocean, and the western Pacific Ocean, and the western part is under America, the Atlantic Ocean, and the eastern Pacific.
The new study has investigated this mystery, using new seismic observations combined with geodynamic models and estimates of how iron alloys behave under high pressure. They found that the eastern inner core under the Banda Sea in Indonesia is growing faster than the western core under Brazil.
This uneven growth is like making ice cream in a freezer that only works on one side – ice crystals form only on the side of the ice cream where cooling is effective. On Earth, uneven growth is due to the rest of the planet absorbing heat more quickly in some parts of the inner core than in others.
But, unlike ice cream, the solid inner core is subjected to gravitational forces that distribute new growth evenly through a stealthy inner flow process, which maintains the spherical shape of the inner core. This means that the Earth is not in danger of capsizing, although this uneven growth is recorded in the speeds of seismic waves in the inner core of the planet.
Estimating the age of the nucleus
So does this approach help us understand the age of the inner core? When the researchers compared their seismic observations with their flow models, they found that the inner core – at the center of the entire core, which formed much earlier – is likely to be between 500 and 1.5 billion years old.
The study notes that the youngest end of this age range is the best fit, although the oldest end coincides with an estimate made by measuring changes in the intensity of the Earth’s magnetic field. Whichever number is correct, it is clear that the inner core is relatively young, between one-ninth and one-third the age of the planet.
This recent work presents a powerful new model of the inner core. However, a number of physical assumptions by the authors would have to be true for it to be correct. For example, the model only works if the inner core consists of a specific crystalline phase of iron, about which there is some uncertainty.
And does that 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 area of the northern half is lower, while the southern half is more mountainous. The crust of the Moon the near side is chemically different from 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.
Thus, although the causes of all these asymmetries vary, Earth appears to be in good company as a slightly asymmetric planet in a solar system of asymmetric celestial bodies.
** This article was originally published on The Conversation by Jessica irving and Sanne cottaar, Senior Lecturer in Geology at the University of Bristol and Lecturer in Global Seismology at the University of Cambridge, you can read it in full here.