The solid inner core of the earth is only a billion years old, new research finds.
Modern Earth is like a layer cake, with a firm outer crust, a hot, sticky mantle, a liquid outer core and a firm inner core. That solid inner core grows slowly like the liquid iron at the core cools and crystallizes. This process helps the turbulent movement of the liquid outer core, which in turn creates the magnetic field that surrounds the earth and helps protect the planet from harmful cosmic radiation.
In other words, the inner core is pretty important.
But not much is known about the history of this iron ball of 1,500 kilometers wide (2,442 kilometers). Estimates of its age range from half a billion years to more than 4 billion years, almost as old as the 4.5-year-old Earth itself. Now researchers have pinched a mini-piece of iron between two diamonds and blasted it with lasers to arrive at a new estimate of 1 billion to 1.3 billion years old – a date range that coincides with a measurable strengthening of the The earth’s magnetic field that happened around the same time.
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“The earth is unique in us solar system in that it has a magnetic field, and that it is habitable, “author Jung-Fu Lin, a geoscientist at the University of Texas at Austin, told WordsSide Science. Finally, our results could be used to reflect on why other planets in our solar system do not have magnetic fields. “
The geodynamo
The Earth’s magnetic field is driven by what scientists call the “geodynamo”. This is the motion of the iron-rich outer core, which transforms the planet into a gigantic, if somewhat messy, magnet. The geodynamo is responsible for Earth’s North Pole and South Pole and the invisible shield of magnetism that flows and picks up charged particles that flow from the sun. These particles would otherwise slowly remove the earth from its atmosphere.
Part of the motion of the inner core is driven by heat, known as the thermal energy source. As the Earth’s core slowly cools, it crystallizes from the inside out. This crystallization process requires energy that can continue the movement of the still liquid outer core. This energy loss from crystallization is called the compositional energy source of the geodynamo, Lin said.
Lin and his team wanted to use experimental evidence to pinpoint the energy of each of these sources. Knowing how much energy would allow them to estimate the age of the inner core.
To do this, the researchers re-created the conditions of the nucleus on a small scale. They heat a piece of iron only 6 microns thick (about the same as the length of a red blood cell) to temperatures up to 4,940 degrees Fahrenheit (2,727 degrees Celsius), and squeeze the sample between two diamonds to match the extreme pressure on Earth’s core. They then measure the conductivity of the iron under these conditions.
A young core
This conductivity measurement allowed the researchers to calculate the thermal cooling of the core available to promote the geodynamo. They found that the geodynamo extracted about 10 terawatts of energy from the cooling core – just over one-fifth of the amount that Earth disappears into space from its surface (46 terawatts, Live Science previously reported).
Once they calculated the amount of energy loss, the researchers were able to calculate the age of the inner core of the earth, Lin said. Knowing the rate of energy loss enabled the researchers to calculate how long it would take to get a solid mass the size of the current core of a blob of molten iron.
The result of 1 billion to 1.3 billion years suggests that the Earth’s core is “actually relatively young,” Lin said.
This estimate is not as young as some estimates, as one published in 2016 in the journal Nature who used similar methods but found that the core was only 700 million years old. Lin said the new experiment used more reliable ways to handle the pressure and temperatures generated at the core, making this younger estimate unlikely.
Ancient magnetic rocks revealed that the magnetic field suddenly became stronger between 1 billion and 1.5 billion years ago, a 2015 study in the journal Nature found. The new era fits in nicely with that evidence, because the crystallization of the inner core would have provided an “impetus” to the magnetic field, Lin said.
There are still questions about the way heat moves in the core, Lin said. Unlike the example they tested, the core is not only iron – it also contains lighter elements such as carbon, hydrogen, oxygen, silicon and sulfur. But the ratios of these light elements are unknown, making it difficult to know how they change the conductivity of the inner core. Lin and his team are working on that now.
“We are trying to understand how the existence of these light elements would actually affect the thermal transport properties of iron at such high pressure, high temperature conditions,” Lin said.
The researchers reported their findings Aug. 13 in the journal Physical assessment letters.
Originally published in Live Science.
