‘Age of Earth’s inner core revised’

Researchers have revised the estimate of the age of Earth’s solid inner core to 1-1.3 billion years from 565 million years old.

• Over the years, estimates for core age and conductivity have gone from very old and relatively low, to very young and relatively high.
• But these younger estimates have also created a paradox, where the core would have had to reach unrealistically high temperatures to maintain the geodynamo for billions of years before the formation of the inner core.
• The new research solves that paradox by finding a solution that keeps the temperature of the core within realistic parameters.
• Furthermore, the experiments help pin down the magnitude of how the core conducts heat, and the energy sources that power the planet's geodynamo -- the mechanism that sustains the Earth's magnetic field, which keeps compasses pointing north and helps protect life from harmful cosmic rays.
• Finding that solution depended on directly measuring the conductivity of iron under corelike conditions – where pressure is greater than 1 million atmospheres and temperatures can rival those found on the surface of the sun.

Looking inside the Earth

• The Earth's interior is composed of four layers, three solid and one liquid—not magma but molten metal, nearly as hot as the surface of the sun.
• Inner core: The deepest layer is a solid iron ball, about 1,500 miles (2,400 kilometers) in diameter.
  • Although this inner core is white hot, the pressure is so high the iron cannot melt.
  • The iron isn't pure—scientists believe it contains sulfur and nickel, plus smaller amounts of other elements.
  • Estimates of its temperature vary, but it is probably somewhere between 9,000 and 13,000 degrees Fahrenheit (5,000 and 7,000 degrees Celsius).
• Outer core: Above the inner core is the outer core, a shell of liquid iron. This layer is cooler but still very hot, perhaps 7,200 to 9,000 degrees Fahrenheit (4,000 to 5,000 degrees Celsius).
  • It too is composed mostly of iron, plus substantial amounts of sulfur and nickel. It creates the Earth's magnetic field and is about 1,400 miles (2,300 kilometers) thick.
• Mantle: The next layer is the mantle. Many people think of this as lava, but it's actually rock. The rock is so hot, however, that it flows under pressure, like road tar. This creates very slow-moving currents as hot rock rises from the depths and cooler rock descends.
• Crust: The crust is the outermost layer of the Earth. It is the familiar landscape on which we live: rocks, soil, and seabed. It ranges from about five miles (eight kilometers) thick beneath the oceansto an average of 25 miles (40 kilometers) thick beneath the continents.

Key-highlights of the study:

• The new research looked at the paradox and found a solution by keeping the temperature of the core within realistic parameters.
• The researchers achieved these conditions by squeezing laser-heated samples of iron between two diamond anvils.
• Result: The new conductivity was measured at 30-50 per cent less than the conductivity of the young core estimate.
• The research suggested that geodynamo was sustained by two different energy sources and mechanisms:
  • Thermal convection (the buoyancy is due to temperature fluctuations)
  • Compositional convection (buoyancy produced by light material released at the inner-core boundary)
• That is, at the time the inner core started to grow, the geodynamo got powered by a new source of energy.
• With improved information on conductivity and heat transfer over time, the researchers made a more precise estimate of the age of the inner core.

Geodynamo paradox

• Earth’s inner core is the innermost geologic layer of the Earth, it is made of iron and is solid. And it is very hot, about 6,000 degrees Celsius.
• The outer core is also iron, but is liquid due to relatively lower pressure.
• As lighter elements rise through the liquid iron of the outer core at different temperatures, they cause convection currents, believed to resemble a dynamo.
• The process is like cream swirling in a mug of coffee.
• The circulation of liquid metal creates electric currents (kinetic energy is converted into magnetic energy) and turns Earth into a giant electromagnet.
• This is how Earth’s magnetic field is generated. The process is called geodynamo, and is fed by convection.

How the age of the inner core was ‘earlier’ calculated?

• The age of the inner core, however, has been essentially calculated by the effectiveness of iron to transfer heat, known as thermal conductivity.
• But there was a paradox with the previously determined younger estimates of the age of the inner core: The core would have had to reach unrealistically high temperatures to sustain the geodynamo for billions of years before the inner core would be formed. 
• The high amount of conductivity of the iron core, however, was impossible because it would have little energy to convection. 
• It also only supported the existence of the geodynamo for about a billion year, whereas other studies have showed that the process has existed for at least 3.4 billion years.

Earth’s magnetic field

• This revised age of the inner core could correlate with a spike in the strength of the Earth's magnetic field as recorded by the arrangement of magnetic materials in rocks that were formed around this time.
• Together, the evidence suggests that the formation of the inner core was an essential part of creating today's robust magnetic fields.
• The Earth’s magnetic field, generated 3,000km below our feet in the liquid iron core, is crucially important to life on our planet.
• It extends out into space, wrapping us in an electromagnetic blanket that shields the atmosphere and satellites from solar radiation.
• The magnetic field is the weakest at the equator, where the dent is located, which makes it worse than if it were at any other place.

Conclusion

The debate about the age of the inner core and the resulting thermal evolution of the Earth is not yet over. More palaeomagnetic data are needed to confirm that the sharp increase in magnetic field strength that we have observed is really the largest in the planet’s history. Furthermore, modelling needs to verify whether some other event could have created the magnetic strengthening at this time.