The Great Iron Ball

Deep within the Earth, far below the layers of rock that form the crust, and even further below the molten rock of the mantle, lies a hot core of Iron and Nickel.  The swirling of the liquid metal creates a flow of charge and produces the magnetic field of the Earth, without which we humans could not survive. But there is still more.  At the centre of the Earth, a part of the liquid metal core, the size of Pluto, cooled into a solid ball of Iron and Nickel.  When in the Earth’s history did it form? This question has been a subject of debate for decades, but now we may have an answer.

The inner core is Earth’s deepest layer. It is a ball of solid iron just larger than Pluto which is surrounded by a liquid outer core. The inner core is a relatively recent addition to our planet and establishing when it was formed is a topic of vigorous scientific debate with estimates ranging from 0.5 billion to 2 billion years ago. Credit: Kay Lancaster, Department of Earth, Ocean and Ecological Sciences

Scientists from the University of Liverpool have narrowed the age of the Iron core to between 1 and 1.5 Billion years.  This is still a very wide range of 500 Million years, but considering how difficult it is to infer the properties of the Earth’s core, we are doing pretty well.

By analysing rock samples, the scientists found that in the period of time 1-1.5 Billion years ago, the Earth magnetic field strength sharply increased.  This increase is the first evidence of a ‘freezing’ of the core, and measuring it gives strong implications for how the geological history of the Earth unfolded.

“The timing of the first appearance of solid iron or “nucleation” of the inner core is highly controversial but is crucial for determining the properties and history of the Earth’s interior and has strong implications for how the Earth’s magnetic field — which acts as a shield against harmful radiation from the sun, as well as a useful navigational aid — is generated.” said Dr Andy Biggin, the study’s lead author and peleomagnetism expert.

“The results suggest that the Earth’s core is cooling down less quickly than previously thought which has implications for the whole of Earth Sciences. It also suggests an average growth rate of the solid inner core of approximately 1mm per year which affects our understanding of the Earth’s magnetic field.”

Dr Biggin added: “The theoretical model which best fits our data indicates that the core is losing heat more slowly than at any point in the last 4.5 billion years and that this flow of energy should keep the Earth’s magnetic field going for another billion years or more.  This contrasts sharply with Mars which had a strong magnetic field early in its history which then appears to have died after half a billion years.”

So whatever is going on with the Earth, it’s cooling slowly enough to keep our magnetic field in business for a long time.  And that’s a small piece of good news for homo sapiens.

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