Magnetic field locked in by ancient crystals

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An artist’s depiction of Earth’s magnetic field deflecting high-energy protons from the Sun four billion years ago (not to scale). – Michael Osadciw / University of Rochester.

Like tiny compasses frozen in time, ancient zircon gems from Western Australia have shown Earth’s magnetic field is at least four billion years old – more than 700 million years older than previous evidence suggested. Because the magnetic field protects the Earth’s atmosphere from destructive solar rays, it raises the possibility that life could have made its debut much earlier than previously thought.

A North American team, led by John Tarduno at New York’s University of Rochester, found the young Earth had a chaotic magnetic field that was, at times, as strong as our field today. They reported their work in Science in July.

It’s “astonishing to be able to probe the magnetic field that far back”, says Louis Moresi, a geoscientist at the University of Melbourne.

Earth formed some 4.5 billion years ago, accreting from dust and proto-planetary fragments swirling around the Sun. Even in the near absolute zero temperatures of space, enough heat was trapped in the colliding mass to melt material in its core. Over time, the radioactive decay of uranium and other ‘hot’ elements has kept the core molten.

Today, if you were to drill through the crust, you’d travel through 2,900 kilometres of rocky mantle before reaching the outer core. You’d then slither through 2,200 kilometres of liquid iron, before hitting a metallic inner core about 1,200 kilometres in diameter – a little smaller than the Moon. The inner core is kept solid by the enormous pressures at the centre of the Earth.

“It’s amazing what they could get out of these little guys”

It’s the liquid outer core, constantly stirred by convection currents, that generates our magnetic field. Just as hot air rises, hot iron rises towards Earth’s cooler surface, carrying heat that escapes through the mantle and crust. As the iron cools, it sinks, providing a constant stirring motion that generates our magnetic field.

The mantle layer is vital to maintaining this motion. If it is too thin, the heat in the core dissipates quickly, along with the magnetic field. If it is too insulating, the vital convection currents shut down.

Thanks to the Earth’s daily rotation and the stabilising solid inner core, the convection currents in the molten outer core have settled into spiralling columns that lie parallel to the Earth’s axis, giving us the elegant north-south dipole our compasses use today. What was it like in the Earth’s youth?

It’s hard to say. Our dynamic planet’s surface is continually crushed, stretched and recycled – thanks to plate tectonics – and rock remnants from the Earth’s earliest days are scarce. But thanks to a zircon crystal dug out of 4.4-billion-year-old sandstone on a Western Australian sheep ranch, we may have a clue. The University of Wisconsin-Madison-led study last year claimed the gemstone was born 100 million years after the Earth itself.

For Tarduno and his team, that report raised an exciting possibility. They realised that like mosquitoes in amber, tiny iron oxide grains also known as magnetite would be trapped inside the zircon as it solidified and could have recorded the ancient magnetic field. His team returned to the same area and collected 25 zircon crystals dated from 3.3 to 4.2 billion years old, and examined them for microscopic magnetite flakes. But a super-sensitive instrument was needed to pick out the magnetic alignment in the grains.

Enter the superconducting quantum interference device (the SQUID). This ultra-high-definition magnetometer can detect faint magnetic fields – as much as 100 billion times weaker than the energy needed to move a compass needle. The SQUID found the magnetite grains harboured magnetic fields of varying strengths – from the equivalent of today’s magnetic field, to 12% of its strength.

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A 4.4 billion-year-old zircon crystal from the Jack Hills region of Western Australia, which is the oldest bit of the Earth’s crust. – John Valley

Before this study, the oldest evidence for a magnetic field on Earth came from South African rocks dated at 3.2 and 3.45 billion years old. Two of the oldest zircons in Tarduno’s study were 750 million years older. “It’s amazing what they could get out of these little guys,” Moresi says, adding they really are “miracle crystals”.

So why does the age of the magnetic field affect life’s appearance on Earth?

Basically, the field shields us from our life-giver – the Sun. Streams of charged particles flow from the Sun, bombarding the inner planets and stripping away their water and atmosphere – unless a magnetic field is strong enough to deflect the onslaught.

Tarduno points to our neighbour, Mars. The barren world once had a magnetic field, but no longer – nor any surface liquid water or atmosphere to speak of. Scientists think that around four billion years ago the Earth and Mars were battered by asteroids. On Mars, the onslaught over-heated its mantle. As the heat gradient between the core and mantle was lost, its core’s convection currents slowed and eventually stopped, switching off its magnetic shield and allowing the solar wind to whisk its atmosphere away. “It may also be a major reason why Mars was unable to sustain life,” he says.

But the Earth was a little larger, and able to weather the storm. Ensconced in its protective magnetic bubble, life could begin to flourish.

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