Date Earth's Magnetic Field Switched On

31 July 2015
Posted by Chris Smith.

The earliest evidence of the Earth's magnetic field has been uncovered in Australian earthrock samples dating back 4.4 billion years.

The protective magnetic cloak cast around our planet is our shield and protector against the maelstrom of incoming radiation that rains in constantly from space.

Without this field in place, the passing solar wind would also whip away our atmosphere and desiccate the planet, a fate which appears to have befallen Mars already.

Geologists still don't understand how the Earth's magnetic field is formed, but current theories suggest that a so-called geodynamo of spinning iron in the planet's core is responsible.

Alongside establishing how this phenomenon operates, another priority is to discover when it first became active, which will provide insights into how the Earth formed and the subsequent evolution of the geology and chemistry of the planet.

One way to do this is to look for the signature of the Earth's magnetic field written into rocks. Before molten magma from the planet's interior cools and hardens, the particles it contains are free to move around.

This means that magnetic materials, like iron, will naturally line up with the prevailing field at the time. Once the rock solidifies, these particles are locked into position, pointing like a compass to what the Earth's magnetic field was doing when they were laid down.

Using extremely sensitive equipment, it's possible to read these tiny magnetic signatures, and if the age of the rock containing them is known, this can reveal the status of the planet's field as it was at the time.

Previously, scientists had managed to perform this sort of analysis on rocks recovered from South Africa that dated from about 3.5 billion years ago. Evidence of a strong magnetic field was detected in these samples, suggesting that the planet's field was already well established by then. What happened before this time, however, no one knew.

Now Rochester University's John Tarduno and his colleagues, writing in this week's edition of the journal Science, have uncovered - and extracted the magnetic signatures from - even more ancient rock samples collected in Western Australia.

The samples they have tested come from the Jack Hills conglomerate, about 800 kilometres north of Perth. Tiny crystals called zircons, trapped inside pebbles, contain magnetic iron particles.

The team were able to establish that the zircons were up to 4.4 billion years old, which is just slightly younger than the planet itself. Critically, the crystals did not bear the hallmarks of previous exposure to extremely high temperatures. Had this been present, it would have wiped out any vestiges of the magnetic signature inside.

Using a device called a SQUID - which stands for superconducting quantum interference device - Tarduno and his colleagues were able to register the presence and intensity of the prevailing magnetic field when the zircons formed.

The field was present from as far back as 4.4 billion years ago, although it was much weaker initially at just 12% of its current intensity, the new results reveal.

This shows that the geodynamo began very early, a finding that will constrain models of the planet's formation, and the subsequent arrival of water and potentially us...

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