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11 December 2014

Protecting planet earth from killer electrons

Our planet has hidden life-protection systems that continue to surprise us.

By Michael Brooks

If you were to design a planet suited for life, it’s hard to imagine improving on the earth. Our planet has hidden life-protection systems that continue to surprise us.

Last month, astronomers discovered a magnetic deflector shield sitting 11,000 kilometres above our heads. It protects the surface of our planet from high-energy-charged particles that would otherwise bombard us with radiation. These “killer electrons” are halted by nothing more than a thin sheet of plasma – a gas made up of charged particles – held in place by the earth’s magnetic field. It’s a puzzling phenomenon: no one knows why such a shield should form.

Life needs shelter from the storms of space. Far from being an empty, neutral environment, space is deadly, packed with cosmic rays, particles that whizz around at near-light speed. This is what will make interplanetary travel so dangerous for human beings.

When astronauts in orbit close their eyes, they are beset by flashes of light under their eyelids. The flashes are stimulated as cosmic rays shear through their eyeballs and brain tissue. We already knew that long stretches in space could lead to radiation-induced cancer. A 2013 study in which mice were irradiated at levels astronauts experience in space showed that they should fear the quick onset of radiation-induced Alzheimer’s disease, too.

On earth, we are protected from this by the magnetic field generated in the sphere of churning, molten metal 3,000 kilometres beneath our feet. This is earth’s outer core. It is powered by heat left over from the planet’s formation, radioactivity within the core and the planet’s spin on its axis.

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It is the motion of flowing metal that creates the magnetic field and thanks to the extraordinary volume of churning metal beneath our feet, earth’s field is strong enough to reach into space. There, its interaction with interplanetary space’s clouds of charged particles has created two fixed cloaks of plasma. They are known as the Van Allen radiation belts and it is at the boundary between them – also a thin sheet of plasma – that the killer electrons have been found to be bouncing away like rocks off a riot shield.

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According to the researchers, this thin sheet of plasma is “an almost impenetrable barrier through which the most energetic Van Allen belt electrons cannot migrate”. If it were not there, it is likely that space-borne electronics, such as those in our GPS satellites, would quickly be fried by the radiation.

It’s not the only way earth’s magnetic field protects us. Life – at least life as we know it – needs oxygen and water. Both exist in abundance in the 480 kilometres of atmosphere above our heads, which is only there because of the earth’s magnetic field.

The field deflects the solar wind, the stream of particles emanating from the sun, which would otherwise blow our atmosphere’s gases off into space. Mars once had a magnetic field that maintained an atmosphere. Once the core of the red planet cooled, its field failed, leaving its atmosphere and surface water to be scoured away.

Earth’s core is gradually solidifying, too, but not quickly enough to cause concern. The solid inner core is growing by roughly a millimetre a year, leaving us a few billion more years of magnetic protection.

We know all this from reading the patterns in seismic waves as they pass through our planet. We discovered the new electron deflector shield thanks to Nasa’s Van Allen probes, a pair of spacecraft sent 11,000 kilometres above earth’s surface. We should eventually be able to work out just how the magnetic shield forms. Until then, we simply have to marvel at the protection that can be generated from a load of churning metal.