Plumbing the Earth's depths

By Ivan Noble
BBC News Online science staff

JOURNEY TO THE CORE

Magnitude 7 quake or equivalent bomb would open crack

Perhaps 100 million kg of iron required to make 'jacket'

Probe travels downwards at about 5 metres per second

A US scientist has described an ambitious plan using a nuclear weapon and a hundred thousand of tonnes of molten iron to send a grapefruit-sized probe to the centre of the Earth.

David Stevenson, of the California Institute of Technology, told BBC News Online that he had come up with the plan, which appears in the journal Nature, to provoke discussion.

"I have quite low confidence that it will work. However, my motivation in writing this paper is to get people to think about ideas like this.

"I believe that there has been insufficient effort in thinking about how you would go down into the Earth," he said.

Protective 'iron jacket'

Professor Stevenson's plan involves creating a crack in the Earth's crust either by detonating a nuclear warhead or by using something which would release a similar amount of energy.

"You fill that crack quickly with liquid iron and with a small, solid probe immersed in that liquid iron. The probe would be perhaps the size of a grapefruit.

Ligo would act as mission control

"The iron being heavier than the surrounding rock causes the crack to keep propagating down and closing up behind as it does so.

"It goes down to the Earth's core at quite high speed, on a timescale of days. As it reaches the core, the probe will send back, using seismic signals, information about what the Earth is made of," he explained.

These transmissions, which would be very faint, could be detected by the supersensitive "listening" equipment at the newly opened Ligo gravitational wave facility in the US.

Such a dramatic mission beneath the Earth's surface, if it worked, would yield important answers to fundamental questions about how the planet works.

"It might tell us the composition of the core, the temperature of the core... the way the Earth formed, the way it separated into liquid core and solid mantle, [and] the way the Earth's magnetic field is generated," Professor Stevenson told BBC News Online.