Plan for an unmanned mission to Earth's core
First, split the ground open with cataclysmic force, then fill it with the world's entire supply of molten iron carrying a small communication probe - and the resulting 3,000 kilometre journey to Earth's core should take about a week, according to a U.S. planetary physicist.
"We would learn a lot more about the nature of Earth and how it works - the generation of the magnetic field, the origin of some kinds of volcanoes, the heat sources inside Earth, the stuff Earth is made of - in short, all the basic questions," he told ABC Science Online.
In his paper, Stevenson argues that "planetary missions have enhanced our understanding of the Solar System and how planets work, but no comparable exploratory effort has been directed towards the Earth's interior".
"Space probes have so far reached a distance of about 6,000 million kilometres, but subterranean probes (drill holes) have descended only some 10 kilometres into the Earth," he writes in his article.
The main barrier to travelling to the core is the dense matter of the Earth's mantle. The energy required to penetrate the mantle by melting is about a thousand million times the energy needed for space travel, per unit distance travelled.
Stevenson's scheme relies on principles observed in 'magma fracturing' - where molten rock migrates through the Earth's interior. He proposes pouring 100 million tonnes of molten iron alloy into a crack of about 300 metres deep in the Earth's surface.
This massive volume of iron, containing a small communication probe, would work its way down to the Earth's core, along the crack, which would open up by the force of gravity and close up behind itself. The crack would open downwards at 5 metres per second, giving a mission timescale of "around a week".
Such 'Earth dives' have not been tried before on any scale, nor is the technology yet available. "No, we can't do it now," said Stevenson. "But the basic scientific principles are understood. The same answer applied to the atomic bomb in 1940."
The initial crack would require a force equivalent to several mega tonnes of TNT, an earthquake of magnitude 7 on the Richter scale, or a nuclear device "with a capability within the range of those currently stockpiled". The amount of iron needed could be as much as the amount produced world-wide in a week.
Heat would be maintained through the release of gravitational energy and the partial melting of silicate rock walls. "But of course, the mantle is hot anyway," said Stevenson, "so once you get below the first 100 kilometres, there are alloys that would never freeze in equilibrium with the mantle." He said the probe would penetrate the outer core but the solid inner core of the Earth would probably stop it from going any further.
The grapefruit-sized probe embedded in the molten iron would contain instruments to measure temperature, conductivity, and chemical composition. It would rely on encoded sound waves to beam data to the surface, as the Earth's interior does not transmit electromagnetic radiation.
One of the existing Laser Interferometer Gravitational-wave Observatories (LIGO), used to detect tiny amounts of gravitational radiation from space, could be reconfigured to read the acoustic frequencies from the probe burrowing beneath.
"My paper is an idea, not a blueprint!" Stevenson told ABC Science Online. "But the physical process involved - with melt moving through the outermost 100 kilometres of earth - is something the Earth does every day."
"This proposal is modest compared with the space program, and may seem unrealistic only because little effort has been devoted to it," he concludes in Nature. "The time has come for action."
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