Earth Probe Plan Would Blast a Path to the Core

National Geographic News
May 14, 2003
A scientist proposes sending a grapefruit-size communication device into the heart of the Earth by blasting a crack in the surface and pouring in a huge quantity of molten iron. The weight of the liquid metal would crack the Earth for more than 1,800 miles (3,000 kilometers), carrying the probe to the planet's core in about a week.

The probe would measure temperature, electrical conductivity, and chemical composition, and would beam back data as encoded sound waves to a surface detector.

David J. Stevenson of the California Institute of Technology in Pasadena outlines the plan in the May 16 issue of the scientific journal Nature.

"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, where equally fascinating scientific issues are waiting to be investigated," Stevenson said in his paper. "I propose a scheme for a mission to the Earth's core, in which a small communication probe would be conveyed in a huge volume of liquid-iron alloy migrating down to the core along a crack that is propagating under the action of gravity."

The proposal might sound ambitious, but it's modest in comparison with the demands of space exploration, Stevenson said.

"We live on the Earth's surface, which divides what is above from what is below. The part above us, the rest of the universe, is mostly empty, mostly unknown…The part below is crammed with interesting stuff and is also mostly unknown, despite its much greater proximity to us."

Stevenson calculated that the energy required to create the crack to launch the probe would be equivalent to a few megatons of TNT, an earthquake of magnitude 7 on the Richter scale, or a nuclear device such as those already possessed by many nations.

It may also be feasible to make use of existing favorable stress environments in the Earth and to avoid the use of nuclear devices, Stevenson said in his paper. "The technological challenge of initiating the crack should be less than that posed by the Manhattan Project," he said, referring to the code name for America's first atomic bomb.

Proven Technologies

According to Stevenson's calculations, it should be possible to send a probe all the way to Earth's core by combining several proven technologies with a few well-grounded scientific assumptions about the workings of the planet.

"We've spent more than [U.S.] $10 billion in unmanned missions to the planets," said Stevenson, who is the Van Osdol Professor of Planetary Science at Caltech. "But we've only been down about ten kilometers [6 miles] into our own planet."

The benefits to science would be significant, Stevenson said, because so little has been directly observed about the inner workings of the Earth. Scientists do not know, for example, the exact composition or even the temperature of the core, and what they do know is based on inferences about seismic data accumulated during earthquakes.

Stevenson said his proposal should be attractive to the scientific community because it is of the same scale, price-wise, as planetary exploration. To date, NASA has flown unmanned missions past all the planets except Pluto, has made a few highly successful soft landings on Mars, has probed the clouds of Jupiter, is getting ready to probe the atmosphere of Titan, and has sent four spacecraft into interstellar space. Sending something into the Earth, Stevenson believes, will have comparable payoffs in the quest for knowledge.

"When we fly to other worlds, we are often surprised by what we find, and I think the same will be the case if we go down."

A Million Tons of Molten Iron

According to Stevenson, the crack that will have to be blasted into the Earth's surface to launch the probe will need to be several hundred meters in depth, and about a foot (30 centimeters) wide, to accommodate a volume of about 100,000 to several million tons of molten iron.

The instant the crack opens, the entire volume of iron will be dropped in, completely filling the open space, he said. Through the sheer force of its weight, the iron will create a continuing crack that will open all the way to the planet's core 3,000 kilometers (1,800 miles) below. Anything on a smaller scale may not work; anything larger will be even more expensive, so Stevenson thinks a crack of those dimensions is about right.

"Once you set that condition up, the crack is self-perpetuating," Stevenson said. "It's fundamentally different from drilling, where it gets harder and harder—and eventually futile—the farther you go down."

The iron will continue to fall due to gravity because it is about twice the density of the surrounding material. Riding along in the mass of liquid iron will be one or more probes made of a material robust enough to withstand the heat and pressure. The probe will perhaps be the size of a grapefruit but definitely small enough to ride easily inside the 12-inch (30-centimeter) crack without getting wedged, Stevenson said.

Inside the probe will be instrumentation for data collection, which will be relayed through low-intensity mechanical waves of some sort. Because radio waves cannot propagate through Earth, this is the only way to get the data transferred, Stevenson said.

Based on the rate the molten iron would fall due to gravity, the ball would move downward into Earth at roughly human running pace (about 10 miles/16 kilometers per hour), Stevenson said.

"Each of the principles involved is based on sound knowledge of crack propagation, fluid dynamics, mechanical-wave propagation, and 'stress states,'" Stevenson said. "If these things didn't already work in nature, we would have no volcanoes and poorly performing bathroom plumbing, but little to fear from a pebble shattering our windshields."

The biggest question should not be the cost, but whether we should pursue the goal of exploring Earth's interior, he said. "That said, I'd suggest we do it if we can keep the cost under [U.S.] $10 billion."

This proposal is modest compared with the space program, Stevenson said, and may seem unrealistic only because so little effort has been devoted to it. "The time has come for action."

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