Probe to Earth’s core is feasible

Dave Stevenson has spent his career working on “swing-by” missions to the other planets. Now he has a modest proposal he’d like to swing by a government agency with a few billion dollars in available funding.

According to Stevenson’s calculations, it should be possible to send a probe into Earth’s core by combining several proven technologies with well-grounded scientific assumptions about the workings of the planet. The probe would sink straight to the core in an envelope of molten iron, sending back temperature readings, compositional information, and other data along the way. Stevenson’s paper, “A Modest Proposal,” appeared this month in the journal Nature.

“We’ve spent more than $10 billion in unmanned missions to the planets,” says Stevenson, the Van Osdol Professor of Planetary Science. “But we’ve only been down about 10 kilometers into our own planet.”

The benefits to science would be significant, he says, because so little has been directly observed about the inner workings of the planet. 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 from seismic data accumulated during earthquakes.

Stevenson says his proposal should be attractive to the scientific community because it is of the same scale, pricewise, as planetary exploration. Sending something into Earth’s core, Stevenson says, will have comparable payoffs in the quest for knowledge.

“The biggest question should not be the cost, but whether we should pursue the goal of exploring Earth’s interior,” he says. “That said, I’d suggest we do it if we can keep the cost under $10 billion.”

Stevenson’s plan calls for a crack to be opened in Earth’s crust, perhaps with some sort of explosion—probably a nuclear bomb. According to his figures, the crack will need to be several hundred meters in length and depth, and about 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. 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 below.

“Once you set that condition up, the crack is self-perpetuating,” he explains. “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. 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 30-centimeter crack without getting wedged.

The probe will contain instruments to collect data, which will be relayed through low-intensity mechanical waves of some sort—probably through deformations of the ball itself—to send out a kind of “Morse code” of data. Because radio waves cannot propagate through the planet, this is the only way to get the data transferred.

Based on the rate at which the molten iron falls due to gravity, the ball would move downward into Earth at roughly a human running pace (about 10 miles per hour), meaning that the entire mission would last a few weeks.

All this may sound to some like science fiction, but Stevenson says each of the principles involved is based on sound knowledge of crack propagation, fluid dynamics, mechanical-wave propagation, and “stress states.”
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