PASADENA, Calif. – 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 some government agency with a few
billion dollars in available funding.
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. The probe would sink straight to the
core in an envelope of molten iron, sending temperature readings,
compositional information, and other data along the way.
"We’ve spent more than $10 billion in unmanned missions to the
planets," says Stevenson, who is the Van Osdol Professor of Planetary
Science at the California Institute of Technology. "But we’ve only
been down about 10 kilometers into our own planet."
The benefits to science would be significant, Stevenson 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 about seismic data accumulated during earthquakes.
Stevenson says 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 (if indeed Pluto is a planet at all), 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."
Stevenson’s plan calls for a crack to be opened in the earth, 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 thousand 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.
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," 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 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 30-centimeter crack without getting wedged.
Inside the probe will be instrumentation for data collection, which
will be relayed through low-intensity mechanical waves of some
sort-probably through deformations of the ball itself to send out a
sort of "Morse code" of data. Because radio waves cannot propagate
through Earth, this is the only way to get the data transferred.
The probe will likely operate with about 10 watts of power, and it
may even be possible to replenish energy and dispense with an
on-board battery by harnessing mechanical energy from the force of
the fall, just as electricity can be generated from falling water.
Such a low power rating will not make it possible to generate very
strong shock waves for data transmission, but strong waves may not be
necessary. In fact, Stevenson further suggests that the Laser
Interferometer Gravitational-Wave Observatory (LIGO) might be
recalibrated in its downtime to track the falling ball.
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 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." 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 is how to initially open the crack," says
Stevenson. "Also, there’s the technological challenge of having a
probe that actually does what it’s supposed to do."
Stevenson says he came up with part of the title "A Modest Proposal"
for his paper, which is appearing in this week’s journal Nature, to
have a bit of fun but at the same time to issue a serious scientific
proposal. He purposely took the title from Jonathan Swift’s famous
essay of the same name. The Swift essay suggests that Ireland’s
terrible economic circumstances could be solved by people eating
their own children, thereby allowing England to continue pillaging
the country’s resources for its own one-sided benefit.
"My proposal is not as outrageous as suggesting one should eat his
own children, but still combines a serious proposal with some
levity," Stevenson says. "Ninety-five percent of the scientists who
read the article may laugh at an enjoyable read, but if the other
five percent seriously consider the goal of probing Earth’s core,
then I’ll be happy."
"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."
