A Modest Proposal
On a warm summer morning, I met with Stevenson at his Caltech office in Pasadena. He was dressed for the weather, wearing shorts, sandals and a short-sleeved shirt. We talked for a while about how the surfaces of Mars and other planets, despite being tens or hundreds of millions of miles away, are far more accessible than Earth’s core.
“Of course, the universe above the Earth is mostly transparent! So you have the wonderful opportunity to use photons to tell you about the rest of the universe,” he says. “But you can’t do that inside the Earth. So the methods we have for seeing inside the Earth, if you will, are actually quite limited.”
Eleven years ago, Stevenson published a paper in the journal Nature outlining a wild scheme to get around some of those limitations. His article, “Mission to Earth’s core — a modest proposal,” described a way to send a small probe directly to the center of the Earth. The article’s title was a nod to Jonathan Swift’s 1729 satirical essay, “A Modest Proposal,” which mocked harsh British policies in Ireland by suggesting that the Irish alleviate their poverty by selling their children as meat to the English gentry. Like Swift, Stevenson wasn’t arguing for the actual feasibility of his idea; the paper was a thought experiment, an exercise to show the literally earthshaking scale of effort that would be needed to probe deep into the planet.
The first step in Stevenson’s journey to the center of the Earth: Detonate a thermonuclear weapon to blast a crack several hundred meters deep in Earth’s surface. Next, pour 110,000 tons of molten iron into the crack. (Stevenson told me that he now thinks 110,000 tons is an underestimate. On the plus side, a nuclear explosion might not be necessary — a million tons of conventional explosives might suffice.) Molten iron, being about twice as dense as the surrounding mantle, would propagate the crack downward, all the way to the core. The crack behind the descending blob of iron would quickly seal itself under pressure from the surrounding rock, so there would be no risk of the crack spreading catastrophically and splitting the planet wide open. Carried along with the sinking iron would be a heat-resistant probe about the size of a football. Stevenson estimated that the molten iron and probe would move at a rate of about 10 mph and reach the core in a week.
The probe would record data on the temperature, pressure and composition of the rock it passed through. Since radio waves can’t penetrate solid rock, the probe would vibrate, transmitting data in a series of tiny seismic waves. An extremely sensitive seismometer on Earth’s surface would receive the signals.
It’s within the reach of current technology to build a probe capable of surviving immersion in molten iron and to collect its data, but what about the rest of the plan? Could some version of Stevenson’s idea possibly work?
“The particular scheme I proposed is probably impractical,” he tells me, mostly because of the enormous quantities of molten iron that would be needed. “But it was not physically ridiculous. Engineering it may have been ridiculous, but in terms of physical principles, I wasn’t violating any laws of physics. I was showing that in a world unrestricted by concerns about how much money you would spend, you could contemplate doing what I described.”
Proposing a realistic mission wasn’t the point of the paper, Stevenson says. He wanted to highlight the limits of what can be known by constructing theories about Earth’s interior from our perch on the planet’s surface. “I wanted to remind people that the history of planetary exploration has told us the importance of going there. Time after time, we have learned things when we arrive at a planet that we had not suspected by looking at that planet from afar. I believe very strongly in this aspect of science.
“There is a danger that we will compartmentalize our understanding of an aspect of the universe by saying to ourselves, ‘OK, we know we can’t go there, so we’re going to build this elaborate story of what’s there based on remote observations.’ And this is what we do for the Earth,” Stevenson continues. “We don’t even know whether the material immediately adjacent to the core is entirely solid or partly solid. We don’t know the character of the core-mantle boundary. There are a lot of questions that would only be answered with precision by going there.”