A Mission to the Earth's Core
Adventure stories involving the exploration of the interior of Planet Earth have a long and distinguished history in science fiction. Jules Verne’s Journey to the Center of the Earth (1864) was perhaps the first such tale. Despite its title, the story involves explorers following the instructions of a seventeenth century runic message on a trip that descends into the crater of an Icelandic volcano and into a long tunnel connecting to a vast cave containing a conveniently phosphorescent ceiling, an ocean, islands, dinosaurs, and mastodons, all many miles beneath the surface of the Earth, but not very close to its center.
Following Verne’s lead and doing considerably more violence to the sciences of geology, paleontology, and physics, Edgar Rice Burroughs wrote seven novels beginning with At the Earth’s Core (1922) that were set in Pellucidar, a "land" occupying the inner surface of a vast spherical hole in the Earth’s hollow interior. Pellucidar had a sizable ocean and more land area than Earth’s outer surface, its own internal sun and moon, and was populated by mastodons, dinosaurs, and an intelligent but rather nasty reptilian species called the Mahars. Burroughs’ various protagonists (including Tarzan) traveled to Pellucidar in a variety of ways, including a mechanical mole machine, arctic pirate expeditions, and a vacuum-filled magnesium dirigible.
Unfortunately, Burroughs got the physics of hollow planets completely wrong. The Mahars, dinosaurs, and explorers would not be pulled to the inner surface of Pellucidar by inside-out gravity. As Isaac Newton first proved, because of the inverse-square law, the pull of gravity anywhere in the cavity of a thick, massive, hollow sphere is zero, because the pulls of gravity from below and above any point exactly cancel. Would-be inhabitants of Pellucidar would find themselves floating around in free fall.
Not immune to the pull of the Earth’s interior myself, I once wrote an almost-published piece about the exploration of the Earth’s core. Near the end of the original manuscript of my hard SF novel Twistor, there was a long scene in which my protagonists David and Vickie, with some help from Boeing Aerospace, built a special "inner-space craft" vehicle that used gravity and the twistor effect (a "rotational" interchange of normal matter and shadow matter) to do a 38 minute in-vacuum free-fall through the Earth’s interior gravitational field to the other side of the planet, sampling snippets of the Earth’s interior all along the trajectory to the center and back and exploring for the first time the "inner space" of our world. Unfortunately, my editor in his wisdom decided to halt the narrative at an earlier point and removed this scene from the published version of the novel, so few people have actually read my inner space adventure.
Now, however, there’s some new writing about the exploration of the Earth’s core, but this time it’s not fiction, but a serious scientific proposal. David Stevenson, a Professor of Planetary Science at CalTech, has proposed mounting an ambitious NASA-style mission to the Earth’s core. He describes his "modest" proposal in a paper recently published in the journal Nature. Since Stevenson has not yet mastered the use of the twistor effect, he describes doing things in a way that cannot be accused of small thinking. Among other resources, he would use a multi-megaton nuclear weapon and an hour’s worth of the net iron production of all the Earth’s iron smelter facilities (~108 kg). In this column, I want to describe this proposal.
Stevenson is faced with the basic problem of how to get through all the rock between the Earth’s surface and its core. Anyone who has ever dug a post hole recognizes the problem. Something like Abner Perry’s mole machine, which supposedly took Perry and David Innes to Pellucidar, couldn’t really do the job. We now have well-engineered digging machines designed for efficient tunneling, but they can’t go down more than a few thousand meters. Deep well-drilling techniques are not much better. The deepest drill hole, dug in the Kola Peninsula in Russia, goes down only 12 kilometers.
To avoid mechanical digging, Stevenson has proposed a more radical approach: melt your way through the rock. It takes about a megajoule of energy to melt a cubic meter of rock, assuming that the rock is already hot enough to be near its melting point. Therefore, melting a tunnel that is 3 square-meters across and 6,380 km long, all the way to the Earth’s center, would require about 2 x 1013 joules of energy. That sounds like a lot of energy, but consider that a large nuclear power plant produces about 8 x 1013 joules per day, so we are in the right energy ballpark. The challenge is to find a vehicle that can withstand the heat and pressure of the Earth’s interior while making the trip.
Stevenson’s "vehicle" is a large blob of molten iron. Iron is very heavy, with a density of 7.87 grams per cubic centimeter, as compared to a density of about 2 g/cm3 for rock. Therefore, a large blob of sufficiently hot molten iron would tend to produce a "China Syndrome," melting its way through the Earth’s crust, losing thermal energy, but gaining gravitational energy as it went. Paleontologists believe that the iron at the Earth’s core got there in just that way.
Stevenson would start the process by finding a suitable fissure in the Earth’s crust, setting off a multi-megaton underground nuclear explosion to widen the fissure to a sizable crack, and then dumping in an instrumented blob of liquid iron (melting point 1535° C) with a mass of about 108 kilograms—the amount of iron in a sphere about 30 meters in diameter. The blob would then assume an elongated shape that would fill a part of the crack and cause the crack to propagate downward under the pull of gravity, melting the path in front while liquid magma flowed around the outside of the iron mass and sealed the path behind. As the iron blob moved downward, despite the high pressure from below, it would achieve a fairly high velocity. Assuming that the iron elongates to melt a path about one meter across, its downward speed would be about 30 meters per second. At that speed, it could reach the Earth’s core in about two and a half days.
The problem with this scheme, of course, is that a blob of molten iron that is subjected to the very high pressures in the Earth’s interior does not provide a very good mode of travel. This rules out manned "inner space flight." The "passenger" would have to be neutral buoyancy micro-miniaturized robotic instrumentation that would relay measurements from the core to the surface of the Earth. How to accomplish that information transfer is also a challenging problem.
Today’s microprocessors are made of silicon (melting point 1410° C) and can’t operate at high temperatures. The instrumentation package would either have to be locally insulated and cooled or would require a presently unknown hot microprocessor technology. Further, getting the measurement information from the probe to the Earth’s surface would be very difficult. There could be no trailing wires, no light beams, and no radio waves, so Stevenson proposes to use acoustic signals with a radiated power of about 10 watts for the duration of the mission. The frequency of the acoustic waves is limited at the high end by absorption by the rock and at the low end by seismic noise and the low rate of information transfer. Stevenson proposes an acoustic signal frequency around 100 Hz for sending signals to a surface detector similar to the LIGO gravity-wave detector, but coupled to, rather than insulated from, the Earth’s vibrations. This, he estimates, should allow the transfer of 10 megabytes or so of information during the several-day duration of the mission.
The power supply for the mission is yet another a challenging problem. Conventional batteries and fuel cells do not tolerate high temperatures any better than microprocessors. The thermoelectric nuclear isotope power generation used in some spacecraft would not work because the molten-iron environment is already hotter than the decaying radioactive isotope. Stevenson proposes to use a Stirling-cycle engine to tap into a part of the energy flow that occurs as the iron melts its way to the core, exploiting the temperature difference between the molten iron and the cooler surrounding rock. To me, that sounds difficult, and I also foresee a problem with the generator that the Stirling engine drives, since most magnetic materials, on which standard generators depend, lose most of their magnetic properties in a high temperature environment.
There are sure to be other problems with the ambitious scheme. If the propagating crack containing the blob splits, it may also split the iron mass into two blobs that may not individually be massive enough to continue propagating to the Earth’s core. Also, the envisioned communication link is one-way. As NASA-watchers know, space probes work best when there is two-way communication, permitting course alterations and program alterations to deal with unforeseen problems. Further, the pull of Earth’s gravity downward diminishes linearly as the probe moves deeper, and I see nothing in Stevenson’s calculations that takes this into account. Presumably there is some critical depth at which the iron blob would stall because the pull of gravity is insufficient to move it further or provide more gravitational energy.
