By Nathalie Angier, The New York Times, May 28, 2012
As if the inside story of our planet weren’t already the ultimate potboiler, a host of new findings has just turned the heat up past Stygian.
A diagram of the Earth's center as a giant ball of fire from the 1678 book "Subterranean World."
Geologists have long known that Earth’s core, some 1,800 miles beneath our feet, is a dense, chemically doped ball of iron roughly the size of Mars and every bit as alien. It’s a place where pressures bear down with the weight of 3.5 million atmospheres, like 3.5 million skies falling at once on your head, and where temperatures reach 10,000 degrees Fahrenheit — as hot as the surface of the Sun. It’s a place where the term “ironclad agreement” has no meaning, since iron can’t even agree with itself on what form to take. It’s a fluid, it’s a solid, it’s twisting and spiraling like liquid confetti.
Researchers have also known that Earth’s inner Martian makes its outer portions look and feel like home. The core’s heat helps animate the giant jigsaw puzzle of tectonic plates floating far above it, to build up mountains and gouge out seabeds. At the same time, the jostling of core iron generates Earth’ magnetic field, which blocks dangerous cosmic radiation, guides terrestrial wanderers and brightens northern skies with scarves of auroral lights.
Now it turns out that existing models of the core, for all their drama, may not be dramatic enough. Reporting recently in the journal Nature, Dario Alfè of University College London and his colleagues presented evidence that iron in the outer layers of the core is frittering away heat through the wasteful process called conduction at two to three times the rate of previous estimates.
The theoretical consequences of this discrepancy are far-reaching. The scientists say something else must be going on in Earth’s depths to account for the missing thermal energy in their calculations. They and others offer these possibilities:
¶ The core holds a much bigger stash of radioactive material than anyone had suspected, and its decay is giving off heat.
¶ The iron of the innermost core is solidifying at a startlingly fast clip and releasing the latent heat of crystallization in the process.
¶ The chemical interactions among the iron alloys of the core and the rocky silicates of the overlying mantle are much fiercer and more energetic than previously believed.
¶ Or something novel and bizarre is going on, as yet undetermined.
“From what I can tell, people are excited” by the report, Dr. Alfè said. “They see there might be a new mechanism going on they didn’t think about before.”
Researchers elsewhere have discovered a host of other anomalies and surprises. They’ve found indications that the inner core is rotating slightly faster than the rest of the planet, although geologists disagree on the size of that rotational difference and on how, exactly, the core manages to resist being gravitationally locked to the surrounding mantle.
Miaki Ishii and her colleagues at Harvard have proposed that the core is more of a Matryoshka doll than standard two-part renderings would have it. Not only is there an outer core of liquid iron encircling a Moon-size inner core of solidified iron, Dr. Ishii said, but seismic data indicate that nested within the inner core is another distinct layer they call the innermost core: a structure some 375 miles in diameter that may well be almost pure iron, with other elements squeezed out. Against this giant jewel even Jules Verne’s middle-Earth mastodons and ichthyosaurs would be pretty thin gruel.
Core researchers acknowledge that their elusive subject can be challenging, and they might be tempted to throw tantrums save for the fact that the Earth does it for them. Most of what is known about the core comes from studying seismic waves generated by earthquakes.
As John Vidale of the University of Washington explained, most earthquakes originate in the upper 30 miles of the globe (as do many volcanoes), and no seismic source has been detected below 500 miles. But the quakes’ energy waves radiate across the planet, detectably passing through the core.
Granted, some temblors are more revealing than others. “I prefer deep earthquakes when I’m doing a study,” Dr. Ishii said. “The waves from deep earthquakes are typically sharper and cleaner.”
Dr. Ishii and other researchers have also combed through seismic data from the human equivalent of earthquakes — the underground testing of nuclear weapons carried out in the mid- to late 20th century. The Russian explosions in particular, she said, “are a remarkably telling data set,” adding that with bombs, unlike earthquakes, the precise epicenter is known.
Some researchers seek to simulate core conditions on a small, fleeting scale: balancing a sample of iron alloy on a diamond tip, for example, and then subjecting it to intense pressure by shooting it with a bullet. Others rely on complex computer models. Everybody cites a famous paper in Nature in 2003 by David J. Stevenson, a planetary scientist at Caltech, who waggishly suggested that a very thin, long crack be propagated in the Earth down to the core, through which a probe in a liquid iron alloy could be sent in.
“Oh, the things we could learn, if only we had unlimited resources,” Dr. Ishii sighed.
The core does leave faint but readable marks on the surface, by way of the magnetic field that loops out from the vast chthonic geodynamo of swirling iron, permeating the planet and reaching thousands of miles into space. Magnetic particles trapped in neat alignment in rocks reveal that the field, and presumably the core structures that generate it, has been around for well over 3 billion of Earth’s 4.5 billion years.
For reasons that remain mysterious, the field has a funny habit of flipping. Every 100,000 to a million years or more, the north-south orientation of the magnetosphere reverses, an event often preceded by an overall weakening of the field. As it turns out, the strength of our current north-pointing field, which has been in place for nearly 800,000 years, has dropped by about 10 percent in the past century, suggesting we may be headed toward a polarity switch. Not to worry: Even if it were to start tomorrow, those of us alive today will be so many particles of dust before the great compass flip-flop is through.
The portrait of the core emerging from recent studies is structured and wild, parts of it riven with more weather than the sky. Earth assumed its basic layered effect as it gravitationally formed from the rich, chunky loam of the young solar system, with the heaviest ingredients, like iron and nickel, migrating toward the center and lighter rocky material bobbing above.
Traces of light, abundant elements that bond readily with iron were pulled coreward, too, and scientists are trying to figure out which mix of oxygen, sulfur or other impurities might best match the seismic data and computer models. Distinct boundaries of state or substance distinguish the different layers — between the elastic rock of the mantle and the iron liquid of the outer core, and between the liquid outer core and the solid inner core.
The core accounts for only one-sixth of the volume of the Earth but one-third of its mass, the great bulk of iron maintained in liquid form by the core’s hellish heat. “Liquid” in this case doesn’t mean molten like lava. “If you could put on your safety gloves and stick your hands into the outer core, it would run through your fingers like water,” said Bruce Buffett, a geologist at the University of California, Berkeley.
“The viscosity is so low and the scale of the outer core so large,” Dr. Buffett added, “that the role of turbulence is a relevant feature in how it flows. Think planetary atmosphere, or large jet streams.” Only in the inner core does pressure win out over temperature, and the iron solidify.
The core’s thermal bounty is thought to be overwhelmingly primordial, left over from the planet’s gravitational formation and mostly trapped inside by the rocky muffler of the mantle. Yet as the hot Earth orbits relentlessly through frigid space, the core can’t help but obey the second law of thermodynamics and gradually shed some of its stored heat.
The heat can be transferred through two basic pathways: conducted straight outward, the way heat travels along a frying pan, or convected out in plumes, the way hot air rises in the atmosphere or soup bubbles in a pot.
Conduction is considered a wasted or even boring form of energy transfer — heat moves, but the Earth does not. Convection, by contrast, is potentially industrious. Convection currents are what ripple through the mantle and shuffle around the tectonic plates, and convection stokes the geodynamo that yields our switching field.
In their report in Nature, Dr. Alfè and his colleagues used powerful computers and basic considerations of atomic behavior to calculate the properties of iron and iron alloys under the presumed conditions of the core. They concluded that the core was losing two to three times as much heat to conduction as previously believed, which would leave too little thermal energy to account for the convective forces that power the Earth’s geodynamo. Hence the need to consider possible sources of additional heat, like stores of radioactive potassium or thorium, or a fast-crystallizing inner core.
Dr. Buffett suggests that water on the surface may also help Earth balance its thermal budget, — by slightly weakening the Earth’s rocky plates and making them more readily churned and recycled in a vigorous, sustainable convective stew.
Life needs water, and maybe its planet does, too.