:: By Lydialyle Gibson

Investigations ::

Roughly 4.5 billion years ago, an asteroid the size of Mars slammed into Earth. So violent was the collision that it hurled back into space giant fragments of planet and asteroid, which consolidated to form the moon. Meanwhile, “the entire planet”—mostly rocks and metals—“turned to liquid in an instant,” says geophysical-sciences professor Bruce Buffett, hammering his fist into the C-shaped curl of his other hand. Melted iron and other heavy metals sank into the earth’s center and created a dense, hot, constantly rotating core. “All of this,” Buffett says, “was happening at once.” Soon afterward emerged the planet’s magnetic field, and over the next several hundred million years, oceans and tectonic plates—all dependent, in one way or another, on the dynamo at Earth’s center.

Convective fluid motions inside the earth’s core—the twisting and stretching, rising and sinking of liquid iron—power the planet like a generator. To interpret the core’s fluid motions and map its landscape of iron crystals and hard structures, Buffett, who earned a Harvard PhD in 1990 and came to Chicago three years ago from the University of British Columbia, uses seismic data, evidence of geomagnetic shifts, and computer simulations. “This is detective work,” he says. Buried 3,219 miles beneath the surface, the core is accessible only indirectly. “There’s always that remoteness. You can’t observe the core directly, so you look at other things and try to infer information about the core that way.”

Analyzing millimeter-sized glitches in the earth’s predicted nutation (a periodic rotation-axis wobble caused by the sun and moon’s gravitational tug), Buffett argues that a thin layer of sediments lie between the planet’s core and molten mantle. Those sediments, impurities dissolved out of the ever-cooling and crystallizing core, magnetically connect core and mantle, he says, altering inner-earth fluid motions and creating a minute drag on the nutation. Buffett first outlined his theory in a Science article six years ago and since then has sought to pinpoint the sediment layer’s size, composition—probably silicate minerals, perhaps iron alloys—and its effect on core convection. At last fall’s American Geophysical Union meeting in San Francisco, he presented a paper investigating the structure and evolution of the sediment layer. His findings show that expelling “lighter elements” from the liquid core intensifies convective vigor and “may increase the power available to drive the geodynamo” that continually regenerates the magnetic field protecting the planet from cosmic rays and other solar-system hazards. 

Deciphering core convection, says Buffett—who chairs Study of the Earth’s Deep Interior, an International Union of Geodesy and Geophysics committee—is crucial to explaining the planet’s other physical processes. “Fluid motions generate magnetic fields that produce currents that then reinforce those fields. It’s kind of like pulling yourself up by your own bootstraps. It sounds circular, and it absolutely is. ... The key is to figure out how the fluid moves through the magnetic field.”

In the past ten years geophysicists have turned to computer simulations and mathematical models to trace the core’s thermal evolution and recreate magnetic-field generation. Collaborating with researchers at Harvard, the University of Toronto, and the University of Washington, Buffett spends much of his time trying to build more realistic and reliable models. “You have these scant few observations, and a lot of things we don’t understand,” he says, “and you try to construct models that can explain what we do see.” Often, models that approximate the magnetic field at the earth’s surface yield “surprisingly discordant” interpretations of core-level magnetic-field structure. “In the deeper interior it’s more difficult to test these things, and so you have lots of theories that are competing at the same time.”

To distinguish between competing theories, Buffett makes new use of existing measurements. Magnetic-field fluctuations, global gravity-field variations, and changes in the length of day offer insight into core dynamics—the rate of fluid motions, core-mantle interactions, the shape and alignment of core crystals. He is developing a framework for using these observations to make inferences about the internal magnetic field and its influence on core convection. “You try to devise ways to help constrain physical processes one way or another,” he says. It’s an exercise in narrowing down.

The earth’s peculiarity presents an additional hurdle to computer modelers. If a normal evolutionary trajectory exists, this planet does not adhere to it. “Earth is the only planet that has liquid water, is the only planet that has plate tectonics, is the only planet that has a magnetic field, is the only planet that has life,” tallies Buffett. “People are sort of feeling around in the dark, but it seems that all of these elements are connected.” Unlike astronomers, who can gaze out at a sky full of stars following well-understood and predictable life cycles, geophysicists must work with “a small number of statistics, and even amongst those, there is extraordinary diversity,” Buffett says. “Even just our neighbors, Venus and Mars, have roughly the same composition as Earth, but completely different paths. What’s going on there?”