Earth’s Burning, Churning Engine

This is a direct copy of a SciPop or news article preserved here because things on the internet have a bad habit of disappearing when you try to find them again. Full credit is given to the original authors and the source.

– Matty

With the discovery of the nature of the inner core, the basic components of Earth’s composition — and even the planet’s evolution from its molten origins — were in place. Or so it seemed until recently. New research has uncovered a flaw in our understanding of the core — specifically, about the manner in which heat energy flows from the core and through the overlying mantle. The problem raises important questions about the age of the inner core, and about how Earth generates its magnetic field, a phenomenon crucial for the existence of life.

– Tim Folger, Journeys to the Center of the Earth

Based on the radioactive dating of ancient rocks, scientists estimate that Earth formed about 4.5 billion years ago. As the molten proto-Earth cooled, its outermost layer hardened into a thin crust. Earth’s mantle also solidified over time, though even now the temperature at the lower mantle is about 4,000 F.

The inner core, once entirely liquid, is slowly solidifying from the inside out, increasing its diameter by about half a millimeter per year, according to some estimates. Iron’s melting point is greater at higher pressure, and as the planet cooled, the extreme pressures at the very center of Earth eventually prevented the iron there from continuing to exist as a liquid. Despite sunlike temperatures, the inner core began to solidify, and it’s been growing ever since. Under slightly less pressure, the outer core — a 1,400-mile-deep, 8,000-degree ocean of iron and nickel — is still hot enough to be fluid. “It would flow through your hands like water,” says Bruce Buffett, a geophysicist at the University of California, Berkeley.

All of Earth’s layers, from core to crust, are in constant motion, caused by the flow of heat. Heat moves through Earth’s interior in two fundamentally different ways: convection and conduction. Convection occurs when heat from below creates motion in the layers above — heated material rises, then falls back again as it cools, only to be heated once more. Convection is what roils a pot of boiling soup. Deep inside Earth, slow-motion convection of rocky minerals in the mantle and heat loss from the cooling solid inner core cause convection in the liquid outer core.

Heat also makes its way through the Earth by conduction — the transfer of thermal energy by molecules inside a material from hotter areas to colder ones — without causing any motion. To continue the soup example, heat is conducted through the bottom of the metal pot. The metal in the pot doesn’t move; it simply transmits, or conducts, heat to the pot’s contents. The same is true inside the Earth: In addition to convection currents moving heated material through the outer core and mantle, heat is conducted through liquids and solids without roiling them.

Researchers have known for many decades that the slow, convective sloshing of liquid iron in the outer core, aided by Earth’s rotation, generates the planet’s magnetic field. As the molten iron flows, it creates electric currents, which generate local magnetic fields. Those fields in turn give rise to more electric currents, an effect that results in a self-sustaining cycle called a geodynamo. Evidence from ancient rocks reveals that Earth’s geodynamo has been up and running for at least 3.5 billion years. (When rocks form, their magnetic minerals line up with Earth’s field, and that orientation is preserved when the rocks solidify, providing geophysicists with a record, written in stone, of the planet’s magnetic past.)

But here is the fundamental problem with our understanding of the geodynamo: It can’t work in the way geophysicists have long believed. Two years ago, a team of scientists from two British universities discovered that liquid iron, at the temperatures and pressures found in the outer core, conducts far more heat into the mantle than anyone had thought possible. “Earlier estimates were much too low,” says Dario Alfè, a geophysicist at University College London, who participated in the new research. “The conductivity is two or three times higher than what people used to think.”

The discovery is vexing: If liquid iron conducts heat into the mantle at such a high rate, there wouldn’t be enough heat left in the outer core to churn its ocean of liquid iron. In other words, there would be no heat-driven convection in the outer core. If a pot of soup conducted heat into the surrounding air this effectively, convection would never start, and the soup would never boil. “This is a big problem,” Alfè says, “because convection is what drives the geodynamo. We would not have a geodynamo without convection.”

Alfè and his colleagues used supercomputers to carry out a “first principles” calculation of heat flow in liquid iron at Earth’s core. By first principles, they mean that they solved a set of complex equations that govern the atomic states of iron. They weren’t estimating or extrapolating from lab experiments — they were applying the laws of fundamental quantum mechanics to derive iron’s properties at extreme pressures and temperatures. The British researchers spent several years developing the mathematical techniques used in the equations; only in recent years have computers become powerful enough to solve them.

“It was exciting and scary because we found values that were very different from what people have used,” Alfè says about the discovery. “The first thing you think is, ‘I don’t want to be wrong with this.’ ”

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