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Evidence for potassium as missing heat source in planetary cores
Motions in the Earth's molten iron core generate convection currents — similar to boiling water, which produce the field.
THERE'S A small problem with Earth's magnetic field: It should not have existed, as Earth's rock record indicates it has, for the past 3.5 billion years. Motions in the Earth's molten iron core generate convection currents — similar to boiling water, which produce the field.
Many sources of heat drive these currents, but the known sources seem inadequate to maintain the field this long. In 1971 University of Minnesota geology and geophysics professor Rama Murthy theorised that radioactive potassium in the core could supply additional heat, but researchers investigating that claim have been unable to obtain reliable experimental data.
In a paper to be published in Nature, Murthy presents experimental evidence for his idea and shows why other researchers have been unable to corroborate it.
The work helps explain how Earth has maintained its magnetic field, which shields the planet from harmful cosmic rays and the constant stream of charged particles from the sun known as the solar wind.
"Earth is losing energy from its surface at a rate of about 44 trillion watts," Murthy said. "About 75 per cent is heat from the mantle (the middle layer, composed of rock), and 20-25 per cent is heat from the core. Measurements of cooling at the core-mantle boundary show too much loss for a core to maintain heat and a magnetic field for 3.5 billion years."
But if radioactive elements such as potassium, and perhaps uranium and thorium, also exist in the core, the heat from their radioactivity could keep the core hot enough to move and maintain the magnetic field, he said.
Earth's core is believed to consist of metallic iron and iron sulphide. Soon after the planet coalesced from a cloud of hot gas and dust 4.5 billion years ago, the core was liquid; since then it has cooled to the point where about 10 per cent is solid.
According to Murthy, a radioactive isotope of the element known as potassium-40 could have been incorporated into Earth's core as it formed. Some scientists have doubted this because potassium is not found in any metal ores at the surface.
However, said Murthy, the core is not pure iron. The presence of sulphur (as iron sulphide) could have allowed potassium to dissolve in the original core material.
Potassium-40, with its half-life of 1.3 billion years, could have supplied enough radioactive heat to keep the core hot enough to maintain the magnetic field for nearly billions of years.
A hot core will dissipate its heat one way or another. If it's only a little hot, the heat will be conducted in a smooth manner into the surrounding mantle, just as heat from warm water will smoothly pass to the container and the air.
But if it's too hot, the heat energy — as in a pot of boiling water — drives convection currents in the core, turning it into a gigantic magnetic generator.
No one can directly study the Earth's core, but scientists can subject samples of core — and mantle — like material to tremendous heat and pressure in the laboratory.
When Murthy and his colleagues subjected samples of iron, iron sulphide and potassium-bearing silicate rock to temperatures and pressures similar to those found at some depth in Earth's mantle, they found that a significant amount of potassium moved from the silicate `mantle' into the metallic iron-iron sulphide `core'.
Extending the results to the temperatures and pressures existing at the actual Earth's core indicated that sufficient potassium could end up in the core to supply the missing heat.
When previous researchers measured movements of potassium into `cores', their data was too scattered to draw any conclusions.
The problem, said Murthy, is that standard procedure calls for the sample material to be polished with oil for analysing its potassium content. But the presence of oil, he found, causes a rapid loss of potassium from the sample.
Instead, Murthy polished his samples dry, using boron nitride powder. The potassium stayed in the samples and produced reliable data.
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