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Evidence For Potassium As Missing Heat Source In Planetary Cores
University Of Minnesota Press Release
May 8, 2003

MINNEAPOLIS / ST. PAUL -- 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 theorized 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 Thursday (May 8) 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 percent is heat from the
mantle [the middle layer, composed of rock], and 20 to 25 percent 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
sulfide. 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 percent 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 sulfur (as iron sulfide) 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 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 sulfide 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 sulfide "core." Extending the results to 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 analyzing 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. 

Because it is white, the boron nitride powder caused a minor
difficulty. "I was afraid to travel with it, because I obtained it
from a company the same week the anthrax scare happened," said
Murthy. "So I shipped the powder to our Washington laboratory."
Murthy and his colleagues performed the work at the Geophysical
Laboratory of the Carnegie Institution of Washington, D.C. 

The next step, says Murthy, is to study how potassium moves into core
material at higher temperatures and pressures, closer to conditions
deep in Earth's mantle. He said similar experiments should be done to
see whether uranium and thorium could also have moved into the core
during the planet's formation. As the sources of Earth's inner heat
get sorted out, Murthy said the new knowledge will refine ideas about
how continents drift. Continents are part of Earth's crust, or thin
outer layer. They are thought to move around by convection currents
in the mantle, but whether those currents exist in the whole mantle
or just the upper part is still an open question, Murthy said.

"We can also simulate the temperature and pressure in the core of
Mars because the planet is much smaller than Earth," said Murthy.
"Mars has more sulfur in its core than Earth, so it ought to have
collected more potassium." The Red Planet once had a magnetic field,
but it was lost. The probable culprit is the small size of Mars,
which caused it to lose too much heat to maintain convection in its
core, said Murthy. He said Mars' core may be entirely liquid now,
losing heat just by conduction, rather than the convection needed to
produce a magnetic field. 

Murthy's colleagues were Wim van Westrenen and Yingwei Fei of the
Carnegie Institution. Van Westrenen is also affiliated with the
Institute for Mineralogy and Petrography in Zurich, Switzerland. The
work was supported by the University of Minnesota, NASA, the Carnegie
Institution and the Swiss National Science Foundation.

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