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.