Sky and Telescope - March 2018 - 17

Today the inner core
has an estimated
diameter of 2,440 km
(1,520 miles), making
it about 70% the size
of the Moon.

FACING PAG E: N ASA- GODDA RD SCIENCE VISUA LIZ ATION ST UDIO / JPL / N AIF;
IRON HE ART: SK Y & TELESCOPE / LE AH TISCIONE; IMAGE: NASA

Earth's Dynamo
The origin of Earth's magnetic ﬁeld can be
traced back to our planet's formation 4.54 billion
years ago. As countless chunks of interplanetary matter
collected into an ever-larger sphere, they delivered abundant
iron, whose high density caused it to sink through the infant
Earth's molten interior toward the center in a process known
as differentiation. Iron became the dominant material in the
core, intermixed with small amounts of several siderophile
("iron-loving") elements such as nickel and sulfur.
Although the core was entirely molten for most of Earth's
history, over billions of years it cooled gradually yet steadily
by conducting its heat outward through the mantle's base.
According to recent research, the inner core started to
solidify into an iron-nickel alloy sometime between 1 billion and 600 million years ago. That's when temperatures
dropped below the point at which these metals can remain
molten under such tremendous pressure - some 3½ million
times the atmospheric pressure at sea level.
Today the inner core has an estimated diameter of 2,440
km (1,520 miles), making it about 70% the size of the
Moon. Its surface temperature is 5,000 to 5,700 K, close to

We depend on a
global electromagnetic
bubble to protect us
from hazardous space
radiation. Is that stalwart
defense wavering?

Mantle
Crust
Outer core
(liquid)

t IRON HEART Earth's iron-nickel
core takes up about half of its radius
and roughly 15% of its volume. Our
planet's magnetic field is generated
by convective motions in the liquid
outer core.

Inner core
(solid)

that of the Sun's photosphere. But at the
lower pressures in the outer core, a layer
2,250 km thick, the iron and nickel remain in
a molten state.
Our planet's magnetic ﬁeld arises in this still-liquid
outer core via what's called a dynamo. Thanks to decades of
seismic studies, laboratory experiments, computer simulations, and other techniques, geophysicists have developed a
model of how this process operates. A planetary dynamo can
arise wherever an electrically conducting ﬂuid undergoes the
cyclic motion known as convection. We see convective currents in a pot of boiling water, with hot bubbles rising to the
top and cooler water sinking to the bottom.
Likewise, the outer core's liquid iron rises, transfers
heat to the lower mantle, becomes denser as it cools, and
then sinks in an ongoing convective cycle. Thanks to iron's
conductive properties, this churning ﬂuid motion generates
strong electrical currents that create our planet's robust
magnetic ﬁeld.
The entire core continues to cool very slowly - just
100°C per billion years - and its solid center continues to
enlarge. Eventually, the last of the outer core's liquid iron
will solidify, turning off the dynamo. But don't hit the panic
button. "It will be billions of years before the inner core
freezes the entire core," says Brad Foley (Penn State University). "There's nothing for us to worry about."
Even though the dynamo arises in the outer core, the
mantle plays a passive but vital role in sustaining the magnetic ﬁeld. As Sabine Stanley (Johns Hopkins University)
explains, "The vigor of the convection is related to how
much heat can escape from the core through the coremantle boundary." If the mantle were ever to block the heat
ﬂowing from the outer core, the convective motions would
grind to a halt and the dynamo would shut down.
Earth's rotation also plays an important role, though it
does not power the dynamo by itself. Instead, our planet's
spin organizes the convective motions in the outer core to
produce a strong dipolar ﬁeld closely aligned with the rotation axis. This gives Earth the outward appearance of having a bar magnet at its center, with invisible lines of force
emanating outward at the north magnetic pole (which is in
the Southern Hemisphere) and coming in at its south pole.
Those lines currently extend about nine Earth radii (57,000
km) into space, where they're balanced by the solar wind.
sk yandtelescope.com * M A RCH 2 018

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Sky and Telescope - March 2018

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