Australian Sky & Telescope - April 2018

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FACING PAGE: NASA-GODDARD SCIENCE VISUALIZATION STUDIO / JPL / NAIF; IRON HEART:


SKY & TELESCOPE /


LEAH TISCIONE; IMAGE: NASA


reversals, an event that would have serious repercussions for
our technology-dependent global economy.
But this concern might be unfounded. Despite its short-
term fluctuations, Earth’s magnetic dynamo has maintained
its powerful dipolar field for billions of years. This is in stark
contrast to our Solar System’s other terrestrial planets, which
raises fascinating questions about whether our planet’s
strong, long-lived magnetic field has been necessary for
preserving our atmosphere and sustaining life.

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 centre 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,
km, making it about 70% the size of the Moon. Its surface
temperature is 4,750° to 5,450°C, close to 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 produce our planet’s robust
magnetic field.
The entire core continues to cool very slowly — just
100°C per billion years — and its solid centre 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 vigour of the convection is related to how
much heat can escape from the core through the core-
mantle 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 organises 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 centre, 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,
km) into space, where they’re balanced by the solar wind.

Short-term chaos
Magnetic signatures preserved in ancient rocks show that
Earth’s dynamo has been operating at more or less its current
healthy level for almost all of our planet’s history. But when
geophysicists zoom in with their equivalent of a microscope,
they see crazy things happening.
For example, they can now track changes in

Today, Earth’s inner
core has an estimated
diameter of 2,440 km,
making it about 70%
the size of the Moon.

W 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)

Outer core
(liquid)

Mantle
Crust
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