Australian Sky & Telescope - April 2018

http://www.skyandtelescope.com.au 15

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