Australian Sky & Telescope - April 2018

18 AUSTRALIAN SKY & TELESCOPE April 2018

CHAOTIC REVERSAL: GARY GLATZMAIER & PAUL ROBERTS; JUPITER: NASA / ESA / A. SIMON; FACING PAGE: NASA-GODDARD SPACE SCIENTIFIC VI

SUALIZATION STUDIO

no measurable global field at all. The Moon and Mars lack

a global field, but their rocks have locked in patches of

ancient crustal magnetism. The most pronounced fields on

Mars appear in a series of magnetic stripes of alternating

polarity in the southern hemisphere. The rocks in these linear

features preserve a record of a dipolar field that experienced

reversals before it shut down eons ago.

Geophysicists wonder why Mercury’s magnetic field is not

stronger, and why Venus can’t whip up a magnetic field at

all. Mercury has a huge iron-rich core that’s at least partially

molten, and Venus has almost the same size, mass and bulk

density as Earth. In one or both cases, the lower mantle

might be acting as an impenetrable lid — inhibiting heat from

escaping from the core, stifling vigorous convection and thus

preventing a dynamo from revving up.

Plate tectonics might be the critical factor that explains

why Earth has a magnetic field and these other worlds do not.

In a process known as subduction, which occurs where crustal

plates collide, slabs of cold oceanic crust sink all the way

down to the core-mantle boundary, making it easy for heat

to escape the core through the lower mantle. This cooling

mechanism drives the churning convective motions in the

outer core that stirs up Earth’s dynamo.

Another recent idea suggests that Venus lacks a global field

because its core has distinct compositional layers, like an

onion, that prevent the kind of wholesale convection that’s

taking place inside Earth.

The lack of present-day magnetism within the Moon and

Mars is less mysterious. Both worlds once had fields, perhaps

substantial ones, but they’re both so small that their interiors

would have cooled off rather quickly. Once the liquid in their

cores froze, any dynamo action would have ceased.

Magnetic fields and life

Earth and its inhabitants have been lucky. Our planet’s

magnetic influence extends deep into space, providing

an invisible force field that at least partially shields our

atmosphere from the ravages of the solar wind and cosmic

radiation.

By shielding a planet’s atmosphere, a strong global

magnetic field can also play a critical role in keeping

liquid water stable on the surface for the billions of

years needed for primitive life to evolve into more

complex forms. It’s easy to draw the conclusion

that having a respectable magnetic field for

billions of years was essential to preserving

Earth’s atmosphere and nurturing the evolution

of complex life.

Anyone making this argument can point to Mars.

The Red Planet has lacked a global magnetic field for

most of its history, with dire consequences for the Martian

atmosphere. NASA’s MAVEN orbiter has documented how

W CHAOTIC

REVERSAL These three

snapshots show the

contorted state of the

dipolar magnetic field in

Earth’s liquid outer core

during three stages in

a computer-simulated

field reversal. Yellow

denotes outward-

directed (with north

polarity) field lines; blue

denotes inward-directed

(south) lines.

The giant planets

Earth’s magnetic field pales in comparison to those

of Jupiter and Saturn. In particular, Jupiter’s mighty

dynamo generates a magnetic field that’s roughly

20,000 times more powerful than ours. Its

magnetosphere extends deep into space, with

the oomph to deflect the solar wind 3 million

km before it reaches the planet. Jupiter’s

magnetic tail extends all the way to the orbit

of Saturn. NASA’s Juno spacecraft recently

found that Jupiter’s field, like Earth’s, changes

significantly from one location to another. This

local variability suggests that the dynamo region is

closer to the planet’s cloudtops than previously thought.

500 years

before midpoint

500 years

after midpoint

Midpoint

FIELD FLIP AHEAD?