Sky and Telescope - March 2018 - 20

Field Flip Ahead?
t 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 computersimulated field reversal.
Yellow denotes outwarddirected (with north
polarity) field lines; blue
denotes inward-directed
(south) lines.

Our Planetary Neighbors

Midpoint

500 years
after midpoint

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 (see page 13).
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.

Although our ﬁeld is considerably weaker than those of the
gas giants, Earth stands alone as the solar system's only
terrestrial planet that can currently muster a strong global
magnetic ﬁeld.
Mercury has a very weak dipolar ﬁeld with an average
surface strength only about 1% that of Earth's. Venus has
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.
Magnetic Fields and Life
Geophysicists wonder why Mercury's magnetic ﬁeld is not
So, geophysically speaking, Earth and its inhabitants really
stronger, and why Venus can't whip up a magnetic ﬁeld at all.
lucked out. Our planet's magnetic inﬂuence extends deep
Mercury has a huge iron-rich core
into space, providing an invisible
that's at least partially molten,
force ﬁeld that at least partially
and Venus has almost the same
shields our atmosphere from the
The Giant Planets
size, mass, and bulk density as
ravages of the solar wind and
Earth's magnetic field pales in comparison
Earth. In one or both cases, the
cosmic radiation.
to those of Jupiter and Saturn. In particular,
lower mantle might be acting as
By shielding a planet's atmoJupiter's mighty dynamo generates a magnetic
an impenetrable lid - inhibiting
sphere,
a strong global magnetic
field that's roughly 20,000 times more powerheat from escaping from the core,
ﬁ
eld
can
also play a critical role
ful than ours. Its magnetosphere extends deep
stiﬂing vigorous convection and
in
keeping
liquid water stable
into space, with the oomph to deflect the solar
thus preventing a dynamo from
on
the
surface
for the billions of
wind 3 million km before it reaches the planet.
revving up.
years
needed
for
primitive life
Jupiter's magnetic tail extends all the way to the
Plate tectonics might be the
to
evolve
into
more
complex
orbit of Saturn. NASA's Juno spacecraft
critical factor that explains why
forms.
It's
easy
to
draw the
recently found that Jupiter's field, like
Earth has a magnetic ﬁeld and
conclusion
that
having
Earth's, changes significantly from
these other worlds do not. In a
a respectable magnetic
one location to another (S&T: Dec.
process known as subduction,
ﬁeld for billions of years
2017, p. 14). This local variability
which occurs where crustal plates
was essential to preservsuggests that the dynamo region
collide, slabs of cold oceanic crust
ing
Earth's atmosphere
is closer to the planet's cloudtops
sink all the way down to the coreand
nurturing
the evoluthan previously thought.
mantle boundary, making it easy
tion of complex life.
20

M A RCH 2 018 * SK Y & TELESCOPE

CH AOTIC RE V ERSA L: G A RY G L AT ZM AIER & PAUL ROBERTS; JUPITER: N ASA / ESA / A . SIM ON;
FACING PAG E: N ASA- GODDA RD SPACE SCIEN TIFIC VISUA LIZ ATION ST UDIO

500 years
before midpoint

Sky and Telescope - March 2018

Table of Contents for the Digital Edition of Sky and Telescope - March 2018