The Solar System

CHAPTER 23 | COMPARATIVE PLANETOLOGY OF JUPITER AND SATURN 507

found that Europa has no magnetic fi eld of its own. It cannot
have a molten conducting core. Tidal heating, however, is impor-
tant for Europa and apparently provides enough heat to keep the
little moon active. In fact, the curving cracks in its crust reveal
the shape of the tidal forces that fl ex it as Europa orbits Jupiter.
If you hiked on Europa with a compass in your hand, you
would detect a magnetic fi eld, but not from Europa itself. Jupiter
rotates rapidly and drags its strong magnetic fi eld past the little
moon. Th at induces a fl uctuating magnetic fi eld at Europa that
would make your compass wander uselessly. Europa’s interaction
with Jupiter’s magnetic fi eld reveals the presence of a liquid-water
ocean lying just 15 km (10 mi) below the icy surface. Th e ocean
may be as deep as 150 km (100 mi) (■ Figure 23-11) and could
contain twice as much water as all the oceans on Earth. It is likely
to be rich in dissolved minerals, which make the water a good elec-
trical conductor and allow it to interact with Jupiter’s magnetic fi eld.
No one knows what might be swimming through such an ocean,
and many scientists hope for a future mission to Europa to drill
through the ice crust and sample the ocean below for signs of life.
Tidal heating makes Europa geologically active. Apparently,
rising currents of water can break through the icy crust or melt
surface patches. Many of the cracks show evidence that they have
spread apart and that fresh water has welled up and frozen
between the walls of the crack. In other regions, compression of
Europa’s crust is revealed by networks of faults and low ridges.
Compression on Earth pushes up mountain ranges, but no such
ranges appear on Europa. Th e icy crust isn’t strong enough to
support ridges higher than a kilometer or so.
Orbiting deep inside Jupiter’s radiation belts, Europa is bom-
barded by high-energy particles that damage the icy surface. Water
molecules are freed and broken up, then dispersed into a

■ Figure 23-11

The gravitational infl uence of Europa on the

passing Galileo spacecraft shows that this

moon has differentiated into a dense core

and rocky mantle. Magnetic interactions

with Jupiter show that it has a liquid-water

ocean below its icy crust. Heat produced by

tidal heating could fl ow outward as convec-

tion in such an ocean and drive geological

activity in the icy crust. (NASA)

Metallic core

Rocky interior

H 2 O layer

Liquid ocean under ice

Ice covering

doughnut-shaped cloud spread round Jupiter and enclosing

Europa’s orbit. Flying past Jupiter in 2002 on its way to Saturn, the

Cassini spacecraft was able to image this cloud of glowing gas.

Europa’s gas cloud is evidence that moons orbiting deep inside a

massive planet’s radiation belts are exposed to a form of erosion

that is entirely lacking on Earth’s moon.

Io: Roaring Volcanoes

Geological activity is driven by heat fl owing out of a planet’s

interior, and nothing could illustrate this principle better than Io,

the innermost of Jupiter’s Galilean moons. Photographs from the

Voyager and Galileo spacecraft show no impact craters at all—

surprising considering Jupiter’s power to focus meteoroids inward

(Figure 23-11b). Th ere is no diffi culty explaining the missing

craters. Over 150 active volcanoes are visible on Io’s surface, blast-

ing enough ash out over the surface to bury any newly formed

craters (■ Figure 23-12). Io is more geologically active than any

other object in the solar system, even more than Earth.

Spectra reveal that Io has a tenuous atmosphere of gaseous

sulfur and oxygen, but those gases can’t be permanent. Even

though the erupting volcanoes pour out about one ton of gases

per second, the gases leak into space easily because of Io’s low

escape velocity. Also, any gas atoms that become ionized are

swept away by Jupiter’s rapidly rotating magnetic fi eld. Th e ions

produce a cloud of sulfur and sodium ions in a torus (a doughnut

shape) enclosing Io’s orbit (■ Figure 23-13).

Th e temperature at the surface averages 130 K (–225°F) and

the atmospheric pressure is very low. Because of the continuous

volcanism and the sulfurous gases, Io’s thin atmosphere is smelly

with sulfur. In fact, the reddish color of Jupiter’s small inner