The Solar System

CHAPTER 23 | COMPARATIVE PLANETOLOGY OF JUPITER AND SATURN 509

Great lava fl ows can be detected carrying molten material
downhill, burying the surface under layer after layer. Sometimes
lava bursts upward through faults to form long lava curtains, a
form of eruption seen in Hawaii. Both of these processes are
shown in Figure 23-12.
What powers Io? It has abundant internal heat, but it is only
5 percent bigger than Earth’s moon, which is cold and dead. Io
is too small to have retained heat from its formation or to remain
hot from radioactive decay. In fact, the energy blasting out of its
volcanoes adds up to about three times more energy than it could
make by radioactive decay in its interior.
Th e answer is that Io is heated by a stronger version of the
kind of tidal heating that has aff ected Ganymede and Europa.
Because Io is so close to Jupiter, the tides it experiences are power-
ful and should have forced Io’s orbit to become circular long ago.
Io, however, is strongly infl uenced by its neighboring moons. Io,
Europa, and Ganymede are locked in an orbital resonance; in the
time it takes Ganymede to orbit once, Europa orbits twice and Io
four times. Th is gravitational interaction keeps the orbits, espe-
cially Io’s, slightly eccentric; and Io, also being closest to Jupiter,
suff ers dramatic tides, with its surface rising and falling by about
100 m. For comparison, tides on Earth move the solid ground by

Visible + ultraviolet

Visual + radio

Ion and neutral camera

Jupiter Io

Io sulfur cloud

Jupiter’s radiation belts wobble

as the planet rotates because

the magnetic field is inclined to

the axis of rotation.

The most intense

radiation belts are

located near Jupiter.

Io plasma torus deep inside

Jupiter’s magnetic field

■ Figure 23-13

Sulfur venting from Io is trapped in Jupiter’s

magnetic fi eld to form a torus around the

moon’s orbit. Ions trapped in Jupiter’s magne-

tosphere form radiation belts that enclose all

four of the Galilean satellites. A fainter torus

of water atoms is located around the orbit of

Europa. (Sulfur cloud: NASA/JPL/Bruce A. Goldberg;

Ion and Neutral image: NASA/JPL/Johns Hopkins Univ.

Applied Physics Lab; Radio: NASA/JPL))

only a few centimeters. Th e resulting friction in Io is enough to

melt the interior and drive volcanism. In fact, there is enough

energy fl owing outward to continually recycle Io’s crust. Deep

layers melt, are spewed out through the volcanoes to cover the

surface, and are later covered themselves until they are buried so

deeply that they are again melted.

Th e four Galilean moons show a clear sequence of more and

more tidal heating the nearer they are to Jupiter. Th e more distant

moons have geologies dominated by impacts, while the closer

moons are dominated by heat fl ow from inside and have few

craters. What a diff erence a few hundred thousand miles makes!

The History of the Galilean Moons

Each time you have fi nished studying a world, you have tried to

summarize its history. Now you have studied a system of four

small worlds. Can you tell their story? To do that you need to

draw on what you have learned about the moons and on what

you have learned about Jupiter and the origin of the solar system

(Chapter 19).

Th e minor, irregular moons of Jupiter are probably captured

asteroids, but the regular Galilean satellites seem to be primor-

dial. Th at is, they formed with Jupiter. Also, they seem to be