The Solar System

522 PART 4^ |^ THE SOLAR SYSTEM

gravitationally so that the orbits they now occupy may diff er

signifi cantly from their earlier orbits.

Th e complex orbital relationships of Saturn’s moons and

their evidently intense cratering suggest that the moons have

interacted and may have collided with each other, with comets,

and with large planetesimals in the past. Nevertheless, as with

Jupiter’s moons, astronomers hypothesize that most or all of

Saturn’s regular moons formed with the planet and that the

irregular moons are captured. As you continue your exploration

of the outer solar system, you can be alert for the presence of

more such small, icy worlds.

up dark, silicon- and carbon-rich material on its leading side. Th e
source of the material covering the leading side of Iapetus could
be meteorites striking and eroding the carbonaceous surface of
the outermost moon, Phoebe, and tossing the resulting dust into
space to be scooped up eventually by Iapetus.
Another odd feature on Iapetus shows up in Cassini images—an
equatorial ridge that stands as high as 13 km (8 mi) in some places.
You can see the ridge clearly in Figure 23-21. Th e origin of this
ridge is unknown, but it is not a minor feature. At that height, it is
over 50 percent higher than Mount Everest, and it extends for a
long distance across the surface. Th at is a big pile of rock and ice.
Th e ridge sits atop an equatorial bulge, and both ridge and bulge
may have formed when Iapetus was young, spun rapidly, and was
still mostly molten.
Saturn’s moons illustrate a number of principles of compara-
tive planetology. Small moons are irregular in shape, and old
surfaces are dark and cratered. Resonances can trigger tidal heat-
ing, and that can in turn resurface moons and outgas atmo-
spheres. Small moons can’t keep atmospheres, but big, cold
moons can. You are an expert in all of this, so you are ready to
wonder where the moons came from.

The Origin of Saturn’s Moons

Jupiter’s four Galilean satellites seem clearly related to one
another, and you can safely conclude that they formed with
Jupiter. No such simple relationships link Saturn’s satellites. Th at
seems to indicate that, unlike Jupiter, Saturn was not enough of
a heat source during that system’s formation to cause the densi-
ties of its regular moons to follow the condensation sequence.
Planetary scientists also suspect that comet impacts have so badly
fractured the regular moons that they no longer show much evi-
dence of their common origin. Understanding the origin of
Saturn’s moons is also diffi cult because the moons interact

SCIENTIFIC ARGUMENT

What features on Enceladus suggest that it has been active?

This argument is based on comparative planetology applied, in this

case, to moons. The smaller moons of Saturn are icy worlds battered

by impact craters, and you can suspect that they are cold and old.

Small worlds lose their heat quickly; and, with no internal heat,

there is no geological activity to erase impact craters. A small,

icy world covered with craters is exactly what you would expect in

the outer solar system. Enceladus, however, is peculiar. Although

it is small and icy, its surface is highly refl ective, and some areas

contain fewer craters than you would expect. In fact, some regions

seem almost free of craters. Grooves and faults mark some regions

of the little moon and suggest motion in the crust. These features

should have been destroyed long ago by impact cratering, so you

must suppose that the moon has been geologically active at some

time since the end of the heavy bombardment at the conclusion of

planet building. The water vents discovered at the south pole of

Enceladus show the moon is still active.

If you look at Titan, Saturn’s largest moon, you see quite a

different world. Build an argument based on a different principle

of comparative planetology. How can such a small world as Titan

keep a thick atmosphere?

PART 4 | THE SOLAR SYSTEM

What Are We? Basic Scientists

People often describe science that has no

known practical value as basic science or

basic research. The exploration of distant

worlds would be called basic science, and it

is easy to argue that basic science is not

worth the effort and expense because it has

no known practical use. Of course, the

problem is that no one has any way of

knowing what knowledge will be of use until

that knowledge is acquired.

In the middle of the 19th century,

Queen Victoria asked physicist Michael

Faraday what good his experiments with

electricity and magnetism were. He

answered, “Madam, what good is a baby?”

Of course, Faraday’s experiments were the

beginning of the electronic age. Many of

the practical uses of scientifi c knowledge

that fi ll your world—digital electronics,

synthetic materials, modern vaccines—

began as basic research. Basic scientifi c

research provides the raw materials that

technology and engineering use to solve

problems, so to protect its future, the

human race must continue its struggle to

understand how nature works.

Basic scientifi c research has yet one

more important use that is so valuable it

seems an insult to refer to it as merely

practical. Science is the study of nature, and

as you learn more about how nature works,

you learn more about what your existence in

this universe means. The seemingly

impractical knowledge gained from space

probes visiting other worlds tells you about

your own planet and your own role in the

scheme of nature. Science tells us where we

are and what we are, and that knowledge is

beyond value.