The Solar System

CHAPTER 23 | COMPARATIVE PLANETOLOGY OF JUPITER AND SATURN 519

The Smaller Moons

In addition to Titan, Saturn has a large family of smaller moons.
Th ey are mixtures of rock and ice and are heavily cratered. Some
of the smallest are probably captured objects and are geologically
dead, but some of the larger moons show traces of geological
activity. You can compare the sizes of a few of these moons in
■ Figure 23-18.
Th e moon Phoebe orbits on the outer fringes of Saturn’s
satellite family, and it moves in the retrograde direction—it
orbits backward. It is quite small, only about 210 km (130 mi)
in diameter, but it is nevertheless the largest of Saturn’s irregular

Titan

Mimas

EnceladusDione Iapetus

Tethys Rhea Hyperion

Phoebe

Earth’s moon

for comparison

■ Figure 23-18

A few of Saturn’s moons compared with Earth’s moon at the

right. In general, larger moons are round and more likely

to show signs of geological activity. Small moons such as

Phoebe and Hyperion are cratered and do not have enough

gravity to overcome the strength of their own material and

squeeze themselves into a spherical shape.

satellites. Phoebe’s surface is dark, with an albedo of

only 6 percent, and heavily cratered (■ Figure 23-19).

Traces of ice are detected where impacts have exca-

vated deeper layers or where landslides have exposed

fresh material. Th e density of Phoebe is 1.6 g/cm^3 , which is high

enough to show that it contains a signifi cant amount of rock. It

seems unlikely that Phoebe came from the asteroid belt, where

ice is relatively rare. It is more likely to be a captured Kuiper belt

object that originated in an orbit beyond Neptune.

Larger regular moons such as Tethys, which has a diameter

of over 1000 km (620 mi), are icy and cratered, but they show

some signs of geological activity. Some smooth areas on Tethys

appear to have been resurfaced by fl owing water “lava,” and long

cracks and grooves may have formed when geological activity

strained the icy crust (Figure 23-19).

Phoebe is a small, dark,

cratered moon with no sign

of geological activity.

Long cracks in the cratered

surface of Tethys show the

larger moon has been

geologically active.

Visual-wavelength images

■ Figure 23-19

Phoebe and Tethys have ancient cratered surfaces, but they differ in inter-

esting ways. Phoebe is only^15 the diameter of Tethys and shows no sign of

internal heat causing surface activity. Tethys has smooth areas and long

cracks on its surface, showing that it has been active. Phoebe is probably a

captured Kuiper belt object, but Tethys likely formed with Saturn. (NASA/JPL/

Space Science Institute)