CHAPTER 23 | COMPARATIVE PLANETOLOGY OF JUPITER AND SATURN 521
elegant dance, these moons exchange orbits and draw apart only
to meet again and again. It seems very likely that these two
moons are fragments of a larger moon destroyed by a major
impact.
In addition to waltzing coorbital moons, the Voyager space-
craft discovered small moonlets trapped at the L4 and L5
Lagrangian points (see Figure 13-5) in the orbits of Dione and
Tethys. Th ese points of stability lie 60° ahead of and 60° behind
the two moons, and small moonlets can become trapped in these
regions. (You will see in Chapter 25 that some asteroids are
trapped in the Lagrangian points of Jupiter’s and Neptune’s
orbits around the sun.) Th is gravitational curiosity is not unique
to the Saturn system, and it indicates that interactions between
moons can dramatically alter their orbits.
Saturn has too many moons to discuss in detail here, but
you should meet at least one more, Iapetus (pronounced Ee-yap-
eh-tus). It is literally an odd ball: Iapetus is an asymmetric moon.
Its trailing side, the side that always faces backward as it orbits
Saturn, is old, cratered, icy, and about as bright as dirty snow. Its
leading side, the side that always faces forward in its orbit, is also
old and cratered, but it is much darker than you would expect.
It has an albedo of only 4 percent—about as dark as fresh asphalt
on a highway (■ Figure 23-22). Th e origin of this dark material
is unknown, but theorists suspect that the little moon has swept■ Figure 23-21
Saturn’s coorbital moons follow nearly identical orbits. The moon in the
lower orbit travels faster and always overtakes the other moon from behind.
The moons interact and change orbits over and over again.
(Not to scale)Coorbital Moons The inner moon orbits
faster and overtakes
the outer moon.The gravitational
interaction pulls the outer
moon backward and the
inner moon forward.The moons have changed
orbits, and the inner moon
begins gaining on the
outer moon.The inner moon will
eventually overtake the
outer moon from behind
once again.As they approach, the
inner moon moves to a
higher orbit, and the outer
moon sinks to a lower
orbit.Visual-wavelength image■ Figure 23-22
Like the windshield of a speeding car, the leading side of Saturn’s moon
lapetus seems to have accumulated a coating of dark material. The poles and
trailing side of the moon are much cleaner ice. The equatorial ridge is 20 km
(12 mi) wide and up to 13 km (8 mi) high. It stretches roughly 1,300 km
(800 mi) along the moon’s equator. (NASA/JPL/Space Science Institute)