The Solar System

536 PART 4^ |^ THE SOLAR SYSTEM

moons orbiting Uranus. Roughly 30 are currently known, but

there are almost certainly more to be found.

Th e smaller moons are all as dark as coal. Th ey are icy worlds

with surfaces that have been darkened by impacts vaporizing

ice and concentrating embedded dirt. In addition, they orbit

inside the radiation belts, and that radiation can convert meth-

ane trapped in their ices into dark carbon deposits to further

darken their surfaces.

More is known about the fi ve larger moons. Th ey are all tid-

ally locked to Uranus, and that means their south poles were

pointed toward the sun in 1986. Voyager could not photograph

their northern hemispheres, so the analysis of their geology must

depend on images of only half their surfaces. Th e densities of the

moons suggest that they contain relatively large rock cores sur-

rounded by icy mantles, as shown in Figure 24-10.

Oberon, the outermost of the large moons, has a cratered

surface, but there is visible evidence that it was once geologically

active (■ Figure 24-11). A large fault crosses the sunlit hemi-

sphere, and dark material, perhaps dirty water “lava,” appears to

have fl ooded the fl oors of some craters.

Titania is the largest of the fi ve moons and has a heavily

cratered surface, but it has no very large craters (Figure 24-11).

Th is suggests that after the end of the heavy bombardment, the

young Titania underwent an active phase in which its surface was

fl ooded with water that covered early craters with fresh ice. Since

then, the craters that have formed are not as large as the largest

of those that were erased. Th e network of faults that crosses

Titania’s surface is another sign of past activity.

gas. If a planet had no moons, its rings would spread out into a
more and more tenuous sheet until they were gone. Th e spread-
ing rings can be anchored by small shepherd moons, which
interact gravitationally with wandering ring particles, absorb
orbital energy, and cause the particles to remain within the rings.
As they gain orbital energy, these small moons move slowly out-
ward, but they can be anchored in turn by orbital resonances
with larger, more distant moons that are so massive they do not
get pushed outward signifi cantly. In this way, a system of moons
can confi ne and preserve a system of planetary rings.

The Moons of Uranus

Uranus has fi ve large regular moons that were discovered from
Earth-based observations (■ Figure 24-10). Th ose fi ve moons,
from the outermost inward, are Oberon, Titania, Umbriel, Ariel,
and Miranda. Th e names Umbriel and Ariel are names from
Alexander Pope’s Th e Rape of the Lock, and the rest are from
Shakespeare’s A Midsummer Night’s Dream and Th e Tempest (an
Ariel also appears in Th e Tempest). Spectra show that the moons
contain frozen water, although their surfaces are dark. Planetary
scientists assumed they were made of dirty ices, but little more
was known of the moons before Voyager 2 fl ew through the
system.
In addition to imaging the known moons, the Voyager 2
cameras discovered ten more moons too small to have been seen
from Earth. Since then, the construction of new-generation tele-
scopes and the development of new imaging techniques (see
Chapter 6) have allowed astronomers to fi nd even more small

■ Figure 24-10

This infrared image shows Uranus, its rings, and its fi ve major satel-

lites. Because of photographic effects, the images of the satellites

are much larger than the satellites themselves. The largest, Titania,

in reality has only 1/32 the diameter of Uranus. The densities of the

fi ve major satellites of Uranus suggest that they contain relatively

large rocky cores with mantles of dirty ice. The size of Earth’s moon is

shown for comparison. (ESO)

Umbriel

diameter

1190 km

Titania

diameter

1610 km

Infrared image

Titania

Umbriel

Miranda

Ariel

Oberon

Ice Ice

Ice

Ice

Ice

Rock Rock Rock Rock Rock

Earth’s moon

Oberon

diameter

1550 km

Ariel

diameter

1160 km

Miranda

diameter

480 km