The Solar System

CHAPTER 24 | URANUS, NEPTUNE, AND THE DWARF PLANETS 537

the other regions. Ariel may have been subject to tidal heating

caused by an orbital resonance with Miranda and Umbriel.

Miranda is a mysterious moon. As you can see in Figure

24-11, it is the smallest of the fi ve large moons, but it appears to

have been the most active. In fact, its active past appears to have

been quite unusual. Miranda is marked by oval patterns of

grooves known as ovoids (■ Figure 24-12). Th ese features were

originally hypothesized to have been produced when mutual

gravitation pulled together fragments of an earlier moon shat-

tered by a major impact. More recent studies of the ovoids show

that they are associated with faults, ice-lava fl ows, and rotated

blocks of crust, suggesting that a major impact was not involved.

Rather, the ovoids may have been created by internal heat driving

large but very slow convection currents in Miranda’s icy mantle.

Certainly, Miranda has had a violent past. Near the equator,

a huge cliff rises 20 km (12 mi). If you stood in your spacesuit at

Umbriel, the next moon inward, is a dark, cratered world
with no sign of faults or surface activity (Figure 24-11). It is the
darkest of the moons, with an albedo of only 0.16 compared
with 0.25 to 0.45 for the other moons. Its crust is a mixture of
rock and ice. A bright crater fl oor in one region suggests that
clean ice may lie at shallow depths in some regions.
Ariel has the brightest surface of the fi ve major Uranian
satellites and shows clear signs of geological activity. It is crossed
by faults over 10 km deep, and some regions appear to have been
smoothed by resurfacing, as you can see in Figure 24-11. Crater
counts show that the smoothed regions are in fact younger than

■ Figure 24-11

The fi ve largest moons of Uranus, shown here in correct relative size scale,
range from the largest, Titania, 46 percent the diameter of Earth’s moon,
down to Miranda, only 14 percent the diameter of Earth’s moon. For a better
view of Miranda, see the next fi gure. (Ariel and Miranda: U.S. Geological Survey,
Flagstaff, Arizona; Other images: NASA)

Oberon has an old, icy,

heavily cratered surface.

Umbriel is a dark, cratered world

with no sign of faults or valleys.

Miranda, the innermost and

smallest of the five main

moons, has the most

extreme signs of past activity.

Deep valleys up to 100 km

across suggest Titania has

had an active past.

The surface of Ariel is

crossed by faults and

broad valleys.