The Solar System

CHAPTER 24 | URANUS, NEPTUNE, AND THE DWARF PLANETS 533

these rings coincides with the orbit of the small moon Mab and

is probably replenished by particles blasted off of the moon by

meteorite impacts. Th e smaller of the rings is confi ned between

the orbits of the moons Portia and Rosalind.

As you read about planetary rings, notice their close relation-

ship with moons. Because of collisions among ring particles,

planetary rings tend to spread outward, almost like an expanding

around its orbit, so the geometry of its interaction with the solar
wind is now diff erent. Unfortunately, no spacecraft orbit Uranus
at present, and there is no way to observe eff ects of these changes
in detail.
Like the magnetic fi eld, the temperature of Uranus can
reveal something about its interior. Jupiter and Saturn are
warmer than you would expect, given the amount of energy they
receive from the sun, and this means that heat is leaking out from
their hot interiors. Uranus, in contrast, is about the temperature
you would expect for a world at its distance from the sun.
Apparently, it has lost much of its interior heat. Yet, it must have
some internal heat to cause convection in the fl uid mantle and
drive the dynamo eff ect. Th e temperature in its core is estimated
from models to be about 8000 K. Th e decay of natural radioac-
tive elements would generate heat, but some astronomers have
suggested that the slow settling of heavier elements through the
fl uid mantle could also release energy to warm the interior.
Laboratory studies of methane show that it can break down
under the temperature and pressure found inside Uranus and
form various compounds plus pure carbon in the form of dia-
monds. If this happens in Uranus, the diamond crystals would
fall inward, warming the interior through friction. Determining
for sure whether a planetwide rain of diamonds actually exists
inside Uranus is probably forever beyond human reach.
For a Jovian world, Uranus seems small and mostly feature-
less. But now you are ready to visit one of its best attractions—its
rings.

The Rings of Uranus

Both Uranus and Neptune have rings that are more like those of
Jupiter than those of Saturn. Th ey are dark, faint, not easily vis-
ible from Earth, and confi ned by shepherd satellites.
Study The Rings of Uranus and Neptune on
pages 534–535 and notice three important points about the
rings of Uranus and one new term:

Th e rings of Uranus were discovered during an occultation

when Uranus crossed in front of a star.

Th e rings are made up of a thin layer of very dark boulders.

Th ey are confi ned by small moons, and except for the outer-

most rings, they contain little dust.

Like the rings of Jupiter and Saturn, the rings around

Uranus and Neptune cannot survive for long periods. All

the Jovian rings need to be resupplied with material from

impacts on moons.

When you read about Neptune’s rings later in this chapter, you
will return to this artwork and see how closely the two ring sys-
tems compare.
Images made with the Hubble space telescope in 2003 and
2005 have revealed two larger, fainter, dustier rings lying outside
the previously known ring system (■ Figure 24-9). Th e larger of

1

2

3

■ Figure 24-9

Two newly discovered rings orbit Uranus far outside the previously known

rings. The outermost ring follows the orbit of the small moon Mab, only

12 km (7 mi) in diameter. The short bright arcs in this photo were caused by

moons moving along their orbits during the long time exposure. (NASA, ESA,

M. Showalter, SETI Institute)

Mab

Ring 2003 U1

Ring 2003 U1

Visual image

Ring 2003 U2