The Solar System

CHAPTER 24 | URANUS, NEPTUNE, AND THE DWARF PLANETS 531

Th e average density of Uranus, 1.3 g/cm^3 ,

tells you that the planet must contain a larger

share of dense materials than Saturn. Nearly all

models of the interior of Uranus contain three

layers. Th e uppermost layer, the atmosphere,

is rich in hydrogen and helium. Below the

atmosphere, a deep mantle must contain large

amounts of water, methane, and ammonia in a

solid or slushy state, mixed with hydrogen and

silicate matter. Th is mantle is sometimes

described as “ice,” but, because of high pressure

and a temperature of a few thousand degrees, it

is quite unlike the earthly material that word

suggests. Th e third layer in the three-layer

models is a small heavy-element core. Many

books refer to the core as “rocky,” but, again,

because of the high pressure and high tempera-

ture, the material is not very rock-like. Th e

term rock refers to its chemical composition

and not to its other properties.

It is a Common Misconception to

imagine that the four Jovian planets are gas

giants. As you have learned, Jupiter and Saturn

are mostly liquid. Uranus and Neptune are

sometimes described as “ice giant” planets, in

recognition of the large proportion of solid

water in their interiors.

Because Uranus has a much lower mass

than Jupiter, its internal pressure is not high

enough to produce liquid metallic hydrogen.

Consequently, you might expect it to lack a

strong magnetic fi eld, but the Voyager 2 space-

craft found that Uranus has a magnetic fi eld

about 75 percent as strong as Earth’s. Uranus’s

fi eld is tipped 60° to the axis of rotation and is

off set from the center of the planet by about 30

percent of the planet’s radius (■ Figure 24-7).

Th eorists suggest that this unexpectedly oddly

oriented magnetic fi eld is produced by a dynamo

eff ect operating not at the center but nearer the

surface in a layer of liquid water with dissolved

ammonia and methane. Such a material would

be a good conductor of electricity, and the rota-

tion of the planet coupled with convection in

the fl uid could generate the magnetic fi eld.

As it made its closest approach to Uranus,

Voyager 2 observed eff ects of the planet’s mag-

netic fi eld. Th is allowed a more precise measure-

ment of Uranus’s rotation period than was

possible from motions of diffi cult-to-detect

cloud features. Th e magnetic fi eld defl ects the

■ Figure 24-5

The atmosphere of Uranus is much colder than that of Jupiter or Saturn, and the only visible cloud
layer is one formed of methane ice crystals deep in the hydrogen atmosphere. Other cloud layers
would be much deeper in the atmosphere and are not visible. The temperature profi le of Neptune is
similar to that of Uranus, and it has methane clouds at about the same place in its atmosphere.

0

100

• 100


• 200

200

Altitude (km)

Atmosphere

of Uranus

Methane clouds

• 300 – 200 – 100 0 100 212

Temperature (°F)

Temperature (K)

100 200 300 400

Jupiter

Saturn

Uranus

Neptune

■ Figure 24-6

A dark cloud, possibly a circulating storm, is visible in this Hubble Space Telescope image
of Uranus. (NASA, ESA, Sromovsky, Fry, Hallel, and Rages)