CHAPTER 24 | URANUS, NEPTUNE, AND THE DWARF PLANETS 529
the atmosphere contains methane, a good absorber of longer-
wavelength photons. As sunlight penetrates into the atmosphere
and is scattered back out, the longer-wavelength (red) photons
are more likely to be absorbed. Th at means that the sunlight
refl ecting off Uranus and then entering your eye is richer in blue
photons, giving the planet a blue color.
As Voyager 2 drew closer to the planet in late 1985, astrono-
mers studied the images radioed back to Earth. Uranus was a pale
green-blue ball with no obvious clouds, and only when the images
were carefully computer enhanced was any banded structure
detected (■ Figure 24-4). A few very high clouds of methane ice
particles were detected, and their motions allowed astronomers to
make the fi rst good measurement of the planet’s rotation period.
You can understand the nearly featureless appearance of the
atmosphere by studying the temperature profi le of Uranus shown
in ■ Figure 24-5. Th e atmosphere of Uranus is much colder than
that of Saturn or Jupiter. Consequently, the three cloud layers of
ammonia, ammonia hydrosulfi de, and water that form the belts
and zones in the atmospheres of Jupiter and Saturn lie very deep in
the atmosphere of Uranus. Th ese cloud layers, if they exist at all in
Uranus, are not visible because of the thick atmosphere of hydrogen
through which an observer has to look. Th e clouds that are visible
on Uranus are clouds of methane ice crystals, which form at such a
low temperature that they occur high in the atmosphere of Uranus.
Figure 24-5 shows that there can be no methane clouds on Jupiter
because that planet is too warm. Th e coldest part of Saturn’s atmo-
sphere is just cold enough to form a thin methane haze high above
its more visible cloud layers. (See Figure 23-15b.)
Th e clouds and atmospheric banding that are faintly visible on
Uranus appear to be the result of belt–zone circulation, which is a
bit surprising. Uranus rotates on its side, so solar energy strikes its
surface with geometry quite diff erent than that for Jupiter and
Saturn. Evidently belt–zone circulation is dominated by the rota-
tion of the planet and not by the direction of sunlight.
Th e Voyager 2 images from 1986 made some astronomers
expect that Uranus was always a nearly featureless planet, but
later observations have revealed that Uranus has seasons. Since
1986, Uranus has moved along its orbit, and spring has come to
its northern hemisphere. Th e Hubble Space Telescope and giant
Earth-based telescopes have detected changing clouds on Uranus,
including a dark cloud that may be a vortex resembling the spots
on Jupiter (■ Figure 24-6). Th e clouds appear to be part of a
seasonal cycle on Uranus, but its year lasts 84 Earth years, so you
will have to be patient to see summer come to its northern
hemisphere.
The Interior of Uranus
Astronomers cannot describe the interiors of Uranus and Neptune
as accurately as they can the interiors of Jupiter and Saturn.
Observational data are sparse, and the materials inside these
planets are not as easy to model as simple liquid hydrogen.
Uranus rotates on its side. When Voyager 2 fl ew past in 1986, the planet’s south
pole was pointed almost directly at the sun. (NASA)Celestial Profi le 9: Uranus
Motion:
Average distance from the sun 19.2 AU (2.87 109 km)
Eccentricity of orbit 0.046
Inclination of orbit to ecliptic 0.8°
Average orbital velocity 6.8 km/s
Orbital period 84.0 y
Period of rotation 17.23 h
Inclination of equator to orbit 97.9° (retrograde rotation)Characteristics:
Equatorial diameter 5.11 104 km (4.01 D⊕)
Mass 8.69 1025 kg (14.5 M⊕)
Average density 1.29 g/cm^3
Gravity 0.9 Earth gravity
Escape velocity 22 km/s (2.0 V⊕)
Temperature above cloud tops 220°C (365°F)
Albedo 0.35
Oblateness 0.023Personality Point:
Most creation stories begin with a separation of opposites, and Greek
mythology is no different. Uranus (the sky) separated from Gaia (Earth)
who was born from the void, Chaos. They gave birth to the giant
Cyclops, Cronos (Saturn, father of Zeus), and his fellow Titans. Uranus is
sometimes called the starry sky, but the sun (Helios), moon (Selene), and
the stars were born later, so Uranus, one of the most ancient gods, began
as the empty, dark sky.