Summary
▶ Uranus was discovered by William Herschel in 1781.
▶ Although Uranus is a Jovian planet, it is signifi cantly smaller and less
massive than Jupiter. Uranus is an ice giant planet about four times
the diameter of Earth. Uranus rotates “on its side,” with its pole nearly
in the plane of its orbit, causing it to have extreme seasons.
▶ The atmosphere of Uranus is mostly hydrogen and helium with some
methane, which absorbs longer-wavelength photons and gives the
planet a greenish-blue color.
▶ The atmosphere of Uranus is so cold that only methane clouds are
visible, and some traces of belt–zone circulation can be seen. As
spring has come to the northern hemisphere, more cloud features have
developed, suggesting that Uranus follows a seasonal cycle and is not
always as bland as it was when Voyager 2 fl ew past in 1986.
▶ Model calculations indicate that Uranus has a small core of dense mat-
ter and a deep slushy mantle of ice, water, and rock. Convection in the
mantle may produce its highly inclined offset magnetic fi eld.
▶ Uranus emits about the predicted amount of heat for a planet at its
distance from the sun, which suggests that it is not extremely hot
inside.
▶ The rings of Uranus were discovered during an occultation (p. 534)
as the planet crossed in front of a star.
▶ The rings of Uranus are composed mostly of boulder-size objects dark-
ened by radiation and trapped among the orbits of small moons. The
ring material is probably produced by impacts on icy moons.
▶ The fi ve large moons of Uranus appear to be icy and old, although
some show signs of geological activity. Miranda appears to have
suffered from dramatic activity, as shown by ovoids (p. 537) on its
surface. Tidal heating is a likely source of the energy that drove this
activity.
▶ Uranus appears to have formed slowly and never became massive
enough to trap large amounts of hydrogen and helium from the solar
nebula. Uranus and Neptune are thought to have formed closer to the
sun and were moved outward by gravitational interactions with Jupiter
and Saturn.
▶ An impact by a large planetesimal while Uranus was forming may have
caused the planet to rotate around a highly inclined axis, but it is
also possible that tidal interactions with Saturn altered the rotation of
Uranus as it was moved outward.
▶ Neptune was discovered in 1846 based on a position computed from
irregularities it was causing in the orbital motion of Uranus.
▶ (^) Neptune is an ice giant quite similar in size to Uranus. It has a core of
denser material; a mantle of ice, water, and rock; and an atmosphere
of hydrogen and helium with traces of methane, which gives the
planet a blue color.
▶ (^) Methane clouds come and go in Neptune’s cold atmosphere and appear
to follow a belt–zone circulation.
▶ (^) Circulation in the liquid mantle gives rise to Neptune’s magnetic
fi eld.
▶ (^) Neptune has more internal heat than Uranus, and the sinking of dense
material, including diamonds, may be adding to the internal heat.
▶ (^) The rings of Neptune probably formed when impacts on moons
scattered icy debris into stable places among the orbits of the
small moons. Forward scattering shows that the rings contain more
small dust particles than those of Uranus. Arcs in the rings appear
to be caused by the gravitational infl uence of a small moon or
moons.
▶ Triton, Neptune’s largest moon, follows a retrograde orbit, and Nereid
follows a long eccentric orbit. These two moons’ odd orbits may have
been caused by the gravitational infl uence of a massive planetesimal
or by the capture of Triton.
▶ Triton has an icy surface and a thin atmosphere of nitrogen. The lack
of many craters, fl ooded areas, and the presence of cracks and faults
suggest that the moon may still be active. Sunlight and heat from the
interior appear to trigger nitrogen geysers in the crust. Tidal heating
during a close approach to Neptune when it was captured into orbit is
another possible source of heat.
▶ Neptune formed slowly, as did Uranus, and never accumulated a deep
atmosphere of hydrogen and helium before the solar nebula was blown
away.
▶ Pluto was discovered in 1930 during a search for a large planet orbit-
ing beyond Neptune and disturbing Neptune’s orbital motion. That
discovery was in a sense an accident, because Pluto is much too small
to affect Neptune, and more observations of Neptune indicate its
orbital motion is actually undisturbed.
▶ No spacecraft has yet visited Pluto, but the New Horizons probe is on
its way for a fl yby in 2015.
▶ Spectroscopic observations indicate Pluto has a frigid crust of solid
nitrogen ice with traces of frozen methane and carbon monoxide.
Its thin atmosphere is mostly nitrogen.
▶ Pluto’s moon Charon orbits in an orbit highly inclined to their orbit
around the sun. Charon and Pluto are tidally locked to face each other.
That means Pluto rotates on its side much like Uranus.
▶ (^) Careful measurements of the brightness of Charon and Pluto on occa-
sions when they move in front of each other have allowed astronomers
to construct low-resolution maps of both objects.
▶ (^) The density of Pluto shows that it and Charon must contain about
two-thirds rock and one-third ices.
▶ (^) The dwarf planets (p. 546) are small bodies that orbit the sun and
do not orbit a planet; they are spherical, like the major planets, but
unlike the major planets they are not large enough to have cleared
their orbital lanes of other objects.
▶ (^) Pluto, another Kuiper belt object named Eris, and the asteroid Ceres
are classifi ed as dwarf planets. About 10 other Kuiper belt objects are
candidate dwarf planets pending the determination of their shapes. At
present more than a thousand Kuiper belt objects are known, most of
which are too small to be dwarf planets.
▶ (^) The dwarf planets grew large in the solar nebula but never became
large enough capture or eject other objects orbiting nearby.
▶ (^) Pluto and similar Kuiper belt objects called plutinos (p. 548) are
caught in a stabilizing 3:2 resonance with Neptune. Many other Kuiper
belt objects orbit in other resonances with Neptune. These objects
are evidence that the Jovian planets interacted soon after formation,
pushed Neptune outward in the solar system, and swept up remnant
planetesimals in these resonances.
Review Questions
- Describe the location of the equinoxes and solstices in the Uranian
sky. What are seasons like on Uranus? - Why is belt–zone circulation diffi cult to detect on Uranus?
- Discuss the origin of the rings of Uranus and Neptune. Cite evidence
to support hypotheses. - How do the magnetic fi elds of Uranus and Neptune suggest that the
mantles inside those planets are fl uid?