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Chapter 24
Uranus, Neptune, and Pluto
Guidepost
In the three previous chapters, we have used our tools
of comparative planetology to study other worlds, and
we continue that theme in this chapter. A second theme
running through this chapter is the nature of
astronomical discovery. Unlike the other planets in our
solar system, Uranus, Neptune, and Pluto were
discovered, and the story of their discovery helps us
understand how science progresses.
As we probe the outer fringes of our planetary system in
this chapter, we see strong evidence of smaller bodies
that fall through the solar system and impact planets
and satellites. The next chapter will allow us to study
these small bodies in detail and will give us new
evidence that our solar system formed from a solar
nebula.
Outline
I. Uranus
A. The Discovery of Uranus
B. The Motion of Uranus
C. The Atmosphere of Uranus
D. The Interior of Uranus
E. The Rings of Uranus
F. The Moons of Uranus
G. A History of Uranus
II. Neptune
A. The Discovery of Neptune
B. The Atmosphere and Interior of Neptune
C. The Rings of Neptune
D. The Moons of Neptune
E. The History of Neptune
Outline (continued)
III. Pluto
A. The Discovery of Pluto
B. Pluto as a Planet
C. The Origin of Pluto and Charon
Uranus
Chance discovery by
William Herschel in
1781,
while scanning the
sky for nearby
objects with
measurable parallax:
discovered Uranus
as slightly extended
object, ~ 3.7 arc
seconds in diameter.
The Motion of Uranus
Very unusual
orientation of rotation
axis: Almost in the
orbital plane.
Possibly result of
impact of a large
planetesimal during
the phase of planet
formation.
Large portions of the
planet exposed to
“eternal” sunlight for
many years, then
complete darkness
for many years!
19.18 AU
97.9o
The Atmosphere of Uranus
Like other gas giants: No surface.
Gradual transition from gas phase to fluid interior.
Mostly H; 15 % He, a few % Methane, ammonia and water vapor.
Optical view from
Earth: Blue color due
to methane,
absorbing longer
wavelengths
Cloud structures only visible after artificial
computer enhancement of optical images
taken from Voyager spacecraft.
The Structure of Uranus’ Atmosphere
Only one layer of
Methane clouds (in
contrast to 3 cloud layers
on Jupiter and Saturn).
3 cloud layers in Jupiter and
Saturn form at relatively
high temperatures that
occur only very deep in
Uranus’ atmosphere.
Uranus’ cloud layer difficult
to see because of thick
atmosphere above it.
Also shows belt-zone structure
 Belt-zone cloud structure must be dominated by
planet’s rotation, not by incidence angle of sun light!
Planetary Atmospheres
(SLIDESHOW MODE ONLY)
Cloud Structure of Uranus
Hubble Space
Telescope image
of Uranus shows
cloud structures
not present
during Voyager’s
passage in 1986.
 Possibly
due
to seasonal
changes of the
cloud structures.
The Interior of Uranus
Average density ≈ 1.29 g/cm3  larger portion
of rock and ice than Jupiter and Saturn.
Ices of water,
methane, and
ammonia,
mixed with
hydrogen and
silicates
The Magnetic Field of Uranus
No metallic core  no magnetic field was expected.
But actually, magnetic field of ~ 75 % of Earth’s
magnetic field strength was discovered:
Offset from
o
center: ~ 30 % Inclined by ~ 60
Possibly due to dynamo in
against axis of
of planet’s
liquid-water/ammonia/methane
rotation.
radius!
solution in Uranus’ interior.
Magnetosphere with weak radiation belts; allows
determination of rotation period: 17.24 hr.
The Magnetosphere of Uranus
Rapid rotation and large inclination deform
magnetosphere into a corkscrew shape.
UV images
During Voyager 2 flyby: Southpole pointed towards sun; direct
interaction of solar wind with magnetosphere  Bright aurorae!
Uranus’s Ring Detection
(SLIDESHOW MODE ONLY)
The Rings of Uranus
Rings of Uranus and Neptune are similar to Jupiter’s rings.
Confined by shepherd moons; consist of dark material.
Apparent motion of
star behind Uranus
and rings
Rings of Uranus were
discovered through
occultations of a
background star
The Rings of Neptune
Ring material must
be regularly resupplied by dust
from meteorite
impacts on the
moons.
Interrupted between
denser segments (arcs)
Made of dark
material,
visible in
forwardscattered
light.
Focused by small shepherd
moons embedded in the
ring structure.
The Moons of Uranus
5 largest moons
visible from Earth.
10 more discovered
by Voyager 2; more
are still being found.
Dark surfaces,
probably ice
darkened by dust
from meteorite
impacts.
5 largest moons all tidally locked to Uranus.
Interiors of Uranus’s Moons
Large rock cores surrounded by icy mantles.
The Surfaces of Uranus’s Moons (1)
Oberon
Old, inactive, cratered surface,
but probably active past.
Long fault across the surface.
Dirty water may have flooded
floors of some craters.
Titania
Largest moon
Heavily cratered surface, but no
very large craters.
Active phase with internal melting
might have flooded craters.
The Surfaces of Uranus’s Moons (2)
Umbriel
Dark, cratered surface
No faults or other signs of
surface activity
Ariel
Brightest surface of 5 largest moons
Clear signs of geological activity
Crossed by faults over 10 km deep
Possibly heated by tidal interactions
with Miranda and Umbriel.
Uranus’s Moon Miranda
Most unusual of the 5 moons detected from Earth
Ovoids: Oval groove patterns, 20 km high cliff near the equator
probably associated with
convection currents in the Surface features are old; Miranda is
no longer geologically active.
mantle, but not with impacts.
Neptune
Discovered in
1846 at position
predicted from
gravitational
disturbances on
Uranus’s orbit by
J. C. Adams and
U. J. Leverrier.
Blue-green color
from methane in
the atmosphere
4 times Earth’s
diameter; 4 %
smaller than
Uranus
The Atmosphere of Neptune
The “Great
Dark Spot”
Cloud-belt structure with high-velocity winds;
origin not well understood.
Darker cyclonic disturbances, similar to Great
Red Spot on Jupiter, but not long-lived.
White cloud features of methane ice crystals
The Moons of Neptune
Unusual orbits:
Triton: Only
satellite in the
solar system
orbiting clockwise,
i.e. “backward”.
Nereid: Highly
eccentric orbit;
very long orbital
period (359.4 d).
Two moons (Triton and Nereid) visible from
Earth; 6 more discovered by Voyager 2
The Surface of Triton
Very low temperature
(34.5 K)
 Triton can hold a tenuous
atmosphere of nitrogen and
some methane; 105 times
less dense than Earth’s
atmosphere.
Surface composed of ices:
nitrogen, methane, carbon
monoxide, carbon dioxide.
Possibly cyclic nitrogen
ice deposition and reDark smudges on the nitrogen ice surface,
vaporizing on Triton’s
south pole, similar to CO2 probably due to methane rising from below
surface, forming carbon-rich deposits when
ice polar cap cycles on
exposed to sun light.
Mars.
The Surface of Triton (2)
Ongoing surface
activity: Surface
features probably
not more than 100
million years old.
Large basins might
have been flooded
multiple times by
liquids from the
interior.
Ice equivalent of greenhouse effect may be one of the
heat sources for Triton’s geological activity.
Pluto
Discovered 1930 by C. Tombaugh.
Existence predicted from orbital disturbances of Neptune,
but Pluto is actually too small to cause those disturbances.
Pluto as a Planet
Virtually no surface
features visible from Earth.
~ 65 % of size of Earth’s
Moon.
Highly elliptical orbit;
coming occasionally closer
to the sun than Neptune.
Orbit highly inclined (17o)
against other planets’ orbits
 Neptune and Pluto
will never collide.
Surface covered with nitrogen ice; traces of frozen methane
and carbon monoxide.
Daytime temperature (50 K) enough to vaporize some N
and CO to form a very tenuous atmosphere.
Pluto’s Moon Charon
Discovered
in 1978;
about half
the size and
1/12 the
mass of
Pluto itself.
Tidally
locked to
Pluto.
Hubble Space Telescope image
Pluto and Charon
Orbit highly inclined against
orbital plane.
From separation and orbital period:
Mpluto ~ 0.2 Earth masses.
Density ≈ 2 g/cm3
(both Pluto and Charon)
 ~ 35 % ice and 65 % rock.
Large orbital inclinations 
Large seasonal changes on
Pluto and Charon.
The Origin of Pluto and Charon
Probably very different history than neighboring Jovian planets.
Older theory:
Pluto and Charon formed as moons of Neptune,
ejected by interaction with massive planetesimal.
Mostly abandoned today since
such interactions are unlikely.
Modern theory: Pluto and Charon
members of Kuiper belt of small,
icy objects (see Chapter 25).
Collision between Pluto and Charon may have caused the peculiar
orbital patterns and large inclination of Pluto’s rotation axis.
New Terms
occultation
ovoid
Discussion Questions
1. Why might it be unfair to describe William Herschel’s
discovery of Uranus as accidental? Why might it be
unfair to describe the discovery of the rings of Uranus
as accidental?
2. Suggest a single phenomenon that could explain the
inclination of the rotation axis of Uranus, the orbits of
Neptune’s satellites, and the existence of Pluto’s moon.
Quiz Questions
1. How do the seasons on Uranus differ from seasons on
Earth?
a. Seasons on Uranus are 84 times longer and more extreme
than on Earth.
b. Seasons on Uranus are 84 times longer and less extreme
than on Earth.
c. Seasons on Uranus are 21 times longer and more extreme
than on Earth.
d. Seasons on Uranus are 21 times longer and less extreme
than on Earth.
e. Seasons on Uranus are longer, more extreme, and in
reverse order of the seasons on Earth.
Quiz Questions
2. What is our current best hypothesis as to how the whole
Uranian system came to have such a large inclination?
a. A large impact during the latter stages of planet building
tipped Uranus on its side.
b. Tidal interactions between Uranus and the other Jovian
planets pulled Uranus onto its side.
c. Magnetic interactions between the Sun and Uranus flipped
Uranus onto its side.
d. Uranus formed outside of the Solar System and was
captured later.
e. The slow rate of rotation of Uranus gives it such little stability
that its rotation axis precesses wildly.
Quiz Questions
3. Both Uranus and Neptune have a blue-green tint when
observed through a telescope. What does this tell you about
their composition?
a. Their atmospheres are composed of mostly hydrogen and
helium.
b. Their atmospheres are composed of mostly carbon dioxide.
c. Their atmospheres are composed of mostly nitrogen.
c. Their atmospheres contain some ammonia.
e. Their atmospheres contain some methane.
Quiz Questions
4. How do we get an accurate measurement of the rotational
period of Uranus?
a. We measure the time for one orbit of a dark spoke in the
rings.
b. We measure the time for the Great Dark Spot to travel once
around.
c. We measure the time for a particular cloud to rotate once
around the planet.
d. We measure the time from one opposition of Uranus to the
next opposition of Uranus.
e. We measure the period of the cyclic fluctuation in the
synchrotron radiation emitted by Uranus.
Quiz Questions
5. In the atmospheres of Jupiter and Saturn we see ammonia,
ammonia hydrosulfide, and water clouds in three distinct
layers. Why don't we see these same three cloud layers in the
atmospheres of Uranus and Neptune?
a. Farther from the Sun it is too cold for these three layers of
clouds to form.
b. These three layers are likely hidden beneath a higher layer
of methane clouds.
c. These chemicals are not present in the atmospheres of
Uranus and Neptune.
d. These condensates form one layer in the atmospheres of
Uranus and Neptune.
e. Uranus and Neptune have no atmosphere.
Quiz Questions
6. Which interior zone of Uranus and Neptune do we suspect
contains the electrically conducting fluid that is responsible for
planetary magnetic fields?
a. The zone of liquid water with dissolved ammonia and
methane.
b. The liquid metallic hydrogen zone.
c. The liquid hydrogen-helium zone.
d. The liquid outer iron core.
e. The heavy element core.
Quiz Questions
7. In what way is Uranus different than the other Jovian
planets?
a. Uranus has no rings.
b. Uranus has no moons.
c. Uranus has a higher density.
d. Uranus has no metallic hydrogen.
e. Uranus has little remaining heat of formation.
Quiz Questions
8. Why is there no liquid metallic hydrogen zone in the interior
of Uranus or Neptune?
a. The temperature is not low enough for hydrogen to become
a superconductor.
b. The hydrogen does not contain sufficient amounts of
deuterium.
c. Uranus and Neptune do not contain hydrogen and helium.
d. The pressure is too low for hydrogen to be metallic.
e. No fusion occurs in Uranus and Neptune.
Quiz Questions
9. How did Uranus and Neptune come to have less hydrogen
and helium than Jupiter and Saturn?
a. The mass fraction of light elements like hydrogen and helium
in the solar nebula decreases with distance from the Sun.
b. Much of their original hydrogen and helium was stripped
away by the gravity of passing stars.
c. Much of their original hydrogen and helium was ionized and
stripped away by the solar wind.
d. Much of their original hydrogen and helium was ionized and
stripped away by interstellar winds.
e. Uranus and Neptune took longer to form than Jupiter and
Saturn.
Quiz Questions
10. What difference in the rings of Uranus and Neptune was
first revealed in observations from Earth-based telescopes?
a. The clumpy ring arcs of Neptune.
b. The difference in the albedo of the ring particles.
c. The difference in the size of the ring particles.
d. The elemental composition of the ring particles.
e. The difference in the dust-to-ice ratio of ring particles.
Quiz Questions
11. What evidence indicates that the rings of Uranus have little
dust and the rings of Neptune contain a lot of dust?
a. The camera lens of Voyager 2 was dust-free until it passed
through the rings of Neptune.
b. The rings of Uranus appear bright in forward-scattered light,
and the rings of Neptune appear dark in forward-scattered light.
c. The rings of Uranus appear dark in forward-scattered light,
and the rings of Neptune appear bright in forward-scattered
light.
d. The rings of Uranus appear bright in back-scattered light,
and the rings of Neptune appear dark in back-scattered light.
e. The rings of Uranus appear dark in back-scattered light, and
the rings of Neptune appear bright in back-scattered light.
Quiz Questions
12. How does a thin planetary ring retain its shape?
a. The tidal force of the planet on the ring particles keeps them
together.
b. The magnetic field of the planet traps the ring particles in a
well-defined orbit.
c. Small moons orbiting just inside and outside the rings
shepherd the ring particles.
d. The gravitational attraction of the ring particles on one
another keeps the ring together.
e. The electrostatic attraction of the ring particles on one
another keeps the ring together.
Quiz Questions
13. What keeps small shepherd moons from breaking apart
within the Roche Limit of a planet?
a. Gravitational attraction of the moon's material.
b. Gravitational attraction by the ring particles.
c. Electrostatic bonds of the moon's material.
d. Gravitational attraction of larger moons.
e. Tidal forces by the planet.
Quiz Questions
14. The discoveries of Uranus, Neptune, and Pluto all came
long after the death of Isaac Newton. How was Newton
involved in the discovery of a new planet?
a. It was the application of Newtonian gravity to the problem of
the orbit of Uranus that led to the discovery of Neptune.
b. It was with a reflecting telescope (the type invented by
Newton) that the planet Uranus was discovered.
c. It was through perceived perturbations by Newtonian gravity
on the orbit of Neptune that a search for a ninth planet was
begun, which eventually resulted in the discovery of Pluto.
d. Both b and c above.
e. All of the above.
Quiz Questions
15. We could divide the Jovian planets into two subclasses, the
Gas Giants and the Ice Giants. Into which groups should we
place the four Jovian planets?
a. The Gas Giants are Uranus & Neptune, and the Ice Giants
are Jupiter & Saturn.
b. The Gas Giants are Jupiter & Saturn, and the Ice Giants are
Uranus & Neptune.
c. The Gas Giants are Saturn & Uranus, and the Ice Giants are
Jupiter & Neptune.
d. The Gas Giants are Jupiter & Neptune, and the Ice Giants
are Saturn & Uranus.
e. The Gas Giants are Saturn & Neptune, and the Ice Giants
are Jupiter & Uranus.
Quiz Questions
16. What is peculiar about the orbits of Neptune's moons Triton
and Nereid?
a. Triton's orbit is around Neptune and Nereid's orbit is around
Triton.
b. Triton's orbit is large and very elliptical, and Nereid's orbit is
very small and circular.
c. Triton's orbit is in the retrograde direction, and Nereid's orbit
is large and very elliptical.
d. Triton's orbit places it inside the Roche Limit of Neptune, and
Nereid's orbit is large and very elliptical.
e. They share similar orbits, and gravitational interactions
cause them to switch orbits each time they meet.
Quiz Questions
17. The surface age of Triton is thought to be about 100 million
years. What is the evidence for such an age determination?
a. The relationship between the rotational and orbital periods of
Triton
b. The thickness of nitrogen snow deposits around the geysers.
c. The degree of tidal heating of Triton due to Neptune.
d. Age dating of meteorites from Triton.
e. The density of impact craters.
Quiz Questions
18. How can worlds like Triton and Pluto have atmospheres
when a larger world such as Ganymede has none?
a. Impacts vaporize ices on these cold bodies.
b. Tidal heating releases gases on these cold bodies.
c. In cold environments, gas molecules have more mass.
d. Gas molecules move more slowly at low temperatures.
e. More frozen gases exist in the colder outer solar system.
Quiz Questions
19. What evidence do we have that Pluto and Charon are
made of mixtures of rock and ice?
a. Spectra show that both bodies have some surface ices.
b. Both bodies have a density of 2 grams per cubic centimeter.
c. The Hubble Space Telescope detected active nitrogen
geysers on Pluto.
d. Both a and b above.
e. All of the above.
Quiz Questions
20. If you visited Pluto and found Charon a full moon directly
overhead, where would Charon be in the sky when it was at
First Quarter phase?
a. At the west point on the horizon.
b. At the east point on the horizon.
c. It depends on the time of day.
d. Directly overhead.
e. Either a or b above.
Answers
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
a
a
e
e
b
a
e
d
e
a
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
c
c
c
e
b
c
e
d
d
d