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Chapter 13
Astronomy Today,
5th edition
Chaisson
McMillan
© 2005 Pearson Prentice Hall
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Chapter 13
Uranus, Neptune, and Pluto
Units of Chapter 13
The Discovery of Uranus
The Discovery of Neptune
Physical Properties of Uranus and
Neptune
The Atmospheres of Uranus and Neptune
Magnetospheres and Internal Structure
The Moon Systems of Uranus and
Neptune
The Rings of the Outermost Jovian
Planets
Units of Chapter 13, cont.
The Discovery of Pluto
Physical Properties of Pluto
The Origin of Pluto
Surface Detail on Planet Pluto
13.1 The Discovery of Uranus
Discovered in 1781 by Herschel; first planet
to be discovered in more than 2000 years
Little detail can be seen from Earth; arrows
point to three of Uranus’s moons:
13.2 The Discovery of Neptune
Neptune was discovered in 1846, after analysis
of Uranus’s orbit indicated its presence
Details of Neptune cannot be made out from
Earth either; arrows again point to moons:
13.3 Physical Properties of
Uranus and Neptune
Uranus and Neptune are very similar:
13.3 Physical Properties of
Uranus and Neptune
Uranus
Neptune
Mass
14.5 × Earth
17.1 × Earth
Radius
4.0 × Earth
3.9 × Earth
Density
1300 kg/m3
1600 kg/m3
13.3 Physical Properties of
Uranus and Neptune
Peculiarity of Uranus: axis of rotation lies
almost in the plane of its orbit. Seasonal
variations are extreme.
13.4 The Atmospheres of Uranus and Neptune
Outer atmospheres of Uranus and Neptune are
similar to those of Jupiter and Saturn
Uranus and Neptune are cold
enough that ammonia
freezes; methane dominates
and gives the characteristic
blue color
13.4 The Atmospheres of Uranus and Neptune
Uranus is very cold; clouds only in lower,
warmer layers.
Few features are
visible:
13.4 The Atmospheres of Uranus and Neptune
Band structure of Neptune is more visible, and
Neptune has internal heat source of unknown
origin:
13.4 The Atmospheres of Uranus and Neptune
Neptune has storm systems similar to those on
Jupiter, but fewer:
13.5 Magnetospheres and Internal Structure
Uranus and Neptune both have substantial
magnetic fields, but at a large angle to their
rotation axes.
The rectangle
within each
planet shows a
bar magnet that
would produce a
similar field. Note
that both
Uranus’s and
Neptune’s are
significantly off
center.
13.5 Magnetospheres and Internal Structure
Magnetic fields of Uranus and Neptune must not
be produced by dynamos, as the other planets’
fields are.
Interior structure of Uranus and Neptune,
compared to that of Jupiter and Saturn:
13.6 The Moon Systems of Uranus
and Neptune
13.6 The Moon Systems of Uranus
and Neptune
Uranus has 27 moons, five of which are major:
Miranda, Ariel, Umbriel, Titania, and Oberon
Very similar to Saturn’s medium-sized moons,
except that all are much less reflective.
Umbriel is the darkest:
13.6 The Moon Systems of Uranus
and Neptune
Miranda is the most unusual; origin of the
cracks and grooves is unknown:
13.6 The Moon Systems of Uranus
and Neptune
Neptune has 13 moons, but only two can be
seen from Earth: Triton and Nereid
Triton is in a retrograde orbit; Nereid’s is highly
eccentric
Triton’s surface has few craters, indicating an
active surface
13.6 The Moon Systems of Uranus
and Neptune
Nitrogen geysers
have been observed
on Triton,
contributing to the
surface features:
13.6 The Moon Systems of Uranus
and Neptune
Also, there appear to be
ice volcanoes:
13.7 The Rings of the Outermost
Jovian Planets
Uranus and Neptune have faint ring systems,
recently detected.
Uranus’s rings
are narrow:
13.7 The Rings of the Outermost
Jovian Planets
Two shepherd moons
keep the epsilon ring
from diffusing:
13.7 The Rings of the Outermost
Jovian Planets
Neptune has five rings, three narrow and two
wide:
13.8 The Discovery of Pluto
Pluto was discovered in 1930. It was thought
to be needed to explain irregularities in the
orbits of Uranus and Neptune, but it turned
out that there were no such irregularities.
13.8 The Discovery of Pluto
Pluto’s orbit is eccentric and inclined to the
plane of the ecliptic; it also crosses the orbit of
Neptune:
13.9 The Physical Properties of Pluto
Pluto’s moon, Charon, was
discovered in 1978.
It is orbitally locked to
Pluto, and about a sixth as
large.
13.9 The Physical Properties of Pluto
Its orbit is at a large angle to the plane of
Pluto’s orbit:
13.10 The Origin of Pluto
The Kuiper belt is outside the orbit of
Pluto, and has many icy chunks.
Current theory is that Pluto is the
nearest, and largest, of these objects.
Summary of Chapter 13
• Uranus, Neptune, and Pluto were all
discovered in the last 350 years
• Uranus and Neptune are similar: gaseous
and cold
• Uranus’s spin axis is almost in the plane of
its orbit
• Surface features are hard to discern on
Uranus, but more obvious on Neptune
• Uranus has no excess heat emission, but
Neptune does
Summary of Chapter 13, cont.
• Uranus’s midsized moons are similar to
those of Saturn
• Neptune’s moon Triton has a retrograde orbit
• Uranus and Neptune both have faint ring
systems
• Pluto was discovered in 1930; its moon
Charon in 1978
• Pluto is thought to be related to objects in
the Kuiper belt