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Uranus and Neptune
Uranus and Neptune
The outermost planets were unknown to
the ancients—they were discovered by
telescopic observations: Uranus in 1781
and Neptune in 1846
Uranus and Neptune have very similar
bulk properties, so it is natural to consider
them together
Discovery of Uranus
 The planet Uranus was discovered by British astronomer William
Herschel in 1781
 Herschel was engaged in charting the faint stars in the sky when he
came across an odd-looking object that he described as “a curious
either nebulous star or perhaps a comet”
 The object appeared as a disk in Herschel’s 6-inch telescope and
moved relative to the stars, but it traveled too slowly to be a comet
 Herschel soon realized that he had found the seventh planet in the
solar system
 Since this was the first new planet discovered in well over 2000
years, the event caused quite a stir at the time
 The story goes that Herschel’s first instinct was to name the new
planet “Sidus Georgium” (Latin for “George’s star”) after his king,
George III of England
Uranus From Earth
 Uranus is in fact just barely visible
to the naked eye, if you know
exactly where to look
 It looks like a faint, undistinguished
star
 Even today, few astronomers have
seen it without a telescope
 Through a large Earth-based
optical telescope, Uranus appears
hardly more than a tiny pale
greenish disk
 With the flyby of Voyager 2 in
1986, our knowledge of Uranus
increased dramatically, although
close-up images of the planet still
showed virtually no surface detail
Discovery of Neptune
 Once Uranus was discovered, astronomers set
about charting its orbit, and quickly discovered a
small discrepancy between the planet’s
predicted position and where they actually
observed it
 The logical conclusion was that an unknown
body must be exerting a gravitational force on
Uranus
 Astronomers realized that there had to be
another planet in the solar system perturbing
Uranus’s motion
Discovery of Neptune
 In the 1840s two mathematicians independently solved the difficult
problem of determining this new planet’s mass and orbit
 British astronomer, John Adams, reached the solution in September
1845
 in June of the following year, the French mathematician Urbain
Leverrier came up with essentially the same answer
 In September a German astronomer named Johann Galle began his
own search from the Berlin Observatory, using a newly completed
set of more accurate sky charts
 He found the new planet within one or two degrees of the predicted
position—on his first attempt
 After some wrangling over names and credits, the new planet was
named Neptune, and Adams and Leverrier (but not Galle!) are now
jointly credited with its discovery
 With an orbital period of 163.7 years, Neptune has not yet
completed one revolution since its discovery
Neptune from Earth
 Distant Neptune cannot
be seen with the naked
eye, although it can be
seen with a small
telescope
 Neptune is so distant that
surface features are
virtually impossible to
discern
 With Voyager 2’s arrival,
much more detail
emerged
Physical Properties
 The radius of Uranus is
4.0 times that of Earth
 The radius of Neptune is
3.9 times that of Earth
 Their masses are 14.5
Earth masses for Uranus
and 17.1 Earth masses
for Neptune
 Large rocky cores
constitute a greater
fraction of the planets’
masses than do the cores
of either Jupiter or Saturn
Rotation of Uranus
 Uranus has a short rotation period
 Earth-based observations of the Doppler shifts in
spectral lines first indicated that Uranus’s “day”
was between 10 and 20 hours long
 The precise value of the planet’s rotation is now
known to be 17.2 hours
 The planet’s atmosphere rotates differentially
However, Uranus’s atmosphere actually rotates faster at
the poles (where the period is 14.2 hours) than near the
equator (where the period is 16.5 hours)
 The rotation of Uranus, like that of Venus, is
classified as retrograde
Rotation of Uranus
 Uranus’s rotation axis lies 98° from the perpendicular
 Relative to the other planets, we might say that Uranus lies tipped
over on its side
 As a result, the “north” (spin) pole of Uranus, at some time in its
orbit, points almost directly toward the Sun
 Half a “year” later, its “south” pole faces the Sun
 The planet experiences the most extreme seasons known in the
solar system
 The poles are alternately plunged into darkness for 42 years at a
time
Rotation of Neptune
 The average rotation period of Neptune’s
atmosphere is 17.3 hours
 Measurements of Neptune’s radio emission by
Voyager 2 showed that the magnetic field of the
planet, and presumably also its interior, rotates
once every 16.1 hours
 Neptune is unique among the jovian worlds in
that its atmosphere rotates more slowly than its
interior
 Neptune’s rotation axis is inclined 29.6°
Atmosphere
 Spectroscopic studies of sunlight reflected from Uranus’s and Neptune’s
dense clouds indicate that the two planets’ outer atmospheres are quite
similar to the atmospheres of Jupiter and Saturn
 The most abundant element is molecular hydrogen (84 percent), followed
by helium (about 14 percent) and methane, which is more abundant on
Neptune (about 3 percent) than on Uranus (2 percent)
 Ammonia, which plays such an important role in the Jupiter and Saturn
systems, is not present in any significant quantity in the outermost jovian
worlds
 Jupiter has much more gaseous ammonia than methane, but moving
outward from the Sun, we find that the more distant planets have steadily
decreasing amounts of ammonia and relatively greater amounts of methane
 The increasing amounts of methane are largely responsible for the outer
jovian planets’ blue coloration
 Methane absorbs long-wavelength red light quite efficiently, so sunlight reflected
from the planets’ atmospheres is deficient in red and yellow photons and appears
blue-green or blue
 As the concentration of methane increases, the reflected light appears bluer
Weather on Uranus
 Uranus apparently lacks any significant internal
heat source
 Because of the planet’s low surface temperature,
its clouds are found only at low-lying, warmer
levels in the atmosphere
 The absence of high-level clouds means that we
must look deep into the planet’s atmosphere to see
any structure
 The bands and spots that characterize flow patterns
on the other jovian worlds are largely “washed out”
on Uranus
 Astronomers have learned that Uranus’s
atmospheric clouds and flow patterns move around
the planet in the same sense as the planet’s
rotation, with wind speeds ranging from 200 to 500
km/h
Weather on Neptune
 Neptune’s clouds and band
structure are much more easily
seen
 Neptune’s upper atmosphere is
actually slightly warmer than that
of Uranus
 Neptune has an internal energy
source—it radiates 2.7 times more
heat than it receives from the Sun
 The cause of this heating is still
uncertain
 Neptune sports several storm
systems similar in appearance to
those seen on Jupiter
 The largest such storm is
comparable to the size of Earth
and is known as the Great Dark
Spot
Magnetospheres
 Voyager 2 found that both Uranus and Neptune have
fairly strong internal magnetic fields—about 100 times
stronger than Earth’s field and 1/10 as strong as Saturn’s
 Uranus and Neptune each have substantial
magnetospheres, populated largely by electrons and
protons either captured from the solar wind or created
from ionized hydrogen gas escaping from the planets
themselves
 Uranus’ magnetic field is tilted at about 60° to the axis of
rotation
 On Earth, such a tilt would put the north magnetic pole
somewhere in the Caribbean
 The magnetic field lines are not centered on the planet
 The internal structures of Uranus and Neptune are different from
those of Jupiter and Saturn, and this difference changes how
their magnetic fields are generated
Internal Structure
 Theoretical models indicate that Uranus and Neptune have rocky
cores similar to those found in Jupiter and Saturn—about the size of
Earth and perhaps 10 times more massive
 The pressure outside the cores of Uranus and Neptune (unlike the
pressure within Jupiter and Saturn) is too low to force hydrogen into
the metallic state, so hydrogen stays in its molecular form all the way
in to the planets’ cores
 Astronomers theorize that deep below the cloud layers, Uranus and
Neptune may have high-density, “slushy” interiors containing thick
layers of water clouds
Moons of Uranus
Moons of Uranus
Uranus' moons are unique in being named
for Shakespearean characters, along with
a couple from the works of Alexander
Pope
Uranus now has 27 known moons
Moons of Uranus
 Oberon and Titania (king and queen of the fairies) are the largest Uranian
moons
 All of Uranus's inner moons appear to be roughly half water ice and half
rock
 The composition of the moons outside the orbit of Oberon remains
unknown, but they are likely captured asteroids
 The darkest of the moons of Uranus is Umbriel and it displays little evidence
of any past surface activity
 Miranda has a surface unlike any other moon that's been seen
 It has giant fault canyons as much as 12 times as deep as the Grand Canyon,
terraced layers, surfaces that appear very old, and others that look much
younger
 Ariel has the brightest and possibly the youngest surface among all the
moons of Uranus
 Cordelia and Ophelia are shepherd moons that keep Uranus' thin,
outermost "epsilon" ring well defined
 There is a swarm of eight small satellites unlike any other system of
planetary moons
 This region is so crowded that astronomers don't yet understand how the little
moons have managed to avoid crashing into each other
Moons of Neptune
Moons of Neptune
 Neptune has a total of 13 known moons
 The largest moon, Triton, is the only large moon in our
solar system to have a retrograde orbit
 Part of its surface resembles the rind of a cantaloupe
 Ice volcanoes spout what is probably a mixture of liquid nitrogen,
methane, and dust, which instantly freezes and then snows back
down to the surface
 Triton's icy surface reflects so much of what little sunlight
reaches it that the moon is one of the coldest objects in the solar
system, about -240 C (-400 F)
 Millions of years from now, Triton will come close enough for
gravitational forces to break it apart - possibly forming a ring
around Neptune
 Proteus, the second-largest, is thought to be right at the
limit of how massive an object can be before its gravity
pulls it into a sphere
Rings of Uranus
 Ground-based observations
revealed the presence of a
total of nine thin rings
 The main rings, in order of
increasing radius, are named
Alpha, Beta, Gamma, Delta,
and Epsilon
 A fainter ring, known as the
Eta ring, lies between the Beta
and Gamma rings, and three
other faint rings, known as 4,
5, and 6, lie between the Alpha
ring and the planet itself
 Voyager 2 discovered two
more even fainter rings
 The rings of Uranus are dark,
narrow, and widely spaced
Rings of Neptune
Neptune is surrounded by five dark rings
Three are quite narrow, like the rings of Uranus
The other two are broad and diffuse, more like
Jupiter’s rings
Moons of Uranus and Neptune
Quiz to follow
Quiz
#1
 By receiving a gravity assist from
_____, Voyager II shortened its travel
time to Uranus and Neptune.
A.
B.
C.
D.
Mars
Pluto
Venus
Jupiter
#2
 How many years did it take for
Voyager II to reach Neptune?
A.
B.
C.
D.
six
nine
twelve
fifteen
#3
 The Hubble telescope showed that
Uranus has pink clouds composed of
_____.
A.
B.
C.
D.
water
carbon
oxygen
methane
#4
The magnetic field of Uranus runs
through one side of the planet.
true
false
#5
 Scientists think that the rings of
Uranus are made of _____.
A.
B.
C.
D.
water vapor
carbon dioxide
liquid hydrogen
coal-like particles
#6
 The largest moon of Uranus is named
_____.
A.
B.
C.
D.
Ariel
Titania
Umbriel
Miranda
#7
 The great dark spot on Neptune is a
_____.
A.
B.
C.
D.
sea
crater
storm
volcano
#8
 Scientists think that Neptune’s core is
_____.
A.
B.
C.
D.
cold and icy
hot and rocky
cool and metallic
warm and gaseous
#9
 Astronomers have identified _____
rings around Neptune.
A.
B.
C.
D.
three
five
seven
nine
# 10
Triton has a dull surface, reflecting
almost no light.
true
false
Answers:
#1
 By receiving a gravity assist from
_____, Voyager II shortened its travel
time to Uranus and Neptune.
A.
B.
C.
D.
Mars
Pluto
Venus
Jupiter
#2
 How many years did it take for
Voyager II to reach Neptune?
A.
B.
C.
D.
six
nine
twelve
fifteen
#3
 The Hubble telescope showed that
Uranus has pink clouds composed of
_____.
A.
B.
C.
D.
water
carbon
oxygen
methane
#4
The magnetic field of Uranus runs
through one side of the planet.
true
false
#5
 Scientists think that the rings of
Uranus are made of _____.
A.
B.
C.
D.
water vapor
carbon dioxide
liquid hydrogen
coal-like particles
#6
 The largest moon of Uranus is named
_____.
A.
B.
C.
D.
Ariel
Titania
Umbriel
Miranda
#7
 The great dark spot on Neptune is a
_____.
A.
B.
C.
D.
sea
crater
storm
volcano
#8
 Scientists think that Neptune’s core is
_____.
A.
B.
C.
D.
cold and icy
hot and rocky
cool and metallic
warm and gaseous
#9
 Astronomers have identified _____
rings around Neptune.
A.
B.
C.
D.
three
five
seven
nine
# 10
Triton has a dull surface, reflecting
almost no light.
true
false