Transcript Lec25-041907

The Outer Worlds:
Uranus, Neptune,
and Pluto
PTYS/ASTR 206
Outer Worlds
4/19/07
Announcements
• Reading Assignment
– Finish Chapter 17
• 6th homework due on Tuesday, April 24
• Quiz on Tuesday, April 24 (will cover all lectures since the last exam
and Chapters 14-17)
• Final Activity next Thursday
– Come prepared with 2 multiple-choice final exam questions
• Next study-group session is Monday, April 23, from 10:30AM12:00Noon – in room 330.
• Public lecture – Tuesday, April 24 7:30PM (this room)
•PTYS/ASTR
Movie206Night – Monday, April 23,
6:30PM (this room)
Outer Worlds
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• “Discovered” by William
Herschel in 1781 (discovered by
chance)
– Herschel was an English
musician and amateur
astronomer
– He used a 6.5” homemade
reflector (it took him over 200
attempts to make it!)
– Prior to its discovery, Uranus
had been seen, but not
recognized as a planet (it had
been designated 34 Tauri)
• Herschel made many
contributions to astronomy,
including the discovery of binary
star systems, and the motion of
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the solar system in space 4/19/07
Discovery of
Uranus
Discovery of Neptune
• The problem:
– Uranus’s orbit didn’t match Newton’s Laws,
with the forces from all known planets at that
time
• Proposed solution – there must be another
planet !
– Bode’s Law gives the distance to be 39 AU
– Its mass must be about the same as Uranus
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Neptune’s Discovery
• John Couch Adams (English
student)
– Predicted its location in 1843
(using Newton’s Laws
including the its force on
Uranus to explain Uranus’s
orbit)
– George Airy (Astronomer
Royal) wouldn’t look!
• Wanted a few more details
worked out
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Neptune’s Discovery
• Urbain Le Verrier (French
Professor)
– Predicted its location in
1846 – published it
– Airy (now interested) did not
have good enough star
charts
– Johann Galle (German
astronomer) did have good
star charts
– LeVerrier convinced Galle
to look for it
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Neptune’s Discovery
• Galle found it in 30 minutes !
• Galle is often credited for being
the discoverer, although Adams
and Le Verrier were the first to
predict its presence
– Instructors note: Why did
Adams get credit? He never
published it !
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Uranus and Neptune from Earth
•
Uranus is visible with the naked eye
under good (dark sky) conditions
•
Neptune is not visible to the naked
eye
•
Both can be seen as planets with
moderate-sized telescopes (rather
than a point-like star)
•
No surface features are visible
without a filter
– Only recently were a groundbased telescope able to discern
surface features on Uranus
using an IR filter and adaptive
optics
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Uranus
Neptune
• Equatorial diameter of 51,800 kilometers
– about 4 Earth diameters
Uranus: Basic Facts
• Mass is about 14.5 Earth masses
• Density =1.3 g/cm3
• Orbital period is 84.01 years
– About the average lifespan of a human
• average distance from the Sun is about
19 AU
• Retrograde rotation
• Rotational period is 17 hours 14 minutes.
• It has 27 moons
– The two largest are Titania and Oberon
– Both are about ½ the size of our moon
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• Equatorial diameter of 49,500
kilometers
– Nearly 4 Earth diameters
– A little bit smaller than Uranus
• Mass is about 17 Earth masses
– A little bit more massive than Uranus
• Density = 1.7 g/cm3
• Orbital period is about 165 years
• It is about 29.8 AU from the Sun
• Rotational period is about 16 hours
• Has 13 moons
– The206
largest
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3/4 the size of our moon
Neptune: Basic Facts
Uranus’s Composition and Appearance
• The atmosphere of Uranus is
composed of
– 83% hydrogen,
– 15% helium,
– 2% methane
• Methane absorbs red light, giving
Uranus its blue-green color.
– Methane on Uranus is 5 times
more abundant than on Jupiter and
Saturn
• It lacks an appreciable internal heat
source, thus there are no visible
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Uranus’s Atmosphere
• Visibly, Uranus appears as a virtually
featureless hazy blue ball.
– Lack of internal heat.
– Clouds on Uranus are cold and
don't billow up above the top haze
layer.
• Seen in false color (at the right),
Uranus reveals atmospheric features
• The atmosphere is arranged into
clouds running at constant latitudes
(like Jupiter and Saturn)
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Uranus has an unusual tilt
• Herschel discovered that moons
were orbiting in a plane
perpendicular to its orbit
– Moons orbit about their parent
body’s equator
• Uranus’s rotation axis is tilted 98o
with respect to the plane of its
orbit about the Sun
• Thought to be the result of a
collision with a planet sized body
early in the solar system
formation
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Neptune’s Atmosphere
• Neptune's atmosphere is
made up of hydrogen,
helium, and methane.
• Neptune's has clouds and
storm systems that revolve
around the planet, but with
wind speeds of 300 m/sec
(700 mi/hr
• These winds are driven by
an internal heat source
(unlike Uranus)
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Neptune’s Clouds
• The clouds seen on Neptune
are frozen methane in the
atmosphere
• Much more cloud activity is
seen on Neptune than on
Uranus
• This is because Uranus lacks
a substantial internal heat
source while Neptune does
– Neptune radiates more
than twice the energy it
receives from the Sun
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The Giant Dark Spot on Neptune
• Similar to Jupiter’s Giant Red Spot
(GRS)
• However, the giant dark spot on
Neptune is not as long-lived as the
GRS on Jupiter
– Disappeared in 1994
– Another re-appeared in the
northern hemisphere in 1995
• Rotates counterclockwise, as does
the GRS on Jupiter
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Uranus and Neptune contain a higher proportion
of heavy elements than Jupiter and Saturn
• Both Uranus and Neptune may have a rocky core (about the size of
Earth) surrounded by a mantle of water and ammonia (like Windex!)
• Electric currents in the mantles may generate the magnetic fields of the
planets
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The Origin of Magnetic Fields on Uranus
and Neptune
• Not massive enough to
possess a liquid metallic
hydrogen layer like Jupiter
• H2O in the interior is under
enough pressure to
induce ionization
– Ionization increases its
electrical conductivity
– An ionized-water
dynamo!
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• The magnetic axes of both Uranus and Neptune are steeply
inclined from their axes of rotation
• The magnetic and rotational axes of all the other planets are
more nearly parallel
• The magnetic fields of Uranus and Neptune are also offset
from the centers of the planets
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Uranus and Neptune each have a system of
thin,
dark rings
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• Discovered by William Lassell
only a month after Neptune was
discovered
•
Triton
(Moon of Neptune)
Orbits Neptune in a retrograde
orbit !
– opposite to the rotation
direction of Neptune
– The only large moon to do
this
• The orbit is also highly inclined
relative to Neptune’s equator
• Density = 2066 kg/m3
– More rock than in Saturn’s
moons
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Probably a captured asteroid!
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• Shows evidence of geological
activity
– Lacks large craters – it is a
very young surface
– Probably tidal heating from
Neptune
– Perhaps extreme tidal
heating in the past
Triton
(Moon of Neptune)
• It also has a tenuous nitrogen
(N2) atmosphere
– 70,000 times less
atmospheric pressure than
Earth
– about 500 times thinner
than the Martian
atmosphere
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Pluto and Charon
• How was Pluto Discovered?
• What is the nature of the
planet and its moon?
• Is Pluto a planet?
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Planet “X”
• Percival Lowell
repeated Adams & Le
Verrier calculations for
planet “X”
– He looked, but never
found it
• After Lowell Died, there
was a tedious search
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• Lowell hired Clyde Tombaugh who discovered
Pluto in 1930
• Pluto was named after the lord of Hades (in
Greek mythology), “PL” are the initials of
Percival Lowell
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Pluto’s Discovery
• Only “planet” found by
an American, and
only planet found in
the 20th century
• Too small to effect the
orbits of other planets
– The calculations of
Lowell didn’t matter
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