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Lecture 23
Outline
• Neptune and Uranus
Neptune and Uranus
Doubling the Solar System
• How can you tell the difference between a
planet and a distant star?
– (assume the both span the same angular
distance)
– Name two objects in the sky that are not stars
Uranus
• Hershel “discovered” it
A Short History of Nearly Everything
By Bill Bryson (Travel Writer and
Humorist)
Neptune’s discovery
• A triumph of modern science
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The Pioneer anomaly
• Is history repeating itself?
Pioneer plaque
http://www.guardian.co.uk/science/2004/s
ep/12/spaceexploration.research
• Some researchers say unseen 'dark matter'
may permeate the universe and that this is
affecting the Pioneers' passage. Others say
flaws in our understanding of the laws of
gravity best explain the crafts' wayward
behaviour.
• As a result, scientists are to press a
European Space Agency (Esa) meeting,
called Cosmic Visions, in Paris this week for
backing for a mission that would follow the
Pioneers and pinpoint the cause of their
erratic movements.
• The reasons for the anomaly have caused
a rift among physicists, … 'Unless there is
really good evidence to the contrary, we
should stick to simple ideas like these
and not go around blaming strange
new types of particle or flaws in general
relativity,' said Professor Martin Barstow,
of Leicester University.
• One proposal put forward is that Newton's
idea that the force of gravity weakens as
distance increases may be incorrect over
very large spaces, and may drop off over
very long distances.
Neptune
• Galileo missed it – noted something that
star seemed to have moved in relation to
other stars.
Yo-hoo
Over here
Neptune’s orbit
• Uranus is at about 20 AU and Neptune is
at about 30 AU. The ratio is 3:2. Does
this mean anything?
Neptune’s orbit
• Uranus is at about 20 AU and Neptune is
at about 30 AU. The ratio is 3:2. Does
this mean anything?
• Saturn’s orbit is 9.5 AU, Jupiter’s orbit is at
5.2 AU.
• Mercury is at 0.38 AU, Venus is at 0.73
AU, Earth is at 1 AU, Mars is at 1.5 AU.
• Both Uranus and Neptune have
atmospheres composed primarily of
hydrogen, helium, and a few percent
methane
• What colors does methane absorb?
• What colors does methane absorb?
• Methane is a greenhouse gas
• Greenhouse gasses tend to absorb long
wavelength photons, which Earth’s surface
emits
• Is the color of Uranus consistent with this?
Bizarro tilt on Uranus
Exaggerated Seasons On Uranus
• Uranus’s axis of rotation lies
nearly in the plane of its
orbit, producing greatly
exaggerated seasonal
changes on the planet
• This unusual orientation may
be the result of a collision
with a planetlike object early
in the history of our solar
system. Such a collision
could have knocked Uranus
on its side
• Which would you guess has a more
active atmosphere?
– Based on distance from Sun
– Based on above pictures
• Neptune looks more active
• But its orbit is 30 AU compared to 20 AU
for Uranus. What is the difference
between amount of energy they receive?
• “Thanks to distance, Neptune
receives less than one-half of the
amount of solar energy than
Uranus.”
• Where did the “one-half” number
come from?
Uranus and Neptune contain a higher proportion
of heavy elements than Jupiter and Saturn
• Both Uranus and Neptune may have a rocky core
surrounded by a mantle of water and ammonia
• Electric currents in the mantles may generate the magnetic
fields of the planets
They should not exist
Bizarro Magnetic Axis
The magnetic
fields of both
Uranus and
Neptune
are oriented
at unusual
angles
• 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
Uranus and Neptune each have a
system of thin, dark rings
Some of Uranus’s satellites show
evidence of past tidal heating
Uranus has five satellites similar to the moderatesized moons of Saturn, plus at least 22 more small
satellites
• Heavily cratered
• Dramatic
topography
• Unfinished tidal
heating?
Triton is a frigid, icy world with a young
surface and a tenuous atmosphere
• Neptune has 13 satellites,
one of which (Triton) is
comparable in size to our
Moon or the Galilean
satellites of Jupiter
• Triton has a young, icy
surface indicative of
tectonic activity
• The energy for this activity
may have been provided
by tidal heating that
occurred when Triton was
captured by Neptune’s
gravity into a retrograde
orbit
• Triton has a tenuous
nitrogen atmosphere