Download 20091127131747!Lecture-23-2009-ASTR111-Weigel

Lecture 14
Outline For Rest of Semester
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Oct. 29th Chapter 9 (Earth)
Nov 3rd and 5th Chapter 9 and Chapter 10 (Earth and Moon)
Nov. 10th and 12th Mars, Venus, and Mercury
Nov. 17th and 19th Jupiter and Saturn
Nov 24th Uranus and Neptune
Nov 26th Thanksgiving
Dec. 1st - Exam 3
Dec. 3rd – Pluto, and the Kuiper Belt
Dec. 8th and 10th – Chapter 7 and 8 (Comparative Planetology I and II)
Tuesday December 15th (7:30 am – 10:15 am) Final Exam
No Reading days are scheduled this semester. (Exam Period begins at
7:30 a.m. on Monday, December 14 and ends on December 21)
Outline
• Quiz discussion
• Review question
• Neptune and Uranus
Roche limit
Some of the small shepherd satellites within Saturn's
ring system are also inside Saturn's Roche Limit. Why
are they not torn apart by tidal forces due to Saturn's
gravity?
1. The Roche Limit applies only to the ring particles, not
to anything as large as a satellite
2. The interaction between Saturn's strong magnetic field
and the magnetic fields generated by the shepherd
satellites helps to hold the satellites together.
3. The Roche Limit only applies to objects held together
by mutual gravitational attraction, not to chunks of rock
like the shepherd satellites.
4. Unlike the ring particles, the satellites are large
enough to produce significant gravitational fields of their
own, and these counteract the tidal forces.
Opposition
Which planet will appear more often at
opposition, Jupiter or Saturn?
1. Same
2. Jupiter
3. Saturn
In-class quiz
• 15 quiz points for any answer
• Triple points if all answers today correct
Earth, Moon, Sun,
Venus, and Mars
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31b
31a
Which case, blue, red,
green, or black, best
represents the
configuration in the
photo?
31c
31d
If you think the black dots
best represent
configuration in photo, text
31d
31b
31a
Which case, blue, red,
green, or black, best
represents the
configuration in the
photo?
31c
31d
If you think the black dots
best represent
configuration in photo, text
31d
32d
32b
32c
32a
Where is the best
placement of moon?
d
b
c
a
Suggested Reading
• Chapters on Uranus and Neptune
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)
3 3 a. Planet always moves in retrograde
3 3 b. Star always appears at same position in sky
3 3 c. Star position repeats after one year, planet
does not
3 3 d. Planet position repeats after on year, star
does not
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
+
=
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.
Atmosphere
• 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?
• 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?
–Red has long wavelength, blue
short.
–So methane absorbing long
wavelength photons is consistent
with it being a greenhouse gas
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?
• “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?
– Energy Flux ~ 1/r2
– Energy Flux Uranus ~ 1/202 = 1/400
– Energy Flux Neptune ~ 1/302 = 1/900
– Ratio is 400/900, which is a little less than 0.5
Interior
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
Moons and Rings
• un14vi03.mov
Movie 16.3
Uranus and Neptune each have a
system of thin, dark rings
Movie 16.3
Discovery of Uranian Rings
Movie
1403002.swf
Movie 16.2
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
Neptune was
3 4 a. predicted to exist through its gravitational
effect on Uranus
3 4 b. discovered in ancient times because it is
a naked-eye object.
3 4 c. found by accident during a search for
comets.
3 4 d. predicted to exist through its gravitational
effect on Saturn.
3 4 e. was discovered using a radio telescope.
Neptune was
3 4 a. predicted to exist through its gravitational
effect on Uranus
3 4 b. discovered in ancient times because it is
a naked-eye object.
3 4 c. found by accident during a search for
comets.
3 4 d. predicted to exist through its gravitational
effect on Saturn.
3 4 e. was discovered using a radio telescope.
Which of the following statements correctly
describes a currently proposed picture for the
formation of Uranus and Neptune?
3 5 a. Uranus and Neptune formed where they are
now with the mass and composition we see
today.
3 5 b. Uranus and Neptune formed farther from the
Sun than they are now and gradually spiraled
inward to their present orbits.
3 5 c. Uranus and Neptune formed closer to the Sun
than they are now and were pushed outward to
their present orbits.
3 5 d. Uranus is a result of the collision of a Marssized object with Neptune.
We know that Neptune contains a larger
amount of heavier elements than Saturn.
We know this because Neptune
a.
b.
c.
d.
3 6 e.
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is smaller than Saturn
has a higher density than Saturn
is farther from the Sun than Saturn
has a lower density than Saturn
is larger than Saturn.