2. Lecture 23

4. Outline For Rest of Semester
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)

5. Outline
Quiz discussion
Review question
Neptune and Uranus

6. 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.

7. Opposition
Which planet will appear more often at opposition, Jupiter or Saturn? 1. Same 2. Jupiter 3. Saturn

8. In-class quiz 15 quiz points for any answer
Triple points if all answers today correct

9. Earth, Moon, Sun, Venus, and Mars

10. astr111 3 rweigel Register for quiz 3 Text to 41411:
Replace with your GMU name
Text to 41411:
astr rweigel
You will receive either one or two texts in response

11. astr111 3 rweigel-jsmith1-hjones7
Register for quiz 3
Separate group member names with hyphen
Text to 41411:
astr rweigel-jsmith1-hjones7
You will receive either one or two texts in response
Late students may not work in group

12. 3 1 c
3 1 b
3 1 a
3 1 d
Which case, blue, red, green, or black, best represents the configuration in the photo?
If you think the black dots best represent configuration in photo, text 3 1 d

13. 3 1 c
3 1 b
3 1 a
3 1 d
Which case, blue, red, green, or black, best represents the configuration in the photo?
If you think the black dots best represent configuration in photo, text 3 1 d

14. Where is the best placement of moon?
3 2 d
3 2 b
3 2 c
3 2 a
Where is the best placement of moon?

15. d b c a

16. Suggested Reading
Chapters on Uranus and Neptune

17. Neptune and Uranus

18. Doubling the Solar System

19. How can you tell the difference between a planet and a distant star
How can you tell the difference between a planet and a distant star? (assume the both span the same angular distance)
Planet always moves in retrograde
Star always appears at same position in sky
Star position repeats after one year, planet does not
Planet position repeats after on year, star does not
3 3
3 3
3 3
3 3

20. Uranus A Short History of Nearly Everything
Hershel “discovered” it
A Short History of Nearly Everything
By Bill Bryson (Travel Writer and Humorist)
German-born musician
At opposition barely bright enough to be seen with naked eye – ancients probably saw it
Plotted in at least 20 star charts between 1960 and 1782

21. Neptune’s discovery + = A triumph of modern science
Could not predict orbit of Uranus using Newtonian Mechanics
Newton’s Law of gravity was suspected to be wrong at great distances – discuss strong force.
Another suggestion was that gravitational pull of another planet was the cause – Uranus accelerated as it approached and decelerated after it passed + =

22. The Pioneer anomaly Is history repeating itself? Pioneer plaque
In 1972, we sent out Pioneer 10 and 11 to survey the solar system and beyond.
About 5 or 10 years ago, people realized that Pioneer 10 and 11 were not in the correct place.
One proposal is that at very far distances, Newtonian gravity does not hold …
Many other proposed causes were tested.
Most recent information is that about 1/3 of the anomaly can be explained by differential heating
Pioneer plaque

23. http://www. guardian. co. uk/science/2004/sep/12/spaceexploration
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.

24. 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.

25. Neptune
Galileo missed it – noted something that star seemed to have moved in relation to other stars.
Yo-hoo
Over here
But background motion is so slow, that “seemed” is all he could really say at that point. (Moves only two degrees per year).

26. 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?

27. 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.

30. Atmosphere

31. Both Uranus and Neptune have atmospheres composed primarily of hydrogen, helium, and a few percent methane
What colors does methane absorb?

32. 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?

33. Red has long wavelength, blue short.
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

34. Bizarro tilt on Uranus

35. 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

37. Which would you guess has a more active atmosphere?
Based on distance from Sun
Based on above pictures

38. 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?

39. “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?

40. 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

41. Interior

42. 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

43. They should not exist

44. Bizarro Magnetic Axis
The magnetic fields of both Uranus and Neptune are oriented at unusual angles

45. 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

46. Moons and Rings
un14vi03.mov
Movie 16.3

47. Uranus and Neptune each have a system of thin, dark rings
Movie 16.3

48. Discovery of Uranian Rings

49. Movie
Movie 16.2

51. Some of Uranus’s satellites show evidence of past tidal heating
Uranus has five satellites similar to the moderate-sized moons of Saturn, plus at least 22 more small satellites

52. Heavily cratered
Dramatic topography
Unfinished tidal heating?

54. 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

55.  Neptune was
3 4
predicted to exist through its gravitational effect on Uranus
discovered in ancient times because it is a naked-eye object.
found by accident during a search for comets.
predicted to exist through its gravitational effect on Saturn.
was discovered using a radio telescope.
3 4
3 4
3 4
3 4

56.  Neptune was
3 4
predicted to exist through its gravitational effect on Uranus
discovered in ancient times because it is a naked-eye object.
found by accident during a search for comets.
predicted to exist through its gravitational effect on Saturn.
was discovered using a radio telescope.
3 4
3 4
3 4
3 4

57. Which of the following statements correctly describes a currently proposed picture for the formation of Uranus and Neptune?
Uranus and Neptune formed where they are now with the mass and composition we see today.
Uranus and Neptune formed farther from the Sun than they are now and gradually spiraled inward to their present orbits.
Uranus and Neptune formed closer to the Sun than they are now and were pushed outward to their present orbits.
Uranus is a result of the collision of a Mars-sized object with Neptune.
3 5
3 5
3 5
3 5

58. We know that Neptune contains a larger amount of heavier elements than Saturn. We know this because Neptune
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.
3 6
3 6
3 6
3 6
3 6