1. Chapter 9 Remnants of Rock and Ice
Asteroids, Comets, and Pluto
Fun clip: the trailer from the movie Armageddon, if you can find it. The file was called armageddon.mov in Feb 2004 but it’s scarce now for some reason.
See also for Phil Plait’s review

2. 9.1 Asteroids and Meteorites
Our Goals for Learning
• Why is there an asteroid belt?
• How are meteorites related to asteroids?

3. Asteroid Facts Asteroids are rocky leftovers of planet formation.
Largest is Ceres, diameter ~1,000 km
150,000 in catalogs, and probably over a million with diameter >1 km.
Small asteroids are more common than large asteroids.
All the asteroids in the solar system wouldn’t add up to even a small terrestrial planet.

4. Asteroids are cratered and not round
Thought question: were these pictures taken from Earth?
One of these has a satellite (Ida is orbitted by Dactyl, lower left)
Asteroids are cratered and not round

5. Why is there an asteroid belt?

6. More than 150,000 asteroids at their predicted locations for Jan 1 2004
On this scale, asteroids are much smaller than the dots used to represent them

7. Why are there very few asteroids beyond Jupiter’s orbit?
There was no rocky material beyond Jupiter’s orbit.
The heaviest rocks sank towards the center of the solar system.
Ice could form in the outer solar system.
A passing star probably stripped away all of those asteroids, even if they were there at one time.
Rocky material condensed everywhere.
B. The students often get confused by the differentiation ideas they’ve just
heard about in planet formation.
D. If a passing star ripped off asteroids, it would have stripped away comets
and planets too. (That may be one explanation for the truncation of the
Kuiper belt…)

8. Why are there very few asteroids beyond Jupiter’s orbit?
There was no rocky material beyond Jupiter’s orbit.
The heaviest rocks sank towards the center of the solar system.
Ice could form in the outer solar system.
A passing star probably stripped away all of those asteroids, even if they were there at one time.

9. Which explanation seems to be the most plausible?
The belt is where all the asteroids happened to form.
The belt is the remnant of a large terrestrial planet that used to be between Mars and Jupiter.
The belt is where all the asteroids happened to survive.
Discussion: A -- planetesimals formed EVERYWHERE in the solar system.
B--there’s not enough mass in the asteroid belt to be the remnant of even
a small terrestrial planet.

10. Which explanation seems to be the most plausible?
The belt is where all the asteroids happened to form.
The belt is the remnant of a large terrestrial planet that used to be between Mars and Jupiter.
The belt is where all the asteroids happened to survive.
But WHY didn’t they form a little planet?

11. If you covered Chapter 8, orbital resonances are covered in the context with the jovian moons and Saturn’s rings.

12. Rocky planetesimals survived in the asteroid belt between Mars and Jupiter because they did not accrete into a planet.
Jupiter’s gravity, stirs up the asteroid orbits and prevents their planet formation.

13. How are meteorites related to asteroids?

14. How are meteorites related to asteroids?
Meteorites are pieces of asteroids - or sometimes planets or the Moon.

15. Meteor: The bright tail of hot debris from the rock
Really useless terminology: “Meteroid” - the rock before it hits the Earth’s surface.
Meteor: The bright tail of hot debris from the rock
Meteorite: A rock from space that reaches Earth’s surface

16. Peekskill, NY:
October 9, 1992
A movie of this meteorite flying through the Earth’s atmosphere is available for public use at
Other meteors/meteorite events available on
The web include the March 26, 2003 Park Forest IL event; August 10, 1972 Grand Tetons, WY (a BIG one skipping off the atmosphere.)

17. Pieces of Asteroids:Meteorite Types
Primitive: Unchanged in composition since they first formed 4.6 billion years ago.
Processed: Younger, have experienced processes like volcanism or differentiation.

18. Primitive Meteorites: simple, all ingredients mixed together
Simple rocks and metal, occasionally carbon compounds and water.
Shiny bits are metal flakes, first to condense.
White features are solidified dropletsof material that splashed out during impacts during accretion

19. Processed Meteorites: shattered fragments of larger objects
Iron from a core
Volcanic rock from a crust or mantle

20. What do we learn from meteorites?
primitive meteorites tell us when solar system formation began.
Processed meteorites tell us what asteroids are like on the inside.
Processed meteorites provide direct proof that differentiation and volcanism happened on asteroids.

21. Meteorites from Moon and Mars
A few meteorites arrive from the Moon and Mars
Composition differs from the asteroid fragments.
A cheap (but slow) way to acquire moon rocks and Mars rocks.
One Mars meteorite generated a stir when scientists claimed evidence for microscopic life in it.

22. What have we learned? • Why is there an asteroid belt?
Orbital resonances with Jupiter disrupted the orbits of planetesimals, preventing them from accreting into a planet. Those that were not ejected from this region make up the asteroid belt today. Most asteroids in other regions of the inner solar system accreted into one of the planets.
• How are meteorites related to asteroids?
Most meteorites are pieces of asteroids. Primitive meteorites are essentially unchanged since the birth of the solar system. Processed meteorites are fragments of larger asteroids that underwent differentiation.

23. 9.2 Comets Our Goals for Learning • How do comets get their tails?
• Where do comets come from?

24. How do comets get their tails?

25. Comet Facts
Formed beyond the frostline, comets are icy counterparts to asteroids.
“Dirty snowballs” = the nucleus
Most comets do not have tails.
Most comets remain perpetually frozen in the outer solar system. Only a few enter the inner solar system, where they can grow tails.

26. When a comet nears the Sun, its ices can sublimate into gas and carry off dust, creating a coma and long tails.

27. Draw This Picture

28. Comets eject small particles that follow the comet around in its orbit
This can cause meteor showers when Earth crosses the comet’s orbit.

29. Meteors in a shower appear to emanate from the same area of sky because of Earth’s motion through space

30. Where do comets come from?

31. Only a tiny number of comets enter the inner solar system - most stay far from the Sun
Oort cloud:
On random orbits extending to about 50,000 AU
Kuiper belt:
On orderly orbits from AU in disk of solar system

32. How did they get there?
Kuiper belt comets align with the plane of planet orbits
Oort Cloud Comets were kicked out of the solar system by the gravity from jovian planets: random orbits

33. What have we learned? • How do comets get their tails?
The vast majority of comets do not have tails. Only those few comets that enter the solar system grow tails. As the comet approaches the Sun its nucleus heats up. Some of the comet’s ice sublimates into gas, and the escaping gases carry along some dust. The gas and dust form a coma and two tails: a plasma tail of ionized gas and a dust tail. Larger particles can also escape, becoming the particles that cause meteors and meteor showers on Earth.

34. What have we learned? • Where do comets come from?
Comets that enter the solar system come from one of two reservoirs in the outer solar system: the Kuiper belt and the Oort cloud. The Kuiper belt comets still reside in the region beyond Neptune in which they formed during the birth of the solar system. The Oort cloud comets are thought to have formed in the region of the jovian planets, and were kicked out to the great distance of the Oort cloud by gravitational encounters with the planets.

35. 9.3 Pluto: Lone Dog or Part of a Pack?
Our Goals for Learning
• What is Pluto like?
• Is Pluto a planet or a Kuiper belt comet?

36. Pluto: the exception Not a gas giant like the other outer planets.
Has a very elliptical, inclined orbit.
By far the smallest planet, and smaller than several moons.
Has a surprisingly large moon Charon, probably formed by a huge comet collision with Pluto.

37. Pluto will never collide with Neptune because of a 3:2 orbital resonance.

38. What is Pluto like?
1978 discovery of Pluto’s moon Charon: Pluto’s mass from Newton’s orbital law.
It has a thin nitrogen atmosphere that will refreeze onto the surface as Pluto’s orbit takes it farther from the Sun.
Pluto is the largest Solar System object that has not been visited by spacecraft.

39. HST’s view of Pluto & Charon

40. Brightness variations during eclipsing orbits showed dirty ice - like comets.

41. Is Pluto a planet or a Kuiper Belt comet?

42. Is Pluto a planet or a Kuiper Belt comet?
Pluto is well beyond Neptune, in the Kuiper Belt.
Inclined orbit is typical of Kuiper Belt comets.
Composition is typical of Kuiper Belt comets, but not any of the other planets.

43. Is Pluto a planet or a Kuiper Belt comet?
Kuiper Belt objects have been found that approach Pluto’s size.
Kuiper Belt comets have similar orbital resonances with Neptune.
Kuiper Belt comets can have moons.
Triton (a captured moon) is even larger than Pluto.
News update: Sedna was discovered Nov 14, It is 3 times farther away than Pluto, it is about 75% the size of Pluto. It may have a moon. It will continue to brighten for the next 72 years. It has a 10,500 year orbit. It might be a representative of the inner Oort cloud. It has a reddish color for unknown reasons.

44. What have we learned? • Is Pluto a planet or a Kuiper belt comet?
Whether Pluto should be called a “planet” is a matter of opinion, but its properties suggest that it is a Kuiper belt comet. Its composition and orbital properties match those of other Kuiper belt comets and do not fit in with the other planets. It is the largest known Kuiper belt comet today, but there may be larger ones still awaiting discovery.
• What is Pluto like?
Pluto is much smaller than any other planet, with an orbit more elliptical and more inclined to the ecliptic plane than that of any other planet. It is made mostly of ices and has a very thin atmosphere of gases that are expected to freeze onto the surface as Pluto moves farther from the Sun in its 248-year orbit. It has a moon, Charon, with a slightly lower density than Pluto, suggesting that Charon may have been formed in a giant impact.

45. 9.4 Cosmic Collisions: small bodies vs. the planets
Our Goals for Learning
• Have we ever witnessed a major impact?
• Did an impact kill the dinosaurs?
• Is the impact threat a real danger or just media hype?
• How do other planets affect impact rates and life on Earth?

46. Have we ever witnessed a major impact?
Smithsonian (SITES) and the Space Telescope Science Institute produced a marvelous minidocumentary of the Shoemaker Levy coverage and real scientist commentary.
Download “Comet Crash”
QT movie (21 MB), broadcast quality mpeg-2 file (124 MB)

47. Comet SL9 caused a string of violent impacts on Jupiter in 1994, reminding us that catastrophic collisions still happen.
Tidal forces tore it apart during previous encounter with Jupiter

48. Chain of craters on Callisto, of another comet torn apart by tidal forces from Jupiter.

50. Impact plume rises high above Jupiter’s surface

55. Did an impact kill the dinosaurs?

56. Mass Extinctions
Large dips in total species diversity in the fossil record.
The most recent was 65 million years ago, ending the reign of the dinosaurs.
Was it caused by an impact?
How would it have happened?

57. No dinosaur fossils in these rock layers
Thin layer containing iridium from impactor
Dinosaur fossils in lower rock layers

58. Iridium - evidence of an impact
Iridium is very rare in Earth surface rocks but often found in meteorites.
Luis and Walter Alvarez found a worldwide layer containing iridium, laid down 65 million years ago.

59. Comet or asteroid about 10km in diameter approaches Earth

64. shows that a large impact occurred at the time the dinosaurs died out,
An iridium-rich sediment layer and an impact crater on the Mexican coast 65 million years ago.
shows that a large impact occurred
at the time the dinosaurs died out,
Map of variable gravity strength; white lines show the outlines of land masses
and the Mexican regions.

65. The Impact Threat: Real danger or media hype?

66. Facts Asteroids and comets have hit the Earth.
A major impact is only a matter of time: not IF but WHEN.
Major impact are very rare.
Extinction level events ~ millions of years.
Major damage ~ tens-hundreds of years.

67. Tunguska, Siberia: June 30, 1908
Several atomic bombs worth of energy.
Tunguska, Siberia: June 30, 1908
The ~40 meter object disintegrated and exploded in the atmosphere

68. Meteor Crater, Arizona: 50,000 years ago (50 meter object)
Crater is 1 km in diameter.
Impact was 20 megatons
Privately owned National Landmark.
Meteor Crater, Arizona: 50,000 years ago (50 meter object)

69. An object 1km in size would produce a mile-high tidal wave
This slide is placeholder in case you would like to download and show the movie noted below (notes field).
A simulation movie is available for public use at
The description is at:
The impact scene in the Hollywood movie from 1998’s Deep Impact is actually pretty good.
An object 1km in size would produce a mile-high tidal wave

70. Chance of death by asteroid: About one in 100,000
From 7-April-2004 congressional testimony of Astronaut Ed Lu, B612 Foundation: 10% chance during our lifetime of a 70 m asteroid, impacting with energy of 10 megatons or 700 Hiroshima bombs. (Senate Subcommittee on Science, Technology, and Space.) Note that 50 meters was the size of the object that created Meteor Crater.
Impacts will certainly occur in the future, and while the chance of a major impact in our lifetimes is small, the effects could be devastating.

71. The asteroid with our name on it
We haven’t seen it yet.
Deflection is more probable with years of advance warning.
Control is critical: breaking a big asteroid into a bunch of little asteroids is unlikely to help.
We get less advance warning of a killer comet…

72. What are we doing about it?
Stay tuned to

73. How do other planets affect impact rates and life on Earth?

74. Gravity from Jovial planets can influence the path of comets and Asteroids. They could protect us or steer one in our direction
Fig 9.20

75. Was Jupiter necessary for life on Earth?
Impacts can extinguish life.
But were they necessary for ‘life as we know it’?

76. What have we learned? • Have we ever witnessed a major impact?
In 1994, we observed the impacts of comet Shoemaker–Levy 9 on Jupiter. The comet had fragmented into a string of individual nuclei, so there was a string of impacts that left Jupiter’s atmosphere scarred for months

77. What have we learned? • Did an impact kill the dinosaurs?
We are not certain whether an impact was the sole cause, but a major impact clearly coincided with the mass extinction in which the dinosaurs died out, about 65 million years ago. Sediments from the time show clear evidence of an impact, and an impact crater of the right age has been found near the coast of Mexico.

78. What have we learned?
• Is the impact threat a real danger or just media hype?
Impacts certainly pose a threat, though the probability of a major impact in our lifetimes is fairly low.

79. What have we learned?
• How do other planets affect impact rates and life on Earth?
Impacts of asteroids and comets are always linked in at least some way to the gravitational influences of Jupiter and the other jovian planets. These gravitational influences have shaped the asteroid belt, the Kuiper belt, and the Oort cloud, and sometimes still help determine when an object is flung our way.