1. Where did asteroids and comets come from?

2. Asteroids and Comets Leftovers from the accretion process
Rocky asteroids inside frost line
Icy comets outside frost line

3. Where are comets and asteroids today?
Most asteroids are in the asteroid belt between Mars and Jupiter.
Most comets are in the Kuiper Belt and the Oort Cloud.

4. Heavy Bombardment
Leftover planetesimals bombarded other objects in the late stages of solar system formation.

5. Origin of Earth’s Water
Water may have come to Earth by way of icy planetesimals from the outer solar system.

6. How do we explain the existence of our Moon and other exceptions to the rules?

7. Giant Impact
…then accreted into the Moon.

8. Odd Rotation
Giant impacts might also explain the different rotation axes of some planets.

9. Review of the nebular theory

10. Thought Question
How would the solar system be different if the temperature everywhere in the proto planetary disc was halved?
Jovian planets would have formed closer to the Sun.
There would be no asteroids.
There would be no comets.
Terrestrial planets would be larger.

11. Thought Question
Which of these facts is NOT explained by the nebular theory?
There are two main types of planets: terrestrial and jovian.
Planets orbit in the same direction and plane.
Asteroids and comets exist.
There are four terrestrial and four jovian planets.

12. When did the planets form?
We cannot find the age of a planet, but we can find the ages of the rocks that make it up.
We can determine the age of a rock through careful analysis of the proportions of various atoms and isotopes within it.

13. Radioactive Decay Some isotopes decay into other nuclei.
A half-life is the time for half the nuclei in a substance to decay.

14. Thought Question
Suppose you find a rock originally made of potassium-40, half of which decays into argon-40 every 1.25 billion years. You open the rock and find 15 atoms of argon-40 for every atom of potassium-40. How long ago did the rock form?
1.25 billion years ago
2.5 billion years ago
3.75 billion years ago
5 billion years ago

15. Dating the Solar System
Age dating of meteorites that are unchanged since they condensed and accreted tells us that the solar system is about 4.6 billion years old.

16. Dating the Solar System
Radiometric dating tells us that the oldest moon rocks are 4.4 billion years old.
The oldest meteorites are 4.55 billion years old.
Planets probably formed 4.5 billion years ago.

17. Meteorites from the Moon and Mars
A few meteorites arrive on Earth from the Moon and Mars.
Composition differs from the asteroid fragments.
This is a cheap (but slow) way to acquire moon rocks and Mars rocks. 17

18. 9.2 Comets Our goals for learning: How do comets get their tails?
Where do comets come from? 18

19. How do comets get their tails?19

20. Comet Facts
Formed beyond the frost line, comets are icy counterparts to asteroids.
The nucleus of a comet is like a “dirty snowball.”
Most comets do not have tails.
Most comets remain perpetually frozen in the outer solar system.
Only comets that enter the inner solar system grow tails. 20

21. Nucleus of Comet A “dirty snowball”
Source of material for comet’s tail 21

22. Anatomy of a Comet Coma is atmosphere that comes from heated nucleus.
Plasma tail is gas escaping from coma, pushed by solar wind (it always points way from the Sun).
Dust tail is pushed by photons. 22

23. Growth of Tail 23

24. Comets eject small particles that follow the comet around in its orbit and cause meteor showers when Earth crosses the comet’s orbit. 24

25. Where do comets come from?25

26. Comets on random orbits extending to about 50,000 AU
Only a tiny number of comets enter the inner solar system; most stay far from the Sun.
Oort Cloud:
Comets on random orbits extending to about 50,000 AU
Kuiper Belt:
Comets on orderly orbits at AU in disk of solar system 26

27. How did they get there? Kuiper Belt comets formed in the Kuiper Belt.
Flat plane aligned with the plane of planetary orbits
Orbiting in the same direction as the planets
Oort Cloud comets were once closer to the Sun, but they were kicked farther out by gravitational interactions with jovian planets.
Spherical distribution
Orbiting in any direction 27

28. Pluto and Other Kuiper Belt Objects
Kuiper Belt objects are basically icy bodies that are the same as comets.
Most comets and asteroids are not spherical.
Only asteroids and larger than about 500km in diameter are spherical.
For these large objects their own gravity is strong enough to make them spherical. 28

29. How big can a comet be? 29

30. Pluto’s Orbit Pluto’s orbit is tilted and significantly elliptical.
Neptune orbits three times during the time Pluto orbits twice—resonance prevents a collision. 30

31. Is Pluto a planet? Much smaller than the eight major planets
Not a gas giant like the outer planets
Has an icy composition like a comet
Has a very elliptical, inclined orbit
Pluto has more in common with comets than with the eight major planets. 31

32. Discovering Large Iceballs
In summer 2005, astronomers discovered Eris, an iceball even larger than Pluto.
Eris even has a moon: Dysnomia. 32

33. Other Icy Bodies
There are many icy objects like Pluto on elliptical, inclined orbits beyond Neptune.
The largest ones are comparable in size to Earth’s Moon. 33

34. Kuiper Belt Objects
These large, icy objects have orbits similar to the smaller objects in the Kuiper Belt that become short period comets.
So are they very large comets or very small planets? 34

35. Is Pluto a planet?
In 2006, the International Astronomical Union decided to call Pluto and objects like it “dwarf planets.” 35

36. What is Pluto like?
Its largest moon, Charon, is nearly as large as Pluto itself.
Pluto is very cold (40 K).
Pluto has a thin nitrogen atmosphere.
The atmosphere refreezes onto the surface as Pluto’s orbit takes it farther from the Sun.
Its axial tilt is small compared to that of the Earth.
Its ellipticity is 0.25 (larger than Earth’s 0.016).
Its perihelion distance is about 30AU.
Its aphelion distance is about 50AU.
Unlike the Earth, Pluto’s seasons are cause by its changing distance from the Sun. 36

37. HST’s View of Pluto and Its Moons37

38. Other Kuiper Belt Objects
Most have been discovered very recently so little is known about them.
Halley's Comet is an example of a comet that is thought to have originally formed in the Kuiper belt and now has an orbit that brings it near the Earth every 76 years. 38

39. Summary
The Kuiper Belt from which comets come contains objects as large as Pluto.
The icy composition of Pluto and other “dwarf planets” is more like large comets than like the major planets.
Large objects in the Kuiper Belt have tilted, elliptical orbits and icy compositions like those of comets. 39

40. Collisions In the Solar System40

41. Recent Impacts in the Solar System
Comet Shoemaker–Levy 9 collided with Jupiter in the mid-1990s.
It was about 10km in size and if it had struct the Earth it might have ended all life on our planet.

42. Impact sites in infrared light42

43. Impact plume from a fragment of comet SL9 rises high above Jupiter’s surface.43

44. Dusty debris at an impact site44

45. Several impact sites 45

46. Did an impact kill the dinosaurs? Mass Extinctions
Fossil record shows occasional large dips in the diversity of species: mass extinctions.
The most recent was 65 million years ago, ending the reign of the dinosaurs. 46

47. Iridium: Evidence of an Impact
Iridium is very rare in Earth surface rocks but is often found in meteorites.
Luis and Walter Alvarez found a worldwide layer containing iridium, laid down 65 million years ago, probably by a meteorite impact.
Dinosaur fossils all lie below this layer. 47

48. Iridium Layer No dinosaur fossils in upper rock layers
Thin layer containing the rare element iridium
Dinosaur fossils in lower rock layers 48

49. Consequences of an Impact
A meteorite 10 km in size would leave a crater about 200km (20 times larger than the meteorite).
The impact sends large amounts of debris into the atmosphere.
Debris would reduce the amount of sunlight reaching Earth’s surface.
The resulting climate change (called impact-winter) may have caused mass extinction. 49

50. Likely Impact Site
Geologists found a large subsurface crater about 65 million years old in Mexico.
200km crater suggests impacting object was ~10 km in diameter.
Impact of such a large object would have ejected debris high into Earth’s atmosphere. 50

51. Tunguska Impact – Siberia 1908
A meteorite of about 40m in diameter exploded about 5km about the Tunguska forest with the force of 15 Megaton bomb and leveling nearly 1000sq miles of Siberian forest. 51

52. 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) 52

53. Facts About Impacts Asteroids and comets have hit Earth.
A major impact is only a matter of time: not IF but WHEN.
Major impacts are very rare.
Extinction level events ~ millions of years
Major damage ~ tens to hundreds of years 53

54. Frequency of Impacts
We can determine how often impacts occur by counting the number of craters of different sizes on the moon and other planets.
Small impacts happen almost daily.
Impacts large enough to cause mass extinctions are many millions of years apart. 54

55. The Asteroid with Our Name on It
We have found about 80% of the estimated 10,000 Near Earth Objects (NEOs).
About 1200 are potentially hazardous.
For large asteroids we may get a few years of warning.
Some hazardous asteroids will come close but none will strike the Earth in the next few years.
How can we avoid the danger of a big impact.
Deflection is more probable with years of advance warning.
Breaking a big asteroid into a bunch of little asteroids is unlikely to help.
We get less advance warning of a killer comet.
They only become bright when the near the sun.
Their highly elliptical orbit means they move very fast close to the sun. 55

56. What are we doing about it?
Stay tuned to 56

57. How do other planets affect impact rates and life on Earth?57

58. Influence of Jovian Planets
The gravity of a jovian planet (especially Jupiter) can redirect a comet. 58

59. Influence of Jovian Planets
Jupiter has directed some comets toward Earth but has ejected many more into the Oort Cloud. 59

60. Was Jupiter necessary for life on Earth?
Impacts can extinguish life.
But were they necessary for “life as we know it”?
Were comet impacts the source of the Earth’s water? 60