1. Chapter 9 Remnants of Rock and Ice Asteroids, Comets, and Pluto There is a strength in numbers: more than a trillion of comets and asteroids are orbiting the Sun. Asteroids and comets falling to Earth have scared our planet and altered the course of biological evolution. They also tell us a lot about the birth of the Solar system.

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 leftover planetesimals. 150,000 in catalogs, and probably over a million with diameter >1 km. Largest is Ceres, diameter ~1,000 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

5. Why is there an asteroid belt? Asteroid belt – where we find the majority of asteroids.

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

8. Why are there very few asteroids beyond Jupiter’s orbit? A.There was no rocky material beyond Jupiter’s orbit. B.The heaviest rocks sank towards the center of the solar system. C.Ice could form in the outer solar system. D.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? A.The belt is where all the asteroids happened to form. B.The belt is the remnant of a large terrestrial planet that used to be between Mars and Jupiter. C.The belt is where all the asteroids happened to survive.

10. Which explanation seems to be the most plausible? A.The belt is where all the asteroids happened to form. B.The belt is the remnant of a large terrestrial planet that used to be between Mars and Jupiter. C.The belt is where all the asteroids happened to survive.

11. During the Birth of solar system planetesimals formed throughout the inner solar system Most of those within Mars’s orbit were ultimately accreted into one of the four planets. Few asteroids that are still orbiting within Mars’s distance are “impacts waiting to happen”. Asteroids in the asteroid belt stay clear of any planets orbit and could survive for billions of years. But WHY didn’t they form a little planet?

12. Jupiter’s strong gravity, through the influence of orbital resonances, stirred up asteroid orbits and thereby prevented their accretion into a planet.

13. How are meteorites related to asteroids?

14. Meteor: only a flash of light caused by a particle entering our atmosphere. Meteorite: A rock from space that reaches Earth’s surface. Usually come from asteroid belt. Question: is there the same number of meteors and meteorites?

15. Peekskill, NY: October 9, 1992

16. Meteorites from Moon and Mars A few meteorites arrive from the Moon and Mars (composition differs from the asteroid fragments). They were blasted from their surface by impacts. 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.

17. 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.

18. 9.2 Comets Our Goals for Learning How do comets get their tails? Where do comets come from?

19. How do comets get their tails?

20. Comet Facts Formed beyond the frostline, comets are icy counterparts to asteroids. “Dirty snowballs” = the nucleus Most comets remain perpetually frozen in the outer solar system. Only a few enter the inner solar system, where they can grow tails.

21. When a comet nears the Sun, its ices can sublimate into gas and carry off dust, creating a coma and long tails. Plasma –gas ionized by the UV from the Sun Sand to pebble sized particles

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

24. Where do comets come from?

25. Kuiper belt: On orderly orbits from 30-100 AU in disk of solar system Oort cloud: 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

26. 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 out there by gravitational interactions with jovian planets: spherical distribution, orbits in any direction.

27. 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.

28. 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.

29. 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?

30. Have we ever witnessed a major impact?

31. Comet SL9 caused a string of violent impacts on Jupiter in 1994, reminding us that catastrophic collisions still happen. It consisted of a string of comet nuclei lined up in a row (tidal forces tore it apart during previous encounter with Jupiter)

32. Chain of craters on Callisto, showing that similar break ups of comets by Jupiter have occurred in the past.

33. Impact was predicted one year in advance and it was carefully monitored when it happened.

34. Impact plume rises high above Jupiter’s surface

36. This painting shows how the impact might have looked like from Io.

37. Each of the black dotes is a scar from one of the SL9 nuclei.

38. And in infrared…

39. Did an impact kill the dinosaurs?

40. Mass Extinctions They are 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?

41. 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.

42. Dinosaur fossils in lower rock layers No dinosaur fossils in these rock layers Thin layer containing iridium from impact Scientists are not sure if the impact was the sole reason for mass extinction, but there is little doubt that it really happened.

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

44. It hit in Yucatan, with a force of a hundred million hydrogen bombs. Scientists identified 65 million years old crater in Yucatan.

45. Hot debris from impact rained around the world causing worldwide iridium rich layer.

46. Impact released dust and smoke which probably stayed in the atmosphere for months, blocking sunlight and causing long harsh winter. Acid rains also caused were also killing vegetation and acidifying water…

47. 25% of all species survived. Notably mammals, which at that time were rodentlike, living underground, and probably having some amount of food stored.

48. An iridium-rich sediment layer and an impact crater on the Mexican coast show that a large impact occurred at the time the dinosaurs died out, 65 million years ago.

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

50. Facts Asteroids and comets have hit the Earth before. Some of these events coincide with past mass extinctions. A major impact is only a matter of time: not IF but WHEN. Scientists have identified more than 600 asteroids larger than 1 km, with orbits close to the Earths orbit. Probably lots of them are still undiscovered. But, major impacts are very rare. Extinction level events happen ~ many tens of millions of years apart. Major damage ~ tens-hundreds of years.

51. Tunguska, Siberia: June 30, 1908 The ~ 40 meter object disintegrated and exploded in the atmosphere. It released energy of several atomic bombs. Once a century event.

52. Meteor Crater, Arizona: 50,000 years ago (50 meter object)

53. 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.

54. 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…

55. What are we doing about it? Stay tuned to http://impact.arc.nasa.gov

56. 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.

57. 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.