1. Asteroids, Comets, Meteors Their Nature, Orbits, and Impacts

2. Asteroids, Comets & Meteors Our goals for learning: Why is there an asteroid belt? Why is there an asteroid belt? How are meteorites related to comets? How are meteorites related to comets?

3. A “typical” Asteroid?

4. Another “typical” Asteroid?

5. Asteroid Facts Asteroids are rocky leftovers of planet formation. Asteroids are rocky leftovers of planet formation. The largest is Ceres, diameter ~1,000 km. The largest is Ceres, diameter ~1,000 km. Usually oblong, irregular shapes Usually oblong, irregular shapes Heavily cratered Heavily cratered Some have moons! Some have moons!

6. Asteroids: cratered typically not round

7. Asteroid Facts There are 150,000 in catalogs, and probably over a million with diameter >1 km. There are 150,000 in catalogs, and probably over a million with diameter >1 km. Small asteroids are more common than large asteroids. 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. All the asteroids in the solar system wouldn’t add up to even a small terrestrial planet.

8. Asteroid Facts Most asteroids orbit in a belt between Mars and Jupiter. Most asteroids orbit in a belt between Mars and Jupiter. Trojan asteroids follow Jupiter’s orbit. Trojan asteroids follow Jupiter’s orbit. Orbits of near-Earth asteroids cross Earth’s orbit. Orbits of near-Earth asteroids cross Earth’s orbit.

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

10. –There was no rocky material beyond Jupiter’s orbit. –The heaviest rocks sank toward 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. Why are there very few asteroids beyond Jupiter’s orbit?

11. Which explanation for the asteroid belt seems the most plausible? The belt is where all the asteroids happened to form. 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 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. The belt is where all the asteroids happened to survive.

12. The belt is where all the asteroids happened to form. 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 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? Which explanation for the asteroid belt seems the most plausible?

13. Orbital Resonances Asteroids in orbital resonance with Jupiter experience periodic nudges. Asteroids in orbital resonance with Jupiter experience periodic nudges. Eventually nudges move asteroids out of resonant orbits, leaving gaps in the belt. Eventually nudges move asteroids out of resonant orbits, leaving gaps in the belt.

14. Origin of Asteroid Belt? Jupiter’s gravity, through influence of orbital resonances, “stirred up” asteroid orbits Prevented accretion into a planet. Jupiter’s gravity, through influence of orbital resonances, “stirred up” asteroid orbits Prevented accretion into a planet.

15. How are meteorites related to asteroids?

16. Meteor Terminology : The bright trail left by a meteorite (what you wish on!) Meteor: The bright trail left by a meteorite (what you wish on!) : A rock from space that falls through Earth’s atmosphere (what you pick up!) Meteorite: A rock from space that falls through Earth’s atmosphere (what you pick up!) Meteor Shower: A series of meteors coming from one area of the sky annually! Meteor Shower: A series of meteors coming from one area of the sky annually!

17. The Leonids, 1998 Other movie clips of meteor & showers Leonids Perseids ABC News ToyotaToyota!

18. Meteorite Types 1. Primitive: Unchanged in composition since they first formed 4.6 billion years ago 2. Processed: Younger, have experienced processes like volcanism or differentiation

19. Primitive Meteorites

20. Processed Meteorites

21. Meteorites from the Moon and Mars Some meteorites found on Earth come from our Moon and Mars Some meteorites found on Earth come from our Moon and Mars Composition differs from the asteroid fragments Composition differs from the asteroid fragments A cheap (but slow) way to acquire moon rocks and Mars rocks A cheap (but slow) way to acquire moon rocks and Mars rocks

22. Comets

23. Comet Facts Formed beyond the frost line, comets are icy counterparts to asteroids. Formed beyond the frost line, comets are icy counterparts to asteroids. The nucleus of a comet is like a “dirty snowball.” The nucleus of a comet is like a “dirty snowball.” Most comets do not have tails. Most comets do not have tails.

24. Comet Facts Most comets remain perpetually frozen in the outer solar system. Most comets remain perpetually frozen in the outer solar system. Only comets that enter the inner solar system grow tails. Only comets that enter the inner solar system grow tails.

25. Anatomy of a Comet Coma (atmosphere from heated nucleus) Coma (atmosphere from heated nucleus) Plasma tail (gas escaping from coma pushed by solar wind) Plasma tail (gas escaping from coma pushed by solar wind) Dust tail (orbiting, & pushed by sunlight) Dust tail (orbiting, & pushed by sunlight)

26. Growth of Tail

27. Deep Impact Mission to study nucleus of Comet Tempel 1 Mission to study nucleus of Comet Tempel 1 Projectile hit surface on July 4, 2005 Projectile hit surface on July 4, 2005 Many telescopes studied aftermath of impact Many telescopes studied aftermath of impact

28. Comets eject small particles that follow the comet around in its orbit and 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. Kuiper belt: In orderly orbits from 30–100 AU in disk of solar system Oort cloud: In random orbits extending to about 50,000 AU Very few comets enter inner solar system; most stay far from Sun.

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

33. How did comets get there? Hypothesis: Kuiper belt comets formed in the belt. Hypothesis: Kuiper belt comets formed in the belt. Supporting Evidence Supporting Evidence –Kuiper Belt comets orbit in flatter planes, –Orbits aligned with the plane of planetary orbits, –Orbiting in the same direction as the planets.

34. How did comets get there? Hypothesis: Oort cloud comets were once closer to the Sun, but they were kicked out there by gravitational interactions with jovian planets Hypothesis: Oort cloud comets were once closer to the Sun, but they were kicked out there by gravitational interactions with jovian planets Supporting Evidence: Supporting Evidence: –spherical distribution around the Sun –orbits in any direction –Existence of other Oort clouds?

35. Have we ever witnessed a major comet/asteroid impact?

36. 1994: Comet SL9 caused a string of violent impacts on Jupiter Tidal forces tore it apart during a previous encounter with Jupiter.

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

38. Artist’s conception of SL9 impact

39. Dusty debris at an impact site

40. Several impact sites

41. Impact sites in infrared light

42. This crater chain on Callisto probably came from another comet that tidal forces tore to pieces.

43. Impacts on Earth!

44. Did an impact kill the dinosaurs?

45. Mass Extinctions Fossil record shows occasional large dips in the diversity of species: 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. The most recent was 65 million years ago, ending the reign of the dinosaurs.

46. Iridium: Evidence of an Impact Iridium: very rare in Earth surface rocks but often found in meteorites. Iridium: 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, probably by a meteorite impact. 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. Dinosaur fossils all lie below this layer.

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

48. Consequences of an Impact A meteorite 10 km would send large amounts of debris into atmosphere. A meteorite 10 km would send large amounts of debris into atmosphere. Debris would reduce the amount of sunlight reaching Earth’s surface. Debris would reduce the amount of sunlight reaching Earth’s surface. The resulting climate change may have caused mass extinction. The resulting climate change may have caused mass extinction.

49. Likely Impact Site Geologists found a large subsurface crater about 65 million years old in Mexico. Geologists found a large subsurface crater about 65 million years old in Mexico.

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

55. Is impact threat real?

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

58. Tunguska, Siberia: June 30, 1908 A ~40 meter object disintegrated and exploded in the atmosphere

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

60. Frequency of Impacts Small impacts happen almost daily. Small impacts happen almost daily. Impacts large enough to cause mass extinctions are many millions of years apart. Impacts large enough to cause mass extinctions are many millions of years apart.

61. The Asteroid with Our Name on It We haven’t seen it yet. We haven’t seen it yet. Deflection is more probable with years of advance warning. 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. 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. We get less advance warning of a killer comet.

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

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

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

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

66. Was Jupiter necessary for life on Earth? Impacts can extinguish life. But were they necessary for “life as we know it”?