Meteorite impacts

1. Meteorite impacts

2. Comparative energies No human in past 1,000 years has been killed by a meteorite

3. Direct observations of meteorite impacts • Tunguska, Siberia, 30 June 1908…a big bang above the Earth’s surface • Shoemaker-Levy 9, July 1994…impacts hitting Jupiter

4. Direct observations of meteorite impacts • In 1954, a 5-kg meteorite crashed through a house in Alabama • the object bounced off a radio and hit the owner in the head

5. Effects upon children

6. Indirect evidence of meteorite impacts • Preserved craters on the continents, mainly the oldest parts (shields) • Lac cratére in northern Québec is a simple crater… • …its rim diameter is 3.4 km, it is 250 m deep, and it is 1.4 Ma in age

7. Location map of some impact craters seen at the surface

8. Lac cratère

9. Meteor crater in Arizona is another simple crater showing rim ejecta

10. Manicouagan • The Manicouagan crater in Québec is a spectacular example of a complex crater • Its original rim has been removed by erosion…the current diameter is 100 km • It has an uplifted central core and outer rings, which are filled by a lake • Its age - 210 Ma - coincides approximately with a large extinction at the end of the Triassic period

11. Manicouagan St. Lawrence River

12. Central uplift

13. Some definitions • Meteoroid: matter revolving around the Sun or any object in planetary space too small to be called an asteroid or a comet • Meteorite: a meteoroid which reaches the surface of the Earth without being vaporized • Meteorites come from larger parent bodies within our solar system

14. Asteroids • Asteroids are rocky fragments which either: • failed to consolidate into a planet, or • represent remnants of a fragmented planet

15. Asteroids and the Asteroid Belt • The Asteroid Belt lies between Mars and Jupiter…there are about 4,000 objects • As asteroids collide with one another, they fragment and send pieces into near-Earth orbits

16. Types of meteorites derived from asteroids • Asteroids have a metallic core and stony silicate mantle • As asteroids fragment, both metallic and silicate pieces are produced Metallic core Stony silicate mantle

17. Stony meteorites (94% of all meteorites) • Two types: • Chondites…contain chondrules…they are very old and primitive • Achondrites…no chondrules Photo of a carbonaceous chondrite (carbon-bearing)

18. Iron meteorites • These consist of nearly pure metallic nickel and iron • This photo shows an iron meteorite named ARISPE

19. Stony-iron meteorites • These are a mixture of the previous two types • Often they are fragmental, suggestive of violent processes • This stony-iron meteorite is named ESTHER

20. Comets • Comets come from the far reaches of the Solar System • They have highly elongate, elliptical orbits which bring them close to the Sun • They mainly consist of ice and dust, thus are referred to as “dirty icebergs” or “dirty snowballs” • They are held together very loosely

21. Comet West, 9 March 1976

22. Impact events • 1. Probabilities • 2. Nature of the event • 3. Consequences • 4. Mitigation

23. 1. Probabilities of a collision • What are the chances of a large meteorite hitting Earth? • As of 2003, ~700 objects with diameters > 1 km known to have orbits which intersect that of Earth • And 30 new objects are discovered each year, with the search only 8% complete!

24. Probabilities - Zebrowski • Zebrowski shows that, on average, collisions of 1 km-diameter objects occur every 250,000 years • Such an impact is sufficient to wipe out most of the human population From Zebrowski (1997)

25. Probabilities - Courtillot • Is Zebrowski’s estimate too high? Courtillot suggests it is about 1 Ma between events • In any case, you can see that these events are both very rare and very destructive From Courtillot (1999)

26. Zebrowski vs. Courtillot • The differences we see on the two graphs give you some idea of the uncertainties involved

27. 2. Nature of the event • Impact cratering is an important process in the history of Earth and other planets • 107 to 109 kg of meteoritic flux strikes Earth each year, mostly in the form of dust

28. Impact events • The cratering process is very rapid • Since the objects travel so fast (4-40 km/second), a huge amount of energy is transferred upon impact

29. Cratering • A blanket of ejecta is dispersed around the crater • rock is fractured, crushed, and broken • In large impact events, the rock can even be vaporized (depending on the type of rock)

30. Cratering (continued) • Very high pressures are reached, resulting in shock metamorphism (pressure-temperature increases) • After the initial compression comes decompression, which may cause the rock to melt

31. Broken rock Ejecta blanket fracturing Simple craters are basically simple bowls With time, the ejecta blanket outside the crater is eroded

32. melt Central uplift Complex craters are generated by rebound of the central core This core, as it decompresses, may melt

33. There are about 200 large, well-preserved impact craters worldwide…BUT…>>200 impact events during Earth’s history This map shows both SURFACE and SUB-SURFACE examples

34. Surface examples

35. Consequences of a large impact event • These would apply for an object of about 1 km or larger • Actually, you may not want to hear the list of death and destruction (or maybe you do)...

36. Consequences 1 • A base surge, similar to a volcanic pyroclastic flow, will be generated by the impact • For a terrestrial impact, rock will be pulverized and/or vaporized, sending up huge amounts of dust into the stratosphere

37. Consequences 2 • For an oceanic impact: • huge amounts of water will be vaporized • runaway hurricanes, called “hypercanes”, may be produced (winds to 1,000 km/hr?) • Global tsunamis will be generated, which will ravage the Earth’s coastlines

39. Isabel, 18 September 2003

40. Consequences 3 • In the short term, global wildfires will be generated by the impact event • These fires will burn uncontrollably across the globe, sending more soot, dust, and gas into the stratosphere

41. Consequences 4 • All this suspended dust and soot will cause global winter and global darkness • Acid rains will fall • Crops will fail catastrophically • The end result will be MASS EXTINCTIONS

42. Consequences 5 • One last interesting point: • The impact likely will trigger devastating quakes around the globe, especially where tectonic stresses are high (i.e., plate margins) • Volcanism (flood basalts) may occur on the opposite side of the globe from the impact, as a result of shock waves travelling through the center of the Earth

43. From Murck et al. (1996)

44. Mitigation • The problem is the possibility of little or no warning • There are proposals to use nuclear weapons and satellites to “shoot down” or destroy such killer objects • For further edification, rent “Armegeddon” from Blockbuster (1998) • Good subject for a paper !

45. Three case studies • Tunguska 1908, Russia • Shoemaker-Levy 9, July 1994, Jupiter • The Cretaceous-Tertiary extinction, 65 Ma

46. Tunguska, Russia, 30 June 1908 • Something big seems to have exploded in the atmosphere • The exact cause is uncertain, but we suspect a comet or a meteor Aerial view of Tunguska Natural Reserve

47. What happened? • The object’s entry appeared to be at an angle of 30-35° • The object shattered in a series of explosions at about 8 km altitude Tree blowdown from the explosions; Note parallel alignment of the trees

48. Big fires • In the central region, forests flashed to fires which burned for weeks • a herd of 600-700 reindeer was incinerated

49. Aligned trees • Trees were felled in a radial sense • About 2,000 km2 were flattened by the blasts

50. What happened? • Our best scientific guess is that it was part of a comet 20-60 meters in diameter… • …no crater was found… • …and no meteoritic debris has been found Felled trees aligned parallel to each other