Impacts and Bio-Extinctions

1. Impacts and Bio-Extinctions

5. Impacts: Three Key Discoveries • 1960: Impacts cause most cratering (Shoemaker) • 1960+: Impacts are inevitable and continuing • 1980: Impacts can have global biological consequences (Alvarez et al.)

6. Impact Cratering: The Barringer Crater

7. Barringer Crater (“Meteor Crater”), Arizona

9. Crater: 1 mile across, age ~ 49,000 yr

10. E. Shoemaker at Barringer Crater and Coesite Sample

11. Shattercone

13. Impact Mechanics

14. Barringer Crater = impact of ~40m diam metallic meteoroid

15. Impact Cratering: Moon Compared to Earth

17. Lunar Impact History

18. Cratered Region on Moon (150 km)

20. The Culprits

24. Evidence for ContinuingImpacts on Earth • Near misses • Impact Geology • Fossil Craters • Tunguska Event • Air bursts

25. 1972 Fireball

26. Meteoroid: ~10-m diameter, 55 km high, velocity 33,000 mph, skipped off atmosphere

27. Ground track of 1972 fireball

29. 35 million year old event, discovered through impact geology (larger of 2 simultaneous impacts)

30. Geological cross-section of Chesapeake impact

31. Tsunami: Waves 1000 feet high

32. Identified Impact Craters Worldwide

34. Identified NoAm impact craters

35. Two ~40 mi diam craters, ~200-300 million years old

36. Tunguska Event (1908) (Crude Reconstruction)

37. Tunguska (Siberia) Impact Site 1908

41. Tunguska 1908 • 20 mile radius of destruction • Heard 300 mi away; dust in atmosphere detected 1000’s of miles away • Likely cause: disintegration of 30m stony asteroid at 10 km altitude • Equivalent: 20 megatons TNT (1000x Hiroshima A-bomb)

42. Impact Energetics • Deposited energy = “kinetic energy” • KE = 1/2 M V2 (from Newton’s laws) • V is large! Orbital velocity ~ 30 km/sec = 66,000 mph • M is large! M = (density) 1/6 pi D3 • Combine, convert to equivalent explosive energy in units of tons of TNT (dynamite) • Result: Impact energy = 250 D3m tons TNT • (Dm is diameter in meters) • Implies • D = 4.3 m yields 20,000 tons TNT = Hiroshima • D = 40 m yields 15 million (mega) tons of TNT = Barringer, Tunguska • D = 10 km (10,000 m) yields 250 mega megatons

43. Comet SL-9: “Planet Buster” scale impacts on Jupiter

44. Impacts and Global Biological Extinctions

46. “In the 570 million year period for which abundant fossil remains are available, there have been five great biological crises, during which many groups of organisms died out. The most recent of the great extinctions is used to define the boundary between the Creataceous and Tertiary periods, about 65 million years ago. At this time, the marine reptiles, the flying reptiles, and both orders of dinosaurs died out...Russell concludes that about half of the genera living at that time perished during the extinction event. ...Deep sea limestones...show iridium increases above the background level at precisely the time of the Cretaceous-Tertiary extinctions, 65 million years ago. Reasons are given to indicate that this iridium is of extraterrestrial origin... A hypothesis is suggested which accounts for the extinctions and the iridium observations. Impact of a large, earth-crossing asteroid would inject about 660 times the object's mass into the atmosphere as pulverized rock; a fraction of this dust would stay in the stratosphere for several years and be distributed worldwide. The resulting darkness would suppress photosynthesis, and the expected biological consequences match quite closely the extinctions observed in the paleontological record...” From Alvarez et al. (1980)