Impacts and Bio-Extinctions. 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.). Impact Cratering: The Barringer Crater.
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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.)
E. Shoemaker at Barringer Crater and Coesite Sample
Evidence for ContinuingImpacts on Earth • Near misses • Impact Geology • Fossil Craters • Tunguska Event • Air bursts
Meteoroid: ~10-m diameter, 55 km high, velocity 33,000 mph, skipped off atmosphere
Ground track of 1972 fireball
35 million year old event, discovered through impact geology (larger of 2 simultaneous impacts)
Identified NoAm impact craters
Two ~40 mi diam craters, ~200-300 million years old
Tunguska Event (1908) (Crude Reconstruction)
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)
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
Comet SL-9: “Planet Buster” scale impacts on Jupiter
“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)