Killer Asteroids and Comets
At 7:17 AM on the morning of June 30, 1908, an asteroid entered the earth's atmosphere in Russian Siberia causing a great explosion at an altitude roughly ten kilometers in the atmosphere. Witnesses in the nearby towns saw a luminous trail across the sky followed by a brilliant fireball. The force of the impact flattened trees 30 kilometers from the central point in the Stony Tunguska River Valley and trees were burned as far as 60 kilometers away. The blast destroyed over 6,000 square kilometers of forest.
A similar but more powerful event may have wiped out the North American megafauna and the Clovis people. Some scientist have theorized that the demise of the mammoth, the saber-toothed cat, and the American camel was due to the hunting skills of the Clovis people and weather changes. However, Richard B. Firestone of the Lawrence Berkeley National Laboratory, thinks that a 10-kilometer-wide comet could have hit North America 12,900 years ago. This event was preceded by an intense blast of iron-rich grains from a supernova explosion that impacted the planet roughly 34,000 years ago.
In support of the supernova and comet impact theory, Firestone and Arizona geologist Allen West have found 34,000-year-old mammoth tusks that are peppered with tiny impact craters apparently produced by iron-rich grains traveling at an estimated 10,000 kilometers per second. These grains may have been emitted from a supernova that exploded roughly 7,000 years earlier and about 250 light years from Earth. Also, the scientists found magnetic metal spherules in the sediment of nine 13,000-year-old Clovis sites in Michigan, Canada, Arizona, New Mexico and the Carolinas. Low-density carbon spherules, charcoal, and excess radioactivity were also found at these sites.
 Chyba, C., P. Thomas, and K. Zahnle 1993. "The 1908 Tunguska Explosion: Atmospheric Disruption of a Stony Asteroid". Nature 361, p. 40-44. [abstract]
 Supernova Explosion May Have Caused Mammoth Extinction, Berkeley Lab, [News Release]
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