There are many interesting facts about Earth with a bearing on biology. For example, the fact that asteroids have repeatedly impacted here over geological time — the larger ones have been the cause of mass extinctions.
Small ones — five to ten meters wide — enter Earth's atmosphere about once a year. But even these relatively small intruders release about the same amount of energy during entry as did the atomic bomb dropped on Hiroshima (most explode harmlessly in the upper atmosphere and are almost completely vaporized before hitting the ground).
In 1908, a large meteoroid or comet fragment, probably with a diameter of over 50 meters, burst over uninhabited forest in the Tunguska valley of central Siberia (see map >>) at an altitude of 5–10 kilometers (3–6 miles). This explosion, known as the Tunguska event, is estimated to have been 1,000 times as powerful as the Hiroshima blast. It scorched and flattened the forest to a distance of 50 kilometers (30 miles) and knocked down horses as far as 375 miles away. Such objects probably hit the Earth about once every 1000 years (Monastersky 1997).
But every 500,000 years or so an asteroid with a diameter of one kilometer (0.62 miles) or more collides with our planet. Even bigger impacts involving 5-km objects take place about once every ten million years.
|
Chicxulub crater
location in Mexico
|
The most recent known impact of this larger size occurred 65 million years ago, off the coast of Mexico's Yucatán Peninsula (see map right) near the modern-day town of Chicxulub (pronounced "tshook-SHOO-loob"). The Chicxulub crater is 180 kilometers (110 mi) in diameter. The explosion released by the impact was equivalent to detonating 2 million hydrogen bombs simulataneously. It is now generally accepted that the Chicxulub event was the main cause of the Cretaceous-Tertiary (K-T) extinction of the dinosaurs (but see the discussion of the dinosaur extinction on this website →).
So the history of geological and evolutionary change on this planet has been a mixture of gradual change and catastrophe. A 500-km asteroid hitting the Earth, such as that shown in the artist's rendering at the top of this page, would virtually sterilize the planet and start evolution over again at square one.
|
| |
A 500-km asteroid impact
|
|
The Tunguska Fireball: Aftermath
(click to enlarge)
|
|
Chicxulub crater
(click to enlarge)
|
|
