13. Colliding worlds

Demise of the dinosaurs and other mass extinction

Catastrophe from the sky

Collisions by objects from outer space have always been a menace to life on Earth. During the planet’s first billion years, the barrage was probably so intense that living things could not exist on the Earth’s surface. After those early times, the rate of bombardment slowed down, so impacts of exceptionally large cosmic projectiles became less frequent. But these giant impacts continued every once in a while, with devastating consequences. The most recent death rock arrived 65 million years ago, resulting in the wholesale removal of life on Earth. Such an abrupt destruction of an entire species of living things by a force of nature is known as a mass extinction.

A thin, worldwide layer of clay, just 0.01 meters thick, provided the initial evidence that an asteroid or comet collision wiped out the dinosaurs. The clay layer was deposited at the right time, and it contained unusual amounts of the rare element iridium that had to come from outer space.

Walter and Luis Alvarez, and their colleagues concluded in 1980 that the iridium deluge came from outside the Earth, delivered by a large asteroid or huge comet that struck the Earth and vaporized about 65 million years ago. According to their hypothesis, the iridium was lofted into the atmosphere along with other debris by the fireball of hot gas created during the collision, and then carried by the winds over much of the globe. The worldwide cloud of iridium-rich dust then slowly filtered back down to the ground where it produced a thin global layer that contained relatively large amounts of iridium. They estimated that a layer 0.01 meters thick covering the entire Earth would be deposited by an asteroid about 10 thousand meters in diameter.

Fig. .. 

After years of searching, the telltale crater was found straddling the northern coastline of the Yucatán Peninsula. It is located below the Mayan village of Chicxulub (pronounced Cheek-shoe-lube, a Maya phase for ""horns of the devil""), and is hence known as the Chicxulub impact basin. The discovery of this crater and the subsequent confirmation of its age at 65 million years led most scientists to accept the impact hypothesis for the demise of the dinosaurs.

Fig. .. 

High-resolution gravity maps in the 1990s revealed the size and structure of the buried Chicxulub crater. Regions of high density and greater gravitational pull are distributed in several concentric rings, with an outermost diameter of at least 180 thousand meters and perhaps as much as 250 thousand to 280 thousand meters. Its size and bulls-eye pattern is similar to the largest impact basins on the Moon and Mercury, created by cosmic collisions during the early days of the solar system about four billion years ago.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University