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Sunday, January 18, 2009
A Smoking Gun for Dinosaur Extinction
It is hard to imagine that one of the largest impact craters on Earth, 180-kilometers (112-mile) wide and 900-meters (3,000-feet) deep, could all but disappear from sight, but it did.
Chicxulub, located on Mexico's Yucatan peninsula, eluded detection for decades because it was hidden (and at the same time preserved) beneath a kilometer of younger rocks and sediments. Size isn't the only thing that makes Chicxulub special. Most scientists now agree it's the "smoking gun" evidence that a huge asteroid or comet indeed crashed into Earth's surface 65 million years ago causing the extinction of more than 70 percent of the living species on the planet, including the dinosaurs. This idea was first proposed by the father and son team of Luis and Walter Alvarez in 1980.
Though the buried giant can't be seen, the impact crater has left subtle clues of its existence on the surface. "When I talk to school children, I describe it like this," says Dr. Gary Kinsland, a geology professor at the University of Louisiana at Lafayette who has been doing research on Chicxulub since 1994. "Put a bowl on your bed, then throw the sheets and blankets over it. All you'll probably see of the bowl now is a subtle depression."
"There is not a big hole anymore," he continues, "but if you look at the rim of the depression on your bed, you'll see that it is still in the same position as the rim of the bowl beneath. That's how surface expression allows us to interpret something about the buried structure."
The view from space lets scientists see some of Chicxulub's surface features that are not nearly so obvious from the ground. Satellite images showing a necklace of sink holes, called cenotes, across the Yucatan's northern tip are what first caught the attention of NASA researchers Drs. Kevin Pope, Adriana Ocampo and Charles Duller in 1990. They were among the first to propose Chicxulub as the impact site linked to the mass extinctions that occurred at the end of the Cretaceous and beginning of the Tertiary geological ages, called the K/T boundary.
For more information : http://www.jpl.nasa.gov/news/features.cfm?feature=8
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