sexta-feira, 21 de março de 2008

A Peruvian mystery

A small space rock crashed in the Peruvian Andes last September, smashing scientists' ideas about what objects are capable of producing impact craters.
Richard Talcott
The 49-foot-wide (15m) Carancas crater in Peru surprised scientists because it formed from the impact of a fragile stony meteorite. Peter Schultz [View Larger Image]
Around noon September 15, 2007, residents in the town of Carancas, Peru, saw a fireball streaking overhead. Eyewitnesses reported the fireball had a luminous head and a white tail. The resulting explosion smashed windows up to 0.6 mile (1 kilometer) away and created a crater 49 feet (15 meters) across. The impact gouged out some 4,500 cubic feet (130 cubic meters) of a dry stream bed and flung it up to 330 yards (300m).

Nothing appeared odd about all this — until scientists examined the scene in detail. Two groups reported on their investigations Tuesday at the 39th Lunar and Planetary Science Conference in Houston. Peter Schultz of Brown University and Thomas Kenkmann of the Museum für Naturkunde, Mineralogie, at Humboldt-Universität Berlin, described their team's startling findings. "This just isn't what we expected," says Schultz. "It was to the point that many thought this was fake."

The surprise arose when the researchers realized the impacting body was a stony meteorite, a fragile type scientists thought would get ripped to pieces as it tore through Earth's atmosphere. Obviously, that didn't happen. The atmosphere typically slows such a meteorite way down, and it leaves a small pit — not a crater — when it hits the ground. "But this meteorite kept on going at a speed 40 to 50 times faster than it should have been going," or about 15,000 mph (24,000 km/h), says Schultz.

Schultz thinks the object survived because it was moving so fast. The fragments of the disintegrating meteorite couldn't escape the fireball's shock wave, and reformed into a more aerodynamic shape. Like a football spiraling through the air, the incoming meteorite easily penetrated the atmosphere.

If Schultz is right, many craters created by stony-meteorite impacts could exist. Unfortunately, weathering would render them unrecognizable within a short period. Already, the Carancas crater is losing many of the characteristics that make it an obvious impact site. "You just wonder how many other lakes and ponds were created by stony meteorites," says Schultz.
Early Earth and Mars

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