Scientists are in agreement that a rock from space crashed into the ground in Arizona 50,000 years ago, carving out a pit 1,240 meters (4,100 feet) across, creating the Barringer crater. But how fast was this rock going when it hit the ground? Where is all the impact-melted rock? Originally thought to be going 20 km/sec (44,000 mph), the meteor should have fractured into pieces which would have covered a larger area.
Scientists have now created a new simulation that calculates the meteor was going only half that speed, and probably came down as a swarm of material and not a single rock.
The iron meteorite was traveling much slower than originally thought, according to the University of Arizona Regents' Professor H. Jay Melosh and Gareth Collins of the Imperial College London Report in Nature (March 10) . Melosh and Collins used sophisticated mathematical models in analyzing how the meteorite would have broken up and decelerated as it plummeted down through the atmosphere. About half of the 300,000 ton, 130-foot diameter space rock would have broken up into pieces before ground impact, said Melosh, and the other would have remained intact and hit at about 12 km/sec (26,800 mph).
That velocity is almost four times faster than NASA's experimental X-43A scramjet (the fastest aircraft flown). However, it was too slow to have melted much of the white Coconino formation in northern Arizona, solving a mystery that has stumped researchers for years.
Scientists have tried to explain why there is not more melted rock at the crater by theorizing that water in the target rocks vaporized on impact or that carbonates in the target rocks exploded, vaporizing into carbon dioxide. The authors stated, "If the consequences of atmospheric entry are properly taken into account, there is no melt discrepancy at all." According to Melosh, "Earth's atmosphere is an effective but selective screen that prevents smaller meteoroids from hitting Earth's surface." When a meteorite hits the atmosphere, the pressure is like hitting a wall. "Even though iron is very strong, the meteorite had probably been cracked from collisions in space," Melosh said. "The weakened pieces began to come apart and shower down from about 8-1/2 miles high. As they came apart, atmospheric drag slowed them down, increasing the forces that crushed them so that they crumbled and slowed more."
At about 3 miles altitude, most of the mass of the meteorite was spread in a pancake shaped debris cloud about 650 feet across. The fragments released a total 6.5 megatons of energy between 9 miles altitude and the surface, Melosh said, most of it in an airblast near the surface. The intact half of the meteorite exploded with at least 2.5 megatons of energy on impact, or the equivalent of 2.5 tons of TNT.
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