The lack of melted rock is blamed on a slow space rock, which broke up before hitting Earth.
The site near Flagstaff was blasted into the face of the Earth about 50,000 years ago, but scientists studying the 570-foot-deep scar in the crust of the Colorado Plateau, were puzzled because there was no melted rock at the site as is typically seen when a space object strikes, said Arizona Regents' professor H. Jay Melosh.
Earlier calculations pegged the speed of the incoming object from space at 34,000 to 44,000 miles per hour, the typical velocity range of such objects. But, last summer, using a sophisticated crater-effects calculation, Melosh and Gareth Collins of the Imperial College, London, determined the space rock was moving at only 26,800 mph, Melosh said.
The object, consisting of 90-percent iron and 10-percent nickel, broke up when it slammed into the outreaches of Earth's atmosphere, a process called atmospheric dispersion.
Then it decelerated as it plummeted toward the surface, he said. "The fragments that actually struck, we're talking a cloud of debris, not a single fragment, joined forces to excavate the crater.
Here's what happened, he explained. When the 300,000-ton, 130-foot-diameter space rock began to break up, "it sprays out in all directions, and when it sprays out, its area increases and its drag increases. That makes it spray out even more." Melosh said.
The result was the deceleration, he said. While 26,800 mph is 10 times faster than the fastest known bullet, it's too slow to melt much of the rock it hit in northern Arizona.
So the mystery that stumped researchers for years was solved, he said. "We should have realized that ahead of time ... but it's one of those things that caught us in a blind spot."
A complete report on the research and findings appears in the March 10 issue of Nature magazine.
Atmospheric dispersion is not unusual, he said. For example, that happened in 1908 when a much bigger object, with very little iron content, entered Earth's atmosphere and raced toward a collision with the surface in Siberia.
"That one never reached the ground at all," he said. "It deposited most of its energy, about 10 megatons worth, at about 10 kilometers (6.3 miles). It was the air blast that knocked down (millions of) trees."
But at Meteor Crater, also known as Barringer Crater, thousands of pieces hit the Earth. In 1909 Daniel M. Barringer, a mining engineer for whom the crater is named, published a map of fragments, listing several thousand in the area that weighed a pound or more, Melosh said.
The rest of the material melted, some of it vaporized, mixed with water droplets and scattered around the area, he said. "The largest piece I've seen is a couple meters across.
"And a lot of fragments, probably the largest, were hauled away and ended up in private collections," Melosh said. "Some fragments were melted down for iron."
The next closest meteor crater to Tucson is Odessa Crater near Odessa, Texas, he said. In all, there are 170 known meteor craters sites in the world.