Researchers long have wondered why there is not more melted rock at Arizona's Barringer Meteorite Crater between Flagstaff and Winslow.

At an impact speed of 34,000 to 44,000 mph, the massive space rock should have melted substantial quantities of the white Coconino geological formation. One possible explanation: The meteor contained large amounts of water, which would have lessened the force of the impact that created the 570-foot-deep, 4,100-foot-wide crater.

But new calculations suggest the rock, after having been broken up in the atmosphere, simply was traveling more slowly than previously believed when it struck 50,000 years ago.

The site "is probably the most studied impact crater on Earth," said astronomer H. Jay Melosh of the University of Arizona. "We were astonished to discover something entirely unexpected about how it was formed."

Using computer models for how such objects would interact with the atmosphere, Melosh and astronomer Gareth Collins of Imperial College London concluded that the 300,000-ton, 130-foot-diameter meteor fractured before it hit the ground, with about half of it dispersing into small fragments.

The remaining half struck the ground at 26,800 mph, about 10 times the velocity of a rifle bullet, but not fast enough to melt a rock of that size, the scientists reported in Nature.

The intact fragment exploded with the energy of at least 2.5 megatons of TNT, they said.