Slowpoke: atmosphere put brakes on meteorite that formed famed crater.

The extraterrestrial object that gouged out northeastern Arizona's Meteor Crater about 50,000 years ago struck Earth at a speed much slower than most scientists had previously proposed.

When a meteorite slams into Earth, the crater that's created usually contains both Earth and space rocks that were melted by the kinetic energy of the collision (SN: 6/15/02, p. 378). Meteor Crater holds much less melted rock than expected, says H. Jay Melosh of the University of Arizona in Tucson.

Previously, scientists had proposed that groundwater at the impact site vaporized and dispersed, taking with it much of the molten rock that had formed when the 40-meter-diameter iron meteorite hit. Now, Melosh and Gareth S. Collins of Imperial College London have come up with an alternate theory to explain the missing melted rock.

Computer simulations of the aerodynamic forces pummeling meteorites blazing through Earth's atmosphere indicate that those objects often break up at high altitudes (SN: 7/19/03, p. 36). When Melosh and Collins used similar models to analyze the voyage of the meteorite responsible fur Meteor Crater, they were surprised to find how much the atmospheric drag seemed to have slowed the incoming object before it struck the ground.

Most extraterrestrial objects enter Earth's atmosphere at speeds between 15 and 20 kilometers per second, fast enough to cross the United States in 4 to 5 minutes. In the new study, the researchers simulated the brief journey of a 300,000-ton iron meteorite piercing the upper atmosphere at 17 km/sec. At an altitude of about 70 km, where the air is less than 1 percent as dense as it is at sea level, the meteorite began slowing down. Aerodynamic forces began to break apart the meteorite when it reached an altitude of around 14 km.

By the time the object reached an altitude of 5 km, its outer layers had fragmented into a cloud measuring about 200 m across and moving at 13 km/sec. A split second later, at a speed of only 12 km/sec, the fragment cloud and the object's 150,000-ton core slammed into the ground.

The object's deceleration, for which scientists hadn't previously accounted, explains the dearth of melted material at Meteor Crater, Melosh and Collins conclude in the March 10 Nature.

Philip A. Bland, a planetary scientist also at Imperial College London, concurs with the new analysis. He adds that even though half of the incoming meteorite was battered to fragments, the craters formed by the core and most of those pieces overlapped to create one massive hole about 1.2 km across. Only a few tons of the original meteorite have been found on the plains surrounding the crater.