Image: Cosmic impact
An artist's conception shows the cosmic impact that scientists believe touched off a mass extinction 65 million years ago.
updated 3/28/2005 5:55:59 PM ET 2005-03-28T22:55:59

The asteroid that struck the Yucatan Peninsula 65 million years ago presumably initiated the extinction of the dinosaurs. The huge collision also unleashed a worldwide downpour of tiny BB-sized mineral droplets, called spherules.

The hard rain did not pelt the dinosaurs to death.

But the planet-covering residue left behind may tell us something about the direction of the incoming asteroid, as well as possible extinction scenarios, according to new research. The falling spherules might have heated the atmosphere enough to start a global fire, as one example.

How the spherules formed in the first place, though, has been a bit of a mystery. One theory is that these half-millimeter-wide (0.02-inch-wide) globules precipitated out of a giant cloud of vaporized rock that circled the planet after the collision.

"That vapor is very hot and expands outward from the point of impact, cooling and expanding as it goes," said Lawrence Grossman of the University of Chicago. "As it cools, the vapor condenses as little droplets and rains out over the whole Earth."

Grossman and Denton Ebel, from the American Museum of Natural History, have shown that this vapor condensation model is consistent with data taken from spherules around the world.

The scientists also found that chemical differences in spherules from the Atlantic and Pacific Ocean imply that the vapor plume initially moved east, which might pinpoint the arrival direction of the asteroid.

Apocalyptic fireball
The spherules populate a 3-millimeter (0.12-inch) layer, called the K-T boundary, which separates the Cretaceous from the Paleogene (formally called the Tertiary) geologic periods. The abrupt disappearance of dinosaur fossils — as well as many marine fossils — above this boundary implies that a major extinction event occurred 65 million years ago.

Around this same time, a city-sized asteroid landed near the present-day town of Chicxulub, Mexico, where traces of a 100-mile-wide crater can still be found.

There is evidence for the asteroid in the unique mineral content of the K-T boundary — specifically a high concentration of iridium. This heavy element is very rare on Earth’s surface but is found in high quantities in meteorites.

The implication is that the energy released in the collision fueled a fireball of vaporized rock that rose above the clouds. In this way the asteroid’s contents — as well as the material at the crash site — were dispersed across the globe.

"The [K-T] layer is thought to be the fallout from the fireball," Grossman told in a telephone interview.

Included in this fallout were glassy spherules, which have been largely transformed due to weathering, but still contain hundreds of spinels — tiny mineral deposits with magnesium, iron and nickel.

"One reason the spinels are important is that most of the original minerals in the spherules are all gone — turned into clay," said Frank Kyte from the University of California at Los Angeles, who was not involved in this work. "The spinels appear to not have been altered."

Oxidizing environment
Because of their pristine state, scientists have studied the spinels in hopes of learning about the cataclysm that created them. One such study concluded that — because the spinels contain metal oxides — they could not have precipitated out of the vapor plume up above the atmosphere.

"The argument is silly," Grossman said.

He knows this because the conclusion drew on his own work describing how spinels are created in meteorites. These spinels formed long ago in the pre-planet gas that surrounded the sun. This gas was mostly hydrogen, so it makes sense that very few oxides — metals bonded with oxygen atoms — are found in meteorites.

But the vapor plume was a different story.

"When you vaporize rock, there is very little hydrogen — whereas 50 percent of the atoms in a rock are oxygen," Grossman said.

As described in the April issue of the journal Geology, Grossman and Ebel performed computer simulations that showed that the spinels in the K-T boundary — with their high concentration of oxides — are consistent with formation inside a liquid droplet condensing out of the impact plume.

These droplets solidified into the spherules, which — according to Kyte — all fell to the ground in a matter of hours or days.

Plume’s weather front
A more detailed analysis of the spherules shows that some of them formed earlier than others.

"The spinels that are found at the Cretaceous-Paleogene boundary in the Atlantic formed at a hotter, earlier stage than the ones in the Pacific," Ebel said.

This implies that the cloud of vaporized rock moved east following the collision — a fact that may provide hints as to how the asteroid hit the Earth. This is important because different sorts of debris get thrown into the sky, depending on whether the asteroid came in at an angle or straight down.

A complete picture of the impact’s geometry and its immediate consequences should help answer questions concerning the eventual effects on the planet’s living creatures.

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