Impacts

Asteroid Rained Glass Over Entire Earth, Scientists Say
Hillary Mayell
for National Geographic News
April 15, 2005

Scientists studying the fallout from a huge asteroid that crashed into Earth 65 million years ago have gained better understanding of the event that most likely took out the dinosaurs and much other life on the planet.

The asteroid that created the Chicxulub (pronounced CHEEK-shoo-loob) crater, located on the Yucatán Peninsula of Mexico, was probably more than 6 miles (10 kilometers) wide, researchers estimate. The resulting crater was 110 to 125 miles (180 to 200 kilometers) wide and very deep. Today it is buried under several miles of limestone and is mostly underwater.

"When this thing came in, it cleaved right through the atmosphere, and you had a huge amount of meteoritic material with a lot of iridium in it smack into the Earth and vaporize totally," said Lawrence Grossman, a geochemist at the University of Chicago.

All that iridium—an element that is abundant in space but rare on Earth—was thrown up in a huge plume of superheated gas, ash, dust, and pulverized rock.

"This high-temperature, high-pressure plume punches through the atmosphere, way up into the stratosphere. Calculations based on the fallout material left behind indicate that this cloud would envelop the globe," said Denton Ebel, a chemist and assistant curator of meteorites at New York City's American Museum of Natural History.

The scientists concluded that minute droplets of superhot liquid rock condensed in the vapor cloud as it expanded and cooled, and then rained down all over Earth.

"Everything at ground zero of course gets wiped out," Grossman said. "But why do species all over the Earth die? The idea is this cloud of dust blocked sunlight. And in order for all these species to die off, it had to take weeks to months for that dust to settle back to Earth and the sun to shine again."

Grossman and Ebel's study is published in the April issue of the journal Geology.

Examining the Iridium Spike

Today the impact debris exists as a thin layer of reddish clay that can be found in rock strata across continents and beneath the ocean floor. The amount of iridium found in this layer is several hundred times higher than normal in some places.

The iridium-rich layer forms what geologists call the Cretaceous-Tertiary boundary (or Cretaceous-Paleogene boundary)—"K-T boundary," for short. In soil below the boundary, fossils of dinosaurs and many other plants and animals that existed during the Cretaceous period (144 million to 65 million years ago) are abundant. In soil layers above the iridium spike, the fossil record tells the story of a world without dinosaurs.

The obvious conclusion is that the layer of debris ejected by the impact draws the line of extinction for the dinosaurs. The K-T boundary ends the age of reptiles and marks the dawn of the age of mammals.

A number of asteroids and comets have hit the Earth without causing worldwide devastation. To determine why this Yucatán event had such a global impact, Ebel and Grossman analyzed the composition of some of the material that fell to Earth. They focused specifically on spinel — a mineral rich in magnesium, iron, and nickel that was contained within the droplets of rock that rained on Earth.

"We think what happened was you got a liquid condensed from this vapor with crystals in it of spinel. And the liquid may have cooled off very quickly so it made glass—spinel in solid glass," Grossman said. "The iridium condensed as the cloud cooled, and fell over a short period of time, creating an enhanced iridium concentration in a very thin layer, which is preserved until this very day."

Millions of years of weathering have turned these glass droplets into clay, but the spinel was impervious to the vagaries of wind and water, and remained intact.

Earlier studies suggested that the tiny glass droplets—which the scientists call spherules—either burned off the asteroid or comet as it came hurtling into Earth's atmosphere, or they splashed out of the crater when the space object first hit.

But the chemical composition of the spinel crystals changes the farther the plume traveled from the impact site, according to Grossman and Ebel.

"We calculated what the composition of the spinel would be as it condensed from the gas as the temperature fell," Grossman said. "The temperature moves from the composition of the Atlantic spinels, which were formed at high temperature, to the composition of the Pacific spinels, which formed at low temperatures.

"This pattern suggests that, as the gas cloud expanded and cooled, the spinels that rained out went from one composition toward the other continuously—and that mimics the composition of the spinels found from the Atlantic to the Indian to the Pacific Ocean," Grossman said.

Ebel and Grossman's calculations also had to take into consideration the likely angle at which the asteroid approached Earth, and the composition of the rock bed it smashed into.

"The angle of the impact dictates what the composition of the vapor will be," Ebel said. "If it came in shallow along the horizon, then most of what would be vaporized would be the sedimentary rocks on the [surface of the Earth]. But if it came in vertical, then it penetrates more deeply and gets into the igneous rocks underneath."

Chicxulub Controversy

Not everyone agrees that the object that created the monstrous Chicxulub crater can be tied to the extinction of the dinosaurs. Gerta Keller, a paleogeologist at New Jersey's Princeton University, argues that the asteroid impact predates the K-T boundary and extinction of the dinosaurs by 300,000 years.

Keller says that Ebel and Grossman are looking at spinels from the K-T boundary, where there is a spike in iridium and abundant spherules. But she says that these samples were not the result of the Chicxulub impact.

"What they should infer is that there is another impact at the K-T boundary, but we don't know much about that impact. All we know is it's iridium rich, spinel rich, but not spherule rich," Keller said.

Keller, however is in a distinct minority.

"The iridium spike could only have occurred as the result of a cataclysmic event," Grossman said. "We've put meat behind the idea that provided the theoretical basis for understanding the specific compositions of the spinels in terms of the model that they are condensates from the vapor."