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 spinela 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 glassspinel 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 dropletswhich the scientists call spheruleseither 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 continuouslyand 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."
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."
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