Impact! Tracking Near-Earth Asteroids
AMNH Scientist Interview: Denton Ebel
Sixty-five million years ago, an asteroid half as long as Manhattan slammed into the tip of Mexico’s Yucatan Peninsula, forming a crater near the present-day coastal village of Chicxulub. Also about 65 million years ago, half of all the plant and animal species alive at the time—including dinosaurs—died out. Coincidence? AMNH geologist Dr. Denton Ebel doesn't think so. His latest research, conducted with the University of Chicago's Lawrence Grossman, also adds never-before-proven details about the impact.
Denton Ebel is the assistant curator of meteorites at the American Museum of Natural History. Ebel is collecting minerals in England in this photo.
Can you see the Chicxulub crater?
It’s mostly offshore, but a tiny bit is on the peninsula. You can actually see this edge from space, but it’s very subtle. The crater is huge. It’s the second-largest on the planet. The asteroid itself would have been about 10 kilometers in diameter to make that crater. Compare that to Mount Everest, which is 7 kilometers high. When the tip of this asteroid was hitting Earth, its top end was up past where you can breathe.
Clearly there’s no asteroid there now. So besides the crater, what evidence do scientists have that one struck?
In the early 1980’s, geologist Walter Alvarez was looking at the fluctuations of meteorites hitting Earth over geologic time. His father, a Nobel-Prize-winning physicist, helped. They went to Italy to study layers of earth sediment, looking for trace elements that are rare on Earth’s surface but common in meteorites. They were surprised to find a significant spike of iridium [an element abundant in some asteroids] in one sediment layer. Since layers of sediment can be dated, the iridium layer turned out to have been deposited 65 million years ago. The Alvarez team figured a giant asteroid impact at that time was responsible.
It turns out that a mass extinction 65 million years ago happens to coincide with this iridium layer. So scientists began looking more closely at fossil data on different species—a lot from this museum—to build up a more complete picture of the cataclysm. Only after this work was the actual crater found, in 1991.
How did scientists find the crater—by satellite?
No. The crater is now mostly buried deep under sediment. When an asteroid impacts Earth, the rocks are reworked in a certain way. Geologists drilling for oil had taken rock cores from the Yucatan. Scientists looked at those cores later on and found these impact signatures. People also found that a layer of spherules [small beads of glassy material] that were thrown up by the impact got thicker and thicker toward the Yucatan area. These spherules are like the splash droplets of melted rock from the impact.
Melted rock from the asteroid itself or from the ground at the impact site?
A combination, but mostly material from the actual rocks on the ground that the asteroid hit. But what you also get with large impacts is a rising column of very hot vaporized rock, including most of the meteorite itself, that forms right where the energy is concentrated.
Kind of like a mushroom cloud from an atomic bomb?
Right—the stem of the mushroom is the material coming up. Then the mushroom cap part spreads out laterally, and the vaporized rock in it condenses into small, molten spherules. When the spherules re-enter the atmosphere, they cool and solidify like little marbles. The impact was so big that the material went into the stratosphere [the atmospheric layer 10-50 km above Earth]. The stratosphere is where the jet stream is. If stuff gets up there, such as all the vaporized iridium from the asteroid and rock from the crater, this air current can spread the material around the world very rapidly—like within a few hours.
Wow. What effect did these glass raindrops have on life on Earth?
When the spherules came back down, friction with the atmosphere created a pulse of hot infrared radiation, like a heating coil in an oven. The heat pulse traveled at the speed of light—much faster than the particles. So all at once, all over the planet simultaneously, the heat pulse basically cooked anything that was out there, cooked it alive. But small creatures that burrow underground, or live underwater, or hibernate in caves would have had a much better chance of surviving. These species are the ancestors of all the species alive today.
