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.
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 speciesa lot from this museumto 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 craterby 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 rapidlylike 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 lightmuch 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.
But the bigger dinosaurs were toast.
Nothing that big survived as far as we know. Some people say nothing bigger than a killer whale survived. Others say nothing bigger than a breadbox! We see evidence of the heat pulse in the sediment layerssoot particles and burned pollen mixed with the spherules.
Literally dinosaur toast. So where does your research come in?
What we did was calculate, for the first time, what the identity and composition of the material condensing from the plume theoretically should have been. And the evidence we have on Earth [the spherules] happens to match our calculations. Also, other scientists thought since spherules were dispersed globally 65 million years ago, their origin must have been many meteorites entering Earth’s atmosphere around the same time. We said, no, you don’t have to invent these hypothetical meteorites, you can get all the global spherules from just one impact instead, one plume which was carried around the world through the stratosphere. And we have the crater to prove it.
What about competing dinosaur extinction theories?
Well, some people think that intense volcanism in India 66 million years ago may have caused global climate change that contributed to the dinosaurs’ extinction. Those eruptions were very slowly oozing out of fissures in the ground and spreading out over large areas. But nothing was shooting high into the air, let alone into the stratosphere. Such oozing volcanoes can cause greenhouse warming in the lower atmosphere, which can raise global temperature over a long period of time. But there's no evidence that that would cause a mass extinction. My research only adds details to the impact that were not there before.
What’s the likelihood of another asteroid as big as Chicxulub happening again?
Every hundred million years is tossed around. The problem with big impacts is that we don’t really know, because these are unique events of which we have very little record. Most asteroid-impact craters get erased over time by erosion, deposition, and/or plate tectonics. Still, the probability is extremely low. And scientists have identified most of the dangerous ones. Put it this way: the probability of someone throwing a meteorite at you is greater than the possibility of you getting hit by one from space.
AMNH: Denton Ebel
AMNH: Dinosaurs — Ancient Fossils, New Discoveries
Learn more about the species that lived before and after the mass extinction 65 million years ago.
Earth Impact Effects Program "Catastrophe Calculator"
Enter the parameters of an asteroid (diameter, speed, etc.), and find out how catastrophic the impact would be (crater, ground shaking, etc.).
Planetary and Space Science Center: Earth Impact Database
Learn details and view photos of asteroid craters worldwide.
More About This Resource...
Supplement a study of astronomy with a classroom activity drawn from this Science Bulletin interview.
- Ask students what they know about asteroids. What are they? How big can they be? What affect can they have on a planet like Earth?
- Have them read the essay (either online or a printed copy).
- As a class discuss the essay. Were they surprised to learn that an asteroid could wipe out animal species such as dinosaurs? What do they think caused dinosaur extinction?