Inch by inch, the team pulled up the skinny core of ghostly white limestone from the ocean floor, gazing at the compressed remains of ancient organisms that died tens of millions of years ago. But then a stark divide appeared as the layers abruptly darkened.
“It was nothing like the stuff above,” recalls Sean Gulick, a co-chief scientist of the expedition and a researcher at the University of Texas at Austin.
This change in the rock marks one of the most catastrophic events in Earth’s history, some 66 million years ago, when an epic asteroid slammed into the sea just offshore of Mexico’s Yucatán Peninsula. The impact triggered a nightmarish sequence of events that sent some 75 percent of plant and animal species spiraling to extinction—including all the nonavian dinosaurs.
Now, by subjecting the rocky core to a battery of tests, including geochemical study and x-ray imaging, the research team has assembled a meticulous timeline chronicling events on that fateful day—sometimes down to the minute. As they report today in the Proceedings of the National Academy of Sciences, the dark layers reveal stunning details, including the sheer amount of material that piled up mere hours after the strike, along with bits of charcoal later left by raging wildfires.
“They can put their fingers on moments in that event,” says Jennifer Anderson, an experimental geologist who studies impact cratering at Winona State University. “The level of detail kind of blows you away.”
While it’s unlikely another asteroid smashup of this magnitude will happen in our lifetimes, significant impacts are inevitable in the larger arc of our planet’s evolution, says Purdue University’s Jay Melosh, who is not part of the study team but who worked on other sections of the crater core. Studying these events helps us more strongly grasp the vulnerabilities of life on Earth, he says.
“It’s not a matter of whether there will be big impacts,” he says, “it’s just a matter of when.”
Drilling into disaster
Previous studies have been slowly piecing together what happened after the so-called Chicxulub impact using a combination of computer models and the geologic fallout found at a smattering of sites around the world. One controversial locale in North Dakota may even capture an entire ecosystem catastrophically tossed by the seismic waves that rippled out from the impact zone.
But the exact details of the chaos that ensued have been an enduring mystery, one that scientists hoped to solve by closely examining the impact crater itself. Layers of sediment had buried the crater over millennia, which prevented roaring winds and water from wearing it away, but also hid it out of reach of eager scientists. To actually touch this infamous moment in our planet’s history, researchers needed to drill.
Scientists started exploring the crater’s structure in 1996 via seismic surveys led by Joanna Morgan, who co-led the latest drilling efforts with Gulick. Along with a second expedition in 2005, that work confirmed the presence of what’s known as a peak ring—a circle of buried mountains that rapidly forms within the largest of impact craters. Such a structure is an ideal place to drill, Gulick says. Not only can it reveal the fundamental processes behind the formation of mega-craters, its elevation places it relatively close to the modern ocean floor, which means easier access.
In the spring of 2016, the team at last sunk metal teeth into the Chicxulub crater, and over the course of two months, they extracted sections of core 10 feet at a time. In total, they collected a slice of Earth about a half-mile long that ca