For billions of years, it waited. A city-sized chunk of primordial space rock circled the solar system somewhere between the orbits of Mars and Jupiter. Earth took shape. Life evolved. And all the while, the space rock just drifted, tumbling end over end like a poorly thrown football. Then, some unknown celestial mechanics shoved this 9-mile-wide projectile out of its orbit. Destination: Earth.
The asteroid belt escapee arrived 66 million years ago. In those last dinosaur days, any skygazing T. rex might have tilted its head in curiosity as a strange, new star grew dozens of times brighter than the sun. Burning through the atmosphere at 45,000 mph, the asteroid’s leading edge hit the Gulf of Mexico while its other end was still higher than a cruising 747.
It excavated a hole nearly 20 miles deep, ripping fault lines down to Earth’s mantle. For two minutes, land behaved like liquid. The open hole left behind by the asteroid quickly filled back in as material rebounded from the depths, building a great ring of peaks around the crater’s center. Twelfth-magnitude earthquakes rocked the region. Cliffs crumbled. A blast of air surged at speeds exceeding 600 mph, bringing hurricane-force winds to what is now North America. Vegetation vaporized. Within the hour, waves hundreds of feet high pounded Texas and Florida. A debris plume erupted above Earth’s atmosphere and rained back down around the globe, creating regional infernos. These were just the opening salvos of hell on Earth.
By chance, the asteroid struck a shallow sea over a sulfur-rich shelf. Once vaporized, the chemicals formed a climate-altering blanket that enveloped the planet and fell as acid rain. Photosynthesis nearly stopped. As forests died, wildfires turned the world’s plants into a layer of soot now found all around the planet. Most of the remaining creatures — from the tiniest plankton to the largest dinosaurs — froze or starved. On land, nothing larger than 55 pounds survived.
“Most damaging were the sulfur and the dust. Those two things made the Earth very dark and cold for an extended period of time,” says Joanna Morgan of Imperial College London, who’s spent her career studying the calamity. Some 75 percent of life vanished, ending the 180 million-year reign of the dinosaurs. But life couldn’t be snuffed out so easily. The survivors emerged from the ash to repopulate the planet. A different kind of creature soon flourished in the dinosaurs’ absence — mammals.
Now scientists have returned to the scene of the crime, seeking answers to fundamental questions about what happened that day. How exactly did that limestone shelf behave like a liquid? Where did the peak ring rocks come from? And what kinds of life were the first to return to ground zero? Researchers are turning back time, layer by layer, drilling down to the time of the Cretaceous — the final dinosaur era — examining rocks and tiny fossils for new details that could solve decades-old controversies.
“It’s the most important natural event on Earth in the last 100 million years,” Morgan says. “It changed the course of evolution.” And if the researchers can find those first species to recolonize the crater, the discovery could teach us not only more about the dinosaurs’ demise, but also how life survived similar events billions of years earlier.
Spotting Ground Zero
Under the fierce May sun, Morgan steps into an open-air basket on a supply vessel floating 19 miles off the Yucatán Peninsula. A crane operator pulls her high above the rough ocean waves, giving her the asteroid’s final view of the Gulf of Mexico.
But there’s no evidence of the apocalypse from here; the coastline isn’t even in sight. Her landing target is the Liftboat Myrtle, a drill rig parked over the planet’s best-preserved large impact scar, now dubbed Chicxulub (pronounced CHICK-soo-loob), after a tiny nearby town.
The rig’s feet stand on the shallow seafloor, just 65 feet underwater, and its platform rises well above the waves, providing a stable base for the drill crew. Morgan is here to sink a diamond-tipped drill bit through nearly a mile of Earth’s crust and collect samples.
Her journey began in 1994. Three years earlier, scientists had linked the Chicxulub crater to Luis and Walter Alvarez’s incredible theory that a 66 million-year-old worldwide layer of iridium — a material common in asteroids, but not on Earth’s surface — proved a space rock’s crash killed the dinosaurs. Clues from around the Caribbean had helped them home in on this missing crater. First, sandy tsunami sediment was found in Texas. Then tiny tektites — glass bits formed during impacts — turned up in Haiti. Eventually, oil-hunting Mexican geologists handed over drill cores from a strange circular feature they’d found in the Yucatán. Inside was shocked quartz, the smoking gun for impact craters. Scientists had started to assume the impact happened in the open ocean. Chicxulub’s surprisingly sulfur-rich location helped explain the environmental devastation. But that was just the beginning.
By the time Morgan became involved, most experts agreed that an asteroid killed the dinosaurs and that it landed in Mexico. But debate still raged over the size of the Chicxulub crater. Some estimated it was nearly twice as big as it really is. Few scientists were studying impacts at the time, and knowing how much energy the asteroid carried depended on knowing the crater’s diameter. Morgan watched as two geologists — Alan Hildebrand of the University of Calgary and Buck Sharpton of the Lunar and Planetary Institute in Houston — sparred over the details.
“There were accusations of voodoo physics from one and the other saying your parents were not married,” Morgan jokes. The scientists were trying to tease out gravitational and magnetic anomalies buried beneath more than half a mile of sediment. The young seismologist saw an opportunity. In 1996, Morgan began a three-month seismic study of the site — the first of its kind. Her team towed a large air gun behind a research vessel, blasting the seafloor with seismic waves that would bounce back, revealing a clearer picture of the crater. The next year, she got the two adversaries, Hildebrand and Sharpton, to sign on as co-authors on a paper in Nature that showed the crater stretched 112 to 124 miles.
Morgan’s next plan was even more ambitious. She wanted to drill Chicxulub. Morgan asked the International Ocean Discovery Program (IODP), a global collaboration of marine research, for more than $100 million to collect six 2-mile-deep cores from around the crater’s center to better understand peak ring formation and the impact’s environmental effects. The IODP sidelined her proposal until she could bring the cost down.
The IODP also called for a 3-D site survey before it would consider Morgan’s proposal. She partnered with University of Texas at Austin geophysicist Sean Gulick, who was already studying the Gulf of Mexico. Again, the researchers towed an array of air guns behind their vessel, this time bouncing more than 35,000 sound waves across a network of 115 land and seafloor seismometers. By 2005, their team had collected enough seismic data to reveal Chicxulub’s exact shape.
Settling the Soot
Fossils from New Mexico and Colorado show that the doomsday asteroid of 66 million years ago may have caused entire forests to burn to the ground. And for decades, some scientists thought that happened because the atmosphere superheated the planet, igniting fires everywhere on Earth as fireballs rained down from the skies.
But David Kring of the Lunar and Planetary Institute has modeled Chicxulub’s immediate aftermath and shown forest fires were likely more regional — some forests lived, while others died. His team proposes that a thermal pulse is to blame, an explosion of heat reaching more than 36,000 degrees Fahrenheit as it spread from the impact site, igniting nearby areas. If that’s correct, forest fires likely spread across southern North America, but stopped before destroying the continent’s northern reaches. The extended fallout also would have started fires on the opposite side of the planet. “There would have been a huge number of ecosystems on Earth at the time, and those ecosystems would have reacted differently,” says Kring.
In research published last year, Joanna Morgan of Imperial College London and her colleagues put that idea to the test by setting pine needles on fire in the lab. The team showed that the thermal pulse from the impact couldn’t ignite the kind of global canopy-replacing wildfires commonly associated with the asteroid. Instead, dry forest litter likely sparked wildfires like the ones forests had evolved with. In Morgan’s version of events, reduced sunlight and re-entering debris may have dried out many of Earth’s plants. Those dead trees and plants later burned as a result.
Either way, we’re sure Earth burned. “What we know is there’s lots and lots of soot, so there must have been lots of fires all across the place,” Morgan says. How did these forests bounce back? A 2014 University of Arizona study of fossilized leaves in North Dakota showed a surprising shift in plant populations. Deciduous plants — those that lose their leaves — fared better than slow-growing evergreens, thanks to their live-fast-die-young strategy. This implies that evergreens were more common before the impact, but fast-growing flowering plants thrived immediately afterward.
Fossil records also commonly show a fern spore spike following the impact, indicating that some spores and seeds survived the fires. This helps explain why some avian dinosaurs lived and others died. A paper published in the journal Current Biology earlier this year looked at birdlike creatures living at the end of the Cretaceous and notes that the survivors — those species that went on to become modern birds — had beaks without teeth, ideal for seeds. Carnivorous species died as their food sources did, but those dinosaurs with toothless beaks could feast on fallen seeds long after plants died.
Beneath the water and sediment, the impact scar looks strikingly similar to Schrödinger crater on the far side of the moon. Both large craters have a circle of hills around their center called a peak ring. Astronomers see such sites all over the solar system, but on Earth, only Chicxulub has an intact peak ring. And despite the rings’ abundance, scientists still don’t understand exactly how this inner circle forms or how land could weaken enough to behave like a liquid in the immediate aftermath, as models predict.
“In addition to all the life stuff — all the extinctions, life coming back, all of that — there’s this fundamental question: How are impacts made?” Gulick says. “And if you want to test the models, this is the place to go because we can get to it without flying to the moon.”
After the 2005 seismic survey, the scientific community gathered in support of a new drilling effort, but at $20 million, the price remained too high. It took the recent decline of world oil markets to drop the price tag to around $10 million before a scaled-down version of the project was feasible. “We’re in a lucky window right now,” Morgan says. “The fact that the oil price is low and the oil rigs aren’t busy is very helpful for us.”
The IODP eventually agreed to finance the project, and after two decades of work, with help from partner groups, Morgan finally set foot on the Liftboat Myrtle.
Checking the Cores
Morgan and Gulick are no strangers to fieldwork. When she’s not studying Chicxulub, Morgan probes the heart of Greece’s Santorini volcano. And Gulick’s seismic studies have taken him from pole to pole, mapping faults and glaciers, and bringing up cores to reveal Earth’s ancient climate.
Standing together on Myrtle’s deck, the two co-chief scientists are ready for more. Both sport dirty red jumpsuits and messy shoulder-length blond locks tucked beneath white hard hats. They’re adorned with the badge of International Ocean Discovery Program Expedition 364: an offshore drill rig under a fireball. As the drill drones on in the background, they share a laugh over rock cores that haven’t seen sunlight since the asteroid impact.
Under Morgan and Gulick’s leadership, Liftboat Myrtle finally sinks its drill bit into the seafloor in April. By month’s end, the crew of international experts and drill operators are reeling in 10-foot sections of core, working shifts around the clock in temperatures well above 100 degrees Fahrenheit. As each cylinder of rock comes up from the deep, onboard specialists rush to record its density, resistivity, temperature and any other data that might change before the cores are examined at a main lab in Bremen, Germany.
Most layers within the cores are wafer thin, but a few stretch several inches in varying shades of gray. Gulick identifies these as ash from Mexican volcanoes that erupted some 50 million years ago. But such local cataclysms can’t compare with the layer that ended the Cretaceous. Scientists predict the boundary layer — a mix of original peak ring materials, tsunami deposits and melted rocks that fell from the sky — should span hundreds of feet. They finish drilling in late May, bottoming out at a depth of nearly a mile.
In one of several air-conditioned shipping containers converted into makeshift-laboratories on the Liftboat Myrtle, Pennsylvania State University micropaleontologist Timothy Bralower lightly breaks up a bit of core rock using a mortar and pestle. Then he dumps the fine fragments onto a glass slide. Countless tiny plankton fossils stare back at him through the microscope. They look like fuzzy grains of quinoa scattered across a black dinner plate. By picking out individual species and comparing them with fossil records, he can tell the team roughly how far back in time they’ve drilled.
It’s a quick and dirty dating technique that reveals when they’re near the most precious core sections — those right before the impact boundary. To prevent contamination, those layers will be sealed and sent intact to Germany, where the 33-person science team will gather in September for a marathon onshore research session, analyzing the samples in 12-hour shifts.
The team hopes these precious rocks preserve a record of the first life to return to ground zero. Models show that seawater quickly returned to the crater, and it may have remained bubbling hot for as long as 2 million years after the impact. Ironically, that means the asteroid that helped destroy life on Earth’s surface could have created a habitat below the waves for certain extreme kinds of life, which today gather around deep-sea vents elsewhere. These organisms feed on chemicals with no need for sunlight, making them a great contender for Chicxulub’s new first residents.
If so, these species could teach us how life on early Earth — and even Mars — survived more than 4 billion years ago, when asteroids constantly bombarded the planet. “Only the kinds of things living in stressful environments would have survived,” Bralower says.
The asteroid impact 66 million years ago — like many before it — fundamentally changed life on Earth. Now, a drill’s impact could alter our very understanding of life.
Were Dinosaurs Already Doomed?
Most researchers agree that an asteroid struck Earth 66 million years ago and wiped out most life. But were dinosaurs dying off before the impact?
In 2016, a long-simmering debate erupted into a roiling boil. An April study in the Proceedings of the National Academy of Sciences examined hundreds of species on the dinosaur evolutionary tree. By looking at the statistics of extinction and speciation events, which happen when new dinosaurs evolve, researchers found signs of decline for many species. Sauropods, a group of large plant-eaters like Brontosaurus, may have started dying off 50 million years before the Chicxulub asteroid impact, they found. And by 40 million years before the impact, more species were dying off than new species were evolving.
What was killing them? This statistical approach can’t isolate a cause, but the team points out plenty of possibilities: continental drift, intense volcanism, climate change and sea level rise.
Another study, this one published in Nature Communications, added fuel to the fire in July. The authors studied well-preserved Cretaceous ecosystems in Antarctica and found two temperature spikes, including a larger one before the impact. The scientists pin the initial warming on staggering volcanic eruptions that created India’s Deccan Traps around the same time period.
However, critics say the fossil record isn’t complete enough for such a sweeping analysis. They point out that other groups, such as marine life and birdlike dinosaurs, show no signs of a struggle.
“Don’t let anyone tell you [the extinction] was gradual,” says Timothy Bralower, a micropaleontologist at Pennsylvania State University.