Life after the asteroid apocalypse
by Adam Mann
Sixty-six million years ago, an asteroid wiped out a huge swath of life on planet Earth. Could similar impacts have helped kick-start life itself?
“Wow, she’s really high above the water,” Sean Gulick thought as his ship chugged toward a drilling platform some 30 kilometers off Mexico’s Yucatan Peninsula. Perched on three sturdy pylons resting on the seafloor, the innovative platform — a “liftboat” known as Myrtle — towered 17 meters above the ocean’s surface.
Researchers needed to keep Myrtle well clear of the waves to steady the drilling equipment that extended from the platform. Their efforts worked: After boring more than 1,300 meters into the seafloor, Myrtle’s crew collected hundreds of intact cores of rock from beneath the surface of the 190-kilometer-wide Chicxulub crater. “I have been on over 25 scientific cruises,” Gulick, a geoscientist at the University of Texas at Austin (UT), wrote on the mission blog during drilling operations in April 2016. “But never on one with as much potential for discovering something truly new.” What the researchers found were indications that living organisms bounced back in the face of extraordinary devastation — in the process, they may have uncovered a clue or two about life’s very beginnings.
Even if most people don’t recognize its name, the Chicxulub crater is probably the most famous impact scar on the surface of our planet. The crater formed a little more than 66 million years ago when a meteorite the size of San Francisco tore through our atmosphere and plowed into Earth’s surface, ending the reign of the dinosaurs. The catastrophe wiped out 75% of all species on the planet and left behind a giant blemish that researchers have been studying for decades.
The impact also left behind several longstanding mysteries. The center of the Chicxulub crater contains a circular range of hills known as a peak ring, and Gulick and his research team ventured to the Yucatan to try to understand how the ring had formed. They also hoped to reconstruct how local life had recovered after the disaster.
Over the past year, analysis of the rock cores drilled out of the Earth has exceeded the team’s initial expectations and offered more than a few surprises. Although large asteroids that smash into the Earth are often regarded as destructive killers — and they are — that is only part of the story. At the crash site, the researchers discovered evidence of a productive ecosystem that thrived in the immediate aftermath of the fiery strike. “It was the opposite of what we expected,” says geophysicist and mission co-chief Joanna Morgan of Imperial College London in the United Kingdom. “It actually ended up being a habitat for life, and quite soon after the impact.”
Other findings lend credence to an even more radical idea. During the first half-billion years of Earth’s history, a constant shower of impactors bombarded the planet at rates that should have rendered the surface uninhabitable. But beneath Chicxulub and other craters, researchers have uncovered fossilized hydrothermal systems created by these impacts. The systems bear a remarkable resemblance to the volcanic chimneys known as black smokers that are found in mid-oceanic ridges. “These vast subsurface hydrothermal systems would have been perfect crucibles for prebiotic chemistry,” says planetary scientist David Kring of the Lunar and Planetary Institute in Houston. Such results suggest that this early incendiary spree might have done more than rain fiery destruction on the planet. The impact’s aftermath could have helped generate the conditions necessary for the creation of life itself.
Ground Zero
In 1980, father-and-son researchers Luis and Walter Alvarez, both at the University of California, Berkeley, hypothesized that an extraterrestrial impactor precipitated the extinction event that took out the dinosaurs. Controversial at the time, the theory was based on a 66-million-year-old worldwide geological layer rich in iridium, an element rare on Earth but common in asteroids. The Alvarez duo eventually succeeded in convincing most of their colleagues, and about a decade later, surveyors found a likely location for ground zero of the apocalyptic event.
The Mexican state-owned oil company Pemex had conducted geological forays into a region near a Yucatan town called Chicxulub in the late 1970s, hoping to find petroleum. Using data on magnetic and gravimetric anomalies in the area, researchers identified a ring-like structure at least 70 kilometers across. In 1991, they tested the oil company’s stored samples and found that they contained shocked quartz, a mineral that forms only in asteroid strikes and nuclear test sites. Five years later, Morgan and her colleagues blasted the seafloor with seismic waves, which bounced back and allowed them to map the full extent of the feature, which sticks out from the Yucatan Peninsula, half onshore and half off.
Chicxulub turned out to be the third-biggest impact crater on Earth and the only one young enough to retain its central peak ring, whose jagged bulls-eye of hills once rose taller than the Eiffel Tower above the crater floor. Such structures have been spotted in large craters on Mercury, Venus, Mars, and the Moon, but nobody was quite sure which aspects of an impact were responsible for creating them. Chicxulub finally offered a chance for geologists to take a hands-on approach to these enigmatic formations. “We have access to the subsurface of Chicxulub, which we don’t have to similarly sized craters elsewhere in the solar system,” says geophysicist Jay Melosh of Purdue University in West Lafayette, IN. “There’s still no better way to learn about the craters then to actually drill in.”
In 1996, Morgan approached the International Ocean Discovery Program (IODP), a global marine research collaboration funded by the governmental agencies of country members, to fund a major expedition that could collect cores from below Chicxulub. But the $100-million price tag of the proposed mission made it a nonstarter. It would take nearly two decades to gather enough additional data and support, and for Morgan to slim down the project to a tenth of its original price, before the collaboration agreed to carry out what became IODP Expedition 364.
In the eons since the Chicxulub impact, the crater had been buried under a half kilometer of limestone sediments. From their perch on Myrtle, the Expedition 364 team used a diamond-tipped drill to bore through this upper layer before switching to coring bits, which only drill the sides of a hole and leave the middle untouched, creating 3-meter-long cylindrical samples of the subsurface. These segments, each a little wider than a hockey puck, provide a window into the geologic layers surrounding the crater’s peak ring.
The samples didn’t come easy — life onboard the vessel was not exactly lavish. “We were living six to a cabin and working in shipping containers turned into labs,” recalls Gulick. “But we didn’t care because of the sheer excitement of discovery.” Morgan says her single most thrilling moment was when the drill reached the peak ring at a depth of around 640 meters below the seafloor and began pulling up “stunningly beautiful” granite blocks. These exquisite orange- and black-speckled samples helped the research team assess two competing theories about the likely mechanism behind peak-ring formation in craters.
Dynamic Collapse
One proposal, the nested melt-cavity hypothesis, suggests that a gigantic sheet of molten rock engulfs a crater’s floor in the moments after a major meteorite impact. The rim of the crater then slumps inward, bumping up against the molten rock and forming a peak ring. In Chicxulub’s case, that ring would contain minerals from the layer of limestone that the asteroid originally burrowed through.
But the core samples support a different theory, known as the dynamic collapse model. It posits that the powerful meteorite blast dug a hole 30 kilometers deep and exposed the rocks beneath to nearly 600,000 times normal atmospheric pressure. For roughly 10 minutes, the ground at the impact site behaved like a viscous fluid, rebounding upward to create a dramatic cascade of debris that fell back and formed the ring-shaped structure. Expedition 364 found granite in the peak ring that originally came from 8–10 kilometers below the surface, heavily favoring this idea.
The result was unexpected, says Melosh. Earlier studies that involved bouncing seismic waves off the crater’s subsurface had suggested it contained rocks with the density of sedimentary minerals such as limestone. But the granite core samples extracted by Myrtle had been so pulverized and fractured by the meteorite impact that their density was lower than normal granite and similar to a sedimentary rock. “To this day we don’t understand what the pattern of fractures was that led to those changes,” Melosh says.
Just as puzzling are the relatively ephemeral effects on the surrounding ecosystem — gleaned from discoveries at Chicxulub that stand in marked contrast to previous findings. Sitting below the Chesapeake Bay in Virginia is another crater, which is both smaller — only 85 kilometers across — and about 30 million years younger than Chicxulub; it is the only other relatively large marine crater whose ancient ecological impact has been explored. Researchers with the US Geological Survey have drilled into the Chesapeake Bay crater and uncovered evidence that the impact caused a 4,000-year-long dead zone on the ocean floor and that marine life in the area took nearly 1 million years to recover its full productivity and diversity.
Because Chicxulub is a larger crater, with a stronger environmental impact, “we expected that we would see something worse,” says micropaleontologist Christopher Lowery, also at UT. But instead, the group’s data revealed that the ecological upheaval from the Chicxulub impactor passed much more quickly. The question was: What made Chicxulub’s aftermath so drastically different?
Death and Destruction
The days after that fateful asteroid strike must have been truly devastating. Smashing into the Gulf of Mexico, the 10- to-15-kilometer-wide bolide generated tsunamis 100 meters high that battered the Caribbean islands and southeastern United States, leaving geologic indications of their destruction as far inland as central Texas. Charcoal deposits indicate that global forest fires might have raged for months. Sulfur-rich aerosols vaporized by the asteroid blast lingered in the atmosphere and blotted out the Sun, starving photosynthetic organisms and causing worldwide temperatures to plunge by an average of 26 ºC for a decade and a half.
In the aftermath of Chicxulub, 90% of Foraminifera species — tiny amoeba-like creatures that produce incredible geometric shells — went extinct around the world. Yet the Expedition 364 cores of sediment layers from just above the crater contain a surprisingly rich diversity of Foraminifera microfossils. Trace fossils, which were left by animals such as worms and small crustaceans burrowing in the seafloor, also appeared rapidly after the event. Geochemical analyses of the samples suggested that large amounts of biomass were being generated in the area just 30,000 years after the cataclysm.
Nevertheless, the ecosystem at Chicxulub was clearly turned upside down by the blast. Photosynthetic phytoplankton struggled to recover for millions of years after the event, for example. Samples from the expedition suggested that there was an order of magnitude less diversity in phytoplankton fossils than in zooplankton such as Foraminifera, which typically feed on phytoplankton. The research team is still puzzling over the difference in these rates of recovery and what the Foraminifera were living on during this period. “It’s counterintuitive,” says earth scientist Timothy Bralower of Pennsylvania State University in University Park, PA, who is studying the phytoplankton fossils.
Part of the answer may lie in the physical landscape surrounding Chicxulub. When the Chesapeake Bay meteorite struck the ground, it carved out a shallow depression that isolated water inside the crater from the rest of the Atlantic Ocean. In contrast, Chicxulub’s tsunamis traveled to the edges of the Gulf of Mexico and then reflected back, filling the crater and rapidly reconnecting it to the larger ocean.
“The whole of the early planet could have been a giant prebiotic reactor.”
— Charles Cockell
Core samples recovered by Expedition 364 also revealed that porous rocks in the center of the Chicxulub crater had remained hotter than 300 °C for more than 100,000 years. Seawater percolating through these rocks would have created an ecosystem analogous to underwater volcanoes that belch mineral-rich water and feed microbes. Although similar structures appeared in the Chesapeake Bay, the water within that crater was stagnant and unable to draw in nutrients from the larger ocean. Lowery speculates that the hydrothermal environment at Chicxulub might have helped nurture the organisms nearby, allowing the productivity of some species to recover much more quickly.
Crucibles of Life
These findings dovetail nicely with a theory that Kring has long advocated, called the impact-origin of life hypothesis. This suggests that asteroid strikes might have provided a jolt of energy that stirred up biochemistry and ignited the process of creating living organisms. “Just like around Yellowstone today, you would have had circulating hot water systems,” says Kring. “And we know from Yellowstone that some very primitive life can persist in those types of environments.”
There are indications that the Earth was already water-rich a mere 100 million years after it coalesced from a ring of dust around the early Sun. Leftover chunks from the solar system’s genesis would have been slamming into the primordial planet around this time, potentially generating energy-rich environments that could persist for millions of years after each crash.
“The whole of the early planet could have been a giant prebiotic reactor,” says astrobiologist Charles Cockell of the University of Edinburgh in the United Kingdom. Craters might have made particularly good crucibles, he adds, because they would contain heat gradients that slowly cooled, favoring the creation of different types of organic molecules at different times. “You can see impacts as generating a whole set of experiments, producing lots of organic material, and then at some point you can imagine that a self-replicating molecule emerged.”
Geneticists have suggested that the earliest common ancestor to all organisms on Earth might have been a hyperthermophile — an extreme heat lover — providing further evidence that the organic Garden of Eden could have been forged by a meteorite. Still, the idea has a long way to go before gaining widespread acceptance.
“As we scientists like to say, ‘It is not unreasonable’,” muses planetary scientist Kevin Zahnle of NASA’s Ames Research Center in Moffett Field, CA. But it is difficult to say whether impacts are better bets for life’s origins compared with alternatives such as volcanic hydrothermal vents, he adds.
“I think it’s a real mistake for anyone to get too polarized about pet environments,” agrees Cockell. “It was a sheer random event that a molecule emerged that happened to be able to self-replicate. Maybe there are lots of environments where that could happen.”
For now, says Kring, the Chicxulub results suggest that the hydrothermal system created by the impact was sufficiently hot and long-lived to favor biochemical experimentation and nurture living organisms. More details about Earth’s earliest days, along with information from the Moon’s well-preserved geologic record, will be necessary to test his hypothesis further.
In either case, the findings coming from Chicxulub continue to astound and bewilder, exposing more layers in this famous story. “I often say that asteroid impacts both giveth and taketh away,” says Melosh. “That’s very much what Chicxulub did. It wiped out the biota at the time, but then set the stage for the recovery of mammals to take over the Earth.”
Published under the PNAS license. More information, including full references, at pnas.org.
