Evidence of giant asteroid strike may be buried under Wyoming | Science

Some 280 million years ago, before the rise of the Rocky Mountains—or even the dinosaurs—a 2.5-kilometer-wide asteroid smashed into the supercontinent of Pangaea, near the eastern border of present-day Wyoming. The impact’s heat and shock wave would have killed anything within 400 kilometers, making it one of the largest asteroid strikes in North American history.

Despite the tumult on Earth’s surface, however, plate tectonics had the last laugh. Intruding oceans and shifting landscapes buried the resulting 60-kilometer-wide crater—about the size of Rhode Island—in a tomb of sand that eventually became sandstone. And there the crater may sit, kilometers down, even to this day.

That’s the scenario painted in new work. Researchers haven’t found the crater itself, but they have identified a series of 31 smaller craters, each no wider than a U.S. football field. These “secondary” craters would have been formed by boulders ejected by the impact, landing up to 200 kilometers away. It is the first time a secondary crater field—commonly seen on other planetary bodies, including the Moon—has been discovered on Earth.

“This is a spectacular finding,” says Brandon Johnson, a planetary scientist at Purdue University unaffiliated with the study. “The work is rigorous and I’m convinced.” The crater field, he adds, which has 60 candidate holes that have yet to be confirmed, has the potential to represent 40% of the craters known on Earth.

The team, led by Thomas Kenkmann, a geologist at the Albert Ludwig University of Freiburg, first discovered the minicraters in 2017 on the northeast flank of Sheep Mountain, at the foot of the Rockies in western Wyoming, near Grand Teton National Park. Fractures in quartz grains found at the base of these depressions were signatures of a shock that can only be created by cosmic impacts. They initially led the scientists to believe the crater field was formed by the hailstorm of an asteroid that broke up in Earth’s atmosphere.

But the team soon discovered more shocked quartz in craters that were farther away—beyond the distance that fragments from a single asteroid could cover. Even more strangely, the craters lacked evidence of rocks or metals foreign to the local geology, as would be expected from meteorites.

Searching for alternative explanations, Kenkmann and his colleagues observed that many of the craters were elliptical. That’s unheard of for impacts from space, which tend to punch circular holes even when they strike at oblique angles because of their speed. And like blood spatter at a crime scene, the axes of these ellipses projected back to a common point.

The craters’ pattern was similar to the rays and streaks of small craters that surround large craters on the Moon, like Tycho. They provide clear evidence that such formations are possible on Earth, the researchers conclude this month in the Geological Society of America Bulletin.

This “compelling” study also raises many questions, says Gareth Collins, a planetary scientist at Imperial College London. Why haven’t secondary craters been found surrounding other asteroid impacts on Earth? “What was so special about this impact?” he says.

Until now, many geologists believed Earth’s thick atmosphere would simply break apart or burn up ejected boulders, preventing a notable crater. But it was hard to know for sure that the atmosphere was the problem, as the erosive force of wind and water can erase these smaller features in decades, Kenkmann says. Earth is good at covering its scars.

Ancient Wyoming had other plans, however. These small cavities, created by boulders up to 8 meters wide falling at up to three times the speed of sound, contained quartz grains that originally formed from wet sand—indicating the boulders crashed into a quiet lagoon of some sort. The craters were soon entombed by hundreds of meters of silt, which turned into shale over the course of 200 million years. It wasn’t until the birth of the Rockies some 75 million years ago that these fossilized craters were exhumed.

The impact that ejected these boulders would have been devastating. “A shock wave runs through the atmosphere that destroys everything,” Kenkmann says. “Nothing would survive.” Several of the depressions examined by the team had signs of melted rock. The boulders alone would not have been able to cause the extreme heat and pressure needed to melt rock. Rather, the melt likely came from plumes of vaporized rock that followed the boulders’ arcing paths.

If the researchers find the primary crater they expect, it would be one of the largest known in the United States. But it would still pale in comparison with the asteroid that formed the 180-kilometer-wide Chicxulub crater off the coast of Mexico, which wiped out the dinosaurs 66 million years ago. Perhaps the Wyoming impact modified the climate for a few years. But there are no major extinctions tied to this time period. “This is a devastating event, but still of regional extent,” Kenkmann says.

Verifying the impact will require a lot more detective work. Unfortunately the Rockies did not unearth the likely site of the primary crater, which by the team’s current best guess is 70 kilometers northeast of Cheyenne, near the Nebraska border, buried 3 kilometers down amid the fossil fuel deposits. There are tantalizing clues from tiny changes in the land’s gravitational pull that some sort of deformity exists down there. But little other evidence has emerged.

That could change soon. Kenkmann will return to Wyoming in April, hoping to verify dozens of other possible secondary craters. And he’s optimistic that records and samples from oil and gas drillers in the region may contain some hint of the primary crater. And ultimately, if their surface-based evidence improves, he hopes to convince science funders to help him drill for the relic crater.

Johnson for one would love to join him in the hunt. He thought this fundamental impact process could only be studied in person by astronauts. But now, he says, in Wyoming, you can be surrounded by secondary craters “without going to the Moon or Mars.”