Earth is constantly being pummeled by space rocks. Several tons rain down on the planet each day in the form of dust. And larger strikes have created more visible features, including giant craters. But which of our planet’s extraterrestrial scars is the oldest?
Researchers reported on Tuesday in Nature Communications that they have pinpointed it, in Western Australia. It was caused by an impact more than 2.2 billion years ago.
Intriguingly, that timing roughly coincides with the end of one of our planet’s ice ages. An impact in the ice would have liberated an enormous amount of water vapor, the researchers suggest, perhaps enough to alter Earth’s climate and catapult the planet out of widespread glaciation.
The Yarrabubba impact structure, about a day’s drive northeast of Perth, isn’t much to look at today. The original crater, believed to have been roughly 40 miles in diameter, is long gone.
“There’s no topography that rises up,” said Aaron Cavosie, a planetary scientist at Curtin University in Perth and a member of the research team.
That’s because the combined effects of wind, rain, glaciation and plate tectonics have scoured several miles off the surface of the planet, effectively erasing the crater. The extent of erosion suggests that the impact structure is very, very old.
Existing clues yield “a pretty giant” age of about a billion and a half years, said Timmons Erickson, a geochronologist at NASA Johnson Space Center in Houston and the study’s lead author. But Dr. Erickson knew that it was possible to do far better, by reading the tiny geological clocks that hide within rocks.
In 2014, Dr. Erickson collected roughly 200 pounds of granitic rocks from Yarrabubba. Back in the laboratory, he and his colleagues placed the rocks in water and added 120,000 volts of electricity. That jolt broke the rocks into sand-size grains. The scientists were looking for grains of zircon and monazite, tough minerals that survive for billions of years and, crucially, incorporate uranium and thorium atoms into their crystalline structure.
Uranium and thorium decay, in a steady dribble over billions of years, into lead. But the searing temperatures of an impact — thousands of degrees Fahrenheit — cause zircon and monazite to recrystallize, a process that drives out lead.
“It’s kind of like cleaning house,” Dr. Cavosie said. “Recrystallization is a bond-breaking process that kicks out the pre-existing lead and thus resets the clock.”
As a result, the relative amounts of uranium, thorium and lead in recrystallized zircon or monazite can be used to calculate how long ago an impact occurred.
Based on measurements of 39 zircon and monazite crystals, Dr. Erickson and his team calculated that the Yarrabubba impact occurred 2.229 billion years ago, with an uncertainty of 5 million years. The next-oldest impact structure, Vredefort Dome in South Africa, is over 200 million years younger.
The age of the Yarrabubba impact structure happens to line up with the end of an ice age, which makes for a compelling coincidence, Dr. Erickson said: “Would an impact event like Yarrabubba be enough to terminate a glacial time in Earth’s history?”
To help answer that question, the scientists modeled the effects of a roughly four-mile-wide impact object striking ice sheets of different thicknesses. They found that more than 100 billion tons of water vapor would have been jetted into the upper atmosphere.
Water vapor is a potent greenhouse gas; suddenly having much more of it aloft could have triggered a warming that ended an ice age, the team suggested. That idea still needs to be tested with climate models, the researchers noted.
Christian Koeberl, a geochemist at the University of Vienna and not involved in the research, agreed. Inferring what might have happened to Earth’s ancient climate is “where things get a lot more speculative,” he said. “We just don’t know the answer to that yet.”
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January 21, 2020 at 11:00PM
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