Where have Earth’s craters gone? Certainly we have the striking Meteor Crater in Arizona, and Chicxulub, which lies beneath Mexico’s Yucatán Peninsula, the 100-mile-wide scar of the meteor that most likely killed off the dinosaurs.
Some of the cosmic battering, from the space rocks that landed in the oceans, did not carve out craters. Others have been erased by erosion and plate tectonics.
Still, there do not seem to be enough craters on our planet, especially from the older eras — just 190 confirmed worldwide.
A new study suggests that geologists cannot find more big dents in Earth’s surface because they were never there.
On Thursday, researchers presented results of a new technique suggesting that the pace of space rocks pummelling Earth and the moon used to be less frequent than it is now, but then doubled or tripled for reasons not yet explained.
“I think we’ve got a good story,” said William F Bottke, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, and one of the authors of a paper published in the journal Science. “We’re changing the impact rate on the Earth by a factor of 2 to 3. That happened 290 million years ago.”
That finding was unexpected, because there is no obvious explanation for why the number of asteroids or comets would jump. This period, which came before the rise of the dinosaurs, was long past the chaotic early days of the solar system.
Other scientists are sceptical, because the research draws its conclusions from a small number of terrestrial and lunar craters. H Jay Melosh, a Purdue University professor and an expert on meteors and impacts, described the paper as “an intriguing idea” but added that he was unconvinced.
“With statistics of small numbers, that doesn’t give me confidence that they’re right,” he said. “You can’t say it’s wrong, either. It’s just not convincing.”
The moon should offer some clues as to what happened on Earth. It is heavily cratered, recording thousands of impacts since it formed some 4.5 billion years ago.
But the ages of lunar craters have often been uncertain. Dating of radioactive elements in the moon rocks brought back by the Apollo astronauts nearly 50 years ago has pinned down the ages of about 10, said Rebecca R. Ghent, a professor of earth sciences at the University of Toronto and an author of the new paper.
Another method used to date some craters is more imprecise. When a crater is new, its inside is usually smooth and pristine. Over time, smaller meteors strike the surface of this interior.
But no one knows the precise impact rate, and counting craters is not straightforward. One impact could scatter smaller rocks across the landscape, resulting in what wrongly appear to be separate, additional impacts. “Then you’re going to get the wrong age,” Ghent said.
She came up with a novel, clever alternative: taking the temperature of a crater.
A fresh lunar crater tends to be surrounded by large boulders that were excavated by the meteor impact. The boulders retain heat when the crater rotates into darkness during the moon’s nights, which last for two weeks at a time.
In older craters, the boulders, battered by micrometeorites for millions of years, turn to dust, which cools quickly at night. Bottke said anyone can observe this phenomenon on a beach at night: The sand is cool, while a rock in the sand is still warm.
A heat-measuring instrument on NASA’s Lunar Reconnaissance Orbiter was able to differentiate warm craters from cooler ones, and the pattern seemed to hold. Tycho, a 53-mile-wide crater known to be young, retained warmth during nighttime, while other, older craters did not.
“It became very clear you could see the rocks in the nighttime data,” Ghent said.
She and her colleagues calibrated the technique using the known ages of craters from the Apollo data. The correlation between the temperatures and the ages were “a tight, tight fit,” she said.
The team then looked at 111 moon craters that were more than 6 miles wide and less than 1 billion years old. They found that there were fewer older craters. The data suggested that the rate of impacts on the moon increased 290 million years ago.
“I was surprised,” Ghent said.
This is not the first time that scientists have suggested this uptick. In 2000, scientists at University of California, Berkeley, came to a similar conclusion based on the dating of glass spheres in moon soil samples. The spheres formed when rocks melted then cooled after impact.
“Their conclusions are broadly consistent with ours,” Paul R. Renne, director of the Berkeley Geochronology Center, who was an author of the earlier paper, wrote in an email.
Since our planet and the moon are close to each other, “we should be able to see the same pattern with the Earth,” said Sara Mazrouei of the University of Toronto, lead author of the new Science paper.
Indeed, with the method they used to date the lunar craters, the scientists found a lower rate of impacts on our planet before 300 million years ago, and almost no craters older than 650 million years.
The absence of older craters might be explained by what geologists describe as the “Snowball Earth” period, when ice covered almost the entire planet. The glaciers would have scoured the surface smooth.
Just looking at the older parts of Earth’s surface, the researchers found fewer craters than would be expected if the rate of impact then were the same as it is now.
To argue against the erosion of craters, the researchers pointed to kimberlite pipes — carrot-shaped rock formations resulting from explosive eruptions. They found a modest amount of erosion in the kimberlites over 650 million years and argue that impact craters would not have eroded much.
The scientists speculate that perhaps the breakup of an asteroid could have generated a bunch of new space rocks that rained down in the inner solar system.
The European Space Agency’s BepiColombo spacecraft, which will arrive at Mercury in 2025, has a similar temperature-measuring instrument, which could detect a jump in impacts at the same time there.
Melosh said a conclusive answer will most likely come from the moon, but not for a while. “All these problems would be solved if we could just get to the moon and date 10,000 craters,” he said.