Existing craters hint that asteroids increasingly pummeled both some 290 million years ago
Dr. A. Parker/SWRI
A new look at the moon’s craters suggests the Earth and moon both suffered a sharp increase in impacts around 290 million years ago, and Earth has kept its biggest scars.
Geologists long assumed that erosion and tectonic activity had erased Earth’s craters so thoroughly that “you couldn’t say anything about the craters on Earth at all,” says planetary scientist Rebecca Ghent (SN: 12/22/18, p. 40). So to figure out how much Earth was pummeled in the past, Ghent and her colleagues turned to the moon.
“We can use our closest neighbor to learn a lot more about the Earth’s history,” says planetary scientist Sara Mazrouei, who worked on the study as a graduate student under Ghent at the University of Toronto.
With no atmosphere and no plate tectonics, the moon’s surface preserves a record of nearly all of its 4.5 billion years of craters. If the moon sat through a hailstorm of impacts, Earth should have experienced the same storm, and therefore the same rate of cratering, the researchers argue in the Jan. 18 Science. But without knowing how old most lunar craters are, it’s unclear if the Earth and the moon suffered impacts constantly or in short bursts.
Ghent realized in 2014 that the youngest craters on the moon were surrounded by large rocks, debris excavated by the impact that formed the crater (SN: 4/14/18, p. 32). Those large rocks absorb heat from the sun during the lunar day and radiate it back out at night in wavelengths of light visible to NASA’s Lunar Reconnaissance Orbiter.
“Right away you could see the young craters popping out,” Ghent says. Older craters, by contrast, were surrounded by rocks that had been beaten down to dust over time, so they didn’t glow as brightly at night.
INCOMING IMPACTS Planetary scientists figured out the ages of more than 100 craters on the moon, which revealed that there was a sharp uptick in impacts 290 million years ago. This movie illustrates craters forming over the past billion years of the moon’s history. Each impact is represented by a burst of light and a pitch, with higher tones representing smaller impacts and lower tones representing larger ones.
Ghent used nine craters whose ages were already known to figure out a mathematical relationship between a crater’s nighttime glow and its age. Then Mazrouei, working by hand, mapped all 111 lunar craters less than a billion years old and wider than 10 kilometers in diameter, and used that map to figure out the cratering rate.
Most lunar scientists assumed that, after an early turbulent period of extreme bombardment more than 3 billion years ago, the moon’s impact rate has been mostly constant. “But we saw an increase,” Mazrouei says — specifically, a jump in impacts by a factor of 2.6 around 290 million years ago.
The team then compared the lunar craters’ sizes and ages with 38 of the largest and most stable craters on the Earth. They lined up almost exactly in their timing and sizes.
To double check that such large craters on Earth weren’t often erased by erosion, the researchers looked at volcanic features called kimberlite pipes near the craters. These carrot-shaped lava tubes change starkly in appearance when eroded. The kimberlite pipes that appeared on the same terrain as the large craters confirmed that very little of either feature had been lost to erosion, Ghent says.
The jump in the impact rate could have been caused by a smash-up in the asteroid belt sending debris toward the inner solar system, says coauthor William Bottke, a planetary scientist at the Southwest Research Institute in Boulder, Colo. In 2007, Bottke linked one such asteroid break-up to the impact that killed the dinosaurs (SN: 9/8/07, p. 148).
Ghent cautions against drawing conclusions about an exact date for that spike in impacts, noting it could have happened tens of millions of years earlier or later than estimated, or in multiple spurts. “I don’t want people to say, ‘Hey, the Permian-Triassic extinction happened during that time. This might have caused it.’ We don’t know that,” she says.
The new finding offers an explanation for a gap in Earth’s craters between 300 million and 650 million years old, Bottke says. “We don’t see fewer craters because of erosion,” he says. “We see fewer craters because the impact flux was lower.”
But the craters’ longevity raised another question. While the moon’s craters date back billions of years, Earth has almost no preserved craters older than about 650 million years. That makes sense if craters are lost constantly to erosion. But if not, where did the older craters go?
Older craters could have been scraped away during a global glacial period called Snowball Earth, Bottke says. Other lines of geological evidence suggest Earth went through deep freezes, the last of which occurred about 650 million years ago.
Other planetary scientists not involved in the study expressed mixed reactions about its methodology and findings.
“That is a major step forward in our understanding of the impact flux in the inner solar system,” says planetary scientist Christian Koeberl of the University of Vienna. “Their method is interesting and new.”
Geologists Thomas Kenkmann and Stefan Hergarten of the University of Freiburg in Germany are more skeptical. In 2015, the duo showed that many of Earth’s craters could still be undiscovered (SN: 7/25/15, p. 5). They’re not convinced that the new study captured all craters, or that Earth’s crater count reflects an increase in impact rate 290 million years ago. It could just be that “younger craters are more likely to have survived,” Hergarten says. “The dataset is really small, so I would not trust in it too much.”
Kenkmann agrees. “Considering the low number of craters for the lunar calculation, and the vague interpretation of the terrestrial record, it appears that a house of cards is being built here.”
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