Lunar Craters Show Massive Asteroid Shower Hit Earth 800 Million Years Ago

Asteroids collide with the moon in this artistic depiction of what an asteroid shower 800 million years ago may have looked like. (Image via Murayama/Osaka University)

(CN) — A massive asteroid shower collided with the Earth and the moon 800 million years ago, resulting in at least eight of the craters we can see on the moon today, according to a trio of Japanese planetary scientists who examined lunar craters to learn about the impact.

Led by Osaka University’s Kentaro Terada, a self-described “cosmo-chemist,” the researchers examined data obtained by the Japanese lunar orbiter KAGUYA, which took images of the lunar terrain used to measure the size of large moon craters and the density of much smaller impacts surrounding the larger ones.

Meteor impacts on the Earth are slowly disguised by continental drift, erosion, volcanism and other resurfacing processes over hundreds of millions of years; ancient impacts have been erased over time.

Our lunar neighbor, it turns out, is a much better record.

“We used the moon as ‘a witness to the history of the solar system,’ because the moon surface has no erosion and well preserves the impact history of the Earth-moon system,” Terada said in an interview with Courthouse News.

This “sheds light on the veiled impact history of the Earth before 600 million years,” he added.

Crater-counting is the key to dating the lunar surface’s historic record.

“‘Crater chronology’ is a well-established technique to derive relative and absolute ages of planetary surfaces, since the Apollo era,” Terada said.

The KAGUYA lunar orbiter captured this image of the moon’s Copernicus crater and small impact sites surrounding it, highlighted and marked in green. Researchers counted the density of these small craters to estimate the age of the impact that caused Copernicus and dozens of other lunar impact sites. (Image via Osaka University)

Co-author Tomokatsu Morota is a crater-counting expert, Terada said, and together with colleague Mami Kato they determined the ages of “main craters,” measuring more than 20 kilometers across, by counting how many small cavities surrounded the main ones.

The team determined that eight of the 59 large craters they were examining had been created at the same time, 800 million years ago — evidence of an ancient meteor shower.

“It is the first moment that made us think, ‘This is big,’” Terada said.

The second moment came soon after: by dating the craters, the researchers realized this coincided with a disruption to the minor planet Eulalia, which measures 100 kilometers in diameter, orbits the Sun every 1,432 days and was disrupted — collided with, broken apart or otherwise impacted for one reason or another — about as long ago as the lunar craters formed.

Terada noted that scientists studying the moon and those studying asteroids usually progress independently.

“But our new finding let us directly link them,” Terada said. “I was so excited.”

Another key piece of evidence is the steady stream of carbon ions emitted from the moon’s surface and detected by KAGUYA, which carries an ion mass spectrometer. The researchers said Eulalia is expected to contain carbon, so its shower could have contaminated the moon and may explain the ions KAGUYA detected.

Terada and his colleagues have previously written about the moon’s emission of carbon, much more than could have been left behind by solar wind or micrometeoroid impacts.

The scientists estimate that the ancient asteroid shower contained a massive amount of meteoroids — at minimum, between 40 and 50 quadrillion kilograms of meteoroids. That’s between 30 and 60 times the size of the Chicxulub impactor that hit 66 million years ago and likely killed three quarters of all plant and animal species on Earth, including the avian dinosaurs.

If these meteors did strike 800 million years ago, the discovery would fit with what scientists know about the Earth’s geologic history: the Cryogenian period, during which the Earth’s two greatest ice ages occurred, would have followed soon after the shower.

The group’s research was published Tuesday in the journal Nature Communications and was funded by Japan’s Ministry of Education, Culture, Sports, Science and Technology.

“NASA and JAXA [Japan’s space agency] are planning to send humankind to the moon. Therefore, we will have more opportunities to look up at the moon in the future,” Terada said. “When you see the full moon, I want you to feel a sense of romance, that you can learn about the past of the Earth by looking at the moon.”

Terada intends to move forward with projects dating the age of extraterrestrial materials such as meteorites and samples obtained from the moon and asteroids. First on the list are samples from the nearby Ryugu and Bennu asteroids.

“If we got the age of 800 million years from Ryugu samples, I would be so excited,” he said.

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