Massive protoplanet formed huge Moon crater, study finds

The asteroid that crashed into the lunar surface, creating the Imbrium Basin (the right eye of the so-called “Man in the Moon”) may have actually been twice as large and 10 times more massive than previously estimated, claims research published Wednesday in the journal Nature.

Earlier estimates based on computer models had suggested that the impactor responsible for the crater’s formation was only approximately 50 miles in diameter, but new analysis of the basin by Pete Schultz, professor of earth, environmental and planetary sciences at Brown University, and his colleagues has found that the object may have actually been much larger.

“We show that Imbrium was likely formed by an absolutely enormous object, large enough to be classified as a protoplanet,” Schultz explained in a statement, adding that this is the first estimate to be based primarily on the moon’s geological features, and that the findings might help explain why the Imbrium Basin is surrounded by unusual terrain including grooves and gashes.

Furthermore, by comparing the impact crater’s size with others on the Moon, as well as some present on Mercury and Mars, the study authors suggest that protoplanet-sized asteroids may have been relatively abundant during the earliest days of the solar system.

Protoplanet-sized impactors common in the early solar system?

Visible even from Earth, the Imbrium Basin is a dark patch in the northwestern quadrant of the moon that is 750 miles long. Surrounding it is a feature known as the Imbrium Sculpture, which is comprised of grooves and gashes radiating out from the center of the basin on its southeastern side and was created by rocks ejected from the crater when it was first formed.

The grooves on the southeastern side suggest the object came from the northwest and struck the surface at an oblique angle, the researchers explained. However, the presence of a second set of grooves which a different alignment, apparently coming from an area to the northwest along the same trajectory as the impactor had long puzzled scientists.

“No one was quite sure where they came from,” Schultz noted. However, he and his colleagues conducted a series of experiments suggesting that the second set of grooves was likely created by shards of the impactor that broke off after it first struck the lunar surface. Based on their analysis of those grooves’ sizes, they were able to determine that the impactor was at least 150 miles (250 km) in diameter – large enough to be classified as a protoplanet – and potentially bigger.

“That’s actually a low-end estimate,” Schultz said. “It’s possible that it could have been as large as 300 kilometers.” His team then conducted similar experiments to determine the approximate size of impactors related to other basins on the Moon and found that they too were bigger than previously estimated, ranging from 100 to 110 km (62 to 68 miles) in diameter.

Based on these findings, and the fact that other worlds have impact craters larger than Imbrium Basin, Schultz believes that the findings indicate that protoplanet-sized asteroids were common at one time. He also believes that the findings explain why moon rocks collected by the Apollo missions had high meteoritic content (the result of intact fragments from the impactors) and that pieces of these large asteroids may be responsible for some of the impacts that took place during the Late Heavy Bombardment some 3.8 to 4.0 billion years ago.

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Image credit: NASA

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