The legendary Man in the Moon has many origin stories, but we may be closer to understanding how he developed one of his defining features. Scientists at Brown University have proposed a new theory based on the geological features of the Moon's surface, suggesting that the Imbrium Basin (the "right eye") is the result of a collision with a protoplanet-sized asteroid much larger than previous estimates.

The early days of our solar system were tumultuous times, with many of the planets and moons as we now know them forming from protoplanets colliding with each other. These small rocky objects are like planetary embyros, and as they slam into each other, they increase in mass until they eventually become planet or moon sized. Mars' two moons were likely formed in this way, and so was our own. The moon's pockmarked face is evidence of how smaller asteroid collisions were even more common, with previous estimates placing the Imbrium impactor at around 50 miles (80 km) wide.

But that figure is based on computer models. For this study, Brown University researchers studied the geological features of the crater and estimated that the object that formed it was 150 miles (241 km) across, bringing it more in line with the size of a protoplanet.

"We show that Imbrium was likely formed by an absolutely enormous object, large enough to be classified as a protoplanet," says Pete Schultz, one of the authors of the study. "This is the first estimate for the Imbrium impactor's size that is based largely on the geological features we see on the Moon."

The features that led to that conclusion include the Imbrium Sculpture, a series of large gashes and grooves that branch out from the 750 mile (1,207 km)-wide crater, most of which lie southeast of the site. That implies that the impacting body came in from the northwest, and debris kicked up as a result littered the area in front of it.

More intriguing was a series of similar scratch marks further northwest, before the crater.

"This second set of grooves was a real mystery," Schultz said. "No one was quite sure where they came from." The team at Brown was able to demonstrate their theory regarding how these came to be, and use them to estimate the size of the impactor more accurately.

Schultz and the team employed the help of a huge cannon at the Vertical Gun Range at the NASA Ames Research Center, which launches objects at speeds of up to 16,000 mph (25,750 km/h). The results of their experiments found that impactors colliding at low-angles, such as the Imbrium object, tend to begin breaking apart on first contact with the ground, a point which can be a long way before the main impact site. The team theorizes that the chunks of the moon impactor that broke off earlier are responsible for those northwestern grooves.

Measuring the distance between those gashes is a good indication of how wide the object was, which is how the team arrived at its figure. And they admit that's a conservative one.

"That's actually a low-end estimate," says Schultz. "It's possible that it could have been as large as 300 km (186 miles)."

The Brown team applied that same technique to other impact sites on the moon, and found that their sizes also tended to be underestimated. Protoplanet-sized asteroids may have been more common in the early years of the solar system than previously thought, the researchers concluded.

"The large basins we see on the Moon and elsewhere are the record of lost giants," Schultz says.

The research appeared in the journal, Nature.

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