Four Billion Years Ago, the Moon May Have Taken a Punch from a Protoplanet

Mare Imbrium, the lunar impact crater that forms the right eye of the pareidolic "Man in the Moon," may have been formed by a protoplanet-sized object colliding with the young Moon, according to a study published Wednesday in Nature.

Authored by Pete Schultz, professor of planetary sciences at Brown University, and David Crawford of Sandia National Laboratories, the new research estimates that an object measuring about 275 kilometers (170 miles) in diameter impacted the lunar surface about 3.8 billion years ago, during a period known as the Late Heavy Bombardment, when planetary collisions were much more common than they are today.

This new size estimate is much larger than previous calculations of the Imbrium impactor's dimensions, challenging prevailing theories about this chaotic era in the history of the solar system.

"We find that the Imbrium basin was formed by an object at least three times larger in diameter, or 30 times more massive [than previous estimates]," Schultz told me. "This has implications for the sizes of even larger impact basins on other planets, not to mention the Earth where the evidence has been largely erased" by geological processes like erosion and plate tectonics.

For Schultz, the tipoff that this ancient impactor may have been in the protoplanetary size range is etched into Mare Imbrium's unique geological features. Stretching about 1,200 kilometers (750 miles) in diameter, the impact basin contains subtle clues about the cataclysmic event that moulded it.

For instance, the crater is lined with scar-like grooves known as the Imbrium Sculpture, especially on its southeast side. This suggests that the impactor was on a northwest trajectory when it collided with the lunar surface at an oblique angle. Debris kicked up by the crash scattered to the southeast, ploughing out massive gashes that sweep radially out from the impact site.

But curiously, there is a second set of grooves located up-range of the main impact site. With the help of the Vertical Gun Range, a cannon located at NASA's Ames Research Center that can shoot projectiles at 16,000 miles per hour, the team identified these unusual features as the scars left by protoplanet chunks that broke off right as the object made first contact with the Moon. Charting out their trajectories and modelling them in computer simulations then led to the new estimate of the Imbrium impactor's impressive heft.

Scientists already know that the Late Heavy Bombardment was, as the name suggests, an era of great turbulence in the solar system. Planetary collisions created colossal craters like Mare Imbrium, and also launched new material into orbit that would have smashed into the paths of the inner planetary bodies over and over.

Now, Schultz and Crawford have demonstrated that many of these impactors may have been much more substantial than past research has imagined.

"This work indicates that there were many more large asteroids in the asteroid belt, not just fragments or left-overs from failed planet formation," Schultz said. "So, this requires thinking about the fate of such large (now-missing) asteroids and how their collisions may have contributed to additional debris during the time of the Late Heavy Bombardment between about 4.3 and 3.9 billion years ago."

"This new research will help determine the size of objects that have struck other planets where similar patterns can be found," he added. "This effort is underway."