Around 3.8 billion years ago, an asteroid measuring more than 150 miles across and roughly the length of New Jersey collided with the Moon, forming the Imbrium Basin, the right eye of the famous Man in the Moon. This new size estimate, published in the journal Nature, proposes an Imbrium impactor with a circumference two times larger and a mass ten times greater than earlier estimates.
“We show that Imbrium was likely formed by an absolutely enormous object, large enough to be classified as a protoplanet,” said Pete Schultz, professor of earth, environmental and planetary sciences at Brown University. “This is the first estimate for the Imbrium impactor’s size that is based largely on the geological features we see on the Moon.”
Previous estimations, according to Schultz, were exclusively based on computer simulations and indicated a circumference of only approximately 50 miles.
These new findings assist to explain some of the Imbrium Basin’s perplexing geological traits. Based on the sizes of other impact basins on the Moon, Mars, and Mercury, the research shows that the early solar system was likely awash in protoplanet-sized asteroids.
Imbrium sculpture
The Imbrium Basin, seen from Earth as a black region in the Moon’s northern hemisphere, is about 750 miles across. Grooves and gashes, large enough to be seen with even tiny telescopes from Earth, encircle the basin, which were produced by boulders blasted out of the crater when it was formed.
The Imbrium Sculpture features radiate out from the basin’s center like spokes on a wheel but are concentrated on the basin’s southeast side. This indicates that the impactor came from the northwest and collided at an oblique angle rather than directly.
However, there is a second set of grooves with a different orientation, in addition to the characteristics radiating from the basin’s center. These appear to have originated in an area to the northwest of the impactor’s track.
The key point is that the grooves made by these chunks aren’t radial to the crater. They come from the region of first contact. We see the same thing in our experiments that we see on the Moon grooves pointing up-range, rather than the crater.
Pete Schultz
“This second set of grooves was a real mystery,” Schultz said. “No one was quite sure where they came from.”
Schultz was able to prove that such grooves were likely generated by portions of the impactor that sheared off on initial contact with the surface utilizing hypervelocity impact experiments at NASA Ames Research Center’s Vertical Gun Range. Schultz was able to determine the magnitude of the impactor thanks to the grooves created by those fragments.
Laboratory impacts
The Vertical Gun Range uses a 14-foot cannon to fire small projectiles at speeds up to 16,000 mph, while impact plates and high-speed cameras record the ballistic dynamics. Schultz found that impactors tend to break apart when they initially make contact with the surface during his studies with low-angle impacts.
The original point of impact is actually behind or “up-range” of the ultimate crater when the impactor’s weight sinks into the surface. The chunks that break off up-range of the final crater continue to scour and groove the surface at a high rate of speed.
“The key point is that the grooves made by these chunks aren’t radial to the crater,” Schultz said. “They come from the region of first contact. We see the same thing in our experiments that we see on the Moon grooves pointing up-range, rather than the crater.”
Following his observations in the lab, Schultz collaborated with David Crawford of Sandia National Laboratories to create computer models demonstrating that the same physics would occur at the massive scales of a lunar impact.
Schultz could utilize the grooves to locate the Imbrium impact spot if he knew how they were made. The groove trajectories might also be used to estimate the impactor’s size because the pieces would have broken off from either side of the impactor.
According to those calculations, the object has a diameter of 250 kilometers (150 miles), which is large enough to be categorized as a protoplanet.
“That’s actually a low-end estimate,” Schultz said. “It’s possible that it could have been as large as 300 kilometers.”
“Lost giants” and the Late Heavy Bombardment
Schultz and his colleagues used similar approaches to estimate the sizes of impactors formed by oblique impacts on numerous additional basins on the Moon. Impactor sizes of 100 and 110 kilometers across were calculated for the Moscoviense and Orientale basins on the Moon’s far side, which are greater than some previous predictions.
Schultz concludes that protoplanet-sized asteroids may have been prevalent in the early solar system based on these new estimations and the fact that the Moon and other planets have even larger impact basins.
“The large basins we see on the Moon and elsewhere are the record of lost giants,” Schultz said.
According to him, the study has a number of other important consequences. The impactors’ surviving shards would have scattered the ancient Moon’s surface, slowly blending in with native soil and rock.
That could explain why the meteoritic content of samples returned from the Apollo missions was so high. This is especially true for Apollo 16, which landed far away from the Imbrium collision.
Furthermore, Schultz’s research suggests that fragments from these giants could be responsible for many of the impacts that occurred during the Late Heavy Bombardment, which lasted from about 3.8 billion to around 4 billion years and is when most of the craters on the Moon and Mercury were formed, according to scientists.
According to the impact models produced by Schultz and Crawford, thousands of the fragments that crumbled off the Imbrium impactor and others would have broken and continued on their way, escaping the Moon’s gravity and floating off into space.
Those fragments would have crossed the Earth and Moon orbits again and again on subsequent journeys around the sun, creating a substantial chance of further impacts. Some of those items would have been a kilometer or two in diameter, huge enough to leave 20-kilometer craters.
“These chips off the old blocks could have contributed significantly to the impact record we see on the Moon and other terrestrial planets,” Schultz said.
Schultz also stated that he is still astounded at how much we can learn by simply glancing up at the Moon.
“The Moon still holds clues that can affect our interpretation of the entire solar system,” he said. “Its scarred face can tell us quite a lot about what was happening in our neighborhood 3.8 billion years ago.”
