Moon craters may overstate intensity of asteroid impacts 4b years ago

On a clear night, the moon’s battered history comes into sharp relief: Even from 240,000 miles away, its largest craters are so massive as to be visible to the naked eye.

Scientists have long thought that such lunar craters arose during a period called the Late Heavy Bombardment (LHB), about 4 billion years ago. During that time, a hailstorm of giant asteroids pummeled the solar system, slamming into the moon, along with young planets like Mercury, Venus, Earth, and Mars.

Evidence for this theoretical period comes mostly from the moon itself. While most traces of Earth’s early history have been wiped away by erosion and tectonic activity, the moon remains as a nearby, relatively untouched, and easily observable relic of the early solar system. In particular, scientists have based most of their theories of that period on the impact basins found on the moon’s near side — the side always facing the Earth — assuming, from the size of its craters and basins, that the moon and other planets endured impacts from massive asteroids.

But now scientists from MIT, the University of Paris, and elsewhere have found that craters on the near side of the moon may not reflect the intensity of asteroid impacts from that period. Instead, much smaller asteroids likely created these craters — a finding that may redefine scientists’ picture of the LHB.

“This is very interesting, because we thought we knew the approximate sizes of impacting asteroids to make the bigger basins on the near side,” says Maria Zuber, MIT’s vice president for research and the E.A. Griswold Professor of Geophysics. “What [this work] indicates is that the flux of large impacting bodies during the Late Heavy Bombardment has been overestimated.”

Zuber and her colleagues publish their results this week in the journal Science.

Different faces of the moon

While massive impact basins pockmark the moon’s near side, its far side contains considerably smaller basins. The discrepancy in crater distribution has puzzled scientists for decades.

To investigate what may have caused this difference, the team obtained data from NASA’s twin GRAIL probes, which orbited the moon from January to December 2012. During its mission, the probes circled the moon, making measurements of its gravity. Zuber and her colleagues used this data to generate a highly detailed map of the moon’s crust, showing areas where the crust thickens and thins; in general, the group observed that the moon’s near side has a thinner crust than its far side.

Katarina Milijkovic, a postdoc at the University of Paris, generated computer simulations of asteroid impacts on the moon by plugging in crustal thickness data from GRAIL (which stands for Gravity Recovery and Interior Laboratory). Milijkovic also incorporated estimates of the moon’s early internal temperatures from thermal modeling, based in part on lava deposits that flooded the large impact basins on the moon’s near side. Scientists have observed that more volcanic activity occurred on the near side, generating higher internal temperatures than on the moon’s far side.

A less catastrophic bombardment

With crustal thickness and temperature data incorporated into the model, Milijkovic then simulated the effects, on both the moon’s near and far sides, of impacts by asteroids of the same size and velocity. She found that identical asteroids would have had very different impacts on the two sides, creating basins on the near side that were as much as twice as large as those on the far side — a result that matches  the size distribution of structures seen today.

Zuber says the near side’s thinner crust and higher temperatures may have made the surface more deformable than the thicker, cooler crust of the moon’s far side. These results, she says, suggest that the LHB may have involved less massive asteroids than scientists have thought.

“I’d certainly been a believer in the Late Heavy Bombardment from looking at those large impact basins. The idea of a Late Heavy Bombardment remains,” Zuber says, “but it will be have to be re-examined.”

Why the near side should have larger impact basins than the far side is puzzling, since previous work has shown that the impact flux on both sides should be about the same. According to the team’s results, the far side of the moon may better reflect the size distribution of asteroids that pummeled the early inner solar system.

“My simulations show that the largest lunar far side basin could have been formed by approximately the same size impactor as the largest impact basin on the lunar near side,” Milijkovic says — even though the latter basin is much larger. “In essence, the Late Heavy Bombardment should have been less catastrophic.”

William Bottke, a planetary scientist specializing in the study of asteroids at the Southwest Research Institute in Boulder, Colo., says scientists have questioned the difference in crater sizes on the moon since the 1960s, when researchers first imaged the moon. Bottke says the recent results may provide a resolution to this puzzle.

“I do think this work has intriguing implications for the earliest time of bombardment of the moon and other worlds, which will be interesting to explore,” Bottke says. “To really make progress, it would be useful to have more physical constraints from the moon itself. Ideally, this would come in the form of samples — rocks are great at telling the stories of worlds.”