Moon's Mysterious Gravity Explained: A Study

When scientists first made contact with the Moon, they found something they didn’t expect: every so often those early probes sent to scout landing sites for Apollo astronauts would suddenly begin plummeting toward the lunar surface before pulling back up, suggesting an uneven gravity.

The reason for this phenomenon, researchers hypothesized, had to do with regions of greater mass below the surface, which they took to calling “mascons.”

However, not until scientists from Massachusetts Institute of Technology and Purdue University were able to map the structure of several of these areas did anyone know exactly how mascons were formed.

Using high-resolution gravity data from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission, the scientists found that the gravitation fields emanating from mascons resembled a bullseye pattern with a center of strong, or positive, gravity surrounded by alternating rings of negative and positive gravity.

They discovered this using GRAIL’s twin probes, Ebb and Flow, which orbited the Moon in tandem, mapping the gravitational field via the changes in distance between themselves.

Next, in order to discover the reason behind the bullseye, the team created simulations of lunar impacts in basins containing lava and basins without and measured their geological repercussions over short and long periods of time.

Sure enough, their experiments yielded the same bullseye pattern, indicating that when an asteroid crashes into the Moon, it sends material flying out that then creates a dense band of debris around the crater’s perimeter. The impact’s shockwave also plays a role by causing reverberations within the crust that then produce a counterwave drawing dense material from the lunar mantle toward the surface. The result is a dense center within a crater and, after hundreds of millions of years, as the surfaces cools and relaxes, a gravitational bullseye is formed.

All of this, says Maria Zuber of MIT’s Department of Earth, Atmospheric and Planetary Sciences, likely combines to create today’s lunar mascons.

“For the first time, we have a holistic understanding of the process that forms mascons,” Zuber, who was also GRAIL’s principal investigator, said in a press release. “There will be more details that emerge for sure, but the quality of the GRAIL data enabled rapid progress on this longstanding question.”

And ultimately, Zuber believes, understanding what gave rise to lunar mascons may help scientists better understand the evolution not just of the Moon, but other planets as well - including Earth.

For example, she said, the period that gave rise to the mascons was likely the Late Heavy Bombardment, a time marked by a blitz of interplanetary collisions and the beginning of single-celled organisms on Earth.

"So knowing what the effect of the impacts on the thermal state of a planet that early tells us about the extreme conditions under which life took hold," she said.