May 30, 2013

Ever since the first satellites were sent to the moon to scout landing sites for Apollo astronauts, scientists have noticed a peculiar phenomenon: As these probes orbited the moon, passing over certain craters and impact basins, they periodically veered off course, plummeting toward the lunar surface before pulling back up.

As it turns out, the cause of such bumpy orbits was the moon itself: Over the years, scientists have observed that its gravity is stronger in some regions than others, creating a “lumpy” gravitational field. In particular, a handful of impact basins exhibit unexpectedly strong gravitational pull. Scientists have suspected that the explanation has to do with an excess distribution of mass below the lunar surface, and have dubbed these regions mass concentrations, or “mascons.” 

Exactly how these mascons came to be has remained a mystery — until now.

Using high-resolution gravity data from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission, researchers at MIT and Purdue University have mapped the structure of several lunar mascons and found that their gravitational fields resemble a bull’s-eye pattern: a center of strong, or positive, gravity surrounded by alternating rings of negative and positive gravity.

To figure out what caused this gravitational pattern, the team created simulations of lunar impacts, along with their geological repercussions in the moon’s crust and mantle, over both the short- and long-term. They found that the simulations reproduced the bull’s-eye pattern under just one scenario.

When an asteroid crashes into the moon, it sends material flying out, creating a dense band of debris around the crater’s perimeter. The impact sends a shockwave through the moon’s interior, reverberating within the crust and producing a counterwave that draws dense material from the lunar mantle toward the surface, creating a dense center within the crater. After hundreds of millions of years, the surface cools and relaxes, creating a bull’s-eye that matches today’s gravitational pattern.

This tumultuous chain of events likely gave way to today’s lunar mascons, says Maria Zuber, the E.A. Griswold Professor of Geophysics in MIT’s Department of Earth, Atmospheric and Planetary Sciences.

“For the first time, we have a holistic understanding of the process that forms mascons,” says Zuber, who is also GRAIL’s principal investigator, and MIT’s vice president for research. “There will be more details that emerge for sure, but the quality of the GRAIL data enabled rapid progress on this longstanding question.”

Zuber and her colleagues have published their results this week in Science.

Mapping a bumpy ride

From January to December 2012, GRAIL’s twin probes, Ebb and Flow, orbited in tandem around the moon, mapping its gravitational field by measuring the changing distance between themselves — a real-time indication of the strength of the moon’s gravitational pull. As the probes got closer to the moon’s surface toward the end of the mission, Zuber recalls, engineers had to adjust the probes’ orbits to counteract the tug of lunar mascons.

“Because the moon’s gravity field is so bumpy, we would put the two spacecraft in a circular orbit, and the orbits immediately became elliptical because the spacecraft got tugged out of their orbit,” Zuber says. “We were always within a week of crashing.”

Despite the impending threat of impact, the probes gathered high-resolution measurements, which Zuber and the GRAIL science team have since translated into detailed gravitational maps. These maps also gave scientists precise measurements of the thickness of lunar crust in any given region of the moon, which Purdue’s Jay Melosh integrated into impact simulations.

Melosh simulated the process of lunar impacts in two similarly sized basins on the near side of the moon — one with lava deposits, the other without. Melosh fed the crustal thicknesses from both basins into the model, then ran the simulation to see how the same impact would affect each region.

According to measurements from GRAIL, the basin containing central lava deposits had a thinner crust than the other basin. After running their simulations, the researchers found that an impact had created a gravitational bull’s-eye pattern in the first basin, but not the second — predictions that matched GRAIL’s measurements.

Making an impact

Why the difference in gravitational signatures? The answer, the group found, lay in the crust’s thickness at the time of impact: Impacts to regions with thinner crust do more damage, easily sending shockwaves into the denser, underlying mantle — which, in turn, draws more dense material to the surface, creating a mascon. Regions with thicker crust, by contrast, are more resistant to impacts and internal upheaval.

“Large impacts happen in seconds to hours,” Zuber says. “The process of how the crust cools off and recovers from such a devastating event, that’s hundreds of millions of years. So we let these models run through time until the surface cools and relaxes. Then what you’re left with is today’s gravity.”

The results from the group’s simulations precisely matched GRAIL’s actual gravity measurements, giving scientists confidence that the simulated impact scenario is indeed what formed the lunar mascons.

While most scientists agree that the moon’s mascons likely arose from large impacts, Laurent Montesi, an associate professor of geology at the University of Maryland, says the precise processes that led to the formation of the mascons has been a mystery since their discovery 45 years ago.

“This paper finally proposes an answer to this longstanding puzzle by including a start-to-finish model of mascon formation,” says Montesi, who did not contribute to this research. “It is now clear that geological processes occurring over millions of years are needed to turn the structure produced immediately by the impact into a mascon. It is remarkable how well the models in the paper reproduce the observed structures.”

Zuber says that knowing what gave rise to lunar mascons may help us understand the evolution of the moon, as well as other planets. The mascons likely formed during a period known as the Late Heavy Bombardment, when the early solar system endured a blitz of interplanetary collisions. The Earth may have undergone even more impacts than the moon, although the resulting craters have since been erased by erosion and plate tectonics. Studying the repercussions of impacts on the moon therefore might offer clues to Earth’s origins.

“This was a very inhospitable time to be at the surface of a planet,” Zuber says. “The tail end of this process is when the first single-celled organisms emerged on Earth. So knowing what the effect of the impacts was on the thermal state of a planet that early tells us about the extreme conditions under which life on Earth took hold."