NASA GRAIL Mission Solves Mystery of Moon's Uneven Gravity

The moon's gravity has puzzled astronomers for years. While other planetary bodies possess uniform gravity, the Earth's moon's gravity is uneven. Now, NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission has uncovered the origin of this uneven gravity, which could help researchers design better lunar-orbiting spacecraft.

In order to make these new findings, GRAIL's twin spacecraft studied the internal structure and composition of the moon in unprecedented detail for nine months. More specifically, they pinpointed the locations of large, dense regions called mass concentrations, or mascons, which are characterized by strong gravitational pull. Mascons lurk beneath the lunar surface and cannot be seen by normal optical cameras.

On a map of the moon's gravity field, a mascon appears in a target pattern. The bulls-eye has a gravity surplus and is surrounded by a ring with a gravity deficit. A ring with a gravity surplus surrounds the bulls-eye and the inner ring. This pattern arises as a natural consequence of crater excavation, collapse and cooling following an impact.

Actually finding these mascons was no easy task, though. GRAIL scientists combined the gravity data from GRAIL with sophisticated computer models of large asteroid impacts and known details about the geologic evolution of the impact craters.

"GRAIL data confirm that lunar mascons were generated when large asteroids or comets impacted the ancient moon, when its interior was much hotter than it is now," said Jay Melosh, a GRAIL co-investigator at Purdue University, in a news release. "We believe the data from GRAIL show how the moon's light crust and dense mantle combined with the shock of a large impact to create the distinctive pattern of density anomalies that we recognize as mascons."

Knowing that these mascons exist means that researchers may be able to understand a little bit more about the geologic consequences of large impacts. In fact, Earth experienced similar impacts in its distant past, which means that by examining the present-day moon we may learn more about our own ancient planet.

"Mascons also have been identified in association with impact basins on Mars and Mercury," said GRAIL principal investigator Maria Zuber of the Massachusetts Institute of Technology in Cambridge in a news release. "Understanding them on the moon tells us how the largest impacts modified early planetary crusts."

The findings are published in the journal Science.