New Theory Explains Moon’s Wonky Gravity

A strange thing happens to satellites when they’re in the Moon’s orbit. When passing over certain craters, they’ve been known to suddenly change course and plummet towards the surface before pulling back up. Scientists now know how these gravitational anomalies formed.

These craters are called mass concentrations, or mascons for short — impact basins that exhibit unexpectedly strong gravitational pull. Scientists have theorized that these craters must contain extra mass. But why this should be the case has remained somewhat of a mystery.

Now, using high-resolution gravity data from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) missions, researchers discovered that mascons have gravitational fields that resemble a bull’s eye pattern: a center of strong gravity surrounded by alternating rings of weak and strong gravity. MIT’s Jennifer Chu explains:

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.

These patterns, therefore, are the result of crater excavation, collapse and cooling following an impact event. The subsequent increase in density and gravitational pull at a mascon's bulls-eye is caused by lunar material melted by the heat of an asteroid impact.

[Sources: NASA and MIT; image: NASA/JPL-Caltech]