The Moon doesn't have a personality, we know that, but boy does it have a habit of screwing around with the trajectory of the expensive metal we send to orbit it. Until recently, NASA wasn't sure why this was occurring, but with the help of the Gravity Recovery and Interior Laboratory (GRAIL) mission's satellites, it's now very clear.
If the acronym "GRAIL' sounds familiar, it's because the aforementioned satellites are the same ones the space agency smashed into the Moon late last year. However, before they met their demise, the satellites "Ebb" and "Flow" recorded nine months' worth of detail on the composition of the Moon, revealing significant "bumps" in its gravity.
These bumps are caused by "mass concentrations" of planetary material, or "mascons" as they're called. They're mostly caused by impacts messing with the Moon's internal structure, as NASA's press release explains:
"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 West Lafayette, Ind., and lead author of the paper. "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."
The image on the right shows a false-colour representation of the "Bouguer" gravity distribution on the Moon, with red being stronger and blue weaker. "Bouguer" gravity, as it's called, describes the attractiveness of the Moon, minus what's contributed by the configuration of the surface and reflects anomalies below-ground.
As you've guessed by now, the unevenness of the gravity field can alter the path of orbiting satellites, causing them to wobble up and down as they traverse the Moon. Nothing like watching a sophisticated contraption worth oodles of dollars plunge to its doom, only to right itself!
Body image: NASA