Sometimes great mysteries hang right over our heads. We’re so used to looking up and seeing the “Man in the Moon” that we often don’t realize that those familiar dark areas on the face of our nearest neighbor are part of a centuries old question that has yet to be answered. Many hypotheses have been put forward and now data from NASA's twin Gravity Recovery and Interior Laboratory (GRAIL) lunar orbiters has provided new insights into how the surface of the Moon formed and how its distinctive “seas” came to be.
The “Man in the Moon” (along with the lady in the Moon, the rabbit, and the beetle) is the result of the dark seas or maria that make up much of the near side of the satellite. These great, meteor-scarred plains have been a familiar sight since prehistoric times and the subject of study over the centuries since the invention of the telescope.
But how they formed is still the subject of debate, which has become more heated over the decades as images sent back by unmanned probes have shown that these huge maria are a rarity in the Solar System. The far side of the Moon, for example, is almost completely lacking in maria and scientists have suggested everything from tidal forces to differences in meteor impact rates as the cause.
The problem in solving the mystery of the maria is that early in its history, the Moon was subjected to a massive bombardment of asteroids. Some of these impacts smashed through the lunar crust and lava rushed out.
It’s a popular bit of trivia to point out that although the “seas” of the Moon aren’t really seas, they were at one time. Billions of years ago, they were seas of molten lava that cooled into great, flat plains. These later came under more asteroid and meteor impacts, which obscured the evidence of the early history of the seas. This flooding and pounding meant that astronomers couldn’t even agree on simple facts, such as how big the seas actually are.
By a careful survey of the Moon’s gravitational field over a period of nine months, NASA was able to build up a map of the Moon’s internal structure. In this case, it was a detailed map of the thickness of the lunar crust, which showed that the near side of the Moon has more large impact basins, which are the maria, than the far side, yet both sides suffered the same number of impacts about four billion years ago. The age was calculated by studying the age of later meteor impacts in and near Oceanus Procellarum and Mare Imbrium.
According to NASA, the most likely explanation is that the near-side hemisphere is rich in radioactive elements, such as uranium and thorium. These kept that side hotter than the rest of the Moon and generated the lava that gushed out to form the seas. It also means that the number of craters on the far side are a better indicator of the bombardment rate in the days when the Solar System formed.
"Impact simulations indicate that impacts into a hot, thin crust representative of the early Moon's near-side hemisphere would have produced basins with as much as twice the diameter as similar impacts into cooler crust, which is indicative of early conditions on the Moon's far-side hemisphere," says Katarina Miljkovic of the Institut de Physique du Globe de Paris.
Launched in 2011, the GRAIL mission was comprised of two unmanned spacecraft named Ebb and Flow that flew in tight formation as part of a mission to chart lunar gravitational anomalies, and resulted in the most detailed gravitational map of any body in the Solar System. In order to prevent contamination of historic lunar landing sites, the craft were deliberately crashed into a mountain near the lunar north pole on December 17, 2012.
The results of the GRAIL study were published in the journal Science.