Where did the Man in the Moon come from? It sounds like a nursery school riddle, but it’s actually a very serious question about the history of our satellite. A major part of the "Man" is the lunar mare or sea called the Oceanus Procellarum or Ocean of Storms; the origin of which has been a matter of scientific speculation for centuries. Now NASA claims that the answer has been found by the Gravity Recovery and Interior Laboratory (GRAIL) orbiter mission, which ended with a controlled impact on the Moon in 2012.
Despite staring at it since time immemorial, looking at it through telescopes for hundreds of years and sending spacecraft to it for half a century, the Moon still holds many mysteries that are surprisingly basic in nature. Even a few years before Apollo 11 landed on the Moon, how the craters formed or even if it had a solid surface or a sea of dust was still an unknown.
One of the biggest mysteries is where the great lunar maria or seas came from. These gigantic lava plains on the near side that make up the features of the Man in the Moon are unique in the Solar System, and how they came to be has puzzled scientists and philosophers going back to the Greeks, with everything from a giant mirror reflecting the Earth’s surface to dead sea beds suggested.
Earth's moon as observed in visible light (left), topography (center), and the GRAIL gravity gradients (right) (Image: NASA/GSFC/JPL/Colorado School of Mines/MIT
The largest of the lunar maria regions is Oceanus Procellarum. It is about 600 mi (2,600 km) wide and is made of of dark volcanic plains, low elevations, and a number of unusual ancient features. Until now astronomers thought it to be the result of a massive asteroid strike on the lunar surface sometime in the Moon’s primordial past. According to NASA, this would have been the largest asteroid impact basin ever to be seen on the Moon. Then came the GRAIL mission.
Launched in 2011, the GRAIL mission consisted of a pair of probes that orbited the Moon in close formation at a very low altitude. By taking careful measurements of their locations and relative distance to one another, NASA scientists were able to build up the most detailed map yet of the Moon's gravitational field. By mapping the anomalies in the gravitational field due to differences in surface density, astronomers now have a much fuller picture of the Moon’s composition and more clues to its past.
One of these is the rectangular pattern of anomalies in the Ocean of Storms, which indicates that it isn’t the result of an asteroid impact, but that different forces were at work in its creation.
"The rectangular pattern of gravity anomalies was completely unexpected," says Jeff Andrews-Hanna, a GRAIL co-investigator at the Colorado School of Mines in Golden, Colorado. "Using the gradients in the gravity data to reveal the rectangular pattern of anomalies, we can now clearly and completely see structures that were only hinted at by surface observations."
What the GRAIL data reveals is a rectangular area marked by giant rifts buried beneath the lava plains that are similar to those found on Earth, Venus, and Mars. NASA says that these rifts may have opened due to some internal forces deep inside the ancient moon that concentrated enough radioactive elements in one spot to create a fountain of molten rock, which burst through the crust and formed the mare. As the lava cooled, it contracted and created fractures similar in patterns to those found on the Saturnian moon Enceladus.
"Our gravity data are opening up a new chapter of lunar history, during which the moon was a more dynamic place than suggested by the cratered landscape that is visible to the naked eye," says Andrews-Hanna. "More work is needed to understand the cause of this newfound pattern of gravity anomalies, and the implications for the history of the moon."
The GRAIL findings were published in the Nature.