The Gravity Recovery and Interior Laboratory (GRAIL) mission has unlocked the mystery of the Moon's "lumpy" gravity, a question which has puzzled astronomers for decades. The twin spacecraft, named Ebb and Flow, were sent to the Moon in order to understand this strange effect that influences spacecraft orbiting the satellite.
An unexpected gravitational pull measured on the Moon tugged on the Lunar Orbiter spacecraft in 1968 as it revolved our our companion satellite. The effect was great enough that it could have posed a danger to the Apollo 11 mission, scheduled for the following year. The cause, NASA determined, was the presence of mass concentrations of rock under the lunar surface they dubbed mascons. This created another dilemma for scientists - what formed these mascons, and why don't they occur on our own planet today?
After 45 years, that mystery has been answered - asteroids created these odd mascon regions and are responsible for the uneven gravitational pull of the Moon. The geological nature of our satellite turns these asteroids into mascons.
Here's how it works. When a large asteroid hits the Moon, it breaks through the low-density crust. The asteroid then enters the denser mantle that sits underneath the crust. This mantle, although it is a solid, is able to flow under pressure.
On the Earth, material from the mantle rushes up to the surface, cools, then sinks back down. One the Moon, however, the mantle material cools too quickly to sink back down to its former layer. This creates an area of uneven gravity, in the form of a mascon.
Mission researchers sent the pair of GRAIL spacecraft buzzing low enough over the Moon - just 34 miles above the surface - that the influence of these formations would pull the spacecraft crashing down, ending the mission.
"[T]hey had to fire their thrusters three times a week. They didn't want to crash before the mission was over," Jay Melosh, co-author of the paper that announced the results, said.
Researchers compared data from the pair of spacecraft with a computer models showing how impacts from large asteroids affect the Moon. Doing so created mascons in the simulations that showed gravitational anomalies like those measured on the Moon.
"Knowing about mascons means we finally are beginning to understand the geologic consequences of large impacts," Melosh said. "Our planet suffered similar impacts in its distant past, and understanding mascons may teach us more about the ancient Earth, perhaps about how plate tectonics got started and what created the first ore deposits," Melosh said.
The mission finally ended when fuel ran out and the GRAIL craft, launched in Sept. 2011, crashed due to the pull of these odd formations.
