"It was known that planets were battered by impacts, but nobody had envisioned that the [Moon's] crust was so beaten up," says Maria Zuber, according to the MIT News Office. Zuber is the E.A. Griswold Professor of Geophysics in MIT's Department of Earth, Atmospheric and Planetary Sciences, and Principle Investigator of the GRAIL mission. "This is a really big surprise, and is going to cause a lot of people to think about what this means for planetary evolution."
Ebb (GRAIL-A) slipped into lunar orbit on December 31st, 2011 and Flow (GRAIL-B) followed about a day later. The two satellites spent the next couple of months synchronizing their orbits so that they were flying in tandem, a fixed distance apart, just 34 kilometres above the lunar surface. The twins are designed so that they can detect the slightest change in the distance between them, down to a few microns — about 1/100th the width of a human hair. Since the strength of the Moon's gravity varies over its surface due to differences in the density of the materials it is made up of, just as Earth's does, and these changes in gravity would have an effect on the satellites' orbit, this allowed the satellites to construct detailed maps of the Moon's gravity and thus reveal the composition of the Moon's crust and interior.
"The data are unbelievably great," said Lindy Elkins-Tanton, director of the Department of Terrestrial Magnetism at the Carnegie Institution of Science, according to Wired. "From a technical standpoint, it is the cleanest, the best, most interpretable data of its kind out there. It's astonishing."
The measurements taken by Ebb and Flow show that the Moon's gravity is strongest around the rims and central peaks of craters, and for volcanic formations, which was expected, however what the scientists did not expect to find was that the lunar crust is just 28 to 40 km deep — much thinner than they previously thought. This thinner crust shows that the interior structure of the Moon is much more like the Earth's than previously thought.
"This is consistent with a hypothesis that the moon is most likely derived from materials that come from the Earth, following a giant impact event," said planetary scientist Mark Wieczorek of Institut de Physique du Globe de Paris, who is a member of the GRAIL team. The giant impact hypothesis — proposed in 1946 by Canadian geologist Reginald Aldworth Daly and receiving more and more support in recent years — suggests that the Moon was formed when the Earth collided with another massive object, likely a planetoid the size of Mars, which created a debris ring around the planet that slowly formed into the Moon.
"The process of building a moon out of that debris should result in a situation where the moon is cooler on the inside and warmer on the outside," said Jeff Andrews-Hanna of the Colorado School of Mines, who is a guest scientist for the GRAIL mission, according to Digital Journal. "And then what naturally happens is that the interior will warm up and expand during that first billion years."
The expansion experienced by the early Moon was evidenced by long 'dikes' of denser material, likely magma seeping up through fissures in the lunar material as the Moon expanded, that the satellites detected below the surface.
"This had been predicted theoretically a long time ago, but there was no direct observational evidence to support this period of early lunar expansion until this GRAIL data." Andrews-Hanna added.
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In addition to the findings of the twin satellites, their performance has been another grand achievement, as they exceeded their design specifications by measuring the changes in distance between them down to as little as several hundreths of a micron, generating far better resolution maps than the scientists anticipated.
"On this mission, with two spacecraft, everything had to go perfectly twice," Zuber said. "Imagine you're a parent raising a twins, and your children sit down at the piano and play a duet perfectly. That's how it feels."