All Cracked Up: A Surprising Look Inside the Moon

The twin GRAIL spacecraft provide new insights into the moon's tortured past

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ARIS MESSINIS / AFP / Getty Images

People admire the moon at Cape Sounion south of Athens in Aug. 2006.

If there’s sibling rivalry in the nearby cosmos, the Moon must be feeling a lot of it these days.  Last week, Mercury was the big story, with the discovery of ice and organic molecules on its sun-blasted surface. This week it’s Mars, thanks, maddeningly enough, to the non-discovery of signs of life by the Curiosity rover. Even the hoary Voyager 1 space probe, whose official mission ended three decades ago, is making news simply by leaving the building.

But now, finally, the Moon is getting some love. Three separate studies of Earth’s cratered companion are being published today in Sciencexpress, the online version of the journal Science, and they’re revealing the moon’s geography and geology (or more properly, selenology) with a precision that’s never been seen before — including a totally unsuspected web of subsurface cracks that crisscross the entire lunar body. It is, exults MIT’s Maria Zuber, lead author of the main paper, “absolutely transformative.”

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That transformation comes courtesy of the GRAIL lunar probe — or rather, probes, since the Gravity Recovery and Interior Laboratory is actually a pair of satellites, informally known as Ebb and Flow, which have been orbiting the Moon in tandem for almost a year now. When they fly over a larger mass of moon below — a mountain, say, or even even a dense concentration of rock beneath the surface —  the excess gravity pulls the lead satellite a little harder, widening the gap between the two. When they come across a deficit of material, such as a crater, the lead satellite slows a bit and the gap shrinks.

These changes are almost absurdly tiny. “We’re measuring changes in velocity of .05 microns per second,” says Zuber, who serves as the mission’s principal investigator. “That’s five times better than we expected, and I calculated that it’s about 1/20,000th the speed of a snail.”

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The GRAILs whip across the Moon’s surface at an average altitude of 34 miles (55 km), and sometimes as low as six (9.7 km). Proximity to the ground lets the satellites make extraordinarily high resolution maps of the Moon’s surface features — and surprisingly, those features account for about 98% of the gravity anomalies the satellites picked up, with only 2% coming from subsurface density variations. “That’s unlike anything we’ve measured for other terrestrial [i.e., rocky] planets,” says Zuber.

What it suggests is that the Moon’s surface was so violently bombarded by asteroid impacts in the first billion years of the Solar System’s history that the crust was thoroughly pulverized and homogenized to a depth of several miles. “We knew it was a violent period,” says Zuber, “but now we can quantify that pretty precisely.” The same bombardment must have affected the Earth. “It makes your head spin,” she says, “thinking of the implications for the development of life [at around that time]. The more you look, the more respect you have for biology”

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The 2% of underground gravity variation GRAIL saw was in  large part due to a network of enormous subsurface cracks up to 300 miles (483 km) long and up to 25 miles (40 km) wide. “They’re amazingly straight,” says Jeffrey Andrews-Hanna of the Colorado School of Mines, lead author of the second paper. “We’ve never seen them in any other data set.”

The cracks, like the homogenous surface, are artifacts of the moon’s earliest days, but they formed in an entirely different way. According to the leading theory of our satellite’s origin, Earth was slammed by a Mars-size planetoid eons ago. The impact vaporized the smaller object along with a significant chunk of Earth’s surface, creating a ring of rocky material orbiting our young —and now wounded — planet.

As the Earth healed, the ring gradually coalesced to form the Moon — at first comparatively gently, as chunks of rock and dust banged into each other, but with increasing violence as the growing Moon’s gravity got stronger and stronger. The interior was relatively cool to start, while the collision-heated exterior was hot. But then the outside cooled and the core warmed up under the pressure of all that overlying rock. “The interior starts to expand,” says Andrews-Hanna, “and the exterior cracks.”

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In a final formative step, the cracks filled with magma, which was denser than the surrounding rock and thus would have a different gravitational tug — a tug which would be detectable in the event that, four billion years later, a species on the nearby Earth sent up a pair of spacecraft to study such things. “We didn’t predict these at all,” says Andrews-Hanna. “That’s an important part of science — you don’t send out a probe to confirm what you know. You send it to be surprised. You expect to see what you don’t expect.”

It doesn’t always turn out that way, of course. The third paper in the trio uses GRAIL data to characterize the overall properties of the Moon’s crust, and here, the observations do confirm what planetary scientists think they know. The giant-impactor theory of the Moon’s formation implies that both Earth and Moon should be made of pretty much the same stuff. The Moon’s overall crustal density suggests that in fact they are.

Impressive as these findings are, these are likely to be just the first of many extraordinary discoveries GRAIL will be making. “We’re really just in the initial stages,” Zuber says. “Our understanding of the Moon is starting to fit together, and we’ve added something to the story. But there’s a lot to come.”

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