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SAN FRANCISCO — The moon is cooling and shrinking today, but early in its history it actually got bigger, scientists have found.
As the moon expanded, molten rock rose from its deep interior to cool and solidify into long gashes buried beneath the surface. For billions of years these fiery scars remained hidden, finally revealing themselves to a pair of spacecraft flying overhead.
The probes, named Ebb and Flow, spotted the rock formations by their gravitational pull. And not just that: the NASA mission has revealed a host of other discoveries, both on the moon’s surface and below it. In producing the best gravity map ever compiled of any planet or moon — Earth included — the mission illustrates how violently the moon’s crust was pummeled by meteorites over eons.
“We see now a picture of the moon being far more broken up and shattered than we’ve seen before,” said Maria Zuber, principal investigator for the twin probes, known collectively as the Gravity Recovery and Interior Laboratory, or GRAIL.
Zuber, an MIT geophysicist, described the results December 5 at a meeting of the American Geophysical Union. Three papers on GRAIL’s first findings appeared online the same day in Science.
Ebb and Flow chase one another as they orbit the moon. As the spacecraft in front feels the tug of something with a little extra gravity, like a mountain, the probe speeds up a tiny bit. The second spacecraft, some 100 to 200 kilometers behind, also speeds up once it gets close enough to feel the same tug. By precisely measuring the changing distance between the two probes, GRAIL researchers can map gravity bumps caused by changing concentrations of mass at or just below the lunar surface.
Every second, mission scientists can detect changes in the spacecraft distance down to 50 nanometers, or billionths of a meter. That’s at least 1,000 times more precise than any previous study of the moon, allowing GRAIL to generate an incredibly detailed gravity map.
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Craters pop dramatically into view. Many are surrounded by rings of fractured rock and blankets of debris kicked up by the crash, with a peak dotting the middle.
The new map reveals that the moon’s crust is between 34 and 43 kilometers thick on average. That’s some 10 to 20 kilometers thinner than scientists had thought, said team member Mark Wieczorek of the University of Paris. The lunar crust is also far more fractured by impacts than anyone had suspected; a full 12 percent of the surface layer is nothing but empty space in churned-up rock deposits. “It really opens a window on what a violent place all terrestrial planets were early in their history,” Zuber said.
The moon’s relatively high porosity confirms that the moon contains roughly the same amount of aluminum as Earth does. That makes sense, because scientists think the moon was born in a cosmic collision when a Mars-sized object smashed into the early Earth. The material kicked up by that blast settled into orbit around Earth, there to coalesce and form the moon.
During the first 500 million to 1 billion years of its existence, the moon was probably hot on the outside and relatively cool on the inside, said Jeffrey Andrews-Hanna, a planetary scientist at the Colorado School of Mines in Golden. Because of that temperature inversion, the moon ballooned slightly, getting wider by some 1 to 10 kilometers, he and his colleagues have calculated.
That period of expansion is when the sheets of molten rock rose from below, penetrating cracks created as the hot moon grew. Only after cooling enough on the outside, some 1 billion years after it was formed, did the moon begin shrinking, Andrews-Hanna said.
In GRAIL’s gravity map, the volcanic scars show up as mysterious linear shapes, up to about 500 kilometers long. They are buried at least 10 kilometers deep, Andrews-Hanna said, and aren’t detectable any other way than with the sensitive gravity measurements.
“We have essentially taken geophysics and are bringing it into the realm of surface geology,” Zuber said.
On Earth, such sheetlike volcanic intrusions are known as dikes, but the lunar versions are so large as to perhaps warrant their own name someday, Andrews-Hanna added.
GRAIL has only a little more time to keep making discoveries. The mission has already finished its main tasks and is orbiting ever closer to the lunar surface. On December 6 the probes are scheduled to pass just 11 kilometers up, with 2-kilometer passes over some mountain peaks, Zuber said.
Eventually GRAIL won’t have enough fuel to keep itself in orbit any longer, and will plunge to the surface doomed by the gravity it studied.
A detailed map of the moon’s gravity reveals variations across the lunar surface. The map shows huge rings around impact craters (red represents areas of high mass concentration; blue, low mass).
Credit: NASA’s Goddard Space Flight Center Scientific Visualization Studio