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Twin Gravity-Mapping Probes Peer Into the Moon’s Secrets

SAN FRANCISCO — Earth’s nearest neighbor – the moon – is rife with mysteries, but twin moon-mapping space probes are helping scientists move closer to answers. Early data from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission, which maps the moon’s gravity field, reveals some surprises.

“We see a picture of the moon being far more broken up and shattered than we’ve seen before,” said planetary scientist Maria Zuber of MIT here at the American Geophysical Union conference Dec. 5. The mission’s early data, also reported today in Science, provides the most detailed view yet of the moon. “This gravity field is the best gravity field for any [body] including Earth,” she said.

Scientists hope that solving the lunar mysteries will help them understand the early solar system. Unlike more distant objects, the moon is relatively easy to study in detail. And unlike the Earth, the solar system’s life history is still written into its wrinkles and pockmarks. Reading those lunar tales requires understanding the moon inside and out.

But the moon always presents the same familiar face to Earth, so scientists needed mobile moon-mapping instruments in its immediate neighborhood. Borrowing from a method used to study Earth’s interior, researchers sent the twin GRAIL probes to the moon. Launched in September 2011, the probes reached lunar orbit one day apart, ringing in the New Year on Dec. 31 and Jan. 1.

Since then, the spacecraft have been zooming in tandem above the moon’s cratered surface, one chasing the other. Differences in the lunar gravity field vary the speed of and distance between the two probes, nicknamed Ebb and Flow. The probes are sensitive enough to detect a change in separation distance of just a few microns – about the width of a human red blood cell.

From an average altitude of 34 miles above the lunar surface, the probes mapped the moon for several months beginning in March; after being granted a mission extension, they swooped in for a closer look in August. Now, the first results are in.

“The data are unbelievably great,” said planetary scientist Lindy Elkins-Tanton of the Carnegie Institution of Science in Washington, D.C., who is not part of the GRAIL team. “From a technical standpoint, it is the cleanest, the best, most interpretable data of its kind out there. It’s astonishing.”

GRAIL’s measurements reveal that gravity fields are strongly associated with features on the lunar surface, such as impact basin rings, central crater peaks, and volcanic landforms. The moon’s crust is also thinner than expected, averaging between 18 and 25 miles deep. Such a thin crust suggests that the moon’s chemical composition is similar to Earth’s. “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, a member of the GRAIL team.

And, for the first billion years of its life, the early moon was expanding. GRAIL data returned evidence of long, magmatic intrusions into the lower crust – sites where magma seeped into cracks and solidified. These intrusions, totaling 3,300 miles in length, suggest the young moon warmed and expanded for about a billion years, with its radius growing between 0.3 and 3 miles.

“As the interior expands, the crust fractures, and that allows magma to well up into those cracks,” said planetary geophysicist Jeffrey Andrews-Hanna of the Colorado School of Mines. “That’s actually the opposite of what’s been happening for most of the lunar history. For the past 3.5 billion years, the moon has been gradually cooling down and contracting.”

Such observations are essential for understanding the moon’s early years, when a primordial magma ocean cooled and its crust formed. Normally, tales of its first 700 million years are invisible, erased by the numerous impact scars that mar the moon’s surface. It’s only below the surface that the story remains. “We’ve never been able to see it before,” said Elkins-Tanton, who studies planetary bodies and has a special interest in the magma oceans that covered young bodies like the Earth and moon. “This is the first mission that actually ‘X-rays’ that part to the point where we can begin to look into the past.”

The lunar highland crust is also more porous and homogenous than expected, the result of numerous early impacts smashing it to bits. But the porosity might hint at something else.

In August 2011, planetary scientist Erik Asphaug of Arizona State University published a theory describing the source of the mysterious, mountainous farside lunar highlands: a smaller, short-lived second moon. For tens of millions of years, the moon and moonlet peacefully coexisted. Then, in a slow-motion collision, the moonlet bumped into the moon. Rather than creating a crater, the moonlet pancaked, sending millions of cubic miles of rocky material sliding across the lunar globe. The landslide-like rubble quickly piled up to a depth of tens of miles, Asphaug describes. “So I think that’s consistent with a deeply porous moon, but obviously there are many ways to generate porosity.”

Asphaug also notes that a deep, porous crust, as interpreted by the GRAIL team, could have captured water delivered by cometary impacts – and that, if coupled with a source of heat, could have provided briefly favorable pockets where life could evolve with the help of materials ejected from Earth.

“We’d never learn about them of course,” he said. “If there was, it would be deeper than the deepest mines on Earth and we’d never know it.”

Many other questions sill remain. Did the moon ever have a churning iron core? Magnetic records in lunar rocks suggests so. Is it still molten? How does the lunar core-crust-mantle behave? The best way to answer some of these questions is to set up a system of seismic monitoring stations on the lunar surface, Elkins-Tanton says.

“Seismic stations would be spectacular,” she said. “That would definitely be the next observation that would make the modeling possible. It would be amazing.”

Video credit NASA