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NASA intentionally sent two space probes crashing into the moon today. They were each about the size of a washing machine, and they've been orbiting the moon for almost a year. The two probes have helped to create the most detailed map ever made of the moon's gravity shield.

As NPR's Veronique LaCapra reports, that's revealing some surprises about the moon's early history.

...detailed map ever made of the moon's gravity shield. As NPR's Veronique LaCapra reports, that's revealing some surprises about the moon's early history.

VERONIQUE LACAPRA, BYLINE: The two little spacecraft had a big challenge: to map the interior of the moon from its crust down to its core. After more than 100 previous moon missions, scientists knew pretty much every nook and cranny of the lunar surface.

MARIA ZUBER: But the inside of the moon was still a - quite a mystery.

LACAPRA: Maria Zuber is a geophysicist at MIT. She's the lead scientist for NASA's GRAIL mission. GRAIL stands for Gravity Recovery and Interior Laboratory. And Zuber says that's because measuring the moon's gravity was the key to seeing what's inside it.

Here's how it works. The two probes - they're called Ebb and Flow - fly in the same orbit around the moon, keeping as close as possible to its surface. Then, say, Ebb flies over something massive, like a mountain.

ZUBER: Well, it has a greater gravitational attraction associated with it, so the first spacecraft speeds up and changes the distance between the two spacecraft.

LACAPRA: By measuring those subtle changes in spacing between the two probes, Zuber and her colleagues could calculate the mountain's gravitational pull. The gravity of underground structures tugged on the probes the same way, so scientists could map what was below the surface too.

BILL MCKINNON: It's a blazing success. It's a tour de force.

LACAPRA: Washington University planetary scientist Bill McKinnon wasn't involved in the lunar probe mission, but he calls GRAIL's scientific discoveries this year's greatest achievement in planetary science.

MCKINNON: People might say, well, what about the Mars lander, you know, Curiosity? That was the engineering feat of the year.

LACAPRA: But McKinnon says in terms of planetary science, nothing can top the GRAIL mission. Jeff Andrews-Hanna is a planetary scientist at the Colorado School of Mines and analyzed some of the GRAIL data. He says some of what scientists saw inside the moon was unexpected.

JEFF ANDREWS-HANNA: We saw these big lines crisscrossing the surface of the moon that didn't show up in any other data set.

LACAPRA: Andrews-Hanna thinks those lines may be solidified, magma-filled cracks, the results of ancient volcanic eruptions. And they're up to 300 miles long and tens of miles wide. But Maria Zuber says one of the biggest surprises was how bad of a beating the moon had taken early in its history. Scientists already knew the moon had been bombarded with asteroids and comets. But when they analyzed the new gravity map...

ZUBER: What we found is that the moon's upper crust was absolutely pulverized.

LACAPRA: Zuber says this suggests that Earth and other planets would have gotten the same treatment. This ancient pummeling would have opened up deep cracks on Mars that might have drained away an early ocean. And on Earth, a fractured crust could have allowed gases to escape helping to form an atmosphere. Zuber says scientists are only at the very beginning of analyzing the data, so there's still a lot more to be discovered. But for the probes Ebb and Flow, today marks the end of their mission. And Zuber is sad to see them go.

ZUBER: It's really a bittersweet feeling for me because these probes have been a part of our lives for the last several years.

LACAPRA: Today, NASA crashed the probes into a mountain near the moon's north pole. The impact wasn't visible from Earth. Veronique LaCapra, NPR News.

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