At about 5:30 p.m. on Monday, two washing machine-sized space probes crashed into the surface of the moon. It was all by design and marked the end of NASA's GRAIL mission. The two probes had been orbiting the moon for almost a year, and they've sent back data that have given scientists an unprecedented look inside our nearest solar system neighbor.
The two little spacecraft had a big challenge: to map the interior of the moon from its crust down to its core. MIT geophysicist Maria Zuber says after more than 100 previous moon missions, scientists knew pretty much every nook and cranny of the lunar surface. "But the inside of the moon was still quite a mystery," she said.
Zuber, the lead scientist for NASA's GRAIL mission, which stands for Gravity Recovery and Interior Laboratory, says 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. One of the probes will fly over something massive, like a mountain.
That mountain "has a greater gravitational attraction associated with it," Zuber says, "so the first spacecraft speeds up and changes the distance between the two spacecraft."
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 in the same way, so scientists could map what was below the surface too.
Bill McKinnon, a planetary scientist at Washington University in St. Louis, calls the mission a blazing success. "It's a tour de force," he says.
McKinnon wasn't involved in the lunar probe mission, but he calls GRAIL's scientific discoveries this year's greatest achievement in planetary science.
"People might say, well, what about the Mars lander, Curiosity? That was the engineering feat of the year," McKinnon said. But in terms of planetary science, McKinnon says nothing can top the GRAIL mission.
Jeff Andrews-Hanna, a planetary scientist at the Colorado School of Mines, analyzed some of the GRAIL data. He says some of what scientists saw inside the moon was unexpected. "We saw these big lines crisscrossing the surface of the moon that didn't show up in any other data set," Andrews-Hanna said.
Andrews-Hanna thinks those lines may be solidified, magma-filled cracks — the result of ancient volcanic eruptions. They're up to 300 miles long and tens of miles wide.
But 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, "what we found is that the moon's upper crust was absolutely pulverized," Zuber said.
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.
"It's really a bittersweet feeling for me, because these probes have been a part of our lives for the last several years," Zuber said.
Today NASA crashed the probes into a mountain near the moon's north pole. That part of the moon was in shadow, so the impact wasn't visible from Earth.