As scientific mysteries go, the moon has long seemed a closed case. Astronauts have bounded across it and returned with vials of its rocky soil. Machines outfitted with sophisticated instruments have circled and crash-landed on it. Been there, done that.

But a resurgence of interest in lunar science, led in part by local scientists, is chipping away at seemingly settled theories and raising new questions about our closest neighbor: how much water is up there, for example, and did the Earth once have two moons?

These aren’t merely matters of astronomical curiosity. The moon holds a frozen record of the early history of the solar system, and understanding the details of its formation and composition may provide valuable insight into the conditions under which life emerged on Earth.

“Look at the battered surface of the moon” said Maria Zuber, a geophysicist at the Massachusetts Institute of Technology. “Earth used to look like that. That record isn’t preserved on Earth anymore.”

Zuber heads GRAIL, a NASA mission in which two washing-machine-sized spacecraft have been circling low over the moon’s surface. They use radio signals to measure slight disturbances in the other’s orbit caused by gravitational changes as they fly over different terrain. The measurements will be used to make a finely detailed map of the moon’s internal structure.

“We don’t expect to find life,” Zuber said, “but we will learn things . . . that [will] help us build a better model of what the Earth was going through” when life emerged.

After the Apollo missions of the late 1960s and early 1970s provided a flood of new samples, data, and measurements, a neat portrait of the moon ­began to emerge. Roughly 4.5 billion years ago, a Mars-sized object slammed into the ­nascent Earth, flinging out hot, molten debris that aggregated rapidly into the moon. It was a violent event that would have caused volatile content, including water, to be lost.