New images of the moon's crust point to a violent past in which it was battered by comets and asteroids during its first billion years, scientists say.
The new findings come from the GRAIL mission, a pair of spacecraft named Ebb and Flow that are orbiting the moon and measuring its gravitational field.
"It was known that planets were battered by impacts, but nobody had envisioned that the [moon's] crust was so beaten up," said Maria Zuber, the Massachusetts Institute of Technology scientist leading the mission.
"This is a really big surprise, and is going to cause a lot of people to think about what this means for planetary evolution," she said in a statement about the findings, to be published this week in the journal Science.
Unlike the Earth's crust, which is repeatedly recycled through the process of plate tectonics, the moon's hard crust dates back billions of years, offering clues to the formation of the solar system, including Earth.
The GRAIL mission has allowed scientists to stitch together a high-resolution map of the moon's gravity, reflecting surface structures such as mountains and craters as well as subterranean features.
The images suggest the moon's crust is 34-43 kilometres thick, considerably thinner than was previously thought, according to Mark Wieczorek, another GRAIL scientist.
"This supports models where the moon is derived from Earth materials that were ejected during a giant impact event early in solar system history," he said.
Around 98 per cent of the crust is deeply fragmented, porous material, the result, scientists say, of very early, massive impacts.
"This is interesting for the moon," Zuber said. "But what it also means is that every other planet was being bombarded like this."