Asteroids and comets colliding with the moon not only pitted its surface but also severely fractured its crust, researchers with NASA said on Wednesday, in a finding that could help crack a Martian puzzle.

On Mars, similar fracturing would have given water on the surface a way to penetrate deep in the ground, where it may remain today, they said.

“Mars might have had an ancient ocean and we’re all wondering where it went. Well, that ocean could well be underground,” planetary scientist Maria Zuber, with the Massachusetts Institute of Technology, told reporters at the American Geophysical Union conference in San Francisco.

The discovery that the moon’s crust is deeply fractured came from a pair of small probes that comprise NASA’s Gravity Recovery and Interior Laboratory, or GRAIL, mission. The identical spacecraft have been following each other around the moon for nearly a year.

Scientists have been monitoring the distance between the two, which changes slightly as they fly over denser regions of the moon.

The gravitational pull of the additional lunar mass causes first the leading probe and then the other one to speed up, altering the gap between them.

The data, assembled into the first detailed gravity maps of the moon, reveal that asteroids and comets cratered the surface and fractured the crust, possibly all the way down to the mantle.

“If you look at the surface of the moon and how heavily cratered it is, all terrestrial planets look that way, including the Earth,” said Zuber, the lead GRAIL scientist.