The strange grooves criss-crossing the surface of the Martian moon Phobos were made by rolling boulders blasted free from an ancient asteroid impact, suggests a new study.
Phobos' grooves, which are visible across most of the moon's surface, were first identified in the 1970s by NASA's Mariner and Viking missions.
While some scientists posited that large impacts on Mars have showered the nearby moon with groove-carving debris, others suggested that Mars' gravity is slowly tearing Phobos apart, and the grooves are signs of structural failure.
To confirm, a team from Brown University in the US designed computer models that showed that boulders ejected from Stickney could have created the puzzling patterns of grooves seen on Phobos.
Stickney is a huge crater at nine kilometres across for a moon the size of diminutive Phobos (27 kilometres across at its widest point).
"These grooves are a distinctive feature of Phobos, and how they formed has been debated by planetary scientists for 40 years," said Ken Ramsley, a planetary science researcher at Brown University who led the work.
"We think this study is another step toward zeroing in on an explanation."
The simulations show that because of Phobos' small size and relatively weak gravity, Stickney stones just keep on rolling rather than stopping after a kilometre or so like they might on a larger body.
In fact, some boulders would have rolled and bounded their way all the way around the tiny moon, explaining why some grooves are not radially aligned to the crater, the researchers noted in the paper published in the journal Planetary and Space Science.
In some cases, the globe-trotting boulders rolled all the way back to where they started -- Stickney, indicating why the crater itself has grooves.
In addition, there are fairly low-elevation areas on Phobos surrounded by a higher-elevation lip, with no grooves at all.
"It's like a ski jump," Ramsley said. "The boulders keep going but suddenly there's no ground under them. They end up doing this suborbital flight over this zone."
"We think this makes a pretty strong case that it was this rolling boulder model that accounts for most if not all the grooves on Phobos," Ramsley said.