In the late 1970s, planetary scientists Lionel Wilson and Jim Head proposed the idea that ejecta -- bouncing, sliding and rolling boulders -- from Stickney may have carved the grooves. For a moon the size of the diminutive Phobos -- 27 kilometers across at its widest point -- Stickney is a huge crater at nine kilometres across. The impact that formed it would have blown free tonnes of giant rocks, making the rolling boulder idea entirely plausible, Ramsley said. Researchers designed computer models to see if the "rolling boulder model" could recreate these confounding patterns. The models simulate the paths of the boulders ejected from Stickney, taking into account Phobos' shape and topography, as well as its gravitational environment, rotation and orbit around Mars.

The models showed that the boulders tended to align themselves in sets of parallel paths, which jibes with the sets of parallel grooves seen on Phobos. The models also provide a potential explanation for some of the other more puzzling groove patterns. The simulations show that because of Phobos' small size and relatively weak gravity, Stickney stones just keep on rolling, rather than stopping after a kilometer or so like they might on a larger body. Some boulders would have rolled and bounded their way all the way around the tiny moon. That circumnavigation could explain why some grooves aren't radially aligned to the crater. Boulders that start out rolling across the eastern hemisphere of Phobos produce grooves that appear to be misaligned from the crater when they reach the western hemisphere.

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That round-the-globe rolling also explains how some grooves are superposed on top of others. The models show that grooves laid down right after the impact were crossed minutes to hours later by boulders completing their global journeys. In some cases, those globetrotting boulders rolled all the back to where they started -- Stickney crater. That explains why Stickney itself has grooves, researchers said. "We think this makes a pretty strong case that it was this rolling boulder model accounts for most if not all the grooves on Phobos," Ramsley said.

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