The idea that Phobos is comprised of a layered rock formation would fit the appearance very well. This raises fascinating questions. Did very large asteroids "accrete" like hailstones over billions of years? If so, what would dictate the varying "rings" laid down? Sure as heck it isn't seasons, like a tree. Or are we dealing with something as large as a busted-up planet?

Tidal fractures would also explain the grooves, if the orientation of the grooves is parallel to Mars' surface, but one wonders if stressing an object like this from either end would give you such neat parallel grooves.

Impacts would give you short streaks, that would not follow the curvature of the moon. Even assuming for some reason that all the impacting bodies came from only one specific direction. These are not impact marks.