Post by Kenneth R. Ramsley, Department of Earth, Environmental and Planetary Sciences, Brown University.
All but one region of Phobos, the largest moon of Mars, is covered by hundreds of valley-like features, usually described as grooves. Most grooves are ~80 to ~200 meters wide and are found in groups of generally parallel members, or families [see Image 1]. Impact craters typically produce slow-moving boulders, and on Phobos there would be little gravity to halt their motions. Did boulders rolling across the surface of Phobos produce the grooves? To answer this question, using a computer model to calculate the fate of rolling boulders, we compare their motions to the geomorphology of the grooves.