Surface and Near Surface Dynamics on Phobos

Phobos as a few small satellites in the solar system is orbiting around its primary inside the Roche limit. Therefore the surface material is loosely bounded and easily ejected by impactors. Whereas dynamics in the close vicinity of Phobos has been studied for both geophysical and navigation reasons, the dynamics on the surface itself has not been studied to the same extent. The gravitational field used here is the ellipsoidal model of Davis, 1981, that describes as well the past and future Phobos as it gets closer to Mars. We look at the trajectory of a test mass for any initial position and velocity. It can exhibit an unusual shape: for some initial positions a gliding test mass released with zero velocity can take off over some distance! Generally the trajectories are not 'down hill' as the motion is strongly dependent on the velocity. We discuss the consequences for material transport on or close to the surface, with in particular the possibility that some of the Phobos groves could have been dug out by rolling blocks.