Dust grains charging in the regions with strong electric potentials (magnetosheath and the wake of Mars) is discussed. It is shown that the stationary state (that corresponds to the absence of electric currents onto a dust grain) is determined by the sum of a given potential in space and the potential of a dust grain induced by its electric charge. It is argued that the absolute value of the electric charge can be strong enough (positive or negative). The motion of such strongly charged grains in the electric fields is discussed. It is shown that dust grains ejected from the surface of Phobos begin to oscillate in the vertical and radial directions with increasing amplitude due to specific resonance quickly occupying significant volume.
N. Borisov (Mon,) studied this question.