A map of north--south subkilometer-scale slope asymmetry on Mars obtained from statistical analysis of along-track MOLA topographic profiles reveals well-defined zonal belts of north--south slope asymmetry at 40--50¿ latitude in both hemispheres. In these narrow anomalous belts the pole-facing slopes are systematically gentler than equator-facing slopes. This asymmetry is especially pronounced for the steepest (>20¿) slopes, in which pole-facing slopes are three times less frequent than >20¿ slopes facing the equator. We interpret these belts to be related to insolation asymmetry. Specifically, we suggest that summertime melting of ground ice on pole-facing slopes occurred during periods of very high obliquity (~45¿) in the past and favored downslope movement of material and reduction of steep pole-facing slopes.