The Mars Orbiter Laser Altimeter (MOLA) data are used to characterize the kilometer-scale surface roughness of Mars. The median absolute value of the differential slope at a given baseline is proposed as a data-derived measure of the surface roughness at this scale. Study of the scale dependence of roughness for the smoothest terrains gives an independent estimate of 20 cm for MOLA ranging accuracy. The baseline lengths from 0.6 to ~20 km are used, and kilometer-scale roughness is mapped for the entire surface. The maps show that different geological units have distinctive roughness characteristics. Scale dependence of roughness for a number of geological units is presented and discussed. The southern polar cap is rougher than the northern at kilometer and subkilometer scale, which suggests differences in the sublimation/condensation balance. The Vastitas Borealis Formation has a distinctive 3-km-scale background surface topography, which suggests a nonvolcanic origin for its upper layer. Young volcanic plains in Amazonis Planitia and the eastern part of Elysium Planitia are very similar to each other in their roughness characteristics and differ from other volcanic plains on Mars, which suggests a distinctive eruption style. There are systematic latitudinal variations of roughness in both the southern highlands and the northern lowlands: terrains at high latitude are smoother at short baselines; the characteristic vertical scale related to this difference is several meters. Processes that could be responsible for formation of this trend include creep of ice-rich near-surface material at high latitudes, treatment of the surface with repetitive deposition and sublimation of seasonal frost, climate-controlled deposition and/or cementation of dust at high latitudes, and repetitive sublimation and accumulation of subsurface ice at low latitudes with climate variations. All mechanisms of origin could operate more effectively under different climate conditions. Relevant morphological observations favor mechanisms involving deposition of smooth blankets at high latitudes. ¿ 2000 American Geophysical Union