Martian surface slopes were calculated at baselines from 0.4 to 25 km using profiles obtained by the Mars Orbiter Laser Altimeter (MOLA) instrument during the aerobraking phase of the Mars Global Surveyor mission. Median slope is proposed as a characteristic measurement of the typical surface roughness at each corresponding scale. Median slope is favored over RMS slope because it is not influenced by the small number of higher slopes at the upper end of the slope-frequency distribution tail. Median slope complements interquartile scale roughness characterization in that it is more sensitive to smaller baseline slopes. A map of the median slope of the northern hemisphere is presented. Median slopes and their scale dependences are used to characterize typical kilometer-scale roughness for a set of geologic units mapped in the northern hemisphere. This analysis demonstrates that many individual units and groups of units are characterized by distinctive surface slopes and that these characteristics are sufficiently different that they hold promise for use in the definition and characterization of units. Characterization of the slope properties of geologic units provides information useful in the interpretation of their origin and evolution. For example, the generally smooth topography of the diverse Vastitas Borealis Formation subunits is dominated by about 3 km, 0.3¿ steep features almost indistinguishable in Viking images. The roughness characteristics of this unit differ from those of other geologic units on Mars and suggest some distinctive process(es) of formation and/or modification of kilometer-scale topography common for all subunits. The similarity of roughness characteristics of the several highland plateau units suggests that kilometer-scale topography was largely inherited from the period of heavy bombardment. The northern polar cap and layered terrains are largely very smooth at small scale. The long, steep-sloped tails of the slope-frequency distributions are compared for the dominant terrain types in the northern hemisphere of Mars and are compared with Earth continents. The Vastitas Borealis Formation found in the northern lowlands differs significantly from both cratered uplands and volcanic plains, and these latter two units can be distinguished from each other on the basis of median slope. Terrestrial continents are smoother than cratered highlands but rougher than the Vastitas Borealis Formation and most volcanic plains. An inventory of all observed slopes much steeper than the angle of repose is presented. Steep slopes occur in the upper parts of tectonic scarps, most likely representing bedrock exposures. The presence of extremely steep slopes in the presumably ductile polar cap provides evidence for geologically recent and/or ongoing formation of these slopes. ¿ 1999 American Geophysical Union