The ratio between co-registered pixels in stereo or repeat image pairs is used to constrain the deviation of the Martian surface from Lambertian reflection due to unresolved shadows at the pixel scale of orbiting imagers. Relative differences between the ratios primarily reflect differences in the effective amount of shadows 'seen' by the sensor in the two measurements, and are thus used as a relative proxy for sub-pixel surface roughness (SR). In-scene atmospheric corrections enable simple application of this 'two-look' approach, which facilitates robust SR mapping at spatial resolutions comparable to the image data. Relative 'two-look' SR estimates can be translated to physical SR parameters through empirical calibration using ground-based measurements. SR variation maps in Gusev crater were derived from THEMIS and HRSC images at 18- and 30- m/pixel, respectively. The latter were calibrated using measurements from Spirit to produce a quantitative clast-coverage map of Spirit's initial traverse in Gusev crater.