Surface slope effects for ripple orientation on sand dunes in López crater, Terra Tyrrhena region of Mars

Ripple orientations on small sand dunes (dunes lacking substantial slip faces) at widely distributed sites across Mars have been documented using High Resolution Imaging Science Experiment (HiRISE) images, in an effort to determine the last formative aeolian sediment transport direction experienced at these locations. Howard (1977) used field measurements and first principles to derive an expression for determining how much the surface slope on a sand dune deflects the orientation of sand ripples with respect to the formative wind direction. A Digital Terrain Model derived from stereo HiRISE images was used to assess the potential deflection of ripples on sand dunes on the floor of López crater on Mars. Three-quarters of the area covered by sand dunes within the DTM has a surface slope <10°, where deflection angles are expected to be <17° (a value that should not pose a major issue for comparison to model-derived winds); such surface slopes are typical of small sand dunes on Mars that lack large slip faces. Sand ripples therefore should be good indicators of the most recent sand-transporting winds that have blown across sand dunes on Mars, as long as areas on or very near to slip faces are avoided.