USE OF DIGITAL TERRAIN MODELS OF MARTIAN SAND DUNES TO ASSESS WIND FLOW PATTERNS

Digital Terrain Models (DTMs) of Martian sand dunes with one meter postings are available from stereo pairs of High Resolution Imaging Science Experiment (HiRISE) images. Several publicly released DTMs of Martian dunes are available from the HiRISE web site, but we have also produced two DTMs of small (low-relief) Martian sand dunes using SOftCopy Exploitation Tool SET (SOCET SET) software at the Astrogeology Branch of the U.S. Geological Survey at Flagstaff, Arizona. Automated feature matching is often difficult on the relatively featureless Martian dunes, but through close attention to a favorable illumination angle and to minimizing the time elapsed between the stereo images, we obtained excellent DTM data where the ripples could provide control points between the stereo images. The DTM data supported a quantitative assessment of the magnitude of the possible deflection of ripple orientation (with respect to the formative wind direction) when a ripple is formed on the sloping surface of a sand dune (Howard, 1987); we conclude that ripple deflection on Martian dunes is minimal (<17°) for small dunes (surface slopes <10°) that lack a substantial slip face. Ripple orientation can therefore be correlated to the direction of the formative surface wind flow over shallow-sloped sand dunes, so that mapped ripple orientations can provide constraints on the most recent wind flow across Martian sand dunes. The DTMs have also been used in freeware (Wind Ninja) to investigate how dune shape may influence the local wind flow over the dunes, with the result that low-relief dunes do not appear to alter significantly the expected flow paths of the surface winds. However, dunes possessing slip faces may have both significant deflection of ripple orientation and altered wind surface flow near the slip face; ripple orientation data for such dunes should only be used well removed from regions where slip faces are present. It is remarkable that HiRISE images, along with the DTMs derived from stereo pairs, allow the documentation of wind flow patterns for individual sand dunes on the surface of another planet.