SURFACE WINDS OVER MARTIAN DUNE TOPOGRAPHY AND THE COLLECTIVE EFFECTS ON RIPPLE FORMATION

Sand dunes have been shown to preserve wind flow patterns in their ripple formations. This study seeks to use ripple orientation mapping and DTM analysis in martian dune study sites to assess recent surface wind flows. We first evaluate possible study sites, selecting those with clear HiRISE frames and in diverse locations. Mapping efforts are ongoing with 24 images now completed. Ripple mapping involves the use of GIS and HiRISE images to draw lines perpendicular to crests across three adjacent ripples. This can document both ripple wavelength and inferred wind direction by the rule of maximum gross bedform-normal transport as explained by Fenton et al. (Icarus, 2014), although with a 180° ambiguity. Howard (GSAB, 1977) quantified how slope angle can influence wind ripple formation. For example, the inferred wind direction of ripples on a 20° slope (where wind parallels the contours) can deviate by 38° from the actual surface wind direction. DTMs are therefore valuable to compare ripple measurements with local slopes. The Soft Copy Exploitation Toolkit has the ability to create a DTM with 1 meter posts from 25 cm/pixel HiRISE stereo pairs and MOLA track data. Though dunes present more processing challenges than other terrain in regards to available tie points, the product is still useful for defining dune-scale structure. This has allowed us to begin analysis of wind directions in 2 DTMs of our own generation, in addition to 1 produced by the USGS. The first study site in Terra Sirenum shows a dominant grouping of measurements toward the SE, where 64% lie in the SSE direction (between 146° and 169°). However, this value does not distinguish between primary ripples on a relatively flat surface and those influenced by topography. Looking at the HiRISE frame, ripples near the crests of linear dunes seem to display wind aligned with the dunes’ long axis (SSE dominant) while ripples near the NE faces do not. Looking at the DTM, there does not seem to be a significant correlation between slope and orientation. This would suggest that ripples near the NE faces are not influenced primarily by the slope, but by another interaction such as form flow. We continue to investigate this site as well as those in Terra Tyrrhena and Iaxartes Tholus for ripples and topography indicating a consistent, recent regional wind regime.