RIPPLE ORIENTATIONS ON MARTIAN DUNES DOCUMENT DIVERSE WIND FLOW PATTERNS

We report here on the statistics of measured orientations for large wind ripples observed on Martian sand dunes as seen in images taken by the High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter. HiRISE images from seven locations widely distributed around Mars (Hellespontes, Gale crater, Nili Patera, North Polar erg, Aonia Terra, Ius Chasma, and Arabia Terra) were imported into JMARS, a web-based Geographical Information System designed to work with planetary imaging data. We selected sites where the sand dunes generally lacked slip faces; if small slip faces were present, we did not make measurements near the slip faces. Measurements involved drawing lines perpendicular to the ripple crests joining three adjacent ripples, in areas where only one ripple orientation was evident. The goal was to use measured ripple orientations to infer wind flow patterns over small sand dunes (that is, we purposely selected low-relief dunes in order to minimize possible slope effects on ripple orientation). The azimuth of the measured lines was exported to Excel where we performed statistical calculations on all of the lines from each site. Statistical values determined were mean (first moment), median, mode(s), standard deviation (square root of second moment), skewness (third moment), and kurtosis (fourth moment). Great diversity of ripple trends exists between the seven sites; two are unimodal and five are weakly bimodal. Most sites have kurtosis values indicative of strong effects from the ‘wings’ of the modes, showing a clear departure from a normal Gaussian (bell-shaped) distribution. Skewness was quite variable, indicating no dominant influence from either wing of the distribution. This report covers sites measured using JMARS; ripple orientations at 33 additional sites were measured in ArcGIS, and results for the 40 sites will be the subject of a future presentation.