Varieties of Barchan Form in the Namib Desert and on Mars

Introduction: Barchans are individual, mobile, crescentic shape dunes, the two horns of which face in the direction of movement. Generally they occur in areas of limited sand supply, with a low precipitation and vegetation cover, on planar surfaces and where winds are unimodal to narrowly bimodal. On Mars, the most common dune is transverse and within the barchan group there is a wide variety of forms [1] (Fig. 1). Morphological classifications: There is little work that describes or explains the morphological variety of barchans. One useful hierarchical classification considers barchans as simple, compound or complex [2]. Another attributes barchan form to a stage in dune evolution [e.g., barchanic shield dune, 3]. Long and Sharp [4] use a ratio between the length of the windward slope (a) and the horn to horn width (c) to categorize barchans: Fat ( 1), Pudgy (0.75) Normal (0.5) and Slim (0.25). This classification may be useful for Martian dunes and give insight into the controls on dune form. Barchan shape is a function of grain size, wind flow velocity, sand saturation, and wind variability [5]. For example, a smaller grain size or higher wind speed both produce a steeper and blunter stoss-side. A low saturation of the interdune sandflow produces an open crescent-moon shaped dune [5]. Dunes in areas characterized by coarse sediment (higher effective wind threshold) and/or low speeds may have lower, longer profiles compared with those in areas with fine sediments and/or strong winds [6]. Dune shape is not necessarily scale invariant. The windward profile has been found to be nearly a straight line for tall dunes, but becomes increasingly convex for smaller ones [7, 8]. Equally, the relative position of the slip face within the whole dune varies with dune size, but also the ratio of horn length to total length increases with the height [9-11]. A satellite image survey of the Namib Desert found that there was a wide range of barchan morphologies present that are more varied and complex than is often assumed. The purpose of this paper is to report on those findings and to present similar forms found on Mars. Figure 1: Variety of barchan forms on Mars. Dunes have variable width/length ratios, horns length and stoss slope curvature. These forms suggest differences in local conditions.