Varieties of barchan form in the Namib Desert and on Mars

Barchan dunes occur in many parts of the Namib Desert of southern Africa and on Mars. Examination of remotely sensed images has shown that there are a variety of morphological types. Using the Long and Sharp (Long, J.T., Sharp, R.P., 1964. Barchan-dune movement in Imperial Valley, CA. Geological Society of America Bulletin 75, 149–156) system, barchans are classified into classic symmetrical barchans, which can be slim, normal, pudgy or fat. These basic dune planforms vary in size up to megabarchans. Dune coalescence dynamics lead to evolution into transverse dune ridges as well as linear ridges. The latter evolve from barchans in three ways: barchan asymmetry, collision of barchan and dome convoys ejected from destabilised barchans and downwind from nebkha fields. A review of modelling studies combined with ground data suggest that the variety of barchan forms reflect an adjustment to the wind and sediment regime. Although further work is required, we suggest that this approach may be useful for determining wind and sediment regimes on planetary surfaces. Many of the dune types in Namibia are also present on Mars suggesting that it is a suitable analogue location for further studies into the controls on dune form.