The Uncritical Solitonic Undulations of Barchan Sand Dunes

Barchan Sand Dunes, Empty Quarter – National Geographic

Among the more interesting landforms generated by Aeolian processes on earth (as well as other planets) are those known as Barchan (literally crescent or arc shaped) sand dunes. Generated with constant wind direction and relatively low sand availability, a Barchan dune can stretch between a few meters up to spans of more than a 100 meters. At a distance these geometric mounds appear as sculptural forms herding en mass across the surface of the desert. An interesting spatio-temporal property of the collective is the behaviour of small fast moving sand dunes passing through larger slower dunes unscathed. This kind movement is similar to the way standing waves (Solitons) pass through each other in sound and light.

Dune formation has been extensively modelled using discreet cellular-based agents. A time-lapsed video of Barchan dune movement would most likely appear as a liquid Cellular Automata-like animation. General sand dune formation relies on the notion of self-organised criticality. Grains of sand pile up on a dune until the dune reaches a critical point and then an avalanche occurs often creating a chain reaction of complex dune formation. Barchan dunes seem to exist on the edge of self-criticality, just about avoiding the inevitable collapse to maintain their form as they march across the desert.

Elsewhere in our Solar System extreme winds on Mars (gusts up to 600 km/hr) also generate fields of Barchan dunes, particularly at its poles where the dunes become frozen.