Dune leapfrogging is deciphered.

Physicists have unraveled how certain wind-driven sand dunes in Morocco and Peru apparently tunnel through slower dunes.

Barchan dunes are massive, crescent-shaped sand piles that move across windswept deserts at speeds up to tens of meters per year. Because smaller dunes outpace bigger dunes and eventually appear in front of them, the small dunes seem to be punching through big ones.

Such behavior has precedents: Solitary waves of light, sound, or water that pass directly through each other are known as solitons (SN: 11/20/99, p.327). Now, a new mathematical model for wind-driven sand, as well as computer simulations of the same, reveal that one Barchan dune can, indeed, act as a soliton and pass through a slower-moving dune.

In the Dec. 11 Nature, Veit Schwammle and Hans J. Herrmann of the University of Stuttgart in Germany outline several simulated dune behaviors, including the soliton scenario. The simulations' outcomes depend on the relative starting heights of the two dunes.

Each setting begins with a small dune rear-ending a big one. In the solitonlike case, wind dumps sand on the rear dune while shrinking the forward one. Next, the now-small, forward dune pulls away from its now-big companion. Such encounters "look exactly like soliton collisions," Hermann says, although the sand grains of one dune don't actually burrow through the other dune.

Other scenarios studied by the scientists include complete swallowing of a tiny dune by a much-larger one and the growth of baby dunes at the ends of a parent dune that has consumed an incoming, slightly smaller dune.--P.W.