Dune trails

A Piece of Mars: The dark dunes in this 0.96×0.54 km (0.60×0.34 mi) scene are slowly migrating towards the lower left. Look closer and you’ll see brighter ripples between the dunes – the biggest ones are ~8m (26 ft) apart. They form trains in the wake of the dunes. Why? This is where the dunes previously marched on by, obliterating the ripples as they went. Ripples form again after the dunes pass by, growing in size and regularity with time. (HiRISE ESP_01746_2570, NASA/JPL/Univ. of Arizona)