The origin of the transverse instability of aeolian megaripples

Flat sand beds subjected to wind stress are unstable, and the wind action results in two types of aeolian sand ripples: normal ripples and megaripples. The distinction between the two types is based on two characteristics: i) the normal ripple pattern usually has a wavelength of up to 30 cm, while the megaripple wavelength is on the order of meters; and ii) unimodal distributions of sand grain size lead to normal ripples, while bimodal distributions result in megaripples. On Mars, the distinction between the two types is more difficult to ascertain because the length scales of normal ripples and megaripples can overlap, and often, there is no detailed information regarding their grain size distribution. Unlike normal ripples, megaripples show transverse instability, whose mechanism remains elusive, resulting in a much larger sinuosity of the crestline than normal ripples. In this study, we investigate the megaripples' transverse instability by using field measurements, wind tunnel experiments and numerical simulations of a three-dimensional ripple model. We show that (a) coarse grains accumulate at megaripple crests, with a corresponding reduction of the lateral sand transport along the crest, and (b) the transverse instability of megaripples is generated by a positive feedback between the height of the crest and the accumulation of coarse grains, with more grains accumulating on the higher portions of the crest. The outcomes of this positive feedback are that the thickness of the coarse grain armoring layer along the crest is not uniform and that it correlates with the crest height. In turn, these height differences drive the transverse instability such that higher portions of the ripple migrate more slowly than the lower sections, creating a wavy crestline. An analysis of Martian ripple images shows variations in the sinuosity index, suggesting that this characteristic can be useful in distinguishing between normal ripples and megaripples on Mars.