Mechanics of Sand Transport - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 48

Fig. 4.16 Bedform wavelength on the flanks of the Tharsis Montes
on Mars, from Lorenz et al. (2014). These mountains span a sufficient
range of elevation that the atmospheric pressure varies by a factor of
almost 10. Although these bedforms are more likely to be ripples
(controlled by the shallowness of the saltation path, see Chap. 5 ) than

dunes (controlled by a saturation length), it is evident that there is an
approximately reciprocal relationship between wavelength and atmo-
spheric density (or pressure)—a trend that holds locally on Mars as
here, but also between planets

Fig. 4.17 Flux instability criterion for bump growth. The streamline
compression by a low hill like a dune causes the maximum stress s max to
be upwind of the crest. If the peak flux (q max ), downwind of s max by a

distance that scales with L sat , is upwind of the crest and thus the crest is
within the deposition zone, the dune will grow (as here), but if downwind,
the dune will shrink. Thus a dune must be a critical size to grow

Mars and Earth, but also over a range of elevations (and thus
atmospheric density) on Martian volcanos (Fig. 4.16 ). That
said, the mechanism of formation of these bedforms is defined
by the shallowness of the saltation path (see Sect. 5.4 ) rather
than the drag length directly, although these quantities share
many factors.

Although the detailed physics of the controlling influ-
ence of the saturation length (and drag length) on dunes is
still being fully explored, it seems to be important. The
influence can be visualized as follows: a dune acts as a
gentle obstacle to the wind, which accelerates (usually by
10-100 %, depending on height and steepness) and thus