Deposits of Sand: Ripples Versus Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 57

Fig. 5.9 Navcam image from
the Opportunity rover of a small
impact crater ('Rayleigh') which
cuts into a ripple, neatly showing
the cross-section of the ripple and
exposing the angle of the layers
in it. The intersection of the
layers with the ripple surface can
be seen behind the crater, and on
the ripple to the right. Image
NASA

features; when both the horizontal and vertical dimension
are scaled by the width of the feature, profile shapes can
be compared directly even for features that differ in scale
by more than three orders of magnitude (Zimbelman et al.
2012; see Fig. 5.17 ). Profile shape can be assessed from
remote sensing imaging data (see Chap. 18 ) , and if com-
bined with an assessment of the dominant particle size
associated with a particular aeolian bedform, the probable
formational mechanism involved in the development of
both large ripples and small dunes should be identifiable
even when detailed field examination may not be possible.
Balme et al. (2008) show that at least some TARs
observed on Mars have identical shapes to some wind
ripples on Earth.

as the saltation path length. The ripple wavelength therefore
relates to the windspeed (as demonstrated in wind tunnel
experiments, e.g., Seppala and Linde 1978) as well as the
particle size and other factors. The largest ripples form
when the grains are coarse and thus can only be moved by
the fastest winds, giving the longest saltation path length.
Similarly, the long, shallow trajectories of grains on Mars
yield large ripples or TARs. On the other hand, the saltation
paths are short enough on Venus and Titan (Kok et al. 2012)
that ripples may be insignificantly small.
The smallest ('elemental') dunes, on the other hand, are
defined by the saturation length, which scales with the drag
length. This in turn depends on the particle size, and the
particle and atmospheric density, but does not depend on the
windspeed. Dunes can grow in size, both by direct grain
accumulation, and by coalescence with other dunes. How-
ever, dune growth ceases when the dune is large enough to
'feel' the top of the planetary boundary layer, which hap-
pens roughly when the dune spacing (typically six times the
dune height) approaches the boundary layer thickness.
Again, there is a consistent progression in size from Venus,
through Titan then Earth, to Mars with the largest elemental

5.5 Controls on Feature Scale

We have discussed various factors controlling the scale of
aeolian features: it is worth a brief recapitulation at this
point. Ripples are defined by the shadow zone of saltating
particles—it is somewhat similar to but not quite the same