Deposits of Sand: Ripples Versus Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 55

(see discussion above regarding wind profiles during sal-
tation). He further argued that the wavelength of wind
ripples (typically *15 cm for simple ripples in fine sand,
Fig. 5.12 ) is therefore the physical manifestation of the
characteristic path of the wind-driven sand.
Sharp (1963) inferred that because aerodynamic (formed
by the wind, as opposed to ripples formed by flowing water)
ripples are observed to start off as small-amplitude, short-
wavelength irregularities in a sand surface, which subse-
quently increase in wavelength as they evolve toward a
steady state, Bagnold's 'characteristic path' concept for a
wind ripple wavelength was suspect. Through geometric
arguments, Sharp (1963) proposed the alternative concept
that ripple wavelength depends on ripple amplitude and the
angle at which the saltating grains approach the sand bed, so
that ripple wavelength is controlled by the length of the
zone shadowed (Fig. 5.14 ) downwind of the ripple from
significant sand grain impacts, and also by the wind velocity
that is the cause of the saltation action.
Anderson (1987) developed an analytical model for the
initiation of sand ripples that result from the growth of
perturbations on an initially flat sand surface. Using the
'splash function' concept derived from wind tunnel exper-
iments by Unger and Haff (1987), Anderson (1987)
modeled the wind tunnel measurements of relatively
low-velocity ejection of grains caused by the impact of
faster-moving saltating grains; he termed this splash process
as 'reptation', from Latin for 'to crawl', to distinguish this
mode of sand transport from saltation, suspension, and
impact creep. Subsequent numerical modeling including
both saltation and reptation (Anderson and Haff 1988)
showed that small, fast-moving ripples overtake and merge
with larger, slower ripples, resulting in the growth of the
mean wavelength for the entire ripple field, along with a
decline in the dispersion of wavelengths (that is, the
observed range of wavelengths decreases as the ripple field
evolves toward a steady state). Both reptation and saltation
have been incorporated into subsequent continuum analyt-
ical models developed for the study of aeolian features on
Earth and Mars (e.g., Momiji et al. 2000; Yizhaq et al.
2004; Yizhaq 2005), and numerical models (e.g., Landry
and Werner 1994).
Bagnold (1941) described some important distinctions
between ripple and dune formation processes: dunes usually
display a slip face dominated by avalanche processes; rip-
ples have the coarsest material collected at the crests with
the finest material in the troughs, whereas the opposite is
true for dunes. When aeolian sand is poorly sorted (that is, a
wide range of particle sizes is present), the coarse grains
become concentrated at the crests of large features that
Bagnold (1941, pp. 153-156) termed 'ridges' rather than
ripples, arguing that the coarse grains are moved by the
impact of saltating sand. Other terms have been used to

Fig. 5.7 Mars dune avalanching. Three images of the same location
(taken by the High Resolution Imaging Science Experiment (HiRISE)
camera on NASA's Mars Reconnaissance Orbiter) taken at different
times on Mars show seasonal activity causing sand avalanches and ripple
changes on a Martian dune. Time sequence of the images progresses
from top to bottom. Each image covers an area 285 9 140 m. The crest
of a dune curves across the upper and left portions of the image. The site
is at 84 north latitude, 233 east longitude, in a vast region of dunes at
the edge of Mars' north polar ice cap. The area is covered by carbon-
dioxide ice in winter but is ice-free in summer. The top and bottom
images show part of one dune about one Mars year apart, at a time of year
when all the seasonal ice has disappeared: in late spring of one year (top)
and early summer of the following year (bottom). The middle image is
from the second year's mid-spring, when the region was still covered by
seasonal carbon-dioxide ice

for sure how they formed, a new (if ungainly) morpholog-
ical term was introduced: Transverse Aeolian Ridges
(TARs—Fig. 5.13 ). To help better understand why a ripple
is not just a very small dune, we must go back to
what several researchers had to say about both ripples and
dunes.
Bagnold (1941, p. 62) described a 'characteristic path'
for well-sorted, wind-blown sand grains, which he argued
was the result of momentum removed from the wind at
the height reached by most of the saltating grains