Deposits of Sand: Ripples Versus Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 58

Fig. 5.10 Portion of a Curiosity MastCam image of a ripple surface, after a sample was scooped out. Note abundant fine brown sand exposed
beneath mm-sized granule coating. Scoop is 4.5 cm wide. Image NASA/JPL

dunes. The same progression, more or less, arises for the
largest dunes. The root cause is the same (atmospheric
density), although the pathway is a little different: denser
atmospheres take more heat to warm up by sunlight, so the
thinnest atmosphere (Mars) has the thickest boundary layer
and thus the largest dunes. The boundary layer depends on
the thermal inertia of the surface, and on the length of the
day, etc., so one could imagine a selection of exoplanets
where the order of elemental dune size actually differs from
the largest dune size.
Normal aerodynamic sand ripples are the smallest
members in a hierarchy of aeolian landforms. Wilson
(1972a, b) documented three distinct scales for aeolian
landforms: ripples (wavelength 0.01-10 m), dunes (wave-
length *10-500 m), and draas (wavelength *0.7-5.5 km)
(Fig. 5.18 ). Note that Wilson's work includes both (small)
sand ripples and (large) granule-coated ripples within the
features he called 'ripples'; we will further explore this
distinction shortly. A region of overlap exists between the
measured dimensions of small sand dunes and large ripples,
but the average sediment size of a 10 m dune is typically
\0.2 mm while surface sediments on a ripple of 10 m
wavelength are [1 mm, and importantly there are no
observed transitional features between these two groups

(Wilson 1972a). Therefore, particle size is an important
characteristic of which class a 10 m-scale aeolian landform
belongs to. Interestingly, the 10 m size of both large ripples
and small dunes is comparable to the smallest aeolian fea-
tures originally identified as TARs on Mars (Wilson and
Zimbelman 2004). Features at all three length scales can be
present at one location at the same time, but they are the
result of differing wind intensities and durations (Wilson
1972b); this interpretation is consistent with the concepts
presented by Sharp (1963), who stated that each aeolian
bedform reaches its own quasi-equilibrium state; i.e., rip-
ples do not grow to become dunes, nor do dunes grow to
become draas (The Arabic word 'draa' means 'arm' and is
often used to refer to megadunes, particularly linear ones,
but also to compound dunes. Because a widely-accepted
formal definition is lacking, we have tended not to use it).

5.6 Sedimentation and Dune Structure

The different modes of particle motion (suspension, salta-
tion, reptation, creep) all deposit particles onto the surface
in different ways. Suspended dust particles generally form a
nearly uniform deposit without obvious layering. Large