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Aeolian Ridge
▶Megaripple
Aeolian Ripple
Sharon A. Wilson
Center for Earth and Planetary Studies,
Smithsonian Institution, National Air and Space
Museum, Washington, DC, USA
Definition
Small (centimeter to meter wavelength) aeolian
bedforms, commonly occurring in trains, created
by the mobilization and accumulation of sand-
sized grains into roughly parallel ridges that are
typically oriented transverse to the direction of
the wind.
Category
A type of ▶ ripple; A type of ▶ aeolian deposit;
A type of ▶ bedform
Description
Ripples are small, regularly repeated depositional
bedforms consisting of sand-sized grains that
develop almost anywhere that sand and wind
occur together (Greeley and Iversen 1985).
Ripples generally form transverse to the wind
direction and their straight to slightly sinuous
crests generally end or bifurcate within a few
meters (Greeley and Iversen 1985, p. 149). Aeo-
lian ripples have wavelengths from 0.01 to 20 m,
heights from a few mm to 1 m, and indices
between 12 and 50 (Leeder 1982). Unlike
dunes, ripples lack slip faces (Thomas 1989;
Greeley and Iversen 1985). The coarsest and fin-
est grains are concentrated at the crests and
troughs, respectively (Bagnold 1941, p. 145),
and they are typically less than tens of saltation
path length long (Bagnold 1941; White 1979).
Ripples can form on free-standing surfaces as
well as on larger aeolian bedforms such as dunes.
Normal ripples on Mars have been observed
by landed missions, but megaripples, termed
▶ transverse aeolian ridges, are large enough to
study with orbital data. While ripples and dunes
on Earth are morphologically distinct, the
increased saltation path length of sand-sized
material on Mars due to the thinner atmosphere
(White 1979) may cause overlap between the
dimensions of large ripples and small dunes
despite their unique formation mechanisms
(Wilson et al. 2003).
Morphometry
The windward-facing (stoss) slopes of the ripple
tend to be 8–10
 and are eroded by saltating
grains, whereas leeward slopes, protected from
impacting grains, are 20–30
 and are steepest
near the ripple crest (Greeley and Iversen 1985).
Ripple wavelength is either controlled by the
saltation path length (Bagnold 1941) or the ripple
height, which may act as a barrier to saltating
particles (Sharp 1963; Fig. 1).
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Ripple Wavelength
It is the horizontal distance between the deepest
points of two troughs or the horizontal distance
between two crests in a rippled sand surface
(Glenn 1979).
Ripple Height
It is the vertical distance from the base of the
ripple to the crest.
Ripple Index
It is the ratio of ripple wavelength to ripple
height, which generally varies inversely with
grain size and directly with wind velocity
(Greeley and Iversen 1985).
Subtypes
(1) Normal ripples (also known as sand ripples,
ballistic ripples, and impact ripples) have
wavelengths of 1–25 cm and heights of
0.5–1.0 cm (Sharp 1963) and are asymmetric
in profile with windward and lee slopes of 10

and 30
, respectively (Mabbutt 1977). Crests
are generally straight or slightly sinuous
and form transverse to wind direction in
well-sorted sand (0.3–2.55 mm).
(2) ▶Megaripples (also known as giant ripples,
granule ripples, erosion ripples, sand ridges,
and pebble ridges) have wavelengths of ~3 m
up to 25 m (Greeley and Iversen 1985) and
form in bimodal sands transverse to the wind
direction. The wind is sufficient to mobilize
the fine material but coarser grains become
concentrated on the ripple crest, resulting in
a more symmetric profile relative to normal
ripples.
(3) Fluid drag ripples (Bagnold 1941), also known
as aerodynamic ripples (Wilson 1972), can
form either longitudinally or transversely
(Thomas 1989) to the high velocity wind
direction in well-sorted fine sand and are char-
acterized by very long, flat ripple forms.
Formation
Ripples form when unidirectional winds are
strong enough to initiate saltation of sand grains
that exploit chance irregularities in the surface
(Bagnold 1941). The windward (stoss) side is
bombarded by wind-transported grains, each
moving downwind at a distance defined by the
average saltation path length of the grain. The
subsequent erosion on the stoss side of the ripple
creates a zone of accumulation on the leeward
side, moving the entire system forward.
In general, ripple wavelength increases with
ripple height (Thomas 1989, p. 238). Sharp
(1963) observed that wavelength increases with
time and suggested ripple wavelength is a function
of ripple height and the angle that saltating grains
impact the bed. Therefore, Sharp (1963) proposed
that ripple wavelength is controlled bywind veloc-
ity and the length of the impact-shadowed zone.
Bagnold argued that ripple wavelength is
a function of the saltation and reptation path
length of the sand grains, which is controlled by
wind speed (Bagnold 1941). However, recent
studies show that saltation trajectories are many
times longer than ripple wavelength. Ripple spac-
ing is about six times the mean reptation path
Aeolian Ripple, Fig. 1 Aeolian ripple development: (1)
stoss slope, (2) ripple crest (coarse grains), (3) lee slope,
(4) ripple trough (sheltered fine grains); dashed lines show
paths of saltating grains (After Pye and Tsoar 1990;
Fig. 6.4, Sharp 1963)
Aeolian Ripple 19
A
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length (Thomas 1989). In the new model, ripples
are controlled by reptation flux. Particles are
entrained into reptation not directly by the wind,
but by the nearly unidirectional bombardment of
saltating particles (Prigozhin 1999). Saltating
grains that impact coarser material cause
reptation or creep or low-velocity movement
along a short path length (Anderson 1987).
These low-energy particles outnumber high-
energy trajectories by about 9:1 (Thomas 1989).
Coarse grains that move by creep processes accu-
mulate on the lee side of the ripple and limit the
height of the ripple (Bagnold 1941) until equilib-
rium is reached and the bedform migrates in the
direction of sand transport (Thomas 1989,
p. 238).
Distribution
Ripples can occur on any large-scale aeolian
bedform (Thomas 1989). On Mars, they com-
monly occur in topographic depressions (e.g.,
craters, troughs, valleys, channels) and are wide-
spread throughout the equatorial and midlatitude
regions of both hemispheres (e.g., Wilson and
Zimbelman 2004; Figs. 2, 3, and 4).
Ripple Patterns
▶Reticulate ridge. In Hellespontus, Mars, trans-
verse ridges show straight, bifurcating, sinuous,
and catenary-to-lunate ripple patterns, oriented
parallel and also orthogonal to dune migration
direction, with a primary component (parallel)
and a secondary component (orthogonal), indi-
cating bimodal winds (Fig. 4). A reticulate or
network arrangement is formed as interference
patterns. Different ripple sets have different albe-
dos that may result from grain size sorting or
mineral separation (Bishop and Wheeler 2010).
Ripple Migration
Ripples show evidence of migration in several
locations on Mars including the North Polar
Erg, Nili Patera, Kaiser, Herschel, Rabe, and
Matara craters but are immobile on several
intracrater dunes in the southern highlands
(Bridges et al. 2011).
History of Investigation
Misconceptions regarding the formation of aeo-
lian ripples are as follows: (1) they form as
a result of a special wave motion, analogous to
water waves, generated by the shear (different
speed) of two adjacent fluids (sand and wind),
and (2) they are microdunes that failed to grow
into regular dunes. Modern conceptions were
developed by Bagnold (1941) and Wilson
(1972). Wilson established the hierarchic system
of aeolian features realizing that ripples and
dunes are not gradational stages, but rather
Aeolian Ripple, Fig. 2 View of ripple crests (covered
with very small hematite spherules) and troughs (scattered
with larger spherules) on the plains between Eagle and
Endurance craters. Field of view is ~52 cm. (Fig. 13.35
from Bell et al. 2008). Meridiani Planum, Mars. Opportu-
nity Mini-TES Reduced Data Record 1T 134662794 RDR
10 00 P3575 N0 A1. Color view from 482 (B), 535 (G),
673 (R) nm images. Sol 73 sequence P2589 (NASA/JPL/
Washington University St. Louis)
20 Aeolian Ripple
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Aeolian Ripple, Fig. 3 The active, dark, mafic El
Dorado ripple field with interfingering inactive, light-
toned coarse-grained ripples on the flanks of Husband
Hill on Mars (Sullivan et al. 2008). (a) Spirit Pancam R1
(436 nm) sol 813; (b) coarse-grained ripple very near the
summit of Husband Hill. Wheel track in right center is
16 cm wide. Spirit Pancam L247 (mosaic of 753, 601,
432 nm), sol 607 (NASA/JPL/Cornell); (c) view from
orbit, scale bar 50 m. HiRISE PSP_003900_1650
(NASA/JPL/University of Arizona)
Aeolian Ripple,
Fig. 4 Three scales and
different orientations of
sand patterns in
Hellespontus/Noachis
Terra, Mars. Portion of
a dark transverse ridge is to
the upper left. Scale bar
50 m. HiRISE ESP_
016036_1370 at 42.68
S,
38.02
E (NASA/JPL/
University of Arizona)
Aeolian Ripple 21
A
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Date:8/7/15 Time:02:59:17 Page Number: 22
represent different mechanisms of formation
(Greeley and Iversen 1985, p. 148 and references
therein).
References
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impact ripples. Sedimentology 34:943–956
Bagnold RA (1941) The physics of brown sand and desert
dunes. Methuen, London
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mometers: ripples and scour flutes of the Strzelecki
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Cambridge, MA, p 340. Leeder, 1982
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ripples. Phys Rev E 60(1):729–733
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Aeolian Sand Deposits
Henrik Hargitai
NASA Ames Research Center / NPP,
Moffett Field, CA, USA
Definition
Windblown and deposited granular material with
particle sizes of 0.0625–2 mm.
Category
A type of ▶ aeolian deposit.
Note
This entry discusses sand deposits in general;
details on bedforms and other aeolian deposits
can be found in the appropriate entries.
Subtypes
Subtypes by organization:
(1) ▶Bedforms
(1.1) ▶Dune
(1.2) ▶Ripple
(1.3) ▶Megaripple
(2) ▶ Sand sheet
(3) ▶Drift deposits (Greeley et al. 2002)
22 Aeolian Sand Deposits