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Geological processes
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granule ripple should require from hundreds to thousands of Earth-years to move 1 cm under present
ms co
ranule
old, 19
r et al
ature regarding different proposed modes of formation of sand rip-
ples is in marked contrast to the general agreement that granule
ripples result from the creep of granule particles induced by the
impact of saltating sand grains; the ?armoring? effect of the gran-
ules contributes directly to spectacular growth of both the height
and wavelength of these features, as compared to typical sand rip-
were more a function of the shadowing effect of the upstream rip-
ple crest than the Bagnold view that sand ripple wavelength was a
manifestation of the average saltation path length. Both the Bag-
nold and Sharp concepts of sand ripples have been explored with
laboratory experiments and associated theory involving both rep-
tation (a ?crawling? motion of the sand grains along the surface)
and saltation (the ?jumping? motion of sand grains well above the
surface). Splash caused by impacting sand grains produces surface
variations that preferentially grow to form typical sand ripples
(Anderson, 1987; Anderson and Haff, 1988, 1991; Anderson and
Bunas, 1993). In spite of the controversy in understanding how
* Corresponding author. Address: CEPS/NASM MRC 315, Smithsonian Institution,
Washington, DC 20013-7012, USA. Fax: +1 202 786 2566.
Icarus 203 (2009) 71?76
Contents lists availab
ru
.e lE-mail address: zimbelmanj@si.edu (J.R. Zimbelman).have been applied to these features in the literature, but here we
use ?granule ripple? to represent all wind-generated bedforms that
are coated by granule-sized particles. Granule ripples are signi?-
cantly larger than wind ripples formed in well-sorted ?ne sand,
yet they are generally smaller than an individual sand dune. Inter-
est in granule ripples increased recently due to results from the
Mars Exploration Rovers (MERs) Spirit and Opportunity, both of
which documented the common occurrence of sand-cored ripples
coated with granule-sized particles (Fig. 1; Greeley, 2004, 2006;
Sullivan, 2005, 2008). No MER observations document movement
of a martian granule ripple, although Spirit recently did document
active sand ripple movement (Sullivan, 2008). The extensive liter-
2. Background
Bagnold (1941) was the ?rst person to present detailed mea-
surements of how sand moves both in the natural environment
and in the laboratory. He pointed out an important distinction be-
tween sand ripples, which he related to the motion of individual
sand grains, and sand dunes, which he described as the result of
the interaction of the cloud of saltating sand with the wind that
drives the grains across the surface (Bagnold, 1941, pp. 149?153,
180?183). Sharp (1963) used ?eld data from sites throughout the
southwestern United States to argue that sand ripple wavelengthsMars surface
1. Introduction
Granule ripples are aeolian bedfor
that is covered by a surface layer of g
ically 1?2 mm in diameter (e.g., Bagn
1963; Ellwood et al., 1975; Fryberge0019-1035/$ - see front matter Published by Elsevier
doi:10.1016/j.icarus.2009.03.033mprised of a sandy core
s, particles that are typ-
41, pp.154?156; Sharp,
., 1992). Several names
ples. This report describes a documented rate of movement for
granule ripples observed at Great Sand Dunes National Park and
Preserve (GSDNPP) in south-central Colorado. The GSDNPP results
are then used to make inferences about potential granule ripple
movement under present conditions on Mars.Keywords:
Earth
atmospheric conditions.
Published by Elsevier Inc.The rate of granule ripple movement on
James R. Zimbelman a,*, Rossman P. Irwin III a, Steve
Andrew Valdez c, Scott Stevens d
aCenter for Earth and Planetary Studies, National Air and Space Museum MRC 315, Smi
b Education Division, National Air and Space Museum MRC 305, Smithsonian Institution
cGreat Sand Dunes National Park and Preserve, 11500 Highway 150, Mosca, CO 81146-
dNational Climatic Data Center, Federal Building, 151 Patton Ave., Asheville, NC 28801-
a r t i c l e i n f o
Article history:
Received 25 July 2008
Revised 13 March 2009
Accepted 13 March 2009
Available online 17 April 2009
a b s t r a c t
The rate of movement for
Great Sand Dunes Nation
induced by saltating sand
averaged 9 m s1 (at a he
tation of sand. Extension o
Ica
journal homepage: wwwInc.rth and Mars
. Williams b, Fred Bunch c,
nian Institution, Washington, DC 20013-7012, USA
shington, DC 20013-7012, USA
8, USA
, USA
nd 10-cm-high granule ripples was documented in September of 2006 at
ark and Preserve during a particularly strong wind event. Impact creep
ed 24 granules min1 to cross each cm of crest length during wind that
t well above 1 m), which is substantially larger than the threshold for sal-
s documented granule movement rate to Mars suggests that a 25-cm-high
le at ScienceDirect
s
sevier .com/locate / icarus
The GSDNPP dune ?eld experiences a bimodal wind regime where
urfa
dic
Icarsand ripples form, there has been general agreement that granule
ripples are the result of creep of the large particles induced through
the impact of saltating sand grains. The granule-coated surface of
sand-cored ripples provides a layer of protection for the underlying
sand, allowing the granule-coated ripples to grow in both height
and wavelength by causing the impacting saltating grains to hit
and rebound off the granules rather than the underlying sand.
Trenching in granule ripples indicates that the granules form a
monolayer on both ?anks of the ripple, but they can be concen-
trated to thicknesses exceeding 1 cm along the ripple crest, with
scattered granules distributed throughout the sand-dominated
core of the ripple (Sharp, 1963). Granule ripples have been de-
scribed in deserts around the world, with reported heights ranging
from 1 to 60 cm and wavelengths of <1?25 m (Bagnold, 1941, p.
155; Sharp, 1963; Greeley and Iversen, 1985, pp. 154?155; Fryber-
ger et al., 1992; Jerolmack et al., 2006).
Early spacecraft images of Mars revealed large dune ?elds, dust
mantles of regional extent, and widespread isolated aeolian bed-
forms on rocky surfaces (e.g., McCauley et al., 1972; Cutts et al.,
Fig. 1. (a) MER Opportunity NavCam image of large ripples on a fractured bedrock s
25 cm/pixel map projected resolution, NASA/JPL/University of Arizona). White dot in
72 J.R. Zimbelman et al. /1977; Greeley et al., 1992). The Mars Orbiter Camera (MOC) re-
vealed in great detail the widespread occurrence of aeolian bed-
forms across Mars (Malin and Edgett, 2001), later supplemented
by the Thermal Emission Imaging System (Hayward et al., 2007)
and the High Resolution Imaging Science Experiment (HiRISE;
McEwen, 2007). HiRISE is now imaging aeolian features at up to
25 cm/pixel resolution (Bridges et al., 2007). Aeolian bedforms on
Mars with wavelengths of 20?80 m, generally oriented transverse
to the inferred wind direction, appear to be nearly ubiquitous at
equatorial and mid-latitudes (Wilson and Zimbelman, 2004; Balme
et al., 2008). Such features, particularly those at the shortest wave-
lengths, fall within the size range that on Earth is populated by
both small sand dunes and large ripples (Wilson, 1972). The
long-lived MERs Spirit and Opportunity (Squyres, 2004a,b) have
provided conclusive evidence that most of the meter-scale aeolian
bedforms at both landing sites are granule ripples (Greeley, 2004,
2006; Sullivan, 2005, 2008). MER results for granule ripples have
been compared to analog features at White Sands National Monu-
ment (WSNM) in New Mexico, where ?eld studies included docu-
mentation of the movement of small (1-cm-high) granule ripples
during periods of over an hour in duration (Jerolmack et al., 2006).
Granule ripples are common along the southern margin of the
main dune mass at GSDNPP, where we studied them in order to ob-
tain accurate topographic pro?les across granule ripples, and in an
effort to assess their rate of movement (Zimbelman et al., 2007).the dominant ?ow is westerly, but with occasional reverse ?ow
from off the Sangre de Cristo Mountains immediately east of the
dunes (Merk, 1960; Johnson, 1967; Janke, 2002; Marin et al.,
2005; Madole et al., 2008). The sand at GSDNPP has a bimodal par-
ticle size distribution, particularly along the southern margin
where Medano Creek supplies fresh material (including the gran-
ule size fraction) from the nearby mountains (Ahlbrandt, 1979).
Several attempts by our group to measure the granule ripple move-
ment over periods of several months proved unsuccessful for a
variety of reasons, but during a visit to GSDNPP in the fall of
2006, it became readily apparent that, under the right conditions,
the timescale over which granule ripples move can be measured
in hours rather than months or years (Zimbelman et al., 2007).
The granule ripples examined at GSDNPP are considerably larger
than those studied by Jerolmack et al. (2006), but results are quite
consistent between both studies, strengthening the credibility of
inferences derived from both studies.ce, sol 795. (b) Portion of HiRISE image of the Opportunity site (PSP_001414_1780,
ates the approximate MER location on sol 795.
us 203 (2009) 71?763. Documented granule ripple movement at GSDNPP
Two of the authors (JZ and RI) visited GSDNPP on September
15 and 16, 2006, where we used a Trimble R8 Differential Global
Positioning System to recover one of our earlier survey lines
across granule ripples. More importantly, this visit happened to
coincide with the passage of a strong low-pressure area to the
north of the park, which resulted in prolonged intense winds from
the south?southwest. Wind records from Alamosa airport (located
45 km SSW of the study area), obtained from the National Cli-
matic Data Center (NCDC), quanti?ed the regional wind strength
(Fig. 2) and wind direction during our visit, both of which are
essentially identical to conditions that we observed locally at
the granule ripple site with a hand anemometer and a compass.
The airport wind data come from sensors mounted high above
the tarmac level (likely well above 1 m height). The NCDC data
are recorded every minute using two-minute running averages;
they reveal a consistent diurnal pattern in wind intensity, with
the strongest winds every afternoon and near-calm conditions
every night, during the weeks leading up to the strong wind
event.
Winds in south-central Colorado were particularly strong dur-
ing our visit, at or above the threshold for sand saltation (5 m s1
at 1 m height; Easterbrook, 1969, p. 290) through most of the
4. Results
The surveyed pro?le across three granule ripples near the loca-
tion of the ?agged ripples was used to obtain accurate shape infor-
mation for the granule ripples (Fig. 4), which showed that the
average stoss slope was 10 and the average lee slope was 7 for
ripples 15 cm in height; each of the surveyed granule ripples
are reasonably symmetrical in shape. Comparison of the Septem-
ber 2006 survey to an April 2006 survey along the same line
showed consistent ripple shapes (suggesting continuity during
transport), but it was inconclusive for rate constraints due to a lack
of information about how many bedforms may have traversed the
line between surveys. The time-stamp on the digital images of the
?agged granule ripples provided accurate duration information;
the 3-cm-high ripple in Fig. 3a moved 2.1 cm in 109 min, and the
10-cm-high ripple in Fig. 3b moved 10.5 cm in 1380 min.
Jerolmack et al. (2006) provide an equation for the creep mass
?ux (qc) of ripples, derived from Bagnold?s (1941) observations
on the rate of dune migration:
qc ? ?1 p?qcH=2 ?1?
where p is porosity, q is particle density, c is the ripple migration
Fig. 2. Wind data for period that ?ags were in place on granule ripples at GSDNPP.
Light grey line indicates 5 m s1, the saltation threshold wind speed at 1 m height.
The ripples shown in Fig. 3 were both ?agged at 0 time on the plot. Data are from
the Alamosa airport, located 45 km SSW of study site, representative of a height
well above 1 m (from NCDC).
J.R. Zimbelman et al. / Icarus 203 (2009) 71?76 73night, and well above threshold throughout all of the daylight
hours (Fig. 2). The NCDC data documented that the strong winds
were consistently from an azimuth of between 190 and 230
(measured clockwise from north) when the wind speed was above
threshold. Prior to conducting the survey along the recovered line,
one of us (RI) placed wire ?ags into the crests of several granule
ripples within 20 m of the survey line. When the surveying was
complete, we were surprised to see that a 3-cm-high granule ripple
had clearly moved in less than 2 h (Fig. 3a). A visit to the site the
following morning allowed us to document the movement of a
nearby 10-cm-high granule ripple over a period of 23 h (Fig. 3b).
By September 16, all of the ?ags on the smaller granule ripples
(such as in Fig. 3a) had fallen over due to the complete dispersal
of these granule ripples, but the ?ags remained in place in the lar-
ger ripples. Sand-induced failure of two digital cameras resulted in
the ripples shown in Fig. 3 being the only ones for which useful
rate information could be derived.
Fig. 3. Flagged granule ripple crests, GSDNPP. (a) Photo taken 109 min after ?ag was pla
granule-coated ripples and sand ripples are in the background. Wind intensity ripped the
after ?ag was placed on crest (of a different ripple than that shown in (a)), indicating 10.5
JRZ, 9/16/06.rate, and H is the ripple height. Jerolmack et al. (2006) used their
observed average ripple migration rate of 0.04 m h1
(=1.1  105 m s1) obtained from staked 1-cm-high granule rip-
ples, porosity of 0.4, and particle density of 2630 km m3 to obtain
a creep mass ?ux at WSNM of 9  105 kg m1 s1 from Eq. (1). If
we assume the Jerolmack et al. (2006) particle density and porosity
values, then Eq. (1) gives creep mass ?ux values of 8 and
10  105 kg m1 s1 for the 3-cm-high and 10-cm-high GSDNPP
granule ripples, respectively. We interpret this favorable compari-
son to indicate that the saltation-induced granule creep ?ux condi-
tions at GSDNPP and WSNM were quite comparable.
Continuity of shape for the moving GSDNPP granule ripples
indicates that the advancing lee side of the ripple sweeps out a par-
allelogram. The area of this parallelogram is equal to the ripple
height times the horizontal length of the crest movement. The par-
allelogram produced by the advancing ripple lee face can be re-
lated to the volume of granules transported over the crest by
considering all values to be normalized per unit crest length. The
volume estimate assumes that the granules on the advancing lee
ced on crest, indicating 2.1 cm movement (to left) of the 3-cm-high ripple. Smaller
?ag material, which is 6.7 cm along the wire. JRZ, 9/15/06. (b) Photo taken 1380 min
cm movement (to right) of the 10-cm-high ripple. Flag material is 6.7 cm along wire.
Icarslope are the primary contributor to the volume of the displaced
ripple (i.e. sand that settles between granules can be ignored).
The ripple in Fig. 3a then indicates a volume of transported gran-
ules of 3 cm (height) times 2.1 cm (crest movement) times 1 cm
(per unit crest length) = 6.3 cm3, and the ripple in Fig. 3b indicates
a transported volume of 10 cm  10.5 cm  1 cm = 105 cm3. These
volumes represent the granules that were transported across each
cm of ripple crest in 109 and 1380 min, respectively. The trans-
ported volume and duration then give 0.058 cm3 per minute and
0.076 cm3 per minute of granules crossing the crest of the ripples
shown in Fig. 3a and b, respectively, per cm of crest length.
Granule transport across a ripple crest can be expressed as the
number of representative granules moved across a unit crest
length per unit time. The granules at GSDNPP are unimodal in size
distribution, with a peak abundance (54 wt.%) between the sizes of
0.5 and 0.75 phi, corresponding to diameters of 1.41 to
1.68 mm (from Fig. 22C of Ahlbrandt, 1979). If the granules are
Fig. 4. Topographic pro?le surveyed across three large granule ripples at GSDNPP (a
local slope of 1.5 has been removed). The surveyed ripples are part of an extensive
granule ripple ?eld; the surveyed ripples are 20 m WNW of the ?agged granule
ripples (Fig. 3) along the margin of the granule ripple ?eld. Vertical exaggeration is
26.6.
74 J.R. Zimbelman et al. /considered to be spherical and 1.5 mm in diameter, then a single
granule has a volume of 1.8  103 cm3. Longwell et al. (1969,
p. 236) gives porosity ranges for sand and gravel as 30?46% and
20?40%, respectively, with median values of 38% for sand and
30% for gravel. If we assume 35% porosity for granule particles,
then the volumes determined above translate to 21 and 27 gran-
ules per minute crossing each cm of the ripple crest length for
the ripples in Fig. 3a and b, respectively. These estimates are con-
sistent with visual observations of tens of granules moving in short
hops up the stoss side of a ripple during periods of tens of seconds.
The similarity of the two numbers suggests that the sand ?ux con-
ditions, which moved the granules through impact creep, were
comparable over the entire time period represented by the ?agged
observations, but the potential variability of the wind during the
two periods of observation should be evaluated.
The NCDC wind data (Fig. 2) corresponds to the total time per-
iod covered by the two ?agged ripples (both crests were ?agged at
time = 0). It does not seem reasonable to use these wind data for
detailed ?ux calculations at GSDNPP since they represent the wind
at a site 45 km from the granule ripple location. However, a general
assessment can be made of wind strength during the two periods
covered by the photographed ripples. The NCDC wind strength val-
ues were summed over both periods and divided by the duration,
giving average wind speeds during the two periods of observation
of 9.3 and 8.9 m s1 (for a height well above 1 m) for the 109 and
1380 min durations covered by the ripples in Fig. 3a and 3b,
should be 12 the sand ?ux on Earth once saltation commences.
The GSDNPP granule movement rate then translates to 290 gran-
ules per minute (per cm crest length) across the ripple crest under
continuous wind above threshold. Almedia et al. (2008) recently
derived threshold friction speeds for Earth and Mars of 0.26 and
1.12 m s1, respectively, through directly solving the motion of
particles through a fully developed turbulent wind ?eld; the ratio
of these fully turbulent threshold friction speeds is 0.23, which re-
sults in a sand ?ux on Mars 2.6 the sand ?ux on Earth once sal-
tation commences. The GSDNPP granule movement rate then
corresponds to 63 granules per minute (per cm crest length) on
Mars using the Almedia et al. (2008) threshold friction speed
values.
The derived granule movement rate for Mars can be used to
estimate how long it could take to produce movement of a granule
ripple on Mars. For a 25-cm-tall granule-coated ripple like those
seen at Meridiani Planum, if the ripple crest moved 1 cm, then
25 cm3 of granules moved across each centimeter of the crest.
For 1.5-mm-diameter spherical granules and 35% porosity (as
was assumed for the GSDNPP calculation), this volume corre-respectively. While this result cannot be taken to indicate equiva-
lent sand ?ux conditions throughout the entire duration of both
time periods, the comparable average wind speeds for both periods
is supportive of the similar volumes and numbers of granules
moved during both periods. Both documented ripples are therefore
indicative of 24 granules per minute (per cm crest length) being
driven across the ripple crest by wind that averaged 9 m s1 (at a
height well above 1 m), which is 1.8 times the saltation threshold
wind speed for sand. Sediment transport rates scale non-linearly
with ?uid velocity, but with only two observation points, use of
the average number of granules moved across both ripple crests
seems preferable to trying to estimate a meaningful standard devi-
ation. The value of 24 granules per minute crossing each cm of rip-
ple crest also can be expressed in units equivalent to the creep
mass ?ux (Eq. (1)) by assuming spherical granules 1.5 mm in diam-
eter and the Jerolmack et al. (2006) particle density, which gives a
mass ?ux of 1.9  104 kg m1 s1, within a factor of two of the
creep mass ?ux calculated above.
5. Application to Mars
The Opportunity rover on Mars observed many large ripples on
the plains of Meridiani Planum (see Fig. 1), with Microscopic Ima-
ger evidence that most of the large ripples are coated with hema-
tite-enriched 1?2 mm particles (Sullivan, 2005; Weitz et al., 2006).
Stereo Panoramic Camera images from Opportunity on sol 794 (in
the immediate vicinity of the ripples shown in Fig. 1) provided
measurements of a typical ripple height of 25 cm, and ripple sur-
face slopes of 5?10, comparable to the slopes measured on large
granule ripples at GSDNPP (Fig. 4). The GSDNPP results for granule
ripple movement can be related to potential granule ripple move-
ment on Mars through consideration of the differing conditions on
the two planets. Atmospheric density, acceleration of gravity, and
friction speed (or shear velocity) on Earth and Mars differ by fac-
tors of 79, 2.7, and 0.14 (for threshold friction speed), respectively,
expressed as Earth values divided by Mars values; values for atmo-
spheric density and acceleration of gravity are from Lodders and
Fegley (1998, pp. 128, 160, 190, 192), and threshold friction speed
are from Greeley and Iversen (1985, p. 53). Sand ?ux is directly
proportional to both atmospheric density and the cube of the fric-
tion speed, and it is inversely proportional to the acceleration of
gravity (Bagnold, 1941, p. 66). Consequently, sand ?ux on Mars
us 203 (2009) 71?76sponds to 9000 granules moved across each centimeter of crest
length. The Mars granule rate derived above indicates that trans-
port of 9000 granules across each centimeter of the ripple crest
for the present martian atmosphere. Saltation transport on Mars
occurs only very occasionally; intervals between saltation events
and Titan. Cambridge University Press, New York.
Dunes National Monument using temporal TM imagery (1984?1998). Rem.
Icarat the Viking Lander sites were estimated to be on the order of 5
Earth-years, with each gust of saltation lasting on the order of
40 s (Arvidson et al., 1983; Moore, 1985; as cited by Almedia
et al., 2008). Almedia et al. (2008) used these Viking Lander obser-
vational constraints to estimate that the fraction of time during
which saltation transport occurs on Mars under present conditions
is 2.5  107. This small saltation frequency then indicates that
1 cm of movement for a 25-cm-high granule ripple could require
from 7.4  109 s = 240 Earth-years (Greeley?Iversen) to
7.0  1010 s = 2200 Earth-years (Almedia). Clearly these esti-
mates suggest that the martian granule ripples would be long-lived
features under conditions typical of the Viking lander sites. This
inference is consistent with observations of old ejecta blocks partly
embedded into Meridiani Planum granule ripples that have been
eroded down to conform with ripple surfaces before the granule
ripples have moved on (Sullivan et al., 2007).
What wind speeds on Mars are comparable to the conditions at
GSDNPP when the granule ripple movement was observed? Sulli-
van (2005) estimated that, at the Opportunity site, recent winds
activated basaltic sand caught in temporary particle traps (e.g.,
small craters) when u*t  2 m/s (the threshold wind friction speed),
which corresponds to a wind speed of 45 m s1 at 1 m height.
Sullivan (2005) concluded that such winds probably moved 1- to
2-mm-diameter hematite fragments through impact creep driven
by the saltating sand, consistent with the inferences made by Jer-
olmack et al. (2006). If 45 m s1 at 1 m height is taken to be the lo-
cal threshold for saltation at the Opportunity site, then the GSDNPP
conditions (1.8 times the saltation threshold wind speed) suggest
that an average wind speed of 80 m s1 at 1 m height may be
comparable at the Opportunity site to the situation when granule
ripple movement was observed at GSDNPP, a value signi?cantly
higher than any wind gust observed at the Viking landing sites over
periods of 4?6 years (e.g., Arvidson et al., 1983; Moore, 1985).
Martian granule ripples possibly could represent paleoclimatic
conditions substantially different from that of the current martian
atmosphere (e.g., the atmosphere may have been denser than it is
today at intervals during the Late Amazonian Epoch; Laskar et al.,
2004). However, 1?2 mm concretions were observed to be com-
mon within the bedrock to the south of the Eagle-Endurance area
(Weitz et al., 2006), effectively removing any need to invoke stron-
ger winds (u*t > 3 m/s) to sort granule-sized particles (through sal-
tation of the concretions) to generate the observed narrow
distribution of size fractions (e.g., Sullivan, 2005; Jerolmack et al.,
2006). Granule movement through impact creep alone now ap-
pears to be suf?cient to explain the observed particle size distribu-
tions on the surface of martian granule ripples (e.g., Weitz et al.,
2006).
6. Conclusions
Granule ripples are prevalent on both Earth and Mars where
sand, granule-sized particles, and wind are abundant. Granule
creep produced by the impact of saltating sand caused observable
movement of granule ripples at GSDNPP during a particularlywould require from 31 min (Greeley?Iversen threshold friction
speeds) to 142 min (Almeida et al. threshold friction speeds) of
continuous saltation-inducing wind to produce 1 cm of ripple
movement.
Wind conditions required for continuous saltation over time
intervals of tens to hundreds of minutes are exceedingly unlikely
J.R. Zimbelman et al. /strong wind event in the fall of 2006. The documented movement
of two GSDNPP granule ripples indicates that 24 granules per
minute (per cm crest length) were driven across the ripple crestSens. Environ. 83, 488?497.
Jerolmack, D.J., Mohrig, D., Grotzinger, J.P., Fike, D.A., Watters, W.A., 2006. Spatial
grain size sorting in eolian ripples and estimation of wind conditions on
planetary surfaces: Application to Meridiani Planum, Mars. J. Geophys. Res. 111,
E12S02. doi:10.1029/2005JE002544.
Johnson, R.B., 1967. The Great Sand Dunes of Southern Colorado. U.S. Geol. Survey
Prof. Pap. 575C, U.S. Gov. Printing Of?ce, Washington, DC, pp. 177?183.
Laskar, J., Correia, A., Gastineau, F., Joutel, F., Levrard, B., Robutel, P., 2004. Long term
evolution and chaotic diffusion of the insolation quantities of Mars. Icarus 170,Greeley, R., Lancaster, N., Lee, S., Thomas, P., 1992. Martian aeolian processes,
sediments, and features. In: Kieffer, H.H., Jakosky, B.M., Snyder, C.W., Matthews,
M.S. (Eds.), Mars. University of Arizona Press, Tucson, pp. 730?766.
Greeley, R. and 27 colleagues, 2004. Wind-related processes detected by the Spirit
rover at Gusev crater, Mars. Science 305, 810?821.
Greeley, R. and 21 colleagues, 2006. Gusev crater: Wind-related features and
processes observed by the Mars Exploration Rover Spirit, Mars. J. Geophys. Res.
111, E02S09. doi:10.1029/2005JE002491.
Hayward, R.K., Mullins, K.F., Fenton, L.K., Hare, T.M., Titus, T.N., Bourke, M.C.,
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Janke, J.R., 2002. An analysis of the current stability of the dune ?eld at Great Sandby wind averaging 9 m s1 (at a height well above 1 m), which
well exceeds the sand saltation threshold wind speed. The rate of
movement of the GSDNPP granule ripples suggests that compara-
ble granule ripples on Mars likely would require from hundreds
to thousands of Earth-years to produce discernable movement un-
der current martian atmospheric conditions.
Acknowledgments
The comments of Nathan Bridges and an anonymous reviewer
were extremely helpful in correcting and clarifying earlier versions
of the manuscript. We are grateful for the support of personnel
from both the Great Sand Dunes National Park and Preserve and
the National Climatic Data Center. This work was supported in part
by research grant NNG04GN88G from the Mars Data Analysis Pro-
gram of NASA.
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