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The Tallest Dunes in the Solar System? Dune Heights on Earth, Mars

The Tallest Dunes in the Solar System ? Dune Heights on Earth, Mars, Titan and Venus
Ralph D. Lorenz1, Lori Fenton2 and Nick Lancaster3 1 Space Exploration Sector, JHU Applied Physics Laboratory,
Laurel, MD 20723, USA. ( 2 SETI Institute, Mountain View, CA, USA. 3Desert Research
Institute, Reno, NV, USA.
We initiated this study, to consider what are the tallest
dunes on each planetary body, largely out of curiosity
given that a somewhat unified perspective on dune
morphology and its relationship to planetary circum-
stances has emerged [1,2]. However, the exercise raises
interesting questions about sand supply and dune
growth and ultimate limits on dune size. The search
for a superlative is always a work in progress: we have
attempted only a preliminary survey here and we wel-
come suggestions of yet larger dunes.
While Mars is often thought to be dune-covered (an
impression reinforced by ripples and drifts commonly
seen by rovers and landers), in fact the area coverage
fraction of Mars by dunes is rather small, with most
dunes, apart from the circumpolar Olympia Undae,
confined in sand traps such as crater basins [3]. Dunes
in some locations have been measured via stereo imag-
ing, or in a few cases by laser altimetry, to be ~200m
high, with most dunes only a few tens of meters tall
(e.g. [3,4]). A prominent exception (Fig.1) is the large
crescent-shaped dune in Russell Crater, which stereo
data (Fig.2) shows to be 450m 600m tall, depending
on the assumed base level (a challenge common to
estimating dune heights elsewhere).
Figure 1. The tallest dune on Mars? A DEM
(H2247_0000_DT4 ) of the large dune in Russell
Crater acquired by HRSC superpimposed on a
THEMIS daytime IR map. The red line denotes the
profile in figure 2. Note that the base level is different
on the two sides of the dune.
Figure 2. Topographic profile from figure 1, indicating
a height of at least ~450m.
Titan is in fact the most dune-covered planetary body
known [1,2] (roughly 15% of its land area is dune-
covered, compared with ~2% for Earth), and the dis-
covery of its giant equatorial fields of linear dunes was
enabled by their large size, making them recognizable
even in the rather coarse (~350m+) resolution radar
data from Cassini [4]. Radarclinometry [5], altimetry,
and near-infrared photoclinometry have been applied to
Titan's dunes, but the tallest appear to be the first ones
recognized, in the Belet sand sea. These have heights
determined to be 100-175m tall.
Venus has very few dunes resolvable in Magellan data.
Only recently [6] has one height determination been
made, using radarclinometric methods. That sudy sug-
gested the Fortuna-Meshkenet dunes (Al-Uzza Undae)
may have heights of 40-80m.
The dunes on our own planet are likely to be the most
contentious in terms of superlative claims: we report
here only claims in the literature. As for Mars and
elsewhere, it is important to distinguish between large
free-standing dunes, dunes that mantle bedrock topog-
raphy, and dunes that form large accumulations of sand
(e.g. Great Sand Dunes, Colorado), which have a
thickness of 100-180 m [7].
Dunes (mostly of complex linear form) with a height of
150 200 m are widespread in sand seas in Namibia
and Arabia. Much larger dunes (height > 200 m) are
commonly of complex star or reversing form and ap-
pear to be associated with areas of multi-directional
and/or opposed wind directions, as well as topographic
obstacles. Such dunes are common in the Lut Sand Sea
of Iran [8], Grand Erg Oriental, Issaouane-N-Irarraren,
and other northern Saharan sand seas.
Dunes with a height of 300 400 m are known from
the Sossus Vlei area of the Namib Sand Sea [9]; the
Badain Jaran sand sea of China [10] and the small Erg
Guidi and Erg Tihodaine in the central Sahara [11].
Dunes exceeding 400 m height have been identified in
the Badain Jaran [10]; the Grand Erg Oriental [12] and
Issaouane-N-Irarraren of Algeria [11]. These appear
to be the tallest dunes reported on Earth: the global
topography datasets from SRTM and ASTER would
now permit a systematic survey.
It has been suggested [14] that dunes may grow until
they reach a height of ~H/12, where H is the thickness
of the planetary boundary layer (PBL): coincidentally,
the dune spacing then tends to ~H. At this point, the
flow over the dune becomes constricted (much like at
an obstacle in shallow water) and the shear at the crest
suppresses further growth. This concept appears to
describe the Namib sand sea, where the PBL grows
from ~300m near the coast to ~3km inland: it is pos-
sible that the PBL thickness may be higher in continen-
tal interiors, especially at the somewhat high elevations
of the Badain Jaran . This PBL thickness argument
appears to be consistent with dune heights on Titan
[15]. The Venus PBL has not been characterized it
may be that dune heights can at least establish a lower
limit on its depth. On Mars, the PBL can be 10km
thick, allowing (in principle) dunes some ~1km high,
but none near this size are observed. Presumably either
there is simply not enough sand, or the winds have not
operated in a constant regime for long enough to allow
sand accumulation at this scale. On Earth, and perhaps
Titan too [16], large (100m+) dunes retain some
memory of the last Croll-Milankovich climate cycle:
400m+ dunes on Mars require longer than a ~100kyr
cyle to grow.
Conclusions: Mars appears to have the tallest dune
known (~450-600m), consistent with it also having the
thickest PBL, although most Martian dunes are much
smaller, either due to local PBL suppression [17], or
due to incomplete growth since climate cycles estab-
lished the present wind regime, or to limited sand sup-
ply. The largest terrestrial dunes appear to be about
400m tall. Titan's dunes appear to have a mature pat-
tern, with few dislocations, suggesting they may have
reached a limiting height of ~175m, although the Titan
heights have not been widely-surveyed. Venus' sparse
dunes have only been measured to be 40-80m: in all
probability, the same issue that limits the number of
dunes on Venus (restricted sand supply) may also limit
their height. The simple-sounding question posed in
the title of this abstract illuminates interesting differ-
ences between dune worlds.
References :[1] Fenton, L. K., et al., Extraterrestrial
Aeolian Landscapes, in Treatise on Geomorphology,
pp.287-312 in vol. 11 Aeolian Geomorphology, pp,
edited by J. Shroder et al., Elsevier, 2013 [2] Lorenz,
R. and J. Zimbelman, Dune Worlds, Springer 2014
[3] Hayward, R. K., L. K. Fenton, and T. N. Titus
(2014), Mars Global Digital Dune Database (MGD3):
Global dune distribution and wind pattern observa-
tions, Icarus, 230, 38–46, [4] Bourke, M. C., et al.
(2006), A comparison of methods used to estimate the
height of sand dunes on Mars, Geomorphology, 81(3-
4), 440452 [5] Lorenz, R. D. et al., The Sand Seas of
Titan : Cassini RADAR observations of Longitudinal
Dunes, Science, 312, 724-727, 2006 [6] Neish, C. D. et
al., Radarclinometry of the sand seas of Namibia and
Saturn's Moon Titan, Icarus, 208, 385-394, 2011 [7]
Lorenz, this meeting [8] Andrews, S., 1981. Sedimen-
tology of Great Sand Dunes, Colorado. In: F.P.
Ethridge, R.M. Flores (Eds.), Recent and Ancient Non
Marine Depositional Environments: models for explo-
ration. The Society of Economic Paleontologists and
Mineralogists, Tulsa, OK, pp. 279-291. [9] Gabriel, A.,
1938. The Southern Lut and Iranian Baluchistan. The
Geographical Journal, 92(3), 193-208. [10] Lancaster,
N., 1989. The Namib Sand Sea: Dune forms, process-
es, and sediments. A.A. Balkema, Rotterdam. [11]
Yang, X. et al. , 2011. Formation of the highest sand
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[12] Wilson, I.G., 1973. Ergs. Sedimentary Geology,
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Les dunes pyramidales du Grand Erg Oriental. Travaux
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[14] Andreotti, B., et al (2009), Giant aeolian dune
size determined by the average depth of the atmospher-
ic boundary layer., Nature, 457(7233), 11203 [15]
Lorenz, R. D.,et al (2010), A 3km atmospheric bounda-
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et al., (2008), The depth of the convective boundary
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C., Hayes, A. G., & Lucas, A. (2015). Sand dune pat-
terns on Titan controlled by long-term climate cycles.
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R. Barnes (2013), Mesoscale Modeling of the Circula-
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Acknowledgement: This work is supported in part
by NASA Grant NNX13AH14G via the Cassini pro-
ResearchGate has not been able to resolve any citations for this publication.
The most recent Cassini RADAR images of Titan show widespread regions (up to 1500 kilometers by 200 kilometers) of near-parallel radar-dark linear features that appear to be seas of longitudinal dunes similar to those seen in the Namib desert on Earth. The Ku-band (2.17-centimeter wavelength) images show ∼100-meter ridges consistent with duneforms and reveal flow interactions with underlying hills. The distribution and orientation of the dunes support a model of fluctuating surface winds of ∼0.5 meter per second resulting from the combination of an eastward flow with a variable tidal wind. The existence of dunes also requires geological processes that create sand-sized (100- to 300-micrometer) particulates and a lack of persistent equatorial surface liquids to act as sand traps.
Over the last half century, spacecraft visits to the many worlds in our Solar System have revealed that the surfaces of no fewer than four planetary bodies are subject to aeolian processes. These worlds beyond Earth include the planets Venus and Mars, as well as a moon of Saturn, Titan. Each body shows the influence of the wind in a unique way, with our understanding strongly controlled by the quantity and type of data returned from spacecraft investigations. Of these worlds, the best studied is Mars, which is prone to dust storms and is freckled with bedforms, yardangs, and wind streaks. Venus is spanned by thousands of wind streaks, but bedforms and yardangs are rare (or unresolved by available data). Now known to be the sandiest world in the Solar System, Titan's equator is belted by vast sand seas made of hydrocarbon grains. Although conditions vary widely across the Solar System, the depositional and erosional processes acting on these worlds are in many ways similar to those observed on Earth. As a result, concepts and methods developed for studying terrestrial aeolian landforms can be applied to their planetary counterparts. The factors controlling dune field sediment state are the same (sediment supply, availability, and wind transport capacity), although they are commonly controlled by very different processes on other worlds. Emergent structures (i.e., bedforms) are self-organized; thus, they are controlled by dune-scale processes regardless of where they form, so that extraterrestrial dune field patterns may be analyzed with the same techniques used on Earth (e.g., pattern analysis, gross bedform-normal transport). Scaling relationships for elementary bedforms have been developed, which correlate well with bedforms formed under varying conditions. However, these relations do not appear to hold everywhere (particularly on Venus and Titan); this may be a result of low resolution data that cannot resolve elementary features of the predicted size. Although not well observed on Venus or Titan, deflation and abrasion are active processes on Mars, scouring dusty surfaces and eroding materials into yardangs and ventifacts. However, it is likely that the timescales of erosion are much longer on Mars than on Earth. Continuing studies show that aeolian landscapes beyond Earth can appear at once hauntingly familiar and exotically alien. Although there are many fundamental unanswered questions about aeolian processes on Mars, Venus, and Titan, it is clear that this juxtaposition occurs because the processes that produce aeolian landscapes on Earth also operate elsewhere, but they do so under vastly different conditions and over very different time- and length-scales.
Eolian and adjacent deposits of Great Sand Dunes, Colorado form a small but sedimentologically complex deposit. Eolian sediments can be subdivided into 3 provinces: 1) low, alkali-cemented dunes forming discontinuous rings around broad, flat-bottomed, ephemeral lakes; 2) undulating, vegetated dunes of barchan, parabolic shrub-coppice, and transverse type, with varying interdune types; 3) high transverse dunes with little or no vegetation and no true interdune deposits. Analysis of a 40-yr span of aerial photographs and field observation of sand transport and cross-bedding dip directions indicate that the main dune mass is accreting vertically and that dune types are growing in complexity. This change from lateral migration to vertical growth most probably reflects Holocene changes in wind regime. The Great Sand Dunes are an example of a localized, cool-climate, intermontane eolian deposit, characterized by extensive fluvial reworking. With its rapid variation in thicknesses and sedimentary structures, such a deposit would be difficult to interpret accurately in the ancient rock record. However, such a deposit could be of economic importance in petroleum and uranium exploration, and in aquifer evaluation. -from Author
Lancaster (geology, Arizona State U.) reports on investigations of dune forms, processes and sediments in the Namib Sand Sea of southwestern Africa, which are used to develop models for the formation of dunes and sand seas and their rock equivalents. Annotation copyright Book News, Inc. Portland, Oregon, USA. Contents: Regional physiographic & climatic setting; Dune morphology & morphometry: Dune sediments; Dune processes; Controls of dune morphology; Accumulation of the sand sea. References; Appendices.
Characterization of dune morphology has historically been based on relationships between dune forms and wind regimes with dune height shown to be sensitive to atmospheric boundary layer depth, sand availability and sediment properties. While these parameters have been used in numerical simulations to model the occurrences of some types of dunes, they cannot alone explain the great diversity in form and size seen on Earth and on other solar system bodies. Here we present results from our studies of dune formation in the Badain Jaran Desert in western China, where Earth's tallest dunes occur. We measured the variability of the dune morphology in this desert on the basis of LANDSAT ETM+ data, and we detected the bedrock landforms beneath the aeolian sands by applying gravity methods. Wind records from stations at the periphery of the desert and SRTM topographical data were examined also to augment the interpretation. Our studies demonstrate that in addition to average wind parameters, dune height is highly sensitive to local geology, subsurface characteristics, and topography, and interactions between changing climate conditions and aeolian and fluvial processes. These additional factors need to be considered in the interpretation and simulation of dunes on Earth. We anticipate that analysis of anomalous dune heights like those seen in the Badain Jaran may also provide critical information on subsurface characteristics and environmental conditions on Earth and on other planetary bodies.
Radarclinometry is a powerful technique for estimating heights of landforms in synthetic aperture radar (SAR) images of planetary surfaces. In particular, it has been used to estimate heights of dunes in the sand seas of Saturn’s moon Titan (Lorenz, R.D., and 39 colleagues [2006]. Science 312, 724–727). In this work, we verify the technique by comparing dune heights derived from radarclinometry to known topography of dune fields in the Namib sand sea of western Africa. We compared results from three different image grid spacings, and found that 350 m/pixel (the same spacing at which the Cassini RADAR data was processed) is sufficient to determine dune height for dunes of similar morphometry to those of the Namib sand sea. At this grid spacing, height estimates derived from radarclinometry are largely representative of, though may underestimate by as much as 30%, or overestimate by as much as 40%, true dune height. Applying the technique to three regions on Titan, we estimate dune heights of 45–180 m, and dune spacings of 2.3–3.3 km. Obtaining accurate heights of Titan’s dunes will help to constrain the total organic inventory on Titan.