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

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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. (Ralph.lorenz@jhuapl.edu) 2 SETI Institute, Mountain View, CA, USA. 3Desert Research
Institute, Reno, NV, USA.
Introduction
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.
Mars
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
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
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.
Earth
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.
Implications:
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.
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Acknowledgement: This work is supported in part
by NASA Grant NNX13AH14G via the Cassini pro-
ject.