Geoscience Reference
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stickiness of particles near the melting point means that
cornices can develop at the brink, rather than slipfaces.
In Antarctica, there exist sets of quite distinctive aeolian
structures, called snow megadunes. These are only 1-8 m
high (measured in the field with GPS), but some 2-6 km
from crest to crest and many tens to hundreds of kilometers
long. Their topographically subdued character meant that
they went largely unnoticed by explorers on land, although
pilots did see them (e.g., Figs. 11.17 and 11.18 ). It was only
satellite imaging (Figs. 11.19 and 11.20 ) that revealed the
vast
extent
of
these
features,
which
cover
some
900,000 km 2 of the East Antarctic Plateau.
The structures move very slowly, if at all (the ice on
which they sit is itself moving at a few meters per year) and
form in the near-continuous strong katabatic winds that
descend across the plateau. The subtle appearance of the
dunes is due to a textural difference between dune and in-
terdune, which is also manifested in the microwave scat-
tering properties (Lambert and Long 2006), making the
dunes very distinct in radar imaging (Fig. 11.18 ). This
textural difference is likely attributable to different recrys-
tallization rates. The spacing of the features (they don't
really meet the definition of dunes or ripples) has been
speculated to be due to a standing wave in the atmospheric
boundary layer, that somehow influences the recrystalliza-
tion process (Fahnestock et al. 2000).
Although the dune topography is very subtle, it has been
measured remotely: large but shallow structures lend them-
selves to measurement by the precise range measurements
from satellite laser altimeters, which yield measurements
consistent with the field determinations (interferometric
radar was also used with similar results) (Figs. 11.19 and
11.20 ). Field access can be challenging (for obvious rea-
sons)—beyond the overall challenges of work in Antarctica,
local mobility by snowmobile and sled can be restricted by
sastrugi, low ridges of packed snow that form in a downwind
direction (visible in Fig. 2.1 ). These form by a combination
of erosion and accretion, somewhat like yardangs or the lee
dunes that form in cohesive sands in China as described by
Rubin and Hesp (2009).
Fig. 11.15 A kite camera image of White Sands. The brilliant white
sand loses much contrast at mid-day, sunset is by far the best time to
observe the morphology. The kite here is at an altitude of about 150 m,
a vehicle is visible at bottom left. Note the serried barchanoid ridges
and flat interdunes—a curved berm is seen at the bottom, where the
sand is bulldozed to keep a loop road and parking area clear for
tourists. Note also the regular wave surface on the dunes at bottom
right, presumably due to this being the destabilization wavelength for
this sediment. Compare the morphology with airborne lidar topogra-
phy in Figs. 18.16 and 18.17 . Photo R. Lorenz
comprise the largest features in the dune field, which is now a
state-protected natural park. A bimodal wind regime occurs
here (see Fig. 3.11 ) , so that the dunes tend to grow vertically
rather than extend horizontally. The Bruneau dunes are being
investigated as analogs to Transverse Aeolian Ridges (TARs)
on Mars, some of which have been shown to have topographic
attributes very similar to reversing dunes on Earth (Zimbel-
man 2010).
11.4
Snow Dunes and Megadunes
Although a variety of different mineral sands on Earth can
form dunes, snow is a quite distinctive material in that the
bulk density of the solid is lower than other minerals (ice is
900 kg/m 3 , compared with *2600 kg/m 3 for quartz) and
snow particles can be appreciably porous, lowering the
density even further. This makes the dynamics of snow
somewhat different—particles trajectories are less ballistic
and more sensitive to turbulent fluctuations in airflow—in
the scaling theory of Claudin and Andreotti (2006, see
Fig. 4.15 ), snow plots as a separate point from sand.
Nonetheless, snow can form local drifts that are essentially
identical to sand dunes, and similar mitigation measures
(vegetation and fences) are sometimes applied to inhibit
transport onto infrastructure like roads and buildings. One
subtlety in snow dynamics is that the porosity allows
compaction to occur in large structures, and the enhanced
11.5
The Role of Vegetation in Dune Motion
Vegetation is a significant component of nearly all desert
areas on Earth. While deserts by definition receive limited
amounts of rain, even minimal quantities of moisture seem
to be sufficient to support a wide range of desert flora. The
hardiness of desert plants is of less concern here than is the
fact that the presence of almost any plant tends to have a
strong impact on the wind flow over the surface. Plants alter
the
boundary
layer
of
the
wind
velocity
profile
by
 
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