Earth Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 120

that planets are complicated places. While one would not
expect a desert near the equator (and indeed LM is not a
desert, having a rainy season), this locale, caught between the
Atlantic and the rain forest, is remarkable for having a wide
expanse of dunes up to 30 m high, with spectacular lagoons
between them (Figs. 11.10 , 11.11 and 1.8 ).
This, of course, begs the question of how the sand
interacts with the lagoons (which the first author can attest
are pleasant to swim in). Sand can move either by saltating
across the interdunes when they are dry, or simply by
marching the slip face forward. The latter process would, if
the lagoons never dried, result in the dune shrinking as it
marched forward. So the existence of this dunefield relies
on the seasonality of rainfall, which arrives December to
May. In the summer and fall, the lagoons often dry out,
allowing the dunes to be built back up, and sand transport to
resume (e.g., de M. Luna et al. 2012). The winds here are
strongly unidirectional (hence the barchan and barchanoid
forms) and cause a remarkable migration rate of 12 m or so
per year into the forest.
An interesting feature of these dunes are scars (small
curved ridges) that they leave behind upwind of the stoss
edge of the dunes. These result from cementation and/or
vegetation at the edge of the lagoon, and appear to form
roughly annually (e.g., Levin et al. 2009) trace about
30 scars behind some example dunes. Although reliant on
vegetation for their formation, they are an interesting analog
to the scars left by gypsum cementation at White Sands and
on Mars (see Chap. 5 ) .

Fig. 11.7 The extreme eastern end of the Rub' Al Khali sees dunes
straddling the UAE—Omani border (dark fence, with observation
towers every couple of kilometers) in this image looking north from a
linear dune. Two other linear dunes are visible, as well as some
barchans on the flat desert floor, made bright by gravels containing
calcrete. To the east of here the gravels (from alluvial fans coming off
the Oman mountains) win and the dunes peter out: to the west and
southwest across the UAE and into Saudi Arabia, the dunes extend for
a thousand kilometers. Photo R. Lorenz

11.3.7 Atacama

Located along the Pacific coast of South America, the At-
acama desert is a coastal desert formed on a plateau about
1000 km long, found mostly in northern Chile, but also
extending into adjacent Andean countries. The Atacama has
the distinction of being the driest place on Earth; average
annual rainfall measured near the Chilean town of Anto-
fagasta is only 1 mm. Some weather stations have never
received any measurable rain, and written records indicate
one portion of the desert perhaps received no rain between
1570 and 1971! An obvious consequence of this limited
rainfall is that very little vegetation occurs within the Ata-
cama desert, limited mostly to a few very hearty cactus.
There are also very few sand dunes here; instead, the Ata-
cama is more of a rocky desert (although there are some
notable barchans on the Peruvian coast). Soils found in this
hyper-arid region have been proposed as analogs to soils on
Mars; in particular, perchlorates identified in the surface
materials analyzed by the Phoenix lander are quite common
within some Atacama soils.

11.3.9 Great Victoria

The largest desert in Australia, the Great Victoria desert is
located in the southwestern part of the continent. This desert
covers 424,000 km 2 (164,000 mi 2 ) and it consists of indi-
vidual sand hills stabilized by grasses, with grassland plains
covering the interdune regions. Like portions of the Ata-
cama desert, some areas within the Great Victoria desert are
dominated by rocks rather than by sand. The Aussie word
'gibber' has become common in the geologic literature for
such rocky plains, which consist of a closely spaced surface
layer of pebbles and cobbles, all glazed with a distinctive
iron oxide coating.

11.3.10 Simpson

Len ¸ ´ is Maranhenses 11.3.8

The Simpson desert is the fourth largest desert in Australia,
forming the 'red center' of the continent. The Simpson
desert covers 176,000 km 2 (68,000 mi 2 ), less than half the
area of the Great Victoria desert, but it contains some of the

This remarkable dune site is a national park on the northeast
coast of Brazil. Compared with the other deserts listed here it
is insignificant (1500 km 2 in area) but it is a useful reminder