Mars Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 134

Martian atmosphere. The two rovers have documented
particles ranging from dust and sand to granules and cob-
bles everywhere along the tens of kilometers of the surface
over which they have ranged.
Sophisticated remote sensing instruments on both landed
and orbiting spacecraft have revealed much about the fine
particles present on the Martian surface. Measurements of
the thermal infrared energy radiated from the surface pro-
vide important clues to the physical size of the constituent
surface materials. As discussed in Sect. 18.4 , thermal inertia
refers to how well (or how poorly) the surface maintains its
temperature through the course of a Martian day. Materials
with high thermal inertia (large blocks and rock outcrops)
tend to stay relatively cool through the day and warm
through the night, in contrast to low thermal inertia mate-
rials (such as a thick dust mantle). When multiple thermal
wavelengths are measured, the distribution of high or low
thermal

of Mars (Fig. 12.9 ), with the resulting collection of dune
forms displaying a considerable diversity across the planet
(Hayward et al. 2007). No obvious sources are recognized
for the identified dune deposits, although the north polar
sand may be derived from a specific dark basal unit within
the polar layered deposits. Dunes within craters appear to be
trapped within the topographic depression rather than
derived from a localized source exposed within the crater
rims. Mars also lacks obvious sinks for mobile sand; the
lack of current oceans makes it difficult to remove sand
from the system except through burial inside impact craters
that became buried by some later event.

12.4 Types of Dunes on Mars

A variety of dune types have been recognized on Mars over
the years, including barchan, barchanoid ridge, transverse,
star, linear, dome, and complex dunes (the last displaying
attributes of two or more dune types). Notably, almost one-
third of the dune fields identified in a global map of sand
dunes on Mars do not seem to fit easily into one of the dune
categories used to describe dunes on Earth (Hayward et al.
2007, 2010, 2012). The formation mechanism for Martian
dunes is thought to be essentially the same as the recognized
aeolian processes that generate similar dune forms on Earth.
However, Martian dunes tend to be up to an order of
magnitude larger than their comparable terrestrial counter-
parts, similar to the large size displayed by many other
Martian landforms. Here we will not necessarily discuss in
detail each of the Martian dune types, but rather present
examples of the more common types for comparison with
dunes on the other planets.
Barchans (see Chap. 6 ) are one of the most readily
identifiable dune types on any planet, due to the distinctive
crescentic shape of the entire dune mass. Barchans and
barchanoid ridges ( Chap. 6 ) are quite common near the
margins of the NPE, particularly where the sand supply
from the erg becomes more widely distributed as the sand
spreads onto the surrounding plains. Barchans are also
found at lower latitudes where the supply of sand is rela-
tively limited, but still sufficient to produce large and
impressive crescent-shaped sand deposits (Fig. 12.10 ).
Some Martian barchanoid ridges are seen in Figs. 6.13 and
6.15 .
One of the most common dune types on Mars is the
transverse ridge ( Chap. 6 ) , comprised of one prominent
crest line that is inferred to be transverse to the orientation
of the wind that blew the sand to its present location
(Fig. 12.11 ). Martian transverse features are often quite
symmetric with respect to the crest axis, making it difficult
to decide from which of the two transverse directions the
driving winds blew, but also suggestive that two nearly

inertia materials within a single pixel can be

quantified, even when the individual particles are not

resolved.
The thermal inertia of windblown deposits within impact
craters on Mars indicates that these deposits respond to
temperature changes as if they were comprised of a uniform
layer of coarse sand (400-600 lm in diameter), which is
larger than the fine sand that makes up most of the dunes on
Earth (100-200 lm in diameter). Spectroscopic measure-
ments of the dark deposits in craters also revealed that they
consist primarily of a basaltic composition, some showing
evidence of minerals called pyroxenes, which are common
in basalt rocks. The Opportunity rover was able to investi-
gate a dark sand deposit (named 'El Dorado') in the
Columbia Hills, confirming the basaltic composition of the
medium-to-coarse sand found there (discussed in more detail
below). Both rovers have also obtained evidence of localized
deposits of sand-sized aggregates that, when pressed upon
by the rover arm, break up into dust; such aggregates may be
similar to 'parna' on Earth, where sand-sized aggregates of
clay particles form dunes around dried lakebeds. Aggregates
on Mars likely could not survive the saltation required for
transport over long distances, but they do appear to con-
tribute to localized collections of sand-sized materials (e.g.,
see Figs. 16-20 of Sullivan et al. 2008).
The observed distribution of sand dunes is widespread
around Mars, but not in a uniform or systematic manner. By
far the largest accumulation of sand dunes occurs in the
sand sea that surrounds the north pole within the 70-80 N
latitude band. Portions of the north polar erg were recently
shown to include the spectral signature of the mineral
gypsum, a material common around the margin of some
saline lakes, leading to speculation that the dune field could
incorporate materials derived from ancient lakebed depos-
its. Outside of the north polar erg, more than 900 sand dune
patches larger than 1 km 2 have been mapped across the rest