Geoscience Reference
In-Depth Information
2
Sand
When we hear the word 'sand', the visual image evoked is
most likely of sand in aggregate—of a pleasant beach or a
massive field of dunes. But the poet William Blake provides
an apt perspective on the inspiration that can come from
contemplating even a single, isolated particle:
short statement points out, it is important to realize that
'sand' formally refers to sediment particle size (which we
will quantify shortly), not to particle composition.
It is conventional too to think of sand as something—
usually quartz (see later)—that is broken down from a larger
mass of bedrock. However, from the viewpoint of sand
being particles of a given size or, broader yet in the context
of this topic, dune-forming material in general, this per-
spective is somewhat parochial. Snow forms dunes, yet is
crystallized in the air from water vapor; Titan's sand may
start in a similar way, perhaps agglutinated somehow on the
surface. Agglutination of dust is often responsible for the
formation of granules that form the crest of ripples on Earth,
and dustballs are suspected as particles in high-altitude
martian bedforms (where the air density is so low that it
would be hard to move solid particles). So material can
grow into sand, not just be broken down into it.
Let's start with snow, as it defines an instructive end-
member in the spectrum of dune-forming material. Because
it often has a very low density (bulk snow can be 100 kg/m 3 ,
about one-tenth that of water, although this is typically due
to porosity between grains or flakes; the effective density of
a fractal snow particle may be typically more like 300 kg/m 3 ,
of course only a third as dense as a solid piece of ice), the
drag at even low air speeds is comparable with its weight,
and thus snow swirls in every eddy of wind, rather than
leaping in little ballistic hops. In this sense, snow dynamics
are sometimes more akin to sand underwater than to sand in
air. A distinctive feature of snow, of course, is that it can be
sticky, and so rather than clean slip faces on the lee side of
dunes or drifts, it can form dramatic overhanging cornices, or
accumulate into long streaks (Fig. 2.1 ).
Evaporite minerals can form dunes, most notably gyp-
sum sand, famous at New Mexico's White Sands. This
material, being both water-soluble and much softer than
quartz, cannot migrate for long before it either becomes
cemented or ground into dust (see also Synkiewicz et al.
2010). Gypsum appears to be a major component of some
Martian dunes, too.
To see a world in a grain of sand,
and heaven in a wild flower,
hold infinity in the palm of your hand,
and eternity in an hour.
(from
'Auguries
of
Innocence',
1803;
in
The
Pickering
Manuscript, published 1863)
For an enchanting and accessible discussion of sand and
its relationship to our planet and its inhabitants, we rec-
ommend Michael Welland's book, Sand: The Never Ending
Story (2009). A good paragraph-length summary of the
microcosm exposed in grains is this by Raymond Siever
who
expanded
more
scientifically
upon
Blake's
poetic
sentiment:
A single sand grain, an irregularly shaped fragment of rock, is
the mute record of former mountains, rivers, and deserts, and of
millions of years of the Earth's upheavals and quiescence. To
make a grain tell us its history, we tear it apart bit by bit to find
out its crystal structure, its chemical composition, its radioac-
tive age, its external shape, and its internal strain. Yet we
cannot tell all we want to know of a sand grain's origin from its
composition alone, any more than we can deduce political
history from human physiology. The context of the state of the
world's continents and oceans at a particular time is the
background. That grain was produced by forces that made the
rock it was eroded from, by the Earth's surface environment
that eroded it from its parent and carried it to a resting place,
and by the internal deformation of the Earth's crust that buried
it. (Siever 1988, p. 1)
Here we attempt to expand upon Siever's terrestrial point
of view, to consider sand and its associated landforms
identified during spacecraft exploration that has been con-
ducted in the other worlds of the solar system.
We can start by returning to the simple question: what is
sand? A typical definition for sand is 'a hard, granular rock
material finer than gravel and coarser than dust'. As this
 
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