Sand - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 23

dominated by even rare exposures of water, Mars sands may
be dry for millions of years and processes that are not
usually noticed on Earth may become dominant. An inter-
esting example is the experiment by Merrison et al. (2010)
who found red coloration emerging in sand simply by
rolling the sand around in a flask of simulated Mars atmo-
sphere (CO 2 at 10 mbar pressure) for a few months, perhaps
indicating a tribochemical process.
Different minerals have different densities, leading to
different mobility in fluids such as water or air. Occasionally
this effect is deliberately exploited to concentrate materi-
als—panning for gold is one example—but more generally
it contributes to an often striking uniformity of composition
in a dune, or at least of layers within a dune (see Chap. 5 ).
The range of densities encountered for sands known in our
solar system at least is actually less (Table 2.3 ) than the
range of densities of sand-forming material on Earth,
although perhaps on some exotic world there are dense
diamond sands!

Unfortunately, we have absolutely no observational
constraints on the composition of sand on Venus. Although
several Soviet landers provided clear evidence that basalt is
the dominant rock type encountered at most of the landing
sites, even then, the process by which sand-size particles are
generated is not obvious. It may even be dominated by
impact; large radar-dark (and thus likely fine-grained or
smooth—see Chap. 18 ) parabolic deposits (Fig. 2.7 ) are
seen around recent impact craters that may contain an
abundance of sand-sized microtektite particles (launched
above the atmosphere from the impact fireball and win-
nowed by the atmosphere). The compositional variations of
sand throughout the solar system are the result of the unique
chemical and atmospheric conditions present on each
planetary body, where different groups of materials end up
being either more or less stable within the corresponding
surface environment. We have no idea, for example, how
methane rain might progressively alter the composition of
Titan's organic sands, if at all.