Introduction - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 2

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Introduction

This topic is about dunes and planets, the interactions of
earth and air.
Sand provides a paradox. It is a solid, and yet it moves.
This paradox applies to rocks in general—moving rocks are,
after all, what geology is ultimately about—but granular
materials on the Earth's surface can move much faster, at
rates that are important to, and observable by, humans. This
is due to the peculiar conditions and processes on our pla-
net, and results in some spectacular landscapes (e.g.
Fig. 1.1 ) in specific regions where the sand (and sometimes
snow) has apparently organized itself into magically regular
structures—dunes.
The ancients knew of four planets, wandering points of
light in the night sky, plus Earth and its moon. The invention
of the telescope led to the discovery of a few more planets and
a few moons, some of which are bigger than the smallest
planet. We know of hundreds of planets (many of which will
have moons too) around other stars. But more importantly
than the astronomical cataloguing of celestial objects has
been the Space Age and, by means of robotic exploration, the
transformation of some of these planetary bodies from mere
dots in the sky to worlds in their own right.
This unmanned space exploration perspective has shown
us that first Mars (Fig. 1.2 ), then Venus, and recently and
remarkably, Saturn's moon Titan have dunes similar to those
on Earth, despite very different conditions and materials.
Thus the formation of dunes and ripples is a general phe-
nomenon, unified by the same physical processes, even
though the conditions under which these processes operate
can be quite different (Fig. 1.3 ). This universality is perhaps
nowhere highlighted better than by the vast sand seas of Titan
where, despite sands made of organic muck, in frigid air four
times denser than ours, on a world with gravity only one-
seventh our own, the landscape is covered in dunes of exactly
the same shape, height and width of the Earth's largest sand
seas. Thus Titan is almost as exotic a world as one can

imagine, and yet standing on its surface are landforms
(Fig. 1.4 ) almost indistinguishable from those on Earth.
Our aim in this topic is to survey dunes on these worlds
with this physical perspective, highlighting the morpholog-
ical similarities and differences that are exposed by dra-
matically improved remote sensing instrumentation at Earth
and elsewhere. Over the 40 or so years of Mars exploration
since dunes were discovered, the quality of orbital images
(in terms of number of pixels per square kilometer) has
improved ten-thousandfold (Fig. 1.5 ), and after the first
pioneering wanderings of the Sojourner rover in 1997
(Fig. 1.6 ), what is now decade of continuous roving across
the surface by its successors has brought a 'field geology'
perspective (Fig. 1.7 ) with millimeter-scale imaging and
advanced scientific instruments brought to bear on individ-
ual ripples, showing us the sands of Mars at the grain level.
Dunes on Earth were first explored by field geologists
and geographers. While many of their traditional techniques
are still applied, new methods, such as GPS and photo-
grammetry using digital images from the ground or the air,
allow us to rapidly and quantitatively measure the shape of
dunes in the field (Fig. 1.8 ). We can now even probe the
internal structure of dunes, revealing the layers that record
the

history of deposition and migration, using ground-

penetrating radar (Fig. 1.9 ).
There have also been spectacular advances in physical
and computational modeling of the processes by which sand
and wind interact: it is now possible to follow the formation
of dunes, and indeed the interactions between dunes, in
silico. A wide range of dune morphologies can be replicated
via the successive application of very simple rules, taming
the bewildering diversity of the landscape under a unifying
algorithmic whip (Fig. 1.10 ). Furthermore, while observa-
tion of real dune formation and motion can take months,
years, or even millennia, their virtual counterparts can be
brought to animated life in seconds on the computer screen.