Introduction - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 7

Fig. 1.9 The internal structure of a crescentic barchan dune in
Morocco is revealed in this Ground-Penetrating Radar (GPR) survey.
The transect (A-A') moves left to right in the downwind direction (top
to bottom in the orbital view) and shows the steeply-dipping former

slip faces that marked the previous position of the dune. Enough time
has elapsed between this survey (where the dune outline and transects
were measured by GPS) and the orbital image at right that the dune
has moved southwards. Image courtesy Charlie Bristow

Fig. 1.10 Modern coupled
simulation of the airflow and
sand transport, using a cellular
automaton sand model (showing
here a 5-pointed star dune).
Streamlines in a lattice gas flow
model are shown, influenced by
the evolving topography
underneath; the streamlines
compress at the dune crest, and
separate, leaving a vortex in the
lee of the dune. Image courtesy
Clement Narteau

a relative term—it depends on what else is going on. For
a dune to exist, it must form or move or repair itself
faster than other processes destroy it. Various geological
processes prevail to different degrees on different plan-
ets: on Earth, for example, there are relatively few
impact craters because fluvial or glacial erosion or other
processes have removed them. There are just a couple
where craters recognizable from orbit are in deserts with
prominent dunes (Fig. 1.12 ), yet this sort of interaction is
common on Titan where erosive activity, while present,
is less vigorous than on Earth. On Mars, in contrast, rain
has not been widespread for billions of years (if ever)
and thousands of craters are visible, acting as traps for
the sand.

On Earth, at least, there are dunes present where dunes
do not presently form or move—they are fossils of a past
climate. This brings another paradox, resurrected into the
wider planetary arena from eighteenth and nineteenth
century Earth science more generally: to what extent does
the landscape we see today represent processes happening
today, versus what processes may have happened in a
catastrophically-different past? This question has come to
the fore in studies of Martian dunes, a few of which have
only recently been observed to move, but others of which
may be very old and cemented by ice or evaporite min-
erals, and even on Titan where some dune-like features
stand distinct in location and orientation from the

majority.