Introduction - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 5

Fig. 1.5 A mosaic of images of dunes in the Proctor crater on
Mars, assembled by J. Zimbelman. a A Mariner 9 image from 1972 (at
*60 m/pixel); b a Mars Global Surveyor image at several m/pixel in
1999; c-e progressive zooms into a massive image (PSP_006780_1320),
from the HiRISE camera on Mars Reconnaissance Orbiter in 2008, with a

resolution of about 50 cm/pixel. From the dunefield perspective we can
now zero in on individual dunes and even see (and observe the migration
of—see Chap. 8 ) the small superposed ripples. (Several of these images
are shown individually at a larger scale in Chap. 12 )

Fig. 1.6 First 'field trip' on Mars. The shallow bedforms in the
foreground are 10 cm or so high, but were hidden from the camera on
the Mars Pathfinder lander by the 'Rocknest' formation. This image
was only possible because the Sojourner rover trekked away from the

lander, revealing new territory. The familiar 'twin peaks' hills (also
visible from the lander) can be seen on the horizon. NASA/JPL image
PIA00965

These computer models also allow us to bridge the real
world and laboratory experiments (which themselves have
reached an impressive level of fidelity (Fig. 1.11 )).
This tremendous arsenal of data and tools has brought a
new level of understanding to planetary dune studies and
lets us offer answers to some basic questions.

2. Are sand dunes the same everywhere? Like the
canonical snowflake, no two dunes are exactly alike.
And yet there is clearly order in the bewitching infinity
of duneforms. The first job in science is usually to
classify objects into groups of more-or-less the same.
While that always entails some subjectivity, it is an
essential simplification. But once that is embraced, the
same sets of forms can be recognized on different plan-
ets, and in the computer, and in water tank experiments,
paving the way for a quantitative link between what we
see on dune worlds, and what wind and time was needed
to make what we see.
3. So what do the dunes tell us? There is a 'Goldilocks'
element to the formation of dunes. The particles
involved must in general not be moving, otherwise they
do not really define a landscape. And yet they must move
often enough to assemble into a dune. 'Often enough' is

1. How do sand dunes develop, move, and change shape?
In order to understand how sand moves, we first need to
establish what sand is, which can be very different in
other planetary environments. But once the different fluid
and gravitational forces that act on a sand grain are
identified, some simple math can tell us the basics about
what winds are required to set grains in motion, although
a prominent theme in modern aeolian studies is how to
tackle the complex and highly fluctuating character of
transport in turbulent winds.