Laboratory Studies - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 195

Fig. 17.2 An X-ray diffraction
image of the Rocknest sand
shadow on Mars, taken by the
Chemin instrument on Curiosity.
The arcs correspond to different
diffraction angles and thus lattice
spacings in the minerals in the
soil; the overall pattern is similar
to that of volcanic rocks in
Hawaii. Image credit: NASA

sample, and the intensity with which the beam is scattered
away at different angles relates to the spacing of atoms in the
crystal lattice, which is characteristic of the mineral com-
position. (Essentially, the crystal structure is acting as a
diffraction grating, which relies on the spacing of the atomic
layers in the lattice being of the order of the wavelength of
electromagnetic radiation being diffracted by Bragg scat-
tering. For minerals, this spacing corresponds to the wave-
length of X-rays although, as we discuss in Chap. 18 , aeolian
ripples can act as Bragg gratings for microwaves in radar
remote sensing.) Laboratory instruments have beamed
X-rays at a sample, and swept an X-ray detector on a
goniometer through a range of angles. A more compact
implementation (the Chemistry and Mineralogy Instrument,
Chemin) has reached Mars for the first time on the Curiosity
rover—the X-rays scattered by a sample are imaged by a
CCD camera, with the various scattering angles forming
circular arcs (Fig. 17.2 ) on the detector whose radius relates
to the lattice spacing in the constituent minerals of the
sample.
Finally, a prominent goal in aeolian studies is age dat-
ing—this

paleoclimate studies. In addition to carbon-dating of buried
organic materials like plants, the principal laboratory tech-
niques here are OSL (Optically-Stimulated Luminescence)
and Thermoluminescence, which are ways of estimating the
time since sand grains were exposed to sunlight. In essence,
energetic particles (cosmic rays, and emission from radio-
active elements in the sand itself) cause defects in the
crystal lattice of sand-forming minerals like quartz, kicking
electrons out of the lattice essentially into metastable stor-
age sites. If these electrons are nudged out of these storage
traps and fall back into holes in the electronic structure of
the lattice, they may emit a photon of light, which can be
measured in sensitive instruments. The amount of light
emitted is a measure of the total dose of radiation seen by
the sand sample since some 'zeroing event' which cleared
the material of trapped electrons. Sunlight acts as a zeroing
process (so obviously the sand samples from a given depth
should not be exposed to light; elaborate protocols for
sampling, using light-proof steel tubes, must be used).
One way of nudging the electrons out of their traps in an
instrument is by heat (hence thermoluminescence). Another
is by exposing the sample to infrared light (the emitted light
is usually in the ultraviolet, and so the stimulating light and

is particularly important in efforts to relate

changing sand mobility and dune morphology to