Laboratory Studies - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 193

17

Laboratory Studies

Although dunes themselves are planetary phenomena, sand
and the processes that make it and shape it can be studied in
the laboratory. Laboratory settings allow more elaborate
analyses of sand than are possible in the field, enabling its
elemental composition to be measured with great sensitivity
and precision. Mineralogical and age analyses can also be
performed. Sand transport can be studied under much bet-
ter-controlled conditions in laboratory wind tunnels than in
the field, and detailed experiments on rates and thresholds,
and deposition patterns including interactions with scale
model topography or even model vehicles can be explored.
Remarkable recent experiments in water tanks expose the
dependence of bedform morphologies on wind direction
variations. A special challenge in the laboratory is to
reproduce in wind tunnels the atmospheric conditions on
other planets.

Krinsley and Doorkamp 1973; Smalley and Krinsley 1979)
and today grains can be studied at the atomic level by
Atomic Force Microscopy (AFM), a technique that has even
been applied on Mars by the Phoenix lander, yielding a
resolution of close to 10 nm, a hundred times better than
optical images.
Various methods are employed to assess the chemical
composition of sands. Traditional approaches using Bunsen
burners 1 and wet chemistry are progressively being sup-
planted by newer methods, although some simple wet
analyses are still useful and efficient (e.g., separating heavy
mineral grains via settling in a dense solution; salinity
measurements by washing the sand and measuring the
electrical conductivity, etc.) A proliferation of modern
techniques (perhaps stimulated by the commercial need to
discriminate them for patent purposes), is characterized by a
blizzard of acronyms: we introduce at least a few in the
following paragraphs.
A revolutionary technique to obtain elemental compo-
sition information is the so-called microprobe (or some-
times EMPA—Electron Micro Probe Analyzer) which uses
an electron beam to stimulate the sample to produce a
spectrum of X-rays; the energy of each line in the X-ray
spectrum is characteristic of an element, and thus by cali-
brating the strength of each line against known materials,
the elemental abundance of the sample (e.g., the relative
amounts of magnesium, iron, etc.) can be measured. Since
the electron beam can be aimed (and is usually conveniently
integrated as part of an electron microscope) to a spot just a
micron or so across, individual grains in a rock, or the
matrix between them, can be separately analyzed.
The same X-ray spectroscopy technique can be used by
using an X-ray source to stimulate the sample instead of an
electron beam (i.e., X-ray fluorescence, XRF), and handheld
XRF analyzers are now available which could be used in the

17.1 Sand Studies

While detailed investigations of sand texture and compo-
sition can be performed in the field, it is often more con-
venient to simply and quickly acquire dozens of samples in
bags or bottles during a field expedition, label them with
their sampling locations, and bring them back to the labo-
ratory. There they can be efficiently and systematically
examined and documented without the risk of notes blow-
ing away in the wind or sand getting under a laptop's
keyboard. Measurements like sieving can also be at least
semi-automated in lab conditions, and access to samples
from other locations for comparisons, and other institutions
(e.g., libraries) for information is easier.
In addition to sieving (or now, more commonly, optical
size classification) and examination of shape and texture
(see Chap. 2 ) , more detail can be derived by studying sands
through a microscope with crossed polarizing filters which
highlight mineralogical differences between the grains.
Even closer textural studies can be made with scanning
electron microscopes (SEMs) (Krinsley and Smalley 1972;

1

E.g. a grain of sand with even a trace of sodium placed into a
Bunsen burner flame will briefly turn the flame a characteristic yellow.