Progress report
Aeolian processes on the
terrestrial planets: Recent
observations and future focus
Robert A. Craddock
Smithsonian Institution, USA
Abstract
Aeolian dune fields have been described on Earth, Mars, Venus, and Titan. The amount and fidelity of data being
returned from orbiting spacecraft and landers have enabled a new era in aeolian studies. This progress report
presents an overview of the latest planetary geomorphic studies characterizing aeolian processes on
extraterrestrial surfaces. Our understanding of aeolian processes on other planetary surfaces comes largely
from Earth analog studies, along with wind tunnel experiments and theoretical modeling. However, an
important difference is that unlike terrestrial dunes most dunes on Venus and Mars are composed primarily
of basaltic particles. Additional research is needed to understand how basaltic particles weather both
physically and chemically so that it will be possible to apply traditional sedimentological concepts, such as
sediment maturity, to understanding aeolian processes on Venus and Mars. It may also be possible to
characterize sediment maturity and provenance through remote sensing data once we have a better
understanding of basaltic sediments. Although there have been a variety of dune forms identified on the
surfaces of the other terrestrial planets, the only dune form found on all of them is linear dunes. Even though
linear dunes are the most common dune forms on Earth, we currently have a poor understanding as to how
they are formed, and additional work is needed to understand these features.
Keywords
aeolian processes, basaltic particles, dunes, Mars, planetary geomorphology, Titan, Venus
I Introduction
Aeolian processes have significantly modified
the surfaces of all the terrestrial planets that have
appreciable atmospheres, including the Earth,
Mars, Venus, and Saturn?s moon Titan. This
progress report presents an overview of the latest
planetary geomorphic studies characterizing
aeolian processes on extraterrestrial surfaces,
and builds on a previous progress report pre-
sented by Tooth (2009). While these studies are
conducted using imagery and remote sensing
data returned from orbiting spacecraft and land-
ers, our understanding of planetary aeolian pro-
cesses is based largely on Earth analogs coupled
with theoretical modeling and wind tunnel
experiments. In particular, Earth analogs are
increasingly important as our knowledge of the
similarities and differences between aeolian pro-
cesses and dune forms on the other planets con-
tinues to grow. Thus, researchers who specialize
in terrestrial aeolian processes or who investi-
gate terrestrial dune forms can provide valuable
Corresponding author:
Center for Earth and Planetary Studies, National Air and
Space Museum, MRC-315, Smithsonian Institution,
Washington, D.C. 20560, USA
Email: craddockb@si.edu
Progress in Physical Geography
36(1) 110?124
? The Author(s) 2011
Reprints and permission:
sagepub.co.uk/journalsPermissions.nav
DOI: 10.1177/0309133311425399
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input into our understanding of extraterrestrial
surface processes. Three areas where Earth ana-
logs are extremely relevant are discussed, includ-
ing studies of basaltic dune fields, laboratory
analyses, and the formation of linear dunes. It is
hoped that the summary presented here may gen-
erate some interest by the terrestrial geoscience
community into investigating aeolian processes
on the other planets.
II Current understanding
1 Venus
Because of the thick carbon dioxide atmosphere
the pressure at the surface of Venus is*90 atm
(9000 kPa), which is equivalent to conditions
experienced at a depth of 900 m in sea water
on the Earth. Despite the extreme differences
compared to typical terrestrial surface condi-
tions, there are theoretical (Iversen et al., 1976)
and empirical data indicating that aeolian pro-
cesses are possible on Venus. In fact, wind tun-
nel simulations show that the threshold velocity
of a 75 mm particle is only*0.28 cm/sec, indi-
cating that large amounts of material could be
easily transported on Venus (Greeley et al.,
1984). Basically, the enormous atmospheric
pressure at the surface of Venus should allow
even large particles to be transported with a
slight breeze. However, the direct evidence for
aeolian features on Venus is limited.
Basilevsky et al. (1985) suggest that small bed-
forms and layered rocks imaged by the Venera
landerswere formedbyaeolianprocesses.Orbital
radar data from the Magellan spacecraft indicate
that wind streaks are frequently associated with
impact craters and some tectonically deformed
terrains, both ofwhichmayprovide the source for
fine-grainedmaterials (Greeley et al., 1992). Lin-
ear wind streaks having a length to width ratio
>20:1 are themost common and widespread aeo-
lian feature on the planet (Greeley et al., 1992,
1995). There are also putative yardangs on
Venus, which occur in a field *40,000 km2
in size, 300 km southeast of Mead crater
(9N, 60.5E). These features are 25 km long by
0.5 km wide on average, with a spacing of
0.5?2 km (Greeley et al., 1992, 1995). They
appear to differ from linear wind streaks in that
they have sharply definedmargins and do not ori-
ginate from any topographic feature. However,
the resolution of the Magellan images is limited,
and interpretation of these features as yard-
angs is complicated by the interfingering of
bright and dark wind streaks throughout the
region (Greeley et al., 1992). It is possible
that the spaces between these features, which
have been interpreted as erosional grooves
(Greeley et al., 1992, 1995), are actually swales
separating linear dunes.
Only two dune fields have been positively
identified on Venus (Greeley et al., 1992, 1995;
Weitz, 1993). The first is centered at 25S,
340E, *100 km north of the 65 km diameter
impact crater Aglaonice, and covers *1300
km2. The dunes in this field are 0.5?5 km long,
and neighboring wind streaks indicate that the
dunes are oriented transverse to the westward
flow of the wind (Figure 1). The source of these
dunes appears to be sediments from an ejecta out-
flow channel that was generated by the formation
of an impact crater (Greeley et al., 1995). A
Figure 1. Some examples of linear features on Venus
thought to have resulted from aeolian processes. (A)
Examples of radar-bright and radar-dark wind streaks
centered at 15N, 60.43E. Magellan spacecraft radar
illumination is from the left at an incidence angle of
46. (B) The radar-dark linear features in this figure are
putative yardangs. Magellan spacecraft radar illumina-
tion is from the left at an incidence angle of 46. Note
the scale bars at the bottom left of each image.
Source: From Weitz (1993)
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second, larger dune field is Fortuna-Meshkenet
located in a valley beween Ishtar Terra and
Meshkenet Tessera and covering*17,000 km2.
Localwind streaks indicate that the dunes are also
oriented transverse to prevailing winds toward
the west (Greeley et al., 1995). Comparison of
images taken eight months apart reveals that
there was no dune movement in the Fortuna-
Meshkenet dune field over this timeframe (Weitz,
1993); however,movementmayhavebeenbelow
the image resolution of*120 m.
Given that it may be unlikely that dune move-
ment would occur over spatial scales sufficient
to be visible in Magellan radar data (i.e. scales
of hundreds of meters), Weitz et al. (1994)
compared Magellan radar imagery collected
from cycle 1 and 2, with differing viewing geo-
metries. In some areas, differences in bright
and dark regions on the surface were apparent
when the data from the two cycles were com-
pared. Weitz et al. (1994) concluded that Bragg
scattering or subpixel reflections from the slip-
faces of microdunes could account for the dif-
ferences in radar brightness, and suggested that
microdunes may be present in parts of the
Southern Hemisphere of Venus, including near
Stowe, Guan, Daosheng, and Eudocia craters,
which could act as sources of fine-grained
material.
It is interesting that more aeolian features
have not been found on Venus given the capac-
ity of the atmosphere to transport sediments
(Greeley et al., 1984). Potentially, surfaceweath-
ering is slow and does not generate many sand-
sized particles. Some other process, such as
impact cratering,may be necessary to comminute
the necessary particles. This seems logical given
the close association many aeolian features have
with craters or tectonic features (Greeley et al.,
1992, 1995; Weitz, 1993). Additionally, dunes
can only be identified in radar data under limited
viewing geometries (Blom and Elachi, 1981,
1987), and it is possible that the Magellan
viewing geometries and resolution are not capa-
ble of identifying all the aeolian features that may
exist on Venus (Greeley et al., 1995,Weitz et al.,
1994).
2 Mars
For over a century we have had indirect evidence
of the importance of aeolian processes on Mars.
Historical telescopic observations showed that
dark features on the Martian surface waxed and
waned with the seasons (e.g. Kahn et al., 1992).
Originally this was interpreted to be the result of
vegetation undergoing seasonal cycles similar to
vegetation in temperate zones on the Earth (Gal-
lant and Hess, 1956). With the advent of space-
craft data, however, it became apparent that dust
from global storms, which occur fairly fre-
quently during Southern Hemisphere summer
(Hartmann and Raper, 1974; Kahn et al.,
1992), were simply hiding dark albedo markings
on the surface. Although there is abundant evi-
dence in the form of valley networks, outflow
channels and modified impact craters indicating
that the early history of Mars supported rainfall
and surface runoff (Craddock and Howard,
2002), aeolian processes have been a persistent
geologic agent for the last 3?4 billion years
(Carr, 2006), and the evidence for aeolian pro-
cesses is ubiquitous at all scales from orbital data
(e.g. Edgett and Christensen, 1994) to lander
images (e.g. Greeley et al., 2004).
Mars exhibits many of the same dune forms
as seen on the Earth, including barchan, trans-
verse, yardangs, as well as star and climbing
dunes (Bourke, 2010; Chojnacki et al., 2010;
Fenton et al., 2005; Greeley et al., 1999; Zimbel-
man and Griffin, 2010). Also, perhaps some of
the most interesting images returned by theMars
Exploration Rovers show a series of large dust
devils advancing across the surface (Greeley
et al., 2006). Although not as common, seif and
linear dunes have also been recognized (Edgett
and Blumberg, 1994; Lee and Thomas, 1995).
Many dunes are located in the floors of impact
craters where sediment could accumulate
(Figure 2). Perhaps one of the most significant
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developments to be made recently is the ability
to determine sediment provenance and pathways
onMars, which is possible because of the variety
of high-resolution images that are available. The
High Resolution Stereo Camera (HRSC) with a
spatial resolution of*10 m/pixel (Neukum and
Jaumann, 2004) and the Mars Context Camera
(CTX) with a resolution of *6 m/pixel (Malin
et al., 2007) can provide the spatial coverage and
resolution necessary to identify and characterize
major dune fields along with their potential
sources. In addition, the High Resolution Ima-
ging Science Experiment (HiRISE) camera with
a spatial scale of 25?32 cm/pixel (McEwen
et al., 2007) and the Mars Observer Camera with
a resolution of 1.4 m/pixel (Malin et al., 1991)
provide the detail necessary to observe small
characteristics of individual dunes. Using these
data, Silvestro et al. (2010) were able to were
able to analyze the nature of complex dune field
patterns located in Aonia Terra (52S, 292.5E)
in the Thaumasia Quadrangle (MC-25) and
determine that there were at least two episodes
of dune construction, indicating that the local
wind regimes changed over time. They also
provided some of the first evidence of distant
sediment transport on Mars (tens of kilometers)
and identified the source areas as layered mate-
rials exposed in pits and crater walls.
High-resolution imagery has also provided
some of the first evidence for recent dune move-
ment on Mars. Silvestro and Fenton (2011) have
begun a systematic search to identify areas of
active sand transport outside of the polar regions
(+65 latitude) beginning with an analysis of
dune fields in the Arabia Terra region of Mars.
Using HiRISE images they found four sites
where active sand transport appears to be occur-
ring. The evidence is subtle, and most of the
areas showing changes are less than a fewmeters
in size. However, such observations are impor-
tant for understanding physical processes on the
surface. Results from such efforts will also lead
to a better understanding of the current Martian
climate and wind regimes.
3 Titan
Because sunlight is so faint at Saturn, it was orig-
inally thought that there would not be enough
energy from solar insolation to drive surface
winds, and thus Titan would not have any active
aeolian processes (Lorenz et al., 2006). How-
ever, climatic models suggest that the tidal pull
bySaturn generates pressure variations inTitan?s
atmosphere capable of driving near-surface
winds (Tokano and Neubauer, 2002). Titan has
a methane-rich atmosphere with a surface pres-
sure of*1.5 atm (146.7 kPa); coupledwith a low
gravity (0.14 g), this results in a threshold wind-
speed of only *10 cm/s necessary to move an
average sand-sized particle (Lancaster, 2006).
Observations from the Cassini spacecraft?s
Radio Detection and Ranging (RADAR) instru-
ment, which has a resolution similar to the
Magellan spacecraft?s radar images of Venus
(Lorenz et al., 2001), show radar dark parallel
features (Figure 3) that appear to be linear
dunes (Lorenz et al., 2006). These features are
Figure 2. Examples of linear dunes found in an
impact crater in Noachis Terra on Mars. The reddish
material is probably dust that has concentrated on
the northeast-facing slopes. Large boulders can be
seen in the dune swales. This image is approximately
500 m across, is centered at 42.66S, 38.02E, and
has a resolution of 0.25 m/pixel.
Source: HiRISE image ESP_016036_1370
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*100?150mhigh, have slopes of 6?10 (Lorenz
et al., 2006), are tens of kilometers long
(Radebaugh et al., 2008), and resemble terrestrial
linear dunes in all respects (Lancaster, 2006;Lor-
enz et al., 2006; Radebaugh et al., 2008, 2010).
However, Titan is an icy satellite, and instead
of consisting of quartz-rich sediments, the dunes
on Titan are most likely composed of ice parti-
cles that were eroded from precipitation and run-
off of liquid methane (Lorenz et al., 2006) or
they consist of hydrocarbon particles that were
generated by photochemistry in Titan?s strato-
sphere and that simply accumulated over time
(Wahlund et al., 2009; Yung et al., 1984).
Although the radar survey of Titan?s surface
is not complete, it appears that the linear dunes
are ubiquitous in the equatorial region between
+30 latitude (Radebaugh et al., 2008), possi-
bly due to Titan?s global atmospheric circula-
tion pattern (Lorenz and Radebaugh, 2009;
Radebaugh et al., 2008). Interestingly, the
slope orientations of the dunes suggests that
they are being driven by westerly winds (Lor-
enz et al., 2006), which is opposite to the wind
directions predicted by global climatic models
(Tokano and Neubauer, 2002). However, more
recent models of global circulation that were
integrated over an entire year on Titan sug-
gests that there may be occasionally fast, tur-
bulent westerlies initiated by the equinoctial
passage of the intertropical convergence zone
around the equator (Tokano, 2010).
The discovery of linear dunes on Titan has
several important implications for understand-
ing aeolian processes in planetary environments.
Dunes have now been recognized on all terres-
trial planets that have an appreciable atmo-
sphere, and aeolian processes are now known
to be literally universal in several meanings of
the word. Generation and transport of sediment
appears to be a basic geologic process on plane-
tary surfaces. Aeolian processes also adjust to
the environment and occur in a range of atmo-
spheric pressures and compositions as well as
variations in surface gravities. It is interesting
to note, however, that the only dune forms that
appear to be ubiquitous are linear dunes.
III Terrestrial analog studies
Terrestrial analogs represent places on the Earth
that, in some respect, approximate the geologi-
cal or environmental conditions thought to occur
on another planetary surface either today or
sometime in the past. Analog studies are impor-
tant for providing the ground truth for interpret-
ing data returned by spacecraft. Results from
analog studies can often improve our under-
standing of geologic processes here on the Earth,
and they are a useful way to test models
Figure 3. Some of the linear dunes on Titan can be
seen in this Cassini radar mapper image. The image is
centered near 19.2S, 257.4W, and covers an area
of 220 by 170 km. North is approximately toward
the top of the image, the radar illumination is from
the right, and the incidence angle is *25. The ver-
tical stripe across the image at its center is a process-
ing artifact.
Source: Cassini radar image PIA11802
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developed to explain the formation of terrestrial
features or processes. Historically, analog stud-
ies have also been used to support astronaut
training (El-Baz, 2011), and have been impor-
tant for developing and testing exploration tech-
nologies, such as the Mars Exploration Rovers
(Cook, 2005). Le?veille? (2010) provides a useful
summary of analog studies that have taken place
over the last 50 years. In particular, there are
three areas of research that promise to increase
our understanding of aeolian processes on the
terrestrial planets, especially in regard to Mars.
1 Weathering of basaltic materials
Dunes composed of volcaniclastic sediments
are found on Venus and Mars. Specifically, the
Venusian and Martian dunes are derived from
basaltic materials, which consist of olivine,
pyroxene, feldspar, and both lithic and vitric
fragments. In contract, terrestrial dunes are
typically composed of quartz-rich sand, but in
some rare instances they may also be composed
of gypsum (Szynkiewicz et al., 2010), carbonates
(Fletcher et al., 2005), or clay aggregates (Dare-
Edwards, 1984). Edgett and Lancaster (1993)
recognized that dunes composed of reworked
volcaniclastic sediments make up a rare but
important additional composition class of terres-
trial dunes. There are only a few places on Earth
where dunes are also derived from a basaltic pro-
venance, including parts of the western United
States, New Zealand, Iceland, and Hawaii
(Edgett and Lancaster, 1993). To date, studies
of these basaltic dunes have been limited.
Edgett (1994) conducted field studies and
an analysis of Thermal Infrared Multispectral
Scanner (TIMS) data for the Shifting Sand
Dunes of Christmas Valley, Oregon. He showed
that differences in thermal emissivity, which
relates to the physical characteristics of the sedi-
ment, can be useful in differentiating active from
inactive dune sand as well as in distinguishing
interdune surfaces. He suggests that additional
infrared spectra of basaltic materials would be
useful in constraining Martian remote sensing
data. As discussed below, utilizing multispectral
data for determining the provenance of sedi-
ments on Mars is promising (e.g. Tirsch, 2009).
More recent work on basaltic dunes has been
conducted in Iceland by Baratoux et al. (2007)
and Mangold et al. (2010). They show that sedi-
ments deposited as the Langjokull glacier
retreats are transported by katabatic and prevail-
ing winds downslope to*8 km away. Abrasion
of local Eldborgir lava flows by the saltating
sand increases the amount of material available
for transport. Eventually olivine is preferentially
sorted so that it increases in abundance down-
wind across the sand sheet. These results suggest
that similar to quartz-rich sediment variations in
the composition of the basaltic material may be
used as a way of inferring maturity and transport
distances.
One of the largest basaltic dune fields on
Earth is in the Ka?u Desert of Hawaii. Gooding
(1982) analyzed some of the materials in this
area and determined that the sand was derived
largely from the Keanakako?i tephra formation,
which is a sequence of ash and tephra that has
been deposited from periodic phreatic eruptions
that Kilauea volcano has experienced over the
last 2000 years (Fiske et al., 2009). Gooding
(1982) suggested that the material in the dunes
he sampled were too well-sorted to have
been emplaced directly from a base surge
(Christiansen, 1979). This is important because
it implies that the dunes in the Ka?u Desert were
formed by aeolian processes and not directly by
volcanic processes, and is thus one of the few suit-
able terrestrial analogs for understanding basaltic
dunes on the other planets.
Many questions about basaltic sediments
remain. For example, how does basaltic material
change physically and chemically during trans-
port?What mineralogy can be used to determine
provenance or transport distance? In the
absence of quartz sand, what are the most
reliable mechanisms for age-dating basaltic (vol-
caniclastic) dunes? How do the characteristics of
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basaltic sand differ between fluvial, glacial, or
aeolian sediment transport? Addressing such
questions through terrestrial analog studies of
basaltic dunes here on Earth could improve our
understanding about aeolian processes on the ter-
restrial planets as well as the nature of dunes seen
on Venus and Mars.
2 Laboratory analyses
Hyperspectral visible and near-infrared (VNIR)
data of Mars have been collected by the
Thermal Emission Spectrometer (TES) on Mars
Global Surveyor (Christensen et al., 2001), the
Observatoire pour la Mine?ralogie, l?Eau, les
Glaces et l?Activite? (OMEGA) spectrometer on
Mars Express (MEX) (Bibring et al., 2005), and
Mars Reconnaissance Orbiter (MRO)/Compact
Reconnaissance Imaging Spectrometer for
Mars (CRISM) (Murchie et al., 2007). TES
collected two types of data, hyperspectral
thermal infrared data from 6 to 50 micro-
meters (mm), and bolometric visible-NIR
(0.3 to 2.9 mm) measurements at a spatial res-
olution of 3 km. OMEGA acquires spectra in
352 contiguous channels covering 0.35?5.1
mm with a spatial resolution of 300 m to
4.8 km (Bibring et al., 2005) while CRISM
collects *10 km wide images from 0.36?
3.9 mm at 18 m/pixel in the high-resolution
targeted mode (Murchie et al., 2009). Among
the many exciting discoveries made by these
instruments is the identification of both Al-
bearing and Fe-bearing phyllosilicates (Bibring
et al., 2006; Poulet et al., 2005) in the older terrain
onMars, suggesting that deep chemical weather-
ingoccurredearly in that planet?s history (Bibring
et al., 2006), which could also be an important
process for producing fine-grained material
suitable for aeolian transport (Craddock and
Howard, 2002).
OMEGA/CRISM have also been used to map
the distribution of Al- and Ca-bearing pyroxenes
and olivine minerals (Mustard et al., 2005). The
results from these instruments compare well in
general to the mineral maps made using TES
over larger footprints (Christensen et al.,
2001). There is a great deal of work needed to
determine the composition of aeolian sand from
these data. Most of the sand is mafic (Fenton
et al., 2003; Poulet et al., 2008; Rogers and
Christensen, 2003; Stockstill-Cahill et al.,
2008; Tirsch, 2009; Tirsch et al., 2011) ranging
from a Type 1 surface composition (Bandfield
et al., 2000), which has a spectrum similar to
basalt, to a Type 2 surface composion, which has
been interpreted as basaltic andesite (Bandfield
et al., 2000) or a basalt containing clay minerals
(Wyatt and McSween, 2002) or amorphous
silica (Kraft et al., 2003). Interestingly, OMEGA
data have also shown evidence for dunes com-
posed of gypsum in the northern polar erg (Fish-
baugh et al., 2007; Langevin et al., 2005;
Szynkiewicz et al., 2010).
There are also a number of recent studies that
suggest VNIR data can be useful for determining
the provenance of aeolian sand on Mars. For
example, Stockstill-Cahill et al. (2008) used var-
iations in mineral abundances observed in TES
multispectral data to determine the provenance
of dark dunes found in Amazonis Planitia cra-
ters. They determined that dark deposits proba-
bly resulted from aeolian erosion of local
basaltic lava flows and were subsequently redis-
tributed within the floors of these craters. Tirsch
et al. (2011) used OMEGA and CRISM data to
show that dark dune materials located in many
Martian craters originated from layered basaltic
materials exposed in the walls or floors of these
craters. Given that similar spectra were observed
in 70 different locations distributed across the
planet, they suggest that the source of the dune
sand may be from a global layer of ash that may
have been deposited from a large explosive
eruption sometime in the past.
In terrestrial dune studies the amount of quartz
present is often used as a way of determining
mineralogical maturity (Blatt et al., 1972; Muhs
et al., 1995). As mentioned previously, the study
by Mangold et al. (2010) suggests that olivine
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abundances may be useful for determining
mineralogicalmaturity in volcaniclastic systems.
A number of investigators have conducted
laboratory analyses to determine how volcaniclas-
tic depositsweather bothmechanically and chemi-
cally, while acquiring spectra to better understand
the remote sensing data we have for Mars.
For example, it has been suggested that
palagonitic tephra from Hawaii volcanoes are
good spectral analogs for Martian soil and dust
because of their general similarities at visible
to near-infrared wavelengths (VNIR, *0.35?
2.5 mm) (Adams et al., 1986; Bell et al., 1993;
Bishop et al., 1998, 2007; Evans and Adams,
1979; Hamilton et al., 2008; Morris et al.,
1990, 1993, 1996; Singer, 1982). Laboratory
analyses of the Keanakako?i tephra (Schiffman
et al., 2000, 2002) indicate that palagonization
occurs in response to hydrothermal alteration
and is typically isolated to caldera-boundary
faults. During palagonization the tephra devel-
ops a quasi- or nano-crystalline rind containing
smectite and other clays that eventually results
in consolidation of the tephra. Pedogenic weath-
ering also occurs, and the resulting products
reflect the local environmental conditions
(Schiffman et al., 2000). Under acidic conditions
(pH <6.0) the tephra undergoes dissolution and
develops opaline crusts on exposed surfaces.
Under neutral conditions (pH ? 6.5?7.8) the
resulting pedogenic product is dominantly
smectite in areas where the mean annual rainfall
is <50 cm/yr and dominantly kaolinite, allo-
phane and imogolite where the mean rainfall
>250 cm/yr. Such analyses are important for
characterizing the phyllosilicates found on Mars
(Bishop et al., 2008; McKeown et al., 2009).
Analyses of tephra from Haleakala, Maui,
indicate that the unaltered tephra are composed
of feldspar, glass, pyroxene, and olivine, and
alteration products include Fe-oxides, phyllosili-
cates, and sulfates, as well as amorphous Al-Si-
bearing material as shown by selected area
electron diffraction (SAED) (Bishop et al.,
2007). Hamilton et al. (2008) collected visible
to near-infrared (VNIR, 0.35?2.5 mm) and
middle-infrared (MIR, 1800?250 cm-1) spectra
of basaltic tephras from Mauna Kea volcano that
were altered under ambient, hydrothermal, and
dry heat conditions. They found that while
MIR spectra of altered tephras identified major
alteration phases (cristobalite, oxide, phyllosili-
cate, and sulfate), the comparison of the tephra
spectra (<45 mm fraction) to dust spectra
retrieved from Mars Global Surveyor and Mars
Exploration Rover instruments did not provide
good spectral matches. Instead, they found that
the best MIR match is a tephra that has a
strong plagioclase feldspar transparency feature
and was altered under dry, high-temperature,
oxidizing conditions. However, this sample was
not a good VNIR analog and is not a process
analog, but it emphasizes the mineralogical
importance of plagioclase feldspar in Martian
dust (Bandfield and Smith, 2003).
In fact, to date no single tephra has been
found to be a good spectral analog across the
VNIR and MIR spectrum, and none of the
tephra samples examined are ideal matches
to Martian spectra at all wavelengths. How-
ever, spectral features have been observed to
vary with decreased temperature and pressure
and spectral features attributable to H2O are
visible (Bishop et al., 2007; Hamilton et al.,
2008). It is possible that basaltic materials col-
lected from other localities may have spectral
signatures that more closely match those from
Mars. More importantly, studies to date have
only analyzed material that has been altered
in situ. It is likely that sediment transport
would affect the bulk composition of Martian
surface materials. For example, the plagioclase
feldspar component observed in the Martian
dust (Bandfield and Smith, 2003) may reflect the
stability or preferential selection of this mineral
following aeolian transport. Laboratory analyses
of basaltic materials that have been physically
transported by aeolian (and fluvial) processes
may help us better understand and interpret
remote sensing data from the terrestrial planets.
Craddock 117
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3 Linear dunes
Linear dunes appear to be the only dune form
that occurs on all the terrestrial planets with an
atmosphere, including Venus, Mars, and Titan.
On Earth they are the most common dune form,
accounting for nearly 40% of all dunes (Bristow
et al., 2000; Lancaster, 1982). Linear dunes
(Figure 4) are characterized by their straight to
irregularly sinuous, elongated shape. Typically
the width of a linear dune is only a few tens of
meters or less, but the length of an individual
dune can often exceed many tens to hundreds
of kilometers. Generally they are found in
semi-arid to arid regions where the regional
wind speeds and directions are highly variable.
Despite their common occurrence, it is still
not clear how they form. Currently there are
three possible models for linear dune formation:
a Linear extension. Twidale and Wopfner (1990)
suggest that sand is derived from a single source
downwind of the dune field and is transported
over great distances as the linear dunes grow
forward along the snout. The sand located in the
swales is either blown off existing dunes or sim-
ply has not yet been incorporated into a dune.
b Wind-rift. There are two slightly different
wind-rift models, but both imply that the dune
sand was derived locally and then transported
over short distances. King (1960) suggested that
linear dunes accrete vertically and uniformly
along the length of the dune. Alternatively, Pell
et al. (1999, 2000) suggested that sand is depos-
ited only in the lee of the advancing dune snout.
The morphology of the dune advances down-
wind, but the sand is not transported any great
distances.
c Lateral migration. This is the latest theory borne
out by studies of dunes in the Namib Desert
(Bristow et al., 2007a) and in the Camel Flat
basin within the Simpson Desert (Hollands
et al., 2006). This theory supports dune forma-
tion primarily from vertical accretion of locally
derived sand. However, it suggests that linear
dunes also migrate laterally over time and
smaller dunes eventually coalesce into larger
ones.
Linear dunes represent some of the largest
dune forms on any planet, which is one of the
reasons a unique solution to their formation has
not been realized. For example, it is difficult to
place any constraints on the age, composition,
and stratigraphy of a linear dune over its entire
length, which can often exceed a few hundred
kilometers. Additionally, ground-penetrating
radar studies (Bristow et al., 2000, 2007b) and
luminescence age-dating (Hollands et al.,
2006;Munyikwa et al., 2000) indicate that linear
dunes are composite structures that are probably
the result of multiple episodes of aeolian activ-
ity. It is not clear how the stratigraphy of linear
dunes may relate to past climates or wind
regimes. Whether linear dunes form in a
bidirectional wind regime (Lancaster, 1982;
Parteli et al., 2009) or helical roll vortices (e.g.
Figure 4. Linear dunes in the Simpson Desert as
seen by an aircraft. These dunes are oriented
towards the northwest, are 10?40 m in height and
can be from one to several hundred kilometers in
length (Craddock et al., 2010). Interdune spacing is
typically between 100 m and 1.5 km and varies as a
function of height (Ambrose et al., 2002).
Source: Photograph by Robert A. Craddock
118 Progress in Physical Geography 36(1)
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Tseo, 1993) will also influence the preservation
of stratigraphy and the record of climatic changes
(Munyikwa, 2005), and in places such as central
Australia vegetation has also perturbed the upper
meter of sand, increasing the difficulty of placing
age-dates and climatic constraints on linear dune
formation (Bristow et al., 2007b). More research
is needed to understand the formation process of
linear dunes and to provide constraints on their
physical properties (e.g. grain size and stratigra-
phy).Additionally, the effects physical properties
have on remote sensing signatures in VNIR, ther-
mal, and microwave wavelengths must also be
assessed in order to better relate our understand-
ing of terrestrial linear dunes to the other planets
(Titus et al., 2008).
IV Summary
Given the increasing amount of high-resolution
imagery for Mars, the fidelity of the data being
returned for Titan, and the availability of data for
Venus, our understanding of aeolian processes
has entered into a new golden age of exploration
and discovery. Terrestrial geoscientists have a
unique opportunity to apply their knowledge and
skills to dune forms and aeolian processes on
these other planets. In particular, this progress
report discusses three areas of Earth analog stud-
ies which could potentially provide important
results. For a broader perspective, there are also
several recent papers that provide a more
community-based consensus of important future
research directions (Bourke et al., 2010; Fenton
et al., 2010; Titus et al., 2010). Obviously, the
aeolian processes discussed here occur in open
systems, so the concept of equifinality is an
important consideration. However, perhaps the
greatest benefit of conducting Earth analog stud-
ies is that they offer a mechanism for testing a
variety of hypotheses. Simply stated, any
hypothesis for explaining the formation of a dune
or some aspect of sediment transport on Earth,
for example, should also be applicable to the sur-
face of another planet. Results from planetary
geological studies will ultimately increase our
understanding of terrestrial processes as well.
Acknowledgements
This manuscript has benefited from many useful
discussions with Lori Fenton, Ron Greeley, Alan
Howard, Ross Irwin, Nick Lancaster, Ted Maxwell,
Gerald Nanson, Rob Sullivan, and Jim Zimbelman
through the years. I thank Tim Warner, Matt Balme,
and two anonymous reviewers for their thorough and
constructive reviews.
Funding
Support for the research that is presented in part has
come from NASA?s Mars Fundamental Research
Program Grant NNX09AC27G and funds from the
George F. Becker Endowment at the Smithsonian
Institution.
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