JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. E1, 10.1029/2000JE001481, 2002
Terrestrial analogs to wind-related features at the Viking
and Pathfinder landing sites on Mars
Ronald Greeley,1 Nathan T. Bridges,2 Ruslan O. Kuzmin,3 and Julie E. Laity4
Received 1 March 2001; revised 12 October 2001; accepted 12 October 2001; published 31 January 2002.
[1] Features in the Mojave Desert and Iceland provide insight into the characteristics and origin of
Martian wind-related landforms seen by the Viking and Pathfinder landers. The terrestrial sites were
chosen because they exhibit diverse wind features that are generally well understood. These
features have morphologies comparable to those on Mars and include origins by deposition and
erosion, with erosional processes modifying both soils and rocks. Duneforms and drifts are the most
common depositional features seen at the Martian landing sites and indicate supplies of sand-sized
particles blown by generally unidirectional winds. Erosional features include lag deposits, moat-like
depressions around some rocks, and exhumed soil horizons. They indicate that wind can deflate at
least some sediments and that this process is particularly effective where the wind interacts with
rocks. The formation of ripples and wind tails involves a combination of depositional and erosional
processes. Rock erosional features, or ventifacts, are recognized by their overall shapes, erosional
flutes, and characteristic surface textures resulting from abrasion by windblown particles. The
physics of saltation requires that particles in ripples and duneforms are predominantly sand-sized
(60 –2000 mm). The orientations of duneforms, wind tails, moats, and ventifacts are correlated with
surface winds above particle threshold. Such winds are influenced by local topography and are
correlated with winds at higher altitudes predicted by atmospheric models.
INDEX TERMS: 5415
Planetology: Solid Surface Planets: Erosion and weathering, 5470 Planetology: Solid Surface
Planets: Surface materials and properties, 6225 Planetology: Solar System Objects: Mars, 6207
Planetology: Solar System Objects: Comparative planetology; KEYWORDS: Mars, aeolian/eolian,
ventifacts, Pathfinder, Viking landers, dunes
1. Introduction
[2] Images and other data from Mars returned by Viking
Landers (VL) 1 and 2 and the Mars Pathfinder (MPF) lander/
Sojourner rover show evidence for atmospheric interaction with the
surface in the form of various aeolian features. These include
deposits and rocks that are thought to have been modified by the
wind. These features have the potential to provide insight into the
nature and evolution of the areas in which they occur and clues for
the climate history on Mars. For example, analysis of the windrelated features at the MPF site suggests that some features were
formed under a paleowind regime different from the current wind
patterns [Bridges et al., 1999; Greeley et al., 2000; Kuzmin et al.,
2001], which might be attributed to a change in climate.
[3] The analysis of the putative aeolian features seen on the
surface of Mars is subject to interpretation. For example, many of
the rocks are pitted, and some investigators have suggested that the
pits are primary features such as vesicles in volcanic rocks [Carr,
1981], chemical weathering etchings, or the scars of microimpacts
[Vasavada et al., 1993]. Others have attributed the features to wind
abrasion [Mutch et al., 1977; McCauley et al., 1979; Greeley et al.,
1982; Greeley and Iversen, 1985; Bridges et al., 1999]. A common
approach in planetary geology to help resolve such uncertainties is
to study terrestrial analogs as a means for understanding the origin
1
Department of Geological Sciences, Arizona State University, Tempe,
Arizona, USA.
2
Jet Propulsion Laboratory, Pasadena, California, USA.
3
Vernadsky Institute, Russian Academy of Sciences, Moscow, Russia.
4
Department of Geography, California State at Northridge, Northridge,
California, USA.
Copyright 2002 by the American Geophysical Union.
0148-0227/02/2000JE001481$09.00
and evolution of planetary features, so long as differences in
environment and other factors are taken into account [El-Baz
et al., 1979]. In this study we review various features at the
landing sites on Mars and then compare them to potential terrestrial
analogs in the field. We first outline the general geology of the
Martian sites and our principal field areas and then compare
common features on the two planets. We conclude with a discussion of the implications for Mars.
1.1. Martian Landing Sites
[4] Three landers and a rover have returned images that provide
important information on Martian aeolian features and processes.
The regional geology of the landing sites is characterized by
Hesperian-aged plains modified by fluvial and impact gardening
after formation of their primary surfaces [Arvidson et al., 1989].
The rock abundances at the sites range from 15% (including
outcrops) at VL 1 to 16% at MPF to 20% at VL 2 [Christensen,
1986; Golombek and Rapp, 1997; Golombek et al., 1999a]. Most
of the rocks are probably basaltic, on the basis of data from the
SNC meteorites, spectral information, and cosmochemical considerations. Pathfinder measurements suggested that many of the
rocks at the MPF site could include andesitic basalts [Rieder
et al., 1997], but the degree to which this reflects a true rock
composition as opposed to a weathering rind and intermixed dust is
uncertain. Furthermore, other igneous rock compositions, impact
breccias, sedimentary rocks, and metamorphic rocks could also be
present, especially at the MPF and VL 1 sites, where outflow
channel deposits could include different rock types.
[5] The general geology of the VL 1 site [Carr et al., 1976] was
mapped from orbital data by Greeley et al. [1977] and described
from lander data by Binder et al. [1977], who noted a variety of
wind-related features, including bright drift deposits and pitted
rocks. McCauley et al. [1979] compared these features with
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5-2
GREELEY ET AL.: TERRESTRIAL ANALOGS TO WIND-RELATED FEATURES ON MARS
candidate terrestrial analogs and concluded that many of the pits on
Martian rocks were indicative of ventifacts, rather than vesicles.
Wind-related features in the VL 1 area seen from orbit include
bright streaks with an average azimuth of 218, similar to the bright
drifts observed from the lander, which have azimuths of 197 ±
14. In contrast, the dark streaks seen from orbit have azimuths that
average 42 [Greeley et al., 1978]. The dark streaks correlate with
the strongest winds measured by VL 1, while the bright streaks
correlate with secondary winds from the opposite direction.
[6] VL 2 data show that wind erosion and deposition played a
critical role in the evolution of the northern plains of Mars. Aeolian
features include drift deposits, ripple patterns, and inferred ventifacts. Sharp and Malin [1984] studied the geology of the Viking sites
and assessed the potential aeolian processes. They suggested that the
ripples and some of the other deposits at VL 2 indicate the presence
of sand and granule-sized particles (2 – 4 mm). They also noted
surface deflation and the appearance of wind-abraded rocks.
[7] The geology of the MPF site was mapped by Rotto and
Tanaka [1995] and Nelson and Greeley [1999] using Viking
Orbiter data prior to the MPF landing. The site was described
locally on the basis of MPF data by Golombek et al. [1999b] and
Ward et al. [1999]. The MPF site is characterized by a complex
landscape of ridges and troughs, hills, craters, surficial deposits,
and rocks of various sizes and shapes, suggesting that the MPF site
has a complex history of resurfacing by fluvial, impact, and aeolian
processes. Aeolian features were first described by Smith et al.
[1997a] and analyzed in detail by Greeley et al. [1999] and Bridges
et al. [1999]. Comparisons with aeolian features on Mars seen from
orbit were made by Greeley et al. [2000] and Kuzmin et al. [2001].
Inferred aeolian features include duneforms, drifts, wind tails,
moats, ripple-like patterns, and wind-abraded rocks.
[8] Many of the features seen on the Martian surface have a
morphologic similarity to aeolian features on Earth. Their sizes
range from several centimeters for ripple-like patterns and small
wind tails to several tens of meters for duneforms and larger drift
deposits. Our goal is to compare these features with possible
terrestrial analogs to gain insight into Martian surface evolution.
1.2. Terrestrial Analog Sites
1.2.1. Mojave Desert field site. [9] Much of our analysis of
terrestrial aeolian features and interpretations of Martian forms is
based on studies in California’s Mojave Desert, an area that has
been investigated extensively by terrestrial and planetary
geologists [e.g., Allen, 1957; Sharp, 1963, 1964, 1978, 1980;
Theilig et al., 1978; Laity, 1987, 1994, 1995]. We focused on the
central part of the Mojave, approximately bounded by the
communities of Baker to the north, Amboy to the south, Kelso
to the east, and Harvard to the west. The geographic location and
climate of the Mojave are ideal for the formation of ventifacts and
other wind-related features. The Mojave Desert is within the Basin
and Range physiographic province and is bounded by the San
Andreas and Garlock faults on the west and by the southward
projection of the Death Valley fault zone on the east. The regional
relief is formed by high-angle faults and includes subparallel
mountain ranges with intervening alluviated basins, which are
often mantled by sand or contain playas. Cenozoic volcanic
rocks occur throughout the area, with the freshest materials
occurring at the Ludlow, Pisgah, and Amboy basaltic centers.
Ventifacts are well preserved in the basalts, which appear to
provide the best analogs to Martian rocks.
[10] The climate of the Mojave Desert is characterized by low
precipitation, low humidity, high summer temperatures, and frequent winds. The annual precipitation ranges from 56 mm near
Ludlow to 102 mm at Barstow. Frontal precipitation dominates from
November through April. There is a minor peak of summer rainfall,
which accounts for about one quarter of the total precipitation.
[11] The current wind regime is determined by the location and
strength of the semipermanent Pacific high-pressure cell and a
desert low-pressure cell. The western three quarters of the area are
dominated by westerlies, which are strong during spring and
summer and abate somewhat in the fall and winter. Mean wind
velocities are lower in the eastern Mojave and directions are more
variable, whereas along the Colorado River there is a distinct
north-south flow regime.
[12] Local and regional topography affect wind velocity and
direction. Geomorphic features such as valleys define major wind
corridors. Wind also accelerates as it moves up the windward
flanks of hills, increasing sand transport and ventifact abrasion.
Frontal storms typically enhance southerly-southeasterly flow
ahead of the front, whereas winds tend to turn to the southwest,
west, or northwest (depending on local topography) behind the
front. Summer thunderstorms can produce strong winds locally,
with wind directions and speeds that are very erratic.
[13] The formation of ventifacts and other aeolian features is
strongly affected by the supply of abradant, which typically is
sand. The principal source of sand is the Mojave River, which
flows 200 km through the desert from its source in the San
Bernardino Mountains to its termination into the Cronese Lake
basins or the Silver and Soda Lake basins. Along the Mojave River
aeolian corridor, sand moves eastward from source areas in the
Lower Mojave Valley, then southeastward through the Devil’s
Playground, terminating at Kelso Dunes, a depositional sink
[Paisley et al., 1991; Zimbelman et al., 1995; Ramsey et al.,
1999]. Evidence of the long-term movement of sand along this
pathway is provided by deeply eroded ventifacts, sand ‘‘ramps,’’
and depositional features throughout the region.
1.2.2. Iceland field site. [14] In many respects, parts of
Iceland, including the region around the Askja volcanic center,
provide an excellent analog for Mars. This area is dominated by
basaltic volcanism and has been modified by strong winds
[Arnalds et al., 2001], periglacial processes, and fluvial activity,
including catastrophic floods. Except for mosses and other lowgrowing plants, the region is generally free of vegetation. Most of
the windblown particles are derived from volcanoclastic materials
and the weathering of lava flows. Despite high levels of
precipitation that make it unlike Mars, many insights into aeolian
features can be gained in this region.
[15] Askja is centered at 65N, 16.7W in north central Iceland
and is one of several calderas of the Dyngjufjoll volcanic complex, a
late Pleistocene structure composed of moberg (subglacial eruptive
products, including tuffs), basaltic lava flows, and some rhyolites.
The complex is part of the neovolcanic zone within the Mid-Atlantic
spreading center and was last active in 1961 [Bamlett and Potter,
1994]. The name ‘‘Askja’’ commonly refers to the general area on
many maps and guides, and this use is adapted here. Although the
entire area contains aeolian features, two sites are particularly
appropriate, one to the west of Askja and one to the northeast.
[16] The western site is in the Dyngjufjalladalur valley cut into
the west flank of Dyngjufjoll and is probably a graben. Scarps on
the eastern side of the valley expose the volcanic sequence of the
complex, including moberg, tuff, and other deposits, which shed
boulders into the valley. Many of these have been ventifacted into
classic forms, described below.
[17] The northeastern site is on the Odadahraun plains, which
lie between Askja and the prominent table mountain, Herdubreid.
The plains consist of lava flows, sheets of windblown basaltic
sands, and ash deposits, some of which are silicic. The latter
deposits are dominated by rhyolitic pyroclastics derived from the
Viti eruption of 1875, which occurred on the eastern margin of the
Askja caldera. This light-colored material was carried eastward,
blanketing much of the area and posing a stark contrast to the dark
basaltic sands and lava flows. The plains in this area contain
abundant aeolian bedforms and surfaces resulting from exhumation
by wind deflation.
[18] As reviewed by Preusser [1976], Iceland lies in a transitional zone between the cool temperate and the subpolar oceanic
GREELEY ET AL.: TERRESTRIAL ANALOGS TO WIND-RELATED FEATURES ON MARS
5-3
Figure 1. Duneforms (arrows) at the Mars Pathfinder (MPF) and Viking 1 (VL 1) lander sites: (a) Mermaid (left
side) and another transverse duneform at the MPF site; both are 3.5 m long; prominent wind direction is from the
northeast (lower left) (MPF image so185), (b) VL 1 duneforms (VL image 12A119), (c) Jenkins barchanoid duneform
(1.5 m long) at MPF seen southeast from the lander (arrow indicates dark band near crest on leeward slope; image
PIA00981), (d) extensive drift deposits (each duneform or drift is 1 m across) northeast from the VL 1 lander,
inferred to be eroded dune deposits (image PIA00531), (e) unnamed barchan dunes southeast from the MPF lander as
imaged from the Sojourner rover behind the Rock Garden; dune in foreground is 1 m across (image so7630).
climates. Its cool summers and mild winters are influenced by mild
maritime and polar air masses, a branch of the relatively warm Gulf
Stream current, and the cold East Greenland and East Iceland
currents. Most of Iceland is within a belt of westerly winds in the
summer and polar easterly winds in the winter. However, the
frequent shifts in weather fronts, including arctic cyclones, lead
to great irregularity in wind patterns.
[19] The station recording weather data closest to Askja is at
Modrudalur, some 50 km to the northeast [Preusser, 1976]. Annual
precipitation is 503 mm, with an average of 182 days experiencing
precipitation. Average mean temperatures range from 7.8 to
9.9C. There are 203 days with temporary snow cover; 145 of
those days see constant snow cover on the ground. These conditions, if extrapolated to the field area, would suggest that aeolian
processes are likely to occur only about half the year, assuming that
wet and/or snow-covered terrain would prevent or retard the
movement of sand and dust by the wind.
[20] With this introduction to the Mars landing sites and the
field areas used as potential analogs, we now compare Martian and
terrestrial features. The comparisons are divided into two types of
features, those composed of particles that have been deposited or
modified by the wind and those associated with wind-modified
rocks.
2. Erosional and Depositional Features
Composed of Windblown Particles
[21] Windblown particles on both Earth and Mars [Greeley and
Iversen, 1985] include material transported in suspension (‘‘dust,’’
or material <60 microns in diameter), saltation (‘‘sand,’’ or
material 60 – 2000 microns in diameter), and creep or reptation
(material >2000 microns in diameter). It should be noted, however,
that the modes of transport and size ranges are more complex. For
example, the expression ‘‘short-term suspension’’ is given for grains
typically 20 – 70 microns in diameter that move sporadically, while
‘‘long-term suspension’’ involves smaller grains carried longer
distances, the distinction being made with reference to the ratio of
the fall velocity to the shear velocity [Tsoar and Pye, 1987].
[22] Windblown particles can accumulate into various deposits,
which, in turn, can be eroded or deflated to form distinctive
features. In this section we consider various small duneforms,
surface lag deposits, ripples, ‘‘moat-like’’ features found in association with small objects such as rocks, exhumed surfaces, and
drifts of windblown deposits distinctive from duneforms.
2.1. Duneforms
[23] Duneforms include accumulations of particles that were
transported primarily in saltation. Reviews of dunes on Earth are
given by Pye and Tsoar [1990], Cooke et al. [1993], and
Lancaster [1995]. Dunes are characterized by a gentle slope
on their windward side leading to the crest of the dune and a
steeper slope on the lee side, called the slip face, that reflects
the angle of repose of the particles. Duneforms that lack slip
faces have been called whalebacks, which often are precursors
to dunes with slip faces [Bagnold, 1941]. Transverse dunes have
their crests oriented perpendicular to the formative winds.
Barchan dunes are crescent-shaped in planform, with the horns
pointing downwind from the formative winds. The term barchanoid is commonly used for dune masses that have suggestions
of a crescent outline and horns but lack the well-developed
planform of the classic barchan.
[24] Distinctive transverse duneforms are seen from landers
only at the VL 1 and MPF sites (Figures 1 and 2). Those at the
MPF site include whalebacks, barchanoid dunes, and small
barchan dunes [Greeley et al., 1999, 2000]. We note that all of
these features are smaller than typical dunes on Earth, other than
whalebacks. Martian whalebacks include Mermaid (Figures 1a
and 3), which is 3 – 4 m by 1 m, <10 – 15 cm high, and has
5-4
GREELEY ET AL.: TERRESTRIAL ANALOGS TO WIND-RELATED FEATURES ON MARS
Figure 2. Drift deposits at the VL 1 site (top; overview toward the southwest from the lander); general sculpted
appearance and concave-up surfaces of the drifts suggest that the site has been deflated (image PIA00393): (a) crosslaminated structures exposed on surface, (b) sculpted contact of bright strata with dark layer in the drift deposits, (c)
patches of dark material in the leeward zone of rocks. Areas shown in a, b, and c are each 1 m wide.
slopes of 8.5 and 12.5 on the windward and leeward sides,
respectively [Ward et al., 1999]. Larger duneforms, such as Roadrunner Ridge (>10 m wide), are found northeast of the MPF lander.
Some of these duneforms have sharp crests, suggesting that they
could be active and, like Mermaid dune, are oriented transverse to
the dominant winds, which are from the NE Smith et al., 1997a]. A
typical barchanoid dune is found 10 m southeast of the MPF
lander (Figure 1c). This feature, named Jenkins, is 1.75 m
measured across the horns. Small transverse duneforms are seen
also at the VL 1 site (Figure 1b).
[25] Barchan dunes were also found at the MPF site. As imaged
by the Sojourner camera, they have sharp crests and occur southwest of the ‘‘Rock Garden,’’ out of view from the MPF lander
camera (Figure 1e). Individual barchans are 1.0 m wide (horn to
horn), 0.9 long, and 10 – 15 cm high and have windward slopes of
<15 [Greeley et al., 1999]. The orientations of the slip faces are
consistent with formative winds blowing from the NNE.
2.2. Surface Lag Deposits
[26] The minimum wind speed needed to initiate movement of
particles as a function of size defines the threshold curve. The
minimum wind is typically expressed as the friction velocity, a
measure of the shear stress exerted by the wind. As shown by
Bagnold [1941], fine sand 100 microns in diameter is the particle
size moved by lowest-speed winds, with both smaller (i.e., especially dust) and larger grains requiring stronger winds. The mass of
the grain is also important in determining threshold. While the
conventional threshold curve is based on grains of the same
mineral composition (nominally quartz of 2.3 gm/cm3), particles
of higher or lower densities but of the same size would have
different wind threshold values. Experiments under Martian atmospheric compositions and densities show that the same threshold
relationship is true on Mars, although the entire curve is shifted to
higher friction velocities because of the lower atmospheric density
[Greeley et al., 1980].
[27] In a deposit of particles of different sizes, fine sands will be
moved by relatively gentle winds. As sand saltates, some of the
finer grains (dust) will be impacted and injected into the atmosphere, where they are carried aloft in suspension. Larger grains, or
grains of higher density, will be left behind, forming a lag deposit.
Often, this deposit is only a single grain thick, as shown in Figure 4
in the Mojave Desert, in which a lag consists of approximately
millimeter-sized basalt fragments overlying (and shielding) lighter
colored sands.
Figure 3. View of Mermaid, a transverse duneform 3.5 m long
(prominent wind is from the lower left) as imaged from MPF
(image so185) and close-up from the Sojourner rover rear camera;
arrow identifies the same rock in both views.
GREELEY ET AL.: TERRESTRIAL ANALOGS TO WIND-RELATED FEATURES ON MARS
5-5
Figure 4. Basaltic rock (35 cm long) and a lag surface in the Mojave Desert, composed of fragments of basalt
approximately a millimeter across, superposed on fine sand and dust; the pen on the rock indicates scale.
[28] Jenkins duneform at the MPF site is characterized by a
distinctive dark band near the crest above its leeward slope (Figure
1c). Mermaid duneform also is characterized by a dark surface
(Figure 3). This dark material could be a layer of a coarse-grained
material left as a lag deposit after the removal of finer-grained
bright dust, similar to that shown in Figure 4. Such deposits are
typical for terrestrial sand patches resulting from cycles of accumulation and deflation. For example, Stengel [1992a] showed
unconformities in dune deposits marked by a layer of slightly
coarser grains separating two sets of strata and representing a
period of deflation.
2.3. Ripples
[29] Ripples constitute the smallest bedform developed in windblown particles. Reviews of the characteristics and formation of
aeolian ripples are given by many authors, including Greeley and
Iversen [1985], Pye [1987], Pye and Tsoar [1990], Cooke et al.
[1993], and Lancaster [1995]. Ripples are defined by a stoss slope
(upwind side with angles typically <10), a crest (which defines the
height with respect to the base), a lee slope (the downwind side
with slopes typically >20), and the wavelength (crest-to-crest
distance). The ripple index (RI) is defined as the ratio of ripple
wavelength to ripple height. In general, the RI varies inversely with
grain size and directly with wind speed [Sharp, 1963].
[30] At least three types of ripples can be recognized: normal,
fluid drag, and megaripples. Normal ripples commonly develop
in sand and reflect the prevailing wind, in which the ripple crest
is generally oriented perpendicular to the wind, although local
slope is also important [Howard, 1977]. Normal ripples are
asymmetric in cross section and can form and change in size
and orientation on timescales as short as a few seconds in
response to wind gusts. They have RI >10 – 15, which distinguishes them from ripples formed in water (RI <10 – 15). The
grains that comprise ripples are typically coarser than the
underlying sand, and the coarsest grains are found on the ripple
crests [Sharp, 1963; Tsoar, 1990].
5-6
GREELEY ET AL.: TERRESTRIAL ANALOGS TO WIND-RELATED FEATURES ON MARS
Figure 5. (a) Megaripples developed in silicic pumice and basaltic sands, overlying basalt flow in Iceland. Ripple
wavelengths average 3 – 5 m (vehicles in background indicate scale); light colored ‘‘ribbons’’ are centimeter – sized
pumice fragments in the troughs of the ripples. (b) Granule ripples and smaller normal ripples in the troughs. The
granule ripples formed first; then a change in wind regime generated the normal ripples (which can form on shorter
timescales). Note the modification to the crests of some of the granule ripples to the upper right of the 20-cm scale
(photograph by G. Erickson, U.S. Geological Survey).
[31] The relationship between saltation path length and ripple
geometry is not well understood. Bagnold [1941] suggested a
direct relationship, but Sharp [1963] disagreed, suggesting that
surface creep and microtopography were more important. As
reviewed by Lancaster [1995], more recent work based on wind
tunnel studies, field observations, and computer models suggests
that reptation-creep length, rather than saltation length, is the
critical factor in ripple formation, generally consistent with Sharp’s
model. Grains in reptation organize into small ripples due to
statistical variations in the saltation cloud; the small ripples merge
to form larger ripples, which achieve a quasi-stable wavelength.
[32] Fluid drag ripples occur in fine-grained sands and some silts
subjected to high winds. They have a linguoid morphology and
appear to form at wind speeds just below those in which grains pass
into suspension. Fluid drag ripples have not been identified on Mars.
[33] Megaripples, also called granule ripples by Sharp [1963]
and ridges by Bagnold [1941], consist of very coarse grains and
usually form two distinct populations. Many megaripples have
normal ripples superposed on their stoss and leeward slopes.
Megaripples occur in many places on Earth. They are characterized
by wavelengths up to 20 m and amplitudes of tens of centimeters.
Figure 5a shows megaripples on the Odadahraun in Iceland. They
have wavelengths of 3 – 5 m and heights of 10 cm. Of particular
interest is the distribution of particles, in which centimeter – sized
fragments of light colored pumice are concentrated in the ripple
troughs and smaller (<0.4 cm), dark fragments of basalt are
concentrated on the crests. Although this is contrary to most
ripples, in which the coarsest grains are on the crest, it demonstrates that the density of the particles is the controlling factor, not
the diameter, in support of the model of ripple formation involving
reptation.
[34] Three ripple-like patterns can be identified on Mars: (1)
small, modified normal ripples at the MPF site, (2) ripple-like
features developed on inferred lag deposits at the MPF site, and (3)
possible megaripples at the VL 2 site. At the MPF site, patches
about 1 by 1.5 m (Figure 6a) are characterized by ribbons of dark
material and intervening brighter, smooth crests with wavelengths
of 2 – 4 cm [Greeley et al., 1999]. These might be considered
normal ripples on Earth, but image resolution is too low to
determine the slopes on these features, and they appear to lack
the distinctive asymmetric cross section. Their crests appear rather
flat. As noted by Stengel [1992b] from terrestrial field observations, this morphology suggests a condition in which the saltation
cloud is undersaturated and the surface is undergoing deflation.
[35] The MPF site shows several areas of possible ripples
(Figure 6). Figure 6b is a close-up of the surface, which is
composed of coarse grains estimated to be 5 mm in diameter
and organized into discontinuous sinuous rows. Although it is not
clear that these are true ripples, we suggest that they formed as a
result of saltation impact and surface creep and could represent the
early stages of ripple formation.
[36] The best-developed ripples are seen at the VL 2 site (Figure
6c), where asymmetric ridges occur in a trough [Mutch et al.,
1977]. The ridges form patterns of 0.5 – 1.5 m wavelength and
0.05 – 0.15 m height, typical of smaller megaripples on Earth.
Particles derived from the surrounding rocky terrain apparently
accumulated in the trough and organized into these ripples.
2.4. Moat-Like Features
[37] Winds encountering small obstacles form distinctive wind
patterns that are often manifested in the surrounding deposits. The
most distinctive of these wind patterns is the ‘‘horseshoe vortex,’’
GREELEY ET AL.: TERRESTRIAL ANALOGS TO WIND-RELATED FEATURES ON MARS
5-7
Figure 6. Ripple-like patterns (a) at the MPF site near the rock Yogi (image 81084), (b) at the MPF site as
weakly developed in inferred lag deposits (image 81088), (c) at VL 2; small megaripples (arrows) are spaced 1 m
apart in a shallow trough (image 21G122-BB3), (d) basaltic rock 30 cm across and ripple patterns formed in sand
of mixed composition in Iceland; the dark (basaltic) grains form the ripple crests; note also the wind tail formed in
the lee of the rock.
which wraps around the obstacle (Figure 7). Flow acceleration
around the obstacle leads to high wind shear stress, resulting in
nonaccumulation of grains and deflation of grains that might have
settled to the surface under more gentle winds. In addition, grains
in saltation strike the obstacle, bounce off, and are caught in the
vortex to be swept downwind. Some of these grains ricochet,
further eroding the particles surrounding the obstacle. The net
result is the formation of a moat-like depression that wraps around
Figure 7. Diagram showing the flow pattern developed in a horseshoe vortex as wind passes over and around a
small obstacle, such as a rock (adapted from Greeley and Iversen [1985]).
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GREELEY ET AL.: TERRESTRIAL ANALOGS TO WIND-RELATED FEATURES ON MARS
Figure 8. Moat and wind tail in association with a 45 cm ventifact in the Mojave Desert. Wind direction is from
lower left to upper right. Sand is accelerated over the rock, strikes the upper surface and bevels it, and creates shallow
flutes and grooves.
Figure 9. Moats (arrows) developed around small rocks on Mars: (a) VL 1 site showing rock 20 cm across.
Prevailing winds are from the left; dark grains have collected on the floor of the trough; elongate dark zones in the
surrounding drift deposits are ‘‘gouges’’ resulting from the blast of the lander descent engines (VL 1 image
11B169.bb2). (b) MPF site, showing the 43 by 22 cm rock Barnacle Bill and its associated moat; prevailing winds are
from the right; wind tail deposits are visible to the left of the rock (MPF image 81008).
GREELEY ET AL.: TERRESTRIAL ANALOGS TO WIND-RELATED FEATURES ON MARS
5-9
Figure 10. View of a lava flow in Iceland that has been mantled by fine-grained deposits and subsequently partly
exhumed: (a) mushroom-shaped structure on horizon is remanent of mantling material (figure indicates scale) (ASU
photograph 2237-A) and (b) close-up of surface in Figure 10a showing preserved flow textures in the lava (ASU
photograph 2242A).
the obstacle (Figure 8). The deepest part is typically on the upwind
side, and the moat becomes shallower extending downwind from
the flanks of the obstacle. Grains with a higher mass than the
average in the deposit often accumulate as a lag deposit on the
moat floor. Moats on Earth develop under conditions of both net
deposition and net deflation on the surrounding surface.
[38] Moat-like features are seen around many of the rocks at
all three sites on Mars. Figure 9a shows two rocks at the VL 1
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GREELEY ET AL.: TERRESTRIAL ANALOGS TO WIND-RELATED FEATURES ON MARS
Figure 11. Oblique aerial view of sand dunes near St. Anthony, Idaho; the dunes migrate across the basaltic flows
but have done little to erode the primary textures and flow features, although some ventifaction has occurred at the
centimeter scale (area shown is 70 m across) (ASU photograph 197-A).
site, both of which have small moats developed on their upwind
sides. Figure 9b shows the rock Barnacle Bill at the MPF site
and its associated moat-like feature and leeward wind-tail drifts.
All three examples show dark material on the floors of the
moats, which we interpret to be lag deposits of higher-density
grains, likely to be of mafic composition from which bright dust
has been removed.
2.5. Exhumed Surfaces
[39] Sedimentary deposits, regardless of origin, can be stripped
by wind erosion, or deflation, exposing the surface upon which
they were originally deposited. Particularly vulnerable to removal
are materials such as dust and volcanoclastic materials deposited
from atmospheric suspension. These can form mantling blankets
that preserve the underlying terrains and then can subsequently be
removed by stronger winds. Similarly, dunes and dune fields can
migrate across terrains, burying surface features, which are reexposed after the passage of the dunes.
[40] Figure 10 shows a wind-deflated surface in Iceland, formerly buried by a deposit of windblown material at least 2 m thick
that protected the surface from erosion. The exhumed surface has
well-preserved, primary flow structures, including basaltic pahoehoe ‘‘ropes.’’ Were it not for the presence of the mushroom-shaped
remnant of the mantling deposit, the former burial of the surface
probably would not have been readily detected. Passages of dunes
also can have little effect on pristine morphology. Figure 11 shows
Holocene basalt flows in the Snake River Plain, Idaho, and sand
dunes that are moving across the flow. At least at the resolution of
the aerial photograph (subcentimeter), the morphology of the flow
features has not been altered.
[41] The MPF site viewed from orbit displays abundant duneforms, most of which have orientations consistent with the duneforms seen from the lander and with the current wind regime
[Greeley et al., 2000]. We suggest that the MPF site has experienced the passage of duneforms across the surface throughout
much of its postfluvial history, leading to repeated burial and
exhumation. During times of burial, rocks would be protected from
wind abrasion, making estimates of rates of abrasion difficult to
determine. Moreover, some areas would be undergoing deflation,
while other areas would be depositional sites.
2.6. Drift Deposits
[42] The term drift deposit is used for any accumulation of
windblown particles not organized into bedforms such as ripples
and dunes. As used here, it can include (1) pristine forms resulting
from deposition in the leeward (or ‘‘shadow’’) zone of rocks or
other obstacles to windflow (Figure 12), (2) remnant deposits of
formerly larger accumulations that have been subjected to deflation, or (3) some combination of deposition and deflation. Most
features seen on Earth fall into the latter category. In addition, some
wind-sculpted drift deposits were originally deposited by nonaeolian processes, such as deposition of fine-grained lake deposits
GREELEY ET AL.: TERRESTRIAL ANALOGS TO WIND-RELATED FEATURES ON MARS
5 - 11
Figure 12. Wind tails on Earth and Mars. (a) Wind tails associated with small rocks and an inferred lag-deposit
surface (the darker areas) at the MPF. Prevailing winds are from the right; area in foreground is 1.4 m wide (MPF
image so32094). (b) Small wind tails on the margin of drift deposits at the MPF site (prevailing winds are from the
left) (MPF image 24060). (c) Wind tails at the VL 1 site, indicating formative winds from the left (PIA image 00393,
part). (d) Wind tail associated with a small rock (25 cm; lower left) in the Mojave Desert. Prevailing winds are from
the left.
that are subsequently subjected to wind erosion. Unfortunately, it is
often difficult to separate among these cases, even on Earth.
[43] The VL and MPF sites show numerous drift deposits, most
of which are found in association with rocks and are called wind tails
(Figure 12a). At VL 1 the largest drifts are >10 m across and appear
to be the eroded remnants of former dunes (Figure 2). Mutch et al.
[1976] and Sagan et al. [1977] suggested that the drift deposits
northeast of the VL 1 lander have been deflated, exposing crossbedded strata. Many of the possible cross beds are aligned perpendicular to the wind tails, suggesting that both the original duneforms
and wind tails were produced by the same wind system [Sagan et al.,
1977]. In addition, the cross laminations in the drift deposits and the
unconformities between individual strata suggest that the former
dunes reflect cycles of deflation and accumulation. Evidence for
wind sculpturing of the strata is shown in Figure 2, where a sharp
contact is seen between the eroded bright strata and the underlying
dark and thinner strata. Small patches of dark material are seen in
front of the drift, which could be local lag deposits.
[44] Drift deposits at the MPF site consist primarily of wind
tails. Similar to wind tails in the Mojave Desert, these range in
length from <1 to 40 cm [Greeley et al., 1999]. As shown in
Figure 12a, most of these occur as isolated deposits in the lee of
rocks on surfaces that appear to be lag deposits. From these
relationships we suggest that the wind tails at the MPF site reflect
a generally deflational wind regime in which the long-term sediment budget is negative.
2.7. Spectral Properties of Martian
Aeolian Materials
[45] In addition to geomorphic interpretations from images,
Imager for Mars Pathfinder (IMP) spectral data provide important
information on Martian particulates, including aeolian materials.
IMP viewed the surface in 12 visible and near-infrared wavelengths
[Smith et al., 1997b]. These data were used to infer or constrain the
mineralogy, relative abundance of ferrous versus ferric phases,
particle size, crystallinity, and degree of compaction in soils [Smith
et al., 1997a; Bell et al., 2000]. We have produced representative
IMP spectra of wind tail, ripples, and duneform materials in which
IMP data were searched for ideal lighting geometries and spectral
coverage and applied the latest calibrations for radiance and reflectance [Reid et al., 1999a, 1999b] for analysis. Raw data were
converted to radiance using CCDCAL Version 2 (provided by R.
Reid), which corrects for saturated pixels, shuttering, dark current,
flat field, and bad pixels. IMP data were converted to reflectance
relative to the radiometric targets using the program SPECTCAL
Version 3. Radiometric target sequences used for the conversion
were close in time to the scene images. This facilitated the computation of scene reflectances using targets under nearly the same
illumination conditions, minimizing diurnal and secular changes in
optical depth and diffuse illumination.
[46] Reflectances for each spot were retrieved using the IL
program IMPSPECT (developed and provided by J. Bell). To
account for slight reflectance offsets between the left and right
‘‘eyes’’ of IMP, all spectra were scaled to the average of either the
440 nm or the 670 nm reflectances. Because reflectances of most
IMP spectra at the MPF landing site peak near 750 nm [McSween
et al., 1999; Bell et al., 2000], the 750/440 nm ratio was used to
define red/blue for each site. The 750 nm filter is contained in
IMP’s right eye, so right eye 440 reflectances were used to
compute the red/blue ratio.
[47] An examination of the spectral shapes and red/blue ratios
of the materials reveals similarities and differences among the
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GREELEY ET AL.: TERRESTRIAL ANALOGS TO WIND-RELATED FEATURES ON MARS
Figure 13. (a) Relative reflectance (R*) versus wavelength for the areas shown in Figures 13b – 13d. (b) IMP threecolor (440, 530, and 750 nm) and 750/440 ratio images of ripples. The scale in upper right corner is in ratio units for
the images in Figures 13b, 13c, and 13d. The boxes show (top) dark and (bottom) light ripples that were measured.
Images are from ‘‘Insurance Pan’’ sequence S0030B, acquired at 1328 LST on Sol 2. (c) IMP three-color (440, 600,
and 750 nm) and 750/440 ratio images of wind tails. The boxes show where wind tails of the rocks (top) Dilbert’s
Boss and (bottom) Barnacle Bill (43 by 22 cm) were measured. Images are from ‘‘Superpan’’ sequence S0182A,
acquired at 1521 LST on Sol 18. (d) IMP three-color (440, 600, and 750 nm) and 750/440 ratio images of Mermaid
duneform (3.5 m long). The top images were taken after the area was slightly disturbed by the APXS Deployment
Mechanism (ADM). Top images are from ‘‘Multispectral Spot’’ sequence S0171B, acquired at 1237 LST on Sol 34.
Bottom images are from ‘‘Superpan’’ sequence S0188B, acquired at 0844 LST on Sol 13.
GREELEY ET AL.: TERRESTRIAL ANALOGS TO WIND-RELATED FEATURES ON MARS
5 - 13
Table 1. Relative Reflectance of Aeolian Materials at the Pathfinder Site
IMP Filter, nm
Subject
BB wind tail
DB wind tail
Dark ripples
Light ripples
Disturbed drift
Undisturbed drift
a
b
a
Box
7
7
11
3
7
7
7
7
11
3
7
7
440
670
750
670/440
750/440
0.040
0.040
0.042
0.050
0.045
0.050
0.184
0.183
NAb
NAb
0.162
0.166
0.221
0.219
0.213
0.291
0.176
0.179
4.62
4.53
NAb
NAb
3.63
3.29
5.57
5.42
5.08
5.84
3.95
3.54
Box size in pixels.
There was no 670 nm image for this sequence.
physical properties and mineralogy of the aeolian materials (Figure
13 and Table 1). The light ripples are the brightest class of
materials and Mermaid is the darkest. There is no appreciable
difference between this material in its undisturbed state and when it
has been slightly disturbed by the Sojourner rover. Two wind tails
(associated with the rocks Barnacle Bill and Dilbert’s Boss) have
nearly identical spectral properties. The dark ripples have similar
spectral properties to the wind tails.
[48] These observations are generally consistent with our overall understanding of the Martian surface, in which soils and soillike materials are thought to become darker and bluer with
increasing particle size, crystallinity, and ferrous/ferric ratio [Bell
et al., 2000]. The high reflectivity and red/blue ratio of the light
ripples indicate that they are probably composed of micron-scale
ferric dust or larger dust aggregates, mixed with only a small
amount of crystalline material. This is similar to the bright soils
classified by Bell et al. [2000]. The darkness of Mermaid is
consistent with a soil that is coarser grained or richer in ferrous
material compared to common bright soil and is within Bell et al.’s
[2000] dark soil classification. The dark color, combined with the
relatively high silica content of Mermaid relative to other soils (2%
and greater), suggests a lag deposit formed by removal of dust
[Moore et al., 1999]. The wind tails and dark ripples have a
brightness intermediate between Mermaid and the light ripples.
Figure 14. Einkanter ventifact from the Mojave Desert formed in basaltic rock. Note the small (0.2 by 1.5 cm)
grooves along the edge of the facets. The pebble above the pen rests in a ‘‘pothole,’’ which appears to have been cut
into the rock by the pebble rocking back and forth by the wind (ASU photograph 4401-H).
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GREELEY ET AL.: TERRESTRIAL ANALOGS TO WIND-RELATED FEATURES ON MARS
Figure 15. Ventifact in Iceland, showing profile which reflects maximum abrasion 20 cm above the surface. This
zone of maximum abrasion results from a combination of higher wind velocities with increasing height above the
surface (and attendant increased particle speeds) and decreasing flux of particles with height, allowing more grains to
strike the rock rather than each other; prevailing winds are from the right (ASU photograph 2241-A).
They may have a mafic composition similar to the putative lag
deposit at Mermaid but are composed of smaller, dust-sized grains.
The lack of bright material in the wind tails could be due to aeolian
removal of surface dust by vortices behind rocks. If correct, this
indicates that the wind tails are sculpted by wind erosion.
3. Wind-Abraded Rocks
3.1. Ventifacts on Earth
[49] On Earth, actively forming ventifacts are found in environments characterized by a supply of abradant (usually sand), sparse
vegetation, strong winds, and topography that allows the free sweep
of wind or that locally accelerates airflow. Ventifacts can form in a
wide variety of rock types, including basalt, granite, marble, and
limestone. Polishing and smoothing of rock surfaces occur in all
lithologies.
[50] The size of the original rock is an important control on the
form of ventifacts. Classic faceted types are described as einkanter,
zweikanter, and dreikanter for one-, two-, and three-ridged forms in
fine-grained rocks (Figure 14). Surface features can include pits,
flutes, grooves, or helical forms. The preservation of ventifacts
varies with climate, rock type (with carbonate ventifacts being
poorly preserved owing to chemical weathering), granular disintegration, and exfoliation. Ventifacts are generally well preserved in
basaltic rocks.
[51] Ventifact formation depends on the wind regime, the
susceptibility of the rock to erosion, and the properties of the
impacting particles (e.g., size, density, speed, and angle of incidence) [Greeley et al., 1982, 1985]. In general, saltating particles
traveling at greater heights have faster velocities, owing to an
increase in wind speed above the ground and longer saltation paths
that allow more time for acceleration by the wind [Anderson and
Hallet, 1986]. Thus, within a general zone of abrasion, erosion
profiles develop with distinct maxima of mass removal (Figure 15),
the height of which is determined by wind speed, sand flux, and
saltation trajectory. As a consequence, many ventifacts develop
semiplanar faces, with the upper part of the abrasion face receding
more rapidly than the lower part. The height of maximum abrasion
appears to be 0.10 m above the surface on level terrain but may
exceed 1.5 m on hillcrests or where local topography enhances
wind velocity [Laity, 1995].
[52] The angle of incidence is also important in ventifact
formation and is a function of the particle trajectory and the
incidence angle of the rock surface being struck. Although the
exact values differ for brittle and ductile targets, in general steep
incident angles of 90 and grazing, low-angle impacts are less
efficient than intermediate angle impacts, in which gouging
increases the efficiency. This is demonstrated by Figure 16, which
shows a metal plate around a power pole which has been abraded
by sand into a pattern reflective of the incident angle. Controlled
experiments show similar results for common rocks, which appear
GREELEY ET AL.: TERRESTRIAL ANALOGS TO WIND-RELATED FEATURES ON MARS
5 - 15
Figure 16. Wind-abraded metal sheet on a power pole in the Mojave Desert, showing effect of incidence angle on
the amount of erosion. The 13.5 cm pen marks the leading face with regard to the incoming windblown grains (zero
incidence angle); butterfly-wing patterns to both sides represent the maximum abrasion for angles between 30 and
45 in which gouging by windblown grains is effective. Note also that abrasion is maximum at a height above the
surface of 30 cm (ASU photograph 2293-A).
to behave as a combination of ductile and brittle materials [Greeley
et al., 1982].
[53] The sizes of abrasion features, such as flutes and grooves,
are also related to wind velocity and are subject to the same
enhancement effects of geometry and topography described above.
Thus these features are largest near the upper surfaces of very large
ventifacts, especially those located on hillcrests and in swales.
Under conditions of high wind velocities and ample sand supply,
grooves in the Mojave Desert can exceed 100 cm long, 23 cm
wide, and 13 cm deep. The depth of flutes and grooves is
consistently less than their width.
3.2. Ventifacts at the Mars Landing Sites
[54] The surfaces of all three landing sites have abundant rocks,
many of which show ventifact features [Bridges et al., 1999].
Rocks at each site exhibit differences, as a class, from rocks at the
other sites. For example, rocks at VL 2 are generally pitted and
have a ‘‘spongy’’ appearance [Viking Lander Team, 1978]. Rocks
at VL 1 are also pitted, but commonly without the ‘‘spongy’’
texture. Rock textures at the MPF site include flutes, grooves,
crusts, and knobby surfaces similar in appearance to sedimentary
conglomerates [Rover Team, 1997; Bridges et al., 1999; Greeley et
al., 1999; McSween et al., 1999; Moore et al., 1999]. Rocks at the
MPF generally have rough surfaces, although some exhibit
strongly forward-scattering photometric behavior (Figure 13),
consistent with a varnished or polished surface [Johnson et al.,
1999]. Rock textures at the MPF site probably include both
primary and secondary features.
3.3. Ventifact Analog Comparisons
3.3.1. Overall shape. [55] The primary shapes of Martian
rocks probably reflect the original processes that emplaced them,
including impact, fluvial, and volcanic events. However, some
rocks at the MPF site have facets and sills suggesting modification
by the wind and periods when the rocks might have been buried or
partly buried by soils. For example, sills at the bases of some rocks
suggest shielding from abrasion (Figure 17), and re-exposure by
deflation [Greeley et al., 1999; Golombek and Bridges, 2000]. This
interpretation is supported by the observation of the facets and
erosional grooves. As noted by Bridges et al. [1999], erosional
grooves on many of the rocks at the MPF site are found mostly on
the upper parts of the rocks. We suggest that the lower parts of
these rocks could have been buried, preventing the formation of
well-defined facets, while the upper parts were exposed, enabling
the development of the grooves. As discussed by Greeley et al.
[2000], the surface at MPF was probably swept by duneforms
repeatedly, with rocks buried, partly buried, exposed, and reexposed; this would lead to highly variable wind abrasion on the
exposed rock surfaces and inhibit the formation of well-developed
facets.
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GREELEY ET AL.: TERRESTRIAL ANALOGS TO WIND-RELATED FEATURES ON MARS
Figure 17. (a) The rock Half Dome (53 cm high) at the MPF site. Arrow points to possible exhumed sill. Note pits
on left edge of rock and flutes on the upper surface (MPF image; scale bar is 10 cm). (b) The pitted and fluted rock
Stimpy at the MPF site. Pits transition to flutes toward the top of the rock (MPF image). (c) Grooves carved into the
rock Flat Top (14 cm high) at the MPF landing sites. Arrow shows a prominent groove; note pits on side of rock
(MPF image).
3.3.2. Surface textures. [56] About half of the rocks at the
MPF site exhibit elongated pits, flutes, and grooves (Figure 17).
Similar to terrestrial ventifacts, pits are generally found on highangle faces, while elongated pits and flutes occur on inclined upper
surfaces, and the grooves are found on the upper surfaces of rocks.
This is consistent with wind abrasion by generally unidirectional
winds, in which the steep windward faces of the rocks are pitted by
near-perpendicular collisions of saltating particles and the upper
GREELEY ET AL.: TERRESTRIAL ANALOGS TO WIND-RELATED FEATURES ON MARS
Figure 18. (a) Pitted basaltic ventifact in the Mojave Desert. Pits
are transitional to shallow flutes along the upper margins of the
boulder. The basal sill of varnished and weathered rock may
represent a zone previously protected from abrasion by a sand
ramp and subsequently exposed by deflation; scale bar is 10 cm
long. (b) Basaltic rock in the Mojave Desert in which the
development of small ventifact grooves have exploited existing
vesicles (ASU photograph 2115-D-3A). (c) Mojave Desert;
weathering crust which has been abraded, forming pits and small
grooves (scale is 30 cm long). (d) The rock Chimp (22 cm high) at
the MPF site showing a surface crust (mosaic of rover right front
camera images 1253233245-S074044 and 1253233615-S074045.
Mosaic has been stretched and sharpened to accentuate detail). (e)
The rock Stimpy (25 cm high) at the MPF site, showing an inferred
crust (superresolution image by T. J. Parker, Jet Propulsion
Laboratory (JPL)).
5 - 17
parts of the rock are gouged by grains hitting at shallower angles,
forming grooves. Although these general relationships are similar
for Martian and terrestrial ventifacts, there are notable differences
in the morphology of specific features. Martian grooves generally
have length to width ratios <4, with most being <2 [Bridges et al.,
1999]. In contrast, grooves in the Mojave have ratios generally >5.
Although the depths of Martian grooves are difficult to measure
because of low image resolution, they appear to be shallower than
many terrestrial grooves.
[57] Pits are seen in rocks at all three landing sites (Figure
17). On Earth, pits form in the initial stages of wind abrasion on
rock faces (Figures 18a – 18c). Primary pits, such as vesicles, are
often preferentially eroded by the wind, resulting in larger pits
[McCauley et al., 1979]. The association of pits with flutes and
grooves on many Martian rocks (e.g., Figures 18d and 18e)
suggests that at least some pits result from wind abrasion. The
fraction of all pits that form from abrasion versus other
processes is, however, unknown.
[58] Some rocks in terrestrial ventifact environments display an
outer crust consisting of a fluted fabric. The terrestrial example
shown in Figure 18a is very similar in appearance to Chimp,
Stimpy, and other rocks at the MPF site. The thickness of the crust
on the Martian rocks is about equal to the penetration depth of the
flutes. The surfaces of many of the Martian rocks, including the
exposed substrate where the crust has spalled, are covered with a
much finer fluted texture. This suggests a sequence of (1) intense
fluting, (2) formation of crust and spallation, and (3) minor fluting.
The large, outer flutes might have acted as repositories that trapped
sand and dust.
[59] On Earth, weathering of feldspars into clays, nucleation
of water onto trapped grains, or other chemical reactions could
lead to the formation of rock crusts. Similar processes might
have acted to form crusts at the MPF site. The formation of
crusts might also have occurred during periods of burial, when
chemical reactions could have occurred between the rock surfaces and the soils.
[60] On Earth, differential erosion occurs on rocks as a function
of resistance to abrasion. Softer materials in rocks are preferentially
eroded, leaving the more resistant materials standing out in relief.
For example, Figure 19a shows a basalt flow containing an ultramafic xenolith surrounded by a depression. Over time, wind has
eroded the softer matrix surrounding the xenolith. Similarly,
volcanoclastic rocks can contain rock fragments of different
materials, as shown in Figure 19c. Similar relationships could
explain the putative ‘‘conglomeritic’’ textures seen at the MPF site
[Rover Team, 1997; Greeley et al., 1999; Moore et al., 1999], such
as on the rock Squash (Figure 19b).
[61] Compositional or textural bands can also result in differential erosion (Figures 20 and 21). Figure 20a shows a terrestrial
basalt consisting of flow bands that are vesicle-rich, vesicle-poor,
and feldspar-rich. The vesicle-rich bands have evolved into
troughs, and the vesicle-poor zones have formed ridges. The
vesicles act as sites preferentially abraded by windblown grains,
whereas less erosion occurs where vesicles are scarce and there are
more feldspar grains. This suggests that the apparent layering
observed in some rocks at the MPF site [Parker, 1998; McSween
et al., 1999] could be flow-banded lavas etched by wind abrasion.
In some cases on Earth, aeolian flutes crosscut flow bands in
volcanic rock (Figure 21a). Possible orthogonal linear features on
rocks at MPF could be similar flutes superimposed on flow bands
(Figure 21b).
[62] Terrestrial rocks can become polished by the wind. Some
rocks at the VL and MPF sites have specular reflections [Guinness
et al., 1997; Johnson et al., 1999] (Figure 22), suggesting wind
polish.
3.3.3. Location. [63] Many of the best-developed ventifacts
at the MPF site (Half Dome, Moe, Stimpy) are found near the
crests of low ridges. This occurrence is consistent with terrestrial
5 - 18
GREELEY ET AL.: TERRESTRIAL ANALOGS TO WIND-RELATED FEATURES ON MARS
Figure 19. (a) Differential erosion of basaltic rock around a more resistant xenolith (8 cm across) in the Mojave
Desert. (b) Rock at the MPF site, showing protruding, rounded rock masses that might also represent differential
erosion (portion of rover right front camera image 1249070145-N027093). (c) Differential wind erosion in Iceland of
a block of tuff (1.75 cm high) containing cobble-sized lithic fragments, which are more resistant to erosion (ASU
photograph 2241-A).
examples in which the most active ventifact formation is found on
slopes where windflow is accelerated.
4. Discussion and Conclusions for Mars
[64] On Earth, aeolian processes are controlled primarily by the
wind regime, the supply of particles, and the surface environment,
including the presence and amount of vegetation. Consequently,
aeolian activity is highly variable on both long and short timescales. For example, long-term climate changes leading to
increased rainfall can inhibit aeolian activity by adding cohesion
to loose particles and promoting vegetation growth, which would
stabilize loose particles. On the other hand, increased rainfall and
runoff can increase the availability of small particles through water
erosion, increasing the potential flux of windblown materials, as
Figure 20. (a) Differential erosion in a flow-banded basalt in the Mojave Desert. Concentrations of vesicles in the
bands are preferentially abraded by windblown sand into grooves (white arrows; scale is 30 cm long). (b) The rock
Mini-Matterhorn (30 cm high) at the MPF site shows suggestions of lineations (white arrows parallel to lineations),
which might represent differential erosion (superresolution image by T. J. Parker, JPL).
GREELEY ET AL.: TERRESTRIAL ANALOGS TO WIND-RELATED FEATURES ON MARS
5 - 19
Figure 21. (a) Wind-abraded flutes (white arrow) cut into a flow-banded basalt (rock is 30 cm wide) in the Mojave
Desert. Unlike the rock shown in Figure 20, these grooves cut across the bands. (b) Possible crosscutting bands in the
rock Yogi (1.14 m high) at the MPF site (superresolution image by T. J. Parker, JPL).
documented by Sharp [1980] in his study of wind abrasion at
White Water Pass, California.
[65] On Mars, where vegetation is not a factor, the presence of
surface water in the past is an important consideration. For
example, periods following putative fluvial activity probably
experienced increased aeolian processes because of the influx of
small particles of fluvial derivation. If the atmospheric density
were also high at that time, we might expect a more frequent
occurrence of winds above threshold, because threshold scales
inversely with atmospheric density. Particles amenable to aeolian
Figure 22. VL 2 site with Sun near the horizon. Arrow points to
a reflective rock surface (20 cm wide) that could be polished by
wind abrasion (VL 2 image 21B124).
activity are also generated by volcanoclastic and impact-cratering
processes, which were more prevalent in the past. Thus one could
argue that the overall supply of small grains has diminished with
time on Mars. This is partly offset by another consideration; on
Earth, oceans and other bodies of water trap some windblown
materials, effectively removing them from the aeolian regime.
While similar traps might have existed in the past on Mars, in
the more recent past, large bodies of water are unlikely to have
existed. Thus most particles generated in the Amazonian period are
likely to be available for aeolian activity.
[66] Short-term timescales are also important in considering
aeolian processes. Significantly more changes can occur during a
single storm than in many weeks or even months of relatively
gentle winds. For example, observations were made of a storm in
the Mojave Desert on 5 March 2000. As the storm front passed
through the area, winds increased from late morning to midafternoon from 8 to 23 m/s (measured 1 m above the ground), all well
above sand threshold. As shown in Figure 23, observations were
made on a small hill where winds were accelerated over the summit
area. Strong gusts were observed, during which the flux of particles
increased significantly, moving drifts of sand several meters in <30
min. As the sand drifts passed over the rocky surface, some rocks
were buried, while others were re-exposed. In late afternoon the
wind direction changed 180 (reflecting the passage of the front),
and although the wind strength diminished somewhat, the direction
of sand flux and attendant abrasion was also reversed. Thus, during
a single storm the flux of windblown particles varied in both
intensity and orientation. Abrasion was also variable, not only as a
function of the flux but also in regard to rock burial and exposure.
Similarly, the deposition, deflation, and orientations of sand drifts
changed during the storm. Drawing on this terrestrial analogy,
short-term aeolian events could also be important on Mars, in
which storm systems and/or gusts might lead to significant changes
on the surface.
[67] The Mars general circulation model (GCM) of Haberle et
al. [1993] can predict dominant winds as a function of season and
time of day and can be used to derive estimates of the magnitude
and orientation of near-surface winds. As applied to the MPF site,
the wind tails are found to correlate well with the predicted highest
seasonal winds in the area [Smith et al., 1997a; Greeley et al.,
1999]. In contrast, the orientations of the ventifacts [Bridges et al.,
1999] and the inferred erosional patterns on craters seen from orbit
do not correlate with any winds predicted by the GCM, suggesting
a change in wind regime [Greeley et al., 2000; Kuzmin et al.,
5 - 20
GREELEY ET AL.: TERRESTRIAL ANALOGS TO WIND-RELATED FEATURES ON MARS
Figure 23. Sand ‘‘ribbons’’ developed during high winds on a 100 m high ridge in the Mojave Desert. The sand
ribbons are many meters in length and protect the underlying rocks from abrasion. Rocks in the linear scour zones are
subject to erosion. The ventifact in the lower left corner of the image demonstrates how sand particles accelerate up
the face and may shoot a meter or more into the air stream. Note also the area of nondeposition in the lee of the rocks,
corresponding to the area of high surface shear stress in the horseshoe vortex (area shown is 1.8 m wide in the
foreground).
2001]. Moreover, ventifacts require much longer times to develop
than drifts of windblown particles, such as wind tails and dunes,
and probably reflect a better ‘‘average’’ of wind regime than more
ephemeral features. We suggest that the ventifacts in the MPF area
formed following the final stages of fluvial activity, which would
have left a supply of sand as an abradant. Their formation was
followed by a change in wind regime, in terms of both direction (as
reflected by the duneforms) and intensity (indicated by the lack of
abrasion features oriented with the duneforms). The present wind
regime is sufficient to transport sand in modest saltation (and
generate dust devils) but apparently is inadequate to accelerate
grains to speeds sufficient to cut abrasional features such as
grooves and flutes.
[68] In conclusion, comparison of features seen at the three
Martian landing sites with terrestrial aeolian features provides clues
to the interpretation of Martian features and the evolution of the
surface. At the MPF site we suggest that features such as
‘‘knobby’’ rocks and lineations previously thought to represent
sedimentary rocks can be explained by aeolian processes and could
be rocks of any origin. We also suggest that the surface at the MPF
site has been buried or partly buried and re-exposed throughout its
history by the passage of duneforms and possible mantling by
particles settled from atmospheric suspension. If correct, this
implies that rates of wind erosion would be very difficult to derive
unless there were some way to establish the time intervals when the
surface was shielded from erosion.
[ 69 ] Comparisons of the Martian and terrestrial features
strongly suggest the presence of abundant sand-sized particles on
Mars. Although the possibility of sand-sized aggregates cannot be
ruled out, the occurrence of ventifact flutes and the various duneforms suggest either holocrystalline grains or aggregates of sufficient strength to withstand repeated saltation.
[70] Acknowledgments. This work was partly supported by the
Planetary Geology and Geophysics Program, National Aeronautics and
Space Administration, through grants and contracts to Arizona State
University and the Jet Propulsion Laboratory. We thank Ray Arvidson,
Kim Deal, and an anonymous reviewer for comments and suggestions,
which improved the manuscript. Graphics, photographic, word processing,
and computer assistance were provided by Sue Selkirk, Dan Ball, Stephanie
Holaday, and Edi Lo, respectively.
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