JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, E08010, doi:10.1029/2007JE003018, 2008
Rock abrasion features in the Columbia Hills, Mars
B. J. Thomson,1 N. T. Bridges,1 and R. Greeley2
Received 8 October 2007; revised 11 April 2008; accepted 22 May 2008; published 9 August 2008.
[1] Wind-abraded rocks (ventifacts) observed along the Mars Exploration Rover (MER)
Spirit traverse in the Columbia Hills reveal evidence for complex wind flow patterns.
Multiple superposed sets of eolian bed forms are evident in rover images and more
broadly in High Resolution Imaging Science Experiment (HiRISE) image coverage.
Formative wind directions inferred from ventifacts are more consistent with smaller,
second-order (T2) eolian ridges rather than larger, first-order (T1) ridges. This suggests that
the direction of recent highest-energy saltation may more commonly be aligned with
the higher-order textures on bed form surfaces, perhaps because of bed form–modified
(secondary) airflow around T1 ridges. Additionally, the lack of ventifacts with formative
wind directions consistent with T1 ridges may indicate that either the ventifact
textures consistent with T1 ridge orientations have been overprinted by abrasion
commensurate with T2 ridges or, alternatively, the rocks were emplaced subsequent
to the formation and stabilization of T1 bed forms. In both cases, T1 ridges appear
fairly immobile and may provide less sand from winds blowing perpendicular to
their ridge crests compared to winds consistent with T2 orientation.
Citation: Thomson, B. J., N. T. Bridges, and R. Greeley (2008), Rock abrasion features in the Columbia Hills, Mars, J. Geophys.
Res., 113, E08010, doi:10.1029/2007JE003018.
1. Introduction
[2] Eolian activity is one of the principal agents of surface
change on Mars and is perhaps the most active process in
the current environment. The term ‘‘eolian abrasion’’ is
somewhat of a misnomer since the wind itself is not an
agent of erosion; rather it is the mobile sedimentary particles
lofted by near-surface winds that shape the surface via
abrasion. Subaerial transport of mobile particles results in
both depositional bed forms and abrasion of exposed rocks
and outcrops. Examples of bed forms include dunes, ripples,
and wind tails. Erosional features include ventifacts, which
are rocks abraded by windblown particles that are common
in arid environments on both the Earth and Mars [e.g.,
Greeley and Iversen, 1985; Laity, 1994; Bridges et al.,
2004]. Formed by repeated impacts of windblown sand
over time, ventifacts provide insight into the formative nearsurface wind direction(s) above the saltation threshold.
Since rocks tend to be longer-lived surface components
than bed forms, ventifacts provide an integrated record of
past and present eolian activity.
[3] Studies of eolian processes at the Mars Pathfinder
landing site reveal an interesting disparity between the wind
directions inferred from ventifacts and eroded crater rims
with that from eolian bed forms [Bridges et al., 1999;
1
Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, California, USA.
2
School of Earth and Space Exploration, Arizona State University,
Tempe, Arizona, USA.
Copyright 2008 by the American Geophysical Union.
0148-0227/08/2007JE003018
Greeley et al., 2000]. A 90° offset between the two types
of wind indicators suggests that local circulation patterns
may have shifted since the ventifacts were formed, possibly
because of climate change. It is not known if this shift
represents a change in only the local conditions or is
perhaps tied to a regional or even global-scale change in
climate. Therefore, the Mars Exploration Rover (MER) sites
provide additional opportunities to assess current and paleowind conditions and determine if there is any evidence for a
similar shift in circulation patterns.
[4] In this paper, we focus on ventifacts observed along
the MER Spirit traverse (Figure 1). In particular, we
examined the distribution and orientation of ventifacts in
the Columbia Hills between Martian solar days (sols) 700
and 810. The long distances being traversed by the rover
(6.1 km through sol 810) are significantly increasing the
available surface area over which ventifacts can be studied.
Previous studies examined the inferred wind directions from
ventifact and eolian bed forms on the relatively flat floor of
Gusev Crater through sol 312 [Greeley et al., 2004, 2006,
2008]. The Columbia Hills are a higher-relief area that
provides an opportunity to examine the effects of topography on ventifact development and expression. We find that
the wind direction inferred from ventifacts in the Columbia
Hills is strongly influenced by topography and generally
orthogonal to that of the major, first-order (T1) bed forms,
but aligned with the smaller, second-order (T2) bed forms
(an example bed form is given in Figure 2). This suggests
that the shaping and imprinting of textures on the ventifacts
may be younger than the T1 bed forms. More broadly, it
indicates that inferences on the direction of recent highestenergy saltation may more commonly be aligned with the
higher-order textures on bed form surfaces as opposed to
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Figure 1. (a) Overview of MER Spirit traverse from sols 1 to 810 overlain on HiRISE image
PSP 001513 1655. Solid yellow line represents traverse from sols 1 to 699; dashed red line represents
traverse from sols 700 to 810 (outlined area). Black box gives location of Figure 1b. (b) Close-up portion
of traverse from sols 700 to 810 in the Columbia Hills. White boxes outline four traverse segments given
in Figure 4. Circles on traverse denote sol way points (labeled with white numbers). Diamonds indicate
locations of mapped ventifacts.
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Figure 2. Example of eolian bed forms at HiRISE resolution (27 cm/pixel, portion of
PSP 001513 1655). Larger ripple labeled ‘‘T1’’ is first-order bed form that is superposed by smaller,
second-order eolian ridges (T2) at near-right angles.
the primary bed forms themselves in areas of complex
topography.
2. Background
2.1. MER Spirit Mission Outline
[5] Each MER rover is equipped with nine cameras
(including stereo components) used to explore their local
environments, including the panoramic camera (Pancam),
the navigation camera (Navcam), the hazard avoidance
cameras (Hazcams), and the microscopic imager (MI).
Pancam is a stereo camera pair mounted on the central
mast that provides high-resolution, multispectral coverage
[Bell et al., 2003]. Also mounted on the central mast is the
Navcam stereo pair [Maki et al., 2003]. Navcam cameras can
acquire a single channel 360° panorama in 12 –14 frames,
albeit at a lower resolution than Pancam (Navcam camera
have 2.1 mrad/pixel; Pancam cameras have 0.27 mrad/pixel).
Hazcams are two pairs of fish-eye, stereo cameras mounted
in the front and rear of the rover beneath the solar array
[Maki et al., 2003]. Their primary function is to acquire
images while the rover is executing drive sequences in order
to avoid obstacles. The final camera is the MI, which is
mounted at the end of the robotic arm or Instrument
Deployment Device (IDD). Akin to a geologist’s hand lens,
the MI provides close-up views of the surface texture of
targets of interest [Herkenhoff et al., 2003].
[6] The MER rovers are capable of operating in a variety
of driving modes that range between fully autonomous to
preprogrammed, ‘‘blind’’ drive segments [Li et al., 2006].
The number of images acquired of the local surroundings
depends upon the drive mode selected, but typically 360°
Navcam panoramas are acquired at the beginning and end
of long traverse segments. Partial and complete panoramas
acquired by Pancam provide higher-resolution coverage that
are more suitable for mapping ventifacts and other targets of
interest, but Navcam and even Hazcam images can be used
to accurately map larger ventifacts for which Pancam data
are not available.
2.2. Site Geology Overview
[7] Spirit touched down on the flat plains of Gusev
Crater, a 160 km diameter impact crater whose southern
wall was breached by the fluvial channel Ma’adim Vallis.
During the first 90 sols of the nominal primary mission
phase, Spirit explored the plains and the nearby Bonneville
Crater (Figure 1) [Squyres et al., 2004]. Plains surfaces on
Gusev floor are dominated by loose dark rocks and sedimentfilled hollows. Chemical analyses of rocks after abrasion
with the RAT (Rock Abrasion Tool) reveal them as basaltic
in composition with phenocrysts, presumably olivine, in an
aphanitic matrix [Gellert et al., 2004; McSween et al., 2004;
Morris et al., 2004; McSween et al., 2006]. No compelling
evidence was found on the plains for lacustrine or aqueous
processes that had been inferred from orbital views of the
regional geomorphology by some authors [Cabrol et al.,
1998; Irwin et al., 2002; Cabrol et al., 2003]. Rather, the
plains appear to be impact-gardened volcanic flood basalts
that overlie any early lacustrine deposits [Grant et al., 2004;
Greeley et al., 2005; Golombek et al., 2006]. After exploring the local plains geology, the rover was directed to drive
to the Columbia Hills, a cluster of low hills that lie some
3 km from the landing site. Superposition relationships
indicate that the Columbia Hills predate the plains, and
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High Resolution Imaging Science Experiment (HiRISE)
onboard the Mars Reconnaissance Orbiter (MRO). The
HiRISE instrument is a high-resolution imager that returns
images with spatial resolution as great at 25 to 32 cm/pixel
from MRO’s nominal 255– 320 km altitude orbit [McEwen
et al., 2007]. Using Time Delay Integration (TDI), each line
of ground observation is imaged up to 128 times to increase
the signal-to-noise ratio [Delamere et al., 2003]. A linear
array of 10 overlapping CCDs provides a 6 km swath
width at red wavelengths (center wavelength is 694 nm).
The central 20% of the image swath is also imaged by two
pairs of additional CCDs sensitive to blue-green and nearinfrared wavelengths (central wavelengths of 536 nm and
874 nm, respectively). Given the typical HiRISE image size
of 6 km by 12 km, the entire Spirit rover traverse can be
acquired in a single image, thus providing essentially
uniform observation conditions over the whole traverse.
Figure 3. Martian ventifact examples from the MER Spirit
site. Color images are Pancam image composites (RGB
equals 753 nm, 535 nm, and 432 nm bands). White arrows
indicate inferred formative wind directions. (a) Example of
grooves cut into outcrop observed on sol 698. (b) Rock with
elongate pits consistent with right-to-left abrasion (Pancam
432 nm band, sol 738). (c) Small aerodynamically sculpted
knob near Home Plate (sol 767). (d) Flute marks on concave
left edge of rock (sol 724).
therefore the hills may preserve evidence of any aqueous
processes associated with the early history of Gusev Crater
[Arvidson et al., 2006].
[8] On sol 157, Spirit reached the base of the Columbia
Hills and began the ascent of the area known as West Spur.
Numerous outcrops and rocks were investigated during the
course of the ascent that painted a more interesting and
diverse picture than the suite of rocks analyzed on the
plains. Many are poorly sorted clastic rocks, suggesting
an origin by impact and/or pyroclastic processes, that have
also undergone various degrees of aqueous alteration
[Squyres et al., 2006]. After driving near the summit of
Husband Hill on sol 630, the rover continued toward a
saddle region within the hills dubbed Interior Valley. On sol
704, the rover approached the field of dark-toned bed forms
dubbed El Dorado. After a week of observation [Sullivan et
al., 2008], the rover continued through Interior Valley to the
Home Plate feature, a low (2 – 3 m) layered plateau [Squyres
et al., 2007]. Numerous targets were investigated en route,
with the rover arriving in the vicinity of Home Plate on sol
744 (Figure 1). After skirting along the northwestern
margins of Home Plate for about 50 sols, the rover then
moved southeast to Low Ridge to position its solar panels to
maximize insolation while it remained motionless during its
second Martian winter campaign.
2.3. High Resolution Imaging Science Experiment Data
Overview
[9] To help identify and colocate small features along the
rover’s path, the traverse was plotted on images from the
2.4. Ventifact and Eolian Bed Form Overview
[10] In the broadest sense, the term ‘‘ventifact’’ refers to
objects sculpted by windblown particles. Rocks subject to
eolian sculpture in terrestrial settings commonly display
facets or cut faces, abrasion-polished surfaces, and scour
marks [Greeley and Iversen, 1985; Laity, 1994]. Here, we
focus on linear scour marks that tend to form under
predominantly unidirectional winds, including elongate pits,
flutes, and grooves (Figure 3). Elongate pits can form from
eolian abrasion alone, though they can also form from
modification of pits initiated by numerous other processes
such as chemical weathering or as primary vesicles that
form as a lava exsolves volatiles when cooling from a
molten state [e.g., Thomson and Schultz, 2003]. Flute marks
are scoop-shaped indentations that typically open downwind
[Sharp, 1949; Laity, 1994]. They are primarily found on nearhorizontal or shallowly inclined surfaces. Grooves, in contrast, are open at both ends and tend to be longer than flutes
[Laity, 1994]. They are commonly found on rock faces that
are parallel or gently inclined to the wind direction.
[11] Ventifact development has been noted in a wide
variety of environments on Earth [Laity, 1994], but ventifacts tend to be most abundant in arid environments where
mechanical erosion outweighs chemical erosion. Given an
abundance of sand and wind coupled with low levels of
recent chemical alteration and erosion on Mars inferred
from geochemical constraints [Haskin et al., 2005; Hurowitz
et al., 2006], it is perhaps unsurprising that ventifacts and
other landforms of eolian abrasion appear to be common on
the Martian surface [McCauley, 1973; Mutch et al., 1977;
Bridges et al., 1999; Greeley et al., 1999, 2004].
[12] High-resolution images of Mars from the HiRISE
instrument provide an unprecedented opportunity to observe
patterns of eolian bed forms over large surface areas
[Bridges et al., 2007]. In this paper, the term ‘‘dune’’ refers
to a ridge or hill-like accumulation of windblown sand with
distinguishable slip faces that is commonly, but not exclusively, dark-toned. A dual dune classification scheme is
necessary to separate morphologic dune types (linear, cresentric, and star [McKee, 1979]) from morphodynamic dune
types (transverse, longitudinal, and oblique [Hunter et al.,
1983]). Transverse dunes migrate in a direction at right
angles to their long axes (meaning the dune crest line is
oriented >75° from the wind vector and resulting sand
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Figure 4. Spirit traverse from sols 700 to 810 divided into four segments. Orientation and location of
ventifacts measured along the traverse are given by position and azimuth of red arrows. Also given in
each segment is a rose plot of ventifact orientations.
transport direction). A longitudinal dune, in contrast,
migrates parallel to its long axis (within 15° of the resulting
sand transport direction). Intermediate between these two
types are oblique dunes (with crest lines oriented between
15 and 75° of the resulting sand transport direction). The
term ‘‘ripple’’ is reserved for two types of features: (1) large
(>1 m wavelength), light-toned ridges of sand that are
covered with a monolayer of coarser granules [Greeley et
al., 2004, 2006], and (2) small subparallel ridges and
troughs (typical wavelengths 1 – 10 cm) of sand that
may or may not be armored with a layer of coarse particles
[Sullivan et al., 2005]. The dominant mode of grain transport in ripples is surface creep [e.g., Sharp, 1963]. In cases
where the classification is ambiguous between dunes and
large ripples (such as in the absence of particle size data
from the MER Microscopic Imager), the general term ‘‘bed
form’’ or ‘‘eolian ridge’’ is applied. Although many of the
larger bed forms are recognizable with lower-resolution
MOC images, HiRISE images have revealed abundant
secondary and even tertiary bed forms that are superposed
on the larger, first-order features [Bridges et al., 2007].
[13] Light-toned, linear bed forms are abundant in Martian trough floors [Malin and Edgett, 2001; Bourke et al.,
2004]. Some researchers have termed these types of features
as ‘‘transverse aeolian ridges,’’ but we do not favor this
nomenclature because it is a blend of morphologic (ridges,
which imply a linear morphology) and morphodynamic
(transverse) descriptors that may not be appropriate in all
cases. Rather, we use the aforementioned terms ‘‘eolian
ridges’’ or simply ‘‘bed forms,’’ except in cases where their
identification as dunes (saltation deposits) or ripples (creepdominated deposits) can be confirmed. Despite the caution
inherent in our terminology, the orientation of the largest (T1)
eolian ridges in the Columbia Hills relative to local topography indicates a transverse origin. Many of these bed forms
resemble eolian ridges elsewhere on Mars that are confined
to narrower troughs, where fluid dynamic models confirm
that wind flow is funneled parallel to the long axes of the
troughs because of the influence of topography [Bourke et
al., 2004]. Eolian ridges in the trough interior are orthogonal
to this direction, indicating that they are transverse features.
Similarly, the crest lines of bed forms in the Columbia Hills
are aligned more or less perpendicular to the long axis of
topographic basins (e.g., Inner Basin in Figure 4), a direction consistent with formation by winds affected by local
topography. This strongly suggests that the eolian ridges in
the Columbia Hills are transverse bed forms, where the bed
form crest line is near perpendicular (15° or less) to the wind
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vector and resulting sand transport direction. That is not to
say, however, that all Martian bed forms are transverse.
Longitudinal bed forms have been recognized along trough
walls [Bourke et al., 2004], and seif-like features have been
observed extending from elongated barchan dune horns
[e.g., Lee and Thomas, 1995; Fenton et al., 2003].
[14] An example bed form in the Columbia Hills is given
in Figure 2. The large, first-order (T1) eolian ridge suggests
formative winds that blow perpendicular to the strike of the
ridge crest, or roughly from the north and south in this
instance (in this paper, we used the terrestrial nomenclature
of Warren and Kay [1987], in which Tn describes a bed
form, with Tn+1 superposed on and orthogonal to Tn). Bed
forms formed under unidirectional winds tend to be asymmetric in cross section with a shallower upwind (stoss) side
and a steeper downwind (lee) slip face that approaches the
angle of response of granular material (about 32°– 34°). The
generally symmetric profile of many bed forms at the Spirit
site suggests reversing winds [Greeley et al., 2006, 2008].
Observations of small (<15 cm wavelength) terrestrial sand
ripples indicate that ripple asymmetry can be reversed over
short time scales (approximately minutes) by strong winds
blowing in the opposite direction [Sharp, 1963]. General
circulation models of large-scale wind flow (computational
grid cell size 7.5° longitude by 9° latitude) over the Gusev
Crater region indicate reversing winds over diurnal and
seasonal time scales [Greeley et al., 2006]. Superposed atop
the T1 eolian ridge in Figure 2 is an array of smaller, secondorder (T2) ridges oriented perpendicular to subperpendicular
(approximately 70° in this example) to the strike of the firstorder ridge crests. It has long been noted that large bed forms
themselves can affect both the speed and direction of incident
wind (see review of Kocurek [1991, pp. 60 – 62]). For
example, deflection of wind flow on the lee flank parallel
to the crest line has been observed in linear dunes [Tsoar,
1983]. This bed form – modified airflow, termed secondary
flow, has been shown to be a function of bed form shape and
the angle between the crest line and incident wind, among
other factors [e.g., Sweet and Kocurek, 1990]. The planimetric configuration of superposed eolian ridges in Figure 2
suggests that larger T1 ridges modify wind flow along their
lengths, and therefore constrain the orientation of T2 forms to
angles that are near perpendicular.
3. Methods
[15] Previously, a catalog of ventifact features observed in
Pancam images was compiled [Stone et al., 2006]. This
catalog, updated through sol 810, includes the sol on which
a particular image was acquired, the rover site number and
position counter, a description of the abrasion texture (if
present), and a qualitative indicator of the utility of the
ventifacts identified as a directional indicator. Once a
candidate ventifact is identified, stereo ranging software is
used to derive the positional information of suitable abrasion textural elements, in particular the endpoints of
grooves, flutes, elongate pits, and linear to sublinear scour
marks. Each pair of points is used to derive a vector in the
local rover coordinate frame that indicates the inferred
formative wind direction [Bridges et al., 1999; Greeley et
al., 2006]. These coordinates are then transformed from
local rover frame into a Mars body fixed coordinate frame
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and plotted along the rover traverse in a GIS. Mapped
ventifacts are overlain on a HiRISE image of the landing
site (PSP 001513 1655, Figures 1 and 4) to compare with
the orientation of local bed forms.
[16] In addition to ventifacts, wind directions inferred
from bed forms were also recorded. In contrast to unambiguous wind indictors such as dust streaks or linear
abrasion grooves on rocks, wind directions inferred from
bed forms are subject to a potentially greater degree of
uncertainty. Bed forms are not simple responses to regional
winds but are the product of a complex interaction between
bed form morphology and wind flow [Livingstone et al.,
2007]. In particular, bed form crest lines are responsive to
the range of all incident winds of sufficient strength to
initiate grain motion. Studies of sand ripples in bidirectional
flows indicate that the crest line orients itself such that bed
form normal (i.e., transverse) sand transport is maximized
[Rubin and Hunter, 1987]. Here, the orientation of T1 and
T2 ridges are measured by taking a linear fit to the crest line.
Because these bed forms are interpreted as principally
transverse, the inferred formative wind directions are taken
to be at right angles to the crest line. Given that the
symmetry of these eolian ridges implies reversing, bidirectional winds, two opposing wind directions are inferred for
each bed form. A caveat with this approach is that if these
inferred transverse bed forms are instead longitudinal or
oblique forms, the formative wind directions would differ
greatly from those construed here.
4. Results
[17] A rose plot of downwind orientations of all ventifacts
identified and mapped between sols 700 and 810 (Figure 5a)
reveals a dominant mode directed to the east, indicating
prevailing formative winds from the west (270°). The
overall distribution of ventifacts in Figure 5a is 40° offset
from the mode of the inferred formative wind directions of
wind streaks and dust devil tracks as determined from orbit
(310°, Figure 5b) [Greeley et al., 2006, 2008], most likely
because of the more complex wind flow patterns in the
Columbia Hills. Ventifact orientations also exhibit a greater
degree of scatter than orbital wind indicators, again suggestive of topography-related wind flow effects.
[18] Dividing the rover traverse between sols 700 and 810
studied here into four segments (Figure 4) reveals significant
variations depending on the local topography. These segments were selected to loosely group ventifacts into natural
clusters that share many common conditions: segment 1
includes the region around El Dorado; segment 2 encompasses Inner Basin; segment 3 defines a connecting valley
between Inner Basin and Home Plate, and segment 4 includes
the region around Home Plate. In segment 1 (sols 700– 714),
only a few rocks are present among the dark-toned bed
forms in El Dorado or the underlying, light-toned ridges.
The rover traverse in segment 2 (sols 715– 734) follows the
western edge of Inner Basin, and the majority of the ventifacts indicate formative winds blowing from generally west
to NW directions. However, a cluster of ventifacts near
Lorre Ridge indicates local winds blowing from the NE,
thus demonstrating the strong influence of local topography.
In segment 3 (sols 735– 742), the rover’s path traces a
narrow valley that connects Inner Basin to the Home Plate
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Figure 5. (a) Rose plot showing orientations of downwind directions of all mapped ventifacts in sols
700– 810. (b) Rose plot from Greeley et al. [2006] showing distribution of inferred downwind directions
from wind streaks and dust devil tracks measured from orbit.
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Figure 6. Compilation of rose plots for each traverse segment. (left) Inferred downwind directions from
ventifacts. Crest line orientation of (middle) first- and (right) second-order bed forms, T1 and T2,
respectively. Dark arrows are inferred formative wind directions (assuming these are transverse bed forms
formed under reversing wind conditions), taken as ±90° from the average measured ridge crest orientation.
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Figure 7. Histograms of the length of eolian ridge crest lines in the Columbia Hills (traverse segments
1 – 4) as measured from orbit with HiRISE data. (a) Histogram of first-order (T1) ridge lengths, with 2 m
bin size. (b) Histogram of second-order (T2) ridge lengths, with 0.5 m bin size.
area. Ventifacts indicate that local winds are funneled along
the length of the small curved valley. Finally, in the area
near Home Plate in segment 4 (sols 743– 810), ventifacts
indicate reversing east-west winds with a dominant mode
indicating prevailing winds from the west.
[19] In addition to ventifacts, T1 and T2 eolian ridges
were also identified in each traverse segment, and a linear fit
to the crest line was used to determine their orientations
(Figure 6). Most T1 ridges measured with HiRISE data have
long axes that are between 5 and 20 m in length, while T2
ridges average between 4 and 6 m in length (Figure 7). In
segment 1, T1 bed form crest lines are oriented NW – SE
(Figure 6). T2 ridges, in contrast, are oriented mostly NE –
SW. The few ventifacts present around the extensive T1 bed
forms south of El Dorado indicate formative winds coming
from the NE; ventifacts near the rover position on sol 700
exhibit more variability, presumably because of local topography. In segment 2, the overall topography is more
confined than in segment 1 – Inner Basin is a small valley
bounded by Lorre Ridge to the west and Allegheny Ridge to
the east (Figure 4). Here, T1 ridges are oriented roughly E–
W, and T2 ridges are again nearly orthogonal to the T1 trend.
In segment 3, while there are numerous well-developed
ventifacts (e.g., Figure 1b), no bed forms are evident in the
HiRISE image (Figure 4). Small submeter wavelength
ripple-like patterns can be observed in rover images along
this segment, but none are large enough to map using the
same technique used in the other segments. Finally, in
segment 4, bed forms were mapped in a region just to the
west of the region outlined in Figure 4. These bed forms are
the closest to the rover path along Home Plate that lie on the
west side of Mitcheltree Ridge. These T1 ridge crest lines
are largely oriented E– W, and superposed T2 forms are
mostly oriented N – S. But owing to the physical separation
between the ventifacts and bed forms in this segment, it is
possible that the ventifacts in this region are more a
reflection of the topography associated with Home Plate.
Therefore, general inferences of the relationships between
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Figure 8. Quantile box plot showing distribution of
orientation offsets between ventifacts and T1 or T2 eolian
ridges. Box midline represents median value; box ends
represent 25th and 75th percentile values. Orientation
offsets were calculated as the difference in azimuth between
inferred wind directions for ventifacts and the nearest eolian
ridge of each type. Since these bed forms appear to have
bidirectional formative winds, this constrains the distribution to values between 0° and 90° (instead of 0° – 180°).
ventifacts and bed forms in this segment should be made
with caution.
5. Discussion and Conclusions
[20] Comparing the inferred ventifact formative winds
with the wind directions inferred from adjacent eolian bed
forms (Figure 6), it is apparent that ventifacts are more
consistent with T2, not T1, eolian ridge trends. This visual
inference can be quantified by comparing the offset in
orientation between the formative wind direction indicated
by each ventifact to the wind direction inferred from the
nearest T1 and T2 ridges (see quantile box plots in Figure 8).
T1 ridges have a median offset 65°, while T2 ridges are
more closely aligned with ventifacts (median offset 25°).
This coincidence of alignment may be indicative of the
activity level of various scales of bed forms and may also
hint at the relative surface ages of ventifacts and bed forms.
[21] Based on MI measurements of bed forms in the
plains [Greeley et al., 2004, 2006], T1 ridges are covered
with a layer of coarse granules, and larger particles require
stronger, less frequent wind events to initiate particle
movement (via saltation-induced creep). From terrestrial
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experience, it is known that the rate of movement of large
ripples covered by a coarse-grained carapace is extremely
low [Zimbelman et al., 2007]. Similarly low movement
rates are also inferred for T1 ripples at the Meridiani landing
site [Sullivan et al., 2007]. In addition, infiltration of dust
particles into T1 ripples trenched by rover wheels on the
Gusev Crater plains is suggestive of inactivity or, at best,
infrequent activity [Greeley et al., 2006].
[22] Faster migration of small, ripple-like features in the
Columbia Hills was dramatically evidenced by Hazcam
images taken before (sol 1260) and after (sol 1265) a period
of strong wind events, showing downwind displacements of
about 2 cm [Sullivan et al., 2008]. T2 ridges in the
Columbia Hills are most likely dominated by sand-sized
particles, although this cannot be confirmed because the
rover did not specifically investigate any T2 forms in the
Columbia Hills. Previous observations of eolian landforms
elsewhere on Mars indicate that surface activity has been
limited both in degree of movement and physical scale (e.g.,
minor erosion of disturbed surfaces at the Viking 1 site
[Moore, 1985] or small avalanches on dune slip faces
[Fenton, 2006]). One rare exception is the erosion and
disappearance of dome dunes that have been recently
documented in the north polar erg [Bourke et al., 2008].
Nonetheless, the general inference can be drawn that
smaller bed forms have a higher activity level than larger
bed forms, especially larger bed forms that are armored by a
layer of coarse particles.
[23] The correspondence of ventifact orientations with T2
ridges is consistent with their inferred greater rates of
movement compared to T1 ridges. More frequent movement
implies a higher number of saltation events and a resulting
greater abundance of abrading particles impinging upon on
rock surfaces. We consider two hypotheses to account for
the observed distribution of ventifact and bed form orientations: (1) that ventifact and T2 ridge orientations may be
largely the result of bed form –modified (secondary) airflow; or (2) the T2 bed forms and ventifacts are the result of
a shift in wind regime (relative to the formative winds
inferred from T1 bed forms). This distinction between these
two scenarios is important because it is tied into the central
question of whether the ventifacts were formed in the
current wind regime, or if they are instead relict features
from a past epoch with different climatic conditions. At the
Mars Pathfinder site, a mismatch in inferred formative wind
directions between ventifacts and bed forms was taken as
evidence of climate change [Bridges et al., 1999; Greeley et
al., 2000]. Since bed forms are easier to overprint and
reorient than abrasion marks carved in rocks, the rocks at
the Pathfinder site were inferred to be relicts from a
previous eolian regime—the rocks appear to be relatively
older than the bed forms.
[24] Several lines of evidence lead us to favor the former
hypothesis, and there are several potential pitfalls with the
latter hypothesis. First, in all locations, T2 bed forms are
oriented perpendicular to near perpendicular to the T1
forms. So if the T2 bed forms are due to a new wind
regime, it seems quite fortuitous that it is exactly orthogonal
to the previous wind regime. Furthermore, not all T2 bed
forms share the same orientation. For example, the T2 forms
in segment 2 are 30° offset from those in segment 1
(Figure 6). Since there is not a single new T2 wind direction,
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Figure 9. Navcam images of T1 bed forms on Gusev Crater plains that lack superposed T2 ridges.
Images acquired on (a) sol 100, (b) sol 2, and (c) sol 111. Object visible in left foreground of Figure 9c is
edge of MER solar panel array.
large-scale topographic control would have to be evoked to
explain the spread in wind orientations (but small-scale
topographic control such that T1 bed forms modify wind
direction would have to remain excluded). Third, the lack of
evidence for the reorientation of T1 bed forms is at odds
with a change in the dominant wind regime. All bed forms
can be considered ideally transverse; oblique and longitudinal bed forms are the result of multiple wind regimes and
can be thought of as ‘‘frustrated’’ transverse forms (except
where anchored by topography). Given sufficient time
under unidirectional wind conditions, the T1 bed form crest
lines should gradually reorient to be transverse to the new
wind direction and maximum sand transport direction
[Rubin and Hunter, 1987]. But this is not observed to be
the case here. Even considering that the larger T1 forms are
probably granule armored and are therefore largely unresponsive to all but the highest wind speeds, it remains
problematic that this new set of winds would be everywhere
orthogonal to the directionality of T1 bed forms.
[25] An additional piece of evidence is ventifact and bed
form orientations on the Gusev plains. In contrast to
ventifacts in the Columbia Hills, inferred wind directions
from ventifacts on the Gusev plains generally agree with
those inferred from first-order (T1) eolian bed forms [Greeley
et al., 2006]. Out on the plains, wind flow is less constrained by topography than in the hills, and therefore the
T1 ripples do not represent a significant impediment to the
wind. Indeed, T1 ripples on the plains appear to lack
superposed T2 ridges (Figure 9), which support the inference that these T1 ripples do not concentrate or significantly modify wind flow. A recent HiRISE image of the
Pathfinder site (PSP 001890 1995) reveals a similar lack
of superposed T2 ridges on bed forms in the plains
discernable from orbit. Therefore, if the T2 bed forms
and ventifacts in the Columbia Hills are the result of shift
in wind direction, this shift is not recorded in any current
or paleowind indicators found throughout the explored area
of the Gusev plains.
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THOMSON ET AL.: ROCK ABRASION IN THE COLUMBIA HILLS
[26] Unlike the ventifacts at the Pathfinder site, we
therefore conclude that there is no strong evidence for a
shift in climate over time as evidenced by a mismatch
between the inferred wind directions from ventifacts and
bed forms. However, although we favor the role of bed
form – modified wind flow, we cannot completely exclude
the hypothesis that the T2 ridges and ventifacts are the result
of a new wind regime. A few issues remain to be fully
explored, such as the survival of T2 bed forms on the
windward flanks of T1 ridges in reversing wind regimes,
and the apparent alignment of some T2 bed forms on
opposite sides of a T1 ridge crest line (such as in Figure 2).
It is possible that some smaller bed forms have migrated onto
the T1 forms from the adjacent, interripple or interdune areas,
but these issues remain the subject of future lines of inquiry.
[27] In either of the two hypotheses explored here, a
puzzle remains as to why no ventifacts are oriented consistent with T1 formative winds. If the rocks had been in place
before the T1 ridges formed, one would expect them to
record some evidence for abrasion consistent with the
directionality implied by T1 forms. Ventifacts generally
have a high preservation potential and thus provide a record
of the integrated energy expended by impacts of all abrading particles on their surface. Therefore, the lack of ventifacts with formative wind directions consistent with T1 bed
forms may indicate that the rock abrasion textures are
relatively younger than the T1 bed forms. It is possible that
more frequent abrasion events consistent with T2 ridge
geometry has massively overprinted or erased any abrasion
marks consistent with T1 ridges. The rocks might still be the
oldest surface constituents, but their resurfaced outer layers
no longer record the oldest winds that have shaped this
region. A related possibility is that most rocks were
emplaced after the formation and stabilization of T1 bed
forms. MiniTES analyses of rocks in the Columbia Hills
suggests many rocks may be ‘‘exotic,’’ i.e., impact
emplaced [Ruff et al., 2006]. Although no superposed
impact craters have yet been recognized on T1 bed forms
that would corroborate their older relative age, the lack of
craters may be due to active T1 flank reworking by
superposed T2 ridges. It is therefore possible that stabilized,
but not static, T1 bed forms are among the oldest features of
the landscape (excluding rock outcrops).
[28] Returning again to the observed differences between
the environments of the Gusev plains and the Columbia
Hills, the inferred greater relative age of the T1 bed forms is
consistent with the hypothesis that ventifact formation in the
Columbia Hills is dominated by bed form – modified airflow
(that is, in a direction commensurate with T2 bed forms). As
evidenced by ventifact orientations on the Gusev plains
[Greeley et al., 2006], abrasion can indeed occur from
winds consistent with the directionality of T1 bed forms.
In the Columbia Hills, however, the larger size and generally closer spacing of T1 forms compared to the plains may
result in more bed form – modified wind flow into directions
consistent with T2 bed forms. In this conceptual framework,
there is a competition between T1 and T2 abrasion potentials
that differ in the two environments (hills versus plains). In
the plains, there appears to be little topographic funneling of
winds such that T1 abrasion dominates, even if the rate is
very low. In the Columbia Hills, there may be a similar low
rate of T1 abrasion, but it appears to be overprinted by a
E08010
higher rate of T2 abrasion. In the Columbia Hills, T1
abrasion may have only dominated in the initial stages of
T1 ripple formation, before topographic funneling of winds
was operative.
[29] Acknowledgments. This paper benefited from detailed reviews
by M. Bourke and J. Zimbelman. This research was supported by a grant
from NASA’s Mars Data Analysis Program and by an appointment to the
NASA Postdoctoral Program (administered by Oak Ridge Associated
Universities under a contract with NASA) and was carried out at the Jet
Propulsion Laboratory, California Institute of Technology, under a contract
with the NASA.
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