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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L20204, doi:10.1029/2007GL031111, 2007
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HiRISE observations of slope streaks on Mars
Frank C. Chuang,1 Ross A. Beyer,2,3 Alfred S. McEwen,4 and Bradley J. Thomson5
Received 23 June 2007; revised 16 August 2007; accepted 5 September 2007; published 24 October 2007.
[1] Images from the High Resolution Imaging Science
Experiment have revealed new details on the morphologic
and topographic characteristics of slope streaks on Mars.
Over 1500 HiRISE images were analyzed with 78 unique
image sites having slope streaks. Images with low sun
illumination reveal that dark slope streaks have topographic
relief where streaked surfaces are lower than their
surroundings. Slope streaks often initiate below localized
features such as rock outcrops, individual boulders, and
impact craters. They are also abundant in great numbers
within the blast zones of small young impact craters 10–
50 m in diameter. These observations suggest that slope
streaks can be triggered by localized disturbances such as
rockfalls and impact blasts. Seismic activity from external
(e.g., impacts) or internal forces could also trigger slope
streaks. The topographic relief and triggering mechanisms
of slope streaks seem to best fit models that involve dry dust
avalanches. Martian slope streaks and meters-thick avalanche
scars are part of a continuum of active mass-wasting features
at meter to sub-meter scales. Citation: Chuang, F. C., R. A.
Beyer, A. S. McEwen, and B. J. Thomson (2007), HiRISE
observations of slope streaks on Mars, Geophys. Res. Lett., 34,
L20204, doi:10.1029/2007GL031111.
1. Introduction
[2] Slope streaks are common features in the equatorial
regions of Mars and are easily recognizable from their
contrast in tone relative to their surrounding surfaces.
These features have been identified since the early
Mariner and Viking missions to Mars and have received
more imaging from recent missions beginning with Mars
Global Surveyor (MGS) [Malin and Edgett, 2001;
Schorghofer et al., 2002; Aharonson et al., 2003]. Early
studies using Viking data suggested that slope streaks
formed as a result of debris weathering, stains from wet
flows, or erosional landslips [Morris, 1982; Ferguson and
Lucchitta, 1983; Williams, 1991]. More recent wet-based
models include briny liquid flows, groundwater piping,
mixed water-dust flows, and ground wetting and/or wick-
ing from salty liquids [Ferris et al., 2002; Motazedian,
2003; Miyamoto et al., 2004; Head et al., 2007]. Drybased models include mass-wasting of dust, granular
flows, and avalanching of heterogeneous dust accumulations along slopes [Sullivan et al., 2001; Treiman and
Louge, 2004; Baratoux et al., 2006].
[3] Although most streaked surfaces have dark streaks,
light-toned streaks are also observed and some individual
streaks transition from light to dark along their paths.
Dark slope streaks are thought to fade over time by
mantling of new dust [Sullivan et al., 2001; Motazedian,
2003]. The widths of individual slope streaks vary, but
are generally a few tens to hundreds of meters across at
their widest location. Their lengths also vary from tens of
meters to a few kilometers. A single slope streak can split
into two separate streaks, form anastamosing patterns, and
develop multiple fingers at its terminus. Deposits at the
termini of streaks are generally absent, not resolved.
Repeat imaging by the Mars Orbiter Camera (MOC)
[Malin et al., 1992] indicates that new slope streaks are
forming within the time between two or more images,
which can be up to several years [Malin and Edgett,
2001; Aharonson et al., 2003]. Gerstell et al. [2004]
identified ‘‘meters-thick avalanche scars,’’ which they
indicated were a separate population of mass-wasting
features from slope streaks. Although avalanche scars
are apparently concentrated in the lower reaches of
Olympus Mons aureole deposits, they are likely to be
as widespread as slope streaks on the Martian surface.
[4] The High Resolution Imaging Science Experiment
(HiRISE) instrument [McEwen et al., 2007a] onboard the
Mars Reconnaissance Orbiter (MRO) spacecraft [Zurek
and Smrekar, 2007] provides the opportunity to study
slope streaks in greater detail than could be done with
MOC. Here we present the morphologic and topographic
characteristics of slope streaks, and their possible triggering mechanisms from the first 7 months of data (11/2006 –
5/2007) during the 2-year MRO Primary Science Phase
(PSP).
2. HiRISE Coverage
1
Planetary Science Institute, Tucson, Arizona, USA.
Carl Sagan Center at the Search for Extraterrestrial Intelligence
Institute, Mountain View, California, USA.
3
NASA Ames Research Center, Moffett Field, California, USA.
4
Lunar and Planetary Laboratory, University of Arizona, Tucson,
Arizona, USA.
5
NASA Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, California, USA.
2
[5] Early in the PSP, most of the slope streak-specific
targets were sites previously imaged by MOC, but also
included sites not imaged by MOC. This study examined
over 1500 HiRISE images from orbits 1300 to 3600 at
resolutions up to 25– 32 cm/pixel. Of these images, 106
(7%) have slope streaks and 22 have stereo coverage to
look specifically for newly formed streaks. Of these
locations with stereo pairs, 3 had new streaks in the
23– 67 days between the two images (mean 41 days).
Overall, 78 unique sites with slope streaks were identi-
Copyright 2007 by the American Geophysical Union.
0094-8276/07/2007GL031111$05.00
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Figure 1. Portion of HiRISE image PSP_003259_1850 (5.0°N, 32.7°E) near Naktong Vallis with dark slope streaks
located along the north facing interior slope of an impact crater. Arrows point to the margin of slope streaks that have
topographic relief where the streaked surface is lower than the surrounding un-streaked surface. The ability to detect
topographic relief is highly dependent on the spatial resolution, low sun illumination, dust thickness, and slope face
orientation in the images. Note how several of the streaks are triggered by impact craters that have dark ejecta. Image
resolution is 54 cm/pixel.
fied. (Table S1 of 78 unique sites with HiRISE image IDs
and their locations is available as auxiliary material.)1
3. Distribution and Observations
[6] Slope streaks and avalanche scars in HiRISE images
occur within the latitudinal range from approximately 39°N
to 28°S, consistent with previous global studies of these
features [Schorghofer et al., 2002; Aharonson et al., 2003;
Gerstell et al., 2004]. Their locations in thermal inertia data
[Putzig et al., 2005] have a range of 30– 293 J m 2 K 1
s 0.5 and a mean and standard deviation of 88 ± 41 J m 2
K 1 s 0.5. Approximately 90% of the values are 116 J
m 2 K 1 s 0.5, consistent with previous estimates of low
thermal inertia at slope streak locations [Schorghofer et al.,
2002, 2007].
3.1. Topographic Relief
[7] Prior to HiRISE, studies of slope streaks had little or
no discussion of discernable topographic relief. Analyses of
MOC images indicated that they did not disturb the preexisting surface and left only a dark or light-toned trail as the
youngest surface feature [Sullivan et al., 2001; Motazedian,
2003]. Analyses of slope streaks from HiRISE data indicate
that these features do have topographic relief as a result of
1
Auxiliary materials are available at ftp://ftp.agu.org/apend/gl/
2007gl031111.
their formation (Figure 1). Images with low sun illumination
reveal that the area within the streak has a slightly lower
surface than its surroundings. The topographic signature and
outer margin of a streak are particularly evident where the
tone of the streaked surface is the same as the surrounding
surface. In addition to low sun illumination, the ability to
detect topographic relief is also dependent on the spatial
resolution, dust thickness, and slope face orientation. For
example, images of slope streaks at the highest possible
resolution, in an area with significant dust coverage, and
along shadowed slopes are optimal conditions for detecting
streak relief. Slope streaks do not consistently have the same
depth and in some cases, a small area (with similar lighting
and viewing conditions) with multiple streaks can have some
streaks with clear topographic relief, and others with little to
no apparent relief.
[8] Shadow length measurements along the edge of two
slope streaks in HiRISE image PSP_001472_1745 indicate
that their depths are 1 m. The thin amount of material
removed is supported by the observation of meter-scale
ripples that can sometimes be traced across both the streak
and un-streaked surfaces (Figure 2). Within the lower
surface of some slope streaks, multiple longitudinal ridgelike structures roughly parallel to the scar margins are
observed (Figure 3). The width of these structures are
typically 1 –2 meters across and ridge pairs can sometimes
be traced to an approximate point below the scar’s source
area.
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Figure 2. Portion of HiRISE image PSP_001472_1745 (5.3°S, 213.7°E) with slope streaks along the east-facing slopes of
a highland escarpment near Mangala Valles. Meter-scale ripples (arrows) can be traced across both streaked and un-streaked
surfaces suggesting a thin layer of material was removed. Shadow length measurements along margin of two slope streaks
indicate depths of 1 m. Image resolution is 26.5 cm/pixel and downslope direction is to the east.
Figure 3. Portion of HiRISE image PSP_002586_1880 (8.0°N, 45.6°E) along the interior slopes of an impact crater
in Terra Sabaea. Each of the light-toned slope streaks that reach the crater floor (to the north-northeast) can be traced to a
point below a small impact crater. Longitudinal ridges parallel to the scar margins are visible within the streaked surface
(arrows). Note how the eastern margin of the right streak traveled over an impact crater without disruption. Image
resolution is 27 cm/pixel.
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Figure 4. Portion of HiRISE image PSP_003569_2035 (23.2°N, 207.6°E) along the interior slopes of an impact crater in
Amazonis Planitia. A boulder that bounced repeatedly downslope towards the northeast has triggered dark slope streaks at
many of its bounce points (dark dashed trail). Image resolution is 29 cm/pixel.
3.2. Triggering Mechanisms
[9] Slope streaks on Mars generally initiate at a small
area or point. From MOC images, these initiation sites often
appear along featureless parts of the slope, but in some
cases, they can start below knobs, spurs, crater rims, or
other areas of localized steepening [Sullivan et al., 2001;
Gerstell et al., 2004]. Detailed examination of HiRISE
images indicates that many of these localized sites have
positive-relief features such as rock outcrops, individual
boulders, and impact craters. Rock outcrops typically form
horizontal ledges, creating a small break-in-slope where the
slope below the ledge appears to be steeper than the overall
wall slope. It is at these steeper parts where many slope
streaks initiate. Large individual boulders along wall slopes
are often eroded materials from the top of the slope that
have rolled or bounced downslope. Figure 4 shows one case
Figure 5. Portion of HiRISE image PSP_002764_1800 (0°N, 226.9°E) in the blast zone north of a cluster of impact
craters west of Tharsis Montes. (left) Image in simple cylindrical projection has thousands of dark slope streaks along the
slopes of the hummocky terrain. The number of dark streaks along concentric arcs at 1, 3, and 5 km distances from the
approximate center of the blast site (white dot) decreases from 305 at 1 km to 65 at 5 km. (right) White boxed area in
left image shows dark slope streaks in detail, many with topographic relief, particularly along north-northwest-facing slopes
that face away from the blast direction. Image resolution is 27 cm/pixel.
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where a rockfall boulder triggered slope streaks at many of
its bounce spots. Small impact craters that trigger a slope
streak are often less than a few tens of meters in diameter
and some have dark radiating ejecta (see Figure 1). Several
HiRISE images of very recent (perhaps <10 yr) impact
craters 10– 50 m in diameter have large blast zones extending many kilometers from the impact site [Malin et al.,
2006; McEwen et al., 2007b]. Within these blast zones, the
terrain is hummocky with hundreds to tens of thousands of
dark slope streaks along the slopes of individual hummocks
(Figure 5). Many of the streaks form within small narrow
chutes with ridge-like margins.
4. Discussion
[10] Evidence of topographic relief within slope streaks
from HiRISE images clearly indicates that their formation
involves removal of material. This observation and those
made by C. B. Phillips et al. (Mass movement within a
slope streak on Mars, submitted to Geophysical Resesarch
Letters, 2007, hereinafter referred to as Phillips et al.,
submitted manuscript, 2007) casts doubt upon previous
slope streak models that suggest staining, wetting, or
wicking of the surface by liquids. Because many slope
streaks occur in areas that have little or no evidence of
fluvial activity in recent geologic time, models involving
liquid water or other types of fluid flow are not required to
explain many, if not all of the observed slope streaks. In
light of these new observations from HiRISE, models that
involve dry dust avalanches seem to best fit the morphologic and topographic characteristics of Martian slope
streaks [Williams, 1991; Sullivan et al., 2001; Miyamoto
et al., 2004; Gerstell et al., 2004; Baratoux et al., 2006].
[11] HiRISE observations indicate that slope streaks and
avalanche scars can be triggered in several different ways
that involve a localized disturbance along a sloped surface.
Weathering and erosion of exposed layers or bedrock near
the top of the slope produces boulders or blocks that fall and
strike the surface below, triggering an avalanche within a
layer of dust. If the initial fall is a considerable distance, the
momentum may be sufficient to cause multiple bounces,
triggering more than one dust avalanche. Figures 1 and 5
show that impacts on the surface or the force of the blast
from impact explosions can also trigger dust avalanches,
forming single or multiple slope streaks. From Figure 1, the
albedo or darkness of the ejecta appears to be as dark as the
streak interior, suggesting that both features were formed
very recently if streaks only remain visible on the surface
for decades, possibly centuries [Schorghofer et al., 2007]. In
Figure 5, we have counted the number of dark streaks along
concentric arcs at 1, 3, and 5 km distances from the
approximate origin of the blast site. The number of streaks
decrease from 305 at 1 km to 65 at 5 km, but the hummocky
terrain of the blast region extends beyond 5 km. This
suggests that the atmospheric forces from the impact only
trigger slope streaks within a limited area. Seismic activity
from external [Davis, 1993] or internal forces could also
directly trigger slope streaks, or indirectly via rockfall
events.
[12] Although we have described many slope streaks and
avalanche scars with clear triggering mechanisms, there are
also many that initiate where no mechanism is observed. In
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these cases, the disturbance may occur by eolian activity
where saltating fine materials, possibly pebble-sized grains
(i.e., objects unresolvable to HiRISE), strike the surface and
trigger dust avalanches. Also, the triggering of slope streaks
below localized features could be due to the preferential
accumulation and avalanching of dust on the downstream
side of objects in the path of wind flow [Baratoux et al.,
2006].
[13] Gerstell et al. [2004] concluded on the basis of a
suite of observations and statistical analyses that metersthick avalanche scars are a distinct class of mass wasting
features separate from slope streaks. While the observed
avalanche scars may be the result of deeper excavation,
many of their lines of evidence, based on MOC observations at the time of their study, are less compelling in
HiRISE images. One characteristic of meters-thick avalanche scars that are less common to slope streaks are their
relatively straight margins unaffected by topographic
obstacles such as small hollows, impact crater rims, and
boulders. This characteristic is likely due to the amount of
material involved where the deeper excavated avalanche
scars forms a potentially more coherent mass movement
with greater momentum, and are less affected by obstacles
along their path. This behavior is not entirely unique to
avalanche scars and has been observed by Phillips et al.
(submitted manuscript, 2007) in some slope streaks near
their apexes along steeper portions of the slope.
[14] Our analysis of dark slope streaks and meters-thick
avalanche scars from HiRISE images suggests that the two
features are related and part of a continuum of active masswasting features at meter to sub-meter scales formed by
dust avalanches. Several lines of evidence support their
relationship: (1) occurrence in the same equatorial latitudes,
(2) formation in similar geologic settings and local regions,
(3) initiation from a point source, and (4) topographic relief
where material has been removed by mass-wasting processes. HiRISE observations indicate that dark slope streaks
without observable depth in MOC images do have topography and are thinner than meters-thick avalanche scars.
While many slope streaks do not appear to have topography
in HiRISE images, this does not indicate that topography is
absent, but that the excavated dust is not sufficiently thick to
be resolved by HiRISE data acquired at the 3 PM local
surface time. The presence and continuing formation of slope
streaks on Mars further highlights the modification of the
current-day surface by mass-wasting and eolian processes.
[15] Acknowledgments. We thank D. Baratoux and N. Schorghofer
for their reviews which helped improve the quality of this manuscript. We
are indebted to the entire HiRISE Team, MRO Project, and Ball Aerospace
& Technologies Corp. whose efforts have produced spectacular HiRISE
images. Image credits for this study: NASA/JPL/University of Arizona.
This work was supported by NASA under subcontract Y432800 issued
through the University of Arizona and the MRO Participating Scientist
Program issued through the Science Mission Directorate. This manuscript is
PSI contribution 422.
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F. C. Chuang, Planetary Science Institute, 1700 E. Fort Lowell Road,
Suite 106, Tucson, AZ 85719, USA. ([email protected])
A. S. McEwen, Lunar and Planetary Laboratory, University of Arizona,
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B. J. Thomson, NASA Jet Propulsion Laboratory, California Institute of
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