DARK SLOPE STREAKS ON MARS: FORMATION, CHANGES, AND FADING. C. B. Phillips1 and C. F.
Chyba2, 1SETI Institute, 515 N. Whisman Rd., Mountain View, CA 94043, phillips@seti.org, 2Princeton University,
cchyba@princeton.edu
Introduction: Dark slope streaks on Mars are an
enigmatic feature type observed at low resolution by
the Viking Orbiters, and at much higher resolution by
the Mars Orbiter Camera (MOC) on the Mars Global
Surveyor (MGS) spacecraft, currently in orbit around
Mars. These dark (and occasionally bright) streaks
have a maximum contrast of about 10% with the
surrounding terrain, and appear to issue from a point
source on a slope that is often associated with
roughness or knobs (Figure 1a). The streaks then
continue downslope for up to a kilometer (or more),
and have a narrow, well-contained shape that spreads
out into a single fan shape or multiple, sometimes
anastomosing or braided, channels as it proceeds
down the slope (Figure 1b, 1c).
Figure 1: Slope Streak Examples
Slope streaks have sharp boundaries, with a nearly
constant brightness throughout the feature, and their
albedo contrast with the surrounding terrain has been
seen to fade over time. They are associated with
particular temperate locations, in regions of low
thermal inertia.
Dark slope streaks are particularly interesting
because they represent one of the most dynamic
geological processes currently affecting the Martian
surface. New slope streaks have been identified in
comparisons between Viking and MOC images, and
new streaks have even observed forming during the
Mars Global Surveyor mission, on timescales as short
as about 100 days. Thus, slope streaks are one of the
most common forms of mass movement currently
taking place on Mars.
Formation Models: Two major classes of models
have been proposed. The dry models suggest that
streaks form through dust movement, while the wet
models suggest that water, ice, and/or brine are
present to lubricate or stain the surface. In the dry
process, oversteepening slopes caused by air fall
deposits of dry dust eventually collapse, forming a
dust avalanche. Williams [1] was one of the first
researchers to propose a version of this model, which
was expanded on in greater detail using MOC images
by [2].
The second main class of models requires a wet
origin for slope streaks, but the role of water varies
between models. Early models such as [3] suggested
that the dark streaks could be stains on the surface
produced by wet, briny debris flows. These flows
could form when a slope intersected an aquifer,
allowing the periodic release of fluid from a wet
subsurface layer. Later models have found other
roles for water. [4] suggest that perhaps water could
lubricate avalanches, or the sublimation of nearsurface ice could trigger mass movements. A
different model [5] modifies the suggestion of [3] by
positing a formation mechanism that includes the
presence of groundwater springs which infiltrate and
saturate the surface, creating the dark streaks.
Change Detection: We are searching the database of
MOC images for overlapping image pairs that
contain slope streaks, and are documenting changes
with an iterative coregistration and ratioing
technique. One such example is shown below in
Figure 2. We hope to find various details of how
slope streaks form, change, or fade over time that
could support one of the two major classes of models.
If slope streaks are found to require the presence of
liquid water, such features could have major
astrobiological significance and be important targets
for future missions.
Figure 2: Ratio example
References: [1] Williams, S. H. (1991) LPS XXII, 1509-1510. [2]
Sullivan, R., et al. (2001) JGR 106, 23607-23633. [3] Ferguson,
H., M., and B. K. Lucchitta (1984) In Reports of the Planetary
Geol. Program 1983, NASA Tech. Memo. TM-86246,188-190. [4]
Schorghofer, N., et al. (2002) Geophys. Res. Lett., 29 (23), 2126.
[5] Ferris, J. C., et al. (2002) Geophys. Res. Lett., 29, 10,1490.
Acknowledgements: The authors acknowledge the contributions
made by Supreet Sidhu and support provided by MDAP.