Dark slope streaks are narrow, avalanche-like features common on dust-covered slopes in the equatorial regions of Mars.Chuang, F.C.; Beyer, R.A.; Bridges, N.T. (2010). Modification of Martian Slope Streaks by Eolian Processes. Icarus, 205 154–164. They form in relatively steep terrain, such as along escarpments and crater walls.Schorghofer, N.; Aharonson, O.; Khatiwala, S. (2002). Slope Streaks on Mars: Correlations with Surface Properties and the Potential Role of Water. Geophys. Res. Lett., 29(23), 2126, . Although first recognized in Viking Orbiter images from the late 1970s,Morris, E.C. (1982). Aureole Deposits of the Martian Volcano Olympus Mons. J. Geophys. Res., 87(B2), 1164–1178. dark slope streaks were not studied in detail until higher-resolution images from the Mars Global Surveyor (MGS) and Mars Reconnaissance Orbiter (MRO) spacecraft became available in the late 1990s and 2000s.Chuang, F.C. et al. (2007). HiRISE Observations of Slope Streaks on Mars. Geophys. Res. Lett., 34 L20204, .
The physical process that produces dark slope streaks is still uncertain. They are most likely caused by the mass movement of loose, fine-grained material on oversteepened slopes (i.e., dust avalanches).Barlow, 2008, p. 141. The avalanching disturbs and removes a bright surface layer of dust to expose a darker substrate. In a study of over 500,000 dark slope streaks, researchers found that streaks were more likely to form in dusty areas instead of moist places. The authors also determined that slope streaks modify less than 0.1% of the martian surface, but transport several global storm equivalents of dust per Mars year; hence, potentially playing a major role in the martian dust cycle."Valentin, T. and A. Valantinas. 2025. Streaks on martian slopes are dry. Nature Communications.
The role that water and other volatiles plays, if any, in streak formation is still debated. Slope streaks are particularly intriguing because they are one of the few geological phenomena that can be observed occurring on Mars in the present day.Kreslavsky, M.A.; Head, J.W. (2009). Slope Streaks on Mars: A New "Wet" Mechanism. Icarus, 201 517–527.Aharonson, O.; Schorghofer, N.; Gerstell, M.F. (2003). Slope Streak Formation and Dust Deposition Rates on Mars. J. Geophys. Res., 108(E12), 5138, .Dundas, C. 2018. HIRISE OBSERVATIONS OF NEW MARTIAN SLOPE STREAKS. 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083). 2026.pdfJunior, C. and T. Statella. 2023. Monitoring martian slope streaks in the northeast of Lysus sulci. Icarus. 406: 115737.Bhardwaj, A., et al. 2018. Are Slope Streaks Indicative of Global-Scale Aqueous Processes on Contemporary Mars? Reviews of Geophysics: Volume 57, Issue 1 p. 48-77
Nature of streaks on Mars Dark slope streaks are albedo features. They appear to the eye as a brightness difference between the streak and the lighter-toned background slope. Usually no topographic relief is visible to distinguish the streak from its surroundings, except in the very highest resolution (<1 m/pixel) images. In many cases, the original surface texture of the slope is preserved and continuous across the streak, as though unaffected by events involved in dark streak formation (pictured left). The overall effect is equivalent in appearance to a partial shadow cast down the sloping surface. These observations indicate that whatever process forms the streaks, it affects only the very thinnest layer at the surface. Slope streaks are only about 10% darker than their surroundings but often appear black in images because the contrast has been enhanced (stretched).Baratoux, D. et al. (2006). The Role of the Wind-Transported Dust in Slope Streaks Activity: Evidence from the HRSC Data. Icarus, 183 30–45.
Albedo features cover the Martian surface at a wide variety of scales. They make up the classical light and dark marking seen on Mars through telescopes. (See Classical albedo features on Mars.) The markings are caused by differing proportions of dust covering the surface. Martian dust is bright reddish ochre in color, while the bedrock and soil (regolith) is dark gray (the color of unaltered basalt). Thus, dusty areas on Mars appear bright (high albedo), and surfaces with a high percentage of rocks and rock fragments are generally dark (low albedo).Barlow, 2008, p. 73. Most albedo features on Mars are caused by winds, which clear some areas of dust, leaving behind a darker lag. In other areas, dust is deposited to produce a bright surface. The selective removal and deposition of dust is most conspicuous around impact craters and other obstacles where a variety of streaks (wind tails) and blotches are formed.Hartmann, 2003, pp. 36–41.
Dark slope streaks are relatively small features. (See A in Photo Gallery.) They differ from larger albedo features in being produced by gravity rather than wind, although wind may contribute to their initial formation.Schorghofer, Aharonson, O.; Gerstell, M.F.; Tatsumi, L. (2007). Three Decades of Slope Streak Activity on Mars. Icarus, 191 132–140, . (See B in Photo Gallery.) The cause of the darkening is uncertain. The particle sizes involved are believed to be very small (sand, silt, and clay-sized particles). No clasts large enough to be imaged are present, and the underlying bedrock slope is never exposed ( i.e., dust is avalanching on a surface of dust).Malin, M.C.; Edgett, K.S. (2001). Mars Global Surveyor Mars Orbiter Camera: Interplanetary cruise through primary mission. J. Geophys. Res., 106(E10), 23,429–23,570. Apparently, other optical, mechanical, or chemical properties are involved in producing the darker tone.
Dark slope streaks commonly share the same slope with other slope streaks of varying tones. The darkest streaks are presumed to be youngest; they have margins that are more sharply defined than streaks that are not as dark. This relationship suggests that streaks lighten and become more diffuse with age, probably because they become covered with fresh dust falling from the atmosphere. Faded dark slope streaks should not be confused with bright slope streaks (discussed below). Dust storms are common on Mars. At times the whole planet is enveloped in a dust storm, as shown in the picture below.
Morphology and occurrence
At moderate resolutions (20–50 m/pixel), dark slope streaks appear as thin, parallel filaments aligned downslope along crater rims and escarpments. They are often straight but may also be curved or sigmoid in shape. (See C in Photo Gallery.) Closer up, dark slope streaks typically have elongated, fan-like shapes (pictured right). They range from about 20 to 200 meters in width and are generally several hundred meters to over 1,000 meters long. Dark slope streaks exceeding 2 kilometers in length are uncommon; most terminate on slope and do not extend further out on to level terrain.
A streak commonly starts at a single point (apex) high on the slope. The apex is often associated with an isolated small ridge, knob, or other area of local steepening. In high-resolution images, a tiny impact crater is sometimes visible at the apex. Slope streaks widen downslope from the apex in a triangular fashion, usually reaching their maximum widths short of the halfway point of their lengths. A single slope streak can split into two separate streaks around an obstacle or form an anastamosing (braided) pattern. (See D and E in Photo Gallery.) Slope streaks commonly develop multiple fingers (digitation) at their downslope ends.
Images from the High Resolution Imaging Science Experiment (HiRISE) on MRO have shown that many slope streaks have relief, contrary to earlier descriptions in which no topographic distinction could be seen between the streaked and adjacent, non-streaked surface. The streaked surface is typically about 1 m lower than the non-streaked surface. This relief is only visible in maximum resolution images under optimal viewing conditions.Phillips, C. B.; Burr, D.M.; Beyer, R.A. (2007). Mass Movement within a Slope Streak on Mars, Geophys. Res. Lett., 34 L21202, .
Dark slope streaks are most common in the equatorial regions of Mars, particularly in Tharsis, Arabia Terra, and Amazonis Planitia (pictured left). They occur between latitudes 39°N and 28°S. At their northern limits, they appear preferentially on warmer, south facing slopes. Curiously, slope streaks are also associated with areas that reach peak temperatures of 275K (2 °C), a temperature close to the triple point of water on Mars. This relationship has led some researchers to suggest that liquid water is involved in dark slope streak formation.
Dark slope streaks do not appear to correlate with elevation or areas of specific bedrock geology. They occur on a wide range of slope textures, including surfaces that are smooth, featureless, and presumably young, as well as older, heavily cratered slopes. However, they are always associated with areas of high surface roughness, high albedo, and low thermal inertia, properties that indicate steep slopes covered with a lot of dust.Putzig, N.E. et al. (2005). Global Thermal Inertia and Surface Properties of Mars from the MGS Mapping Mission. Icarus, 173 325–341.
It has been suggested that streaks could form when accumulations of dry ice start subliming right after sunrise. Nighttime CO2 frost is widespread in low latitudes.Heavens, N., et al. 2017. WIDESPREAD LOW-LATITUDE DIURNAL CO2 FROST ON MARS. Lunar and Planetary Science XLVIII (2017). 1485pdf.
Formation mechanism
Researchers have proposed a number of mechanisms for dark slope streak formation. The most widely held view is that the streaks are the result of dust avalanches produced by dry granular flowValentin, T. and A. Valantinas. 2025. Streaks on martian slopes are dry. Nature Communications. on oversteepened slopes. Dust avalanches resemble loose snow avalanches on Earth. Loose snow avalanches occur when snow accumulates under cold, nearly windless conditions, producing a dry, powdery snow with little cohesion between individual snow crystals. The process produces a very shallow trough (slough) on the surface of the snow, which from a distance appears slightly darker in tone than the rest of the slope.
Other models involve water, either in the form of spring discharges,See Ferris et al. (2002) for a discussion. wet debris flows, or seasonal percolation of chloride-rich brines. Using data from the Mars Odyssey neutron spectrometer, researchers found that slope streaks in the Schiaparelli basin occur in areas predicted to yield between 7.0 and 9.0 weight percent Water Equivalent Hydrogen (WEH) in contrast to typical background values of less than 4% WEH. This relationship suggests a connection between high WEH percentages and the occurrence of dark slope streaks. However, any process that requires voluminous amounts of water (e.g., spring discharges) seems unlikely because of the overall thermodynamic instability of liquid water on Mars.
Another model proposes that dark slope streaks are produced by ground-hugging density currents of dry dust lubricated by carbon dioxide (CO2) gas. In this scenario, a small initial slump at the surface releases CO2 gas adsorbed onto subsurface grains. This release produces a gas-supported dust flow that moves as a tenuous density current downslope. This mechanism may help explain slope streaks that are unusually long.
Some observations suggest that dark slope streaks can be triggered by impacts. Pictures acquired by CTX in 2007 and 2010 showed a new streak appeared in the aureole of Olympus Mons. A follow-up image from HiRISE showed that a new crater at the top of the streak. The researchers concluded that the impact triggered the new slope streak. Another streak connected with an impact was found in the Arabia quadrangle.