Dark Slope Streaks
Encyclopedia
Dark slope streaks are narrow, avalanche
-like features common on dust-covered slopes in the equatorial regions of Mars
. They form in relatively steep terrain
, such as along escarpment
s and crater
walls. Although first recognized in Viking Orbiter
images from the late 1970s, 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.
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). The avalanching disturbs and removes a bright surface layer of dust to expose a darker substrate. 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.
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
).
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). 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 crater
s and other obstacles where a variety of streaks (wind tails) and blotches are formed.
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. (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). 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).
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.
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 lead 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.
s produced by dry granular flow on oversteepened slopes. Dust avalanches resemble loose snow avalanche
s 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, wet debris flows, or seasonal percolation
of chloride
-rich brine
s. 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.
features forming on the surface of present-day Mars. New streaks were first identified by comparing images from the Viking Orbiters
of the 1970s to images of the same locations taken by the MGS
Mars Orbiter Camera
(MOC) in the late 1990s. The presence of new streaks showed that slope streaks are actively forming on Mars, on at least annual to decade-long timescales. A later, statistical treatment using overlapping MOC images spaced days to several years apart showed that slope streaks may form on Mars at a rate of about 70 per day. If accurate, this rate suggests that slope streaks are the most dynamic geologic features observed on the surface of Mars.
Dark slope streaks fade and disappear at a much slower rate than new ones appear. Most streaks identified in Viking images are still visible after decades, although a few have vanished. Researchers infer that streaks appear at a rate 10 times faster than they disappear, and that the number of slope streaks on Mars has increased in the last three decades. This imbalance is unlikely to have persisted for geologically significant periods of time. One possible solution to the imbalance is that streaks last for centuries, but are wiped clean en masse after extremely rare but fierce dust storms (storms of a magnitude not observed on Mars since Viking). After the storm subsides, a thick layer of fresh dust is deposited to begin a new cycle of streak formation.
and minor fluvial
(water-related) activity occupying opposite endpoints. Gullies
are another feature common on slopes in the mid-latitude southern hemisphere of Mars They have received much attention in the literature but are not discussed here.
instrument on MRO
suggest that meters-thick avalanche scars and slope streaks are related and part of a continuum of active mass wasting
features formed by dust avalanches.
(RSL), the features received a considerable amount of media attention. RSLs are narrow (0.5 to 5 meters) dark markings that preferentially occur on steep, equator-facing slopes in the southern hemisphere between latitudes 48°S to 32°S. Repeat HiRISE
images show that the markings appear and grow incrementally during warm seasons and fade in cold seasons. RSLs bear only a superficial resemblance to dark slope streaks. They are much smaller in width and have a different pattern of geographic occurrence and slope properties than dark slope streaks. RSLs seem to occur on bedrock slopes with seasonally high surface temperatures of 250–300K (-23–27°C). These location may favor the flow of briney
fluids emerging from seeps at certain times of the Martian year. Unlike RSls, dark slope streaks appear to occur sporatically throughout the Martian year, and their triggering seems unrelated to season or large regional events.
-dominated terrains in the arctic
and Antarctic
regions of Earth. They are zones of enhanced soil moisture that route water downslope over the top of the permanently frozen ground just below the surface (ice table
). Although water tracks have not been specifically identified on Mars, several researchers have noted their morphological and spectroscopic similarity to Martian slope streaks. Like dark slope streaks, water tracks are narrow, sublinear features elongated in the downslope direction. They typically display a slight darkness relative to their surroundings and show little or no detectable relief. During peak flow conditions, they appear as damp, darkened, patches of soil that are generally less than 60 m wide and several hundred meters long. The dark surface discoloration vanishes in frozen water tracks during winter, rendering them nearly undetectable.
Avalanche
An avalanche is a sudden rapid flow of snow down a slope, occurring when either natural triggers or human activity causes a critical escalating transition from the slow equilibrium evolution of the snow pack. Typically occurring in mountainous terrain, an avalanche can mix air and water with the...
-like features common on dust-covered slopes in the equatorial regions of Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
. They form in relatively steep terrain
Terrain
Terrain, or land relief, is the vertical and horizontal dimension of land surface. When relief is described underwater, the term bathymetry is used...
, such as along escarpment
Escarpment
An escarpment is a steep slope or long cliff that occurs from erosion or faulting and separates two relatively level areas of differing elevations.-Description and variants:...
s and crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
walls. Although first recognized in Viking Orbiter
Viking program
The Viking program consisted of a pair of American space probes sent to Mars, Viking 1 and Viking 2. Each spacecraft was composed of two main parts, an orbiter designed to photograph the surface of Mars from orbit, and a lander designed to study the planet from the surface...
images from the late 1970s, dark slope streaks were not studied in detail until higher-resolution images from the Mars Global Surveyor
Mars Global Surveyor
The Mars Global Surveyor was a US spacecraft developed by NASA's Jet Propulsion Laboratory and launched November 1996. It began the United States's return to Mars after a 10-year absence. It completed its primary mission in January 2001 and was in its third extended mission phase when, on 2...
(MGS) and Mars Reconnaissance Orbiter
Mars Reconnaissance Orbiter
Mars Reconnaissance Orbiter is a NASA multipurpose spacecraft designed to conduct reconnaissance and Exploration of Mars from orbit...
(MRO) spacecraft became available in the late 1990s and 2000s.
The physical process that produces dark slope streaks is still uncertain. They are most likely caused by the mass movement
Mass wasting
Mass wasting, also known as slope movement or mass movement, is the geomorphic process by which soil, regolith, and rock move downslope under the force of gravity. Types of mass wasting include creep, slides, flows, topples, and falls, each with its own characteristic features, and taking place...
of loose, fine-grained material on oversteepened slopes (i.e., dust avalanches). The avalanching disturbs and removes a bright surface layer of dust to expose a darker substrate. The role that water and other volatiles
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...
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.
Nature of streaks on Mars
Dark slope streaks are albedoAlbedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
features. They appear to the eye as a brightness difference between the streak and the lighter-toned background slope. Usually no topographic
Topography
Topography is the study of Earth's surface shape and features or those ofplanets, moons, and asteroids...
relief
Terrain
Terrain, or land relief, is the vertical and horizontal dimension of land surface. When relief is described underwater, the term bathymetry is used...
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
Normalization (image processing)
In image processing, normalization is a process that changes the range of pixel intensity values. Applications include photographs with poor contrast due to glare, for example. Normalization is sometimes called contrast stretching...
).
Albedo features cover the Martian surface
Martian Surface
The study of surface characteristics is a broad category of Mars science that examines the nature of the materials making up the Martian surface. The study evolved from telescopic and remote-sensing techniques developed by astronomers to study planetary surfaces...
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
Classical albedo features on Mars
The classical albedo features of Mars are the light and dark features that can be seen on the planet Mars through an Earth-based telescope. Before the age of space probes, several astronomers created maps of Mars on which they gave names to the features they could see. The most popular system of...
.) The markings are caused by differing proportions of dust covering the surface. Martian dust is bright reddish ochre
Ochre
Ochre is the term for both a golden-yellow or light yellow brown color and for a form of earth pigment which produces the color. The pigment can also be used to create a reddish tint known as "red ochre". The more rarely used terms "purple ochre" and "brown ochre" also exist for variant hues...
in color, while the bedrock
Bedrock
In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...
and soil (regolith
Regolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...
) is dark gray (the color of unaltered basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
). 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). 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
Deposition (geology)
Deposition is the geological process by which material is added to a landform or land mass. Fluids such as wind and water, as well as sediment flowing via gravity, transport previously eroded sediment, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of...
of dust is most conspicuous around impact crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
s and other obstacles where a variety of streaks (wind tails) and blotches are formed.
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. (See B in Photo Gallery.) The cause of the darkening is uncertain. The particle sizes involved are believed to be very small (sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
, silt
Silt
Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...
, and clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
-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). 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).
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 sigmoidSigmoid
Sigmoid means resembling the lower-case Greek letter sigma or the Latin letter S. Specific uses include:* Sigmoid function, a mathematical function* Sigmoid colon, part of the large intestine or colon...
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
Apex (geometry)
In geometry, an apex is the vertex which is in some sense the highest of the figure to which it belongs.*In an isosceles triangle, the apex is the vertex where the two sides of equal length meet, opposite the unequal third side....
) 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
Anastomosis
An anastomosis is the reconnection of two streams that previously branched out, such as blood vessels or leaf veins. The term is used in medicine, biology, mycology and geology....
(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
HiRISE
High Resolution Imaging Science Experiment is a camera on board the Mars Reconnaissance Orbiter. The 65 kg , $40 million instrument was built under the direction of the University of Arizona's Lunar and Planetary Laboratory by Ball Aerospace & Technologies Corp....
) on MRO
Mars Reconnaissance Orbiter
Mars Reconnaissance Orbiter is a NASA multipurpose spacecraft designed to conduct reconnaissance and Exploration of Mars from orbit...
have shown that many slope streaks have relief
Terrain
Terrain, or land relief, is the vertical and horizontal dimension of land surface. When relief is described underwater, the term bathymetry is used...
, 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.
Dark slope streaks are most common in the equatorial regions of Mars, particularly in Tharsis
Tharsis
The Tharsis region on Mars is a vast volcanic plateau centered near the equator in Mars’ western hemisphere. The region is home to the largest volcanoes in the Solar System, including the three enormous shield volcanoes Arsia Mons, Pavonis Mons, and Ascraeus Mons, which are collectively known as...
, Arabia Terra
Arabia Terra
Arabia Terra is a large upland region in the north of Mars in that lies mostly in the Arabia quadrangle. It is densely cratered and heavily eroded. This battered topography indicates great age, and Arabia Terra is presumed to be one of the oldest terrains on the planet...
, and Amazonis Planitia
Amazonis Planitia
Amazonis Planitia is one of the smoothest plains on Mars. It is located between the Tharsis and Elysium volcanic provinces to the west of Olympus Mons in the Valles Marineris region of the Memnonia quadrangle, centered at...
(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
Triple point
In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases of that substance coexist in thermodynamic equilibrium...
of water on Mars. This relationship has lead 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.
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 avalancheAvalanche
An avalanche is a sudden rapid flow of snow down a slope, occurring when either natural triggers or human activity causes a critical escalating transition from the slow equilibrium evolution of the snow pack. Typically occurring in mountainous terrain, an avalanche can mix air and water with the...
s produced by dry granular flow on oversteepened slopes. Dust avalanches resemble loose snow avalanche
Loose snow avalanche
A loose snow avalanche is an avalanche formed in snow with little internal cohesion among individual snow crystals. Usually very few fatalities occur from loose snow avalanches, as the avalanches have a tendency to break beneath the person and are usually small even having a path as small as a few...
s 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
Spring (hydrosphere)
A spring—also known as a rising or resurgence—is a component of the hydrosphere. Specifically, it is any natural situation where water flows to the surface of the earth from underground...
discharges, wet debris flows, or seasonal percolation
Percolation
In physics, chemistry and materials science, percolation concerns the movement and filtering of fluids through porous materials...
of chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
-rich brine
Brine
Brine is water, saturated or nearly saturated with salt .Brine is used to preserve vegetables, fruit, fish, and meat, in a process known as brining . Brine is also commonly used to age Halloumi and Feta cheeses, or for pickling foodstuffs, as a means of preserving them...
s. Using data from the Mars Odyssey Neutron Spectrometer, researchers found that slope streaks in the Schiaparelli basin
Schiaparelli (Martian crater)
Schiaparelli is an impact crater on Mars named after Giovanni Schiaparelli located near Mars' equator. It is in diameter and located at latitude 3° South and longitude 344°. A crater within Schiaparelli shows many layers that may have formed by the wind, volcanoes, or deposition under...
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
Gravity current
In fluid dynamics, a gravity current is a primarily horizontal flow in a gravitational field that is driven by a density difference, hence gravity currents also sometimes being referred to as "density currents"...
of dry dust lubricated by carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
(CO2) gas. In this scenario, a small initial slump at the surface releases CO2 gas adsorbed
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
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.
Formation rate
Slope streaks are one of the few geomorphicGeomorphology
Geomorphology is the scientific study of landforms and the processes that shape them...
features forming on the surface of present-day Mars. New streaks were first identified by comparing images from the Viking Orbiters
Viking program
The Viking program consisted of a pair of American space probes sent to Mars, Viking 1 and Viking 2. Each spacecraft was composed of two main parts, an orbiter designed to photograph the surface of Mars from orbit, and a lander designed to study the planet from the surface...
of the 1970s to images of the same locations taken by the MGS
Mars Global Surveyor
The Mars Global Surveyor was a US spacecraft developed by NASA's Jet Propulsion Laboratory and launched November 1996. It began the United States's return to Mars after a 10-year absence. It completed its primary mission in January 2001 and was in its third extended mission phase when, on 2...
Mars Orbiter Camera
Mars Orbiter Camera
The Mars Orbiter Camera or Mars Observer Camera was a scientific instrument on board the Mars Observer and Mars Global Surveyor spacecrafts...
(MOC) in the late 1990s. The presence of new streaks showed that slope streaks are actively forming on Mars, on at least annual to decade-long timescales. A later, statistical treatment using overlapping MOC images spaced days to several years apart showed that slope streaks may form on Mars at a rate of about 70 per day. If accurate, this rate suggests that slope streaks are the most dynamic geologic features observed on the surface of Mars.
Dark slope streaks fade and disappear at a much slower rate than new ones appear. Most streaks identified in Viking images are still visible after decades, although a few have vanished. Researchers infer that streaks appear at a rate 10 times faster than they disappear, and that the number of slope streaks on Mars has increased in the last three decades. This imbalance is unlikely to have persisted for geologically significant periods of time. One possible solution to the imbalance is that streaks last for centuries, but are wiped clean en masse after extremely rare but fierce dust storms (storms of a magnitude not observed on Mars since Viking). After the storm subsides, a thick layer of fresh dust is deposited to begin a new cycle of streak formation.
Similar and related features
Dark slope streaks occur in association with or superficially resemble a number of other small-scale, slope-related features on Mars. These include bright slope streaks, avalanche scars, and recurring slope lineae. Water tracks are features that occur in the polar regions of Earth. They resemble dark slope streaks and recurring slope lineae, but have not yet been described on Mars. Many of the slope features on Mars may originate through a continuum of processes with dry mass wastingMass wasting
Mass wasting, also known as slope movement or mass movement, is the geomorphic process by which soil, regolith, and rock move downslope under the force of gravity. Types of mass wasting include creep, slides, flows, topples, and falls, each with its own characteristic features, and taking place...
and minor fluvial
Fluvial
Fluvial is used in geography and Earth science to refer to the processes associated with rivers and streams and the deposits and landforms created by them...
(water-related) activity occupying opposite endpoints. Gullies
Martian Gullies
First discovered on images from Mars Global Surveyor, Martian gullies may be the site of recent liquid water. Gullies occur on steep slopes, especially on the walls of craters. Gullies are believed to be relatively young because they have few, if any craters. Moreover, they lie on top of sand...
are another feature common on slopes in the mid-latitude southern hemisphere of Mars They have received much attention in the literature but are not discussed here.
Bright slope streaks
Bright slope streaks are streaks that have a lighter tone (about 2%) than their surroundings. (See F in Photo Gallery.) They are much rarer than dark slope streaks, but both types of streaks have similar morphologies and occur in the same regions of Mars. Evidence suggests that bright slope streaks are older than dark slope streaks. New bright slope streaks have never been observed, and dark slope streaks can be seen overlying bright slope streaks in some images, indicating that the former are younger than the latter. It is likely that bright slope streaks form from old dark slope streaks that have transitioned past a partially faded stage. This supposition is supported by geographical evidence indicating that bright slope streaks are slightly more common in regions where the formation rate of new dark slope streaks is low. In other words, areas with relatively many bright streaks tend to be less active and contain a higher population of old dark streaks.Avalanche scars
Areas with abundant slope streaks also contain an apparently distinct class of avalanche scars. The scars resemble slope streaks in morphology and size. (See G in Photo Gallery) They are typically several meters deep and hundreds of meters long. They begin at a single point (sometimes a small, barely resolved impact crater) high on a slope. The edges radiate downslope in a triangular fashion. In about half of the documented examples, a low-lying mound of debris is visible at the downslope end. Originally called “meters-thick avalanche scars,” these features were thought to be distinct from slope streaks. However, higher-resolution images from the HiRISEHiRISE
High Resolution Imaging Science Experiment is a camera on board the Mars Reconnaissance Orbiter. The 65 kg , $40 million instrument was built under the direction of the University of Arizona's Lunar and Planetary Laboratory by Ball Aerospace & Technologies Corp....
instrument on MRO
Mars Reconnaissance Orbiter
Mars Reconnaissance Orbiter is a NASA multipurpose spacecraft designed to conduct reconnaissance and Exploration of Mars from orbit...
suggest that meters-thick avalanche scars and slope streaks are related and part of a continuum of active mass wasting
Mass wasting
Mass wasting, also known as slope movement or mass movement, is the geomorphic process by which soil, regolith, and rock move downslope under the force of gravity. Types of mass wasting include creep, slides, flows, topples, and falls, each with its own characteristic features, and taking place...
features formed by dust avalanches.
Recurring slope lineae (warm-season flows)
In the summer of 2011, a paper appeared in Science describing a new class of slope features with characteristics that suggest formation by seasonal releases of liquid water. (See H in Photo Gallery.) Called recurring slope lineaeLinea
Linea is Latin for 'line'. In planetary geology it is used to refer to any long marking, dark or bright, on a planet or moon's surface. The planet Venus and Jupiter's moon Europa have numerous lineae; Saturn's moon Rhea has several.-See also:...
(RSL), the features received a considerable amount of media attention. RSLs are narrow (0.5 to 5 meters) dark markings that preferentially occur on steep, equator-facing slopes in the southern hemisphere between latitudes 48°S to 32°S. Repeat HiRISE
HiRISE
High Resolution Imaging Science Experiment is a camera on board the Mars Reconnaissance Orbiter. The 65 kg , $40 million instrument was built under the direction of the University of Arizona's Lunar and Planetary Laboratory by Ball Aerospace & Technologies Corp....
images show that the markings appear and grow incrementally during warm seasons and fade in cold seasons. RSLs bear only a superficial resemblance to dark slope streaks. They are much smaller in width and have a different pattern of geographic occurrence and slope properties than dark slope streaks. RSLs seem to occur on bedrock slopes with seasonally high surface temperatures of 250–300K (-23–27°C). These location may favor the flow of briney
Brine
Brine is water, saturated or nearly saturated with salt .Brine is used to preserve vegetables, fruit, fish, and meat, in a process known as brining . Brine is also commonly used to age Halloumi and Feta cheeses, or for pickling foodstuffs, as a means of preserving them...
fluids emerging from seeps at certain times of the Martian year. Unlike RSls, dark slope streaks appear to occur sporatically throughout the Martian year, and their triggering seems unrelated to season or large regional events.
Water tracks
Water tracks are little-studied slope features common in permafrostPermafrost
In geology, permafrost, cryotic soil or permafrost soil is soil at or below the freezing point of water for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of...
-dominated terrains in the arctic
Arctic
The Arctic is a region located at the northern-most part of the Earth. The Arctic consists of the Arctic Ocean and parts of Canada, Russia, Greenland, the United States, Norway, Sweden, Finland, and Iceland. The Arctic region consists of a vast, ice-covered ocean, surrounded by treeless permafrost...
and Antarctic
Antarctic
The Antarctic is the region around the Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica and the ice shelves, waters and island territories in the Southern Ocean situated south of the Antarctic Convergence...
regions of Earth. They are zones of enhanced soil moisture that route water downslope over the top of the permanently frozen ground just below the surface (ice table
Water table
The water table is the level at which the submarine pressure is far from atmospheric pressure. It may be conveniently visualized as the 'surface' of the subsurface materials that are saturated with groundwater in a given vicinity. However, saturated conditions may extend above the water table as...
). Although water tracks have not been specifically identified on Mars, several researchers have noted their morphological and spectroscopic similarity to Martian slope streaks. Like dark slope streaks, water tracks are narrow, sublinear features elongated in the downslope direction. They typically display a slight darkness relative to their surroundings and show little or no detectable relief. During peak flow conditions, they appear as damp, darkened, patches of soil that are generally less than 60 m wide and several hundred meters long. The dark surface discoloration vanishes in frozen water tracks during winter, rendering them nearly undetectable.
Photo gallery
Dark streaks and related features appear in the images below. To see the features described in the caption and text, it may be necessary to enlarge the image by clicking on it.Recommended reading
- Barlow, N.G. (2008). Mars: An Introduction to Its Interior, Surface, and Atmosphere; Cambridge University Press: Cambridge, UK, ISBN 978-0-521-85226-5.
- Hartmann, William, K. (2003). A Traveler’s Guide to Mars: The Mysterious Landscapes of the Red Planet; Workman: New York, ISBN 0-7611-2606-6.