Dark slope streak
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 escarpments 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 and Mars Reconnaissance Orbiter 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. 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.
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 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. 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.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. 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 is dark gray. Thus, dusty areas on Mars appear bright, and surfaces with a high percentage of rocks and rock fragments are generally dark. 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 and blotches are formed.
Dark slope streaks are relatively small features. They differ from larger albedo features in being produced by gravity rather than wind, although wind may contribute to their initial formation. The cause of the darkening is uncertain. The particle sizes involved are believed to be very small. No clasts large enough to be imaged are present, and the underlying bedrock slope is never exposed. 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. Dust storms are common on Mars. At times the whole planet is enveloped in a dust storm, as shown in the pictures below.
Morphology and occurrence
At moderate resolutions, 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 in shape. Closer up, dark slope streaks typically have elongated, fan-like shapes. 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 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 pattern. Slope streaks commonly develop multiple fingers at their downslope ends.
and gully deposits. The geographical distribution indicates that gullies and slope streaks are different phenomena.
Images from the High Resolution Imaging Science Experiment 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. 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, 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.
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.
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 flow 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 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 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 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 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 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.
Research, published in January 2012 in Icarus, found that dark streaks were initiated by airblasts from meteorites traveling at supersonic speeds. The team of scientists was led by Kaylan Burleigh, an undergraduate at the University of Arizona. After counting some 65,000 dark streaks around the impact site of a group of 5 new craters, patterns emerged. The number of streaks was greatest closer to the impact site. So, the impact somehow probably caused the streaks. Also, the distribution of the streaks formed a pattern with two wings extending from the impact site. The curved wings resembled scimitars, curved knives. This pattern suggests that an interaction of airblasts from the group of meteorites shook dust loose enough to start dust avalanches that formed the many dark streaks. At first it was thought that the shaking of the ground from the impact caused the dust avalanches, but if that was the case the dark streaks would have been arranged symmetrically around the impacts, rather than being concentrated into curved shapes.
The crater cluster lies near the equator 510 miles) south of Olympus Mons, on a type of terrain called the Medusae Fossae formation. The formation is coated with dust and contains wind-carved ridges called yardangs. These yardangs have steep slopes thickly covered with dust, so when the sonic boom of the airblast arrived from the impacts dust started to move down the slope.
Using photos from Mars Global Surveyor and HiRISE camera on NASA’s Mars Reconnaissance Orbiter, scientists have found about 20 new impacts each year on Mars. Because the spacecraft have been imaging Mars almost continuously for a span of 14 years, newer images with suspected recent craters can be compared to older images to determine when the craters were formed. Since the craters were spotted in a HiRISE image from February 2006, but were not present in a Mars Global Surveyor image taken in May 2004, the impact occurred in that time frame.
The largest crater in the cluster is about 22 meters in diameter with close to the area of a basketball court. As the meteorite traveled through the Martian atmosphere it probably broke up; hence a tight group of impact craters resulted.
Dark slope streaks have been seen for some time, and many ideas have been advanced to explain them. This research may have finally solved this mystery.
Formation rate
Slope streaks are one of the few geomorphic 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 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. After the storm subsides, a thick layer of fresh dust is deposited to begin a new cycle of streak formation. A recent study published in Icarus found that they last about 40 years. The researchers looked at a region in Lycus Sulci with Viking images and with CTX images from the Mars Reconnaissance Orbiter. The ones first observed with Viking have all gone, but have been replaced with new ones.