Dark slope streak

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 ( PIA03170 fig1duststroms.jpg|Mars without a dust storm in June 2001 (on left) and with a global dust storm in July 2001 (on right), as seen by Mars Global Surveyor ==Morphology and occurrence==

camera on the Mars Reconnaissance Orbiter. 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. 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. 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==

. It is located in the middle left of this picture. Tharsis Tholus is just off to the right. 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. wet debris flows, 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. 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. File:ESP 054066 1920newstreak.jpg|New streak that was caused by a recent impact that created a small crater, as seen by HiRISE. Location is 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 (72 feet) 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. Image:2764streaks.jpg|Image indicates crater cluster and curved lines formed by airblast from meteorites. Meteorites caused airblast which caused dust avalanches on steep slopes. Image is from HiRISE. Image:2764streaksclose.jpg|Close up of previous image along light/dark boundary. Dark line in middle of image shows border between light and dark area of curved lines. Green arrows show high areas of ridges. Loose dust moved down steep slopes when it felt the airblast from meteorite strikes. Image is from HiRISE. Dust devils have even been observed to start the formation of dark slope streaks. A team of researchers found an increase in dark slope streaks after S1222, a marsquake that was detected by the Insight lander. ==Formation rate==

between February 1998 and November 1999, as seen by Mars Orbital Camera (MOC). 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 (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. ==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 wasting and minor fluvial (water-related) activity occupying opposite endpoints. 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 and I in Photo Gallery.) Called "recurring slope lineae" (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 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. Water tracks Water tracks are little-studied slope features common in permafrost-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. ==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. Image:Slope Streaks Crater PIA11312.jpg|A. Dark slope streaks are the tiny, linear albedo features along the SE wall of the crater. Compare with the far larger, wind-related albedo feature (oval patch at center top of image). This image is a THEMIS VIS from the Mars Odyssey spacecraft. It is about 25 km wide. North is at top. Image:Slope Streak Dust Devil.jpg|B. This dark slope streak may have been initiated by winds from dust devil. A thin dust devil track is visible across the apex of the slope streak. This Mars Reconnaissance Orbiter (MRO) HiRISE image is 1.8 km across and based on Schorghofer et al., 2007, p. 136, Fig. 5. Image:Dark streaks in Diacria.JPG|C. Dark streaks in Diacria quadrangle, as seen by the Mars Orbiter Camera (MOC) on Mars Global Surveyor (MGS). Image:Slope Streak Diverting PIA08672.jpg|D. Dark slope streaks in Phlegra Dorsa region as seen by MOC. The streak near the center of image has been diverted around a small hill. The image is about 3 km (1.9 mi) across. Image:Braided_Slope_Streak.jpg|E. Braided (anastomosing) slope streak in Lycus Sulci region as seen by MOC. The morphology resembles features produced by fuidized flow. The image is about 3 km (1.9 mi) across. Image:Bright Slope Streaks PIA03587.jpg|F. Both dark and bright slope sreaks occur together on the wall of this impact crater in Arabia Terra as seen by MOC. Photometric analysis shows that the brightness of the streaks is inherent and not due to lighting conditions or viewing geometry. Image:Avalanche Scars PSP 003239 1870.jpg|G. Shallow avalanche scars associated with dark slope streak. The slope streak has the same sharp apex and triangular-faceted morphology as the scars, suggesting that both types of features have a similar origin. Image is from HiRISE. Image:Slope Lineae PIA14479-43 946-710.jpg|H. Recurring slope lineae as seen by HiRISE. Image:PIA17934-MartianSlope-SeasonalDarkFlows-20140210.jpg|I. Seasonal Dark Flows, "Recurring Slope Linae" (RSL), on Martian slopes (2 November 2007). Image:Pedestaltop22919.jpg|Dark slope streaks near the top of a pedestal crater, as seen by HiRISE under the HiWish program. Image:Streaks22919.jpg|Dark slope streaks and layers near a pedestal crater, as seen by HiRISE under the HiWish program. Image:23677streaks.jpg|Young and old dark streaks, as seen by HiRISE under HiWish program. Location is Diacria quadrangle. File:55107 1930streaks.jpg|Dark slope streaks, as seen by HiRISE under the HiWish program Arrows show how boulders affected the shape of the streaks. File:55107 1930streaksclose.jpg|Dark slope streaks, as seen by HiRISE under the HiWish program Arrows show how boulders affected the shape of the streaks. File:Mesa with dark slope streaks in Lunae Palus quadrangle, Mars 01.jpg|Wide view of mesas with dark slope streaks, as seen by HiRISE under HiWish program File:Mesa with dark slope streaks in Lunae Palus quadrangle, Mars 02.jpg|Large group of dark slope streaks along a mesa wall, as seen by HIRISE File:Dark slope streaks in Lunae Palus quadrangle, Mars 03.jpg|Close view of end of dark slope streaks, as seen by HiRISE File:Mesa with dark slope streaks in Lunae Palus quadrangle, Mars 04.jpg|Dark slope streaks along a mesa wall, as seen by HIRISE Picture is about 1 km across. File:Dark slope streaks in Lunae Palus quadrangle, Mars 05.jpg|Close view of end of dark slope streaks, as seen by HiRISE File:ESP 055285 2030-animationstreaks.gif|Animation showing changes in streaks on slopes of ridges within the Olympus Mons Aureole. The changes happened in 6 years. Picture was named HiRISE picture of the day for January 13, 2024 == References ==