Slope Streaks in the Antarctic Dry Valleys: Characteristics, Candidate For- Mation Mechanisms, and Implications for Slope Streak Formation in the Mar-

نویسندگان

  • J. W. Head
  • D. R. Marchant
  • J. L. Dickson
  • J. S. Levy
چکیده

Introduction: Slope streaks on Mars, first observed in Viking Orbiter images [1], were imaged in detail by MGS MOC [2,3], and have been observed to form and change over less than decadal time periods [3]. Typically displaying a low albedo relative to their surroundings, they show no detectable relief, are elongated downslope, usually <200 m in width, and usually extend for distances ranging from a few hundred meters to <2 km; length/width ratios range from ~5 to 30. They typically occur on steep slopes associated with massifs, escarpments and crater interior walls; no correlation with substrate of a particular age are seen. New streaks are typically the darkest, some relatively bright streaks are observed, and occasional brightness variations are observed within streaks; quantitative estimates of brightness contrast with local background are ~10% for dark streaks and ~2% for bright streaks. Streaks typically follow local topography closely, and often are digitate, or branch and fan in their distal (downhill) regions [2-8]. Streaks are not evenly distributed on Mars, occurring in some low latitude regions [4,5]; further, they occur exclusively in regions of low thermal inertia, steep slopes, and only where peak temperatures exceed 275 K; northernmost streaks form preferentially on warmer southfacing slopes [5]. Changes in slope streaks are observed only if the observation interval includes the warm season [5]; areas of most abundant slope streaks are positively correlated with GRS/NS elevated hydrogen levels [9,10]. These characteristics have led to a range of hypotheses for slope streak origin, including: 1) dry dust avalanches (with avalanching dust revealing the darker subsurface which then brightens with time as the avalanche scars are healed) [2,11], 2) dust avalanches controlled by wind [8], 3) wet debris flows [1], 4) both wet and dry debris flows [12], and 5) erosive fluvial processes from spring discharge, where melting is aided by hydrothermal activity or hypersaline aquifers [7,13]. Slope Streaks in the Antarctic Dry Valleys: The hyperarid, cold polar desert environment of the Antarctic Dry Valleys represents one of the most Mars-like temperature-precipitation environments on Earth [14,15]; the ADV are characterized by three major microenvironments, widespread permafrost, and extremely low, Mars-like erosion rates. There is currently no hydrothermal activity in the Dry Valleys region. Analysis of aerial photographs and satellite images of the South Fork of upper Wright Valley revealed the presence of streaklike features on the north-facing slopes of the valley wall (Fig. 1). These streaks typically display a low brightness relative to their surroundings, show no detectable relief, are elongated downslope, are usually <60 m in width, and extend for distances ranging from ~100-400 m; length/width ratios range from ~10 to 35. They occur on steep slopes (10-30) associated with the southern wall of Wright Valley in colluvium derived from bedrock scarps and massifs [15], which are exposed ~400-600 m above the valley floor. New streaks are typically the darkest, some relatively bright streaks are observed, and occasional brightness variations are observed within streak. Streaks follow local topography closely, are often digitate and branching distally, and show brightness differences related to variations in surface roughness at the meter scale. Slope streaks in South Fork (Fig. 1) occur exclusively on the warmer, equator-facing slopes.

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Slope streaks in the Antarctic Dry Valleys: Characteristics, candidate formation mechanisms, and implications for slope streak formation in the Martian environment

Figure 2. Slope Streaks in the ADV; Left: digitate streaks, image width 220 m; Right: parallel streaks and effect of local topography at lower left; image width 260 m. Slope is 20-30, top to bottom. Slope streaks in the Antarctic Dry Valleys: Characteristics, candidate formation mechanisms, and implications for slope streak formation in the Martian environment J. W. Head, D. R. Marchant, J. L. ...

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تاریخ انتشار 2007