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首页> 外文期刊>Journal of Geophysical Research. Biogeosciences >Mass movement slope streaks imaged by the Mars Orbiter Camera
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Mass movement slope streaks imaged by the Mars Orbiter Camera

机译：火星轨道相机成像的质量运动坡度条纹

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摘要

Narrow, fan-shaped dark streaks on steep Martian slopes were originally observed in Viking Orbiter images, but a definitive explanation was not possible because of resolution limitations. Pictures acquired by the Mars Orbiter Camera (MOC) aboard the Mars Global Surveyor (MGS) spacecraft show innumerable examples of dark slope streaks distributed widely, but not uniformly, across the brighter equatorial regions, as well as individual details of these features that were not visible in Viking Orbiter data. Dark slope streaks (as well as much rarer bright slope streaks) represent one of the most widespread and easily recognized styles of mass movement currently affecting the Martian surface. New dark streaks have formed since Viking and even during the MGS mission, confirming earlier suppositions that higher contrast dark streaks are younger, and fade (brighten) with time. The darkest slope streaks represent similar to 10%) contrast with surrounding slope materials. No small outcrops supplying dark: material (or bright material, for bright streaks) have been found at streak apexes. Digitate downslope ends indicate slope streak formation involves a ground-hugging flow subject to deflection by minor topographic obstacles. The model we favor explains most dark slope streaks as scars from dust avalanches following oversteepening of air fall deposits. This process is analogous to terrestrial avalanches of oversteepened dry, loose snow which produce shallow avalanche scars with similar morphologies. Low angles of internal friction typically 10-30(i) for terrestrial loess and clay materials suggest that mass movement of (low-cohesion) Martian dusty air fall is possible on a wide range of gradients. Martian gravity, presumed low density of the air fall deposits, and thin (unresolved by MOC) failed layer depths imply extremely low cohesive strength at time of failure, consistent with expectations for an air fall deposit of dust particles. As speed increases during a dust avalanche, a growing fraction of the avalanching dust particles acquires sufficient kinetic energy to be lost to the atmosphere in suspension, limiting the momentum of the descending avalanche front. The equilibrium speed, where rate of mass lost to the atmosphere is balanced by mass continually entrained as the avalanche front descends, decreases with decreasing gradient. This mechanism explains observations from MOC images indicating slope streaks formed with little reserve kinetic energy for run-outs on to valley floors and explains why large distal deposits of displaced material are not found at downslope streak ends. The mass movement process of dark (and bright) slope streak formation through dust avalanches involves renewable sources of dust only, leaving underlying slope materials unaffected. Areas where dark and bright slope streaks currently form and fade in cycles are closely correlated with low thermal inertia and probably represent regions where dust currently is accumulating, not just residing. [References: 38]
机译：最初在维京轨道飞行器图像中观察到在陡峭的火星斜坡上呈扇形的暗条纹，但由于分辨率的限制，不可能做出明确的解释。由火星全球测量师（MGS）航天器上的火星轨道相机（MOC）拍摄的图片显示了无数个暗坡条纹的例子，这些条纹在明亮的赤道区域分布广泛（但不均匀），以及这些特征的个别细节在Viking Orbiter数据中可见。深色斜坡条纹（以及稀有的明亮斜坡条纹）代表了目前影响火星表面的最普遍且最容易识别的大规模运动之一。自维京号以来，甚至在MGS任务期间，都形成了新的深色条纹，这证实了早先的假设，即对比度较高的深色条纹较年轻，并且随着时间的推移而褪色（变亮）。最暗的斜坡条纹与周围的斜坡材料形成相似的10％对比度。没有小露头供应深色：在条纹顶点发现了物质（或明亮的物质，用于明亮的条纹）。数字化的斜坡下端表明斜坡条纹的形成涉及地面支撑流，该地形流受到较小的地形障碍物的偏转。我们偏爱的模型解释了大多数黑暗的斜坡条纹，这是由于空气下落沉积物过度加深后尘埃雪崩造成的疤痕。此过程类似于过度陡峭的干燥雪地雪崩，这些雪崩会产生具有相似形态的浅雪崩疤痕。对于陆地黄土和粘土材料，内部摩擦的角度通常较低，通常为10-30（i），这表明（低内聚力）火星尘土飞降的质量运动可以在很宽的梯度范围内进行。火星重力，假定的低空沉积物密度以及薄薄的（未由MOC解决）失效层深度意味着在失效时的内聚强度极低，这与对空气颗粒沉积尘埃的预期一致。随着尘埃雪崩过程中速度的提高，越来越多的雪崩尘埃颗粒会获得足够的动能，使其在悬浮状态下损失到大气中，从而限制了雪崩前沿下降的动量。随着雪崩前沿的下降，质量损失到大气的速率被不断夹带的质量所平衡，平衡速度随着梯度的减小而减小。该机制解释了从MOC图像中观察到的结果，这些结果表明形成的斜纹几乎没有储备动能，可用于向谷底跳动的跳动，并解释了为什么在下斜纹的末端没有发现大量的远端移位沉积物。通过粉尘雪崩形成的深色（和明亮）斜坡条纹的整体运动过程仅涉及可再生粉尘源，而使下面的斜坡材料不受影响。当前形成深色和明亮的倾斜条纹并周期性地消失的区域与低热惯性紧密相关，并且可能代表了当前正在积聚灰尘的区域，而不仅仅是存在。 [参考：38]

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