Modification of Martian Slope Streaks

Introduction: Slope streaks, commonly dark features in the high albedo, low thermal inertia, dust-rich equatorial regions of Mars, have been observed in images from all orbiting spacecraft since Mariner 9 [1-9] and are thought to fade or brighten over time due to mantling of dust [2,3,8]. Recent studies using images from the High Resolution Imaging Science Experiment (HiRISE) camera [10] have shown that under conditions of low sun illumination, highest possible spatial scale, sufficiently thick dust deposits, and slopes facing away from the sun, slope streaks do have relief on the order of a meter or less [9]. This study focuses on features that were not identified or were uncommon in previous studies including ridge-like structures, terminal deposits, and mantle deposits [11]. Data and Methods: Approximately 2,346 HiRISE images from orbits 1300 to 4700 were studied, including a repeat of images up to orbit 3600 by [9]. Some individual HiRISE images beyond orbit 4700 were also studied, but they were not systematically evaluated like those up to orbit 4700. The images have spatial scales of 25-100 cm/pixel with a total of 135 unique sites (~6%) having slope streaks. Photocli-nometry, or shape-from-shading, was used to estimate the heights and/or depths of surface features in HiRISE images. In this study, profiling photoclinometry was applied in which slope information from individual pixels are modeled along a down-sun line, producing a profile of heights [12,13]. Slope Streak Features: Three features are observed that were previously unapparent or unresolved in MOC and HiRISE images: ridge-like structures, terminal deposits, and mantle deposits. Approximately 32% (44/135) of images with slope streaks have ridge-like structures and 8% (11/135) have terminal deposits. The percentage of slope streak images with mantle deposits is more difficult to ascertain and only a few images have unambiguous examples. Ridge-like structures. These features are typically a series of ridges along the bed of a streak. The number of ridges varies depending on the width and length of the streak, but are typically in the tens to hundreds for any given streak. The ridge axes are often, but not always transverse to the downslope direction of the streak and cover most of the bed. In some cases, the entire length of the bed is covered by ridges. Ridge morphology varies from linear to slightly arcuate, sometimes sinuous segments. The ridges are generally solitary, but their ends may sometimes merge with an adjacent ridge or the …