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Heights and Slopes on Mars North Polar Scarps Using HIRISE Point-to-Point Stereo Measurements

Given the challenge and scientific importance of measuring vertical elevation distances and slopes in high-relief environments over 100s mto ~1 kmscales (such as steep north polar scarps), we develop a technique to make such measurements that yields results more accurate than MOLA (in these situations) yet is not as complex and resource-intensive as producing a full scale DEM. The technique yields height and slope between two given points within a stereo pair of images. Estimates of errors and examples of results are given. Introduction: The High Resolution Imaging Science Experiment on Mars Reconnaissance Orbiter has recently completed its second season of observation of Mars' north polar region [1]. Among other accomplishments, HiRISE has imaged polar layered deposits in unprecedented detail and shown steep polar scarps to be geologically dynamic environments [2-4]. Steep scarps (found around the perimeter of the north polar topographic dome and Chasma Boreale, as opposed to gentler interior polar troughs) generally include the best sections through the two main types of layered deposits: the classic polar layered deposits of ice with varying dust content, and the underlying “basal unit” composed of alternating bright, resistant, likely ice-rich beds and dark, sandy, relatively easily eroded dark layers [3,6]. While some of the questions raised in pre-MRO descriptions of the basal unit [7-9] have been cleared, the basal unit remains complex. Much of the irregular, discontinuous, and pinching-out behavior of bright layers can be seen to likely result from highly contrasting erosional styles of bright and dark layers, intermittent cover by loose dark material, and greatly varying amounts of scarpwards erosion along the exposed section (forming alcoves where layers recede towards the cliff face and promontories where the layers extend away from the cliff face) [3,6]. Large-scale bright layers are seen to be relatively parallel and continuous, although some evidence for possible gentle undulations exists. This complex expression complicates interpretation of layering. In addition, high-resolution imaging now allows us to identify specific features for which dimensional measurements would advance our understanding of the stratigraphy and erosional processes. Examples include layer thicknesses, from which relative fractions of bright and dark materials in the basal unit may be estimated; vertical distribution of layers, from which regular frequencies or cycles of deposition may be identified; and slopes of exposures or secondarily emplaced material, from which dynamic aspects of currently active masswasting processes can be described [2-4]. Current MOLA-derived topographic data is insufficient for such studies, especially in terrain of high-frequency relief. With a shot spacing of ~300-400 m, a spot size of ~150 m, and interpolation of combined measurements to 512 pixels per degree (~110 m per pixel), neither MOLA profiles or gridded products are for topographic studies on the scale of ≤ 100 m [10]. Methods: There are three main ways in which HiRISE image data can be used to gain relief information for a scene. Anaglyphs are image products in which a “stereo pair” of images taken from different viewing angles are projected in identical fashion and then viewed in red and blue channels of an RGB image. Anaglyphs spectacularly display how differential erosion has shaped steep scarp exposures, and relative relief and slope information help make sense of otherwise confusing stratigraphy as mentioned above. While extremely helpful, anaglyphs still leave open questions such as: how high are certain scarp sections? how thick are layers (packages), and what are the slopes involved?. The most thorough topographic analysis requires construction of a full digital elevation model, or DEM [11]. This process is very time and computer intensive, and hence production is limited. The one DEM in polar region so far is in a polar trough [12] The third way to glean relief information gives the height and horizontal distance between a chosen pair of points (say, points A and B) using a pair of stereo images (images 1 and 2) along with known spacecraft observation parameters and the relative pixel locations of the points in each image. Our current method balances precision with simplicity of function. The main parameters involved are emission angle (the angle between the normal to the local surface and a line to the spacecraft) and pixel scale (distance covered by an individual pixel) in each image. Sensitivity of this calculation to the input parameters has been preliminarily assessed. Over distances of several thousand pixels, the resulting height difference between two points is minimally sensitive to perturbations of the emission angle on the scale of .01 degrees, giving variations of height measurements on the order of ± .1%, and moderately sensitive to perturbations of the 2479.pdf 40th Lunar and Planetary Science Conference (2009)