HiRISE Observations and Slope Stability Analysis of Rotational Landslides in Bahram Vallis, Mars

Bahram Vallis is a narrow-winding valley that terminates at the circum-Chyrse basin (20-22 N, 301-304 E) on Mars. The valley has numerous circular to sub-circular alcoves along its slopes, some with large deposits directly below on the valley floor. These features are evident from earlier image datasets of Mars including THEMIS, MOC, and HRSC, and are indicative of mass-wasting processes that have occurred along the valley walls. A portion of Bahram Vallis was recently imaged by the HiRISE camera at 25 cm/pixel resolution (PSP_003460_2015 and PSP_003605_2015) in a location with both alcoves and valley floor deposits. Several features are apparent, a well-defined circular crown and main scarp (i.e., failure area), tensional cracks along the crown margin, and a well-defined accumulation zone with slump deposits. The general appearance of these landslides is similar to terrestrial rock and/or loose earth rotational landslides. The Bahram landslides are also different from other well-documented landslides on Mars, such as those in the Valles Marineris canyon system that have lobate forms and longitudinal and/or transverse ridges on their surfaces. To assess the possible landslide surface and stability of valley walls in Bahram Vallis, we have applied a "method of slices" modeling technique commonly used in geotechnical engineering along two MOLA topographic profiles across the valley and alcove walls. A landslide slip surface (circular arc) is fitted between two points, a point at the top of the slope closest to the landslide crown, and another at the base of the slope near the deposit terminus. The valley wall materials are divided into slices and the sum of their moments (driving moment) is divided by the total resisting moment of the mass to produce a "factor of safety" ratio value (F.S. < 1 = failure). Assuming the wall materials are a layer of regolith above a infinitely deep mass of rock, the initial estimated F.S. for the two profiles is 84-87, indicating relatively stable walls. Although these values suggest current wall stability, a different combination(s) of rock and regolith properties or the addition of saturated materials in the model could produce significantly different (i.e., lower F.S.) results. Future analysis of these landslides may include using a stereo-derived digital elevation model from HiRISE images and software for landslide modeling that can apply horizontal ground accelerations (seismic impulses) as an additional parameter.