A RECENT MUDFLOW IN THE NILI FOSSAE REGION OF MARS: MORPHOLOGY AND NUMERICAL SIMULATIONS
Résumé
Introduction: There is a wide morphological range of landslides on Mars [1-3]. They can mobilize large volumes of material: estimates range from 10 6 to 10 12 m 3 [4]. Those formed during Amazonian are interpreted as being dry landslides [e.g., 5,6]. This study focus-es on a small landslide, which we estimate to have a volume of 10 6 m 3 , located in the Nili Fossae region. We performed a morphological analysis of this landslide and found similarities with mudslides on Earth. Terrestrial mudslides necessarily involve the presence of liquid water. The morphological similarities between the martian landslide and terrestrial mudslides raises the question of the potential role of water on Mars. To analyze the rheology of the martian landslide we conduct numerical simulations using the numerical model SHALTOP [e.g., 7,8]. The initial results from our simulations lend support to our interpretation that the landslide in question flowed like a terrestrial mudslide and had a viscous, rather than granular, behavior. Dry simulations could not reproduce the mass distribution of the landslide, as well as being poor matches from terrestrial analogy. Approach: To conduct the morphological analysis of the landslide we used High Resolution Imaging Science Experiment (HiRISE) images with a resolution of 25-50 cm/pix and ConTeXt imager (CTX) at 6m/pix. In order to perform the numerical simulation we created a Digital Elevation Model (DEM) at 2 m/pix from HiRISE stereo images using the Ames Stereo Pipeline [9]. We estimated the pre-landslide topography using the methods in Conway & Balme [10] and then calculated the mobilized volume. The DEM and the estimated release volume were used as an inputs to SHALTOP. The aim of the simulations was to vary the types of dry or viscous behavior in order to reproduce the runout and the mass distribution of the deposit of the landslide. For dry flows we used the Pouliquen and Forterre frictional law which parametrizes a dense granular flow in terms of Froude number, friction coefficient , velocity and thickness of the flowing layer [11]. For viscous flows we used the Bingham frictional law, which parametrizes amongst others the yield stress and viscosity [12]. Morphological results: The landslide studied here is located in a 25 km diameter impact crater to the north of the Nili Fossae. The landslide detached from the inner crater wall with a slope of 21°. It is composed of at least of two depositional lobes (Fig. 1a). The smallest is about 1000 m long by 230 m wide. The