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Small Impacts Trigger Dust Landslides on Mars

Introduction: The HiRISE imaging system recently confirmed the suggestion from MOC [1] that new small (~1 km diameter) dark spots on Mars are the sites of recent (< 10 years old) small impact craters. At present, >70 such impacts have been imaged by the HiRISE system at scales ranging from 0.25 to 0.32 m/pixel. All of these new impact sites have been discovered over dust-mantled regions where small impacts produce dark spots that are easily detected from orbit. Our work focuses on a small impact in PSP_002764_1800. The impact produced a cluster of five relatively large craters and several smaller craters, of which the largest is ~21.7 m in diameter, located at 0 1’ 56.81” S, 226 55’ 30.61” E. This crater cluster formed in plains of the Amazonian Upper Medusae Fossae formation [2,3], initiating approximately 10 small landslides on steep slopes of apparent yardangs that traverse the HiRISE image [4]. This impact site was re-imaged 230 days later via PSP_005665_1800 and appears unchanged except for different illumination angles. This region has a high albedo and low thermal inertia inidicative of a dust cover. We classified and mapped the location and orientation of 64,336 small landslides in the vicinity of the impact and computed the spatial density of the landslides as a function of distance from the largest crater. Our goal is to determine whether the impact caused the landslides and to discriminate between two possible trigger mechanisms: Seismic shaking or airblastinduced slope failure. Method: We superimposed a 500 m x 500 m grid over the image to establish consistent areas for landslide counting. Five classes of landslides, CL1-CL4, and CLFaint, were defined based on their relative albedo and morphology (Figure 1). CL1 are dark, with sharply defined edges, longitudinal grooves, welldefined headscarps and downslope deposits. CL2 are similar to CL1, but lack obvious grooves and have higher albedos. CL3 are typically less than half as long as CL1 and CL2s, have higher albedo and lack welldefined headscarps. CL4 possess degraded margins, the highest albedo and a mottled appearence. CLFaint are blurred, narrow, and faint with higher albedos. Using Environment for Visualizing Images (ENVI v4.5), we defined a grid system North (N), South (S), and West (W) of the impact that encompass the majority of the landslides. The N region was 3x6 cells, the S: 3x5, and the W: 3x7. Line segments were drawn on the image to locate and classify each type of landslide. Results: The impact fell nearly on the boundary between two distinct terrains. To the N are ridges, interpreted as mega-yardangs, spaced ca. 375 m apart, while to the S the terrain is dissected into mesoyardangs spaced at ca. 25 m. The difference in spacing of the ridges is reflected in a difference in the average lengths of the landslides: Slopes are longer in the N quadrant and so the landslides are both longer and less numerous in this direction: an average spatial density of types CL1-CL3 of 2,836 km in the N, compared to 5,141 km in the S and 5,987 km to the W. The S lies entirely in the finer-textured terrain and 14 out of 21 grid cells in the W lie in this terrain.