Valles Marineris landslides: Evidence for a strength limit to Martian relief?

Unresolved controversies in Martian geology surround the role of active tectonics and a wetter climate early in Mars history, and particularly the history and amount of liquid water at or near the surface. Among the various lines of evidence brought into such debates are the massive landslides along the walls of Valles Marineris, which generally have been interpreted as resulting from marsquakes, and therefore necessitating active tectonics, under either wet or dry conditions. We analyze Valles Marineris landslides using digital elevation data from the Mars Orbiter Laser Altimeter (MOLA) and find that a relief limit consistent with the intact strength of evaporites or other weak sedimentary rock defines an upper bound to the length and relief of unfailed slopes, as would material with the strength properties of basalt lithology subjected to ground accelerations of about 0.2 g. In contrast to prior interpretations of Valles Marineris landslides, we propose an alternative, complementary hypothesis that does not require significant pore-water pressures or ground acceleration based on the close correspondence between back-calculated material strength properties and values consistent with portions of the chasm walls at least locally being composed of relatively weak materials, such as potentially frozen evaporites and/or mixtures of ash fall or flow deposits, ice, hydrated salts and lava flows.