Experimentally Derived Thresholds for Windblown Sand on Mars

Aeolian sand transport frequently occurs on Mars despite wind speeds rarely exceeding predicted thresholds for motion. This dissonance is problematic for understanding aeolian processes and the resulting geomorphologic responses in contemporary and ancient atmospheres. To address the apparent discrepancy between speeds required to initiate sand motion and transport observed from orbital and in situ observations, we conducted a series of wind tunnel experiments at Martian atmospheric pressures and used buoyancy as the similitude parameter to simulate the force required to initiate particle motion on Mars. Shear velocities were derived from velocity profiles corresponding to the onset of sporadic transport, which can induce downwind cascading motion. Here, we find that threshold shear velocities are slower than previously thought by a factor of 1.6 to 2.5. Measured wind speeds on Mars exceed our observed thresholds, thus offering one mechanism behind the dissonance between observations of transport below previous thresholds of motion.