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. Plain Language Summary Robotic lander and satellite images show sand movement by wind frequently occurs on Mars. Martian wind speeds, however, rarely exceed the minimum wind speed thought to be required to move sand. To resolve this dilemma, we conducted wind tunnel experiments to determine if sand could move at slower wind speeds than previously predicted. Previous experiments used the onset of continuous motion throughout a wind tunnel to define the minimum wind speeds required to move sand on Mars and ignored sporadic transport that can occur at slower wind speeds. Here, we use the onset of sporadic bursts of sand movement in a wind tunnel to define the minimum wind speed for sand motion on Mars. These bursts can induce a cascade of motion that develops from discrete patches and grows exponentially into continuous transport, which has the potential to produce significant landform change on Mars. We find the minimum speeds necessary to initiate sporadic bursts of motion are slower than previous model estimates by a factor of 1.6 to 2.5. Our results offer one explanation for abundant ripple and dune movement and dust emission under current thin-atmosphere climate conditions and suggest winds have sculpted the Martian landscape over billions of years.