Threshold windspeeds for sand on Mars: Wind tunnel simulations

Wind friction threshold speeds (u *t ) for particle movement (saltation) were determined in a wind tunnel operating at martian surface pressure with a 95 percent CO 2 and 5 percent air atmosphere. The relationship between friction speed (u * ) and free‐stream velocity (u ∞ ) is extended to the critical case for Mars of momentum thickness Reynolds numbers (Re θ ) between 425 and 2000. It is determined that the dynamic pressure required to initiate saltation is nearly constant for pressures between 1 bar (Earth) and 4 mb (Mars) for atmospheres of both air and CO 2 ; however, the threshold friction speed (u *t ) is about 10 times higher at low pressures than on Earth. For example, the u *t (Earth) for particles 210 µm in diameter is 0.22 m s −1 and the u *t (Mars, 5 mb, 200 K) is 2.2 m s −1 .