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 CO2 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 (Reϑ) 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 CO2; 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 &mgr;m in diameter is 0.22 m s-1 and the u*t (Mars, 5 mb, 200 k) is 2.2 m s-1.