Mars: Wind Friction Speeds for Particle Movement

Wind friction threshold speeds (V,t) for particle movement were determined in a low pressure boundary layer wind tunnel at an atmospheric pressure of 5.3 mb. The results imply that for comparable pressures on Mars, the minimum V,t is about 2.5 m/sec, which would require free-stream winds of 50 to 135 m/sec, depending on the character of the surface and the atmospheric conditions. The corresponding wind speeds at the height of the Viking lander meteorology instrument would be about a factor of two less than the free stream wind speed. The particle size most easily moved by winds on Mars is about 160 •m; particles both larger and smaller than this (at least down to about 5 •m) require stronger winds to initiate movement. The results presented here are in general agreement with previously reported values of V, t for particles 12 •m to 300 •m derived from one atmosphere tests, but are inconsistent with values for particles larger than about 300 •m.