Results of the imager for Mars Pathfinder windsock experiment

The Imager for Mars Pathfinder (IMP) windsock experiment measured wind spee ds at three heights within 1.2 m of the Martian surface during Pathfinder l anded operations. These wind data allowed direct measurement of near-surfac e wind profiles on Mars for the first time, including determination of aero dynamic roughness length and wind friction speeds. Winds were light during periods of windsock imaging, but data from the strongest breezes indicate a erodynamic roughness length of 3 cm at the landing site, with wind friction speeds reaching 1 m/s. Maximum wind friction speeds were about half of the threshold-of-motion friction speeds predicted for loose, fine-grained mate rials on smooth Martian terrain and about one third of the threshold-of-mot ion friction speeds predicted for the same size particles over terrain with aerodynamic roughness of 3 cm. Consistent with this, and suggesting that l ow wind speeds prevailed when the windsock array was not imaged and/or no p articles were available for aeolian transport, no wind-related changes to t he surface during mission operations have been recognized. The aerodynamic roughness length reported here implies that proposed deflation of fine part icles around the landing site, or activation of duneforms seen by IMP and S ojourner, would require wind speeds >28 m/s at the Pathfinder top windsock height (or >31 m/s at the equivalent Viking wind sensor height of 1.6 m) an d wind speeds >45 m/s above 10 m. These wind speeds would cause rock abrasi on if a supply of durable particles were available for saltation. Previous analyses indicate that the Pathfinder landing site probably is rockier and rougher than many other plains units on Mars, so aerodynamic roughness leng th elsewhere probably is less than the 3-cm value reported for the Pathfind er site.