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 µm in diameter is 0.22 m s−1 and the u*t (Mars, 5 mb, 200 K) is 2.2 m s−1.

Estimation of the rate of aeolian abrasion of rocks on Mars requires knowledge of (1) particle flux, (2) susceptibilities to abrasion of various rocks, and (3) wind frequencies on Mars. Fluxes and susceptibilities for a wide range of conditions were obtained in the laboratory and combined with wind data from the Viking meteorology experiment. Assuming an abundant supply of sand‐sized particles, estimated rates range up to 2.1×10−2 cm of abrasion per year in the vicinity of Viking Lander 1. This rate is orders of magnitude too great to be in agreement with the inferred age of the surface based on models of impact crater flux. The discrepancy in the estimated rate of abrasion and the presumed old age of the surface cannot be explained easily by changes in climate or exhumation of ancient surfaces.We consider the primary reason to be related to the agents of abrasion. Either windblown grains are in very short supply, or the grains are ineffective as agents of abrasion. At least some sand‐sized (∼100 μm) grains appear to be present, as inferred from both lander and orbiter observations. High rates of abrasion occur for all experimental cases involving sands of quartz, basalt, or ash. However, previous studies have shown that sand is quickly comminuted to silt‐ and clay‐sized grains in the martian aeolian regime. Experiments also show that these fine grains are electrostatically charged and bond together as sand‐sized aggregates. Laboratory simulations of wind abrasion involving aggregates show that at impact velocities capable of destroying sand, aggregates form a protective veneer on the target surface and can give rise to extremely low abrasion rates.

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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 µm; particles both larger and smaller than this (at least down to about 5 µm) require stronger winds to initiate movement. The results presented here are in general agreement with previously reported values of V*t for particles 12 µm to 300 µm derived from one atmosphere tests, but are inconsistent with values for particles larger than about 300 µm.