New formulations valid for wide ranges of particle diameter and density and gas density are presented for prediction of saltation threshold speed for small particles. A low‐air‐density wind tunnel was used to extend the range of previous investigations and to separate the effects of Reynolds number and interparticle forces of cohesion. The new formulations are used to predict saltation threshold for atmospheric conditions on the surface of the Earth, Mars, and Venus.

The eolian transport of surface material on the planet Mars is estimated from results of low-pressure wind tunnel testing and theoretical considerations. A semiempirical relation is developed that will estimate the total amount of surface material moving in eolian saltation, suspension, and surface traction. The estimated total mass movement of surface material per unit width time on the surface of Mars is q = 2.61p(V, -V,t)(V, + V,t)2/g (g/cm s), wherep is the density of the atmospheric gas, g is the acceleration due to gravity, and V, and V,t are the friction speed and saltation threshold friction speed, respectively. A flat surface composed of particles of nearly uniform size is assumed. A change in the mean particle size changes the threshold friction speed V,t. The path lengths of saltating particles and wavelengths of surface ripples can vary as much as a factor of 2 if the surface temperature varies from 150 to 250 K. The angles between particle paths and the horizontal surface are calculated to be lower on Mars than on earth, and particles travel much faster on Mars than on earth. The ratio of final particle velocity to threshold friction speed, VF/V,t, is found to be several times that of saltation on earth. l NTRODUCTION This paper presents three aspects of the movement of surface material by wind on Mars and assembles them to estimate transport rates. First, the governing equations of motion are presented and their numerical solutions. These are discussed with applications to Mars. Second, an analytical theory is presented from which one may estimate the surface flux. Last, results from low-pressure wind tunnel experiments are presented and are used to complete the semiempirical relation-

High-speed motion pictures (2000 frames/s) of saltating spherical glass microbeads (of diameter 350–710 μm and density 2·5 g/cm3) were taken in an environmental wind tunnel to simulate the planetary boundary layer. Analysis of the experimental particle trajectories show the presence of a substantial lifting force in the intermediate stages of the trajectories. Numerical integration of the equations of motion including a Magnus lifting force produced good agreement with experiment. Typical spin rates were of the order of several hundred revolutions per second and some limited experimental proof of this is presented. Average values and frequency distributions for liftoff and impact angles are also presented. The average lift-off and impact angles for the experiments were 50° and 14° respectively. A semi-empirical procedure for determining the average trajectory associated with given conditions is developed.

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.

[1] An apparatus has been fabricated to simulate terrestrial and Martian dust devils. Comparisons of surface pressure profiles through the vortex core generated in the apparatus with both those in natural dust devils on Earth and those inferred for Mars are similar and are consistent with theoretical Rankine vortex models. Experiments to determine particle threshold under Earth ambient atmospheric pressures show that sand (particles > 60 mm in diameter) threshold is analogous to normal boundary-layer shear, in which the rotating winds of the vortex generate surface shear and hence lift. Lowerpressure experiments down to $65 mbar follow this trend for sand-sized particles. However, smaller particles (i.e., dust) and all particles at very low pressures ($10-60 mbar) appear to be subjected to an additional lift function interpreted to result from the strong decrease in atmospheric pressure centered beneath the vortex core. Initial results suggest that the wind speeds required for the entrainment of grains $2 mm in diameter (i.e., Martian dust sizes) are about half those required for entrainment by boundary layer winds on both Earth and Mars.