Martian dust devils: laboratory simulations of particle threshold

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