Transition Between Aeolian Saltation and Suspension on Earth and Mars

Observations by the Mars Exploration Rover (MER) vehicles have stimulated progress toward understanding aeolian processes on Mars. The transition between aeolian saltation and suspension on Mars appears to occur at smaller particle sizes than previously predicted. The explanation may involve how response time of a particle to wind-related turbulent eddies is likely to differ significantly between terrestrial and martian aeolian environments. On Earth or Mars, wind-blown grains move primarily through saltation (bouncing of grains along primarily ballistic trajectories that are distorted horizontally by wind drag) or suspension. Each saltating particle also can cause other similarly-sized particles to move much shorter distances when the particle re-impacts the bed, and can also move larger particles in creep. Suspension involves longer particle trajectories that are not primarily ballistic, when turbulent eddy wind drag overwhelms gravitational forces that would otherwise cause grains to fall back to the surface. From terrestrial studies, the transition between saltation and suspension can be predicted where the ratio of turbulent eddy wind speeds and terminal fall speed of the grain are roughly equal. For quartz sand and typical terrestrial wind conditions, the transitional particle size typically is 50-70 microns. Smaller silt- and clay-sized particles are likely to be lofted directly into short-term or long-term suspension, while larger, sand-sized particles are likely to saltate (although higher wind energies can shift the transitional particle size to larger diameters, sending larger grains into suspension). Active terrestrial sand dunes commonly have particle sizes several times the minimum transitional grain size. The same physics applied to Mars predicted the smallest particles capable of saltation will be four times larger---about 200 microns---than on Earth, and that, analogous with Earth, the mean particle size for martian dunes should be several times greater still (i.e., coarser than is typical for terrestrial dunes). However, recent field evidence collected by MER is inconsistent with these predictions, revealing well-formed, active ripples of 100 micron basaltic sand---particles about half the minimum predicted saltation size for Mars. Particles lofted into suspension must, by definition, be more responsive to the accelerations and decelerations of turbulent eddies than to gravity, and we propose that particle response time to these eddy accelerations and decelerations is a significant factor. In addition to requiring that the eddy perturbation winds are stronger than the suspended particle fall speeds, the eddies must also have enough time to act on the particles so that the particles react to the eddies. If particle/eddy interaction times are insufficient, the particles effectively ignore the eddies and no suspension can occur, regardless of the relative magnitudes of the perturbation winds and the particle fall speed. The time with which a particle can respond to flow-induced drag forces (its stopping time or Stokes number) can be compared with the time over which a particle experiences a consistent force from an eddy. An order-of-magnitude calculation shows that for Earth, the particles respond sufficiently fast to the eddies that this condition is automatically met (Stokes number is small). However, for Mars the particles respond relatively slowly to the eddies compared to eddy lifetimes, and thus the condition is not always met (the Stokes number may be large).