Martian dust devils: Laboratory simulations of particle threshold

Abstract

[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.

Dust devil speeds, directions of motion and general characteristics observed by the Mars Express High Resolution Stereo Camera

“…Laboratory work (Neakrase et al, 2006;Greeley et al, 2004;Greeley et al, 2003) has been conducted to simulate dust flux within dust devils, the directions of motion, and particle lifting under Martian conditions to understand the basic dynamics and characteristics of dust devils. The nearly global coverage of…”

THEMIS VIS and IR observations of a high‐altitude Martian dust devil

“…A low-pressure core often forms at the center of a developing vortex, causing a difference in pressure between the top and bottom of the surface particle layer. This pressure differential creates a suction effect which easily entrains dust-sized particles as small as 1 mm on Mars [Fisher et al, 2005;Ferri et al, 2003;Greeley et al, 2003] which are typically lifted only by high winds combined with saltating sand-sized particles [Greeley et al, 1992]. A visible dust devil forms if the vortex gains sufficient strength to entrain these particles from the surface.…”

In situ measurements of particle load and transport in dust devils

“…Dust devils lift material by their intense swirling winds (Balme et al, 2003b) and perhaps also by a suction effects at the cores (Greeley et al, 2003;Balme and Hagermann, 2006). To estimate the effect of dust devils on the climate requires data describing their size frequency distribution, and describing the amount of dust that they lift.…”