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.

Saltation threshold on Earth, Mars and Venus

[1] The ripple field known as “El Dorado” was a unique stop on Spirit's traverse where dust-raising, active mafic sand ripples and larger inactive coarse-grained ripples interact, illuminating several long-standing issues of Martian dust mobility, sand mobility, and the origin of transverse aeolian ridges. Strong regional wind events endured by Spirit caused perceptible migration of ripple crests in deposits SSE of El Dorado, erasure of tracks in sandy areas, and changes to dust mantling the site. Localized thermal vortices swept across El Dorado, leaving paths of reduced dust but without perceptibly damaging nearly cohesionless sandy ripple crests. From orbit, winds responsible for frequently raising clay-sized dust into the atmosphere do not seem to significantly affect dunes composed of (more easily entrained) sand-sized particles, a long-standing paradox. This disparity between dust mobilization and sand mobilization on Mars is due largely to two factors: (1) dust occurs on the surface as fragile, low-density, sand-sized aggregates that are easily entrained and disrupted, compared with clay-sized air fall particles; and (2) induration of regolith is pervasive. Light-toned bed forms investigated at Gusev are coarse-grained ripples, an interpretation we propose for many of the smallest linear, light-toned bed forms of uncertain origin seen in high-resolution orbital images across Mars. On Earth, wind can organize bimodal or poorly sorted loose sediment into coarse-grained ripples. Coarse-grained ripples could be relatively common on Mars because development of durable, well-sorted sediments analogous to terrestrial aeolian quartz sand deposits is restricted by the lack of free quartz and limited hydraulic sediment processing.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2008JE003101

Wind-driven particle mobility on Mars: Insights from Mars Exploration Rover observations at "El Dorado" and surroundings at Gusev Crater

[1] Dust devils, particle-loaded vertical convective vortices found on both Earth and Mars, are characterized by high rotating wind speeds, significant electrostatic fields, and reduced pressure and enhanced temperature at their centers. On Earth they are subordinate to boundary layer winds in the dust cycle and, except possibly in arid regions, are only “nuisance-level” phenomena. On Mars, though, they seem to support the persistent background atmospheric haze, to influence the surface albedo through the formation of “tracks” on the surface, and to possibly endanger future exploration because of their high dust load and large potential gradients. High-resolution numerical simulations and thermophysical scaling models successfully describe dust devil–like vortices on Mars, but fitting dust devil action into the Martian global dust cycle is still problematic. Reliable parameterizations of their erosional abilities and solid temporal and spatial distribution data are still required to build and test a complete model of dust devil action.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005RG000188

Dust devils on Earth and Mars

Wind friction threshold speeds for particle movement 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 and free-stream velocity is extended to the critical case for Mars of momentum thickness Reynolds numbers between 425 and 2000. It is determined that the dynamic pressure required to initiate saltation is nearly constant for pressures between 1 bar and 4 mb for atmospheres of both air and CO2.

Threshold windspeeds for sand on Mars: Wind tunnel simulations

Orbital change experiments with a Mars general circulation model

Wind friction threshold speeds 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 wind friction threshold speed 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 microns; particles both larger and smaller than this (at least down to about 5 microns) require stronger winds to initiate movement.

Mars: Wind friction speeds for particle movement

Saltation threshold experiments conducted under reduced gravity conditions

Dust on Mars: New Values for Wind Threshold

Simulated Martian aeolian abrasion and the creation of "aggregates".

An open circuit wind tunnel designed to operate in a large vacuum chamber was built at NASA-Ames to investigate saltation threshold, flux, deflation rates, and other aeolian phenomena on the planet Mars. The vacuum chamber will operate at pressures as low as 4 mbar, and the tunnel operates at windspeeds as high as 150 m/sec. Either air or CO2 can be used as a working fluid. It was found that, to a first order approximation, the same dynamic pressure was required at Martian pressure to entrain or saltate particles as was required on Earth, although wind and particle speed are considerably higher at Martian pressure. A 2nd wind tunnel, designed to operate aboard the NASA KC-135 0-g aircraft to obtain information on the effect of gravity on saltation threshold and the interparticle force at 0-g, is also described and test data presented. Some of the experiments are summarized and various aspects of low pressure aeolian entrainment for particles 12 to 100 micron in diameter are discussed, some of them unique to low pressure testing and some common in Earth pressure particle transport testing. The facility, the modes of operation, and the materials used are described.

/pdf/saltation-thresholds-and-entrainment-of-fine-particles-at-3lzw5kkz18.pdf

R. Leach

Papers

Mars: Wind friction speeds for particle movement

Saltation threshold experiments conducted under reduced gravity conditions

Dust on Mars: New Values for Wind Threshold

Simulated Martian aeolian abrasion and the creation of "aggregates".

Saltation thresholds and entrainment of fine particles at Earth and Martian pressures