Experimental (wind tunnel) investigations into aeolian entrainment: application to extraterrestrial environments

Data on aeolian sediment entrainment are critical for understanding sediment behavior throughout the Solar System. Such data are commonly derived from wind tunnel experiments. On-going work in the Titan Wind Tunnel is... [ view full abstract ]

Data on aeolian sediment entrainment are critical for understanding sediment behavior throughout the Solar System. Such data are commonly derived from wind tunnel experiments. On-going work in the Titan Wind Tunnel is providing new data on aeolian sediment entrainment for a range of fluid pressures (1-20 bars), sediment densities (1100-4740 g cm-3), and grain sizes (<~100-1000 μm). Our experiments are tuned to investigate two sources of uncertainty in aeolian entrainment models. 1) The effect of density ratio on threshold wind speed and entrainment processes: Previous experimental work in the Venus Wind Tunnel implemented a term that included the particle-to-fluid density ratio. The threshold wind speed was inferred to be a continuous function of this density ratio term, which was high for low density-ratio conditions (thick atmospheres) and low for high density-ratio conditions (thinner atmospheres). However, since its initial postulation ~30 years ago, this density ratio expression has not been independently verified, and the physical processes that underlie the density ratio term have never been elucidated. Our threshold wind speed experiments over a range of grain densities and fluid pressures are providing data to test the applicability of the density ratio term. We are also collecting and analyzing high-speed video to derive the physical entrainment mechanisms (e.g., fluid vs impact entrainment) as a function of pressure. We are comparing our experimental results with the output of a numeric model of sediment entrainment to further understand the controlling processes. 2) The effect of interparticle force on threshold wind speed: The interparticle force controls the threshold wind speed at small grain sizes. In widely used threshold wind speed models, this force is a power-law function of grain diameter, but the value of the exponent has varied between n=2 (assumed) and n=2.5 (fitted to wind tunnel data). A lack of error bars on these decades-old data prevents us from resolving this question through a goodness-of-fit calculation. To address this issue, we are analyzing our new and better documented wind tunnel data for a range of particle sizes and densities with the same approach previously used in the empirical derivation of the exponent value, n. In addition, we plan to collect threshold wind speed data for desiccated vs non-desiccated sediments and evaluate the effect on the threshold wind speed as a function of water content. The results of this work will elucidate aeolian processes on Titan (under different paleoclimates), Mars, and even Earth.