AEOLIAN FLUID THRESHOLD ON MARS: IS CLOSURE POSSIBLE BETWEEN WIND TUNNEL RESULTS AND OBSERVED SURFACE CHANGES? (Invited Presentation)

Wind-driven particle mobility has long been a central problem of Mars science, and was addressed by Nathan Bridges in his analyses evaluating wind-induced surface changes. Boundary layer wind tunnel experiments indicate fluid threshold wind friction speeds u_*tf required to mobilize beds of granular materials on Mars are several times higher than on Earth. While not unexpected, these results have been challenging to incorporate into interpretations of martian surface activity, particularly in the context of other analyses indicating that boundary layer wind friction speed u_* on Mars generally is well below u_*tf for sand- and, especially, dust-sized particles. This presentation examines these long-standing challenges and considers several factors that help achieve at least partial closure between wind tunnel u_*tf experiments and observations showing that wind-related surface changes are common at the MSL site and elsewhere on Mars: (1) Hand lens-quality images have revealed that surface dust occurs as weak, sand-sized, low-density aggregates of fine particles that are easily mobilized. (2) Rover observations show that silicate-density sand grains are abundant constituents of the martian regolith, disproving the Viking-era "kamikaze" sand destruction scenario in which very high u_* values were assumed. (3) Numerical models indicate that impact threshold u_*ti for sand on Mars is a small fraction of u_*tf (unlike on Earth), allowing saltation at lower wind speeds more compatible with climate model predictions and in situ anemometry. (4) Lower gravity on Mars enables a low-flux saltation mode to be initiated and sustained entirely below boundary layer u_*tf. (5) Turbulent boundary layers in wind tunnels do not replicate larger scales of turbulence of full-scale planetary boundary layers, such as transient eddy structures that inject downward from higher in the boundary layer to raise surface shear stress and initiate grain movements. (6) Wind tunnel u_*tf experiments commonly utilize flat beds for reproducibility, but these results must be related carefully to natural aeolian surfaces on Mars having greater relief, where shear stress concentrated transiently on higher, more exposed points is more likely to initiate saltation flux under conditions involving lower, spatially-averaged boundary layer u_*.