AEOLIAN THRESHOLD ON TITAN: RESULTS FROM EXPERIMENTS IN THE TITAN WIND TUNNEL

Aeolian sedimentary landforms hold important information about resurfacing processes, material transport, and minimum wind speeds at the time of formation. Titan, the largest satellite of Saturn, exhibits evidence for pervasive aeolian sedimentation in the form of extensive dunes. Although minimum (threshold) wind speeds necessary to entrain sediments on Titan can be estimated through extrapolation from experiments at non-Titan conditions, the accuracy of such extrapolations is uncertain. We have conducted wind tunnel experiments under Titan analog conditions using the high-pressure Titan Wind Tunnel (TWT) in the Planetary Aeolian Laboratory at the NASA Ames Research Center. By supporting pressurization of air at standard temperature to 12 bars, this tunnel allows reproduction of the kinematic viscosity of Titan’s 94K nitrogen-rich atmosphere, and thereby of the particle friction Reynolds number, viscous sublayer thickness, and balance of lift and drag forces at the sand-atmosphere interface. Titanian “sand” likely consists of organic tholins or tholin-coated ice grains, with a predicted density of 400-1500 kg m^-3. To capture the range of possible grain densities and sizes with sufficient data to reliably describe those various threshold curves, our experimental matrix includes 27 unique combinations of density, grain size, and grain size range. Because threshold speed is proportional to (gravity / atmospheric density) ^0.5, friction speeds derived in the tunnel are converted to friction speeds on Titan by multiplying by [(1.4/9.8)(13.7/5.1)]^0.5 = ~0.6. Our results show moderate agreement with threshold curves of Iversen and White [1982] (IW) and a poor fit to threshold curves based on Shao and Lu [2000], both of which are based on experiments at non-Titan conditions. Compared to IW, our experimental curves indicate a smaller particle size for minimum threshold friction speed and a lower threshold speed at small grain sizes. These results are applicable to ~spherical, non-cohesive particles. Actual Titan particles, which may be non-spherical, loosely bonded complex aggregates, may have different thresholds. The results of these experiments serve as boundary conditions with which to model more complex particle properties and atmospheric behavior on Titan and other outer Solar System bodies.