P12B-05
New Titan Saltation Threshold Experiments: Investigating Current and Past Climates

Monday, 14 December 2015: 11:20
2007 (Moscone West)
Nathan Bridges1, Devon M Burr2, John Marshall3, James K Smith4, Joshua P Emery2, Sarah M Horst5, Emily Nield2 and Xinting Yu5, (1)Applied Physics Laboratory Johns Hopkins, Laurel, MD, United States, (2)University of Tennessee, Knoxville, TN, United States, (3)SETI Institute Mountain View, Mountain View, CA, United States, (4)NASA Ames Research Center, Moffett Field, CA, United States, (5)Johns Hopkins University, Earth and Planetary Science, Baltimore, MD, United States
Abstract:
Titan exhibits aeolian sand dunes that cover ~20% of its surface, attesting to significant sediment transport by the wind. Recent experiments in the Titan Wind Tunnel (TWT) at NASA Ames Research Center [1,2] found that the threshold friction speed needed to detach Titanian “sand” is about 50% higher than previous estimates based on theory alone [3], a result that might be explained by the low ratio of particle to fluid density on the body [1].

Following the successful completion of the initial Titan threshold tests, we are conducting new experiments that expand the pressure range above and below current Titan values. The basic experimental techniques are described in [1], with minor updates to the instrumentation as described in [2]. To reproduce the kinematic viscosity and particle friction Reynolds number equivalent to that expected for Titan’s nitrogen atmosphere at 1.4 bars and 94 K requires that TWT be pressurized to 12.5 bars for air at 293K. In addition to running experiments at this pressure to reproduce previous results [1] and investigate low density (high density ratio) materials, TWT pressures of 3 and 8 bars are in the experimental matrix to understand threshold under past Titan conditions when the atmospheric pressure may have been lower [4]. Higher pressures, at 15 and 20 bars in TWT, are also being run to understand the putative effects of low density ratio conditions. Our experimental matrix for this follow-on work uses some of the same materials as previously used, including walnut shells, basalt, quartz, glass spheres, and various low density materials to better simulate the gravity-equivalent weight of Titan sand.

For these experiments, the TWT is now equipped with a new high pressure Tavis transducer with sufficient sensitivity to measure freestream speeds of less than 0.5 m s-1 at 12.5 bars. New techniques include video documentation of the experiments. We are also investigating methods of measuring humidity of the wind tunnel environment and electrostatic forces to assess their effect on threshold.

[1] Burr, D.M. et al. [2015], Nature, 517, 60-67.

[2] Burr, D.M. et al. [2015], Aeolian Res., in press

[3] Iversen, J.D. and B.R. White (1982), Sedimentology, 29, 111-119.

[4] Charnay, B. et al. [2014], Icarus, 241, 269-279.

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