Titan's Aeolian Saltation Threshold Conditions: Initial Experimental Results

Saturn's moon Titan has extensive sand dune fields nearly circling its equator between latitudes +/-30˚, as revealed by radar and infrared imagery from NASA's Cassini spacecraft (Lorenz et al., 2006). The dunes are largely linear dunes, with a few barchan or barchanoid dunes (Ewing et al., 2014). Curiously, the dunes are oriented roughly west-to-east rather than the east-to-west orientation predicted by Titan atmospheric circulation models. However, winds above five km are west-to-east, implying occasional coupling of momentum from that altitude to the surface (Charnay et al., 2015) resulting in rare, above-threshold winds (Tokano, 2010). Additionally, the sand's provenance is likely airfall of hydrocarbon aerosols called tholins formed from UV-photolyzed atmospheric methane (Barnes et al., 2008). Yet, the current methane component of Titan's atmosphere is only 0.5 -1 Gyr old. Together, these constraints raise the age of the sand, and implications for landscape modification.

We present results from experimental wind tunnel measurements from the NASA Ames Planetary Aeolian Laboratory's Titan Wind Tunnel (TWT) revealing the threshold friction speeds u*_t for modern and paleo atmospheric pressures on Titan. The TWT is a closed-loop, pressurized wind tunnel that simulates aeolian dynamic conditions on Titan. Partial dynamic similarity to Titan's conditions is achieved by matching the kinematic viscosity; 12.5 bars in the TWT corresponds to Titan's modern 1.5 bar atmosphere. The differences in gravity between Earth and Titan, as pertaining to grain saltation profiles, is not accounted for.

We find, as expected, that u*_t decreases with increasing pressure but is in the range of 0.03-0.2 m/s compared with 0.3-1 m/s for Earth or 1-2 m/s for Mars conditions (Kok et al., 2012; Sullivan and Kok, 2017). Ongoing work is quantifying the vertical mass profile of lofted grains and refining u*_t. The mass profile, along with u*_t, will constrain the numeric aeolian transport code COMSALT (Kok and Reno, 2009) to allow accurate modeling of grain transport and sedimentary evolution of Titan's surface. The proposed DragonFly mission could test the resulting predictions in situ on Titan by observing grain motion by slowly increasing local "wind" with its flight propellers.