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CHARGING OF TITAN DUNE GRAINS : EFFECT ON SEDIMENT TRANSPORT

We quantify the triboelectric charging behavior of Titan dune grain analogues, and discuss implications for sediment transport. Introduction: Dunes are a dominant geomorphic unit on Titan in terms of spatial coverage [1]. The dunes are the largest surface reservoir of hydrocarbons [2] and are likely an important component of the global organic photochemistry reservoir. Titan’s dunes appear to be formed from eastward sediment transport [3,4,5], in contrast with the dominant westward wind flow in the equatorial region (as inferred by General Circulation Models) [6,7,8]. Work involving climate modelling, the scaling of terrestrial sediment transport equations, and wind tunnel experiments has shown that high saltation thresholds (with respect to Titan’s ~m/s wind speeds) exist on Titan [9, 10]. This conclusion implies that the dunes are insensitive to slow easterly winds, and respond exclusively to rare, faster winds, proposed to be westerly in nature [6]. Given the ability of high saltation thresholds to explain many of the geomorphic characteristics of Titan’s dunes, it is important that the mechanisms for generating such high thresholds be understood. Burr et al. 2014 indicate the importance of low particle-to-fluid density ratio on Titan. However, an additional parameter affecting saltation on Titan, which has received little attention to date, is that of particle cohesion as a result of interparticle forces [10]. While empirical expressions for the saltation threshold [11, 12] have included interparticle forces such as van der Waals and electrical double layer forces, further experimental work is required to quantify and understand the effects of electrostatic forces on particle mobility under Titan atmospheric conditions. Fluidized particle collections (volcanic plumes, dust devils, and industrial fluidized beds) readily acquire charges via triboelectric processes (frictional and contact electrification) and can influence the granular dynamics of some systems [13]. For example, during the pneumatic transport of pharmaceutical powders, tribocharging can result in particle agglomeration, particles sticking to pipe walls, and even flow disruption [14]. While the mechanisms that permit an initially neutral granular medium to become charged when mobilized are poorly constrained, experimental work has shown that the magnitude and polarity of acquired charges depend on the physical and chemical properties of the particles as well as environmental conditions [15]. The triboelectric charging of Titan’s hydrocarbon dune grains likely differs from the electrification of silicate grains on Earth. These differences are not only driven by the material properties of the particles themselves, but by the specific environmental conditions on the moon’s surface. Titan’s elevated atmospheric pressure and exceedingly dry environment may allow grains to charge with enhanced efficiency [16]. On Earth, inertial forces tend to dominate over electrostatic forces for dense silicate particles. Conversely, the large charges that could develop on Titan’s low-density hydrocarbon grains may result in comparably high electrostatic forces capable of influencing the dynamics of aeolian sediment transport. In this study we quantify the tribocharging behavior of a number of materials (including pure hydrocarbon particles and several Titan analog materials, e.g. walnut shells) in near-Titan conditions. We also compare the charging of these materials to that of silicates. Methods: To investigate the triboelectric charging behavior of proposed Titan grain materials we developed the apparatus shown schematically in Fig. 1a. The device consists of a 15 cm-long ramp attached to a servo. 125 μL of particles are placed on one end of the ramp, which is held initially in a horizontal position. The sample is sprayed with an ionizing air gun to dissipate any initial charge on the particles. We note that the ramp’s surface is coated with particles of the same composition as the sample, ensuring that tribocharging results from particle-particle interactions only. Experi-