Electrification of sand on Titan and its influence on sediment transport

Journal name:
Nature Geoscience
Volume:
10,
Pages:
260–265
Year published:
DOI:
doi:10.1038/ngeo2921
Received
Accepted
Published online

Abstract

Triboelectric, or frictional, charging is a ubiquitous yet poorly understood phenomenon in granular flows. Recognized in terrestrial volcanic plumes and sand storms, such electrification mechanisms are possibly present on Titan. There, dunes and plains of low-density organic particles blanket extensive regions of the surface. Unlike Earth, Titan hosts granular reservoirs whose physical and chemical properties possibly enhance the effects of charging on particle motion. Here we demonstrate in laboratory tumbler experiments under atmospheric conditions and using organic materials analogous to Titan that Titan sands can readily charge triboelectrically. We suggest that the resulting electrostatic forces are strong enough to promote aggregation of granular materials and affect sediment transport on Titan. Indeed, our experiments show that electrostatic forces may increase the saltation threshold for grains by up to an order of magnitude. Efficient electrification may explain puzzling observations on Titan such as the mismatch between dune orientations and inferred wind fields. We conclude that, unlike other Solar System bodies, nanometre-scale electrostatic processes may shape the geomorphological features of Titan across the moons surface.

At a glance

Figures

  1. Experimental apparatus.
    Figure 1: Experimental apparatus.

    a, The operation during the charging period. b, The system during the measurement period.

  2. Surface charge densities and charge-to-mass ratios.
    Figure 2: Surface charge densities and charge-to-mass ratios.

    a, Charge density distributions for three organic granular compounds (biphenyl, naphthalene and polystyrene). b, Charge densities for three silicate samples. c, Charge-to-mass ratio distributions for the same three organic granular compounds. d, Charge-to-mass ratios for three silicate samples. Note that both the charge densities and charge-to-mass ratios in experiments with organics have maximum magnitudes one order of magnitude higher than those recorded in the experiments with silicates.

  3. Electrostatic-to-inertial ratio.
    Figure 3: Electrostatic-to-inertial ratio.

    Inset: photograph of a multiple-particle polystyrene aggregate in our experiments. Main panel, the range of EIR as computed from equation (1) for the average ranges of charge densities for organic and silicate materials. The solid portions of the curves bound the expected range of EIRs based on the average spans of charge density estimated for organic (maroon, labelled ‘organics) and silicate (blue, labelled ‘silicates) grains. The bins separated by the vertical dotted lines report the expected fraction of grains in our experiments with a given charge density. The horizontal dotted line demarcates the boundary between inertia- and electrostatically dominated systems (EIR = 1).

  4. Electrostatics and the saltation threshold.
    Figure 4: Electrostatics and the saltation threshold.

    a, Titan. The thick, lowest curve in the left panel is the frictional threshold without electrostatic effects taken into account. Dotted curves above this lowest curve show the modified saltation threshold given the charge densities indicated in the right panel (the average charge densities of all three organic materials). The lengths of the bins in the right panel give information about the relative frequency at which a specific charge density appeared in our experiments. The solid tan curve shows the higher-than-expected saltation threshold including the particle-to-fluid density ratio modification. b, Earth. Even when particles are charged to their highest value, the effects of electrostatics are much less prominent in the terrestrial case.

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Author information

Affiliations

  1. School of Earth & Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332, USA

    • J. S. Méndez Harper,
    • G. D. McDonald,
    • J. Dufek,
    • J. McAdams &
    • J. J. Wray
  2. School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA

    • J. S. Méndez Harper
  3. Jet Propulsion Laboratory, Pasadena, California 91109, USA

    • M. J. Malaska
  4. Department of Earth and Planetary Science, University of Tennessee—Knoxville, Knoxville, Tennessee 37996, USA

    • D. M. Burr
  5. Department of Astronomy, Cornell University, Ithaca, New York 14853, USA

    • A. G. Hayes

Contributions

J.S.M.H. designed experimental apparatus and measurement electronics, conducted experiments, and performed analysis. G.D.M. conducted experiments and helped with analysis. J.D. provided support with theoretical models of grain motion. M.J.M. provided support regarding material selection. D.M.B. provided support regarding aeolian dynamics on Titan. A.G.H. provided initial conceptualization of the project. J.M. provided laboratory support and conducted experiments. J.J.W. provided support regarding remote sensing.

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The authors declare no competing financial interests.

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