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Methane storms as a driver of Titan’s dune orientation

Nature Geoscience volume 8, pages 362–366 (2015)Cite this article

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Abstract

The equatorial regions of Saturn’s moon Titan are covered by linear dunes that propagate eastwards1,2,3. Global climate models (GCMs), however, predict westward mean surface winds at low latitudes on Titan, similar to the trade winds on Earth1,4. This apparent contradiction has been attributed to Saturn’s gravitational tides1, large-scale topography4 and wind statistics5, but none of these hypotheses fully explains the global eastward propagation of dunes in Titan’s equatorial band. However, above altitudes of about 5 km, Titan’s atmosphere is in eastward super-rotation, suggesting that this momentum may be delivered to the surface. Here we assess the influence of equatorial tropical methane storms—which develop at high altitudes during the equinox—on Titan’s dune orientation, using mesoscale simulations of convective methane clouds6,7 with a GCM wind profile that includes super-rotation8. We find that these storms produce fast eastward gust fronts above the surface that exceed the normal westward surface winds. These episodic gusts generated by tropical storms are expected to dominate aeolian transport, leading to eastward propagation of dunes. We therefore suggest a coupling between super-rotation, tropical methane storms and dune formation on Titan. This framework, applied to GCM predictions and analogies to some terrestrial dune fields, explains the linear shape, eastward propagation and poleward divergence of Titan’s dunes, and implies an equatorial origin of dune sand.

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Figure 1: Two-dimensional (altitude–longitude) simulation of the evolution of a storm under Titan’s conditions at the equator during equinox.
Figure 2: Wind speed in the gust front.
Figure 3: Storm impact on the RDD.
Figure 4: Analogy between linear dunes on Titan and in Rub’al-Kali desert on Earth.

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Acknowledgements

We thank J-Y. Grandpeix, F. Forget, A. Spiga and J. Leconte for helpful discussions. We acknowledge financial support from the UnivEarthS LabEx program of Sorbonne Paris Cité (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02), the French National Research Agency (ANR-12-BS05-001-03/EXO-DUNES) and the Centre National d’Etudes Spatiales. B.C. acknowledges support from an appointment to the NASA Postdoctoral Program at NAI Virtual Planetary Laboratory, administered by Oak Ridge Affiliated Universities.

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Authors and Affiliations

  1. Virtual Planetary Laboratory, University of Washington, Box 351580 Seattle, Washington 98195, USA,

    Benjamin Charnay

  2. Laboratoire de Météorologie Dynamique, IPSL/CNRS/UPMC, 75252 Paris, France

    Benjamin Charnay & Sébastien Lebonnois

  3. Southwest Research Institute, Boulder, Colorado 80302, USA

    Erika Barth & Scot Rafkin

  4. Institut de Physique du Globe de Paris, CNRS-UMR 7154, Sorbonne Paris Cité, Université Paris-Diderot, 75238 Paris, France

    Clément Narteau

  5. Laboratoire Astrophysique, Instrumentation et Modélisation, CNRS-UMR 7158, Université Paris-Diderot, CEA-SACLAY, 91191 Gif sur Yvette, France

    Sébastien Rodriguez & Antoine Lucas

  6. Laboratoire de Matière et Systèmes Complexes, CNRS-UMR 7057, Sorbonne Paris Cité, Université Paris-Diderot, 75013 Paris, France

    Sylvain Courrech du Pont

Authors
  1. Benjamin Charnay
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  8. Antoine Lucas
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Contributions

B.C. developed the idea of the methane storm control. E.B. and S.R. developed and ran the mesoscale model. S.L. and B.C. developed and ran the GCM. B.C. analysed the simulations. C.N. and S.C.P. provided the dune growth mechanism and Fig. 4b. A.L. provided the denoised image and orientations of Titan’s dunes. B.C. wrote the paper with significant contributions from all the authors in interpreting the results and editing of the manuscript.

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Correspondence to Benjamin Charnay.

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The authors declare no competing financial interests.

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Charnay, B., Barth, E., Rafkin, S. et al. Methane storms as a driver of Titan’s dune orientation. Nature Geosci 8, 362–366 (2015). https://doi.org/10.1038/ngeo2406

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