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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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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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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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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DOI: https://doi.org/10.1038/ngeo2406
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