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The formation of Titan's dune controlled by tropical methane storms

13-04-2015

The equatorial regions of Titan (the largest moon of Saturn) are covered by a large field of linear dunes propagating eastward. This direction is opposite to mean winds predicted by climatic models, constituting one of Titan's great mysteries. By combining results from a regional model applied to methane clouds with a Titan global climate model, a French-American team showed that the formation of Titan's dunes should be controlled by rare tropical methane storms producing strong eastward gusts dominating the sand transport. These results explain the shape, orientation and direction of propagation of Titan's dunes, and provide clues on the origin of Titan's sand. This study involves researchers from the Laboratoire de Météorologie Dynamique (CNRS/IPSL/UPMC/ENS/Ecole Polytechnique),  the Southwest Research Institute in Boulder and other laboratories (IPGP, AIM, MSC) affiliated to the CNRS, the CEA and the University Paris Diderot, and is published in Nature Geoscience on 13 April 2015.

Image of tropical methane clouds at the equinox.

Titan is Saturn's largest moon. It is surrounded by a thick atmosphere mostly composed of nitrogen (around 98%) with a small fraction of methane, which can condense into clouds. One of the greatest surprises of the Cassini-Huygens mission was the discovery of a large dune field (the largest in the Solar System) in the equatorial band. These dunes are linear, tens of kilometers long, about one hundred meters high and parallel to the equator. The nature of Titan's sand is unknown but planetary scientists believe that it is composed of hydrocarbon polymers, kind of soot coming from methane photolysis in the atmosphere. The direction of propagation of the dunes has been determined by analysing their morphology around topographic obstacles and their termination.  Radar images indicate that all dunes propagate eastward in the equatorial regions. However, climate models applied to Titan predict that mean surface wind should be westward at these latitudes, similar to trade winds on Earth. Therefore, Titan's dunes propagate in the opposite direction of predicted winds, constituting a great enigma.

Despite these predictions for surface winds, winds flow eastward everywhere at higher altitude. This phenomenon is called super-rotation. A coupling between this super-rotation and dune formation may solve the mystery of the eastward dune propagation. The researchers have thus proposed a possible coupling through methane storms occurring during the equinoctial season in the equatorial regions. These storms are rare but particularly violent. They are large convective systems developing up to high altitudes. Using a Titan regional weather model, the researchers showed that methane storms should produce strong downdrafts, flowing eastward when reaching the surface. These strong gusts dominate sand transport and propagate dunes eastward.


Illustration including Saturn and a radar of Titan's dunes.

By combining these results with wind patterns from a Titan global climate model and by using analogies with the formation of linear dunes on Earth, the researchers proposed a general scenario for the formation of Titan's dunes. This scenario explains most major features of Titan's dunes such as their shape, orientation, direction of propagation, size and spacing. It also reveals that Titan's sand has necessarily been formed in the equatorial regions, whereas it was previously proposed to have been formed in polar regions where there are methane seas. Finally, the strong gusts produced by tropical methane storms likely imply the occurrence of sand or dust storms, very common during storm events over terrestrial deserts. This study therefore reveals that aeolian processes on Titan are controlled by rare and strong events making Titan a world much more dynamic than expected.



Source

"Methane storms as a driver of Titan's dune orientation". B. Charnay, E. Barth, S. Rafkin, C. Narteau, S. Lebonnois, S. Rodriguez, S. Courrech du pont, A. Lucas, Nature Geoscience, http://dx.doi.org/10.1038/ngeo2406



Contact

Benjamin Charnay , Tél. : (33) 1.44.27.4972

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