Titan’s dune fields scanned in the microwave: revealing their true nature

Post by Dr. A Lucas, CNRS Research Scientist, Université de Paris, Institut de physique du globe de Paris, CNRS, F-75005, Paris, France

Titan, Saturn’s largest satellite, has proved to be a world that is both strange and yet so familiar to us. Mountains, lakes, drainage systems and dune fields (Images 1-2) cover its surface. Methane on Titan occupies a similar position to water on Earth. It participates in climatic cycles. Moreover, its photodissociation in the upper atmosphere is responsible for the soot rains that fall on the surface of this icy world. The fate of these grains composed of organic materials is just as essential. Indeed, winds sometimes mobilize them. Over long, very long periods of time, this granular transport is responsible for the formation of vast dune fields located at the equator. But after 13 years of exploration by the Cassini probe, these dunes have not revealed all their secrets. In particular, their morphodynamics are widely debated. Are these bedforms remains from an old time, are they still active today? What is their growth rate? And what is their resulting sediment flux?

Fig1_new

Image 1: Despeckled T8 swath SAR image over the Belet sand dunes located at the Equator of Titan. The dark longitudinal features are the micro-wave absorbent dunes composed of sand made of organics molecules. The bright areas are rough topographic reliefs revealing the icy bedrock beneath the organic sediment cover. Glints (bright spots) are detectable of the crest of some dunes due to specular reflection on their avalanche side.

In order to unlock their secrets, Lucas et al. have developed a model of the RADAR response of these giant dunes. Indeed, the Cassini probe, clad with high-precision instruments, carried a RADAR imager (ku-band) capable of traversing the thick atmosphere to the surface. These echoes were analyzed from every angle. Thanks to the space probe’s overflights, a wide range of observation geometries, unique even for terrestrial observations, could be collected. By taking advantage of the physical formalism of the multiple interactions between RADAR echoes, surface and sub-surface, an inversion of the surface properties of dune fields was carried out.

T8_Dunes

Image 2: Despeckled T8 swath SAR image over another part of the Belet sand dunes located at the Equator of Titan. See Image 1 for interpretation of the features.

In particular, the roughness and dielectric properties have been finely tuned. First, this work showed that the mountains surrounded by the dunes were mainly composed of water ice and had very rough surfaces. But the main results of this work show that the sedimentary coverage between the dunes and the interdunes is not of the same nature. The grains have been segmented into the finest grains that are involved in building the dunes themselves, while the largest grains are located at the foot of these aeolien beforms. These new observations are comparable to what we know on Earth. Indeed, such granular segregation is observed in different places, particularly in the Kumtagh in China and in the Ténéré Desert in Niger (Image 3). The dunes that are found there have very particular morphodynamics. Their growth occurs by advection along their crest, in the resultant drift direction of sand flows. This work also emphasized the possible presence of avalanche phases on the dunes. Moreover, this work teaches us that the sediments located in the interdunes do not participate, or only slightly, in sediment transport under the assumed wind conditions. There is a phenomenon of armouring in the interdunes that keeps the largest grains on the ground. These results are important because they require a review of our understanding of wind transport at Titan’s equator. On a global scale, it has probably been overestimated compared to the first estimate.

Nevertheless, we still need to understand the wind conditions that are responsible for these dune structures. The recently selected DragonFly mission will help us to better understand these fascinating geomorphological processes.

Bilmakumtag_red

Image 3: Granular segregation and avalanche phases in terrestrial desert (top) Ténéré desert, Niger. (bottom) Kumtagh desert in China.  Credits: montage A. Lucas, Y. Callot (Niger, 1976), G. Steinmetz and C. Narteau (China, 2011).

Further Reading

Lucas, S. Rodriguez, F. Lemonnier, A. Le Gall, S. MacKenzie, C. Ferrari, Ph. Paillou, C. Narteau, 2019. Texture and composition of Titan’s equatorial sand seas inferred from Cassini SAR data: Implications for aeolian transport and dune morphodynamics, Journal of Geophys. Research Planets, doi:10.1029/2019JE005965, also available on arXiv:1702.02881, 2019

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