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IAG Planetary Geomorphology Working Group

Featured image for September 2007:

Longitudinal dunes on Saturn's moon Titan

Image and caption contributed by Dr. Jani Radebaugh

Dunes on Titan[Click for full resolution]

The Cassini spacecraft is in orbit around Saturn, and occasionally flies close to one of its many icy moons. Because of specially designed instruments on Cassini, the surface of Saturn's largest moon, Titan, enshrouded in a thick, hydrocarbon haze-rich atmosphere, has been observed for the first time by this spacecraft. The RADAR instrument, operating in Synthetic Aperture Radar mode at 2.17 cm, can peer through the haze to see directly to the surface. One of the most interesting discoveries about Titan's surface processes is the presence of thousands of dunes, mostly in Titan's equatorial regions. These features are dark to RADAR and the near-infrared (from other Cassini instruments, the ISS and VIMS), indicating that their composition is likely organic particulates that have rained out of the mostly nitrogen and 1.5 bar surface pressure atmosphere. The dunes are almost strictly longitudinal in form, similar to dunes found in many of Earth's large sand seas, such as in the Namib, Saharan, Arabian, and Australian Simpson deserts. These features form mostly parallel to, rather than transverse to, the dominant wind direction. They may elongate downwind due to slightly oblique, perhaps seasonal winds. The dunes are 1-2 km wide, with 1-3 km dune spacing, and can extend over 100 km, similar in form to terrestrial sand sea dunes.

Dunes on Titan can be seen to interact with topographic features. In the image above from the 21st orbit around Titan in December 2006, radar-dark dunes divert around radar-bright or speckled ridges as wind flows from W-E. Dunes divert both to the north and the south of the obstacles, and seem to end abruptly at certain features that may have slopes prohibitively steep for the dunes to climb. On the lee side of topographic features the dunes resume their course and continue down wind. Dunes taper off toward higher latitudes, perhaps because sand supply decreases away from low latitudes. Further study of Titan's dunes will help refine models of global wind patterns and regional wind interactions with topography. In addition, it will help us understand erosion, transport, and deposition of dune particles, and will provide constraints on global climate, as dunes form in generally dry areas.

Cassini RADAR SAR image is north up, with resolution ~300 m. RADAR illumination direction and inclination angle is indicated by the open arrow. Image courtesy of the NASA Cassini Project.

Further Reading:

Elachi, C., et al., (2006) Titan Radar Mapper Observations from Cassini’s T3 fly-by. Nature 441, 709-713, doi:10.1038/nature04786.

Lancaster, N. 1995. The Geomorphology of Desert Dunes. Routeledge, London and New York, 290 pp.

Lorenz, R. D., et al., (2006) The Sand Seas of Titan: Cassini RADAR observations of Longitudinal Dunes. Science 312, 724-727.

Lunine, J. I., et al., (in press) Icarus, in press.

Radebaugh, J., R. et al., (in revision) Dunes on Titan observed by Cassini Radar, Icarus.

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