PerspectivePlanetary Science

Linear Dunes on Titan

Science  05 May 2006:
Vol. 312, Issue 5774, pp. 702-703
DOI: 10.1126/science.1126292

Land forms and deposits created by the dynamic interactions between granular material and airflow occur on several planetary bodies, including Earth, Mars, and Venus. Recent orbital image data indicate that such aeolian (wind-produced) landforms and deposits also occur on Titan, as originally predicted by Greeley (1). As reported on page 724 of this issue, Lorenz and colleagues have analyzed high-resolution radar image data of Titan and conclude that extensive areas of dark linear features are directly comparable to large linear desert dune fields on Earth (2). This interpretation provides intriguing new information on the nature of surface processes on one of Saturn's moons, one of the few in the solar system known to possess an atmosphere.

The recognition of landforms on other planetary bodies involves comparison of orbiter and lander images with terrestrial analogs (see the figure). Because morphologically similar landforms are assumed to be formed in essentially the same manner on different planetary surfaces, this approach can indicate the types of surface processes and environments that occur on an unfamiliar landscape, provided that the fundamentals of the landforms and processes are well understood on Earth (3). Such an approach has been applied most successfully on Mars, where studies of terrestrial analogs of landforms and deposits go back to the recognition of dunes on Mariner 9 images in the early 1970s (4). It has also been used to understand airflow and aeolian processes on Venus (5). In many instances, the need to understand the physics of surface processes on other planetary bodies has generated fundamental studies of such processes on Earth. The report by Lorenz et al. is an excellent example of the application of well-chosen Earth analogs to understand geomorphic processes and wind patterns on Titan.

The dunes on Titan are very similar in geometry (width, crest-to-crest spacing, and length) to linear dunes in Namibia and the Rub al Khali of Arabia and show patterns and interactions with preexisting topography that have direct parallels in terrestrial desert regions. On Earth, existence of linear dunes indicates a moderate supply of sand and seasonally varying winds from two directions oblique to the dune and separated by less than 180°. Such winds tend to promote extension of the dune in the direction of the mean transport vector with some lateral migration of the form (6, 7).

Patterns in a sea of sand.

(Top) Plot of height versus crest-to-crest spacing of linear dunes in Namibia, Kalahari, and Australia, compared to Titan dunes. Data for terrestrial dunes are from field measurements and aerial photographs (6). Titan data are interpreted from figure 1 of (2). Note that Titan dunes are directly comparable in size and spacing to Namibian examples. (Left) Landsat 7 image of linear dunes in the central Namib Sand Sea. Dune crests are spaced ∼2.5 km apart; dunes are 100 to 150 m high. (Right) Ground photograph of linear dunes in Namib Sand Sea. Dunes shown are ∼100 m high.

CREDIT: LANDSAT IMAGE FROM USGS/EROS, SIOUX FALLS, SD; GROUND PHOTO BY N. LANCASTER

Occurrence of aeolian features on any planetary body indicates the existence of a sufficient supply of sand-sized sediment, winds to transport that sediment, surface conditions (such as lack of vegetation and low surface moisture) that make it possible for the wind to transport sediment (sediment availability), and topographic and/or meteorological conditions that promote deposition of the transported sediment (8). Production of sand-sized sediment can be achieved by a number of processes, including explosive volcanism that produces ash and tephra, cratering and meteorite impacts, and weathering and erosion by water and/or ice. On Titan, Lorenz et al. suggest that sand-sized material could be produced by fluvial processes, based on the existence of channel systems. Alternatively, atmospheric deposition could produce agglomeration of hydrocarbon solids of sand size.

Assuming that material of sand size is available, existence of aeolian landforms indicates that wind is currently capable of transporting such material, or has been capable of transporting sand-sized material in the geologic past. On Titan, the low gravity and high atmospheric density combine to produce conditions in which material of sand size can be transported by winds with a velocity as low as 0.1 m s−1. Atmospheric models and limited observations by landers indicate that these conditions are currently met on Titan. This is in contrast to Mars, where winds capable of transporting the materials found in aeolian landforms appear to be very rare in modern atmospheric conditions (9). Movement of large volumes of sediments across the surface of Titan to form sand seas also indicates generally dry surface conditions.

The accumulation of sand-size material in certain areas to form sand seas or dune fields implies spatially varying wind and sediment-transport patterns in which the influx of material into an area exceeds the outflux, resulting in deposition of sediment. Such spatial variations in wind energy are likely the product of global and regional circulation patterns, with topography playing a local role, as in terrestrial deserts.

Recognition of extensive linear dune fields on Titan provides further evidence of the variety of processes on planetary surfaces and the wonders of our solar system. The existence of such dunes constrains the nature of surface and atmospheric processes in the exotic environment of this moon of Saturn by providing evidence for the existence of granular material and its large-scale transport by wind.

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