Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p51g1193r&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P51G-1193
Other
[5460] Planetary Sciences: Solid Surface Planets / Physical Properties Of Materials, [6005] Planetary Sciences: Comets And Small Bodies / Atmospheres, [6055] Planetary Sciences: Comets And Small Bodies / Surfaces, [6281] Planetary Sciences: Solar System Objects / Titan
Scientific paper
Eolian dunes occur on Earth, Venus, Mars, and Titan, distinguishing them as one of the more widespread landforms in the solar system. On Earth, unidirectional winds blowing over loose, noncohesive sand produce crescentic-shaped dunes with crests oriented normal to the sand-transport direction (transverse dunes), but roughly half of Earth's large deserts are covered with longer-crested dunes (linear dunes) that are commonly oriented parallel or oblique to the resultant sand-transport vector (longitudinal dunes and oblique dunes, respectively). Such linear dunes form in at least two situations: (1) directionally bimodal winds blowing over loose sand, and (2) unimodal winds blowing over sediment that is vegetated, cohesive, sheltered by upwind topography, or otherwise locally stabilized. This study documents an example (Qaidam Basin, China) where a downwind increase in sediment cohesiveness (due to salt and mud incorporated from the local land surface) causes dunes to change orientation from transverse to longitudinal, and the work presents a compilation of related situations where stabilization of dune sediment has been reported to produce linear dunes. This family of stabilized dunes functions dynamically as self-extending "sand-shadow" or lee dunes. Loose sediment accumulates locally on these dunes, where it becomes stabilized, thereby allowing the dune itself to function as an obstacle that induces subsequent deposition farther downwind. Linear dunes on Titan previously have been interpreted as forming in the first situation listed above: bimodal winds blowing over loose sand. Because Titan's sand is believed to be composed of hydrocarbons or nitriles, however, the hypothesized loose, non-sticky nature of the sand has surprised researchers. In addition, the previous hypothesis of bimodal winds requires that north-south tidal flow be stronger than west-east zonal flow, which also was unexpected. The new hypothesis presented here—that Titan's dunes formed by unidirectional winds blowing over cohesive or sticky sand—resolves these two puzzles, cannot be ruled out with existing observations, and has grossly different implications regarding Titan's sediment properties, surface moisture, and wind regime. Satellite image of dunes in the Qaidam Basin, China. Change in sediment properties causes a change from transverse to longitudinal orientation of the dunes. Transverse dunes are higher in elevation than the longitudinal dunes and composed of noncohesive sand. Longitudinal dunes are cemented with silt, clay, and salt acquired locally.
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