Interpretation Of Wind Regime of Bagnold Dunes In Gale Crater, Guided By Third-Generation Models Of Dune Formation
Abstract
HiRISE images show that barchan dunes in the Bagnold dune field merge and change abruptly into linear dunes as they migrate southward (Fig. 1). Models of the conditions required to produce this kind of change have evolved substantially over the past half-century. First-generation models (pre-1980s) generally considered dunes to be either transverse or parallel to the sand-transport direction and interpreted winds accordingly. These models required drastic changes in winds to convert barchans to linear dunes (from unidirectional to bi-directional winds). Second-generation models used experiments and theory to quantify the orientation of transverse, oblique, and longitudinal dunes in bi-directional wind regimes, but these models also require substantial differences in wind regime between barchans and linear dunes (Rubin & Hunter, 1987, Science). Third-generation models—which are still in their infancy—have used lab experiments and stability analysis to show that where the bed is partially starved, a surprisingly weak secondary wind can induce grossly different dune morphology and orientation (Courrech du Pont, et al., 2014, Geology). For example, adding a secondary wind oriented at an angle of 150° to the main wind and having a magnitude of only 20% of it will convert barchans to linear dunes on a partially starved bed. We interpret the wind regime of the Bagnold dunes using lab experiments, third-generation dune models, observations of migration of superimposed ripples, and wind-models. Several characteristics of these dunes make starved-bed models appealing. First, the bed is, in fact, partially starved. Second, the change from barchans to linear dunes coincides with changes in sand coverage and occurs over an extremely short distance ( a single dune wavelength). In this situation, the secondary wind required to produce linear dunes can be nearly an order of magnitude weaker than the main mode, allowing the required abrupt change in winds to be less extreme. Figure 1. HiRISE image of Bagnold dunes, showing rapid downwind (north-to-south) change in morphology from barchans to linear dunes. Image: NASA/JPL/University of Arizona.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMEP43D..05R
- Keywords:
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- 1824 Geomorphology: general;
- HYDROLOGYDE: 1862 Sediment transport;
- HYDROLOGYDE: 5415 Erosion and weathering;
- PLANETARY SCIENCES: SOLID SURFACE PLANETSDE: 5419 Hydrology and fluvial processes;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS